external corrosion control for infrastructure sustainability...

Manual of Water Supply Practices

M27

External Corrosion Control for Infrastructure Sustainability

Third Edition

American Water Works Association

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American Water Works Association6666 West Quincy AvenueDenver, CO 80235-3098awwa.org

Manual of Water Supply Practices — M27, Third Edition

External Corrosion Control for Infrastructure Sustainability

Copyright © 2014 American Water Works Association

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information or retrieval system, except in the form of brief excerpts or quotations for review purposes, without the written permission of the publisher.

DisclaimerThe authors, contributors, editors, and publisher do not assume responsibility for the validity of the content or any consequences of its use. In no event will AWWA be liable for direct, indirect, special, incidental, or consequential damages arising out of the use of information presented in this book. In particular, AWWA will not be responsible for any costs, including, but not limited to, those incurred as a result of lost revenue. In no event shall AWWA’s liability exceed the amount paid for the purchase of this book.

AWWA Senior Manager of Editorial Development and Production: Gay Porter De NileonAWWA Senior Technical Editor/Project Manager: Martha Ripley GrayCover Art: Cheryl ArmstrongProduction: Janice Benight Design StudioAWWA Senior Manuals Specialist: Molly Beach

Library of Congress Cataloging-in-Publication Data

External corrosion. External corrosion control for infrastructure sustainability. -- Third edition. pages cm. -- (Manual of water supply practices ; M27) Includes index. ISBN 978-1-58321-966-9 -- ISBN 978-1-61300-245-2 1. Corrosion and anti-corrosives. 2. Water-pipes--Corrosion. I. American Water Works Association. II. Title. TA462.E98 2013 628.1’5--dc23 2013022195

Printed in the United States of America

ISBN-13 978-1-58321-966-9 eISBN-13 978-1-61300-245-2

Copyright © 2014 American Water Works Association. All Rights Reserved.

©vAWWA Manual M27

Figures

1-1 Metals used in a typical gate valve, 31-2 Metals used at a typical water-service-to-main connection, 41-3 Metals used in a pipe-repair clamp, 4

2-1 The four basic elements of a galvanic corrosion cell: anode, cathode, electrolyte, and electronic path, 8

2-2 Chemical reactions in a typical galvanic corrosion cell, 92-3 Galvanic cell formed with nonuniform electrolyte and electrodes of a

single metal, 102-4 Creating a galvanic cell with a single piece of metal in a nonuniform

electrolyte, 112-5 Contrasting conventional current flow with electron movement in a galvanic

corrosion cell. Current flow in the electrolyte is by ion transport, 122-6 A typical electrolytic corrosion cell, 132-7 Direct-current transportation system as a source of current causing electrolytic

corrosion, 152-8 Measuring cell voltage with a voltmeter, 152-9 Concentration cell (crevice) corrosion, 192-10 Pitting corrosion (arrows indicate positive current flow), 192-11 Impingement corrosion, 202-12 Photograph of sulfur joint corrosion failure of gray iron pipe caused by a

combination of corrosive characteristics of the sulfur joint material used to pack the joint and MIC, 21

2-13 Stress, fatigue, and fretting corrosion, 222-14 Selective corrosion, 232-15 Dezincification of a brass valve seat, 242-16 Atmospheric corrosion, 24

3-1 The four-pin system of soil-resistivity testing, 263-2 Use of a single probe for testing soil resistivity, 273-3 Quad-box for testing resistivity of a water-saturated soil sample, 273-4 Testing soil pH, 283-5 Testing for pipe-to-soil potential, 293-6 Failure caused by stray current, 313-7 Example joint bond: coat exposed wire, thermite weld, and steel after welding, 323-8a Unrestrained joint per AWWA M11 (AWWA C200-type steel pipe), 373-8b Restrained joint per AWWA M11 (AWWA C200-type steel pipe), 373-9 Reinforced concrete cylinder pipe (AWWA C300-type concrete pipe), 403-10 Prestressed concrete lined cylinder pipe (AWWA C301-type concrete pipe), 403-11 Prestressed concrete embedded cylinder pipe (AWWA C301-type concrete pipe), 403-12 Bar-wrapped concrete cylinder pipe (AWWA C303-type concrete pipe), 41

4-1 Cathodic protection system (galvanic cell using sacrificial anodes), 524-2 Details of a sacrificial anode installation, 544-3 Details of an impressed-current system, 57


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4-4 Corrosion caused by electrical discontinuity in a cathodically protected pipeline, 584-5 Corrosion of a metal structure in the vicinity of a cathodically protected structure, 594-6 Three methods for polyethylene encasement of ductile-iron pipelines, 64

5-1 Schematic of the four requirements for atmospheric corrosion, 685-2 The effects of orientation on atmospheric corrosion, 695-3 Crevice corrosion or pack rust, 715-4 Stress-related galvanic corrosion of fasteners, 725-5 Application of coating system, 75

6-1 Typical cathodic protection system for ground storage reservoir, 836-2 Typical cathodic protection system for elevated tank, 84


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2-1 Galvanic series of selected metals and alloys (in seawater), 172-2 Typical soil corrosion cells resulting from nonuniform electrolyte conditions, 22

3-1 Soil-test evaluation for ductile-iron pipe (10-point system), 35 3-2 Soils grouped in order of corrosive action on steel, 383-3 Relationship of soil corrosion to soil resistivity, 383-4 Guidelines for use of AC pipe based on pH of acidic soils, 443-5 Corrosion guidelines for AC pipe for soluble sulfate in water and soils, 443-6 Stainless steel alloys and their corrosion resistance, 453-7 Suitable applications for stainless steels, 46

4-1 Magnesium anode factors, 55

5-1 Comparison of uniform corrosion rates of different metals in the atmosphere, 70

Tables


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Corrosion control is a practical science that allows relatively reliable predictions of corrosive conditions and, more importantly, effective prevention or mitigation strategies to control corrosion where economically appropriate. Faced with how best to install and protect a transmission or distribution system to minimize corrosion, today’s water utility manager must compare the cost of implementing reliable corrosion protection with the benefit of ensuring an extended life for pipelines and appurtenances.

Both the technology and the economics involved in external corrosion control are complex, requiring a logical and well-considered approach by utility managers, operators, and consulting engineers, all of whom must be familiar with local conditions and available options. Not all environments are corrosive, not all materials corrode, and there is no single answer to all corrosion problems. In any given situation, the corrective measure selected must be at once the most appropriate for the material and environment involved and eco-nomically feasible.

This manual is addressed primarily to the professional water utility operator, whose objective is to provide safe drinking water to the public. The title has been revised for the third edition to reflect the basic premise that corrosion control is an attempt to extend the useful life of water infrastructure. The text is intended to give the reader an understand-ing of how and why corrosion occurs, how the corrosion potential of an environment is evaluated, and how many of the proven corrosion prevention and control measures oper-ate. For readers who believe a review of theoretical concepts of basic chemistry and basic electrical circuit theory is necessary beyond that which is addressed herein, the relevant sections of Basic Science Concepts and Applications* are recommended.

Updates in this third edition were made to the figures, tables, and text, including dis-cussion of the effects of chemical environment on pipe materials, pitting and crevice corro-sion, common water pipe coatings, and recommended protective methods for specific pipe materials. Topics added or expanded on include stray current corrosion, microbiologically induced corrosion (MIC), oxygen-concentration cells, crevice corrosion of stainless steel, galvanic corrosion, and stainless steel in aboveground environments. New emphasis was placed on using field and lab measurements to determine corrosive conditions.

The general principles and examples presented in this manual are not intended to replace the services of a knowledgeable corrosion engineer. However, methodical appli-cation of the principles introduced—i.e., determining the cause of corrosion, analyzing its extent, and considering appropriate procedures for prevention or mitigation—will lay the foundation for an effective corrosion control program that will benefit the public and the utility alike.

In the selection and application of corrosion monitoring, protection, and mitigation materials, the operator must confirm that materials in contact with the water supply have been tested and certified in accordance with the standards and regulations relevant to the water utility involved.

*Basic Science Concepts and Applications, 3rd ed., American Water Works Association, Denver, Colo-rado (2010), ISBN 9781583217788.

AWWA Manual M27

Preface


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Contents

AWWA Manual M27

List of Figures, v

List of Tables, vii

Preface, ix

Acknowledgments, xi

Chapter 1 Importance of Controlling External Corrosion . . . . . . . . . . . . . . . . . . . . . . . 1 Corrosion: Occurrence and Implications, 2 Economics of Corrosion Control, 6 References, 6

Chapter 2 Chemistry of Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Basic Electrochemistry of Corrosion, 7 Chemistry of Corrosion in Water Systems, 14

Chapter 3 Evaluating the Potential for Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Field and Laboratory Measurements, 26 Stray Currents, 31 MIC (Microbiologically Influenced Corrosion), 33 Effects of the Chemical Environment on Common Water Pipe

Materials, 34 References, 47

Chapter 4 Corrosion Control and Protection of Buried Pipelines . . . . . . . . . . . . . . . 49 Coatings and Linings, 50 Cathodic Protection, 52 Materials Selection, 60 Trench Improvement, 60 Protective Methods for Specific Pipe Materials, 60 References, 65

Chapter 5 Atmospheric Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 How Metals Corrode in the Atmosphere, 68 Types of Corrosion That Can Be Expected, 70 Methods of Control, 73 Coating Evaluation, 77 Stainless Steel in Aboveground Environments, 77

References, 77Chapter 6 Corrosion Control of Water Storage Tanks . . . . . . . . . . . . . . . . . . . . . . . . . 79

Corrosion of Water Tanks, 79 Corrosion Prevention for Water Tanks, 81 Conclusion, 86 References, 86

Glossary, 87

Index, 91

List of AWWA Manuals, 97


AWWA MANUAL

M 27

Chapter 1

Importance of Controlling External Corrosion

Corrosion is the deterioration of a material or its properties because of a reaction with its environment. Deterioration of pipelines, valves, pumps, and associated equipment due to external corrosion is an important concern for many water utilities. At one time, corrosion was accepted as inevitable in many soils, and extra thickness for metal piping was often specified to extend the piping’s useful life. Today, a variety of techniques are available to eliminate or significantly reduce external corrosion. Determining the need for such corrosion-control measures and selecting the most appropriate techniques are the primary topics of this manual. Corrosion is generally defined as deterioration of a metal or an alloy by reaction with its environment. For the purposes of this manual, corrosion also includes the dissolving of other water system materials through contact with water or soil.

This chapter first presents a brief introduction to the science and terminology of corrosion and then discusses economic implications and managerial responsibility for external corrosion-control programs.

After completing this chapter, the reader should be able to 0 Define corrosion.

Recognize certain environmental conditions and items of water supply equipment that are often associated with external corrosion problems.

Understand the cost to remedy and the extent of the corrosion problem.

Recognize potential hazards to public health and safety that may result from corrosion.

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©2 EXtErnal Corrosion Control for infrastruCturE sustainability

AWWA Manual M27

• Understand the basic economic questions that must be asked when selecting mea-sures for corrosion control.

• Recognize the responsibilities for a corrosion-control effort that must be assumed by various utility personnel.

Corrosion: oCCurrenCe and impliCationsCorrosion is a natural phenomenon. Metals are normally found in their stable, oxidized (corroded) form in nature. Iron ores, for example, are found as iron oxides. These oxides are chemically reduced in the refining process to produce useful metal, with the iron atoms in the elemental (unoxidized) form. In the presence of oxygen and water, or under certain soil and electrical conditions, refined iron tends to return to its more stable form, iron oxide (rust). Some waters and some soils are especially favorable to corrosion. The US Federal Highway Administration (FHWA 2002) performed a two-year study on the direct costs associated with metallic corrosion in nearly every US industry sector, from infra-structure and transportation to production and manufacturing. The study provides cur-rent cost estimates and identifies national strategies to minimize the effect of corrosion. Results of the study show that the total annual estimated direct cost of corrosion in the United States is $276 billion ($36 billion for drinking water and sewer systems).

potentially Corrosive ConditionsSeveral conditions increase the likelihood that corrosion will occur in a water utility system:

• Dissimilar metals or alloys in contact with each other and with a common media, such as water or soil.

• Great variances in soil in contact with metal or alloys.• Naturally occurring corrosive soil.• Atmospheric corrosion.• Environmental contamination of soil with chemical waste, cinders, mine wastes,

salts, or other refuse.• Stray current corrosion, including exposure to stray direct-current earth currents

from transit systems. • Microbiologically influenced corrosion.These conditions, discussed briefly in the following sections, are examined in detail

in chapters 2 through 4 of this manual. Where such conditions occur, the water utility staff should be especially alert to the selection of materials and preventive measures that will minimize the effects of corrosion.

Dissimilar metals. Iron and copper are among the metals used in water system pip-ing, valves, pumps, and other equipment. For each application, the manufacturer selects a metal with appropriate properties. There is no single ideal metal or alloy that can satisfy the many requirements of water system equipment.

Unfortunately, whenever two dissimilar metals are immersed in a common corro-sive medium (soil or water) and then placed in contact with each other, the likelihood of corrosion significantly increases. The extent of corrosion depends on the characteristics of the corrosive medium and the metals involved. Figures 1-1 through 1-3 illustrate common


IMPORTANCE OF CONTROLLING EXTERNAL CORROSION 3

uses of dissimilar metals and alloys in water systems. Each of these situations, among many others, poses a potential condition for corrosion.

Soil variances. The composition of soil can vary from point to point and with changing soil depth. In many cases, a single metallic unit (pipe, well casing, valve, etc.) may be in contact with two or more completely different soil types. Whenever th is situation occurs, the likelihood of corrosion increases. The severity of the corrosion will depend on the soils and the metal involved.

Naturally corrosive soils. As noted, some soils tend to promote corrosion. As a general rule, swamps, bogs, peat, and soils with high salt content are corrosive. Low, poorly drained soils are more likely to be corrosive than soils in well-drained areas. The corrosivity (also called aggressiveness) of a given soil can be determined by sampling, testing, and analysis.

Atmospheric corrosion. Equipment and facilities may experience corrosion due to exposure to the atmosphere, brought on by acid rain, salt spray, industrial chemicals, and other factors.

Cap Screw (Stee )

Operat ng Nut (Iron)

Bo ts and Nuts (Stee )

Stem (Bronze)

Body and Bonnet (Cast Iron)

Gate Nut (Bronze)

Gate (Cast Iron)

Wedge (Bronze)

Figure 1-1 Metals used in a typical gate valve

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4 EXTERNAL CORROSION CONTROL FOR INFRASTRUCTURE SUSTAINABLTJTY

Water Man (Iron)

Corporal on Stop (Brass) ServceLne (Copper or Ga van zed Stee )

Figure 1-2 Metals used at a typical water-service-to-main connection

Bots(A oyStee) Band (Sta n ess Stee )

Lugs (Ma eab e Iron)

Figure 1-3 Metals used in a pipe-repair clamp

Environmental contamination. In many urban areas, the history of street surfacing may offer clues concerning potential corrosion of underground water system materials. Older streets were often surfaced with cinders and later paved Cinders are aggressive to most pipe and valve materials, and their presence is a warning that serious corrosion may

Coal bottom ash has been used as an aggregate material in flowable fill mixes. Bot- tom ash (and boiler slag) is composed principally of silica, alumina, and iron, with smaller percentages of calaum, magnesium, sulfates, and other compounds. The composition of

occur.

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©importanCE of Controlling EXtErnal Corrosion 5

AWWA Manual M27

the bottom ash or boiler slag particles is controlled primarily by the source of the coal and not by the type of furnace. Due to the salt content and, in some cases, the low pH of bottom ash and boiler slag, these materials could exhibit corrosive properties. When using bottom ash or boiler slag in an embankment, backfill, subbase, or pipe base course, the potential for corrosion of metal that may come in contact with the material is of concern and should be investigated prior to use.

The presence of chloride salts can create corrosive soils. Steel reinforcement in con-crete, iron, copper, brass, and many other materials in common use may be subject to attack if elevated concentrations of chlorides are present in the environment. Heavy use of deicing salts and chemicals on streets and highways can also be a potential source of corrosion.

Finally, sites where chemical contamination has occurred, such as refuse dumps, landfills, and mine or industrial waste disposal areas, may cause deterioration of water utility materials. Such locations should be avoided if possible. However, if alternative loca-tions are not feasible, the potential for corrosion must be considered.

Stray current corrosion. ASTM G15, Standard Terminology Relating to Corrosion and Corrosion Testing, defines stray current corrosion as “corrosion caused by electric cur-rent from a source external to the intended electrical circuit, for example, extraneous current in the earth.” Although stray current corrosion may sometimes be caused by alter-nating current in areas with a very high alternating-current density, it is normally associ-ated with direct current.

Stray current corrosion is normally localized and will occur at locations on the struc-ture where the direct current is discharged back into the earth. In areas of stray current influences, electrically continuous pipelines accumulate a greater magnitude of stray current flow than electrically discontinuous pipelines. Sources of stray direct current normally include cathodic protection systems, direct (DC) powered streetcars or trains, welding equipment, and mine/industrial equipment.

A more detailed discussion of stray current corrosion is given in chapter 3 (Evaluat-ing the Potential for Corrosion).

Microbiologically influenced corrosion (MIC). ASTM G15 defined microbiologically influenced corrosion as “corrosion inhibited or accelerated by the presence or activity, or both, of microorganisms.” MIC-related corrosion normally takes the form of pitting, as compared with generalized corrosion. Four primary forms and mechanisms of MIC have been proposed and published (Pope and Morris 1995).

1. One is in instances where a biofilm, a film composed of families of low-nutrient bacteria, forms on the metal surface, creating a differential aeration cell.

2. Another occurs when various mutually beneficial bacteria create a colony housed in a biodome, thereby setting up a corrosion cell by cathodic depolarization.

3. Still another condition may exist in which the biological waste material from these bacteria within the biodome presents a strong acid concentration, which can rap-idly perforate the metal substrate.

4. Lastly, conditions can occur that provide for iron-reducing bacteria to flourish. In this instance, bacteria that respire iron (Fe), or utilize Fe in their electron recep-tor for energy, become citizens of the colony represented in a particular biodome.

To date, there is no widely accepted field method without laboratory analysis to pos-itively identify MIC responsible for, or contributing to, corrosion. MIC continues to be studied and defined to develop reliable field test methods and also to allow more defini-tive control mechanisms to be developed. A more detailed discussion on MIC is given in chapter 3.



AWWA Manual M27

implications of CorrosionAside from the 2002 FHWA report, information is limited that details costs incurred by the public water supply industry due to corrosion-produced losses. Given the extent and wide variety of materials used in water systems, the amount is certainly substantial.

In addition to the financial impact of repair, replacement, labor, and equipment, other more important costs impact the public as a result of corrosion. The health of water consumers may be threatened whenever extensive corrosion breaches the sanitary integ-rity of the water system. The ever-present danger of backflow of contaminated liquid into the drinking water system is further increased when water pressure is interrupted to facil-itate repairs on corroded wells, pumps, treatment equipment, pipes, valves, and services.

Another concern is that public safety depends heavily on an adequate supply of pressurized water for fire control. Low pressures and insufficient water can result in the growth of small fires into disasters that cause injury, death, and destruction of property. Uncontrolled corrosion can be a major contributor to the problems of unreliable or inad-equate fire-control systems.

Controlling corrosion in water utility systems can contribute greatly to cost savings, public health protection, and public safety.

eConomiCs of Corrosion ControlTwo primary considerations are involved in any decision regarding corrosion control. The first and more important is the protection of public health and safety. The second is eco-nomics. Both private and governmental utilities must operate effectively and efficiently.

In either case, faced with decisions regarding the best corrosion-control programs to implement, water utility staff must determine which actions will produce the lowest over-all cost and the highest return on capital. The staff must decide which alternative is prefer-able: (1) minimize initial costs and accept higher maintenance costs and shorter equipment life or (2) marginally increase initial investment by specifying corrosion-control proce-dures that will reduce maintenance and extend the life of components.

Economic evaluations are commonly the province of the design engineer and util-ity management. Determining a reasonable estimate for the anticipated life of alternative installations requires considerable engineering expertise and experience. However, much of the required data is empirical and depends on knowledge of the system and local envi-ronmental conditions.

referenCesUS Federal Highway Administration (FHWA), Office of Infrastructure Research and Development. 2002.

Corrosion Costs and Preventive Strategies in the United States. Report FHWA-RD-01-156 from CC Technolo-gies Laboratories Inc. http://www.nace.org/uploadedFiles/Publications/ccsupp.pdf (accessed July 2013).

Pope, D.H., and E.A. Morris. 1995. Some Experiences with Microbiologically Influenced Corrosion of Pipe-lines. Materials Performance, 34 (5): 24.


AWWA MANUAL

M 27

Chapter 2

Chemistry of n Corrosion

Corrosion in water utility components affects a large number of materials, occurs under a wide range of environmental conditions, and can have greatly varying effects. Nonethe- less, the physical mechanisms underlying corrosion are essentially unchanged from one situation to another. In this chapter, basic chemical and electrical reactions of corrosion are explained, and several water utility system configurations where corrosion commonly occurs are described.

After completing this chapter, the reader should be able to Explain the basic corrosion reactions.

Recognize an anode and a cathode on a corroded piece of equipment.

Explain the difference between a galvanic corrosion cell and an electrolytic corrosion cell. Determine which metal will corrode in a bimetal couple.

Understand how polarization and passivation can retard corrosion.

Recognize several common types of corrosion and understand why they occur.

BASIC ELECTROCHEMISTRY OF CORROSION Corrosion of metals and their alloys is an electrochemical process; that is, a corrosion reaction involves both chemical reactions and the flow of electrons. Two basic types of com- sion occur in a water system: galvanic corrosion and electrolytic corrosion. This section discusses the basic physical configurations and chemistry of each type.

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8 EXTERNAL CORROSION CONTROL FOR INFRASTRUCTURE SUSTAINABILITY

Galvanic Corrosion The galvanic corrosion process is identical to the reactions in an electrical battery, in which electrical current is generated by immersing two dissimilar metals, called electrodes, in a chemical solution (electrolyte) and connecting them with an external conducting wire, referred to as the electronic path. Figure 2-1 shows the elements of a galvanic corrosion cell. Note the solid arrows indicating the direction of electron movement and the black/ gray arrows indicating positive current flow. Numerous figures throughout the document illustrate corrosion cells of various types. Most of these show electron flow as "e-." Cur- rent flow through the electrolyte is by ion transport and not electron flow. Free electrons do not exist in the electrolyte.

In the galvanic cell, corrosion takes place at the surface of the electrode where electrons are generated to travel through the metal path. This electrode is called the anode. The conducting solution (water, soil, or some other chemical solution) is called the electrolyte. The electrode to which electrons flow is called the cathode. These four elements-the anode, cathode, electrolyte, and electronic path-must exist before corrosion can occur.

Electron Flow (e-)

~ositive Current FIOW

Figure courtesy of NACE International.

Figure 2-1 The four basic elements of a galvanic corrosion cell: anode, cathode, electrolyte, and electronic path

AWWA Manual M27

Copyright 0 2014 American Water Works Association. All Rights Reserved.

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CHEMISTRY OF CORROSION 9

Chemistry. The basic electrochemical reactions occurring in a galvanic corrosion cell are fairly simple. In the cell shown in Figure 2-2, the iron (Fe) anode on the left is corroding. Some of the iron atoms release electrons, which travel across the electronic path and enter the cathode. This loss of electrons changes the iron atoms from elemental iron (Fe") to ferrous iron (Fe2+) and then to ferric iron (Fe3+), leaving them with a strong positive charge. Some of the molecules of the electrolyte, in this case pure water (H,O), are naturally separated into hydrogen ions (H+) with positive charges and hydroxyl ions (OH-) with negative charges. The positively charged iron atoms are attracted to the negative OH- ions. The attraction causes the iron atoms to leave the anode and enter the electrolyte, where they combine with OH- ions. As atoms are lost, the metal surface of the anode deteriorates. This deterioration is corrosion. Note that the products of corrosion, Fe(OH), and Fe(OH),, may accumulate on or near the corroded surface.

At the cathode, the negatively charged electrons arrive from the metallic path. The electrons are attracted to the positively charged H+ ions in the electrolyte. The attraction causes the electrons to leave the cathode and combine with the H+ ions, forming hydrogen gas (H,). The gas may accumulate on the surface of the cathode. Note that the metal of the cathode does not corrode. In fact, the reactions within the corrosion cell actually protect the cathode from corrosion. This fact forms the basis for cathodic protection (chapter 4).

Electron Flow (e-1 - ~""""""""","

Positive Current Flow

HI Gas

Electrolyte' (H,O=H'+ O H 3

' The charge carrier in the dectrolyte Is ions, not electrons.


Figure 2-2 Chemical reactions in a typical galvanic corrosion cell

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When different materials are used for the electrodes and electrolyte, the chemical reactions will be slightly different. For example, under field conditions, the water forming the electrolyte of the corrosion cell may contain dissolved oxygen. If dissolved oxygen is added to the water in the corrosion cell just described, the reaction at the cathode will combine the oxygen, water, and electrons to yield hydroxyl ions (OH-) instead of hydrogen gas. Oxygen can react with the surface layer of hydrogen, thereby removing it as a depo- larizing agent. Because the layer of hydrogen forms a coating of sorts, and behaves as a resistor, its removal accelerates the cathodic reaction and corrosion activity. Changing the chemical composition of the electrolyte or the materials acting as the electrodes in a corrosion cell can cause great variations in the severity of the corrosion. However, no matter what materials are involved, it will always be the anode that corrodes and sends electrons into the electronic path, and it will always be the cathode that is protected from corrosion.

Nonuniform electrolytes. The type of galvanic cell just described has two different metals immersed in a single, uniform electrolyte. A second type of galvanic cell occurs when two pieces of the same metal are immersed in an electrolyte of uneven composition, as shown in Figure 2-3. Because of the uneven nature of the electrolyte, a corrosion cell can develop. One piece of metal will become the anode-it will corrode and feed electrons into the electronic path. The other piece of metal acts as the cathode-it will be protected from corrosion and will feed ionic charge carriers into the electrolyte.

As illustrated in Figure 2-4, the cell just described can be modified so that a single piece of metal acts as the anode, cathode, and electronic path. Figure 2-4A shows the same configuration as Figure 2-3-two electrodes of the same metal in a nonuniform

Electron Flow (e-)


Figure 2-3 Galvanic cell formed with nonuniform electrolyte and electrodes of a single metal

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A.

