jean emmanuel 2011

7/24/2019 Jean Emmanuel 2011

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UNIVERSITY OF MAURITIUS

A Study on Degradation

of Carbon Steel inAtmospheric ConditionBENG (HONS) MECHANICAL

ENGINEERING

JEAN EMMANUEL CHAN YOUN SEN

3/30/2011

FACULTY OF ENGINEERING


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TABLES OF CONTENTS

LIST OF TABLES vi

LIST OF FIGURES viii

ACKNOWLEDGEMENT x

DECLARATION FORM 17

ABSTRACT xi

LIST OF ABBREVIATION xii

CHAPTER 1 1

INTRODUCTION 1

1.0 INTRODUCTION 2

1.1 DEFINITION OF CORROSION 3

1.2 CONSEQUENCES OF CORROSION 3

1.2.1 Economic Dimension 4

1.2.2 Health Dimension 5

1.2.3 Safety Dimension 5

1.3 ATMOSPHERIC CORROSION IN MAURITIUS 6

1.4 CLIMATE IN MAURITIUS 7

1.5 AIMS AND OBJECTIVES 8

CHAPTER 2 9

LITERATURE REVIEW 9


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2.0 INTRODUCTION 10

2.1 MECHANISM OF CORROSION REACTION 10

2.2 ATMOSPHERIC CORROSION STAGES 12

2.2.1 Initial Stages of Atmospheric Corrosion 12

2.2.2 Intermediate Stages of Atmospheric 12

Corrosion

2.2.3 Final Stages of Atmospheric Corrosion 13

2.3 PARAMETERS AFFECTING ATMOSPHERIC 14

CORROSION

2.3.1 Time of Wetness 14

2.3.1.1 Rain 14

2.3.1.2 Temperature 15

2.3.2 Sulphur Dioxide 15

2.3.3 Airborne Chlorides 16

2.3.4 Other Corrosive Agents 16

2.4 SURFACE ROUGHNESS 16

2.4.1 Surface Roughness Parameters 17

2.4.1.1 Ra 18

2.4.1.2 Rq (rms) 19

2.4.1.3 Rz 19

2.4.1.4 Other Parameters 20

CHAPTER 3 21

METHODOLOGY 21

3.0 INTRODUCTION 22


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3.1 ATMOSPHERIC EXPOSURE 23

3.1.1 Material 23

3.1.2 Exposure Test Sites 24

3.2 MASS LOSS 26

3.3 SURFACE ROUGHNESS MEASUREMENTS 26

3.3.1 Sampling Length 28

3.4 MEASUREMENT OF ACTUAL 29

CORROSION PENETRATION

3.4.1 Specimen Preparation 30

3.4.2 Depth Measurement 34

3.4 CONCLUSION 36

CHAPTER 4

37

RESULTS, ANALYSIS & DISCUSSION

37

4.0 INTRODUCTION 38

4.1 VISUAL INSPECTION OF THE SURFACE 39

4.2 CORROSION LOSS ANALYSIS 40

4.3 SURFACE ROUGHNESS MEASUREMENT ANALYSIS 42

4.3.1 Cut off Selection Methodology 42

4.3.2 Cut off Selection Methodology Results 44

4.3.3 Roughness Values Analysis 45

4.4 CHANGES IN DEPTH OF CARBON STEEL 49

POST EXPOSURE

4.4 CONCLUSION 51


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CHAPTER 5 52

GENERAL DISCUSSION, CONCLUSION & 52

RECOMMENDATION

5.0 GENERAL DISCUSSION AND CONCLUSION 53

5.1 RECOMMENDATION 54

APPENDIX A 56

A.1 UNIFORM CORROSION 56

A.2 GALVANIC CORROSION 56

A.3 PITTING CORROSION 57

A.4 CREVICE CORROSION 57

APPENDIX B

B.1 ROUGHNESS 58

B.2 WAVINESS 58

B.3 SPACING 59

B.4 HYBRID PARAMETERS 60

APPENDIX C 62

C.1 CORROSION LOSS RESULTS 62

C.2 CUT OFF SELECTION 62

C.3 MEASUREMENT OF DEPTH WITH MICROSCOPE 65


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LIST OF TABLES

CHAPTER 3

Table 3.1 Composition of carbon steel exposed 23

Table 3.2 Recommended cut -offs for different surface finishes 29

CHAPTER 4

Table 4.1 Identification codes of specimens 40

Table 4.2 Corrosion loss of the specimens 41

Table 3.3 Roughness values at 0.25mm cut off 42



Table 3.6 Required cut off according to ISO 4288 43

Table 4.3 Required cut off values 44

Table 4.4 Set of measurements of specimen thickness 49

APPENDIX C

Table C.1: Corrosion Loss 62

Table C.2: Cut off test at 0.25mm 63



Table C.5: Thickness of reference specimens 65

Table C.6: Thickness of specimens of study A 65

Table C.7: Thickness of specimens of study B 67


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LIST OF FIGURES

CHAPTER 2

Fig. 2.1: Atmospheric corrosion of iron 11

Fig. 2.2: Cross section through a rough surface showing roughness heights 17

and surface spatial wavelengths

Fig. 2.3: Graphical representation of Ra 18

Fig. 2.4: Graphical representation of Rqand Ra 19

Fig. 2.5: Graphical representation of Rz 20

CHAPTER 3

Fig. 3.1: Specimen test coupon 27

Fig. 3.2: Profile showing a roughness wavelength of 0.25mm with an Ra 28

of about 20m

Fig. 3.3: Nikon Microscope ME600 equipped with stage micrometer 30

Fig. 3.4: Specimen Transformation 31

Fig. 3.5: Hand Bench Shear 32

Fig. 3.6: Vertical milling machine 33

Fig. 3.7: Specimen being milled along the longitudinal edge of cut 33

Fig. 3.19: Nikon Microscope ME600 equipped with a stage micrometer 34

Fig. 3.20: Specimen fixed firmly at right angle to the slide using plasticine 35

Fig. 3.21: Specimen on slide and positioned on stage 35


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CHAPTER 4

Fig. 4.1: Coupon freshly removed from rack (left) and cleaned (right) 39

Fig. 4.2: Formation of rust layer and cracks 45

Fig. 4.3: Graph of Ra(m)against Corrosion loss (m) 46

Fig. 4.4: Graph of Rq (m)against Corrosion loss (m) 47

Fig. 4.5: Comparison between Raand Rq 47

Fig. 4.6: Graph of RzDIN (m)against Corrosion loss (m) 48

Fig. 4.6: Graph of Change in Thickness (m)against Corrosion Loss (m) 50


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ACKNOWLEDGEMENT

First of all, I would like to thank Mr. Surnam for proposing this project to me. He actedas my supervisor and provided me with assistance throughout the course of the project.

I am also very grateful for his guidance and sound advices which helped me doing this

project at my maximum potential.

