201 a document

201A

Corrosion Basics

201A Ch1 Introduction to Corrosion

Section 1: Introduction

IInnttrroodduuccttiioonn

Welcome to the Corrosion 201-A course, which provides details on how to recognize

and treat twelve different forms of corrosion that adversely affects systems,

equipment, and infrastructure throughout industrial, and government communities.

Since the impact of corrosion is particularly devastating to the Department of

Defense, this course focuses on the prevention and treatment of corrosion of

weapon systems, support equipment, and the associated infrastructure. Through

this course, the learner will be exposed to engineering content and concepts.

TThhee CCoorrrroossiioonn PPoolliiccyy aanndd OOvveerrssiigghhtt OOffffiiccee

The DoD Corrosion Policy and Oversight office (CPO) is responsible for

implementing the congressionally-mandated Corrosion Prevention and Mitigation

Program; and has assembled this course to help the defense acquisition community

better understand and implement corrosion-related responsibilities. The CPO's

operation is directed and controlled by Title 10, Section 2228, Instruction

5000.67, and the DoD Corrosion Prevention and Mitigation Strategic Plan.

DDooDD SSttrraatteeggiicc PPllaann

The DoD strategic plan articulates DoD corrosion program policies, strategies and

objectives; prevent, detect and treat corrosion, transcend traditional corrosion

control methods by implementing modern corrosion control techniques throughout

design, fabrication, manufacturing, operation, maintenance and disposal

processes.


Section 2: Terms and Definitions

DDeeffiinniittiioonn ooff TTeerrmmss

Lets begin with some basic definitions.

Corrosion and oxidation are both terms that are used to describe the

deterioration of materials. However, corrosion is often associated with the

deterioration of metals and their alloys while oxidation is more commonly

used to describe the deterioration of non-metallic materials.

Deterioration is the loss or degradation of performance or properties such

as strength, durability, and appearance.

Material is the stuff that we make things out of, such as metals, polymers,

ceramics, composites, nano-technology items, etc.

Reaction is the response of material to the environment.

Environment is the surroundings to which a material is exposed.

Environmental Properties include chemical composition, form of moisture, and

operating temperature. Flow, wet, dry cycles, stress, and abrasion can also

be factors. The environment will affect the type and rate of corrosion as

well as the damage that results.

CCoonnddiittiioonnss oonn CCoorrrroossiioonn SSeevveerriittyy

Materials, particularly metals and metal alloys vary widely in their resistance

to the corrosive effects of the environment. Three conditions determine the

potential severity of corrosion damage.

1. the corrosion resistance of the metal or alloy or the oxidation resistance

of other materials.

2. the length of time the metal or alloy or non-metallic material is exposed to

the corrosive environment, and

3. the severity of the environment

These three conditions must be considered carefully when selecting materials and

designing systems and facilities that will be operated or used in specific

corrosive environments.


Section 2: Terms and Definitions

CCoosstt ooff CCoorrrroossiioonn

The choice and use of materials in corrosive environments is also an economic

decision. In a study funded by the Department of Transportation in the year 2000:

the annual cost of corrosion in the United States was estimated at $276 billion

and that figure continues to rise. More recent DoD studies estimated the annual

cost of corrosion within the military is over $20 billion.

The objective is to reduce this annual cost by at least one-third. Corrosion

reduces scarce resources, system and facility readiness, performance, and safety.

CCoorrrroossiioonn DDeecciissiioonn PPooiinnttss

Therefore, effective management decision-making is needed at all organizational

levels. Complex interactions between multiple disciplines are required for

effective corrosion prevention and mitigation. Decision-makers such as designers,

engineers, logisticians, manufacturers, operators, and maintenance personnel must

consider several items in material selection. They must:

choose alloy classes or other classes of materials that are less prone to

corrosion;

design to avoid high risks or plan mitigation;

specify production and manufacturing practices to avoid detrimental effects

or to mitigate effects;

recognize high risks and use effective tools, techniques, and practices to

mitigate these risks;

perform failure analysis by identifying the relevant forms of corrosion to guide

root cause analysis and corrective action.


Section 3: Additional Resources

CCoorrrroossiioonn IInnffoorrmmaattiioonn RReessoouurrcceess

The DoD has established corrosion technology information sources. At any time,

you may click on the RESOURCES button for additional information within this

course. Other corrosion technology information is available through NACE

International, ASM International, and SSPC (The Society for Protective Coatings)

and others.


Section 4: 12 Types of Corrosion

1122 TTyyppeess ooff CCoorrrroossiioonn

There are multiple forms (or types) of corrosion, depending on how they are

categorized.

For the purpose of this course, we will discuss 12 types. Each of which is

described in terms of its physical appearance, form, and structure of the damage

resulting from corrosion attack.