C.

electrolyte. In Figure 2-4B, the two electrodes have been placed in direct contact with each other, eliminating the connecting wire. In Figure 2-4C, the two separate electrodes have been replaced by a single block of metal, which has one area acting as a cathode, another as an anode, with electrons flowing through the block. In Figure 2-4D, the block of metal, which could be a piece of pipe, has been totally buried in the heterogeneous electrolyte, which could be soil. However, the effect remains the same as in the original configuration-the metal loss occurs at the anode.

Because soil is often nonuniform, a possibility always exists that such corrosion cells can develop around underground metal structures. Whether the cell causes significant damage depends on the corrosivity of the soil as an electrolyte and the type of metal or alloy involved.

Current flow. Up to this point, the operation of the galvanic corrosion cell has been described in terms of the movement of electrons. In practice, corrosion cell chemistry is often discussed in terms of electrical current flow, also called conventional or positive cur- rentflow. Because of a historical misunderstanding about the nature of electricity, this conventional current flow is considered to move in the direction opposite to the movement of the electrons. Thus, where electrons leave iron atoms in the anode to travel into the electronic path, the conventional current flow is said to go in the other direction, leaving the surface of the anode and entering the electrolyte. At the cathode, the conventional current flows from the electrolyte into the cathode.

Restated in terms of conventional current flow, the basic galvanic corrosion reaction is as follows (Figure 2-5): electrical current is generated by immersing dissimilar metals

D.


Figure 2-4 Creating a galvanic cell with a single piece of metal in a nonuniform electrolyte

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Electron Flow {e-)

Positive Current Flow

- ~""""""","""""


Figure 2-5 Contrasting conventional current flow with electron movement in a galvanic corrosion cell. Current flow in the electrolyte is by ion transport

or alloys in a uniform electrolyte or by immersing a single metal or alloy in a nonuniform electrolyte. The current travels from the anode through the electrolyte to the cathode and returns through the electronic path. At the location of current discharge from the anode into the electrolyte, a loss of anode metal occurs. This loss of metal is corrosion. Corrosion control is the process of reducing, eliminating, or reversing the current flow, thus reducing or eliminating the corrosion.

Electrolytic Corrosion The galvanic corrosion cell discussed in the previous section requires an anode, a cathode, an electrolyte, and an electronic path." The reactions in the cell generate an electrical current. The configuration of an electrolytic cell is similar to a galvanic cell, but the electrolytic cell does not generate an electrical current. Instead, the corrosion reaction is

"The committee considered changing the terminology used in the first two editions of this manual from return current path to metal path or electronic path. In the corrosion community and NACE publications, metal path is the common term. Metal path is not exactly correct because current can also be carried by semiconductors, soil, graphite (carbon) in iron, and graphite or carbon electrodes used by CP systems. Some soils are rectifying and can change AC to DC based on their ability to conduct more efficiently in one direction of current flow than the other. The soils in this case behave as a semicon- ductor diode. Carbon is not a metal but conducts electricity, including electrons related to corrosion concerns, very well. Another example of a nonmetallic current path is stray current corrosion, which occurs when current flows from a source through the soil to an underground structure and that structure sustains damage at the location where the current exits to return to its source. In this scenario, no metal path connects the current source to or from the damaged structure. This is all to say the return current path is more descriptive and accurate than metal path. After much discussion, return current path was changed to electronic path as the preferred wording in all occurrences in M27.

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driven by a direct-current source originating outside the cell. However, the end result is the same-corrosion at the anode.

Figure 2-6 shows an electrolytic corrosion cell. The four basic elements of the galvanic cell are still necessary: anode, cathode, electrolyte, and electronic path. In addition, an outside direct-current source, located within the electronic path, must exist to drive the reaction. As current is forced through the corrosion cell by the outside current source, corrosion occurs at the anode.

When electrolytic corrosion develops in metals exposed to soil or water, it can be severely damaging. Several situations may arise in which the external direct current needed to drive electrolytic corrosion can occur in the area of a water distribution system. Direct-current-driven train or subway systems can be a source of stray current if the rails (the system’s intended electronic path) are not completely electrically isolated from the soil (Figure 2-7). If underground pipe or other metallic equipment picks up a portion of this current, it will corrode at the point where the current ultimately leaves to rejoin the original circuit. Other sources of stray direct current are electric welding equipment grounded to underground utilities and stray direct current from nearby cathodic protection systems.

Alternating current does not have the same magnitude of effect on metal at the point of discharge as direct current and generally does not cause corrosion. In some cases, alternating current at very high current densities can cause corrosion.

Electron Flow (e-)

Positive Current Flow -

~.11111111....111111II


Figure 2-6 A typical electrolytic corrosion cell

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Chemistry of Corrosion in Water systemsAll corrosion can ultimately be explained in terms of the principles discussed in the ear-lier sections of this chapter. However, the nature and severity of corrosion within a water system will vary greatly depending on the materials involved, the electrolyte, the physical configuration of the cell, and the environment. This section discusses the calculation of corrosion rates and considers the effects of different materials that may be involved in a galvanic cell. It also explains two physical/chemical processes that may reduce the severity of corrosion and briefly describes a number of common corrosion configurations found in water systems.

Calculating Corrosion rateCorrosion reactions occur in accordance with well-understood physical laws. By deter-mining the conditions surrounding a corrosion cell, the theoretical rate of corrosion can be predicted. Actual field conditions will vary the calculated rate somewhat, but predictions are still an important tool in determining the need for corrosion-control measures.

Cell voltage. The chemical reactions occurring in a galvanic corrosion cell force elec-trons through an electronic path. When the electronic path is disconnected, the force—the potential for current flow—still exists, exhibited as the difference between the electrical charges of the anode and the cathode. As in any electrical circuit, this electrical force is known as potential, or voltage. The voltage across the electrodes of a galvanic cell that is not connected to an electronic path is called the cell voltage, cell potential, or voltage differential. Voltage is a relative measure of driving force for corrosion between two electrodes. It can be measured by connecting the leads of a voltmeter to the two electrodes, as shown in Figure 2-8.

The chemical reactions within a galvanic corrosion cell will vary depending on the materials and electrolyte that comprise the cell, and the cell voltage will vary accordingly. For corrosion cells created as part of a battery designed to generate electrical current, the potential can be several volts. For corrosion cells commonly occurring in water utility piping systems, the voltage will usually be less than 1 V, and it is commonly measured in millivolts (1,000 mV = 1 V). The cell voltage is the driving force that pushes the electrons through the electronic path—the greater the voltage, the greater the current, and the more rapid the rate of corrosion.

Ohm’s law and Faraday’s laws. The effects discussed in the previous paragraph can be expressed mathematically. Ohm’s law states that the current flow through an electronic path will be equal to the voltage (the cell potential) divided by the resistance of the circuit. Thus,

=I E

R(Eq 2-1)

Where:

I = current flow (A)

E = potential or voltage difference (V)

R = resistance (in ohms)



Third Rail or Overhead Power Line


Figure 2-7 Direct-current transportation system as a source of current causing electrolytic corrosion


Figure 2-8 Measuring cell voltage with a voltmeter

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According to Faraday’s law, the rate at which an anode of a given metal corrodes var-ies with the magnitude of electrical current flow and the electronic properties of the metal. For example, the corrosion rate for iron is 20 lb (9.1 kg) per ampere of current per year. Using Ohm’s and Faraday’s laws, the rate of current flow and the rate of corrosion can be calculated wherever the corrosion cell current is known.

Galvanic Corrosion materialsGalvanic corrosion cells can occur in many combinations of metals, alloys, and electrolytes found in all potable water production and distribution systems. In considering corrosion cells caused by differential metal combinations (known as galvanic or bimetal couples), it is essential to know which of the two metals involved will act as the anode and, therefore, cor-rode. It is also important to know how great the potential for corrosion will be for any two metals. The galvanic series for a given electrolyte supplies the answers to these questions.

The galvanic series. Table 2-1 shows a galvanic series typical for seawater. When any two metals in the series are combined with an electrolyte and an electronic path to form a galvanic cell, the metal nearer the top of the series will be the anode and corrode; the metal nearer the bottom will be the cathode and be protected. The metals on the cathodic end of the series are said to be more noble than those on the anodic end.

The farther apart two metals are in the series, the greater the cell voltage they will produce across the electronic path when immersed in the electrolyte. The cell voltage and the resistance of the circuit will determine how much current will flow and, therefore, how rapidly corrosion of the anode will occur. Thus, metals that are widely separated in the galvanic series will produce corrosion cells that demonstrate a high rate of corrosion, even when the electrical resistance of the circuit is high. Metals that are in close proxim-ity in the series will produce cells where little or no corrosion occurs, because the voltage produced will seldom be sufficient to force a significant corrosive current to flow.

For the series shown in Table 2-1, the cell exhibiting the greatest corrosion would be formed with an anode of magnesium and a cathode of platinum, immersed in seawater and connected through an electronic path. The galvanic series shown in Table 2-1 is accu-rate where the electrolyte is seawater. Freshwater or moist soil exhibits slight variations in the arrangement of the metals in a galvanic series, and the magnitude of the cell voltage generated with electrodes of any two metals would also vary.

Galvanic materials in water systems. In most water systems, different equipment can contain components made of steel, gray iron and ductile iron, stainless steel, lead, brasses, copper, and bronzes. Many components are exposed only to treated water and may be sub-ject to internal corrosion within the electrolyte of the treated water. The subject of internal pipe corrosion is beyond the scope of this manual. However, combinations of metals exposed externally do occur, notably where services, valves, or pumps are connected to mains.

For example, a brass curb stop connected to a wrought-iron service can be a source of considerable corrosion, as illustrated by the distance between brass and wrought iron in the galvanic series. A large valve may comprise several metals and alloys in its construc-tion, with two alloys commonly used on the outside exposure: steel for nuts and bolts and cast iron for the valve body. It would appear that the two alloys are too close together in the galvanic series for corrosion to occur; however, the tendency for steel to act as an anode in a cell with cast iron is greatly magnified by the difference in surface areas between the small bolts and nuts and the much larger cast-iron valve body and bonnet. The same effect occurs in gray- or ductile-cast iron mechanical-joint bolts and nuts, as well as many other configurations within a water utility system where a small surface area of relatively anodic metal is in contact with a large surface area of relatively cathodic metal.


©ChEmistry of Corrosion 17

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Table 2-1 Galvanic series of selected metals and alloys (in seawater)* at 25°C

Anodic, Active End at Top of Series (Read Down)† Manganese bronze

Naval brass

Magnesium Nickel (active)

Magnesium alloys Inconel—76% Ni, 16% Cr, 7% Fe (active)

Zinc Yellow brass

Aluminum 52SH Aluminum bronze

Aluminum 4S Red brass

Aluminum 3S Copper

Aluminum 2S Silicon bronze

Aluminum 53S-T Ambrac—5% Zn, 20% Ni, 75% Cu

Alclad 70% Cu, 30% Ni

Cadmium Comp. G bronze—88% Cu, 2% Zn, 10% Sn

Aluminum 17S-T Comp. M bronze—88% Cu, 4% Zn, 6.5% Sn, 1.5% Pb

Aluminum 24S-T Nickel (passive)

Mild steel Inconel—75% Ni, 16% Cr, 9% Fe (passive)

Wrought iron Monel—70% Ni, 30% Cu

Gray iron and ductile iron 18-8 stainless steel, Type 304 (passive)

Ni-resist 18-8, 3% Mo stainless steel, Type 316 (passive)

13% Cr stainless steel, Type 410 (active) Titanium

50–50 lead–tin solder Silver

18-8 stainless steel, Type 304 (active) Graphite

18-8, 3% Mo stainless steel, Type 316 (active) Gold

Lead Platinum

Tin

Muntz metal Cathodic, Noble End at Bottom (Read Up)†

Source: ASTM G82, Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance. ASTM International, 2002. West Conshohocken, Pa.* Each environment has its own specific galvanic series. The relative positions of the various metals and alloys may vary slightly

from environment to environment.† In a galvanic cell of two dissimilar metals, the more active metal will act as the anode and be corroded, while the more noble

metal will act as the cathode and be protected.



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One good method of counteracting this differential effect of surface area is to pro-vide greater nobility in the smaller metal unit. For example, ductile-iron pipe can be fur-nished with alloy steel coupling bolts containing copper, nickel, and chromium. This can minimize corrosion reactions, even in fairly conductive electrolytes.

The most severe galvanic corrosion occurs when two metals are installed that are widely separated in the galvanic series. However, due to intrinsic manufacturing non-homogeneity, even a single piece of metal equipment can have slight variations in the composition of its metal surface from one point to another. With a sufficiently strong elec-trolyte, these variations can produce microscopic galvanic corrosion cells, which can even-tually expand and produce major deterioration of the metal.

polarization and passivationTwo different phenomena that may reduce the rate of corrosion are polarization and pas-sivation. Where appropriate, a design engineer may select materials that encourage these effects.

Polarization. As the products of a corrosion reaction build up at the anode and cath-ode, the voltage difference between the two tends to diminish. This is called polarization. The effect is likely to be greater at the cathode. For example, in acid solutions, hydrogen gas can literally blanket the cathode surface, tending to retard the flow of current. The hydrogen layer may be partially removed as hydrogen gas combines with oxygen gas, forming water. This is more pronounced on a comparatively large cathode because the hydrogen is spread thinly over a large surface area. Hence, it is preferable to have a large anode and small cathode than the reverse.

Even so, the anode can become polarized by a layer of oxide film or other compound that has the effect of forming a barrier between the metal and its environment. Some met-als polarize more easily than others; therefore, metal selection can be an important part of engineering.

Passivation. Passivation is a condition in which a given metal behaves in the manner of a more noble metal than is indicated by its position in the galvanic series. It is a condi-tion of electrochemical activity where the initial corrosion products of certain metals pro-vide protection of the base metal.

Passivation of steel in concrete. Steel embedded in concrete becomes corrosion resis-tant. The curing of concrete is a process called hydration, where the cement in freshly placed concrete develops a high alkalinity. The alkalinity of the concrete forms a passive oxide film on the surface of the steel due to the initial corrosion reaction. The passive film that forms iso-lates the steel from the environment and stops further corrosion as long as the film is intact. That alkalinity, in the presence of oxygen, stabilizes the initial corrosion film on the surface of the embedded steel and ensures protection while the alkalinity is maintained. Typically, concrete would have a pH of 12 or above because of the presence of calcium hydroxides. When passivated, the steel behaves in the manner of a more noble metal than is indicated by its position in the galvanic series. That is also seen by measuring its potential to a half cell. Typically, bare steel will have a potential of –500 mV to –700 mV to a copper–copper sulfate half-cell; passivated in concrete, its potential rises to –200 mV.

specific types of Corrosion in Water systemsIn water utility systems, situations commonly occur in which a combination of environ-mental factors, physical configurations, and materials can accelerate corrosion. The fol-lowing paragraphs describe some of those conditions and their effects.

Concentration cells. Concentration cells (Figure 2-9) are localized occurrences of cor-rosion that often develop at crevices in metal units due to the uneven diffusion of oxygen.



Oxygen content inside the crevice may be low compared with that in the environment just outside the crevice. In this case, metal near the lower oxygen concentration will become anodic, and corrosion will occur within the crevice.

Pitting corrosion. When protective films covering a metal, metallic layers, or the uniform surface of a metal break down, corrosion occurs as pitting (Figure 2-10). The metal within the pit area is anodic and the surrounding area is cathodic.

Impingement corrosion attack. As a result of the manufacturing process, many metals can form protective surfaces under normal operating conditions. Physical impact, such as a strong stream of water, can break through this protective surface and produce corrosion (Figure 2-11). Pumps are especially vulnerable to this type of attack because of cavitation, the abrasive effect caused by the collapse of bubbles formed by extreme pressure differentials within the pump mechanism.

Low Oxygen


Figure 2-9 Concentration cell (crevice) corrosion

Electrolyte


Figure 2-10 Pitting corrosion (arrows indicate positive current flow)

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4- Flow


Figure 2-11 Impingement corrosion

Microbiologically influenced corrosion (MIC). MIC refers to corrosion brought about by the presence and/or activities of microorganisms in biofilms on the surface of the corroding material. Most materials, including metals, some polymers, glass, and ceramics, can be degraded in this manner.

A number of microbiological organisms, notably bacteria, fungi, and algae, have been associated with corrosion damage in water systems. These microorganisms can influence corrosion by effects such as differential aeration cells, the production of corrosive species such as mineral and organic acids, ammonia, and reduction of sulfate to sulfide. The various types of microbes tend to act synergistically in the corrosion of materials with their interactions typically being of a complex nature.

By-products of sulfate-reducing bacteria cause bacteriological corrosion (Figure 2-12). These bacteria live in environments where little or no oxygen is present and the pH is near neutral. Their life processes reduce sulfates to sulfides that are excellent electrolytes and depolarizers and are otherwise aggressive to metal surfaces.

In the metabolic process of any living thing, there is an electrodynamic balance, as energy is either required or released during the functioning of the organism. In some species of bacteria, these energy changes induce a measurable electric current. When corrosion-causing microorganisms reproduce on the surface of iron pipe, electrobiochemical removal of iron occurs, causing corrosion. Sulfate-reducing bacteria have been found to cause the electrode potential of stainless steel to shift 150 mV to the negative side.

Microorganisms can participate in iron and steel corrosion by creating conditions favorable to electrochemical reactions. For example, their metabolism can change the surface film resistance with such products as sulfuric and organic acids. In other instances, slime deposits may shield some areas so that they become anaerobic even with considerable oxygen present in the water. The anaerobic sites on the metal surfaces become anodic to the aerobic areas.

The most important bacteria associated with corrosion are the sulfate reducers or sulfide formers. They are classified as either nonsporulating (Desulfovibrio) or spore-forming (Desulfotomuculum, which include the Clostridium nigrificuns).

Sulfate-reducing bacteria use the sulfate ion as the terminal electron accepter for their respiration. These organisms are also autotrophic in using an inorganic source of carbon. Energy is obtained by the reduction of the sulfate ion by hydrogen, which can come from molecular hydrogen or organic compounds. In metabolism, the organisms can use molecular hydrogen for producing H,S. Corrosion usually results by cathodic depolarization, and it is greatly accelerated by the organism’s consumption of hydrogen.

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I

Figure courtesy of U.S. Pipe and Foundry Company.

Figure 2-12 Sulfur joint corrosion failure of gray-iron pipe caused by a combination of corrosive characteristics of the sulfur joint material used to pack the joint and MIC.

The net result is that iron is contjnuously removed from the anodic area, causing a pitting type of corrosion. FeS is cathodic to metallic h, and its presence accelerates corrosion.

Other bacteria, such as Thiobucillus fhioxiduns, can oxidize H,S to obtain energy for metabolism. The sulfuric acid produced is highly corrosive and forms an anodic concentration cell that further accelerates attack. The pH of this reaction has been recorded at less than 0.2.

The iron bacteria Sphaerotilus, Gullionella, and Crenothrix use energy produced from converting ferrous oxide to ferric hydroxide. The ferric hydroxide forms a sheath on the organism and such sheaths deposit on the walls of pipes, creating small areas of iron hydroxide. Anaerobic conditions exist under such areas, developing a suitable environment for growth of sulfate-reducing bacteria.

With the continued production of iron, additional iron bacteria develop on the deposits, thus contrihting to the iron scale on the pipe. The cycle feeds upon itself and produces sharply defined pits.

Oxygen concentration cells. When two portions of the same metal receive oxygen at different rates, a corrosion cell is established and two types of oxidation may occur: dehy- drogenation and loss of electrons. The hydrogen atom can disassociate into a proton and

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an electron. Electrons are transported by electron carrier systems, while the hydrogen ion is temporarily absorbed by the buffer medium of the celL The bottom of the pipe is usually anodic to the top of the pipe due to this phenomenon.

Soil corrosion. Most soil-induced corrosion occurs in soil with high electrical conductivity (low electrical resistivity), which makes the soil an effective electrolyte. Nonuni- forxnity, chemical contamination, or differential aeration (areas of high and low oxygen content) can increase the problem. Sometimes low-resistivity soils act in combination with sulfate-reducing bacteria to create extremely aggressive conditions. Table 2-2 lists some typical soil corrosion cells that result from variances in electrolyte along a metal surface.

The pH of the soil also may be significant. Acidic soils with pH below 5.0 are generally aggressive. A neutral pH (6.5-7.5) usually indicates less corrosive conditicms unless sulfate-reduc~g bacteria are involved High-pH soils are not normally aggressive to ferrous metals; however, they are usually high in soil salts, which can result in low soil resistivities and makes these soils good electrolytes.

Dry soils make poor electrolytes, thus moisture is generally a recpkment $or soil corrosion. Constantly dry soils seldom cause corrosion problems.

Stress, fatigue, and fretting corrosion. Physical reactions and corrosion can combine to rapidly deteriorate metal and alloys, as in stress, fatigue, and fretting corrosion (Figure 2-13). Tensile stresses, cyclic stresses, or high-frequency vibrations acting indepen- dently may or may not cause material failure, but a corrosive environment can enhance or accelerate the deterioration of metal.

Table 2-2 Typical soil corrosion cells resulting from nonuniform electrolyte conditions Anodic Area CathodicArea

Low oxygen High= axygen Low resistivity Higher resistivity Sulfate-reducing bacteria No sulfate-reducing bacteria Water-saturated soil Drier soil

clean soil cleansoil

Organic contamination (dead vegetation, leaves, refuse) Contact with highly conductive material (cinders, coal, salts)

A. Stress Corrosion R Fatigur Corrosion C Fretting Corroslon


Figure 2-13 Stress, fatigue, and fretting corrosion

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Selective corrosion. Selective corrosion singles out one element of an alloy (Figure 2-14). One material that may suffer selective corrosion is common yellow brass, which consists of approximately 30 percent zinc and 70 percent copper. In certain wet environments, zinc may be lost from the brass, leaving a porous copper mass with greatly reduced strength. This effect is termed dezincification. Figure 2-15 illustrates dezincification of a brass valve seat.

Atmospheric corrosion. Atmospheric corrosion (Figure 2-16) requires atmospheric humidity and an oxidizing agent, usually oxygen. Most water treatment plants have areas where materials are constantly exposed to very humid conditions. The result is oxidation, and in the case of iron or steel, rusting. In addition to oxygen, the halogens- chlorine, fluorine, iodine, and bromine-are extremely aggressive oxidizing agents in such environments.

Atmospheric corrosion may tend to cause uniform corrosion attack. Oxidizing agents (acids, chlorine, etc.) dissolve the corrosion product layer and oxidation continues.

Dezincification


Figure 2-14 Selective corrosion

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Figure courtesy of U.S. Pipe and Foundry Company.

Figure 2-15 Dezincification of a brass valve seat


Figure 2-16 Atmospheric corrosion

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AWWA MANUAL

M 27

Chapter 3

Evaluating the Potential for Corrosion

External corrosion of water utility piping and equipment is not inevitable. Certain combinations of materials in certain environments may suffer serious corrosion at a rapid rate, while other configurations may be relatively immune. For the purposes of both design and operations, it is important to be able to predict the occurrence and severity of corrosion.

For metallic corrosion to occur, four elements are required an anode, a cathode, an electrolyte, and an electronic path. Dissimilar metals and alloys are commonly found in contact in water utility installations, and even a single piece of metal can simultaneously act as anode, cathode, and electronic path. In either case, the electrolyte is a key variable in determining where and to what extent corrosion will occur. For external corrosion, the electrolyte is the environment: the soil or water in contact with the equipment.

Many environmental conditions will not support corrosion. Other environments serve as a damaging electrolyte for some metals, alloys, or metallic pairs. To evaluate an environment for a given material, experience provides the best guide. In the absence of experience, as in the case of new locations, analytical procedures should be used. If either experience or analysis shows the environment to be aggressive, corrosion prevention or control should be initiated.

This chapter details the relationships between specific materials and environmental conditions commonly found in water utility installations. It concludes with a discussion of monitoring for stray electrical currents that may cause electrolytic corrosion.

After completing this chapter, the reader should be able to Understand the influence of environment on the corrosion process.

Realize that a given environment may cause corrosion in one material but not in another.

Be alert to many of the environmental conditions that cause corrosion.

25


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Have a general understanding of the systems used to determine whether soils are corrosive to various materials. . Understand how stray direct current can affect underground metallic structures and recognize some of the common sources of stray current.

FIELD AND LABORATORY MEASUREMENTS

Soil Resistivity It is g e n d y accepted that soil resistivity is one of the most important ksts to be conducted when evaluating the potential corrosivity of a soil. A soil-resktivity test ydds the r e a p rocal of conductivity; low soil resistivity indicates that it will serve well as an electrolyte. Low-resistivity soils tend to be corrosive. Resistivity, reported in ohm-centimeters (ohm-cm), represents the average of the electrical resistances across each cubic centimeter of soil in a given volume. The test may be accomplished by several means. To determine average resis tivity from near ground surface to pipe depth or below, the four-pin system may be used (Figure 3-1). The equipment and test procedure for this test is described in ASTM GS7 (Test Method for Field Measurement of !%A Resistivity Using the Wenner Four-Electrode Method).

The four-pin system has limitations because it may average dry topsoil with wet- ter subsoil, contaminated soil with clean soil, etc. A single soil probe (Figure 3-2) may also be used. The probe is suitable for spe&c readings at various soil depths, enabling the surveyor to search out the lowest resistivity soil that may come in contact with pipe. Because soil moisture affects resistivity and moisture in the field may vary unpredictably, a soil sample may also be collected from pipe depth for laboratory testing. The quad-box (Figure 3-3) or a similar unit makes it possible to water-saturate the soil and simulate the most aggressive conditions likely to occur after pipe installation. The equipment and test procedure for this test are described in A ! 3 M G187 (Test Method for Measurement of Soil Resistivity Using the Two Uectrode Soil Box Method).

So Surface

Figure 3-1 The four-pin system of soil-resistivlty testing

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EVALUATING THE POTENTIAL FOR CORROSION 27

So Surface

Figure 3-2 Use of a single probe for testing soil resistivity

Stan ess-Stee End P ates

Noncondud ng S des and Base

Figure 3-3 Quad-box for testing resistivity of a water-saturated soil sample

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28 EXTERNAL CORROSION CONTROL FOR INFRASTRUCTURE SUSTAINABLITY

Information on FH of soil is w d as an aid in evaluating the corrosivity of a soil environment Soils with a pH below 4.0 are severely acidic in nature and thus are considered aggressive to most piping materials. Neutral pH (6.5-75) indicates that the soil may mpport sulfate-rdudng bacteria if other characteristics are suitable. Soils with a lugh pH (8.5-14.0) possess high concentrations of dissolved salts and usually exhiiit low resistivity. Figure 3 4 illustrates equipment for measuring pH. shown are a pH meter, a pH-sensing electrode, and a reference electrode. The pH-sensing electrode and reference electrode can also commonly be h n d combined into a single electrode referred to as a combination pH electrode. The equipment and test procedure for this test is described in ASTM G51 (Test Method for Mea- suring pH of Soil for Use in Corrosion Testing).