The technicians in the Mechanical Workshop also played an important role in the

smooth running of my project. They were of great help in providing whatever piece of

equipment I needed or how to handle them. Mr Bhanjee taught me how to manipulate

the optical microscope; Mr. Dunesh explained to me how to operate the millingmachine, Mr. Seetohul gave me some tricks in using the Surtronic 3+ and Mr. Chu was

there to help me whenever I needed it.

My friends were also there to support me in dire times, boosting my morale whenever I

felt giving up and always cheering me up. I dedicate a big thank you to them, especially

Davina, who was there to accompany me during most of my all-nighters.

And finally, I would like to thank my parent, without whom, I would not have the

opportunity to complete this project as they provided me with all my financial support.


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ABSTRACT

Atmospheric corrosion is a problem of growing concern in Mauritius, especially sincethere has been a growing use of carbon steels over the past years according to the CSO.

However atmospheric corrosion effects in Mauritius are relatively unknown as very few

studies have been carried out in that respect. The aim of the project was to analyze the

behavior of carbon steel in atmospheric Mauritian environment.

Carbon steel test specimens were exposed across different sites of the island for some

times. Afterwards they were removed and the mass loss was measured which enable

the calculation of their respective corrosion loss. This was then used as a reference for

further tests.

Surface roughness measurements were also done in order to analyze the effects of

atmospheric corrosion on the surface. The correlation with corrosion loss was also

found as obeying a logarithmic regression method.

Thickness measurement using a microscope was also performed and the change in

thickness is evaluated. This is also correlated with corrosion loss and was found to have

a logarithmic relationship.


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LIST OF ABBREVIATION

EDM Electro Discharge Machining

RH Relative Humidity

TOW Time of Wetness


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CHAPTER 1

INTRODUCTION


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1.0 INTRODUCTION

Atmospheric corrosion is a major problem in most countries around the world. It has

been a recurring problem for Man since iron was first extracted from it ore about

4000 BC (Leygraf & Graedel, 2000). It is the cause of premature failure of structural

parts and outdoor equipments such as bridges, steel building roofs or even airplanes

fuselage which results in human injury or even loss of life.

Atmospheric corrosion is often seen as an inevitable stage of any metallic product

since the day it is manufactured and put in service. Certainly corrosion cannot be

avoided but it can be, at least, be reduced or prevented at most. Scientists and

researchers have carried out a lot of studies on the matter as the problem became

more and more serious over time.

It is only recently that engineers could use their arsenal of tools at their disposal to

study atmospheric corrosion as it is a very complex and hard-to-predict phenomena.

Even in Mauritius, very few studies have been undertaken on the matter which

remains largely empirical but is not appropriate for us. As atmospheric corrosion is

heavily dependent on the environment and the use of steel is growing in the island

which has known unprecedented development, it makes sense that a study on the

surface degradation of carbon steel in Mauritian atmospheric condition is carried

out.


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1.1 DEFINITION OF CORROSION

Corrosion is a naturally occurring process. It is the tendency of any metal or alloy to

return to their naturally occurring compound called ores commonly found in nature

which are more thermodynamically stable. It can be defined as the destructive and

unintentional attack of a metal. However, it is electrochemical in nature and is a

surface phenomenon. The corrosion reaction mechanism involves transfer of

electron from the metal which undergoes an oxidation reaction to another chemical

species which, then, undergoes a reduction reaction. These are usually water,

atmospheric oxygen and so on (Callister, 2007). Considering the metal M, this can

be expressed by the following reaction

MMz+

+ ze-

The metal M loses z electrons and becomes a positively charged ion, Mz+

. This

explains the material loss as metal oxides are usually formed on the surface. The

metal which has been corroded is called the anode and the electron acceptor is called

the cathode.

1.2 CONSEQUENCES OF CORROSION

Corrosion affects the daily life of everyone and their surroundings in many aspects. I

has both direct consequences, in the sense that it affects the use of useful service

components, and indirect consequences as well by the fact that any cost incurred due

to corrosion by manufacturers or higher bodies is passed on to the consumer.

Corrosion affects in a lot of different ways, namely:

Economic

Health

Safety


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1.2.1 Economic Dimension

Billions of dollars are lost each year because of it. In 1975, the National Institute of

Standards and Technology has estimated that the annual cost due to corrosion in the

United States is between $9 billion to $90 billion. Various technical bodies such as

the National Association of Corrosion Engineers corroborated the validity of these

figures at that time (Schweitzer, 2007).

The cost in the previously mentioned study was evaluated more precisely at $82

billion, or 4.9% of its gross national product (GNP). 60% of the cost was estimated

to be unavoidable. The rest could have been prevented by the use of appropriate

corrosion inhibition methods available at that time.

If nothing else had changed, the costs of metallic corrosion would have risen to

almost $350 billion annually by 1995, $139 billion of which would have been

avoidable (ASM International, 1999).

The costs of corrosion can be can categorized in two sections, namely:

Design, manufacture and construction Materials selection, such as replacement of carbon steel with stainless

steel

Additional material, such as larger corrosion allowances

Mitigation and prevention of corrosion by the use of coatings,

sealants, inhibitors, and cathodic protection, including the cost of

labor and equipment

Management

Corrosion-related inspections

Corrosion-related maintenance, repairs, replacement of parts,

inventory of backup components, rehabilitation, and loss of

productive time (Cost of Corrosion, 2010).


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In Mauritius, no such statistics are available and there is no reliable estimate of the

cost of corrosion for the island. This would require a collaboration of all acting

members of each economic sector to obtain valid results.

1.2.2 Health Dimension

There has been a notable increase in the use of metallic prosthetic devices in the

body recently. Examples are pins, hip joints, plates and pace makers. Although new

alloys and implementation techniques have been developed, corrosion is still a

problem. Examples include failures through broken connections in pacemakers,

inflammation caused by corrosion products in the tissue around implants, and

fracture of weight-bearing prosthetic devices (Kruger, 2001)

The health aspect is also very important in plants which produce a lot of waste water

or have large chimneys releasing tones of smoke in the atmosphere. Internal

corrosion of distribution pipes can cause the contamination of water or smoke with

heavy metal or toxic particles. This is even more acute for water distribution plants.

1.2.3 Safety Dimension

There have been many accidents in the past owing to or partly due to a failed

metallic component put in service. A failed component may seem inconsequential at

first but can be greatly aggravated by the circumstances leading to severe human

injury or loss of lives. Two examples of such accidents are listed below.

The structural failure on April 28, 1988 of a 19 year old Boeing 737,

operated by Aloha airlines, was a defining event in creating awareness of

aging aircraft in both the public domain and in the aviation community. This


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aircraft lost a major portion of the upper fuselage in full flight at 24,000 feet,

near the front of the plane (Miller, 1990). Multiple fatigue cracks were

detected in the remaining aircraft structure, in the holes of the upper row of

rivets in several fuselage skin lap joints (Wildey, 1990).