Section 5: Identifying the Types

GGeenneerraall CCoorrrroossiioonn OOvveerrvviieeww

General or uniform corrosion is the most readily observed and detected of the

forms of corrosion. It occurs over most of the entire surface of the metal or

alloy thats exposed to the environment as shown here.

It is the type of corrosion for which we have the most knowledge and means to

prevent or control. Because steel is prone to general corrosion, it accounts for

the greatest amount of alloy loss.

PPiittttiinngg CCoorrrroossiioonn OOvveerrvviieeww

Here is an example of severe pitting corrosion.

Pitting is found on metals in areas where the passive film has broken down. Where

the passive film remains stable, there is little or no pitting. Anodes form where

the local breakdown occurs. Significant, serious corrosion is evident at those

anodic sites. The surrounding area becomes a cathode. The corrosion growth

process is autocatalytic. That means that once a pit starts to grow, the solution

becomes more corrosive and the pit becomes self-propagating. The environment,

within the pit, becomes progressively more corrosive and corrosion cells self-

propagate more quickly.

It is more likely for corrosion to continue in the pit rather than for corrosion

to initiate in another location, even if the process is interrupted and

reactivated.



CCrreevviiccee CCoorrrroossiioonn OOvveerrvviieeww

Crevice corrosion can be the result of design or manufacturing flaws, and it is

often associated with the use of flanges and bolts, tight contact between two

metals, or tape wrapped around a material.

Crevice corrosion occurs on passive metals. Crevices form underneath deposits on

a material. In such restricted conditions, the environment becomes much more

corrosive. If the passive film breaks down, anodes form in the restricted area

beneath the crevice. The outer areas in the freely exposed solution become the

cathode. The result is a localized autocatalytic attack that can accelerate very

rapidly and cause severe damage.

Crevice corrosion also occurs under protective coatings applied to a metal

surface to isolate it from a corrosive environment. Corrosion is induced at flaws

in coatings bonded to the metal surface. The environment penetrates the flaw and

initiates accelerated corrosion in a large area beneath the coating.

CCoorrrroossiioonn FFaattiigguuee OOvveerrvviieeww

If a metal is subject to pitting corrosion, the pits can act as stress risers

that increase the effective level of stress at the bottom of the pit.

Intergranular corrosion can result in the formation of stress risers that have a

similar effect. The alternating, or cycling stresses, further weaken the corroded

metal and corrosion fatigue results.

DDeeaallllooyyiinngg OOvveerrvviieeww

Most metals used in engineering applications are not pure metals but are alloys,

consisting of two or more elements designed to achieve specific properties. For

example, brass is a mixture of copper and zinc.



Dealloying is the corrosion of one or more of the component metals of an alloy.

The remaining material may retain the original size and shape of the alloy, but

has greatly reduced strength and ductility.

EErroossiioonn CCoorrrroossiioonn OOvveerrvviieeww

Flow-assisted corrosion, or erosion corrosion, occurs or is accelerated when

there is a relative movement between a metal and its environment such as the flow

of fluids through pipes. Abrasive particles in the environment exacerbate erosion

corrosion.

FFrreettttiinngg CCoorrrroossiioonn OOvveerrvviieeww

Fretting is direct wear that is due to small relative motions between two

surfaces that are under load and move relative to each other. The relative

movement is usually cyclic and occurs at a fairly rapid rate. Fretting of either

surface accelerates and increases the effects of corrosion.



GGaallvvaanniicc CCoorrrroossiioonn OOvveerrvviieeww

The next type of corrosion we will look at is Galvanic corrosion. As its name

implies, it is corrosion due to the galvanic action between two or more

dissimilar metals, alloys, or electrically conductive non-metals.

This is one of the more common forms of corrosion in complex components because

of the wide use of dissimilar metals in the design and manufacture of equipment

and structures.

HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt OOvveerrvviieeww

Hydrogen embrittlement can result from absorption of hydrogen that promotes

brittle fracture of the metal. Hydrogen embrittlement is more prevalent in alloys

with a high yield strength.

EExxffoolliiaattiioonn aanndd IInntteerrggrraannuullaarr OOvveerrvviieeww

This is an example of exfoliation, which is intergranular corrosion occurring

along an elongated grain structure, resulting in a flaking off of the grain

layers.



The aluminum alloy shown is particularly susceptible to exfoliation when joined

using steel fasteners. If the fasteners were aluminum, intergranular corrosion

would have progressed more slowly. As time progresses, you can see the aluminum

around the periphery of the fastener begin to lift and eventually appears to come

apart.

Intergranular corrosion. Metals and alloys have crystalline structures. This

means that the atoms have periodic alignments or stacking of many atoms in atomic

planes to make a crystal. Most commonly used alloys are composed of aggregates of

small crystals called grains, where one periodic stack of atoms forms a crystal

interface with other stacks having slightly different orientations. The

resulting aggregate is called a polycrystalline metal. The regions where these

grains meet are called grain boundaries and can have a different composition and

structure than the individual grains themselves. When this difference in

chemistry and structure leads to a more corrosion-prone grain boundary, corrosion

occurs preferentially at the grain boundaries. This is called intergranular

corrosion.

SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg OOvveerrvviieeww

Stress corrosion cracking is the corrosion-induced propagation of cracks when the

material is under a sustained tensile stress while being exposed to a corrosive

environment.

The tensile stress may be due to an applied load, residual stress from forming

and fabrication, or a combination of the two. It is particularly insidious

because the combined stresses and corrosion can cause unexpected structural

failure.

SSttrraayy CCuurrrreenntt CCoorrrroossiioonn OOvveerrvviieeww

Stray current corrosion occurs when a metal structure inadvertently gets in the

path of current flowing in the environment between 2 other structures.



This shows an underground tank, such as might be used to store gasoline, with an

impressed current cathodic protection system to reduce the rate of corrosion.

Cathodic Protection will be described later, but it involves the passage of

current from an anode to the part being protected through the ground as in this

case or maybe through seawater.

If there is a nearby metallic structure, such as a buried pipe, the path of least

resistance for the current might be through the pipe. Where that current leaves

the pipe to go to the tank, the pipe is polarized to a higher potential and can

corrode at an accelerated rate. Stray Current Corrosion can be mitigated by

connecting the pipe to the tank electrically, and thus protecting both the pipe

and the tank. This type of corrosion can affect most metals buried in soil or

immersed in water. Sources of electricity throughout equipment and facilities can

provide stray alternating or direct current which may cause corrosion of metal

materials in the path of the stray current.


Section 6: Understanding the Corrosion Cell

TThhee CCoorrrroossiioonn CCeellll

All electrochemical cells consist of four components: First, there must be an

anode where electrons are generated through an oxidation reaction such as metal

corrosion. Second, there must be a cathode where the electrons liberated from the

anode are consumed by a cathodic reaction. Third, there must be a metallic

conductive path that electrically connects the anode and cathode. The anode and

cathode might be two different metals that are electrically connected or they

might be different sites on the same piece of metal. Finally, the metal must be

exposed to an electrolyte consisting of water or a solution that contains and

transports ions and contains a cathodic reactant.

These four components must be present for corrosion to occur or for the operation

of any electrochemical cell, like a battery.

Electrons, liberated during oxidation, move freely along the metallic path but do

not move freely in the electrolyte. Conversely, ions move freely in the

electrolyte but not along the metallic path. It should be noted that an ion is an

atom or molecule that has lost or gained one or more electrons, making it

positively or negatively charged. A negatively-charged ion, which has more

electrons than protons, is known as an anion. A positively-charged ion, which

has fewer electrons than protons, is known as a cation.

In corrosion, the corrosive environment is the electrolyte. A typical corrosive

environment is seawater, which contains chloride and many other types of ions.

Other corrosive environments might be a liquid held in place by a solid substance

such as soil or concrete.

Metal corrosion is the destructive dissolution of metal through an oxidation

reaction at the anode of an electrochemical cell. Metal atoms oxidize to produce

positively charged metal cations such as Zn+2 or Fe+2 that are released into the

environment. The electrons, released by the oxidation reaction, travel through

the metal to the cathode. As with all electrochemical cells, corrosion requires a

cathode where the electrons liberated by the corrosion reaction at the anode are

consumed by some reduction or cathodic reaction. The rate of metal corrosion or

oxidation at the anode is always equal to the rate of reduction at the cathode.

This reduction reaction consumes cathodic reactants usually supplied from the

electrolyte.

Typical cathodic reactants in corrosion are chloride ions, dissolved oxygen but

can involve other species. In acids, the primary cathodic reaction is the

reduction of H+ ions to form hydrogen gas. In neutral environments, such as

seawater, the concentration of H+ ions is low and the cathodic reaction is often

the reduction of dissolved oxygen gas molecules to form hydroxyl anions. Cations


Section 6: Understanding the Corrosion Cell

migrate and diffuse through the electrolyte to the cathode while anions migrate

and diffuse toward the anode.

Electrochemical cells that form on corroding metal surfaces or in metal cracks or

crevices require these four components and behave in the same manner as

batteries. The difference is that the electrical current produced by a battery is

captured to light a bulb, start an engine in a car, or power a device such as a

laptop computer.

Corrosion cells behave like a shorted battery, where the anodes and cathodes are

directly in contact with each other. The local anodes and cathodes might move

across the surface of the piece of metal resulting in uniform attack. In this

arrangement, the electrons cannot be captured as the short circuit prevents this.

In a battery the separation of anode and cathode and the travel of electrons from

the anode to cathode through this circuit, enable the electrons and electrical

energy to be used such as in the example of the light bulb. So, in our attempts

to thwart corrosion, we try to design structures and components to eliminate at

least one of the four electrochemical cell components, and thus prevent the

oxidation and reduction reactions characteristic of electrochemical cells.