Pipe-to-Soil Potential Pipe-to-soil potential (also called structure-to-electrolyte potential) is the electrical potential between the metal and a reference electrode placed in contact with the earth. The reference cell is one-half of a battery. In the commonly used copper-copper sulfate reference electrode (CSE), the "half-cell" consists of a copper rod in a saturated solution of copper sulfate crystals. The other half of the cell is the metal in the soil electrolyte.

The reference cell has a stable, constant potential against which the voltage of another metal can be measured, analogous to measuring elevation with reference to sea level When the potential between a pipe and reference electrode in contad with the earth is measured, the potential includes not only the voltage between the pipe and reference cell but also the voltage created by currents flowing through the resistance of the earth. This measurement is the most useful test in evaluating stray-current effects and cathodic protection effectiveness.

Soil surface

Figure 3-4 Testing soil pH

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When a pipe is connected to the positive terminal of a digital voltmeter and the reference cell is connected to the negative terminal, tlw meter will give a negative reading, indicating that the pipe is negative to the reference electrode. A negative or minus sign dwuld be placed m front of the voltage reading. Note, however, that an analog-type voltmeter or potentiometer would read backward with these connections, and the refemce cell must be hooked to the positive terminal and the pipe to the negative terminal so that the needle will read upscale. Again, a negative sign must be added to indicate that the pipe is negative to the reference cell even though the analog meter has moved in a positive direction. Figure 3-5 illustrates the arrangement for measuring pipe-to-soil potential with a digital voltmeter.

The best type of connection is an insulated test lead, brazed or thermite-welded to the pipe. However, a connection may be made to a valve, blowoff, or other accessible appurtenance, provided that it is electrically continuous with the pipe.

Pipe-to-soil potentials measured with the reference electrode placed at intervals along and over the centerline of an electrically continuous pipeline may vary, indicating anodic (corroding) and cathodic (noncorroding) areas of the pipe. Readings taken with the reference electrode offset from the centerline of the pipe can also help locate anodic areas. Offset readings identical to the pipe alignment readings indicate neither cathodic nor anodic conditions and, if consistent, rule out the existence of stray current.

So Surface

Figure 3-5 Testing for pipe-to-soil potential

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AWWA Manual M27

If pipe-to-soil potentials become less negative as a reference cell is moved closer to the pipe, then current is flowing toward the pipe and the pipe is cathodic. If pipe-to-soil potentials become less negative as the reference cell is moved away from the pipe, then the pipe is anodic and tends to corrode. This is true for natural galvanic corrosion cells, such as between dissimilar metals and alloys, or anodic and cathodic areas on the same metal or alloy, and where the pipe is under cathodic protection. But, if the pipe is losing current due to stray current, the pipe-to-soil potential close to the pipe will be less negative (and may even go positive) compared with those potentials farther from the pipe.

Current span and line CurrentLine current measurements are useful for identifying stray or long-line currents on a con-tinuous pipeline and for locating suspected corrosion areas. Line current can be deter-mined by measuring the voltage between two points on a pipeline, then calculating the amplitude of current required to produce that potential in a line of the given size and material. The test procedure uses a high-resistance voltmeter (with millivolt ranges), with the terminals connected to test leads from the pipe. For most steel pipe with welded or bonded joints, current flow in amperes can be determined by the formula:

I = KV (Eq. 3-1)

Where: I = current flow between the two points (A) K = 4.0 W/L, a conversion factor indicating the resistance of the pipe (A/mV) W = the weight-per-length of the steel pipe (lb/ft) L = length of the span (ft) (calculation of K assumes welded or bonded pipe

joints, steel resistivity of 15.5 ohm-cm, and steel density of 490 lb/ft3) V = measured voltage differential (mV)

Similar calculations can be made for other types of metallic pipelines.The test span can be calibrated by causing a known direct current to flow across the

span (using test points outside the two IR span test points) to obtain the conductivity of the span, in amperes/millivolt.

Similarly, current in a span can be determined by applying a direct current opposite the stray current between two test points, then adjusting the applied current until the measured potential between the test points is zero. The applied counter-current is equal to the current flowing along the pipe. A series of current-span tests along a pipeline can help locate areas of discharge or assimilation of stray current.

Line currents can also be measured by means of a clamp-on ammeter, using clamps sized to fit the pipe. Clamps can be manufactured for virtually any size pipe. These are most useful above grade or in vaults where the clamp can be attached without excavating the full diameter of the pipeline.

Ground Voltage GradientsTesting ground voltage gradients helps to determine whether current is flowing through the soil in the vicinity of an existing or proposed underground structure. It also can indi-cate the direction of any current flow and thus help locate its source. When plotted along the pipe alignment, results of the tests may even be helpful in locating suspected anodic areas on a pipeline.

Ground voltage gradients are tested with a high-resistance millivolt meter and two matched copper–copper sulfate half-cells. Half-cells may be checked by placing them side by side in the soil surface. A zero-millivolt reading shows that they are properly matched.



With the half-cells placed in the soil surface at a selected spacing, any millivolt potential is recorded. If such a potential exists, the presence of earth current is implied. Measuring the gradient is accomplished by making a series of such readings in a straight line. "Leapfrogging"-leaving one half-cell in place and carrying the other past it to a new location of equal spacing along the alignment of the study-makes the test proceed more quickly, but care must be taken to reverse the sign for each new measurements. significant variations in the ground voltage may indicate anodic and cathodic sections of the pipeline. The test can also confirm the existence of areas of concern that were determined previously using pipe-to-soil tests.

STRAY CURRENTS Direct-current electricity flowing through the soil near a metal pipeline or structure can cause electrolytic corrosion of the metal or alloy. The electricity is referred to as stray current because it has strayed away from the path intended for it in the circuit where it orig- inated. Stray current is primarily a problem for long, electrically continuous pipelines, which can form a parallel pathway for the current in its original circuit. Figure 3-6 illustrates a failure attributable to stray current. Electrically segmented pipelines (those using rubber-gasketed joints that are not bonded) are usually not vulnerable to stray-current corrosion, unless they are near the direct-current source. Bonding of rubber-gasketed joints may be necessary if the line is to be monitored for possible stray current or is cathodidly protected. Figure 3-7 Wtrates a bonded joint.

Courtesy of U.S. Pipe and Foundry Company.

Figure 3-6 Failure caused by stray current

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TOP of pipe

W \

lnsu ated bond w re

\ I T / \ Rubber gasket jo nt

Therm te wed w re connect on

courtesy of AECOM

Figure 3-7 Example joint bond: coat exposed wire, thermite weld, and steel after welding

When planning a new pipeline, or if stray direct current is suspected of causing corrosion, the first step in analyzing the problem is to review the possible sources of direct current. Common sources indude impressed-current cathodic-protection systems, direct- current-powered transit systems, arc-welding equipment, and direct-current transmission systems. E&ds from most other sources vary with operation. If a local corrosion-control coordinating committee exists, its members are usually well informed on such sources. Information on such committees is available horn NACE International (http://www.nace. 0%). If no such c d t t e e is active, consult other utility operators.

Several electrical tests are commonly used far the analysis of stray current in the field. The tests measure a voltage differential or line current. The pipe-to-soil test measures the voltage between the pipe and the soil, using a reference electrode. The current-span test measures the voltage between two points on the pipeline. The ground-whge gradient test measures the voltage between two points in the soil. When test leads are placed at appropriate intervals along the pipeline, these tests allow the line to be monitored both for the existence of environmental stray currents and, to some extent, for the existence of active corrosion. The tests and observations are best applied to long, electrically continuous metal pipelines.

Various electrical tests described in the previous section under Field and Laboratory Measurements are used to investigate and monitor stray electrical currents. Note that any attempt to monitor a pipeline requires detailed records showing the location of outlets, connections to other lines, pipeline appurtenance structures (such as air valves and manholes), and proximity to foreign lines or structures that could influence monitoring data.

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©Evaluating thE potEntial for Corrosion 33

AWWA Manual M27

miC (miCroBioloGiCally influenCed Corrosion)MIC is one manifestation of the effect of soil bacteria on corrosion. Some general facts about microbes follow (Little and Wagner 2001).

• Not all microbes need oxygen to live. Some strict aerobes must have oxygen. Other strict anaerobes cannot exist in oxygen. And facultative microbes can live with or without oxygen but may prefer one over the other.

• Microbes live by having a receptor for their required energy source. Energy sources can be many different things. In terms of MIC, different microbes may seek sul-fate ions, hydrogen, oxygen, iron, and other elements.

• Microbes can endure extremes of pH and temperature. Some like acids, some like high alkalinity, and others like neutral pH. Some microbes can survive boiling in water for over an hour.

• They like to reproduce, some faster than others. The significance of the repro-duction rate is that a MIC problem can occur rapidly, even before anyone would suspect it.

• Current research (Taga and Bassler 2003, Winans and Bassler 2008) has shown that a mechanism exists whereby chemical signals even allow microbes to communicate.

• Microbes can resist many chemicals by chemically breaking down antibiotics and trapping the degraded material in a slime wall or by mutating to provide a defense mechanism.

The following four primary forms and mechanisms of MIC have been proposed and published (Pope and Morris 1995):

1. The most commonly accepted mechanism for MIC is in instances where various mutually beneficial bacteria create a colony housed in a biodome, thereby setting up a corrosion cell by virtue of cathodic depolarization. Those bacteria that can exist under anaerobic conditions (absence of oxygen) at the pipeline surface have the ability to reduce any sulfates present and consume hydrogen in the process. Consumption of hydrogen at the pipe surface acts to depolarize cathodic areas and permits more rapid consumption of the metal by galvanic corrosion cells. The bacteria, then, do not directly attack the pipe but provide conditions conducive to a more rapid attack by existing corrosion cells, which are normally partially stifled by the development of an insulating polarization film of hydrogen.

2. Another mechanism occurs when a biofilm, a film composed of families of low-nutrient bacteria, forms on the metal surface, creating a differential aeration cell in which oxygen-poor regions on the metal become anodes.

3. Still another condition may exist in which the biological waste material from these bacteria, within the biodome, presents a strong acid concentration that can rapidly corrode and perforate the metal substrate.

4. Lastly, conditions can occur that provide for iron-reducing bacteria to flourish. In this case, bacteria that respire iron (Fe), or use Fe in their electron receptor for energy, become citizens of the colony represented in a particular biodome.

To date, there is no widely accepted field method without laboratory analysis to posi-tively identify MIC as responsible or contributing to corrosion. MIC continues to be stud-ied and defined to develop reliable field test methods and also to allow more definitive control mechanisms to be developed.



AWWA Manual M27

effeCts of the ChemiCal enVironment on Common Water pipe materialsThe effects of soil, water, and air on a pipe vary greatly depending on the pipe material. This section discusses environmental effects on and evaluation procedures for materials commonly used in water utility pipe and appurtenances.

Gray-iron and ductile-iron pipeSeveral soil evaluation procedures for gray-iron and ductile-iron pipe installations have been developed. Because of the complexity of subsurface corrosion, no system is univer-sally applicable and no system can replace the need for expertise in the field of corrosion control. The evaluation system originally developed and recommended by the Ductile Iron Pipe Research Association, commonly referred to as the 10-point system, is discussed in detail in the following text. This soil evaluation procedure is specific to gray-iron and ductile-iron pipe and should not be applied to other materials. Additional information on soil evaluation, corrosion, and corrosion protection of ductile-iron pipe may be found in AWWA Manual 41, Ductile-Iron Pipe and Fittings.

Appendix A of ANSI/AWWA C105/A21.5-10, Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems, covers the soil-survey tests, observations, and interpreta-tions that comprise the 10-point system. The soil evaluation system was introduced in 1968 and adopted as an appendix to the standard in 1972. It was based on field experience with operating gray-iron pipelines, where soil-test results were recorded along with the age of the pipe and its condition, including extent and type of corrosion. Most of the experience was with gray-iron pipe. As ductile-iron pipe became more widely available, it was neces-sary to determine whether the same tests and evaluation could also be accurately applied. Field research has demonstrated that the overall corrosion rates of ductile and gray iron are such that the soil evaluation system can be applied to both with equal accuracy.

The evaluation procedure is based on the result of five soil tests and certain obser-vations: (1) soil resistivity, (2) pH, (3) oxidation-reduction (redox) potential, (4) sulfides, and (5) moisture. For a given soil sample, each result is evaluated and assigned points according to its contribution to corrosivity. The points for all five criteria are totaled. If the sum exceeds 10, the soil is considered corrosive to gray-iron or ductile-iron pipe, and cor-rosion will likely occur unless protective measures are taken (as discussed in chapter 4). Table 3-1 lists the scoring criteria for the various tests. The following sections provide additional detail on each area of evaluation.

Soil resistivity. See description in previous section titled Field and Laboratory Measurements.

pH. See description in previous section titled Field and Laboratory Measurements.Oxidation-reduction (redox) potential. A test of the oxidation-reduction potential

indicates the degree of soil aeration. Low or negative results indicate that the soil is anaer-obic and can support sulfate-reducing bacteria. The redox test can be performed with the same meter used to measure pH, using a platinum electrode in conjunction with the refer-ence electrode used for pH. As with the pH electrodes, these electrodes can be combined into a single combination redox electrode. The sample should be protected from exposure to the atmosphere until tested.



AWWA Manual M27

Table 3-1 Soil-test evaluation for ductile-iron pipe (10-point system)

Soil Characteristics Based on Samples Taken

Down to Pipe Depth Points*

Soil Characteristics Based on Samples Taken

Down to Pipe Depth Points*

Resistivity—ohm-cm† Redox potential

<1,500 10 > +100 mV 0

≥1,500–1,800 8 +50 to +100 mV 3.5

>1,800–2,100 5 0 to +50 mV 4

>2,100–2,500 2 Negative 5

>2,500–3,000 1 Sulfides

>3,000 0 Positive 3.5

pH Trace 2

0–2 5 Negative 0

2–4 3 Moisture

4–6.5 0 Poor drainage, continuously wet 2

6.5–7.5 0‡ Fair drainage, generally moist 1

7.5–8.5 0 Good drainage, generally dry 0

>8.5 3

Source: Based on Appendix A in ANSI/AWWA C105.*Ten points—corrosive to ductile-iron pipe; protection is indicated.†Based on water-saturated soil box.‡If sulfides are present and low or negative redox-potential results are obtained, three points shall be given for this range.

Sulfides. Soils containing elevated sulfide concentrations usually indicate the pres-ence of sulfate-reducing bacteria. The sulfides test is qualitative and accomplished by introducing a solution of 3 percent sodium azide in 0.1N iodine into a test tube containing a small quantity of soil obtained from pipe depth. If sulfides are present, they catalyze a reaction between sodium azide and iodine with the release of nitrogen. The chemical reaction is

2NaN3 + I2 → 2NaI + 3N2 ↑SodiumAzide

Iodine SodiumIodide

NitrogenGas

Moisture. The sample should be protected from exposure to the atmosphere until tested. Since the soil moisture may vary throughout the year, general drainage character-istics are recorded rather than specific moisture content.



AWWA Manual M27

Experience. In addition to the analytical tests just described, which will indicate the ability of the soil to cause corrosion, notes on prior experience in the area are extremely important. In many cases, experience can yield the best predictions of soil corrosivity.

Other methods. Another soil corrosivity analysis method is a proprietary mathemat-ical model for ductile-iron pipe employed by DIPRA (DDM,™ or Design Decision Model). That system of analysis includes all the factors discussed for the 10-point system but takes into account additional considerations related to the concentrations of chlorides in the soil and whether bi-metallic couplings such as corporation stops or connections to other pipe materials will be present. Also added to this system is an evaluation regarding the influ-ence of the groundwater that may be present and whether that groundwater will fluctuate, such as would be found in tidal areas, on a multiple-times-daily basis into and out of the pipe zone.

Another soil aggressivity assessment system widely used by corrosion engineers is the nine-point system described in the Handbook of Cathodic Corrosion Protection (von Baeck-mann et al. 1997). The information on the nine-point system is presented in Section 4.2.1.

Also, as noted in Appendix A of ANSI/AWWA C105/A21.5, there is a need to recog-nize that certain soils and installations are known to be corrosive regardless of the results of any testing that might be conducted. Examples include cinders, landfills, and peat bogs. Such “known corrosive environments,” when encountered, indicate that the likelihood of a corrosion problem would be significant if the pipe were to be installed as manufactured without corrosion prevention or control measures.

steel pipeSteel pipe applications include buried and exposed pipelines conveying water and treat-ment plant and process piping systems conveying water. Modern steel water pipe gen-erally consists of a watertight steel cylinder that is lined with cement mortar or other protective lining and then coated with cement mortar or polymer-based dielectric coating. The steel cylinder is the main structural element of the pipe; however, when used, the cement–mortar lining and coating also contribute to pipe stiffness. Steel pipe is assembled in the field with welded or rubber-gasketed joints (Figure 3-8). For additional information, refer to AWWA Manual M11, Steel Pipe—A Guide for Design and Installation. Bare steel pipe is not recommended for direct burial unless it is coated with cement mortar or dielectric coating, encased in concrete, or pressure grouted in a tunnel liner application.

The potential for external corrosion of steel pipe depends on subsurface environ-mental conditions. Soil chemical and physical analyses (e.g., pH, moisture content) and the presence of stray current are important factors that impact the design and operation of corrosion control measures. Soil resistivity is generally the most important parameter for judging soil corrosivity with respect to steel pipe. As a general guideline, Tables 3-2 and 3-3 summarize the extent of corrosion attack on buried bare steel pipe as related to soil characteristics and soil resistivity. Additional information on the principles of corrosion and corrosion control can be found in AWWA Manual M11. For cement–mortar-coated steel pipe, refer to the section on concrete cylinder pipe in this chapter.



Grout placed alter installation (or heat shrink sleeve if bonded dielectric coating used)

Cement-Mortar Coating (may be substituted with bonded dielectric coating)

Joint bonding (if specified)

Rubber 0-Ring Mortar placed Cement Mortar Lining Gasket after installation

Courtesy of Northwest Pipe Company.

Figure 3-8a Unrestrained joint per AWWA M11 (AWWA C2OO-type steel pipe)

Grout placed altw installation (or heat shrink sleeve if bonded

dielectric coating used)

Cement Mortar Coating (may be substituted with

bonded diel- coating)

courtesy of Northwest Pipe Company.

Figure 3-8b Restrained Joint per AWWA M11 (AWWA C2OO-type steel pipe)

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Table 3-2 Soils grouped in order of corrosive action on steel

Aeration and Soil Group Drainage Characterization Soil Types

I-Lightly Good Uniform color and no mottling anywhere Sands or sandy loam Corrosive in soil profile; very low water table. Light, textured silt loams

Porous loams or clay loams thoroughly oxidized to great depths

11-Moderately Fair Slight mottling (yellowish brown and Sandy loams Corrosive yellowish gray) in lower part of profile

(depth 18-24 in.); low water table. Soils would be considered well drained in an agricultural sense, as no artificial drainage is necessary for crop raising.

Silt loams Clay loams

111-Severely Poor Heavy texture and moderate mottling clay loams Corrosive close to surface (depth 6-8 in.); water clays

table 2-3 f t below surface. Soils usually occupy flat areas and would require artificial drainage for crop raising.

IV-Unusually Very poor Bluish-gray mottling at depths of 6-8 in.; Muck Corrosive water table at surface or extreme imper- Peat

meability because of colloidal material contained.

Tidal marsh Clays and organic soils Adobe clay

Table 3-3 Relationship of soil corrosion to soil resistivity

Soil group Description Resistivity, ohm-cm

I Excellent 10,000-6,OOO

11 111 IV

Good Fair Bad

6,000-4,500 4,500-2,OOO

2,oooo

Concrete Pressure Pipe, Bar-Wrapped, Steel Cylinder Type Concrete cylinder pipe is a general designation given to pipe manufactured with a watertight steel cylinder and reinforcing bars or prestressing wire, all embedded in a cement- rich concrete or mortar encasement. Three types are currently being produced in the United States and Canada: reinforced concrete cylinder pipe (Figure 3-9), prestressed concrete cylinder pipe (Figures 3-10 and 3-11), and bar-wrapped concrete cylinder pipe (Figure 3-12). These types are described in detail in AWWA C300, Standard for Reinforced Concrete Pressure Pipe, Steel-Cylinder Type; AWWA C301, Standard for Prestressed Concrete Pressure Pipe, Steel-Cylinder Type; and AWWA C303, Standard for Concrete Pressure Pipe, Bar-Wrapped, Steel-Cylinder Type.

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Grout P aced Afler lnsta at on

C rcurnferent a Rod Re nf0mrnent

Stee Spgot R ng Rubber Gasket

Long tud na Rod Re nforcement

Grout Paced Stee Cy nder After lnsta at on

SteeBe Rng

Figure 3-9 Reinforced concrete cylinder pipe (AWWA C300-type concrete pipe)

Concrete cylinder pipe has demonstrated consistent long life when properly installed in most natural environmental conditions. This is attriiable to the protection hydrated Portland cement affords embedded carbon steel. The high pH environment provided by the hydrated cement paste protects the normal oxide film found on the embedded steel surfaces, which passivates the steel. This alkali passive protection prevents corrosion unless aggressive ions like chlorides attack the passive film or acidic conditions reduce the pH of the cement paste substantially.

Because of the passivating effect, the potential between uncorroded steel encased in mortar or concrete and a copper-copper sulfate reference electrode (CSE) is normally between 0 mV and -300 mV. The potential of corroding steel encased in mortar or concrete will be substantially more negative. This difference in potential between corroding and noncorroding steel can be useful in monitoring for possible corrosion activity.

In the majority of environments, no additional protective requirements are necessary for concrete cylinder pipe. However, in some circumstances precautionary measures should be taken to maintain pipeline integrity. These circumstances include (1) high-chloride environments, which can depassivate steel; (2) stray-direct-current interference from man-made structures; (3) high-sulfate, severe-acid, or aggressive carbon dioxide conditions, which can chemically attack the concrete or mrtar coating; and (4) atmospheric exposure, which can deteriorate the cement-mortar coating. Each of these conditions is discussed in the following sections. Protective measures for these environmental conditions are more fully discussed in the AWWA Manual M9, Concrete Pressure Pipe.

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Grout P aced Rubber Gasket After lnsta at on

Stee Ba Rng

Cement-Mqrtar Coat ng

See Cy nder

H gh-Tens e W re

Concrete Core Mortar Po nt ng After lnsta at on

f Requ red

Stee Sp got R ng

Figure 3-10 Prestressed concrete lined cylinder pipe (AWWA C301-type concrete pipe)

Grout P aced Prestressed After lnsta aton Cernent-Mortar Coat ng H gh-Tens e W re

Cement M o m Paced n F e d

Stee Cy nder Stee Sp got R ng See Be Rng Concrete Core

Rubber Gasket

Figure 3-11 Prestressed concrete embedded cylinder pipe (AWWA C301-type concrete pipe)

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Grout P aced After lnsta at on Rod Re nforcement

See Be Rng CernepMortar Coat ng

Cernmt-Mortar or Concrete L n ng

Figure 3-12 Bar-wrapped concrete cylinder pipe (AWWA C303-type concrete pipe)

Chlorides. The breakdown of passivation and possible steel corrosion can occur in sound concrete only if certain negative ions (anions) are present at the steel surface. One anion that is of practical concern is the chloride ion. When exposed to a lugh concentration of chloride ions and oxygen, steel encased in concrete will corrode. The threshold chloride ion concentration required at the steel surface to initiate corrosion is quite high-about 0.02 movL (approximately 700 mg/L). However, in the absence of impressed (externally generated) voltages, oxygen must be replenished at the steel surface to sustain corrosion In high-chloride environments such as continuous€y submerged ocean outfalls (approximately 20,000 mg/L chloride), concrete cylinder pipelines do not experience damaging corrosion oi embedded steel. This is due to the extremely low rate of oxygen diffusion through the saturated mortar coatings.

Concrete cylindex pipelines buried in lugh-chloride soil environments must be evaluated differently from those continuously submerged in seawater. In soils that do not become totally dry in the pipe zone, such as cohesive soils, the mortar or concrete exterior will remain moist, limiting the access of oxygen to the encased steel. If the chloride ion concentration in these soils exceeds 400 ppm and if soil resistivity as measured in the maximum natural moisture state, is lower than 1,500 ohm-cm, it is prudent to apply additional protection.

Cathodic interference. The section on stray direct current at the end of this chapter covers evaluation of the environment for corrosive conditions that could result from inter- fering direct current in the area of the pipeline.

Sulfate soils. Soils containing elevated concentrations of sodium, magnesium, and calcium sulfates are designated sulfate soils ( o h incorrectly termed alkali soils) and are cluefly found in parts of western &ada and the West and Southwest of the United States. Under certain circumstances, these soils can be aggressive to concrete structures, particularly those in contact with the soil but with partial atmospheric exposure Such installations can result in the accumulation of high-sulfate concentrations in the concrete or mortar due to capillary action and evaporation This buildup results in expansive deterioration of the Portland cement paste. Completely buried concrete pipelines are not subject to this type of sulfate buildup. Completely buried concrete pipe can be susceptible to expansive chemical sulfate attack in high-sulfate-bearing soils where groundwater contact is routine. In these chemical reactions, the sulfate ions of various metals including

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AWWA Manual M27

magnesium and sodium react with the calcium hydroxide in cement paste to form calcium sulfate or gypsum, which in turn reacts with tricalcium aluminate in the cement paste to form tricalcium sulfoaluminate or ettringite, which is a much larger compound. This causes microcracking and expansive degradation of the concrete.