On December 15, 1967 at approximately 5 p.m., the U.S. Highway 35 bridge

connecting Point Pleasant, West Virginia and Kanauga, Ohio suddenly

collapsed into the Ohio River. At the time of failure, thirty- seven vehicles

were crossing the bridge span, and thirty-one of those automobiles fell with

the bridge. Forty- six individuals perished with the buckling of the bridge

and nine were seriously injured. Along with the numerous fatalities and

injuries, a major transportation route connecting West Virginia and Ohio was

destroyed, disrupting the lives of many and striking fear across the nation

(LeRose, 2001).

1.3 ATMOSPHERIC CORROSION IN MAURITIUS

Mauritius is developing very fast at an unprecedented rate during the last decade.

And, indeed steel is part of it and is used increasingly each year for the construction

of big structures, roof shelters, guarding rails and so on. For the year 2008 alone, the

total import of iron and steel in Mauritius totaled Rs 3675 millions for a total

quantity of 114,000 tonnes (Central Statistics Office, 2009).

Low carbon steel and medium carbon steel are the types of steel used most often.

This is because they are cheap, relatively strong, and malleable and offers a good

balance of properties versus cost. Whenever large quantities of steel are needed such

as for the construction of steel buildings or steel bridges, carbon steel is used. It is

also used for the manufacture of smaller structures such as solar water heaters,

doors, fences, gates or road signs.


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Unfortunately carbon steel is not corrosion resistant. As soon as the manufactured

product is put in service, it will start to corrode being in contact with the atmosphere.

Furthermore there have been no serious studies concerning corrosion of carbon steel

in atmospheric condition in Mauritius and as such information on this phenomenon

is scarce. Corrosion is not taken seriously by Mauritians as their effects in the short

term are not visible. It is seen as an inevitable process that every carbon steel

product is subject to. Signs of corrosion can be seen all over Mauritius. Rusted steel

roof, corroded bodies of old cars, rusted road sign displays or rusted outdoor gates

are some examples. It is taken as granted and is thought to be inherent to the use of

the product. Increased awareness of corrosion control methods can bring Mauritians

up to date on the matter.

1.4 CLIMATE IN MAURITIUS

Mauritius has two kinds of climate. Below the 400-meter level on most of the

windward (southeastern) side of the island and below 450 meters on the leeward

side, a humid, subtropical climate prevails. Above these altitudes, the climate is

more temperate, but there is no sharp break, and variations in exposure, altitude, and

distance from the sea produce a wide range of patterns. The island has two seasons.

The hot and wet summer lasts from November through April. February is the

warmest month with temperatures averaging 27 C in the lowlands and 22 C on the

plateau. Cyclone season runs from December through March, and the storms, which

come from the northeast, have caused much destruction on the island over the years.

Winter, lasting from May through October, is cool and dry, influenced by the steady

southeasterly trade winds. July is the coolest month and has average temperatures of

22 C in the lowlands and 16 C in the plateau. Rainfall is abundant, ranging from

90 centimeters per year in the western lowlands to 500 centimeters in the tableland--

an average of 200 centimeters per year overall. Nonetheless, the high rate of

evaporation and uneven distribution necessitate irrigation. Humidity is frequently

above 80 percent (Mauritius-Climate , 1994).


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Factors affecting atmospheric corrosion are temperature, time of wetness, wind

velocity, humidity, presence of atmospheric pollutants, airborne salinity and others.

Given the climate prevalent in Mauritius, the atmosphere in the island greatly favors

atmospheric corrosion of carbon steel. There is abundant amount of sunshine time

and rainfall time at all times during the year. The humidity level is often above the

critical value of 80% and, being a small island, Mauritius is surrounded by sea,

which places it in a category of serious airborne salinity level.

1.5 AIMS AND OBJECTIVES

The project aims at studying the effects of atmospheric corrosion on carbon steel

locally in Mauritius. Carbon steel specimens of standard shape and size have been

exposed to the open atmosphere at specific different sites around the island for a set

amount of time exposure.

The objectives of the project would then be to:

Analyze the reduction in thickness of the specimens after being subjected to

atmospheric corrosion.

Analyze the surface roughness characteristics of the base metal of the

corroded samples. This would lead to the development of models to predict

corrosion loss based on surface roughness.


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CHAPTER 2

LITERATURE REVIEW


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2.0 INTRODUCTIONThe objective of the project is to assess the effects of atmospheric corrosion on carbon

steel in the local Mauritian environment. However there have not been many serious

studies in the past covering this matter. This literature review will gather the research

and findings of other published papers and documenting them.

The following topics have been compiled and documented:

Mechanism of corrosion reaction

Parameters affecting atmospheric corrosion

Surface roughness

2.1 MECHANISM OF CORROSION REACTION (Roberge, 2000)

Being electrochemical in nature, atmospheric corrosion requires the presence of an

electrolyte. Above a certain critical humidity level, thin-film invisible electrolytes

tend to form on the surface of carbon steel under atmospheric exposure conditions. The

critical humidity level is not constant and depends on the corroding material, the

tendency of corrosion products and surface deposits to absorb moisture, and the

presence of atmospheric pollutants.

Atmospheric corrosion proceeds by cathodic and anodic reactions. The latter reaction

involves the dissolution of the metal into the electrolyte while the former is considered

as an oxygen reduction reaction which is plentiful in the atmosphere. For iron,

Anode Reaction: 2Fe2Fe2+

+ 4e

equation (2.1)

Cathode Reaction: O2+ 2H2O + 4e-4OH

-equation (2.2)


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Fig. 2.1: Atmospheric corrosion of iron

The anodic and cathodic reactions for the atmospheric corrosion of iron (main

constituent of carbon steel) are illustrated schematically in Figure 2.1. Besides an anode

and a cathode, a metallic path (iron) connecting both electrodes and the presence of an

electrolyte (e.g. water containing salt) are required. As soon as there is a difference in

potential between the anode and the cathode, a complete circuit is set up and corrosion

takes place.

Adding equation (2.1) and equation (2.2) leads to the following stoichiometric equation:

2Fe + O2+2H2O2Fe(OH)2 equation (2.3)

The compound with the chemical formula 2Fe(OH)2is known as ferrous hydroxide. Its

color is greenish black and is quite unstable in the presence of dissolved oxygen. It

quickly turns into ferric hydroxide, Fe(OH)3, which is reddish-brown in color and is

known as rust.

4Fe(OH)2+ 2H2O + O24Fe(OH)3 equation (2.4)


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2.2 ATMOSPHERIC CORROSION STAGES (Leygraf & Graedel, 2000)

Atmospheric corrosion occurs through multiple stages over time. From the initial stages

of corrosion which occurs in mere seconds when exposed to atmosphere; to theintermediate stages; to the final stages, which occur after years or even decades of

exposure.