201A Ch2 External Factors

Section 1: Metallurgy and Material Microstructure

PPaassssiivvee MMeettaall CCoonnssiiddeerraattiioonnss

There are two important considerations when using passive metals to prevent

corrosion: (a) the corrosion resistance of the alloy, and (b) the corrosivity of

the environment. For success, (a) must be great enough to withstand (b).

CCoorrrroossiioonn TTyyppeess:: MMeettaalllluurrggyy

Metallurgy can affect forms of corrosion. Three forms of corrosion damage are

related to metallurgy and metallurgical behavior.

Pitting corrosion. In this sample, manganese sulfide inclusions from the steel

production process corrode and create pits on the metal surface.

Intergranular corrosion, in this sample of a stainless steel alloy, chromium was

depleted at the grain boundaries resulting in preferential corrosion.

Dealloying. Grey cast iron is composed of ferrite and graphite platelets forming

a lamellar structure. Since ferrite is prone to corrosion and graphite is

corrosion resistant, the ferrite phase corrodes away, leaving a weak structure of

graphite flakes.

IInntteerrggrraannuullaarr CCoorrrroossiioonn:: CChhrroommiiuumm CCaarrbbiiddee PPrreecciippiittaattiioonn

Intergranular corrosion is directly related to the metallurgical behavior of

grain boundaries. Precipitation of chromium carbides (adjacent grain boundaries)

and subsequent migration of these chromium carbides into the grain boundary

regions, may lead to intergranular corrosion when exposed to a corrosive

environment.


Section 1: Metallurgy and Material Microstructure

Heat treatment forms precipitates that significantly strengthen aluminum. These

precipitates decrease the corrosion resistance of the alloys. As corrosion

progresses, exfoliation begins, and thin sheets of alloy peel away from the

material structures.

FFaabbrriiccaattiioonn PPrroocceessss:: WWeellddiinngg

The fabrication process and resulting structure can also affect an alloys

vulnerability to corrosion and physical characteristics. Welds can be highly

prone to corrosion because of differences in structural composition and

morphology. Welding can also result in high residual stresses in the weld and the

adjacent heat affected zone of the metal.


Section 2: Electrical Potential

EElleeccttrriicc PPootteennttiiaall DDiiffffeerreennccee

Previously, we discussed the electrochemical corrosion cell and the fact that the

difference in electrical potential between two areas of the metal surface is the

driving force behind the corrosion cell performance. This was evident in crevice

corrosion where loss of a passive film forms anodic and cathodic areas. Now we

address the corrosion cell where potential difference is due to dissimilar metals

in contact with each other, different environments, and different locations.

GGaallvvaanniicc SSeerriieess:: SSeeaawwaatteerr

Galvanic corrosion results from the potential difference created by dissimilar

metals in electrical contact.

This graph shows the potential difference between metals in seawater. The

horizontal scale shows electrical potential compared to a reference electrode in

seawater from most negative at the left to more positive at the right.

When two metals in the series come in contact, they form a galvanic couple. The

more positive member of the couple will be the cathode. The more negative will be

the anode. The graph also shows the position of active and passive alloys in the

solid boxes.


Section 2: Electrical Potential

GGaallvvaanniicc CCoorrrroossiioonn:: CCaatthhooddee--AAnnooddee RRaattiioo

The relative areas of the electrodes in a galvanic corrosion cell have a

significant impact on the rate of corrosion. The design of structures and systems

with dissimilar metals should consider the galvanic couple characteristics and

avoid high ratios of cathode to anode areas.

GGaallvvaanniicc CCoorrrroossiioonn:: SShhiippss

The classic case of galvanic corrosion is potential difference due to dissimilar

materials. A common example is associated with ship hulls.

Ship hulls are fabricated from steel. Hulls can be in electrical contact with

propellers, which are fabricated from bronze, which is an alloy of copper. Copper

alloys are more noble than steel, which promotes steel corrosion. Ship hull

corrosion can be off set by using sacrificial anodes. Sacrificial anodes are made

of zinc, a more active metal than copper. The zinc anodes corrode instead of the

steel. An example would be to use enough zinc beneficial galvanic action to

offset bronze detrimental galvanic action.

GGaallvvaanniicc CCoorrrroossiioonn:: AAiirrccrraafftt

Another example of dissimilar metal galvanic corrosion is corrosion of aluminum

surfaces and structures on aircraft where aluminum and steel used in aircraft

fabrication become galvanically coupled. Aluminum corrodes because the steel is

more noble. Electromagnetic interference (EMI) materials can contain silver or

other conductive particles. These conductive particles may be more noble and

therefore trigger galvanic action with the aluminum.


Section 3: Environmental and Solution Chemistry

CCrreevviiccee AAeerraattiioonn CCeellllss

Aeration cells, commonly formed during crevice corrosion, can create a potential

difference caused by dissimilar oxygen concentration environments.