Acidic soils. Significant chemical attack of concrete cylinder pipe in acidic soils usually occurs only in unnatural soils affected by contamination, such as cinders, mine wastes, or industrial dumps. Acidic conditions in natural soils are usually caused by milder, less aggressive acids formed by decaying vegetation. The effect of acidic soils on pipelines installed in low-pH conditions depends on the extent to which acidic conditions at the pipe surface can be replenished by groundwater movement. Generally speaking, no problems will occur in soil having a pH of 4 or higher where there is little groundwater movement with respect to the pipe, such as in fine-grained silts or clay. Under these condi-tions, the soil water at the pipe surface is neutralized by the alkalinity in the concrete or mortar. Where acidic soils are encountered and where considerable movement of ground-water with respect to the pipe is anticipated, such as granular soils, the total acidity should be measured to determine if supplemental protection is necessary.

Aggressive carbon dioxide. Carbon dioxide and moisture react with mortar or con-crete to produce calcium carbonate, which has a lower pH than calcium hydroxide. This process, called carbonation, is usually limited to a thin exterior surface layer on concrete pipe due to the formation of the relatively insoluble calcium carbonate blocking further penetration of carbon dioxide. This surface carbonation is not detrimental to the pipe for many years. Penetrating carbonation, especially in the aqueous phase, can occur under certain conditions and lead to corrosion of reinforcing steel due to the lowering of the pH of the protective mortar or concrete in contact with the reinforcing steel. Carbon dioxide generation in the soil due to decaying vegetation or geothermal activity combined with relatively soft groundwater and highly permeable soil can lead to penetrating carbonation. If these conditions are suspected, testing of groundwater for aggressive carbon dioxide is recommended. When the performance history of existing buried concrete structures or a current soils and groundwater analysis indicate a potential problem, supplemental pre-cautions should also be considered.

Atmospheric conditions. Although concrete cylinder pipe is normally buried below ground, it occasionally becomes necessary to make an aboveground installation. The atmo-spheric environment is drastically different from buried conditions. Atmospheric condi-tions can bring alternate wetting and drying, rapidly changing temperatures between night and day, possible freezing and thawing cycles, and exposure to atmospheric car-bonation. These exposure conditions over the life of the pipe can lead to deterioration of the protective concrete or mortar pipe exterior. In general, the conditions associated with atmospheric exposure can be expected to reduce pipe life unless the pipe is coated or other additional protective measures are taken.

Copper pipeAlthough copper may tend to resist subsurface corrosion, certain soil and environmen-tal conditions can cause deterioration. Specific soil chemistries that may cause problems include (1) highly alkaline organic soils where the ratio of chloride and sulfate to carbon-ate is high; (2) elevated concentrations of organic and inorganic acids; (3) poor aeration, which supports sulfate-reducing bacteria activity; and (4) elevated chloride, sulfide, and/or ammonia concentrations. Corrosion of copper by a soil can be aggravated by the applica-tion of fertilizer or heavy lawn sprinkling, especially if the soil is poorly drained. Copper should not be embedded directly in cinders or in tidal marshes where it may be subject to attack by sulfur compounds.



AWWA Manual M27

Copper in contact with concrete often acts as a cathode to nearby copper exposed to soil, causing the pipe in the soil to corrode. Premature failure of the copper at the con-crete-to-soil interface in foundations and concrete slabs can occur. Copper pipe should be sleeved in polyethylene (PE) as it passes through concrete.

Another hazard to underground copper is the practice of using metal water service lines as a ground for electrical systems. When copper service lines are used as a ground, and connected to either conducting or nonconducting mains, corrosion damage can some-times occur where current leaves the copper, even if the current is alternating.

In domestic plumbing with parallel hot and cold water piping, underground hot-water copper pipes can act as anodes relative to the cold-water copper pipes. If the two systems are electrically connected, as can occur at the hot-water heater, the hot-water pipes can experience accelerated corrosion by thermogalvanic action. This can be overcome by using an isolation union between the two lines.

asbestos–Cement pipeAsbestos–cement (AC) pipe was formerly available in the United States in two basic types, which were distinguished by the free lime content of the pipe. Type I pipe had no limit on uncombined calcium hydroxide; Type II pipe had 1 percent or less uncombined cal-cium hydroxide.* In the United States, only Type II AC pipe was produced. In addition to asbestos fiber and Portland cement, Type II pipe contains approximately 30 percent finely ground silica. The curing process was completed in high-pressure steam autoclaves, at a temperature of approximately 300°F (150°C). The series of compounds known as hydro-garnets, which result from steam curing of cement products, are very stable and highly resistant to the action of sulfate solutions.

Guidelines for the use of AC pipe under various soil conditions were published in AWWA C400, Standard for Asbestos–Cement Pressure Pipe, 4 In. Through 16 In. (100 mm Through 400 mm), for Water Distribution Systems; AWWA C402, Standard for Asbestos–Cement Transmission Pipe, 18 In. Through 42 In. (450 mm Through 1,050 mm), for Water Supply Service; AWWA C403, Standard for the Selection of Asbestos–Cement Transmis-sion Pipe, Sizes 18 In. Through 42 In. (450 mm Through 1,050 mm), for Water Supply Service; and ASTM C500, Standard Methods for Testing Asbestos–Cement Pipe. These guidelines establish parameters that are intended to define conditions where indefinite life expectancy can be anticipated. The above-referenced AWWA standards were with-drawn in November 2008.

Table 3-4 shows guidelines for the use of AC pipe in acidic soils based on minimum pH factors alone. AC pipe may or may not perform satisfactorily in acid soil environments having pH values below those listed in this table. To determine the suitability of AC pipe in soils having lower pH values, each situation should be evaluated individually, taking into consideration all aspects of the soil environment that affect corrosiveness to AC pipe.

Table 3-5 classifies aggressiveness to AC pipe of soluble sulfates in water and soils. Type I AC pipe will be attacked to various degrees by all but the nonaggressive levels of sulfate concentrations in waters and soils. Type II AC pipe is resistant to all levels of soluble sulfates.

* Tests for uncombined calcium hydroxide are made in accordance with ASTM C500, available from the American Con-crete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331-3439.



Table 3-4 Guidelines for use of AC pipe based on pH of acidic soils

Minimum pH of Acidic Soils When Usinn AC Pipe

Water Conditions Within Soil Environment Type 1 Type = Essentially quiescent 5.0 4.0

Rapidly moving or grossly cyclic 6.3 5.5 Mildly fluctuating 5.5 5.0

Source: AWWA C403

Table 3-5 Corrosion guidelines for AC pipe for soluble sulfate in water and soils

so, in soil Sulfate Aggressiveness P p m

Nonaggressive 1,000 and less

Mildly aggressive 1,000 to 2,000 Moderately aggressive 2,000 to 20,000 Highly aggressive 20,000 and above

Source: AWWA C403

Thermoplastic Pipe PVC and PE are the thermoplastic materials most commonly used in water distribution. These materials do not corrode in the sense that metals do. Being nonconductors, they are immune to corrosion by galvanic or electrochemical effects. They are also unaffected by polarily active materials, such as aqueous acids, bases, and salts. Consequently linings, coatings, and cathodic protection are not required with thermoplastic piping. However, certain substances can adversely affect thermoplastic pipe’s structural properties through direct chemical attack, oxidation, or environmental stress cracking. Pipeline contact with such substances can occur in instances of subsurface contamination by leaking gasoline storage tanks, solvents, aromatic hydrocarbons, ketones, some detergents, and certain oils. The nature and extent of their effect depend on the following:

Type or nature of the chemical(s) present

Concentration of the chemical(s)

Duration or persistence of exposure

Temperature

Pipe material, i.e, PVC or PE Possible effects range from slight swelling with little loss of properties to severe soft-

ening and significant loss of tensile strength to permeation of some chemicals species through the pipe wall. Some PE materials are also subject to environmental stress cracking when simultaneously subjected to stress and certain organic liquids. AWWA standards for PE pipe prohibit the use of those grades of PE. PVC pipe intended for aboveground applications should be protected from ultraviolet radiation by wrapping or coating the pipe. Alternatively PVC pipe may be formulated with special additives for aboveground applications (e.g., PVC aboveground irrigation pipe).

When thermoplastic pipe is considered for use with harmful chemical reagents or solvents, the pipe’s suitability for that exposure must be evaluated. For more information regarding PVC pressure pipe’s environmental resistance, consult AWWA Manual M23,

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PVC Pipe-Design and Installation or the Uni-Bell Handbook ofPVC Pipe: Design and Construc- tion. For information on the environmental resistance of PE pipe, see AWWA Manual M55, PE Pipe-Design and Installation or the Plastics Pipe Institute?

Stainless-Steel Pipe The types of corrosion discussed here are those that would most likely affect stainless steel, when buried, submerged in water, covered by silt and mud, or exposed to the atmosphere. The corrosion resistance of stainless steels is dependent upon its chromium content and other specific elements, such as nickel, molybdenum, and nitrogen (Table 3-6). The austenitic and duplex (austenitic-ferritic) stainless steels are the materials of construction most commonly used within the water industry due to their very low general corrosion rates. These materials form thin and tightly adherent chromium oxide films, which provide them with their surface protection. If damaged, these protective films rapidly reform when exposed to oxygen-containing environments.

Corrosion of stainless steel pipe in soil. The primary considerations for stainless steels are soil resistivity, acidity/alkalinity (pH), and chloride content, which can be affected by location (coastal; city winter salting for de-icing). The use of selective pipeline bedding materials, such as sand, loams, and fine stone, can help with drainage away from the pipe, especially in aggressive acidic and/or high-chloride soils.

Both the 300-series austenitic alloys and the duplex nonlean stainless steels have performed well when buried in coastal-plain soils and also in soils with relatively low resistivities, e.g, 500 ohm-cm levels. This is based upon a European soil study (Cunat 2001) and studies undertaken by Gerhold et al. (1981) after eight years of buried exposure. (NOTE: Thirty- year NIST exposures on steel and stainless-steel surfaces were recently investigated by Idaho National Laboratory [Adler Flitton 20051, which showed the same positive performance as the eight-year trials.) Likewise, a Japanese study (JSA/NDI 1988) undertaken in sandy loam and loam soils showed the stainless-steel materials Types 304t304L and 316/316L to exhibit excellent corrosion resistance. However, selecting higher-alloyed corrosion-resistant alloys with higher levels of chromium and molybdenum may be appropriate when soil resistivity levels fall below 2,000 ohm-cm. Alternatively, consideration canbe given to the use of protective external coatings or cathodic impressed-current systems.

Table 3-6 Stainless-steel alloys and their corrosion resistance Stainless Steel Type Alloy UNS # %Ni %Cr %Mo PREN

Austenitic 3041304L S30403 8-12 18-20 - 19 316/316L S31603 10-14 16-18 2-3 25

Duplex 2205 S31803 4.5-6.5 21-23 2.5-3.5 34.5 2304 s32304 3-4.5 21.5-23.5 0.2 25.5 2101 - 1.5 21.5 0.3 25.5 2003 S32003 3 4 19.5-22.5 1.5-2.0 28.5

2507 S32750 6-8 24-26 7-8 42.5

NOTE: UNS-United Numbering System; Ni-nickel content; G-chromium content; Mo-molybdenum content; PREN-pitting resistance equivalent number.

* Plastics Pipe Institute, 105 Decker Court, suite 825, Irving TX, 75062.

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Pitting corrosion of stainless-steel pipe. When the oxide film becomes damaged and cannot be repaired environmentally, or where the concentration of chlorides can accumulate in sufficient quantity to penetrate the oxide film, pitting attack or crevice corrosion can occur. In the case of stainless steels, this form of localized corrosion attack can be of a pinhole nature, which generally does not impact the structural stability of the pipeline. Selection of the correct grade of stainless steel will help overcome this type of attack. This can be defined by ranking the pitting resistance value for an alloy.

The pitting resistance equivalent number (PREN) value is calculated on the chemistry of each alloy. The higher the number, the greater the resistance to chloride-bearing environments. Hence, Type 316/316L would offer higher resistance to aggressive soils than Type 304/304L (refer to Table 3-6). Alloy 2507 is the alloy with the highest resistance to pitting corrosion for the alloys shown in Table 3-6. In most cases, Type 304/304L performs well, buried in a wide range of soil chemistries, especially those with high-resistivity values, e.g, 5,000 ohm-cm and above.

The PREN value can be calculated using the following formula:

PREN = % Cr + 3.3Mo + 16N

Where: Cr = Chromiumcontent Mo = Molybdenumcontent N = Nitrogencontent

Alloy selection, using this method, would be more pertinent to those soil environments that have high chloride values, low-resistivity soil values, and are continuously wet.

Stainless-steel pipe in submerged and waterlogged conditions. Under these circumstances, concern for pitting and crevice corrosion becomes more significant. Selection of the appropriate stainless steel is important, especially in handling stagnant water conditions or salinebrackish waters (Table 3-7). Where the level of chlorides is greater than 250 ppm, then either 2304 or the molybdenum-bearing grades of stainless steel, such as Type 316 stainless steel or the 2203 and 2205 duplex stainless steel, should be considered. For brackish waters and seawater-laden soils and waters, where the chloride levels can be 3,600 pprn chloride and greater, higher-alloy grades of stainless steel may be necessary to handle these aggressive sets of conditions.

Table 3-7 Suitable applications for stainless steels Alloy UNS # Application

3041304L

316/316L

S30403 Acceptable for fresh waters with up to 200 pprn chlorides; pitting and crevice corrosion unlikely to occur

Acceptable for waters up to 1,000 pprn chlorides; pitting and crevice corrosion unlikely to occur

S31603

2205 s31803 Acceptable for waters up to 3,600 pprn chlorides

2507 S32750 Acceptable for seawater environments

Seek guidance from a corrosion specialist when dealing with high-chloride environments (greater than 1,000 pprn chlorides)

NOTE: UNS-United Numbering System

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Stainless-steel pipe in aboveground environments. The tightly adhering oxide scale provides a very corrosion-resistant surface to most aboveground environments. Exposure to marine or saline conditions will necessitate regular cleaning (wash-down) practices to remove salt buildup from sprays and windborne salt particles in order to ensure their optimum corrosion performance. Usually Type 316/316L stainless steel and the duplex stainless materials provide the appropriate selection for external piping used in buildings; pipelines spanning waterways; and architecture, bridge railings, and other hardware exposed to marine environments. In urban and inland environments, stainless steels all perform well and have long service lives with minimal maintenance.

Type 316/316L stainless steel has performed very well as air lines in wastewater treat-ment plants and for aboveground hardware, such as railings, bolting, catwalks, cabinetry, and exterior lighting in both wastewater treatment and water treatment plants, where residual chlorine-/chloride-borne vapors or sprays may be present. This alloy has per-formed well in these environments with minimum maintenance, although regular clean-ing is recommended to maintain performance capabilities, along with adequate venting in enclosed spaces, such as piping galleries where these vapors may accumulate and con-dense on the metal surfaces.

referenCesAdler Flitton, M.K. 2005. Underground Corrosion after 32 Years. Idaho National Laboratory, Idaho Falls, ID,

EMSP project No. 86803.ANSI/AWWA C105/A21.5, Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems. American

Water Works Association, Denver, CO.ANSI/AWWA C200, Standard for Steel Water Pipe, 6 In. (150 mm) and Larger. American Water Works Associa-

tion, Denver, CO.ANSI/AWWA C300, Standard for Reinforced Concrete Pressure Pipe, Steel-Cylinder Type. American Water

Works Association, Denver, CO.ANSI/AWWA C301, Standard for Prestressed Concrete Pressure Pipe, Steel-Cylinder Type. American Water

Works Association, Denver, CO.ANSI/AWWA C303, Standard for Concrete Pressure Pipe, Bar-Wrapped, Steel-Cylinder Type. American Water

Works Association, Denver, CO.ASTM C500, Standard Methods for Testing Asbestos–Cement Pipe. Available from the American Concrete

Institute, Detroit, Mich.ASTM G51, Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing. West Conshohocken,

Pa.: ASTM International. www.astm.org. ASTM G57, Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four Electrode

Method. West Conshohocken, Pa.: ASTM International. www.astm.org.ASTM G187, Standard Test Method for Measurement of Soil Resistivity Using the Two Electrode Soil Box

Method. West Conshohocken, Pa.: ASTM International. www.astm.org.AWWA C400, Standard for Asbestos–Cement Pressure Pipe, 4 In. Through 16 In. (100 mm Through 400 mm),

for Water Distribution Systems. American Water Works Association, Denver, CO. [Withdrawn 2008.] AWWA C402, Standard for Asbestos–Cement Transmission Pipe, 18 In. Through 42 In. (450 mm Through

1,050 mm), for Water Supply Service. American Water Works Association, Denver, CO. [Withdrawn 2008.]AWWA C403, Standard for the Selection of Asbestos–Cement Transmission Pipe, Sizes 18 In. Through 42 In.

(450 mm Through 1,050 mm), for Water Supply Service. American Water Works Association, Denver, CO. [Withdrawn 2008.]

AWWA Manual M9, Concrete Pressure Pipe. American Water Works Association, Denver, CO.AWWA Manual M11, Steel Pipe—A Guide for Design and Installation. American Water Works Association, Den-

ver, CO.AWWA Manual M23, PVC Pipe—Design and Installation. American Water Works Association, Denver, CO.AWWA Manual M41, Ductile-Iron Pipe and Fittings. American Water Works Association, Denver, CO.



AWWA Manual M27

AWWA Manual M55, PE Pipe—Design and Installation. American Water Works Association, Denver, CO.British Stainless Steel Association. The Operational Guidelines and Code of Practice to Support Drinking

Water Directorate, www.bssa.org.uk. Cunat, P. J. 2001. Corrosion Resistance of Stainless Steels in Soils and Concrete. Ceocor, Biarritz.Ductile Iron Pipe Research Association (DIPRA). 2004. The Design Decision Model™ for Corrosion Control of

Ductile Iron Pipelines, DIPRA, Birmingham, AL.Flitton, M.K. Adler. Underground Corrosion after 32 Years. Idaho National Laboratory, Idaho Falls, ID, EMSP

project 86803.Gerhold, W.F., E. Escalante, and B.T. Sanderson. 1981. The Corrosion Behavior of Selected Stainless Steels in Soil

Environments. U.S. Department of Commerce, National Bureau of Standards, Washington, DC (NBSIR 81-2228).

Japanese Steel Association and Nickel Development Institute (JSA/NDI). 1988. A Report on the Performance of Stainless Steel Pipe for Water Supply in Underground Soil Environments, Vols. 1 and 2.

Kain, R.M., A.H. Tuthill, and C. Hoxie. 1984. The Resistance of Types 304 and 316 Stainless Steels to Crevice Corrosion in Natural Waters, Journal of Materials for Energy Systems, ASM, Vol. 5, No. 4, March.

Little, B.J., and P. Wagner. 2001. Microbiologically Influenced Corrosion. In Peabody’s Control of Pipeline Corro-sion, 2nd ed., ed. R.L. Bianchetti. Houston, Texas: NACE.

Pope, D.H., and E.A. Morris. 1995. Some Experiences with Microbiologically Influenced Corrosion of Pipe-lines. Materials Performance, 34 (5): 24.

Romer, A., G.E.C. Bell, S. Duranceau, and S. Foreman. 2004. External Corrosion and Corrosion Control of Buried Water Mains. American Water Works Association Research Foundation, Denver, CO. ISBN 1-58321-347-3.

Taga, M.E., and B.L. Bassler. 2003. Chemical Communication Among Bacteria. PNAS 100 (Suppl 2) 14549-14554. http://www.pnas.org/content/100/suppl.2/14549.full doi: 10.1073/pnas.1934514100.

Uni-Bell PVC Pipe Association, Handbook of PVC Pipe: Design and Construction, 5th ed. Dallas, Texas: Uni-Bell.Von Baeckmann, W., W. Schwenk, and W. Prinz. 1997. Handbook of Cathodic Corrosion Protection, 3rd ed. Hous-

ton, Texas: Gulf Professional Publishing. Winans, S.C., and B.L. Bassler, eds. 2008. Chemical Communication Among Bacteria. Washington, DC: ASM

Press. ISBN 978-1-55581-404-5.


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Chapter 4

Corrosion Control and Protection of Buried Pipelines

Where tests and observations indicate that corrosion will occur in a new or existing installation, steps must be taken to control or mitigate it. A variety of methods and combinations of methods are available for corrosion control. All are based on the principle, discussed in detail in chapter 2, that four elements are required to support corrosion: an anode, a cathode, an electrolyte, and an electronic path. In a buried pipeline, the anode (where damage is sustained) and the cathode occur on the pipe surface, the electronic path is via the wall of the pipe, and the electrolyte is the soil. If any one of these elements can be eliminated, then corrosion can be controlled.

This chapter discusses several corrosion control systems used by water utilities, including bonded coatings, cement-mortar coatings, cathodic protection, polyethylene encasement, selection of corrosion-resistant materials, and environmental alteration The operational advantages and disadvantages of each method are covered.

After completing this chapter, the reader should be able to Understand the function of coatings in the control or mitigation of corrosion.

Know the basic procedures used for cathodic protection.

Know how to avoid combinations of metals or alloys that cause bimetal corrosion cells.

Have an understanding of electrolyte (environment) alteration for corrosion control or mitigation. Recognize specific precautions required for certain pipe materials in certain environments.

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CoatinGs and lininGsCoatings, linings, and encasements are used to reduce or mitigate corrosion of buried pipelines. They act by creating a barrier between the metal and the surrounding soil, the potentially corrosive environment. The effectiveness of a coating or lining depends on its degree of integrity (freedom from pinholes called holidays and cracks), its ability to bond to metal (if a bonded coating), and its ability to insulate against the flow of electric current.

Coatings, linings, and encasements fall into the following three general categories:• Passivating, bonded Portland cement mortar and concrete coatings and linings.• Dielectric, bonded coatings and linings.• Unbonded polyethylene encasement.Portland cement mortar and concrete are materials that control corrosion due to

the passivating effect of high-pH Portland cement on steel. Passivation inhibits corrosion from occurring. Mortar and concrete are used on all concrete pressure pipe (AWWA C300, C301, C302, and C303) to protect the encased steel in the pipe. Mortar coatings and linings are also used on steel pipelines (AWWA C205) and as a lining in most ductile-iron pipe (AWWA C104).

Dielectric, bonded coatings and linings, such as coal-tar enamels, epoxies, wax- and cold-applied tapes, and polyurethanes control corrosion by acting as a physical barrier to corrosive constituents in the soil or transported water. Dielectric, bonded coatings and lin-ings are used in steel pipelines (AWWA C200) as discussed in several AWWA standards, such as C203, C210, C213, C214, C215, and C222.

Unbonded polyethylene encasement offers some of the qualities of a bonded coating, such as dielectric strength. It is mainly an environmental improvement. Its purpose is to isolate the pipe from an aggressive soil and provide a benign environment underneath. It is used on ductile-iron pipe (AWWA C105).

Although modern coatings dramatically reduce or mitigate corrosion in water utility structures, the user must be aware of the following:

• Factory-applied coatings may be damaged during shipping, handling, storage, or construction if not handled per AWWA standards. It is prudent to inspect coat-ings per AWWA standards prior to installation and make field repairs as needed. Some factory-applied coatings cannot be repaired to their original quality.

• If a pinhole or holiday exists in a bonded dielectric coating in a corrosive environ-ment, corrosion may develop that will undermine the coating and cause pitting.

• If a metal with a bonded dielectric coating is under cathodic protection, excessive current can cause coating disbondment.

Bonded dielectric coatings in corrosive environments should be supplemented by cathodic protection. The properties of coatings and cathodic protection systems are syn-ergistic; coatings greatly reduce the cost of the cathodic protection system, while cathodic protection mitigates corrosion of exposed metal at pinholes and flaws (holidays) in the coatings. In most instances, lower maintenance and life-cycle costs are the result.

The following paragraphs briefly describe water pipeline coating materials in AWWA standards.

Polyethylene encasement (AWWA C105). Loose polyethylene encasement was first used experimentally in the United States in 1951 for protection of gray-iron pipe in corro-sive environments. The first field installation of polyethylene wrap on gray-iron pipe in an


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operating water system was in 1958. Since 1958, polyethylene encasement has been used extensively on ductile-iron pipelines in severely corrosive soils throughout the United States on pipe ranging in size from 3 in. to 64 in. (76 mm to 1,600 mm) in diameter.

Coal-tar enamel and tape (AWWA C203). The history and performance of coal-tar enamel coatings on water pipelines have been extensive and excellent since the mid-1930s (AWWA C203). The primary reason for its excellent performance is due to the roughly ⅛-in. (typically applied) thickness of the coating and its low absorption and permeability. In the past decade or two, coal-tar enamel has been used less often due to health and envi-ronmental concerns for the workers exposed to the material during coating and pipe man-ufacture. Coal tar is a suspected carcinogen, and the volatile organic compound (VOC) content and odor the material emits during application have led to stringent permitting requirements for its use in populated areas, which have forced some companies to refrain from applying coal-tar enamel to pipe.

Portland cement mortar (AWWA C205) and concrete (AWWA C300, C301, C302, and C303). The history and performance of mortar coatings on water pipelines have been extensive and excellent for more than 100 years (AWWA C205, C301, and C303). One of the first Portland cement–mortar-coated steel pipelines was installed in the city of St. John, New Brunswick, Canada, in 1855 (Bardakjian 1995, Bardakjian and Hausmann 2007). A section of the pipeline was removed from service due to line relocation in 1963 after 108 years of service, and the interior and exterior were found to be free of corrosion. Cement–mortar-lined-and-coated steel pipe was first used in the United States in late 1800s. Some of the first pipelines were in service for almost a century by the time the first national standard was written (AWWA C205). Portland cement mortar and concrete also has the advantage that minor cracks can heal in water exposure. The minimum thickness of mortar coatings is ³⁄₄ in.

Liquid epoxies (AWWA C210), fusion-bonded epoxies (AWWA C213), tape sys-tems (AWWA C214), and extruded polyolefin (AWWA C215). Until 1978, Portland cement mortar and coal-tar enamel were the only two coating materials listed in AWWA standards. During the next 11 years, from 1978 to 1988, four additional coating systems (liquid and fusion-bonded epoxies, tape, and extruded polyolefin) were added. Tape and extruded polyolefin coatings are roughly ¹⁄₃₂ to ¹⁄₁₆ in. thick. The liquid epoxies per AWWA C213 have a minimum thickness of 0.016 inch, and the thickness of the fusion-bonded epoxies per AWWA C213 range from 0.012 to 0.015 inch.