2.2.1 Initial Stages of Atmospheric Corrosion

Surface Hydroxylation

This is the instant reaction of water vapor with the metal surface which occurs within a

fraction of a second. The bond formed maybe either in molecular or dissociated form.This results in the formation of metal-hydroxyl bonds for the former and metal-oxygen

bonds for the former. As surface hydroxyl groups (-OH) are generated, these act as sites

for further water adsorption on metal surfaces. This is the precursor to the formation of

thin layers of electrolytes on the metal surface.

Adsorption and Absorption of Water

Upon further exposure to the atmosphere, subsequent atmospheric water is adsorbed in

molecular form on the surface, leading to a surface less conducive to rapid combination

with water. The first layer of water has a high degree of ordering relative to the

substrate because of its proximity to the solid surface. As amount of layers increase,

they become more mobile with a higher degree of random orientation. From three

monolayers or thicker, aqueous films possess properties close to bulk water.

2.2.2 Intermediate Stages of Atmospheric Corrosion

Gas Deposition

The aqueous phase acts as a solvent for atmospheric constituents in either gaseous or

particulate form that deposit into the liquid layer and which are known to affect the

atmospheric corrosion process. Gaseous atmospheric constituents include nitrogen

dioxide (NO2), sulphur dioxide (SO2), and ammonia (NH3). One particulate form of

atmospheric constituent is sodium chloride (NaCl), which is readily present in marine

atmospheres.


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Change in Layer Liquid Chemistry

The deposition of constituents into the aqueous phase results in a number of chemical

and electrochemical reactions. Important chemical reactions which can occur are the

transformation of SO2 into sulphurous acid (HsSO4) or the transformation of NO2 into

nitric acid (HNO3). As the thickness of the liquid layer varies continuously, its chemicalcomposition changes continuously as well. Hence the pH of the aqueous layer may

change from neutral at high liquid layer thickness to highly acidic at low liquid layer

thickness.

Nucleation of Corrosion products

When the concentration of ion pairs in the liquid layer eventually reaches

supersaturation, the ion pairs will precipitate into a solid phase. The nucleation of

precipitated species is facilitated by the heterogeneous nature of the substrate surface,

particularly by solid-state defects, which can act as nucleation sites.

2.2.3 Final Stages of Atmospheric Corrosion

Coalescence of Corrosion Products

With prolonged exposure, the number of precipitated nuclei and their size increase until

eventually they completely cover the metal surface. These precipitates at this stage are

normally referred to as corrosion products. The brown rust layer often seen on

carbon steel is a common designation for clearly visible and often observed corrosion

products. There are other products which are invisible to the naked eye such as passive

films.

Aging and Thickening of Corrosion

While being affected by a number of continuously varying parameters such as relative

humidity or liquid layer thickness, the corrosion product layer undergoes a 3-stepsequential growth. The first step occurs during the increase of the liquid layer thickness

when part of the corrosion product may dissolve; the second involves the coordination

in the liquid layer of dissolved metal anions and counterions; and the third step occurs

during the decrease of the liquid layer thickness when the newly coordinated ion pairs

reprecipitate. The repeated cycles continuously cause the layer of corrosion products to

change its properties.


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When the corrosion products finally become stable and acquire properties which no

longer change with time, then it can be said to have a constant corrosion rate. The time

to reach stable conditions can vary from years to decades, or even centuries.

2.3 PARAMETERS AFFECTING ATMOSPHERIC CORROSION

The rate of atmospheric corrosion is influenced by a number of parameters which are

atmospheric gases, atmospheric particles and weathering parameters. Being

electrochemical in nature, the presence of an electrolyte is mandatory and, thus, the time

of wetness greatly influences atmospheric corrosion. Atmospheric corrosive gases such

as carbon dioxide (CO2) or hydrogen sulfide (H2S) also degrade carbon steel. Sea salt,

mainly sodium chloride (NaCl), behaves as atmospheric corrosive particles and

catalyzes the formation of acid electrolytes.

2.3.1 Time of Wetness (Schweitzer, 2007)

Time of wetness (TOW) is defined as the length of time during which the metal surface

is covered by a film of water, i.e. an electrolyte film, therefore allowing atmospheric

corrosion to take place. This is a very complex variable which depends on factors such

as relative humidity (RH) whereby the film of water is greatest when RH80%, the

temperature of the air and the metal surface above 0 C, the duration and frequency of

rain, fog, dew, and melting snow, as well as the amount of sunshine time and rainfall

time.

2.3.1.1 Rain (Schweitzer, 2007)

Rain precipitation has three effects on carbon steel from the point of view of

atmospheric corrosion. They are:

1) It creates thicker layers of electrolytes on the metal surface as compared to fog

or dew.

2)

A phase layer of moisture containing ions such as H

+

and SO42-

which stimulatecorrosion is formed.

3) It washes away any atmospheric corrodants which may have deposited on the

surface during the preceding dry period.


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The first two effects promote atmospheric corrosion whereas the third one decreases

corrosion rate for carbon steel. Which effect predominates depends on certain

conditions such as weather, type of corrosion atmosphere and so on.

In a strongly polluted atmosphere, corrosion on the skyward side of steel plates is

substantially lower than on the downward side. In a strongly polluted atmosphere where

dry deposition is considerably greater than deposition of sulphur pollutants, the washing

effect of rain takes over. In a less-polluted area, the situation is reversed, which

indicates that the corrosive action of rain, in this case, is more important.

2.3.1.2 Temperature (Schweitzer, 2007)

The effects of temperature affecting corrosion are also very complex as it has dualeffects depending on conditions similar to rain effects. An increase in temperature

increases the rate of chemical and electrochemical reactions as well as diffusion rates.

Consequently an increase in temperature promotes corrosion rate. On the other hand, an

increase in temperature also decreases the relative humidity and causes the more rapid

evaporation of the surface electrolyte, causing a decrease in corrosion rate as time of

wetness is reduced.

2.3.2 Sulphur Dioxide (Roberge, 2000)

Sulphur dioxide is mainly produced as the product of the burning of sulphur-containing

fossil fuels, especially in urban and industrial atmospheres. It plays a vital role in

atmospheric corrosion as it is readily adsorbed onto the metal surface, has a high

solubility in water and has a tendency to form sulphuric acid in the presence of an

aqueous layer of electrolyte. Sulphate (SO42-

) ions are formed by the oxidation of

sulphur dioxide according to the following equation:

SO2+ O2+ 2e-SO4

2- equation (2.5)

This reaction, i.e. the formation of sulphate ions, is considered as the principal factor

which accelerates atmospheric corrosion of iron and steel. This ultimately leads to the

formation of iron sulphate (FeSO4) which is a corrosion product found in industrial

atmospheres and in the layers of the metal surface.


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2.3.3 Airborne Chlorides (Roberge, 2000)

Chlorides are considered as one of the most corrosive components responsible for the

degradation of steel. It is commonly found in relatively large amounts in marine

atmospheres and coastal areas because of the close proximity of the sea. The

concentration of chlorides varies according to the direction and strength of the wind. Italso decreases when moving further inland.