Low oxygen concentration produces a negative potential.

High oxygen concentration produces a higher potential.

Since the potential of the anodic area in the crevice corrosion concentration

cell is much more negative than the potential in the cathodic area, the potential

difference accelerates corrosion cell activity.

CCoorrrroossiioonn IInniittiiaattiioonn aanndd GGrroowwtthh FFaaccttoorrss

Environmental differences and the associated chemistry affect the initiation and

growth of corrosion.

DDiiffffeerreennttiiaall AAeerraattiioonn CCeellll

Aeration cells are oxygen concentration cells where the amount of oxygen varies

from one location to another.

The area of higher oxygen concentration has a higher electrical potential.

The area with low oxygen concentration has a lower electrical potential.

Current flows from the area of lower potential the anode through the

solution to the area of higher potential the cathode, forming an

electrochemical corrosion cell.

Micro scale corrosion cells can form in naturally formed crevices and under

deposits of precipitates created during fabrication. Oxygen is consumed beneath

those crevices and deposits, and cannot be replenished because of the

microstructure but there is a high amount of oxygen remaining in the bulk

material.



CCrreevviiccee CCoorrrroossiioonn:: CCrriittiiccaall CCrreevviiccee CChheemmiissttrryy

Critical crevice chemistry is the primary

mechanism that causes crevice corrosion. This

classic experiment demonstrates the results of

critical crevice chemistry. The solution

chemistry is shown at the bottom.

The solution in the crevice becomes more corrosive.

Critical chemistry develops where the passive film

breaks down. The crevice becomes more corrosive because

of oxygen depletion. Chloride ions build up and the

solution becomes more acidic.

As the process continues, the anodic area becomes

increasingly more negative than the freely exposed

metal, and this further accelerates the critical

chemistry activity.

AAggggrreessiivvee CChheemmiiccaall SSppeecciieess

Aggressive chemical species, such as chlorides, are often present naturally in

the bulk environments or are added during production processes. Chlorides are

particularly detrimental because they can break down passive films and increase

solution conductivity so current can flow more rapidly. Oxygen can also be

detrimental because of the aeration effects. Inhibitors are important to thwart

the effects of such aggressive chemicals.

MMiiccrroobbiiaall--IInnfflluueenncceedd CCoorrrroossiioonn

Microbial action in biofilms, or within deposits on metal surfaces, can change

the chemical environment and significantly increase corrosivity. For example,

sulfate ions are moderately non-corrosive chemical species. Sulfate reducing

microbes will convert sulfate ions to sulfide ions, which are very aggressive.



Pipelines, storage tanks and other locations that have sulfate in the water, and

where conditions are conducive to microbial activity, can suffer from severe

Microbial Influenced Corrosion (MIC).

HHeeaatt EEffffeeccttss

Hot metal surfaces and surfaces with high heat transfer rates require special

consideration.

Water dripping onto a hot metal surface can cause very corrosive conditions.

A moist surface or a moist material, such as insulation in contact with a hot

metal surface, can cause increased corrosivity.

Corrosion under insulation is a classic example, where moisture wetting the

insulation comes in contact with the hot metal surface and the resulting

concentrated solution triggers corrosion. When high heat levels are transferred

across a metal surface, corrosion rates are significantly higher than if the

metal is immersed in the solution that is at the same temperature as the metal.

201A Ch3 Stress and Hydrogen Damage

Section 1: Mechanical Stress and Wear

CCoorrrroossiioonn TTyyppeess:: MMeecchhaanniiccaall SSttrreessss aanndd WWeeaarr

Lets consider mechanical stress and wear related problems where a combination of

the mechanical stress and corrosive environments result in corrosion damage.

Additionally, we will address hydrogen related problems where hydrogen absorbed

into the metal causes damage.

SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg

In those forms of corrosion caused by mechanical stress and wear, mechanical

stresses work in conjunction with the environment and can cause cracking of

otherwise durable materials. Mechanical stress and wear can cause Stress

Corrosion cracking and corrosion fatigue. Abrasive wear from impact type forces

can cause erosion corrosion and fretting.

Stress corrosion cracking is also called environmental cracking, anodic stress

corrosion cracking, chloride stress cracking, sulfide corrosion cracking,

hydrogen stress cracking, and so on. Regardless of the terminology, three

conditions must prevail for stress corrosion cracking to occur.

1. Tensile stresses are always present on the outer fibers of the materials.

These stresses act to pull apart the outer surfaces.

2. A metal susceptible to stress corrosion cracking in the given environment

like sensitized stainless steel.

3. A corrosive environment containing a specific chemical that can trigger

stress corrosion cracking a chloride ion in solution for example.

These three critical conditions must be present simultaneously and eliminating

any one of these conditions will prevent stress corrosion cracking.

SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: EExxaammpplleess

These pictures are examples of stress corrosion cracking failures on different

alloys. They show that stress corrosion cracking is highly localized and the

cracks at the metal surface deeply penetrate the metal. Adjacent metal shows no

damage.



SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: MMeettaall aanndd CCoorrrroossiivvee EEnnvviirroonnmmeenntt

This table shows some of the combinations of metal alloy systems and environments

that cause stress corrosion cracking.

Carbon steels are susceptible to stress corrosion cracking in concentrated

caustic alkaline environments, concentrated nitrate solutions, and in carbonate,

bicarbonate type solutions.

Austenitic stainless steels suffer stress corrosion cracking in hot concentrated

chloride solutions.

High strength aluminum alloys are vulnerable to stress corrosion cracking in

marine environments and/or corrosive environments of chloride, bromide, or iodide

solutions.



SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg:: DDeeggrreeee ooff DDaammaaggee

The degree of stress corrosion cracking varies with several parameters as shown

here.

As the magnitude of tensile stress on the outer surface of the metal increases,

the more rapidly cracking occurs. If threshold stress is not exceeded, no stress

corrosion will occur. As the concentration of critical chemicals in solution

increases, cracking occurs more readily. Geometries such as a notch, a crack, or

a sharp corner on the material, can concentrate stress, and accelerate crack

initiation and propagation.

SSttrreessss CCoorrrroossiioonn CCrraacckkiinngg MMiittiiggaattiioonn

There are several ways in which to prevent or control stress corrosion cracking.

The best approach is to choose the right materials for a given environment. If

susceptible materials are used, design and production process specifications may

be created that avoid stresses. Austentic stainless steels with precipitated

chromium carbides and the resultant chromium depletion is just such a susceptible

material, so additional mitigation needs to be considered.

Likewise, high-strength aluminum alloys can also be susceptible depending on

their composition and heat treatment. Shot peening is a process to generate

compressive stresses at the surface and can reduce susceptibility to stress

corrosion cracking.



CCoorrrroossiioonn FFaattiigguuee

Corrosion fatigue is the combined action of corrosion and cyclic stresses that

initiates cracks through metal that would otherwise be ductile and durable.

Frequent landings and take-offs contributed to corrosion fatigue and failure of

the Aloha Airlines aircrafts aluminum alloy fuselage. Crevice corrosion

contributed to the initiation of the cracking. During flight, an entire section

of the fuselage broke away from the aircraft.

CCoorrrroossiioonn FFaattiigguuee:: SSttrreessss CCoonncceennttrraattoorrss

Stress concentrators can promote corrosion fatigue as shown here.

Stress concentration occurs at the root of scratches and at sharp corners. Cracks

initiate more readily at these sites resulting in increased fracture rate,

shortened corrosion fatigue life, and reduced strength.

CCoorrrroossiioonn FFaattiigguuee MMiittiiggaattiioonn

In order to control and mitigate corrosion fatigue, select materials with

enhanced corrosion fatigue strength or higher endurance limits; reduce residual

tensile stresses in manufactured parts by selecting effective design and

fabrication processes.


Section 2: Abrasive Wear from Impact Type Forces

EErroossiioonn CCoorrrroossiioonn

Erosion corrosion is material deterioration due to the combination of chemical

action and mechanical abrasion or wear. The severity of attack is more than that

of either chemical corrosion or abrasion alone. The degree of erosion can vary

from slight, to moderate, to heavy.

Erosion corrosion is prevalent where high velocity or turbulent fluids of other

materials flow over a metal surface.

EErroossiioonn CCoorrrroossiioonn VVuullnneerraabbiilliittyy

The degree to which a metal surface is vulnerable to erosion corrosion depends on

the following factors:

surface film content and characteristics

passive film durability

corrosion product layers, and

the velocity of liquids or solids over the materials surface. The higher

the velocity; the more aggressive the erosion corrosion.

There is a critical velocity below which there is little or no corrosion.

Similarly, turbulence, which tends to scrape or scour the surface; or impingement

of fluids, gases and particulate matter will increase the erosion. In order to

mitigate erosion corrosion

1. design and build systems to avoid turbulent flow.

2. Place deflector plates in high velocity areas

3. protect welded and other susceptible areas from fluid flow.

Material selection and changing the environment can also mitigate erosion

corrosion.

FFrreettttiinngg CCoorrrroossiioonn

Fretting corrosion is corrosion where two moving

metal surfaces in simple small relative motions make

rubbing contact when the interface is subjected to

vibrations and compressive loads. In this highly

magnified view of metal surfaces in contact, the

surfaces appear smooth but there are mountain


Section 2: Abrasive Wear from Impact Type Forces

tops that contact each other when the surfaces rub together.

When those points of contact fuse, small amounts of metal become dislodged,

causing metal particle buildup. These small metal particles oxidize and

essentially become efficient grinding compounds thus, fretting is self-

propagating since these corrosion products cause more fretting. Disassembly of

the interfacing surfaces reveals the loss and a surface covered with corrosion

products.