Petrolatum and petroleum wax tape (AWWA 217). These coatings were introduced in the AWWA standards in 1990. This standard provides the minimum requirements for cold-applied petrolatum and petroleum wax tape to the exterior of steel water pipelines for underground and underwater installation for long-term corrosion protection. The coat-ing system consists of a primer and a fabric tape that is encapsulated and coated on both sides with the petrolatum or petroleum wax. The tape coating is applied after the primer and serves as a corrosion barrier.

Polyurethane (AWWA C222), polyamide (AWWA C224), fused polyolefin (AWWA C225), and fusion-bonded polyethylene (AWWA C229). These bonded coatings were introduced in the AWWA standards for steel water pipelines starting in 1999 with poly-urethane and ending in 2008 with fusion-bonded polyethylene. They do not have the extensive performance history of Portland cement mortar and coal-tar enamel. They are typically applied at a minimum thickness of 0.009 to 0.09 inch depending on the coating and pipe diameter.



CATHODIC PROTECTION Cathodic protection is a system for reducing corrosion of a metal structure by installing an external anode and making the entire pipeline the cathode of a corrosion cell. Conven- tional direct electrical current flows from the anode through the soil onto the protected pipeline, then back to the anode through an insulated copper wire (the electronic path). Proper design of a cathodic protection system results in corrosion control intended to overcome any naturally occurring anodes on the pipeline by providing an external anode that is consumed over time. If need be, the design can incorporate the possibility of replac- ing the anode@) and extend protection. While cathodic protection cannot be expected to completely eliminate corrosion, it can reduce the corrosion rate to acceptable levels that provide a minimum desired service life for the pipeline. The cathodic protection methods discussed in this section can be used for steel (dielectric or mortar coated), cast-iron, ductile-iron, copper, and concrete pressure pipe (reinforced, prestressed, and pretensioned).

To form the protective galvanic or electrolytic cell, the same four elements are required as for a naturally occurring corrosion cell: a n anode, a cathode, an electrolyte, and an electronic path The cathode is the protected structure itself, which must be electrically continuous to ensure complete protection. The conductive electrolyte is the corrosive soil, water, or concrete environment. The anode and the electronic path are added, as shown in Figure 41. To generate the needed current for a sacrificial anode cathodic protection system, appropriate anodes are selected from the galvanic series; and to generate the needed current for an impressed current cathodic protection system, a direct current source is provided.

So Surface


Figure 4-1 Cathodic protection system (galvanic cell using sacrificial anodes)

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A qualified corrosion engineer should conduct the design of effective, economically feasible cathodic-protection systems. The examples in this chapter are not intended as a reference for design, but rather to provide an overview of processes and equipment and illustrate conventional system designs. Details such as anode size, anode material, and ter-minal box design will vary considerably from one installation to another. Options for con-necting anodes to the various pipe materials, test stations, or bonding the joints of a buried pipe include Cadwelding, pinbrazing, or mechanical connections. For more detailed infor-mation on cathodic protection, refer to NACE Standard SP0100, SP0169, and other appli-cable publications as provided in the References section at the end of this chapter.

Note: Systems that incorporate cathodic protection require routine monitoring and maintenance to ensure proper protection is provided. Follow the cathodic protection sys-tem designer’s recommendations regarding this issue.

sacrificial anode systemsCathodic protection installations in which a galvanic cell is created are referred to as sac-rificial anode systems.

Operating principles and common uses. The sacrificial anodes, also called galvanic anodes, are made of metals or alloys that are electronegative to the structure that must be protected—that is, they are nearer to the anodic end of the galvanic series. When inserted in the same soil as the structure and electrically connected to it by an electronic path, the sacrificial anode becomes the anode of a galvanic cell and corrodes, generating an electri-cal current. The structure becomes the cathode and is protected. Details of a typical sacri-ficial anode installation are illustrated in Figure 4-2.

Sacrificial anode protection may be used selectively in “hot spot” areas that have been located by soil survey procedures. This requires that rubber-gasketed joints be elec-trically bonded throughout the protected area. The same protection can also be used to protect steel tanks, well casings, steel piping, and other water treatment equipment. Advantages and disadvantages of sacrificial anode systems include the following.

Advantages• No external power is required.• Relatively low installation costs

and minimal maintenance.• Systems seldom cause

interference with foreign structures.

Disadvantages• Driving voltages are limited.• Current output is limited.• Operation is effective only in relatively

low-resistivity soils.• Anodes must be replaced frequently

if the pipes are uncoated or if coating deterioration occurs in high-current-demand environments.



Exotherm c Wed (Cadwe d)

Epoxy Putty or Mastc

lnsu ated Copper W re

Figcue &esy of NACE International.

Figure 4-2 Details of a sacrificial anode installation

Design and construction. Magnesium and zinc metals can be used as sacrificial anodes to protect iron and steel structures. Magnesium anodes are probably the most widely used, because they create a higher cell voltage than zinc anodes. Magnesium anodes are typically used in soils with resistivities above 1,500 ohm-cm. Zinc anodes are typically used in soils with resistivities less than 1,500 ohm-an. To determine the current output of a magnesium anode, the anode and correction factors must be known Table 4-1 lists anode factors based on the required pipe-to-soil (Pb) potential.

Generally, protection is achieved with a negatively polarized potential of at least -850 mV with cathodic protection applied, measured with respect to a saturated copper- copper sulfate reference electrode ccmtacting the electrolyte (NACE International SF'O169). An alternative measure of protection is a minimum of 100 mV of cathodic polarization between the structure surface and a stable reference electrode contacdng the electrolyte PACE International SPO169). Current output of the anode is calculated using the following formula.

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CORROSION CONTROL AND PROTECTION OF BURIED PIPELINES 55

Table 4-1 Magnesium anode factors

Magnesium' Dimensions Anode Desired Correction Factor Anode Weight, Factor, PIS Potential, (Magnesium),

3 (1) 3 x 3 x 4.5 (76 x 76 x 144) 0.53 -0.70 1.14 5 (2) 3 x 3 x 7.5 (76 x 76 x 191) 0.60 -0.80 1.07

Ib (kg) in. (mm) f 2, Y

9 (4) 3 x 3 x 13.5 (76 x 76 x 343) 0.71 -0.85 1.00

17 (8) 3 x 3 x 25.5 (76 x 76 x 648) 1.00 -0.90 0.93 32 (15) 5 x 5 ~ 2 1 (127 x 127 x 533) 1.06 -1.00 0.79 50 (23) 8 diameter x 15 (203 diameter x 381) 1.09 -1.10 0.64

These factors are based on typical anode dimensions.

(Eq 4-1) 150, O O O h P

I =

Where: I = current output (mfl)

f = anode factor, from Table 4-1 y = correction factor, from Table 4-1 P = average electrolyte resistivity (ohm-cm)

For example, for a P/S potential of -0.90 V, a 17-lb magnesium anode, and an average resistivity of 1,000 ohm-cm, the anode current output would be

150,000xl.0x0.93 1,000

I = = -14omA

The life expectancy of a sacrificial anode is inversely proportional to the current flow. The actual life varies with the materials (e.g., zinc versus magnesium), the installation, and the utilization factor (a measure of the allowable anode consumption before it needs to be replaced, typically 85 to 90 percent). Nominal zinc anode consumption is roughly 25 pounds per amp-year while magnesium anodes are consumed at just under 17 pounds per amp year.

The contact resistance of a vertically installed anode can be calculated as follows:

Where: R, = resistance to earth (ohm) L = length of anode (cm) d = diameter of anode (cm) p = resistivity (ohm-cm)

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Thus, it can be seen that the anode and chemical backfill geometry can have a great effect on the resistance. The current output is related to the resistance in accordance with Ohm’s law:

=I ER

(Eq 4-4)

Where: I = current (A) E = voltage (V) R = resistance (ohm)

The anode factors developed by Tefankjian (1972) are the results of the foregoing computations. For example, compare the utility of a 17-lb Galvomag magnesium anode (3 in. × 3 in. × 25.5 in.) packaged in a 6-in. × 28-in. chemical backfill with a 17-lb high-purity magnesium anode (4 in. × 4 in. × 17 in.) packaged in a 6.5-in. × 19-in. chemical backfill. The former will provide lower contact resistance (owing to its greater length) and a higher driving voltage. Thus, in accordance with Ohm’s law, the magnesium anode will provide more current for cathodic protection.

impressed-Current systemsCathodic protection systems using an external power-current source are called impressed-current systems, or rectifier ground bed systems.

Operating principles and common uses. As illustrated in Figure 4-3, impressed-current systems include a rectifier unit supplying direct electrical current and a series of anodes inserted into the soil (called the ground bed). Electric current from the positive terminal of the rectifier is applied to the ground bed anodes, where it is driven (impressed) into the earth. The current travels through the earth to the protected structure. The struc-ture carries the current to a connecting wire, which returns it to the negative terminal of the rectifier. As the current flows, the structure—acting as the cathode of the electrolytic cell—is protected from corrosion, and the anodes in the ground bed corrode.

Typically, an impressed-current system is used to protect large structures or long stretches of pipeline. Advantages and disadvantages of the system include

Advantages• Large driving voltages are

possible.• Higher (virtually unlimited) cur-

rent levels are available.• The system is suitable for high-

resistivity soils.• Larger structures and longer pipe-

lines can be protected.• Anodes are more easily monitored

and when necessary, replaced.

Disadvantages• Installation costs are higher than for

sacrificial anode systems.• Maintenance and operating costs

are higher than for sacrificial anode systems.

• There is a danger of damage to other metallic structures or pipes from stray currents.


CORROSION CONTROL AND PROTECTION OF BURIED PIPELINES 57

So .Surface


Figure 4-3 Details of an Impressed-current system

Design and construction. Design of an impressed-current system requires that the corrosion control engineer carefully evaluate the anode materials and ground bed configuration, the current supply unit, the electrical continuity cd the protected structure, the potential for stray current corrosion of nearby structures, and a number of environmental variables.

The anodes are usually composed of graphite, high-silicon cast iron, or mixed metal oxide coated titanium. Tlwre are four basic configurations of ground bed installation In the horizontally remote configuration, the ground bed is placed some distance from the protected structure to effect a wide spread of current. In the deep well (vertically remote) configuration, anodes are placed in deep holes to spread current or to protect deep structures, such as well casings. In the distributed Configuration (see Figure 43), anodes are located to protect spedfic struidures, such as tanks, or are distributed along a pipeline. This configuration reduces interference with other underground structures and the shielding effect one structure may have for another. In the horizuiitaf configuration (also called the purullel configuration), a continuws anode is installed parallel to the protected pipeline. This provides good current coverage and reduces interference with other structures.

The current supply unit, as shown in Figure 4-3, includes a 110-V AC power supply, a meter, a switch box with a circuit breaker to protect the rectifier, a step-down transformer to reduce the voltage, and a rectifying element to change the alternating current to direct current. The rectifying elements are usually silicon diodes.

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The electrical continuity of the protected structure is essential, because it acts as part of the electronic path. If a break occurs in the electrical continuity, perhaps at a pipe joint, the electrical driving force of the system wil l force the current into the soil producing metallic corrosion at the point where the current leaves the protected structure cPig.re 44).

Structures that lie in the area of the protected st ruchw but are not electrically continuous with it may also be corroded by the cathodic pmtectio n currents (Figure 4-5). This problem, called stray current corrosion, is a major hazard when impressed-current systems are close to water lines. The extent of stray-current corrosion depends on the location, size, and configuration of the foreign structure. The problem may be mitigated by electrically bonding the foreign structure to the protected s t r u m . Refer to Chapter 3 for a discussion of stray-current monitoring, which must be integral to any cathodic protection system.

Environmental variables that must be considered when designing an impressed- Lment cathodic protection system include

DetaiIs of the structure to be protected.

Average soil resistivity.

Currentrequirements. . Locations and types of other structures and pipelines.

Availability of power supply.

Property easements.

Power Supp y

so surface

Figure courtesy of NACE lntemational.

Figure 4-4 Corrosion caused by electrical discontinuity in a cathodlcally protected pipeline

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CORROSION CONTROL AND PROTECTION OF BURIED PIPELINS 59

Power Supp y

So Surface

Figtire courtesy of NACE International.

Figure 4-5 Corrosion of a metal structure in the vicinity of a cathodically protected structure

In designing a system to serve a pipeline, soil-survey data are reviewed and an area of low resistivity chosen for the anode ground bed. Once the area is selected, a current- requirement test is performed. The results of this test aid in selecting the rectifier and determining the potential effeck on foreign structures. With the current requirement established, the entire system is designed to have as low a circuit resistance as feasible. Lower circuit resistance will result in lower initial and continuing power costs.

Sigruficant contrihtions to circuit resistance are made by the rectifier, the earth, the anode-to-earth and structure-toearth interfaces, and the cables connecting the rectifier to the anodes and to the structure. The anode-to-earth resistance depends on the placement number, d spacing of the anodes. This resistance can be reduced by surrounding each anode with coke breeze. Formulas for estimating de to -ea r th resistance are available. The number of anodes can also be mathematicdy determined if anode-to-earth resistance, soil resistiviv, anode spa&& anode length, and anode radius have been determined.

Stray Current and Cathodic Protection Interference When stray current from direct-current sources such as electric railways or impressed- current cathodic protection systems exists along the proposed right-of-way one of the following should be implemented

Eliminate the stray direct current if possible.

Apply a dielectric barrier to the pipe exterior.

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• Make the pipeline electrically continuous and provide test stations so it can be determined if stray direct current is being picked up or discharged. If direct cur-rent is being picked up, provide the means to safely discharge it.

Additional discussion on stray current is given in chapter 3 under the section titled Stray Currents.

materials seleCtionEarlier chapters discussed materials and combinations of materials that are vulnerable to corrosion under certain conditions. Based on that discussion, the following rules of thumb may be helpful in reducing corrosion problems often associated with water and waste-water utility equipment:

• Avoid the use of combinations of metals that lie far apart in the galvanic series.• When small metal units are in direct contact with larger masses of metal, they

should be more noble than the larger mass. For example, mechanical bolts of steel alloyed with copper, nickel, and chromium will be more noble when used in duc-tile-iron pipe joints.

• Provide the best available coatings and paints in enclosed, humid environments within the treatment plant and pumping stations, especially those where chlo-rine-bearing atmosphere occurs.

• Avoid extensive reliance on sacrificial coatings, such as galvanizing (zinc) on per-manent structures.

• Recognize the passivating effect of structural constituents. For example, concrete on steel and noble metals as constituents of steel alloys will all increase corrosion resistance.

• Watch for sites of stress, fatigue, fretting, and crevice corrosion, and specify met-als that are most resistant to these problems.

trenCh improVementGenerally, trench improvement of corrosive soils to reduce their corrosive tendencies is impractical. Extensive studies have been made of bedding and backfilling around under-ground pipe with sand, limestone, limestone screenings (fines), dolomite, sand plus 10 percent cement, and select soil. Although some reduction in corrosion incidence is real-ized with most of these treatments, it has been observed that, in many instances, the sub-stitute fill material eventually takes on the characteristics of the surrounding soil. The long-term result can be inadequate protection.

proteCtiVe methods for speCifiC pipe materialsExperience has shown that for any given pipe material, certain protective measures or combinations of protective measures are generally most effective and most economically feasible. This section briefly considers several of the pipe materials commonly used in water utility systems, with attention to the protective measures that have often been found appropriate.


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Many utilities and pipeline designers take a more conservative approach than pre-scribed by AWWA pipe standards. Risk-based decisions often consider an analysis of cost/benefit and also risk/consequence. Criteria for corrosion control might consider factors of corrosivity, pipe size and importance to the system, and installation and maintenance cost.

DIPRA sponsors a proprietary mathematical model for ductile-iron pipe based on the 10-point system of AWWA C105, but the model additionally considers the likelihood and consequences of a failure. That model provides corrosion control recommendations beyond the polyethylene encasement alone that is prescribed in AWWA C105. A similar 25-point system by others, published in NACE Materials Performance (Spickelmire 2002) and updated in 2012 (Spickelmire 2012), entails a wider range of corrosion factors, conse-quences, and control criteria.

steel pipeThe chapter on Principles of Corrosion and Corrosion Control of AWWA Manual M11, Steel Pipe: A Guide for Design and Installation, provides a detailed discussion of corrosion protection methods used for steel pipe. Please refer to AWWA Manual M11 for supple-mental information. Coatings for corrosion control of steel pipe are effective when prop-erly used. They are considered to be the primary line of defense against corrosion of steel pipeline systems. AWWA has developed standards for protective coatings for buried steel pipelines. The coatings currently covered include coal-tar enamel, cement–mortar coating, cold-applied tapes, extruded polyolefin, liquid and fusion-bonded epoxy, polyurethane, polyamide, fused polyolefin, and fusion-bonded polyethylene. A light-colored coating system is recommended for steel pipe exposed to the atmosphere, and recoating may be necessary at time intervals dependent on the exposure condition.

The following corrosion control provisions are recommended for all buried steel pipeline:

• Apply a cement–mortar or other protective lining per AWWA standards.• Apply a cement–mortar or dielectric coating per AWWA standards.• Install and handle the pipe in accordance with AWWA C604, project specifica-

tions, and good construction and installation practices.• Electrically bond the gasketed joints of installed pipe (welded joints are internally

bonded) and provide test stations at appropriate intervals to permit monitoring.• Fill the exterior joint recesses with cement mortar or other protective system com-

patible with the coating system used.• Check for holidays and repair any accidental damage to the coating prior to

backfilling.• Fill the interior joint recesses or lining hold-back areas with cement mortar or

other protective system, over a properly prepared substrate, compatible with the lining system used.

To provide corrosion protection, under most conditions, cathodic protection of dielectrically coated steel pipe is appropriate. For supplemental discussion of cathodic protection methods for dielectrically coated steel pipe, refer to NACE Standard SP0169. For supplemental protection of cement–mortar-coated steel pipe in certain environmental conditions, refer to the discussion in the section on concrete pressure pipe in this chapter and to NACE Standard SP0100.


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Copper pipeCopper exposed to aggressive conditions should be isolated from the environment with an inert moisture barrier, a wrapping of insulating tape, a coating of an asphaltum paint, or with another appropriate material. Cathodic protection may also be necessary.

Concrete pressure pipeThe chapter on Design Considerations for Corrosive Environments of AWWA Manual M9, Concrete Pressure Pipe, provides a detailed discussion of corrosion protection methods used for concrete pressure pipe. Portions of that chapter are repeated here for easy reference, but the reader is encouraged to refer to AWWA Manual M9 for supplemental information.

Where the chloride content of the soil dictates the need for supplemental protection of concrete pressure pipe (see chapter 3), one of the following should be provided:

• A moisture barrier to protect the pipe exterior and/or electrical continuity (bonded joints) and a system for monitoring corrosion activity.

• Silica fume in an amount equal to 8 to 10 percent of the cement content added to the exterior mortar or concrete cover over the reinforcing steel.

• Cathodic protection if monitoring of the pipeline detects the onset of corrosion.If concrete pressure pipe will be installed in granular soils with a pH of 5 or less, the

total acidity should be determined. If total acidity is 25 meq/100 g of dry soil or greater, one of the following measures should be considered:

• Backfill the pipe zone with consolidated clay.• Backfill the pipe zone with consolidated limestone.• Provide an acid-resistant membrane or barrier coating on the pipe exterior.• Use silica fume in an amount equal to 8 to 10 percent of the cement weight in the

exterior mortar or concrete over the reinforcing steel.In any soil with a pH below 4, the pipe should be installed in an envelope of con-

solidated clay or with an acid-resistant membrane or barrier coating on the pipe exterior.When a potential problem with aggressive carbon dioxide is indicated by a soil and

groundwater analysis (see chapter 3), one of the following steps should be considered:• Encase the pipe in a clay backfill.• Apply a membrane, a sealer, or a barrier coating to the pipe exterior.For concrete pressure pipe continuously submerged in seawater, Type II cement,

which contains 8 percent or less tricalcium aluminate (C3A), should be specified for sulfate resistance. In soils with more than 2,000 ppm water-soluble sulfate, one of the following measures should be considered:

• The C3A content of the cement in the exterior concrete or mortar should not exceed 5 percent.

• Silica fume in an amount equal to 8 to 10 percent of the cement weight should be added to the external concrete or mortar covering the reinforcing.

• Apply a membrane or barrier coating to the pipe exterior.When concrete pressure pipe will be installed without earth cover, additional pro-

tective measures are desirable due to the long-term atmospheric exposure. The external surface of the pipe should be sealed with a coating material that is compatible with con-crete or mortar substrates to prevent the admittance of moisture. A light color should be selected for the sealer to reflect sunlight and avoid large temperature differentials. Sched-uled inspections and maintenance touch-up of the protective treatment are considered good practice.



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ductile-iron pipeThe chapter on External Corrosion Protection in AWWA Manual M41, Ductile-Iron Pipe and Fittings, provides a detailed discussion of corrosion protection methods used for ductile-iron pipe. Portions of that section are repeated here for easy reference, but the reader is encouraged to refer to AWWA Manual M41 for supplemental information.

Ductile-iron pipe is manufactured in 18- and 20-ft (5.5- and 6-m) lengths, and all modern joints use rubber gaskets. Because of this, a ductile-iron pipeline is a series of electrically segmented units. Experience has demonstrated that the joints retain resistance even after years of burial. Older gray-iron pipe was installed using other types of joints, such as caulked lead, sulfur compound, and cement, but the resistance of those joints is high enough that such pipelines are also considered electrically noncontinuous. When joint continuity of gray- or ductile-iron pipe is needed for application of cathodic pro-tection or stray-current mitigation, the joints must be electrically bonded together with insulated bond wires, which are normally attached by means of exothermic welding or pin brazing procedures. On ductile-iron pipe with epoxy or other special polymeric lin-ings, the pipe manufacturer should be consulted for bonding recommendations to prevent damage to the lining.

After years of study by DIPRA evaluating adherent coatings, polyethylene encase-ment, trench improvement, cathodic protection, and other protection methods, test results led to development of ANSI/AWWA C105/A21.05, Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems. Polyethylene encasement is the most common external cor-rosion control method which has been recommended by the ductile-iron pipe industry over the past 50 years for installations of ductile-iron pipe in corrosive soils. Since its first evaluation in 1951, this method of control has been successfully used to protect over 300 million feet of gray- and ductile-iron pipe in the United States.

Polyethylene encasement is an engineered corrosion control system manufactured using specially designed virgin material with specific thickness and mechanical require-ments, e.g., strength, elongation, propagation tear resistance, impact resistance, and dielectric strength, that are specified in national and international standards. Recycled polyethylene is not used in the manufacture of the film. As shown in Figure 4-6 in this section, protection is achieved simply by encasing the pipe with a tube or sheet of loose polyethylene at the trench immediately before installation. Of the three encasement meth-ods shown, Method A is the most widely used.

For installations of polyethylene encasement below the water table or in areas subject to tidal action, it is recommended that tube-form polyethylene be used, with both ends sealed as thoroughly as possible with adhesive tape or plastic straps at the joint overlap. It is also recommended that circumferential wraps of tape or plastic straps be placed at 2-ft (0.6-m) intervals along the barrel of the pipe to help minimize the space between the pipe and the polyethylene. For more information on the installation or taping of ductile-iron pipe encased in polyethylene, refer to ANSI/AWWA C105/A21.5.

Polyethylene encasement is not a bonded coating, although it offers some of the qual-ities of a coating, such as dielectric strength. It is mainly an environmental improvement. First, it reduces the environment to a very thin space between the pipe and the loose wrap; second, it excludes direct exposure to corrosive soil. It does allow the entrance of ground-water into the annular space between the pipe and wrap; however, the corrosive charac-teristics of the water are soon depleted by the action of initial corrosion, usually oxidation. Extensive use of polyethylene encasement has demonstrated its effectiveness for gray- and ductile-iron pipe in the following respects:



12 in. (305 mm) Minimum

Method A: One length of polyethylene tube for each length of pipe, overlapped at joint.

12 In. (305 mm) Minimum

Method 6: Separate pieces of polyethylene tube for barrel of pipe and polyethylene tube or sheet for joints. The polyethylene over joints overlaps tube encasing barrel. (Method B is not recommended for bolted-type joints unless an additional layer of polyethylene is provided over the joint area, as in methods A and C.)

12 in. (305 mm) Minimum

Method C: Pipe completely wrapped with flat polyethylene sheet.

Figure 4-6 Three methods for polyethylene encasement of ductile-iron pipelines

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• It provides a uniform environment for the pipe, which eliminates local galvanic corrosion cells.

• It offers good electrical shielding, which resists the assimilation of stray direct current.

• Because it is installed on the pipe at the time of pipeline construction, it is not sub-ject to damage during shipment.

• Pinholes and minor damage to the loose wrapping material may not significantly diminish its protective ability.

• Rips, punctures, or other significant damage to the polyethylene film may dimin-ish its protective ability and should be repaired.

• Initial cost of material and installation are very low, and there are no maintenance costs.

Research has shown that polyethylene encasement is a viable corrosion control sys-tem for ductile- and gray-iron pipe in most environments. However, other options includ-ing cathodic protection and stray-current control should be considered for “uniquely severe environments,” as defined in appendix A of ANSI/AWWA C105/A21.5.

Cathodic protection can be applied to manufactured asphaltic-coated ductile-iron pipe or polyethylene-encased iron pipe. Over 30 years of experience and testing have dem-onstrated that the use of cathodic protection improves the effectiveness of polyethylene encasement by protecting the pipe where the film may be damaged or not have been prop-erly applied, resulting in exposed surfaces. The methods are not exclusive, and synergy results when they are used in combination.

referenCesANSI/AWWA C104/A21.4, Standard for Cement–Mortar Lining for Ductile-Iron Pipe and Fittings. American

Water Works Association, Denver, CO.ANSI/AWWA C105/A21.5, Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems. American

National Standards Institute and American Water Works Association, Denver, CO.ANSI/AWWA C200, Steel Water Pipe, 6 In. (150 mm) and Larger. American Water Works Association, Denver,

CO. ANSI/AWWA C203, Standard for Coal-Tar Protective Coatings and Linings for Steel Water Pipelines—Enamel

and Tape—Hot Applied. American Water Works Association, Denver, CO.ANSI/AWWA C205, Standard for Cement–Mortar Protective Lining and Coating for Steel Water Pipe—4 in.