Airborne salinity increases with the presence of chlorides and chloride salts are

deposited on the surface of the metal, which greatly affects corrosion. They are

deposited as droplets or as crystals. This increases markedly corrosion rates. Apart from

the enhanced surface electrolyte formation by hygroscopic salts such as NaCl and

MgCl2, direct participation of chloride ions in the electrochemical corrosion reactions

also happen.

In ferrous metals, chloride ions have a tendency to compete with hydroxyl groups for

combining with ferrous cations produced in the anodic reaction. While ferrous iron salts

are fairly stable, iron chloride salts are relatively more soluble, therefore stimulating

further the corrosion effect.

2.3.4 Other Corrosive Agents

The parameters discussed in section 2.3 are part of the main atmospheric sources of

atmospheric corrosion. There are many more not discussed in detail but needs to be

mentioned at least. These are:

Dust, dew and fog

Carbon dioxide, nitrogen compounds (NOx) and ozone

Sheltering from particles, rainfall and sunshine

Wind velocity

2.4 SURFACE ROUGHNESS

Surface topography characterizations have many important applications such as friction,

lubrication and wear (Thomas, 1999). Therefore surface parameters are vital in required

performance such as efficiency of heat exchanger tubes which is better when their

surfaces are slightly rough rather than highly finished. Other applications which work


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best with some degree of roughness are brakes and clutch plates. On the other hand, if a

thin film of lubrication is to be maintained between two sliding parts, the surface

roughness should be small enough so that the peaks do not protrude out of the oil film,

even under severe conditions. Otherwise there will be wear. Hence controlling or

predicting surface parameters is very important as this affects to a large extent the

properties of a component (Sharma, 2008).

2.4.1 Surface Roughness Parameters

Roughness is defined as the deviation from a true flat surface, resulting in finer

irregularities in the surface texture which are inherent to the production process.

(Bennett, 2003)

Fig. 2.2: Cross section through a rough surface showing roughness heights and

surface spatial wavelengths. Note that the height scale is in angstroms and thelength scale is in micrometers (1 m = 10,000 )

It can be quantified by certain specific amplitude parameters which are known as Rx

values where x is a subscript depending on the definition of the parameter. This type of

parameter characterizes the vertical deviation of the surface from the mean line and is


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defined by mathematical formulas which represent statistically the surface by a single

number.

2.4.1.1 Ra

Fig. 2.3: Graphical representation of Ra

This is defined as the average roughness of all points from the mean center line across

the sampling length and is defined by the following formula,

It is the most commonly used parameter in industry. This is because it is readily

available in even the less sophisticated instruments such as the Surtronic 3+ as

calculations required are relatively not complex as compared to other parameters. Hence

it is a very stable and repeatable parameter which is useful in industry where time and

repeatability are essential. Furthermore it is a good parameter for distinguishing random

type surfaces and, by nature, corrosion produces random surfaces,


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2.4.1.2 Rq (rms)

Fig. 2.4: Graphical representation of Rq and Ra

It is the root mean square average roughness of the surface taken along the assessment

length and is the rms parameter corresponding to Ra. It is defined by the formula,

Rqis more sensitive to peaks and valleys than Raas the amplitudes are squared in the

former.

2.4.1.3 Rz

It is also called the ten-point height as it is the average peak-to-valley profile roughness

of 5 successive sampling lengths, i.e. the evaluation length and is defined by the

following formula,


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Fig. 2.5: Graphical representation of Rz

Rzis more sensitive to peaks and valleys as opposed to Raand Rqas the parameter

works with maximum heights and deepest valleys instead of averages.

2.4.1.4 Other Parameters

The surface parameters covered previously in section 2.4.1 are the most commonly used

ones. However there are many other families of surface parameters which define surface

topography and are available in the ISO standards. Some examples are given below.

Roughness Parameters Waviness Parameters

Spacing Parameters

Hybrid Parameters

Further notes on the above family of surface parameters are included in the appendix.


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CHAPTER 3

METHODOLOGY


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3.0 INTRODUCTION

This chapter contains details about the exposure of the carbon steel specimens in

different sites across Mauritius. Post-exposure, all the steps and procedures involved in

experimentation would be discussed.

Details pertaining to how the study would be carried out, under what conditions

and on which sites will be described first.

After having obtained the mass loss, further operations taken to study the

specimen will be discussed.

Measurement of surface roughness using the Talysurf will be explained.

The steps taken in order to prepare the samples for being used with the optical

microscope will be discussed as well.

Details and procedures for using the Nikon optical microscope will be analyzed.


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3.1 ATMOSPHERIC EXPOSURE (B.Y.R. & Oleti, 2008)

The samples used in the project were obtained from previous atmospheric exposure of

carbon steel samples because of time constraints. Corrosion monitoring techniques

require exposure time to be around 1000 hours if corrosion rate is estimated to be

around 0.05 mm year-1

, i.e. corrosion loss around (Roberge, Corrosion Inspection andMonitoring, 2007)

The exposure of the carbon steel specimens was carried out in two distinct studies; one

was called study A and another one subsequently called study B. The type of carbon

steel used and sites of exposure was specific to each study. Only the time of exposure

was the common to both studies, i.e. 1 year. Each test coupon was clearly marked

using alphabetical stamps.

3.1.1 Material

Carbon steel plates were used in both study A and study B. Their size was 150 mm x

100 mm x 3 mm and was cut from the same sheet for each study. The carbon steel used

in each study was of different percentage composition of alloying elements which are

listed in the table below.

Table 3.1 Composition of carbon steel exposed

The main characteristic used in the classification of steel is the percentage composition

of carbon as alloying element. Low carbon steel (mild steel) is classified as steel having

approximately 0.05% to 0.26% carbon content with up to 0.4% manganese content and


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medium carbon steel is classified as steel having approximately 0.29% to 0.54% carbon

content with 0.60 to 1.65% manganese content (Key to Metals AG, 1999).

Hence, based on this, the carbon steel plates used for study A would be classified asmedium carbon steel and the carbon steel plates used for study B would be classified as

low carbon steel (mild steel).

3.1.2 Exposure Test Sites

The sites chosen for the exposures were chosen such that the test specimens were

exposed to the full effects of the weather and were exposed to different types of

environment, which are listed below (Roberge, 2000):

Rural

This is the least corrosive type of environment and normally does not contain pollutants

such as sulphur dioxide or chlorides but it does contain organic and inorganic

particulates. The principal corrosive agents are moisture, oxygen and carbon dioxide.

Urban

This is similar to the rural type given that there very little or no industrial activity in this

type of environment. However there are additional contaminants of SOx and NOx it

because of the more densely populated area and increased traffic activity.

Industrial

The atmosphere is heavily polluted with sulphur dioxides, phosphates, nitrates and

chlorides because of intense industrial activity.

Marine

This type of environment is characterized mostly by its location nearby coastal areas.