Parameters that affect fretting are:

1. the amount of stress on contacting surfaces, which makes movement difficult

2. the amount of oxide debris on the surfaces

3. presence of vibration or surface rubbing during transportation of products

that are in contact with each other

4. degree of relative motion between surfaces, and

5. presence of oxygen and moisture between interfacing surfaces.

FFrreettttiinngg CCoorrrroossiioonn MMiittiiggaattiioonn

In order to mitigate and control fretting:

1. Implement effective design and material selection, fabrication, and

operating procedures.

2. Maintain good alignment of rotating parts.

3. Make surfaces rougher in order to reduce slippage.

4. Apply loads that will lock interfacing parts together and reduce relative

motion.

5. Use low viscosity fluids with corrosion inhibitor and;

6. Apply corrosion preventative compounds.


Section 3: Hydrogen-related Damage

HHyyddrrooggeenn AAbbssoorrppttiioonn

Now, let's consider hydrogen-related damage to metals caused by the metals

absorption of hydrogen.

There are several sources of hydrogen on metal surfaces. Hydrogen, the smallest

atom, can move from the surface into the metal, for example, be absorbed.

Hydrogen atoms can move in the metal along the interstitial sites in the metals

crystalline lattice. The hydrogen atom can collect at points of tensile stress,

causing metal embrittlement and hydrogen stress cracking. Blistering and, in some

alloys, hydride formation are additional forms of hydrogen damage.

HHyyddrrooggeenn AAttoomm

Hydrogen is a by-product of the corrosion reaction. Remember that corrosion

occurs at the anode where metal goes into solution as metal ions or at the

cathode, the reduction of a hydrogen ion [H+] (as in an acid) comes to the metal

surface accepts an electron and becomes a hydrogen atom. The hydrogen atom can

recombine with another hydrogen atom and form a hydrogen gas bubble or can

accumulate on the surface and be absorbed by the metal.



An electroplating process can generate hydrogen, and large amounts of hydrogen

atoms can be absorbed by the metal. If very high pressure gaseous hydrogen is

present, hydrogen molecules can disassociate and hydrogen atoms can be absorbed

by the metal. If hydrogen sulfide is present in the solution when corrosion is

occurring, absorption of hydrogen by the metal occurs more readily and greater

amounts are absorbed.

HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt

Let's distinguish the three forms of hydrogen damage.

Hydrogen embrittlement is a loss of ductility from the absorption of hydrogen.

Reactions that take place during the corrosion process or during electroplating

cause absorption of large quantities of hydrogen, which are absorbed and retained

in the metals crystalline structure. If a load is applied to the structure, or

there are residual tensile stresses, the metal can crack and break without

warning. The higher the materials strength, the more susceptible it is to

hydrogen embrittlement.

HHyyddrrooggeenn EEmmbbrriittttlleemmeenntt MMiittiiggaattiioonn

Hydrogen embrittlement mitigation methods include:

1. Baking: Heat the parts in an oven at moderate temperature for a length of

time that allows the hydrogen to leave the metal before it cracks.

2. Material selection: Select lower strength metals for the particular

application, if possible. And,

3. Processing: change the processing or exposure conditions to avoid hydrogen

absorption.

HHyyddrrooggeenn BBlliisstteerriinngg

Hydrogen blistering occurs as a result of internal delaminations, or blisters,

forming from the recombination of hydrogen within the metal. As hydrogen is

absorbed by the metal, it forms a gas in a weak area within the metal.

Recombination of the hydrogen atoms into gas causes the buildup of very high

pressures, which can deform the metal.

Corrosion is the primary source of hydrogen in many cases and hydrogen sulfide

can exacerbate the problem.



HHyyddrriiddee FFoorrmmaattiioonn

Hydride formation is another form of hydrogen damage that occurs in materials

such as titanium and zirconium. High levels of hydrogen absorption in these

alloys can cause the formation of metal hydrides. Metal hydrides form along

crystallographic planes. When the hydride forms, its volume is greater than the

metal, which creates high stresses in the material. Hydrides tend to be brittle

and crack readily under stress.

As shown in the micrographs, hydrides formed in a very orderly pattern along

crystallographic planes and the resulting crack or fracture reduces material

ductility.

201A Ch4 Stray Current and Miscellaneous

Section 1: Stray Current Corrosion

SSttrraayy CCuurrrreenntt CCoorrrroossiioonn OOvveerrvviieeww

Now, we will discuss a few special cases of corrosion.

The first of which is Stray Current Corrosion caused by currents that can flow

from Direct Current (DC) sources such as electric railroads or between pipelines

through the soil, seawater or other conductive environments.

Cathodic protection currents on pipelines in close proximity can result in stray

currents.

There may also be induced AC currents, resulting from the co-location of buried

metal structures and high power transmission lines.