(100 mm) and Larger—Shop Applied. American Water Works Association, Denver, CO.ANSI/AWWA C210, Standard for Liquid-Epoxy Coating Systems for the Interior and Exterior of Steel Water

Pipelines. American Water Works Association, Denver, CO.ANSI/AWWA C213, Standard for Fusion-Bonded Epoxy Coatings for the Interior and Exterior of Steel Water

Pipelines. American Water Works Association, Denver, CO.ANSI/AWWA C214, Standard for Tape Coating Systems for the Exterior of Steel Water Pipelines. American

Water Works Association, Denver, CO.ANSI/AWWA C215, Standard for Extruded Polyolefin Coatings for the Exterior of Steel Water Pipelines.

American Water Works Association, Denver, CO.ANSI/AWWA C217, Petrolatum and Petroleum Wax Tape Coatings for the Exterior of Connections and Fit-

tings for Steel Water Pipelines. American Water Works Association, Denver, CO.ANSI/AWWA C222, Standard for Polyurethane Coatings for the Interior and Exterior of Steel Water Pipe and

Fittings. American Water Works Association, Denver, CO.


66 EXtErnal Corrosion Control for infrastruCturE sustainability

AWWA Manual M27

ANSI/AWWA C224, Standard for Nylon-11-Based Polyamide Coating System for the Interior and Exterior of Steel Water Pipe, Connections, Fittings, and Special Sections. American Water Works Association, Denver, CO.

ANSI/AWWA C225, Standard for Fused Polyolefin Coating Systems for the Exterior of Steel Water Pipelines. American Water Works Association, Denver, CO.

ANSI/AWWA C229, Standard for Fusion-Bonded Polyethylene Coating for the Exterior of Steel Water Pipe-lines. American Water Works Association, Denver, CO.

ANSI/AWWA C602, Standard for Cement–Mortar Lining of Water Pipelines in Place—4 in. (100 mm) and Larger. American Water Works Association, Denver, CO.

ANSI/AWWA C604, Standard for Installation of Buried Steel Water Pipe—4 In. (100 mm) and Larger. American Water Works Association, Denver, CO.

AWWA Manual M9, Concrete Pressure Pipe. American Water Works Association, Denver, CO.AWWA Manual M11, Steel Pipe: A Guide for Design and Installation. American Water Works Association, Denver,

CO.AWWA Manual M41, Ductile-Iron Pipe and Fittings. American Water Works Association, Denver, CO.Bardakjian, H. 1995. Quality Enhancements of Cement–Mortar Coatings. In Advances in Underground Pipeline

Engineering, ASCE, New York, 734–744.Bardakjian, H. and D. A. Hausmann. 2007. Corrosion Protection of Large Diameter Welded Steel Pipelines

with Cement Mortar Coatings. In Pipelines 2007: Advances and Experiences with Trenchless Pipelines Projects, ASCE, Reston, VA.

Hall, S.C. 1987–2011. Corrosion Control of Concrete Pressure Pipe, Proceedings Western States Corrosion Seminar, Western Area NACE International, www.westernstatescorrosion.org.

Kroon, D.H., D. Lindemuth, S. Sampson, T. Vincenzo. 2004. Corrosion Protection of Ductile Iron Pipe, Corrosion Paper No. 4046, Houston, TX.

NACE SP0100. Cathodic Protection to Control External Corrosion of Concrete Pressure Pipelines and Mortar Coated Steel Pipelines for Water or Wastewater Service. NACE International, Houston, TX.

NACE SP0169. Control of External Corrosion of Underground or Submerged Metallic Piping Systems. NACE International, Houston, TX.

Peabody, A.W. 2001. Peabody’s Control of Pipeline Corrosion, 2nd ed. R.L. Bianchette, ed. NACE International, Houston, TX.

Romer, A., Graham E.C. Bell, S. Duranceau, and S. Foreman. 2004. External Corrosion and Corrosion Control of Buried Water Mains, American Water Works Association Research Foundation, Denver, CO.

Schiff, M.J. 1993. Impressed Current Cathodic Protection of Polyethylene Encased Ductile Iron Pipe. Corrosion Paper No. 583, Houston, TX: NACE International.

Spickelmire, B. 2002. Corrosion Considerations for Ductile Iron Pipe. Materials Performance, 41(7):16.Spickelmire, B. 2012. Corrosion Considerations for Ductile Iron Pipe: A Consultant’s Perspective. NACE Interna-

tional 57th Annual Appalachian Underground Corrosion Short Course, May 15, 2012, Morgantown, W.Va.Tefankjian, D.A. 1972. Application of Cathodic Protection. Materials Protection and Performance: 11:11.


AWWA MANUAL

M 27

Chapter 5

Atmospheric n Corrosion

Atmospheric corrosion, like corrosion underground or inside of water-containing pipes, is caused by electrochemical reactions. We define afmosphm'c corrosion as corrosion that occurs anywhere other than in direct contact with a continuous electrolyte such as soil or water. Examples of waterworks subject to atmospheric corrosion include:

Pipes on bridge crossings, either on top of or suspended beneath the bridge.

Pipes inside of treatment facilities that are exposed to the air.

Valves in manholes or pits where there is no water or soil surrounding the pipe.

The exterior of storage tanks and standpipes.

Exposed metal in treatment plants and wells. As in underground or submerged corrosion, four things must be present for atmo-

spheric corrosion to occur. Figure 5-1 illustrates these conditions. First, both an anode and a cathode must be present. On a single metal, these can be small, transient, or static local areas on the metal. In a bimetallic couple, the cathode is the more electropositive metal and the anode is the more electronegative. Current will flow from the anode to the cathode, which will result in metal loss at the anode. Also, a metallic path must exist for the electrons that are produced at the anode to reach the cathode. Lastly, an electrolyte must be present for ion movement. The major differences between underground or submerged corrosion and atmospheric corrosion are the thickness of the electrolyte and the time of exposure to the electrolyte (i.e., time of wetness.) While the electrolyte in underground and submerged corrosion can be almost infinite, the amount of electrolyte in atmospheric corrosion is very thin. It can vary from obviously wet film from rain to invisible.

The remainder of this chapter focuses on factors that influence atmospheric corrosion and how it is controlled.

67



Mo sture Layer

Figure 5-1 Schematic of the four requirements for atmospheric corrosion

After completing this chapter, the reader should be able to Understand how and why metals corrode in the atmosphere.

Be able to identify the types of corrosion that can be expected on metals exposed to the atmosphere.

Understand the various means of mitigating the corrosion process on metals exposed to atmospheric conditions.

Understand how to prevent corrosion of stainless steel in water treatment plants.

Effects of the Environment Seven environmental factors influence atmospheric corrosion: moisture, air pollutants, temperature, the presence of poultices (e.g., mud), solar radiation and wind, orientation, and sheltering. These factors are discussed in the following paragraphs.

Moisture is the most important factor. Moisture provides the electrolyte necessary for the anodic and cathodic reactions to occur. As previously stated, moisture may be present only in a very small film, usually from rainfall or condensation. Below a threshold relative humidity, corrosion will not occur. The threshold relative humidity is dependent on both the material and the presence of air pollutants. For steel, the threshold relative humidity is approximately 60 percent.

The concept of time-of-wetness is important in atmospheric corrosion. Time-of-wetness refers to the amount of time that moisture exists on the surface. Obviously, the longer moisture exists on the surface, the longer time corrosion has to deteriorate the surface. A structure often has areas that have a moisture film longer than others (shaded or sheltered surfaces), and these tend to corrode more than surfaces that lose their moisture film early.

Air pollutants increase the corrosiveness of the moisture film. The most damaging air pollutants that accelerate corrosion are sulfur oxides (which form sulfuric acid, which in turn attacks passive layers and corrodes iron), chlorides (from deicing salts and marine environments), particulates, and aerosols (which retain moisture and other pollutants on

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ATMOSPHERIC CORROSION 69

the surface). Other corrosion-causing pollutants are nitrogen oxides, ammonia (causes stress corrosion of brass), hydrogen sulfide (attacks copper and steel), carbon dioxide, and ozone. Air pollutants, notably sulfur dioxide, decrease the threshold relative humidity, causing corrosion to occur at lower air moisture contents.

Temperature affects the relative humidity and dew point, time of wetness, and kinet- ics of the corrosion reaction. The &iu point is the point at which the combination of relative humidity and temperature causes moisture to condense on a surface.

A podtice is an area of trapped and retained moisture and air pollutants, such as accumulations of soil, dirt, organic matter, or other similar medium, that can hold moisture and pollutants against the metal surface, resulking in increased corrosion at that surface interface.

Solar radiation and wind affect the time of wetness. Wind affects the accumulation of particulates. Chlorides, moisture, or other contaminants blown by the wind can be deposited on surfaces, causing local corrosion in the direction of the pmahng wind. Solar radiation (ultraviolet light exposure) also affects the life of protective coatings on the sdace.

The orientation of a surface affects the extent to which atmospheric corrosion will occur (Figure 5-2). A surhce that is shaded from sunlight wi l l tend to stay moist longer than one that is heated by sunlight. The longer moisture stays on the surface, the more corrosion (time of wetness). similarly, a surface that is shieded from the wind will tend to stay moist longer and suffer more corrosion H&cmtal surfaces facing up will be washed by rain and will not collect contaminants; however, if the d c e is not clrained and moisture collects, so can contaminants, leading to inaeased corrosion. Downward-facing horizontal surfaces corrode more than their upward-facing counterparts because they collect condensation.

Sections of infrastructure that are sheltered from the elements may be corroded less than those that are exposed if they remain relatively dry. On the other hand, in conditions where moisture can condense on sheltered objects, the sheltering can prevent drying. Moisture can diffuse through seals and condense inside an enclosure, resulting in a buildup of moisture. The temperature inside sheltered environments can rise higher than outside the endosure, increasing corrosion rates.

More Corros on

Less corros on

Figure 5-2 The effects of orientation on atmospheric corrosion

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The previous discussion listed many factors that contribute to atmospheric corrosion. In fact, any number of factors can be present and act on any structure, and effects will vary in different areas of the structure as a consequence of changes in orientation, sheltering, and exposure to sunlight and wind. The total environment needs to be taken into account when designing corrosion control.

TYPES OF CORROSION THAT CAN BE EXPECTED Metals corrode in the atmosphere likewise to those that are immersed. However, those characteristics specific to atmospheric corrosion are discussed in this section.

Uniform Corrosion Uniform corrosion is a general wastage of metal in which the entire surface seems to corrode at the same rate. Since atmospheric corrosion does not always occur at a constant rate (for reasons to be discussed), corrosion products sometimes form in a layered or flaking fashion. In the case of carbon steel, a large volume of corrosion products can build up. By contrast, stainless steel develops a very thin and tightly adhering oxide surface layer that provides it with a very low uniform corrosion rate (see Table 5-1). Chromium is the primary element in providing corrosion resistance to stainless steel.

Pitting Corrosion Pitting is a type of localized corrosion that occurs at discrete sites. Generally, a pit can be described as a hole or cavity penetrating into the metal from the metal surface with a surface diameter similar to or less than its depth. Pitting and uniform corrosion often occur together in atmospheric corrosion. In the case of stainless steel, the presence of chloride ions can result in the penetration of the protective oxide surface layer, when in sufficient quantity, to cause pitting attack of the surface.

Crevice Corrosion Crevice corrosion is a particular form of pitting that occurs between adjoining surfaces usually due to oxygen-concentration cell effects. The crevice can be two metals in contact or a metal against a nonmetal. Crevices can also be formed on metals as a shielded area caused by the buildup of debris or deposits. The form of crevice corrosion that occurs most in atmospheric exposure occurs between two closely spaced steel objects, such as back-to-back angles on a pipe support bracket. The volume of corrosion products is

Table 5-1 Comparison of uniform corrosion rates of different metals in the atmosphere

Rural Urban Industrial Marine Carbon steel 4-65 23-71 26-175 26-104 Weathering steel 5-27 4-68 7-85 Stainless steel (300 series) 0.002-0.01 0-0.02 <0.03-0.03

Copper 0.35-0.51 1.27-3.05 0.15-1.52 Galvanized steel 0.2-3 2-16 2-16 0.5-0.6 Aluminum* 0.01 1 0.8-13 0.4-0.6

NOTE: Corrosion rates p; p = lo4 m = 0.00005 in

'Source: iS"h4 STP 558, Corrosion in Natural Enmronments, 1974; ASM Handbook Volume 13, Corrosion, 1987.

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Lmnsiderably larger than the actual volume of metal corroded. The extra volume of the corrosion produ& exerts a tremendous force on the joint. Forces as high as 10,000 psi (68,950 kPa) can be exerted. One common effect of this force is the spalling of concrete on bridges contaminated with deicing salts. This force can ac-tually deform the metal. Tlus type of corrosion is often called pack rust, as shown in Figure 5-3.

Gevice corrosion in stainless steel requires the presence of chlorides and may occur at wetted or moisture-laden tight joints and connections involving metal-to-metal or metal-to-nonmetal contact. An example of the latter is moisture-adsorbent gaskets or where glues or tightly adhering substances are present on the surface of the metal. Lack of oxygen in the crevice results in adjacent oxygenated and deoxygenated surface areas that create anodic and cathodic sites, resulting in oxygen cell crevice corrosion. Crevice geometry and tightness of the gap, along with the amount of chlorides built up, will determine the likelihood of crevice corrosion occurring. In the presence of tight gaps, chlorides can be adsorbed into the gap through capillary action, resulting in acid metal chloride corrosion attack between the metal interfaces. Lower grades of stainless steels are more vulnerable to crevice corrosion, particularly in chlorideladen coastal environments. However, corrosion is avoidable by careful design and good alloy selection.

Crevice corrosion of stainless-steel pipe. Gevice corrosion may occur at tight joints and connections involving metal-to-metal and metal-to-rummetal contact (e.g., gaskets), in the presence of water. This is dependent on gap geometry and tightness, along with the presence and quantity of chloride in the environment. Type 304 stainless steel is more sensitive to crevice geometry than to chloride content in the environment, at least up to

Gevice corrosion can also occur in instances in which an oxygen differential is created on the metal surface due to tightly adhering substances, such as adhesives, labels, or “sticky,” dense surface deposits. This oxygen * creates anodic and cathodic sites adjacent to one another, which in the presence of water and chlorides may result in crevice corrosion.

1.~00 p p

crev ce corros on or pack rust

Figure 5-3 Crevice corrosion or pack rust

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Galvanic corrosion. Galvanic corrosion occurs when two or more metals are connected to one another and exposed to a liquid environment by which an “electric battery” effect is created. The further apart these metals are from one another electrochemically, the greater is the corrosion driving force and attack of the anodic metal component. Galvanic corrosion as it occurs in the atmosphere is usually limited to the area around the junction between the two metals because the moisture film is very thin, and current attenuation occurs within a short distance. Long-line galvanic corrosion cells, such as exist in pipe-lines, do not occur in atmospheric corrosion.

Area ratios between the anode and cathode will also affect the amount of corrosion that occurs on the anodic metal. For example, a steel screw (anode) connected to a stainless steel sheet (cathode) will suffer more corrosion than if the sheet were steel and the screw stainless steel because the area of the steel sheet anode is relatively larger than the screw. Knowledge of this type of mechanism can be useful in structural design.

Environmental cracking. Three types of environmental cracking occur: stress cor-rosion cracking, hydrogen embrittlement, and corrosion fatigue. Stress corrosion happens when the metal has a tensile stress and a corrodant is present. Examples are stainless steels and aluminum in chloride environments, or brass in ammonia environments. In the case of stainless steel, chloride stress corrosion cracking rarely occurs below 140°F.

Hydrogen embrittlement occurs when atomic hydrogen (hydrogen charging) enters a susceptible metal subjected to stress. Hydrogen charging can occur due to corrosion reactions or excessive cathodic protection. For example, galvanized high-strength steel fasteners are subjected to stress corrosion cracking under highly corrosive conditions because the galvanic corrosion between the zinc and steel substrate generates hydrogen that is absorbed by the steel.

Corrosion fatigue occurs when the cyclic stresses exceed the corrosion fatigue limit for a material.

Figure 5-4 illustrates another type of stress-related corrosion caused by a galvanic corrosion cell between stressed and unstressed parts of a component. This type of corro-sion is often found on fasteners where the heads and nuts are more stressed than the body of the fastener.

Figure 5-4

Bolt falls out

Stressed parts corrodeStressed parts corrode

Stress-related galvanic corrosion of fasteners


©atmosphEriC Corrosion 73

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methods of ControlMethods of corrosion control include selection of appropriate materials, attention to design, and application of coatings.

materials selectionCommon structural metals include carbon steel, weathering (high-strength low-alloy [HSLA]) steel, galvanized steel, stainless steel, aluminum, and copper. Each will corrode in the atmosphere, but at vastly different rates. Table 5-1 shows a comparison of typical corro-sion rates for these common alloys.

This table shows the wide variations in the way different alloys perform in atmo-spheric corrosion. Carbon steel, because it is inexpensive, is the most commonly used metal in construction, but it corrodes at the highest rate. However, it can be protected readily, as will be discussed.

Weathering steel contains alloying elements that improve its atmospheric corrosion resistance. Weathering steel performs well in atmospheric exposure where conditions are not too severe. (A marine environment is too severe because of its exposure to salts.) Weathering steel must also be able to dry out once wetted; otherwise it does not develop its protective scale. Therefore, weathering steel should not be used in enclosed areas or at tight-fitting crevices where it cannot dry out.

Galvanized steel is a good material for most applications. Galvanized steel is carbon steel clad with a thin layer of zinc that is about 0.0035 in. at a 2-oz/ft2 coating. Once the zinc is depleted from corrosion, the steel will corrode at the same rate as carbon steel.

Stainless steel performs the best, and copper and aluminum also perform well. Cop-per has long been used both for its architectural appeal in roofing and trim and because of its excellent corrosion resistance. In exposed installations, copper forms over time an attractive and protective bluish-green (verdigris) surface film.

Several types of stainless steel are notable for their corrosion resistance. The most common are the austenitic stainless steels, Types 304/304L and 316/316L, which offer both corrosion resistance and life-cycle cost effectiveness and are used extensively for piping and bolting materials. In addition to corrosion resistance, the duplex and lean grades of duplex stainless steel offer higher design strengths. In general, Type 304/304L is suitable for rural environments and Type 316/316L for most coastal sites. Occasional washdowns with low-chloride waters, particularly in areas sheltered from the cleaning effect of rain, will ensure the good appearance of the stainless steel is maintained. In severe conditions, alloy 2205 or higher grades of stainless can provide additional corrosion resistance.

designCare in the design stage can often prevent corrosion problems from occurring. The goal in designing to control corrosion is to prevent, or lessen, one or more of the four elements needed for corrosion to take place (see Figure 5-1). The following are important points to consider when designing to prevent corrosion.

• Choose metals that are relatively close in the galvanic series of metals. This is not always possible. In fact, it is sometimes necessary to use different metals to take advantage of the corrosion resistance of some metals or to benefit from small-cathodic to large-anodic areas, e.g., bolting in tanks, where other methods of control are not practical. When it is necessary to combine different metals and a galvanic problem is likely, use dielectric insulation or a sealant between them or apply a coating to the cathode. Note it is important not to coat the anode alone as holes or damage at the coating could lead to concentrated attack at those sites from the then relatively large cathode area.



AWWA Manual M27

• Avoid crevices. Make back-to-back angles large enough so that the surfaces can be maintained (painted); otherwise, seal the joint with caulk or weld metal.

• Provide drainage. Design to avoid water entrapment if possible; otherwise, use weep holes where water cannot collect. Use large enough holes such that the hole will not clog too quickly.

• Avoid debris entrapment.• Design corrosion control to be independent. Do not rely on the corrosion control sys-

tem of other applications for your structure. Provide protection sufficient to pro-tect your structure if it becomes exposed to the full environment. For example, in the case of a pipe crossing beneath a bridge with expansion joints, assume that the gutter system for the expansion joints will fail.

CoatingsAn effective coating system starts with a good set of specifications. Such specifications tell the contractor what is expected and provide a means of monitoring the coating process. Good specifications include sections specific to the coating itself in addition to routine contract provisions: type of structure to be coated, coating to be used, method of surface preparation, method of application, and inspection provisions.

The two general coating types are barrier and sacrificial. Barrier coatings provide a barrier between the surface and the environment. Sacrificial coatings provide not only a barrier but also some cathodic protection at defects in the coating. Sacrificial coatings are composed of active metals; they are typically the same metals used as galvanic anodes in cathodic protection.

Examples of sacrificial coatings are electroplated zinc, hot-dipped zinc (“galva-nized”), metallized zinc; aluminum-zinc; hot-dipped aluminum (“aluminized”); electro-plated cadmium (usage in decline for environmental reasons); and zinc-rich paint (liquid applied).

Examples of barrier coatings are oil-based alkyd, silicone alkyd, and acrylic paints. Barrier coatings are easy to apply, less sensitive to surface preparation when compared to high-performance coatings, and initially less costly than high-performance coatings. Gen-erally, they have a shorter life than high-performance coatings in severe environments.

High-performance coatings include epoxy, polyurethane, vinyl, and zinc-rich coat-ings. These have a higher initial cost and are sensitive to surface preparation but have a longer life than standard maintenance coatings if applied correctly.

To be successful, any coating needs to have the following properties: weatherability, corrosion protection, appearance retention, and maintainability.

A third class of coatings that is often considered when dealing with pipe crossings is standard underground pipe coatings. These include the following:

• Coal-tar and asphalt. These are not recommended for atmospheric exposure, because coal tars and asphaltic coatings degrade rapidly when exposed to ultra-violet light.

• Petrolatum. This is an acceptable coating for atmospheric exposure when applied either with an external coating or self-hardened for sunlight exposure.

• Extruded polyethylene. This is also a good coating, but must be formulated for ultraviolet light exposure.

• Fusion bonded epoxy. This coating will chalk at the rate of about ½ mil per year, so a topcoat should be applied if appearance retention is important.



Tape coatings. These must be formulated for ultraviolet light exposure.

Polyurethane. Coatings are applied as a system in one or more coats. Figure 5-5 illustrates this. The

primer forms the main bond with the substrate and is also the main corrosion-prevention element of the system. The topcoat forms the top layer, provides the weathering layer and aesthetic surface, and protects the primer from damage. An intermediate layer is frequently used to add thickness to the system, which increases the life of the coating system.

Coating selection depends on Structure criticality. This refers to the effects of corrosion-related failure and aesthetics. It is important to tailor the coating to the environment. Exposure environment. Severe environments (e.g., marine, heavy industrial) require more durable coatings. In general, the high-performance coating systems perform longer in more corrosive environments. We-cycle cost. Consider the true cost of the coating system, not just the initial cost. A less expensive, low-durability coating will require several recoats within the life expectancy of the structure, while a more durable, more costly coating might require fewer recoats. The more costly coating initially could be the less expnsive one in the long run. Life-cycle benefits may also be considered in seled- ing one of the stainless-steel materials, which do not need coatings for their protection or repeated cleaning, preparation, and recoating- Ease of maintenance. This refers to ease of spot repair and recoatability of the coating. A high-performance coating might be justified if maintenance will not be performed. Environmental considerations. If previous coatings contain lead, overcoating may be feasible, or the old coating may have to be removed. If the old primer is intact, a coating suitable for topcoating might be economically justified. However, keep in mind that eventually the old coating will have to be removed. The costs of rig- ging and scaffolding are often the overriding cost factors. If this is the case, then removal of the old coating might be justified. Where regulatory restrictions on volatile components apply, users can select coatings that conform to the volatile organic compouad (VOC) regulations.

Topcoat

Interned ate

Pr me1

Subsbate

Figure 5-5 Application of coating system

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AWWA Manual M27

When coatings are used, surface preparation for receiving the paint coating becomes important. Surface preparation is perhaps the most critical part of coating. Its purpose is to remove surface contamination and create a profile (“tooth”) to which the primer will stick. In general, high-performance coatings require better surface preparation than ordinary maintenance coatings. The exceptions are the surface-tolerant epoxies and moisture-cured urethanes. As a general rule, the more severe the environment, the better the surface prep-aration should be. Surface preparation is normally specified by the NACE International and Society for Protective Coatings (SSPC) standards. Typical standards are as follows (SP = surface preparation):

SP2 Hand Tool Cleaning Normal maintenance coatings

SP3 Power Tool Cleaning Same + surface tolerant coatings

SP5 White Metal Blast Cleaning High-performance coatings

SP6 Commercial Blast Cleaning Some high-performance coatings

SP10 Near-White Metal Blast Cleaning High-performance coatings

SP11 Power Tool Cleaning to Base Metal High-performance coatings

SP12 Surface Preparation and Cleaning High-performance coatings of Metals by Waterjetting Prior to Recoating

Coating can be applied by various methods depending on the coating, structure, and environmental conditions. The manufacturer should always be asked to provide recom-mendations when there is any doubt. Always apply the primer before the surface changes, usually within the same 8-hour shift. Otherwise, rust bloom can occur and require fur-ther surface cleaning. Application of the topcoat(s) should follow the manufacturer’s rec-ommendations regarding cure time and recoat intervals. Too fast a recoat could result in solvent entrapment, resulting in disbondment of the topcoat. Too long an interval between coats could result in disbondment, if the maximum recoat interval has been exceeded (important with epoxies and polyurethanes). All coating should be done with an eye on environmental conditions. The manufacturer’s specifications regarding temperature and humidity must be followed. The temperature of the surface being coated should always be at least 5˚F above the dew point, otherwise condensation on the metal could affect the coating cure and bond.

The work should always be inspected by a third-party inspection group, if possible. The purposes of inspections are to

• Confirm specifications are met.• Conduct tests for dew point, temperature, surface profile, surface preparation

methods and procedures, surface profile and cleanliness, coating dry film thick-ness, recoating intervals, and containment and waste disposal compliance.

• Maintain a written record of events.Inspection costs about 10 percent of the total painting cost; good inspection pays for

itself in improved life of the coating system.Because of the protective nature of their tightly adherent oxide coatings, austenitic

and duplex stainless steels require no further coating system for atmospheric corrosion resistance, unless the coating is purely aesthetic, or avoidance of galvanic coupling is a primary concern. Likewise, copper is not coated in order to allow for a desired verdigris finish to form, as with roofing, for aesthetic reasons.


©atmosphEriC Corrosion 77

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CoatinG eValuationThe objective behind evaluation of atmospheric corrosion is usually to test various prod-ucts, mainly coatings, to determine how well they perform in the particular environment of your system. Coating an entire structure to see how the coating will perform can do this. However, it is more practical to test many coatings at one time, using test panels, such as 6 in. × 12 in., made of the same material as that on which the coating would be applied, usually steel. The coating is applied in the manner consistent with the manufacturer’s specifications. The panel may be scribed or contain irregularities such as moisture traps or bolts. The test panel is installed in a representative location and examined periodically for appearance and ability to prevent corrosion. Long-term tests on a large number of coat-ings can be conducted in this manner. Using test panels to evaluate coatings is described in ASTM G50, Practice for Conducting Atmospheric Corrosion Tests. It is possible to run exposure tests at a commercial test facility located in a corrosive environment (e.g., marine or industrial facility).