Fine chloride particles blown by the wind are deposited on metal surfaces and this tends


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to be highly corrosive. It is usually dependent in a large extent on wind speed, wind

direction and distance from the sea.

Three sites were chosen for study A as follows,

The University of Mauritius at Reduit, which is considered a rural region.

The Mauritius Meteorological Services at Vacoaswhich is an urban region.

The Ministry of Agriculture at Palmarwhich is found in a rural region.

As for study B, four sites of exposure were selected as follows,

Reduit (same as for study A)

Belle Mare which purely a marine zone.

Port Louis which is an industrial environment and, given its close proximity to

the shore line, a marine type as well.

St Julien dHotman which is a rural site.

Prior to exposure the specimens were cleaned using acetone. Stains of rust found on

some of the mild steel samples were dealt with a rust remover beforehand. According to

ISO 8565 for atmospheric corrosion testing for metals and alloys, the samples were then

exposed to the atmosphere for a period about 1 year.

For study A, samples were removed at 1 , 3, 6, 9, 12, 15 and 18 months from start of

exposure for mass loss analysis. There were 7 successive removals

For study B, samples were removed at 2 , 7, 12 and 19 months from start of exposure

for mass loss analysis. There were 4 successive removals.


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3.2 MASS LOSS

Before the test coupons were exposed, each one was weighed to the nearest 0.001g.

This is the sample mass before exposure. After specific time periods, the carbon steel

test coupons are removed and cleaned from their corrosion products according to BS

7545 (1991), which are then reweighed. The difference between the sample mass beforeand after exposure is the mass of metal corroded and, as a result, can be used for the

determination of corrosion loss.

The mass loss values were also already determined prior to the start of this project as

exposure time was very long and time was limited as one year for completion of this

project.

The values of the corrosion loss were then calculated and some other parameters such as

time of exposure or density of carbon steel can be used for the calculation of corrosion

penetration, also commonly as thickness loss of material, as shown below.

CP =

equation (3.1)

Where,

CP = corrosion penetration, mm

W = mass loss, g

= density of specimen, g/mm3

A = area of specimen, mm2

The value obtained for CP will only be an average value for the time it has been

exposed.

3.3 SURFACE ROUGHNESS MEASUREMENTS

After removal of the corrosion products using ISO 8565 (1995), the surface roughness

of the base metal skyward surfaces were analyzed using a surface profilometer to assess

of any trend or changes in surface topography.


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Fig. 3.1: Specimen test coupon

The instrument used was the Taylor Hobson Surtronic 3+ with a stylus diamond tip

radius of 2 m. The stylus diamond tip radius of 2 m was preferred to the 5 m one

because the smaller tip radius measures the finer and deeper valleys that bigger ones

cannot.

The coupon was of dimension 150mm x 100mm and its thickness was of 3mm. The

surface roughness measurements were taken on face which was skyward during

exposure. Besides, it was taken along a small longitudinal area in the middle of the

skyward face as shown in figure 3.1.


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3.3.1 Sampling Length

The sampling length in surface roughness measurement is the length of assessment

acting as a filter to remove unwanted wavelengths from a surface. For example a

sampling length, which is often written as cut off length, of 0.8mm will allow

wavelengths below 0.8mm to be analyzed whereas wavelengths above 0.8mm will beconsidered as waviness and their amplitudes will be severely reduced.

However the selection of cut off value is vital for the accuracy of the measurements

since roughness values fluctuate when the cut off is altered.

Fig. 3.2: Profile showing a roughness wavelength of 0.25mm with an Raof about

20m

The hypothetical profile in figure 3.9 shows that roughness amplitude of Ra of about

20m is obtained for a roughness wavelength of 0.25mm. It the surface was analyzed

with a 0.08mm cut off value, the value for Rawould be virtually zero and with a cut of

0.25mm, the Ra value would be about 10m. Only at larger cut offs, say 0.08mm or

2.5mm, that the correct value of 20m would be obtained. At cut offs of 8mm and

higher, the profile would include large waviness errors and this would increase thevalue of Ra.

Furthermore manufacturing process and material also influence the selection of the

sampling length. According to ISO 4288 pertaining to rules and procedures for the


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assessment of surface texture, the choice of cut off depend on the different surface

finish processes and can be chosen based on the following table.

PeriodicProfiles Non-Periodic Profiles Cut-off SamplingLength/

Evaluation

Length

Spacing

Distance

Sm(mm)

Rz(m) Ra(m) c(mm) c(mm)/L

> 0.013 - 0.04 To 0.1 To 0.02 0.08 0.08/0.4

> 0.04 - 0.13 > 0.1 - 0.5 > 0.02 - 0.1 0.25 0.25/1.25

> 0.13 - 0.4 > 0.5 - 10 > 0.1 - 2 0.8 0.8/4

> 0.4 - 1.3 >10 - 50 > 2 - 10 2.5 2.5/12.5

> 1.3 - 4.0 > 50 > 10 8 8.0/40

Table 3.2 Recommended cut -offs for different surface finishes (ISO 4288 1996)

Atmospheric corrosion implies removal of material from the surface and it can be

treated within reasonable limits to a manufacturing process with no control over the

process parameters unlike EDM where the discharge current can be adjusted or milling

where the feed rate can be increased or lessened as required. Corrosion is therefore

expected to produce an irregular texture on the carbon steel surface and would produce

non-periodic profiles. According to table 3.2, then the value of the cut off would dependon the value of Raand Rz

Tests were then carried out on each specimen to determine the required cut off

according to ISO 4288 (1996). This involved taking surface roughness measurements

with all available cut offs on the Surtronic 3+, i.e. 0.25mm, 0.8mm and 2.5mm, and the

appropriate cut off was chosen based on the results obtained.

3.4 MEASUREMENT OF ACTUAL CORROSION PENETRATION

The corrosion penetration rate obtained through mass loss analysis in section 3.2 is an

average value for the amount of time the carbon steel test coupons have been exposed to

the atmosphere. However the actual corrosion penetration is not uniform across the

surface and some localized corrosion penetration run much deeper through the metal


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than the value obtained by equation (3.1) and should not be overlooked as structural

components strength and service life will be affected by the actual corrosion

penetration.

It is a good idea to know the deepest penetration of corrosion on the samples. It can then

be compared to the corrosion penetration rate and an idea of the discrepancy between

the average penetration and the actual penetration can then be obtained.

The atmospheric corrosion process is not uniform across the samples and in order to

measure the penetration at different locations, direct measurement of the thickness of

the carbon steel samples after exposure is preferred instead of mass loss analysis. This

can be achieved with an instrument, the Nikon Microscope ME600 equipped with a

stage micrometer giving a precision of up to 0.001mm, which is available for use in the

UOM metallurgy laboratory.