Current can enter and flow through the buried or immersed metal structures

because they have lower electrical resistance than the environment. Severe

corrosion damage occurs when the current flows back into the environment. The

damage tends to be highly localized and penetration rates are rapid.

SSttrraayy CCuurrrreenntt CCoorrrroossiioonn MMiittiiggaattiioonn

In order to mitigate or control stray current corrosion:

1. Ground the stray current so it doesnt flow from the metal surface directly

into the electrolyte but flows through the grounding device.

2. Electrically bond together co-located systems to avoid areas where current

can flow to and from electrolytes.

3. Use sacrificial anodes along with impressed current in cathodic protection

systems. The sacrificial anodes provide a path to ground.

AACC IInndduucceedd CCuurrrreennttss

AC induced currents are caused by high voltage AC power lines that induce current

flow in the buried support structures and co-located buried steel structures and

pipelines that can propagate induced current flow.

The corrosion damage occurs where there are induced currents flowing from the

metal and to the soil or other electrolyte. Corrosion damage by induced currents

is exacerbated by High AC voltage levels.


Section 1: Stray Current Corrosion

AACC IInndduucceedd CCuurrrreenntt CCoorrrroossiioonn:: CCaauussee aanndd EEffffeecctt

These photos and illustrations show some causes and effects of AC induced current

corrosion. Sharing the right-ofway between power lines and pipelines - note

trench operations for construction of a buried pipeline between power lines.

Induced current corrosion a defect in the coating along with induced AC

currents caused severe corrosion, which penetrated the pipeline and caused a leak

18 months after installation. Disbonded coating was also observed. If an induced

current flows in a structure with a high quality coating one with very few

defects, the current density at the isolated defect will be much higher than in a

structure with many defects.

Induced AC current superimposed on the DC current in a typical cathodic

protection system can exacerbate corrosion initiation and damage.


Section 2: Miscellaneous Forms

MMiisscceellllaanneeoouuss TTyyppeess ooff CCoorrrroossiioonn

There are a number of instances of corrosion that cannot be conveniently

classified as one of the forms of corrosion previously identified.

Sequential corrosion processes. Corrosion of a multi-component, multi-layer

coating systems. Special Case: filiform corrosion, which is a specific type of

corrosion degradation of coated metal products.

Coupled corrosion processes, where one form triggers another specific form.

SSeeqquueennttiiaall CCoorrrroossiioonn PPrroocceessss

These photos depict an example of the sequential corrosion process. A manganese

sulfide inclusion in the steel was at the outer surface of a metal sample. When

the metal sample was placed in a corrosive brine solution, the sulfide corroded

and a pit was formed. A pit in the metal surface is shown at high magnification

in the upper figure. This and other pits became initiation sites for stress

corrosion cracks growing into the metal.

The lower figure shows multiple corrosion sites (black oxide-covered surface on

the fractured face). Rapid fracture occurred (light area) by mechanical overload

of the uncracked area.



MMuullttiillaayyeerr CCooaattiinngg SSyysstteemm

There are multiple coating layers, each to protect each other. For example, in

an automobile theres cleaning, a pretreatment which is mostly in organic and the

pretreatment is meant to protect the metals substrate against corrosion. A primer

goes on top of that after the pretreatment is dried and cured and the primer is

meant to protect the pretreatment. A top coat goes on top of that and the top

coat is meant to protect the primer. So in other words, its a three separate

coating steps all working as a system.

MMuullttiillaayyeerr CCooaatteedd MMeettaall CCoorrrroossiioonn

If some external damage occurs which cuts through those coating layers, corrosion

of the steel will be determined by the depth and width of the penetration, the

life of the organic coating and primer, and the zinc layer cathodic protection.



The actual corrosion of the steel itself follows after the galvanic corrosion of

zinc stops thus multi-layer coated metal corrosion is also a sequential

corrosion process.

FFiilliiffoorrmm CCoorrrroossiioonn

Filiform corrosion of both coated steel and coated aluminum is shown in the

schematic. Defects in coatings allow the electrolyte to contact the metal

surface.

Anodes and cathodes on the metal surface under the coating complete the formation

of corrosion cells. These filiform corrosion cells migrate along the surface of

the metal in a pattern similar to mole tunnels in a lawn. The corrosion cells

leave corrosion products that discolor the metal surface beneath the coating. The

result does not cause significant structural damage but causes user concern due

to the unpleasant appearance.

CCaavviittaattiioonn CCoorrrroossiioonn

Cavitation corrosion is a somewhat unique form of corrosion that results from the

formation and collapse of bubbles on a metal surface. These bubbles form and

collapse in areas of rapid pressure drops such as on the blades of ship

propellers. Low pressure generates the bubbles and a following wave of high

pressure collapses them. Under magnification, the mechanical damage to the

surface appears like metal peening or grit blasting processes.

201 a document

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