Tests of various metals, such as aluminum, galvanized steel, and aluminized steel, can also be done using test panels. ASTM G1, Preparing, Cleaning and Evaluating Corro-sion Test Specimens, discusses this type of test.

In any type of exposure test program, many types of environments can exist on the same structure. It is important to test under the worst-case conditions as well as “normal” conditions.

stainless steel in aBoVeGround enVironmentsThe tightly adhering oxide scale on stainless steels provides a very corrosion-resistant surface to most aboveground environments. Exposure to marine or saline conditions will necessitate regular cleaning (washdown) practices to remove salt buildup from sprays and windborne salt particles in order to ensure their optimum corrosion performance. Usually Type 316/316L stainless steel and the duplex stainless materials provide the appropriate selection for external piping used in buildings, for pipelines spanning waterways, and for architecture, bridge railings, and other hardware exposed to marine environments. In urban and inland environments, stainless steels all perform well and provide long service lives with minimal maintenance.

Chlorine gas is a common oxidant used in water treatment plants, which often results in chlorine-contaminated atmospheres within the plant, especially in piping gal-leries. Corrosive condensates can form on the external piping and other metallic compo-nents, such as cabinetry, railings, and ladders. If left, it will lead to incipient pitting attack and staining.

In water treatment plants, ventilation of open areas will mitigate much of the sur-face corrosion attack on stainless-steel equipment. Regular washing maintenance of these stainless-steel structures will minimize pitting and staining attack on these units.

referenCesAilor, W.H. 1982. Atmospheric Corrosion. John Wiley & Sons: New York.ASM Handbook. Vol. 13: Corrosion. 1987. ASM International: Materials Park, OH.ASTM G1, Preparing, Cleaning and Evaluating Corrosion Test Specimens. ASTM International, West Con-

shohocken, PA.ASTM G50, Practice for Conducting Atmospheric Corrosion Tests. ASTM International, West Conshohocken,

PA.



AWWA Manual M27

ASTM STP 558, Corrosion in Natural Environments. ASTM International, West Conshohocken, PA.Carter, J.P., P.J. Linstrom, D.R. Flinn, and S.D. Cramer. 1987. The Effects of Sheltering and Orientation on the

Atmospheric Corrosion of Structural Metals. Materials Performance, July, 32–35.Haynie, F.H. 1974. The Economics of Clean Air in Perspective. Materials Performance, April, 33–38.Komp, M.E. 1987. Atmospheric Corrosion Ratings of Weathering Steels: Calculation and Significance. Materi-

als Performance, July, 42–44.Mattson, E. 1982. The Atmospheric Corrosion Properties of Some Common Structural Metals: A Comparative

Study. Materials Performance, July, 9–19.Perez, F.C. 1984. Atmospheric Corrosion of Steel in a Humid Tropical Climate—Influence of Pollution, Humid-

ity, Temperature, Solar Radiation and Rainfall. Materials Performance, April, 170–175.Tuthill, A.H., et al. 1998. Effect of Chlorine on Common Materials in Fresh Water. Metals Performance, 37 No. 11

(Nickel Development Institute Reprint No. 14 049).USEPA. 1985. Environmental Factors Affecting the Corrosion of Weathering Steel. US Environmental Protection

Agency, Research Triangle Park, NC.


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M 27

Chapter 6

Corrosion Control of Water Storage Tanks

Protecting the submerged internal surfaces of steel water storage tanks from corrosion by cathodic protection is sound engineering practice supported by industry standards (ANSI/AWWA D104, ANSI/AWWA D106, NACE International SP0388, and NACE Interna- tional SP0196). The use of this technology has resulted in demonstrated economic benefits (Drisko 1972, Meyers and Obrecht 1975, SPFA 1977, Klimco 1977). Because many of the tanks contain potable water for consumption by the public, all system materials in contact with the water should be certified in accordance with ANSI/NSF 61, Drinking Water Sys- tem Components -Health Effects.

After completing this chapter, the reader should be able to Understand mechanisms that can initiate corrosion of steel submerged in water storage tanks.

Understand the application of coatings and cathodic protection for mitigating corrosion in water tanks.

Recognize that regular maintenance is required for sustainability of the tank performance.

CORROSION OF WATER TANKS The corrosion of submerged metallic structures is caused by electrochemical activity. In freshwater tanks, corrosion activity usually results in a concentrated pitting attack that more quickly penetrates the steel plate than if the corrosion were more uniformly distributed on the metal surface. This attack is initiated by the development of anodic and cathodic areas on the submerged metal surfaces. Anodic areas will suffer metal loss, whereas cathodic areas will not.

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mechanisms of CorrosionA number of mechanisms can initiate corrosion of steel submerged in water storage tanks.

Uniform corrosion. Although steel appears to be homogeneous on first glance, close inspection shows that it is quite irregular, consisting of multiple grains of metal that are electrically discrete. Thus, some will be anodes and others will be cathodes. The corrosion attack will usually appear as random, closely spaced pits.

Stressed metal. Steel that is under stress will be anodic to unstressed steel. In tanks, these stresses can be caused by welding or through bending or forming (without stress relieving), bolting, and riveting. Most often, the fastener (bolt or rivet) will be anodic to the adjacent plate.

Dissimilar metal corrosion. The use of different metals in direct contact with each other will establish a corrosion cell in which the more noble metal will be cathodic and the more active metal will be anodic. Examples of such cells in tanks are copper or stainless-steel heater coils in industrial fire protection tanks and weld seams in which the metal-lurgy of the welding rod differs from the base plate metal.

Crevice corrosion. A corrosion cell develops at crevices where oxygen concentrations are lower than those in the adjacent area. The anodic area will be in the crevice, with the nearby surface areas cathodic. In water tanks, these develop most commonly between the head and plate of bolted or riveted plates and between the overlapping areas of unsealed plates.

Differential in oxygen concentrations. When steel is immersed in water and some of the steel surface is exposed to a relatively oxygen-enriched water compared with other sections of the surface, the area deprived of oxygen will be anodic in relation to surfaces exposed to highly oxygenated water. This phenomenon is often observed in bare or poorly coated tanks in which the lower submerged surfaces are heavily corroded (greater depths having lower oxygen levels), whereas the upper areas show little corrosion. Even more common is vertical striation corrosion. Deep, vertical gouges, sometimes several inches or even feet long, occur on the uncoated submerged surfaces of the tanks. This common corrosion phenomenon is caused by the development of an initial corrosion pit that gen-erates soft, oozing corrosion products. Gravity causes these products to move down the side of the tank wall, shielding the lower surface from oxygen and rendering it anodic. The shielded surface begins to corrode, generating more corrosion products. The process continues down the wall of the tank.

Coating pinhole corrosion. When the internal surfaces of tanks are coated with a dielectric material (e.g., epoxy), the corrosion activity will be concentrated at the holi-days (holes) in the coating. The breaks in the coating can result from mechanical damage, improper surface preparation, or merely microscopic voids in the coating surfaces.

The corrosion currents will concentrate at the holidays and result in high corrosion current densities at these locations. Even though a good coating reduces the total metal loss, a complete penetration of the metal surface occurs more quickly than if the tank were not coated.

The need for corrosion control. Many other factors can influence the rate at which corrosion processes will proceed in water tanks. Among the most significant are elec-trolyte resistivity, water flow rates, the extent of anodic surface areas relative to cathodic areas, iron ion concentrations, temperature, and water level fluctuations.

Virtually all potable waters are corrosive with respect to steel. Almost all tank own-ers require that tanks be coated as part of a regular maintenance coating program or sim-ply be entirely recoated periodically. Therefore, the question is not whether water tanks are subject to corrosion. The question is, what is the most effective and economical means of corrosion mitigation?


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Corrosion preVention for Water tanksThe interior surfaces of water storage tanks should be protected from corrosion by coating and cathodic protection. The coating reduces the amount of bare metal in contact with the water, while the cathodic protection system provides corrosion protection at the unavoid-able voids in the coating. AWWA and NACE International standards that address cathodic protection of the internal, wetted surfaces of steel water storage tanks are noted in the references.

Coatings and Cathodic protectionA four-year study by the US Navy (Drisko 1972) concluded:

Cathodic protection has been shown to be an economical and effective method for protecting the immersed interiors of steel water tanks. Cathodic protec-tion of a water tank can double or triple the time between recoatings. Thus, cathodic protection and protective coatings should be considered as compo-nents of a single corrosion control program for water storage tanks. Automati-cally controlled cathodic protection systems utilizing one reference half-cell should be installed in steel water storage tanks throughout the naval shore establishment.

Another report (Meyers and Obrecht 1975) concluded:Supplemental cathodic protection should be used to prevent the corrosion that can occur at “holidays” in organic coatings.

A brochure prepared by the Steel Plate Fabricators Association (SPFA 1977) summarizes:The modern techniques of corrosion control using coatings and cathodic pro-tection can reduce the initial cost and will extend the life of the steel storage tanks indefinitely.

The cost of installing cathodic protection is a fraction of the cost of repainting the submerged surface area of most tanks. Because cathodic protection can double or triple the time between recoatings, the economics of using cathodic protection for corrosion con-trol on water tanks are apparent.

design of Cathodic protectionFor any water tank, several variables must be considered in designing a cathodic protec-tion system. Among these are water chemistry, coating, tank design, and tank operations.

Variations in water chemistry that affect cathodic protection include the water source, total dissolved solids, total hardness, dissolved oxygen, total alkalinity, pH, tem-perature, and bacteria.

Variations in tank coatings include the generic type and condition of the coating. Tank designs and shapes include ground storage reservoirs, standpipes, and elevated water storage tanks. Tanks can be used as water storage facilities, pressure regulators, fire protection reservoirs, or any combination of these.

Because of these numerous variables, it may seem unreasonable to expect a limited number of cathodic protection designs to accommodate all. Yet, this is possible, because certain conditions in water tanks and treatment equipment favor standardization.

First, the electrolyte (i.e., the water) in which the metal is submerged is relatively uniform. Second, although structural shapes vary, tanks are all basically a combination of cylinders and gradual curves. These conditions contrast with those found in under-ground pipelines where considerable irregularity can occur in electrolyte and piping



AWWA Manual M27

configuration. Third, and most responsible for permitting standardized design, cathodic protection systems for water tanks are almost always of the impressed-current type. Great flexibility can be built into an impressed-current system to meet various requirements without adding greatly to the cost. Magnesium and other types of galvanic anode materi-als are a viable alternative for well-coated tanks.

When designing a cathodic protection system for a water storage tank, the designer must determine the total amount of current required for protection, the resistivity of the water, the anode materials, the anode geometry, and the service life.

The most important factor, however, is the type and condition of the coating. Although several coatings are suitable for the interiors of water storage tanks, epoxy coat-ings are most commonly used.

When a high-quality coating is applied and used with cathodic protection in accor-dance with AWWA standards and manufacturers’ specifications, a useful life of 20 or more years can be expected. When it is realized that an uncoated water tank of 10-million gallon capacity can require as much as 150 A of current, whereas a similar well-coated tank might need as little as 5 to 10 A, the value of a quality coating system used in conjunc-tion with cathodic protection is apparent.

A coating serves a different function on a cathodically protected structure. If coating is the only means of controlling corrosion, then blistering, peeling, or cracking of 1 to 2 percent of the coated surface can cause failure of the paint system to control corrosion. In most cases, accelerated attack will occur.

In a cathodically protected tank, comparable coating deterioration is compensated for by a small increase in the cathodic protection system’s electrical current output. The coating’s major function with cathodic protection is to reduce the cost of power and the rate of anode consumption. Repainting the submerged portions of the structure can, therefore, be done less frequently.

Cathodic protection anodesThe most commonly used cathodic protection anodes are mixed metal oxide coated tita-nium wires. The anodes are suspended vertically from the roof or in a circular hoop con-figuration to conform to the geometry of the tank (Figures 6-1 and 6-2). The installation technique has several advantages. Primary among these is the fact that it provides the most even distribution of current for optimum performance, tailored to the size and shape of the particular tank.

This suspension technique is ideally suited for achieving a long design life in a tank subject to icing conditions. The flexible suspension system is designed so the anode mate-rial cannot come in contact with the tank ice during normal fluctuation in water level. As a result, the tank receives cathodic protection year round without the need for yearly replacement of ice-damaged anodes.

The hoop suspension system also eliminates the need for hand holes (plus covers and insulated support brackets) and the maintenance problems involved with these. There is no need to cut numerous holes in the top of the tank. These hand holes were also sub-ject to their own corrosion problems, as well as being a source for contamination if they were not tight-fitting. With the hoop system, the limited number of small metal hangers required for the suspension material can be easily welded to the inside of the tank before the coating is applied.



AWWA Manual M27

Figure 6-1

60

8 — Eye r ngs we ded to tank wa . Even y spaced every 60 ft at a he ght of 6 ft above the tank f oor

8 — 5/16- n. bra ded po yester rope supports8 — J m buoy f oats

1 — .062- n. p at n zed/n ob um sp ra ed around a 5/16- n. po yester rope support

2 — Submers b e reference e ectrodes

A submerged w re to be #10 AWG RHW/USE

1 — 30 Vo t, 15 Amp TASC V rated, automat c rect f er

1 — 1- n. pressure entranc ng f tt ng1 — 2- n. × 2- n. ang e we ded to tank she to fac tate system grounds

115 V/60 Hz/1θ AC supp y

30

128

24 8"

HWL

Typical cathodic protection system for ground storage reservoir

automatic transformer-rectifierSeveral automatic control systems have been developed to compensate for changes in cur-rent requirements for cathodic protection. These changes result from fluctuating water levels, changes in water chemistry, and coating deterioration.

The US Navy report (Drisko 1972) states:An automatically controlled cathodic protection system for steel water tank interiors is a reliable safeguard for ensuring that the immersed surfaces are receiving full protection from corrosion without excessive use of current or damage to the coating.


84 EXTERNAL CORROSION CONTROL FOR INFRASTRUCTURE SUSTALNABILITY

6 - Eye r ngs we ded to tank s dewa equa y spaced every 60 R * 6 n. at a he ght of 8 ft above the tank t oor (ow water ne)

6 - 5 / 1 ~ n. bra ded PO yester rope supports

6 - F otat on buoys

1 - 0.062- n. d ameter p at numln ob um anode w re

50

37

7c

A submerged w re to be 81 2-7 AWG RHWNSE

2 - Submers b e reference e ectrode 7 2 - Eye rngs weded to the ns de of rser

ppe, 180 ftapart 2 n. down from top sde

1 - -031- n. p at n zedlnob urn r ser anode paced a ong the eni re ength of the r ser, 5 ft off the bottom

1 - 30V. 8 Amp automat c potent a contro ed red f er

1 - Pressure entranc ng f ti ng

115 VBCI Hzne AC supp y

Figure 6-2 Typical cathodic protection system for elevated tank

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AWWA Manual M27

The study by Meyers and Obrecht (1975) concludes:Regardless of the coating efficiency, cathodic protection systems should be auto-matic potential controlled. This will ensure that any exposed steel immersed in the water is continuously being protected without excessive use of current or blistering of the coating.

Automatic voltage-control units incorporate a circuit that compares the difference in potential between the reference electrode and the tank against a preset internal volt-age. When the structure-to-reference electrode varies from the optimum value, the circuit operates to increase or decrease the anode current so that the optimum voltage will be maintained.

Meyers and Obrecht (1975) state:The success of an automatic potential controlled cathodic protection system, however, depends upon how accurately the tank-to-water potential can be measured. Voltage drop error between the steel and a permanently installed copper–copper sulfate reference electrode must be eliminated if effective cor-rosion control is to be obtained.

The best method to eliminate the voltage drop in the potential measurement is to reduce the current flowing from the anode to the tank wall to zero. This can be done by interrupting the current output from the rectifier unit. The reading taken immedi-ately after the anode current is interrupted, called an instant-off reading, is maintained between –0.850 V and –1.050 V in a properly operating system.

This technique requires only one reference electrode, with no manual balancing or field compensations necessary. Through the use of solid-state components, the output of anode current for the rectifier is turned off for a fraction of a second, during which time the tank-to-electrolyte potential is measured by the reference electrode and electronically stored in a memory circuit. This memorized reading is then compared with a preset opti-mum potential and is used to control the output of current during the remaining time period, after which the system recycles. These systems are also equipped with a corrosion-potential–monitoring voltmeter, which is located in the circuit so that it measures and indicates the instant-off protective level maintained in the tanks.

The units in this type of system are equipped with an automatic gain control so that the system responds to a given deviation in potential and is automatically adjusted to match specific conditions. The advantages over manual control lie not only in auto-matically maintaining precise protective levels regardless of changes in water level, water chemistry, and coating deterioration, but also in affording a means of reading the level of protection. Continuous corrosion control can be ensured by monitoring the tank’s poten-tial voltmeter.

design lifeMost of the components in cathodic protection systems for water storage tanks and water treatment equipment have a design life of 20 years. The electrical components and recti-fier units are capable of continuous operation over this length of time with little attention. Mixed metal-oxide coated titanium wire anode systems can easily be designed for a 20- to 40-year life. Sacrificial anode systems are typically designed for a 10-year life.



AWWA Manual M27

maintenanceA complete inspection of the cathodic protection system should be performed annually. For manual systems, annual maintenance should include readjustment of the system to maintain appropriate operating levels for the coming year. For systems with seasonal anodes, this service should include the replacement of the anode material as required. For all manual and automatic systems, the annual inspection should include a potential pro-file of the submerged structure along with visual and electrical tests.

ConClusionCathodic protection for the submerged surfaces of water storage tanks is an effective and economical method of corrosion control. Although bare steel tanks can be protected, both coating and cathodic protection are usually recommended for new installations.

Automatically controlled, solid-state rectifiers, which measure the maintained poten-tial without IR drop error, are recommended. A flexible, long-life anode system using wire anodes is recommended. Vertically suspended, long-life, wire anodes can be used for tanks when icing conditions do not exist. All systems must be properly maintained to ensure continued effective operation and optimum corrosion control.

referenCesANSI/AWWA D104, Standard for Automatically Controlled, Impressed-Current Cathodic Protection for the

Interior Submerged Surfaces of Steel Water Storage Tanks. American Water Works Association: Denver, CO.

ANSI/AWWA D106, Standard for Sacrificial Anode Cathodic Protection Systems for the Interior Submerged Surfaces of Steel Water Storage Tanks. American Water Works Association: Denver, CO.

ANSI/NSF 61, Drinking Water System Components—Health Effects. NSF International: Ann Arbor, MI.AWWA. 2010. Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair. Stephen Meier, ed.

American Water Works Association: Denver, CO.Drisko, R.W. 1972. Surveillance and Automatically Controlled Systems for Cathodic Protection of Water Tank Interi-

ors. Department of the Navy, Naval Civil Engineering Lab, Technical Report R7605.Klimco, R. 1977. Cathodic Protection Guards Cleveland’s Water System. American City and County Magazine

(March).Meyers, J.R. and Obrecht, M.F. 1975. Corrosion Control for Potable Water Storage Tanks. Presented at Sixth Inter-

national Congress on Metallic Corrosion, Sydney, Australia (Dec. 3–9).NACE International SP0196. Standard Practice, Cathodic Protection, Galvanic Anode for Internal Submerged

Surfaces of Steel Water Storage Tanks. NACE International: Houston, TX.NACE SP0388. Standard Practice, Impressed Current Cathodic Protection of Internal Submerged Surfaces of

Steel Water Storage Tanks. NACE International: Houston, TX.Steel Plate Fabricators Association (SPFA). 1977. New Ideas for Painting Steel Storage Tanks. Steel Plate Fabrica-

tors Association, Inc.: Chicago, IL.


©97AWWA Manual M27

M1, Principles of Water Rates, Fees, and Charges, #30001

M2, Instrumentation and Control, #30002M3, Safety Practices for Water Utilities, #30003M4, Water Fluoridation Principles and Practices,

#30004M5, Water Utility Management, #30005M6, Water Meters—Selection, Installation, Testing,

and Maintenance, #30006M7, Problem Organisms in Water: Identification and

Treatment, #30007M9, Concrete Pressure Pipe, #30009M11, Steel Pipe—A Guide for Design and Installation,

#30011M12, Simplified Procedures for Water Examination,

#30012M14, Recommended Practice for Backflow Prevention

and Cross-Connection Control, #30014M17, Installation, Field Testing, and Maintenance of

Fire Hydrants, #30017M19, Emergency Planning for Water Utilities, #30019 M20, Water Chlorination/Chloramination Practices and

Principles, #30020M21, Groundwater, #30021M22, Sizing Water Service Lines and Meters, #30022M23, PVC Pipe—Design and Installation, #30023M24, Dual Water Systems, #30024M25, Flexible-Membrane Covers and Linings for

Potable-Water Reservoirs, #30025M27, External Corrosion: Introduction to Chemistry

and Control, #30027M28, Rehabilitation of Water Mains, #30028M29, Fundamentals of Water Utility Capital Financing,

#30029M30, Precoat Filtration, #30030M31, Distribution System Requirements for Fire

Protection, #30031

M32, Computer Modeling of Water Distribution Systems, #30032

M33, Flowmeters in Water Supply, #30033M36, Water Audits and Loss Control Programs, #30036M37, Operational Control of Coagulation and Filtration

Processes, #30037M38, Electrodialysis and Electrodialysis Reversal,

#30038M41, Ductile-Iron Pipe and Fittings, #30041M42, Steel Water-Storage Tanks, #30042M44, Distribution Valves: Selection, Installation, Field

Testing, and Maintenance, #30044M45, Fiberglass Pipe Design, #30045M46, Reverse Osmosis and Nanofiltration, #30046M47, Capital Project Delivery, #30047M48, Waterborne Pathogens, #30048M49, Butterfly Valves: Torque, Head Loss, and

Cavitation Analysis, #30049M50, Water Resources Planning, #30050M51, Air-Release, Air/Vacuum, and Combination Air

Valves, #30051M52, Water Conservation Programs—A Planning

Manual, #30052M53, Microfiltration and Ultrafiltration Membranes for

Drinking Water, #30053M54, Developing Rates for Small Systems, #30054M55, PE Pipe—Design and Installation, #30055M56, Nitrification Prevention and Control in

Drinking Water, #30056M57, Algae: Source to Treatment, #30057M58, Internal Corrosion Control in Water Distribution

Systems, #30058M60, Drought Preparedness and Response, #30060M61, Desalination of Seawater, #30061

AWWA Manuals


©Acknowledgments

This is the third edition of AWWA Manual M27, retitled External Corrosion Control for Infrastructure Sustainability. Members of the Corrosion Committee involved in its develop-ment and approval included the following:

Andrew E. Romer, Chair, AECOM, Orange, Calif.David H. Kroon, Vice-Chair, Corrpro/Aegion, Houston, TexasGraham E. C. Bell, Secretary, HDR Schiff, Claremont, Calif.

Steven E. Cooper, Uni-Bell PVC Pipe Association, Louisville, Ky.Stuart L. Greenberger, Portland Water Bureau, Portland, Ore.John M. Grocki, Splash Inc., Enfield, Conn.David T. Haines, NACE International, Houston, TexasSylvia C. Hall, Ameron International Corporation, South Gate, Calif.L. Gregg Horn, Ductile Iron Pipe Research Association, Hoover, Ala.Mike Horton, US Pipe & Foundry Company, Birmingham, Ala.Brent D. Keil, Northwest Pipe Company, Pleasant Grove, UtahStephen Lamb, Splash Inc., Huntington, W. Va.Donovan Larson, Jefferson County Water District No. C-1, Barnhart, Mo.Tim Lorenzo, Raven Lining Systems, Broken Arrow, Okla.Richard Mielke, Northwest Pipe Company, Raleigh, N.C.Randy Moore, Utility Service Co., Inc., Atlanta, Ga.Randy Nixon, Corrosion Probe Inc., Centerbrook, Conn.Gene Oliver, American Cast Iron Pipe Company, Birmingham, Ala.Carol Powell, Nickel Institute, Shropshire, UKShah Rahman, Northwest Pipe Company, Saginaw, TexasRobert A. Ryder, Kennedy/Jenks Consultants, San Francisco, Calif.Jon Turner, Phoenix Civil Engineering Inc., Ventura, Calif.Jose L. Villalobos, V&A Consulting Engineers Inc., Oakland, Calif.Yaofu Zhang, Russell Corrosion Consultants Inc., Columbia, Md.

Members of the Corrosion Committee involved in the development and approval of the second edition (2004) of AWWA Manual M27, External Corrosion—Introduction to Chemistry and Control, included the following:

Franklyn W. Pogge, Chair Lake Barrett David P. Prosser

Graham E.C. Bell W. Harry Smith Steven R. Piper Troy F. Stroud Nicholas G. Pizzi Robert P. Walker

xiAWWA Manual M27


©AWWA Manual M27xii

Members of the Corrosion Committee involved in the development and approval of the first edition (1987) of AWWA Manual M27, External Corrosion—Introduction to Chemistry and Control, included the following:

Robert E. Behnke, ChairmanW. Harry Smith, Vice-Chairman

Joe A. Willett, Secretary G.S. Allen Donald R. Reedy M. Lamar Bowden R.C. Robinson W.C. Creasman Stanley R. Saylor J.R. Easterly Melvin J. Schiff Lewis R. Keyser Troy F. Stroud Donald E. Knibb David L. Tippin Gerald L. Mahon Harry K. Uyeda Philip I. McGrath Jr. Jose L. Villalobos J.H. Miller Charles F. Voyles Franklyn W. Pogge Donald M. Waters Allan L. Pool Robert Zeef


©87AWWA Manual M27

aggressive. Corrosive.anode. That part of a corrosion cell that is discharging current into the electrolyte and is

corroding. Electrochemical oxidation occurs at this electrode. Contrast with cathode.backflow. The entrance of water or other liquid from any but the normal source of a

potable water supply system.bacteriological corrosion. Corrosion that results from the by-products of sulfate-reduc-

ing bacteria in media of very low or no oxygen content.bimetal couple. A type of corrosion where two different metals or alloys are in contact

with each other in a common medium.cathode. That part of a corrosion cell that is receiving current and is protected. Electro-

chemical reduction occurs at this electrode. Contrast with anode.cathodic protection. Reduction of corrosion by making the metal a cathode. This is done

by causing direct current to enter the structure’s surface at all locations.coating. An application of an adherent or mechanically bonded material between metal-

lic structures and their environment (electrolyte) that in a general sense provides im-proved protective properties.

concentration cell. A corrosion cell involving two identical electrode materials, with cor-rosion resulting from mechanical, physical, or chemical differences of the environ-ments adjacent to the two electrodes.

conductivity. The ease with which an electrical circuit allows current to flow. Conductiv-ity, measured in mhos or siemens, is the reciprocal of resistivity.

copper–copper sulfate electrode (reference electrode, or CSE). A copper rod partially immersed in a copper sulfate solution inside a cylindrical tube with a porous bottom, creating the cathodic half of a galvanic cell. The copper sulfate provides excellent soil contact—much better than would the bare copper rod. A reference electrode made with this cell is used in making field measurements of pipe-to-soil potentials.

corrosion (general). Deterioration of a material by reaction with its environment.corrosion (metallic). Deterioration of a metal by reaction with its environment. This is

nature’s way of returning refined metals to their natural state. Rust is iron oxide—iron ore also is iron oxide.

corrosion cell. The arrangement of an anode and a cathode in contact with a common electrolyte in such a manner that current discharges from the anode into the electro-lyte. Corrosion cells are either galvanic, which generate electrical current, or electro-lytic, which are driven by an outside electric current.

corrosion control. The reduction in the rate of deterioration or the total elimination of the environmental impact on water utility systems.