Fig. 3.3: Nikon Microscope ME600 equipped with stage micrometer

3.4.1 Specimen Preparation

The carbon steel specimens with initial dimensions of 150mm x 100mm x 3mm cannotbe used with the Nikon Microscope ME600 as they are too large and bulky to be

handled on the stage of the microscope. Ideally a thin strip of metal cut along the length

of the specimen can be used, say a 150mm x 15mm x 3mm. This is roughly the

dimension of the strip, in figure 3.1.


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The strip in the middle of the specimen along its length is preferred for measurement of

the depth because,

This area of the metal affected by atmospheric corrosion is most representative

of a real-life situation of a service component exposed to the atmosphere since it

surrounded by bulk material on all sides. This part of the specimen was also chosen for surface roughness measurements.

For the sake of consistency, it makes sense to use this region for depth

measurement as well.

The challenge is to convert a 150mm x 100mm x 3mm plate into a 150mm x 15mm x

3mm strip.

X Y

t

Fig. 3.4: Specimen Transformation

The cutting of the specimen from X to Y has been done in several steps as described in

the following methodology. Great care was taken to ensure reliability of results and

many precautions were followed in the methods described below.

Bench Shear


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Fig. 3.5: Hand Bench Shear

The outlines of the strip, like in figure 3.4, were carved onto the surface using a scriber.

Then it was cut along the outline in a shear sheet metal operation.

Milling Machine

Before they are machined further, it is a good practice to reduce the degree of bending

as far as possible to ensure a good gripping whenever it is being machined and to

maintain parallelism to the maximum possible when required. A metalworking bench

vice commonly found in the mechanical workshop has been used to do so. The bent

specimens have been placed between the jaws of the vice while they were closed

afterwards. The large forces applied, akin to forging processes, caused the specimens to

return to its initial unbent shape to some extent.

The specimen is then machined with the vertical milling machine available in the UOM

mechanical workshop as seen in figure 3.6. It is used to correct the deformed and rough

surface produced by the previous shearing operation by removing material along its


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longitudinal edges where it was sheared, corresponding to a reasonable depth of cut, of

say 3 mm, on each side (see figure 3.7).

This will produce a new surface along the edges where it was cut which is smooth,sharp and clean. The strip of metal can now be used with the Nikon Microscope ME600

equipped with a stage micrometer to measure its depth to an accuracy of 0.001 mm.

Fig. 3.6: Vertical milling machine

Fig. 3.7: Specimen being milled along the longitudinal edge of cut


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3.4.2 Depth Measurement

The specimen which had been prepared by applying measures mentioned in section

3.4.1 is used with the Nikon Microscope ME600 equipped with a stage micrometer. Its

depth, i.e. thickness, has been measured with the stage micrometer which offers an

accuracy of 0.001mm. Only the deepest valleys are measured as only the deepest levelof corrosion penetration is of interest here.

Fig. 3.19: Nikon Microscope ME600 equipped with a stage micrometer

The specimen was held at right-angle to the slide using parallels and great was care was

taken while fixing it firmly using plasticine that it remained at right angle to the slide.

The slide was then positioned on the stage and there was no further hand manipulation.

Any relative movement of the slide needed was done via the respective knobs which

moved the stage in the X, Y or Z directions.

On each specimen, ten readings of the thickness measurement are taken along the length

of the metal stripe with 5mm interval between each reading. A magnification of 5X was


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used since it would time-consuming to measure ten readings per specimen over 20

specimens, which meant a total of 200 readings

Fig. 3.20: Specimen fixed firmly at right angle to the slide using plasticine

The initial thickness of the carbon steel test coupons before exposure was 3mm. While

observing the thickness under the stage microscope with a magnification of 5X, the

length measured with the stage microscope would theoretically be 15mm. However the

scale of the filar micrometer onboard the microscope eyepiece can read a maximum of

10mm. Hence each reading must be done in a two-step process.

Fig. 3.21: Specimen on slide and positioned on stage


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3.4 CONCLUSION

Many carbon steel test coupons were exposed to the atmosphere and they were

affected by atmospheric corrosion, causing material loss. After the time of

exposure, the specimens retrieved and this mass loss measured. From that, the

average corrosion penetration rate was calculated

The effect of atmospheric corrosion on the surface roughness values was also

monitored. After exposure, measurement of the surface roughness values was

carried on the specimens using the Taylor Hobson Surtronic 3+.

The actual corrosion penetration was also measured. First the specimens were

machined with a bench shear and milling machine to turn them into a thin metalstripe. These were then measured using Nikon Microscope ME600 equipped

with a stage micrometer to find the actual thickness over ten locations.


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CHAPTER 4

RESULTS, ANALYSIS &DISCUSSION


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4.0 INTRODUCTION

This chapter contains all the various results obtained from the experiments carried on

the carbon steel test specimens after having been subjected to atmospheric corrosion.

The analysis of these results obtained through various experiments will be interpreted so

that meaningful conclusions can be drawn out and discussed thoroughly.

Results obtained are:

Corrosion penetration determined from mass loss in grams through difference

between mass of coupon before and after exposure in different sites across

Mauritius. This will be used as a reference.

Surface roughness measurement values by the use of a Taylor Hobson Surtronic

3+ surface profilometer on the metal surface.

The thickness of the test coupons after exposure along different locations among

the longitudinal length.

These results will be analyzed and interpreted to assess the effects of atmospheric

corrosion as follows:

The values for corrosion loss obtained will be correlated with the surface

roughness measurement values obtained and any resulting patterns or

relationship will be identified and discussed.

The value of corrosion penetration obtained by the weight loss method is an

average value only, i.e. a uniform thickness across the whole surface. However

the true thickness is much less than that. The smallest thickness for each test

coupon will be located along the longitudinal length and measured using a

Nikon Microscope ME600 equipped with a stage microscope. The true change

in thickness will be compared against the average corrosion loss obtained by the

mass loss method.


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4.1 VISUAL INSPECTION OF THE SURFACE

After exposure to the atmosphere for the duration of the time of exposure, the

specimens are removed and a visual inspection is carried out on them which allow first-

hand examination of the corroded carbon steel surface and preliminary detection of

discontinuities such as cracks, corrosion or surface finish.

Fig. 4.1: Coupon freshly removed from rack (left) and cleaned (right)

Figure 4.1 show the coupon with identification code R2 when it has been freshly

removed from the rack and after it has been cleaned according to ISO 8565 (1995)

which has been obtained by scanning the surface. There is a thick layer of rust formed

on the skyward surface of the specimen and it can be seen clearly that the surface did

not corrode uniformly. Part of the rust layer is thicker as evidenced by the areas which

are more reddish brown whereas areas where corrosion is lighter, the layer is thinner as

it becomes more yellowish. There are also brown spots which indicate even thicker rust

layer.