CSE. See copper–copper sulfate electrode.current. The movement of electricity through a circuit, measured in amperes (A). Con-

ventional electrical current flow is said to move through an external circuit from the positive terminal (cathode) to the negative terminal (anode) of a galvanic cell; electrons actually move in the opposite direction, from negative to positive. Electrical current is analogous to flow rate in a water pipe.

Glossary



AWWA Manual M27

current span. The amount of current moving through a pipe, calculated using the mea-surement of the difference in voltage between two locations along the pipeline.

dezincification. Selective removal of zinc from an alloy, such as brass.dielectric coating. A coating that does not conduct electricity.electrode. One of two pieces of metal that are immersed in an electrolyte to form a corro-

sion cell. The corroding electrode, from which electrical current enters the electrolyte, is called the anode. The protected electrode is called the cathode.

electrolyte. The ionically conductive media in which the anode and cathode of a corro-sion cell are immersed. Soil is the electrolyte of greatest interest for water pipe.

electrolytic corrosion cell. A cell in which an external direct current generates the corrosion.

electron (e–). A constituent of an atom with a negative charge. Electrons flow in the direc-tion opposite to current flow.

electronic path. The movement of electrons through a complete electrical circuit.environment. The surrounding materials and conditions that influence the water utility

system.environmental alteration. The selection of an electrolyte (backfill material) to eliminate

or reduce the rate of corrosion. Also referred to as trench improvement.Faraday’s law. A law of electrochemistry that states that the amount of any substance dis-

solved or deposited in electrolysis is proportional to the total electric charge passed.fatigue. The process of progressive, localized permanent structural change occurring in a

material subjected to fluctuating stresses less than the ultimate tensile strength of the material that may culminate in cracks or complete fracture after a sufficient number of fluctuations.

fatigue corrosion. See stress corrosion.fretting corrosion. Deterioration at the interface between contacting surfaces as the result

of corrosion and slight oscillatory slip between the two surfaces.galvanic corrosion. Accelerated corrosion of a metal because of an electrical contact with

a more noble metal or nonmetallic conductor in a corrosive electrolyte.galvanic series. A list of metals and alloys arranged according to their relative corrosion

potentials in a given electrolyte.ground‑voltage gradient. The voltage difference between each two of a series of locations

along the ground surface.half cell. (1) The single oxidation or reduction half reactions in the complete electrochem-

ical cell. (The potential of a half cell can only be calculated from the thermodynamic properties of its components [thermodynamic use].) (2) Commonly used in the field to refer to a reference electrode, but this is not a preferred use.

holiday. A void or discontinuity in a coating that will allow the passage of electrical current.

hot spot. An area of soil found by survey and analysis or experience to be more corrosive than surrounding soil.

impingement corrosion. A form of erosion-corrosion generally associated with the local impingement of a high-velocity, flowing fluid against a solid surface.

impressed‑current system. A cathodic protection system using an outside source of pow-er, converting it to direct current, and injecting it into the soil through an anode bed. Also called a rectifier-ground-bed system.


©AWWA Manual M27

glossary 89

ion. One of the electrically charged particles produced by the disassociation (breakup) of a chemical compound.

mitigate. To moderate or make less severe.Ohm’s law. A fundamental law of electricity that states that the current I flowing in a

circuit is equal to the voltage E divided by the resistance R. Thus, I = E/R, where I is in amperes, E in volts, and R in ohms.

oxidation. (1) Loss of electrons by a constituent of a chemical reaction. (2) Corrosion of a material that is exposed to an oxidizing gas at elevated temperatures.

oxidation‑reduction potential (redox potential). The electrical potential (in millivolts) between platinum and reference electrodes inserted in soil, measured with a pH meter. A low potential indicates low soil aeration.

passivation. A condition of metal or alloy, usually at its surface, causing it to behave in a more noble manner.

pH. The hydrogen ion activity of a media. Values range from 0 to 14. A pH of less than 7 is acidic, 7 is neutral, and greater than 7 is alkaline.

pH meter. An instrument used for the electronic determination of pH. pH can also be measured with colorimetric or other analytical chemical methods.

pipe‑to‑soil potential. The potential measured between an underground metal, such as pipe, and a copper–copper sulfate electrode. Any two connected metals develop an electrical potential (driving force in volts) when in contact with a common media, such as soil.

pitting corrosion. Highly localized corrosion that causes penetration into the metal at a few spots.

polarization. Retardation of the corrosion process by the buildup of protective layers on an electrode, usually the cathode or anode.

polar solvent. A solvent compound whose molecules are polarized. That is, the elec-trostatic charge on one side of the molecule is relatively positive and, consequently, relatively negative on the other, due to unequal sharing of electrons in the covalent bonding between the atoms. Polar solvents separate the structural units of the solvate by surrounding each ion with a cluster of solvent molecules held to the positive or negative ion by the oppositely charged end of the solvent molecule. Water (H2O) and methanol (CH3OH) are examples of highly polar solvents.

polyethylene encasement. A polyethylene film placed around gray-iron or ductile-iron pipe to prevent corrosion.

potential. The force available to drive an electrical current through a circuit, measured in volts (V). It is analogous to pressure (head) in a water pipe.

rectifier‑ground‑bed system. See impressed-current system.redox potential. See oxidation-reduction potential.reduction. Gain of electrons by a constituent of a chemical reaction.resistance. The tendency of an electrical circuit to retard the flow of current. It is mea-

sured in ohms, and is analogous to friction factor in a water pipe.resistivity. A measure of the effective resistance of a media, such as soil, over a given dis-

tance. It is measured in ohm-centimeter.resistivity meter. An instrument that uses batteries and is used to measure the average

resistance to current flow in a media.



AWWA Manual M27

return current path. The metallic connection between the anode and cathode of an elec-trochemical cell.

return on investment. A method to determine whether any proposed course of action will prove to be economical, in comparison with other possible alternatives.

sacrificial anode. An anode of metal less noble than a metal structure to be cathodically protected. It is sacrificed by corrosion to create cathodic conditions at the protected structure.

sacrificial‑anode system. A cathodic protection system in which the driving voltage for the protective current is generated by a galvanic corrosion cell, with the protected structure being the cathode.

single probe. A probe, usually 4–4½ ft (1.2–1.4 m) in length, used to measure resistivity soil around tip or point of probe.

soil box. A device used to determine resistivity of a confined volume of soil. soil corrosion. Corrosion of underground materials resulting from soil conditions.soil resistivity. An indication of difficulty with which a soil conducts electrical current.

The average electrical resistance of a volume of soil.stray current. Direct current traveling through the earth around an existing underground

structure. If stray current enters and leaves that structure, corrosion will occur.stress corrosion. Corrosion that acts on metal at points of tensile stress, working, or vibra-

tion wear.sulfides. A chemical compound of sulfur with another element. A result of the life pro-

cess of sulfate-reducing bacteria.voltage. See potential.

referenCesNote: Some definitions are reprinted with permission from NACE and ASTM (NACE International, 1440 South Creek Drive, Houston, TX 77084; ASTM International, PO Box C700, West Conshohocken, PA 19428; NACE/ASTM G193-11a, Standard Terminology and Acronyms Relating to Corrosion). NACE Standards are revised periodically. Users are cautioned to obtain the latest edition; information in an outdated version of the standard may not be accurate.

ASM Handbook. 2003. Glossary of Terms. In Vol. 13A, Corrosion: Fundamentals, Testing, and Protection. ASM International: Materials Park, OH.


This page has been reformatted by Knovel to provide easier navigation.

INDEX

Note: f. indicates figure; t. indicates table; n. indicates (foot)note

Index Terms Links

A

Aerobic microbes 33

Aggressive, defined 87

Anaerobic microbes 33

Anodes

defined 87

in electrolytic corrosion 13 13f.

in galvanic corrosion 8 8f.

Anodic metals 16 17t.

Asbestos-cement (AC) pipe 43

and aggressiveness of soluble sulfates 43 44t.

curing of 43

guidelines for use in acidic soils 43 44t.

standards 43

Type I (no limit on calcium hydroxide) 43

Type II (1% or less calcium hydroxide) 43

Atmospheric corrosion 22 24f. 67–68

and air pollutants 68–69

anode 67 68f.

avoiding crevices 74

avoiding debris entrapment 74

and barrier coatings 74 75f.

cathode 67 68f.

coating application 76

coating evaluation 77

coating inspection 76

coating selection factors 75

and coatings 74–76 75f.

control methods 72–76

corrosion fatigue 72

defined 67

Index Terms Links


Atmospheric corrosion (Cont.)

design in control of 73–74

and dew point 69

electrolyte 67 68f.

environmental cracking 72

environmental effects 68–70

examples of 67

four conditions for 67 68f.

galvanic 72 72f.

hydrogen embrittlement 72

independent corrosion control 74

and materials selection 73

metallic path 67 68f.

and moisture 68

and orientation of a surface 69 69f.

and poultices 69

providing drainage 74

and sacrificial coatings 74 75f.

selecting metals in galvanic series 73

and sheltering 69

and solar radiation 69

stainless steel in aboveground environments 77

stress corrosion cracking 72 72f.

and surface preparation for coating 76

and temperature 69

and time-of-wetness 68

and wind 69

B

Backflow, defined 87

Bacteria 5

acidic waste from 5

iron-reducing 5 33

and microbiologically influenced corrosion (MIC) 20–22 21f.

Bacteriological corrosion, defined 87

Barrier coatings 74 75f.

Bimetal couples 16

defined 87

Index Terms Links


Biodomes 5

Biofilm 5

C

Carbon steel 70t. 73

Cathodes

defined 87



Cathodic protection 9 52–53 52f.

defined 87

impressed-current systems 56–59 57f. 58f. 59f.

and interference from stray current 59–60

sacrificial anode systems 53–56 54f.

Cell potential 14

Cell voltage 14

measuring with voltmeter 14 15f.

Clostridium nigrificans 20

Coal-tar enamel and tape coatings 50 51 74

Coatings, linings, and encasements 50

barrier coatings 74 75f.

coal-tar enamel and tape 50 51 74

coating, defined 87

coating application (atmospheric corrosion) 76

coating inspection (atmospheric corrosion) 76

coating selection factors (atmospheric corrosion) 75

dielectric, bonded coatings and linings 50 51

extruded polyolefin 51

fused polyolefin 51

fusion-bonded polyethylene 50 51

handling 50

liquid and fusion-bonded epoxies 50 51

passivating Portland cement mortar and

concrete coatings 50 51

petrolatum and petroleum wax tape 51 74

polyamide 51

polyurethane 50 51 75

sacrificial coatings 74 75f.

Index Terms Links


Coatings, linings, and encasements (Cont.)

and surface preparation for coating

(atmospheric corrosion) 76

tape systems 50 51 75

testing coatings (atmospheric corrosion) 76–77

unbonded polyethylene encasement 50–51

Concentration cells 18–19 19f.

defined 87

Concrete cylinder pipe 38

and acidic soils 39 42

and aggressive carbon dioxide 39 42

and atmospheric exposure 39 42

bar-wrapped 38 41f.

corrosion control measures 62–63

and high-cloride environments 39 41

Portland cement and long life 39

prestressed 38 40f.

reinforced 38 39f.

and stray-direct-current interference 39 41

and sulfate soils 39 41–42

Concrete Pressure Pipe (M9) 39 62

Concrete pressure pipe, bar-wrapped, steel

cylinder type. See Concrete cylinder pipe

Conductivity, defined 87

Copper pipe 42

in contact with concrete 43

corrosion control measures 62

in parallel hot and cold water piping 43

and problematic soil chemistries 42

used as ground 43

Copper–copper sulfate electrode, defined 87

Copper–copper sulfate reference electrodes

(CSEs; half-cells) 28 30–31

and concrete cylinder pipe 39

Corrosion 1–2

chemistry of, in water systems 14–24

concentration cells 18–19 19f.

defined 1 2 87

Index Terms Links


Corrosion (Cont.)

and dissimilar metals 2–3 3f. 4f.

electrochemistry of 7–13

and environmental contamination 4–5

fatigue type 22 23f.

financial and health implications of 6

fretting type 22 23f.

impingement attack 19 20f.

industrial costs of (US) 2

microbiologically influenced (MIC) 5 20–22 21f. 33

as natural phenomenon 2

and naturally corrosive soils 3

and oxidized forms of metals 2

and oxygen concentration cells 21–22

piting 19 19f.

potentially corrosive conditions 2–5

and reduced water pressure 6

selective 22 23f. 24f.

soil type 22 22t.

and soil variances 3

stray current 5

stress type 22 22f.

types of, in water systems 18–24

See also Atmospheric corrosion; Crevice

corrosion; Electrolytic corrosion;

Galvanic corrosion; Microbiologically

influenced corrosion (MIC); Soil

corrosion; Storage tank corrosion;

Stress corrosion

Corrosion (metallic), defined 87

Corrosion cell, defined 87

Corrosion control 49

cathodic protection 52–60

coatings, linings, and encasements 50–51

for concrete cylinder pipe 62–63

for copper pipe 62

defined 87

for ductile-iron pipe 63–64 65f.

Index Terms Links


Corrosion control (Cont.)

materials selection 60

protective methods for specific pipe materials 60–65

as reduction, elimination, or reversal of

current flow 12

for steel pipe 61

and trench improvement 60

Corrosion fatigue 72

Corrosion rate 14

and cell voltage 14 15f.

and Faraday’s law 16

and Ohm’s law 14

Corrosive soils 3

Crenothrix 21

Crevice corrosion 70–71 71f.

of stainless-steel pipe 71

CSE. See Copper–copper sulfate reference electrode

Current flow formula 30

Current, defined 87

Current span

defined 88

testing 30

testing, and stray currents 32

D

Design Decision Model (DDM) 36 61

Desulfotomaculum 20

Desulfovibrio 20

Dezincification, defined 88

Dielectric coating, defined 88

Dielectric, bonded coatings and linings 50

and excessive current 50

DIPRA 63

See also Design Decision Model

Dissimilar metals 2–3 3f. 4f.

in typical water system equipment 3 3f. 4f.

Index Terms Links


Ductile-iron pipe

corrosion control measures 63–64 65f.

and Design Decision Model (DDM) 36

and moisture 34

and oxidation-reduction (redox) potential 34

and pH 34

and prior experience 36

and soil resistivity 34

and sulfides 34

10-point soil corrosion potential evaluation 34 35t.

Ductile-Iron Pipe and Fitings (M41) 34 63

E

Electrode, defined 88

Electrolytes

defined 88



nonuniform, in galvanic corrosion cell with

electrodes of single metal 10 10f.

nonuniform, in galvanic corrosion cell with

single piece of metal 10–11 11f.

Electrolytic corrosion 12–13

anode 13 13f.

cathode 13 13f.

direct-current source 13 13f. 15f.

electrolyte 13 13f.

electronic path 13 13f.

Electrolytic corrosion cell, defined 88

Electron, defined 88

Electronic path

defined 88

discussion of terminology 12 12n.



Environment, defined 88

Environmental alteration, defined 88

Index Terms Links


Environmental contamination 4

chemical waste 5

chloride salts 5

cinder paving 4

coal bottom ash 4-5

Environmental cracking 72

Epoxy coatings, liquid and fusion-bonded 50 51 75

F

Facultative microbes 33

Faraday’s law 16

defined 88

Fatigue, defined 88

Fatigue corrosion 22 23f.

Fretting corrosion 22 23f.

defined 88

G

Gallionella 21

Galvanic corrosion 8

anode 8 8f.

atmospheric 72 72f.

basic electrochemical reactions 9 9f.

cathode 8 8f.

cell with nonuniform electrolyte and

electrodes of single metal 10 10f.

cell with single piece of metal in

nonuniform electrolyte 10–11 11f.

and conventional (or positive) current flow 11–12 12f.

defined 88

different reactions with different materials 10

electrolyte 8 8f.

electronic path 8 8f.

Galvanic corrosion materials 16

and galvanic series 16 17t.

in water systems 16–18

Galvanic couples 16

Galvanic series, defined 88

Index Terms Links


Galvanized steel 70t. 73

Gray-iron pipe

and moisture 34

and oxidation-reduction (redox) potential 34

and pH 34

and prior experience 36

and soil resistivity 34

and sulfides 34

10-point soil corrosion potential evaluation 34 35t.

Ground-voltage gradient

defined 88

testing 30–31

testing, and stray currents 32

H

Half cell, defined 88

Handbook of Cathodic Corrosion Protection 36

Holiday, defined 88

Hot spot, defined 88

Hydrogarnets 43

Hydrogen embrittlement 72

I

Impingement corrosion

attack 19 20f.

defined 88

Impressed-current systems 56 57f.

advantages and disadvantages 56

common uses 56

current supply unit 57 57f.

deep well configuration 57

defined 88

design and construction 57–59 57f. 58f. 59f.

distributed configuration 57 57f.

electrical continuity and discontinuity 58 58f.

environmental variables 58–59

horizontally remote configuration 57

operating principles 56

Index Terms Links


Impressed-current systems (Cont.)

and stray-current corrosion 58 59f.

Ion, defined 89

Iron bacteria 21

L

Line current testing 30

M

Materials Performance (NACE) 61

Metal path 12n.

Metals

corrosion rates in atmosphere 70t. 73

See also Anodic metals; Dissimilar metals;

Nobility of metals

Microbiologically influenced corrosion (MIC) 5 20–22 21f.

biodomes and cathodic depolarization 33

biofilm and differential aeration cells 33

biological waste material and acid concentration 33

general facts about 33

and iron-reducing bacteria 33

and microbial receptors 33

and microbial reproduction 33

and microbial resistance to chemicals 33

and pH 33

primary forms of 33

and temperature 33

Millivolt meters 30

Mitigate, defined 89

N

Nobility of metals 16 17t.

and passivation 18

O

Ohm’s law 14 56

defined 89

Index Terms Links


Oxidation, defined 89

Oxidation-reduction potential (redox potential),

defined 89

P

Passivation 18

defined 89

of steel in concrete 18

PE Pipe—Design and Installation (M55) 45

Petrolatum and petroleum wax tape coatings 51 74

pH

and corrosivity potential 28 28f.

defined 89

pH meter, defined 89

Pipe-to-soil potential 28–30 29f.

defined 89

and stray currents 32

Piting corrosion 19 19f. 70

defined 89

Plastic pipe. See Thermoplastic pipe

Plastic Pipe Institute 45

Polar solvent, defined 89

Polarization 18

defined 89

Polyamide coatings 51

Polyethylene coatings

extruded 74

fusion-bonded 50 51

Polyethylene encasement

defined 89

unbonded 50–51

Polyolefin coatings

extruded 51

fused 51

Polyurethane coatings 50 51 75

Portland cement mortar and concrete coatings 50 51

and passivation 50

Index Terms Links


Potential 25–26

current-span measurements 30 32

defined 89

ground-voltage gradients 30–31 32

line current measurements 30

and pH 28 28f.

pipe-to-soil 28–30 29f. 32

and soil-resistivity testing 26 26f. 27f.

and stray currents 31–32 31f.

structure-to-electrolyte 28

PVC Pipe––Design and Installation (M23) 44–45

R

Reduction, defined 89

Resistance, defined 89

Resistivity, defined 89

Resistivity meter, defined 89

Return current path 12n.

defined 90

Return on investment, defined 90

S

Sacrificial anode, defined 90

Sacrificial anode systems 53 54f.

advantages and disadvantages 53

anode contact resistance (vertically

installed) formula 55

anode current output formula 54–55

common uses 53

current output relation resistance 56

defined 90

design and construction 54–56 55t.

operating principles 53

Sacrificial coatings 74 75f.

Selective corrosion 22 23f. 24f.

Single probe, defined 90

Soil box, defined 90

Index Terms Links


Soil corrosion 22 22t.

defined 90

relationship to soil resistivity 36 38t.

and steel pipe 36 38t.

Soil resistivity, defined 90

Soil-resistivity testing 26

four-pin system 26 26f.

quad-box 26 27f.

single-probe system 26 27f.

Soil variances 3

Sphaerotilus 21

Stainless steel 70t. 73

corrosion resistance in aboveground

environments 77

Stainless-steel pipe 45

in aboveground environments 47

alloys and their corrosion resistance 45 45t.

corrosion in soil 45

crevice corrosion 71

piting corrosion 46

piting resistance equivalent number (PREN) 46

in submerged and waterlogged conditions 46 46t.

suitable applications for 46 46t.

See also Steel pipe

Standards

ANSI/AWWA D104 79

ANSI/AWWA D106 79

coal-tar enamel and tape coatings (AWWA C203) 50 51

Drinking Water System Components—

Health Effects (ANSI/NSF 61) 79

epoxy coatings, liquid and fusion-bonded

(AWWA C210) 50 51

NACE International SP0196 79

NACE International SP0388 79

NACE SP0100, SP0169 53 54 61

petrolatum and petroleum wax tape

coatings (AWWA C217) 51

polyamide coatings (AWWA C224) 51

Index Terms Links


Standards (Cont.)

polyethylene coatings, fusion-bonded

(AWWA C229) 50 51

polyethylene encasement (AWWA C105) 50–51 61

polyolefin coatings, extruded (AWWA C215) 51

polyolefin coatings, fused (AWWA C225) 51

polyurethane coatings (AWWA C222) 50 51

Portland cement mortar and concrete

coatings (AWWA C205, 300, 301, 302, 303) 51

Practice for Conducting Atmospheric

Corrosion Tests (ASTM G50) 77

Preparing, Cleaning and Evaluating

Corrosion Test Specimens (ASTM G1) 77

Standard for Asbestos–Cement Pressure

Pipe, 4 In. Through 16 In. (100

mm Through 400 mm), for Water

Distribution Systems (AWWA C400) 43

Standard for Asbestos–Cement

Transmission Pipe, 18 In. Through 42

In. (450 mm Through 1,050 mm), for

Water Supply Service (AWWA C402) 43

Standard for Concrete Pressure Pipe,

Bar-Wrapped, Steel-Cylinder Type

(AWWA C303) 38 51

Standard for Polyethylene Encasement for

Ductile-Iron Pipe Systems (ANSI/

AWWA C105) 34 36 61 63

Standard for Prestressed Concrete Pressure

Pipe, Steel-Cylinder Type (AWWA C301) 38 51

Standard for Reinforced Concrete Pressure

Pipe, Steel-Cylinder Type (AWWA C300) 38 51

Standard for the Selection of Asbestos–

Cement Transmission Pipe 18 In.

Through 42 In. (450 mm Through

1,050 mm), for Water Supply Service

(AWWA C403) 43

Standard Methods for Testing Asbestos–

Cement Pipe (ASTM C500) 43

Index Terms Links


Standards (Cont.)

tape system coatings (AWWA C214) 50 51

Test Method for Field Measurement of Soil

Resistivity Using the Wenner Four-

Electrode Method (ASTM G57) 26

Test Method for Measurement of Soil

Resistivity Using the Two Electrode

Soil Box Method (ASTM G187) 26

Test Method for Measuring pH of Soil for

Use in Corrosion Testing (ASTM G51) 28

Steel, and corrosion rates in atmosphere 70t. 73

Steel pipe 36

coatings and linings 36

corrosion control measures 61

joints 36 37f.

relationship of soil corrosion to soil resistivity 36 38t.

soils corrosive to 36 38t.

See also Stainless-steel pipe

Steel Pipe–A Guide for Design and Installation (M11) 36 61

Storage tank corrosion

automatic transformer-rectifier in control of 83–85 86

cathodic protection, design of 81–82

cathodic protection anodes (hoop

suspension) 82 83f. 84f.

coating, and cathodic protection 81 82 86

coating, need for 80

and coating pinholes 80

control standards 79

corrosivity of potable waters to steel 80

in crevices 80

design life of protection systems 85

and differential in oxygen concentrations 80

and dissimilar metal 80

electrochemical activity as cause of 79

inspection and maintenance of protection systems 86

and stressed metal 80

uniform 80

Stray current corrosion 5 12n.

Index Terms Links


Stray currents 31

and arc-welding equipment 32

and bonded joints 31 32f.

and cathodic protection interference 59–60

defined 90

and direct-current transmission systems 32

and direct-current–powered transit systems 32

failure caused by 31 31f.

and impressed-current cathodic-protection

systems 32

and impressed-current systems 58 59f.

sources 32

Stress corrosion 22 22f.

cracking 72 72f.

defined 90

Sulfate-reducing bacteria 20

Sulfide-forming bacteria 20 21

Sulfides, defined 90

T

Tape system coatings 50 51 75

Thermoplastic pipe 44

effects of chemical attack, oxidation, or

environmental stress cracking 44

immunity to corrosion 44

standards 44–45

Thiobacillus thioxidans 21

U

Unbonded polyethylene encasement. See

Polyethylene encasement

Uniform corrosion 70 70t.

V

Voltage differential 14

Voltmeter 14 15f.

Index Terms Links


W

Weathering steel 70t. 73

external corrosion control for infrastructure sustainability...

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