After removing the layer of rust through cleaning processes according to ISO standards,

the specimen has been scanned again as shown in figure 4.1 (right). Small pits are seen


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all over the surface and they are more concentrated in the middle, albeit irregularly

scattered. This is why surface roughness measurements were carried out in the middle

and the thin metal strip for thickness measurement was taken from the middle section as

well. Hence it is concluded that although the base metal carbon steel is covered with

layers of corrosion products during exposure, it has also been attacked underneath as

proven by these small pits. This is probably due to the formation of cracks in the rust

layer. These pits also grow larger and deeper with corrosion loss.

4.2 CORROSION LOSS ANALYSIS

The variables from equation (3.1) for calculation of corrosion loss of the test coupons

are the mass loss, m (g), density of specimen (g/mm-3) and area of specimen, A (mm-

2). Variable is constant for all specimens since same dimensions were used for

specimens of both study A and study B. However variable is different for each studyas the composition of low carbon steel used in study B and medium carbon steel used in

study A are different (See table 3.1).

Area, A = 31500 mm

2

Density of low carbon steel, b (Study B) = 0.0077 g/mm3

Density of medium carbon steel, a (Study A) = 0.0075 g/mm3

A code was stamped on each specimen before atmospheric exposure for identificationpurposes. This code was written according to exposure site and study as show in table

4.1 below. Each code was followed by a random number stamped with it for

identification purposes for specimens within the same site.

Site Code Site Code

Palmar MB Reduit R

Reduit RB Port L:ouis P

Vacoas VB St Julien dHotman S

Belle Mare B

Study A Study B

Table 4.1 Identification codes of specimens


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Results for the calculated corrosion loss of the specimens are shown in ascending order

of corrosion loss in tables 4.2.

Code Massloss (g) Days ofexposure Corrosion loss(m)

VB08 1.861 47 7.88

P29 2.876 580.2 11.86

R2 2.953 78.9 12.17

RB02 3.982 175 16.86

S18 5.148 90.1 21.22

S5 5.614 90.1 23.15

RB20 7.55 506.1 31.96

VB22 7.656 532 32.41

S22 8.023 225.3 33.08B29 8.23 196.9 33.93

R18 9.419 213.1 38.83

B26 10.385 580.2 42.8

R28 11.289 596 46.54

P1 13.159 196.1 54.25

P22 18.202 573.1 75.04

Table 4.2 Corrosion loss of the specimens

The results obtained were as expected, i.e. the corrosion loss increases with respect to

the number of days it has been exposed.

The specimen test coupons which were used for the computation of corrosion loss were

taken from many different sites and there are many different factors affecting corrosion

in each site, e.g. temperature, time of wetness, wind velocity, rainfall time and so on.

Although the variable of site could have been eliminated by taking all coupons fromthe same location for computation of corrosion loss, this was not done because the aim

of this project is to assess the surface roughness properties with corrosion loss and an

investigation of the true thickness after corrosion.


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Furthermore there are many types of corrosive environment classified as rural, urban,

industrial and marine across Mauritius. The test coupons were taken from different sites

to get a good range of corrosion loss for correlation with surface roughness

measurements and true change in thickness.

4.3 SURFACE ROUGHNESS MEASUREMENT ANALYSIS

As described in section 3.3, surface roughness measurements have been carried out on

some specimens to analyze the behavior of the carbon steels surfaces against

atmospheric corrosion. Each specimen had an appropriate sampling length with which

surface roughness measurements were performed using the Taylor Hobson Surtronic 3+

surface profilometer.

4.3.1 Cut off Selection Methodology

The Taylor Hobson Surtronic 3+ surface profilometer offers a choice of 3 cut off values

to work with, i.e. 0.25mm, 0.80mm and 2.50mm. In terms of roughness, there are five

amplitude parameters and one spacing parameter available. Values obtained were Ra,

Rq, Rz, Rt, Ryand Sm.

The required cut off for each carbon steel test coupon is different since corrosion is

dependent on many factors, many of which are uncontrollable. To find the required cutoff for each test coupon, the methodology described below has been applied.

Surface roughness measurement is taken on each test specimen using all the

available cut offs on the Surtronic 3+ and the roughness values corresponding to

the specific cut off used are noted, especially Raand Rz. Tables 3.3 to 3.4 below

show some typical values obtained for three test coupons stamped as RB13, B10

and B16.

Lc= 0.25mm CR

Ra/m Rq/m RzDIN/m Rt/m Ry/m Sm/m

RB13 1.52 2.04 7.4 12 10.7 106

B10 0.82 1.06 4.3 5.7 5 61

B16 1.1 1.44 5.4 8.9 7.4 79

Table 3.3 Roughness values at 0.25mm cut off


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Lc= 0.80mm CR


RB13 2.18 2.8 12.8 16.4 15.2 148

B10 1.22 1.6 8 11.5 10.6 109B16 1.8 2.46 12 14.9 14 146


Lc= 2.50mm CR


RB13 2.4 3.32 18 23.5 22 149

B10 1.24 1.6 8.1 12 11.8 149

B16 2.82 3.72 20.2 24.5 23.6 206


Using Raand Rzvalues as reference to be compared against table 3.2, the

required cut off is deduced. Table 3.6 below shows a typical result for the three

samples used above.

According to 0.25mm

test

According to 0.80mm

test

According to 2.50mm

test

Ra Rz Ra Rz Ra Rz

RB1

3

0.8 0.8 0.8 2.5 2.5 2.5

B10 0.8 0.8 0.8 0.8 0.8 0.8

B16 0.8 0.8 0.8 2.5 2.5 2.5

Table 3.6 Required cut off according to ISO 4288

The results show that the required cut off itself varies according to the cut off used for

the test. For B10, the required cut off is definitely 0.8mm and therefore only the

roughness values obtained while using a cut off of 0.8mm will be considered for further

analysis.

For RB13 and B16, the choice for required cut off is between either 0.8mm or 2.5mm.

The selection is decided by the cut off which emerged as the required one according to

ISO 4288 ANDwas the also the cut off which was the smallest one as smaller required


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cut off means a finer surface and more detail is captured by the profilometer. In that

case it is 0.8mm for both RB13 and B16.

4.3.2 Cut off Selection Methodology Results

An appropriate cut off value was required for surface roughness measurement of each

specimen as atmospheric corrosion was not uniform on the metal surfaces and the

surface irregularities are not spread regularly. Table 4.3 shows the required cut offs of

the specimens in ascending order of corrosion loss. They have been obtained according

to ISO 4288 (1996).

Required cut off

(mm)

Corrosion loss

(m)VB08 0.8 7.88

P29 0.8 11.86

R2 0.8 12.17

RB02 0.8 16.86

S18 2.5 21.22

S5 2.5 23.15

RB20 2.5 31.96

Table 4.3 Required cut off values

Test coupons with relatively small cut corrosion loss have a required cut off of 0.8mm

and as corrosion loss increases, the required cut off increases to 2.5mm. It can be

deduced that during the first stages of corrosion that the base metal has a fine surface

such as a 0.8mm cut off is sufficient to capture enough surface irregularities for

statistical averaging. As corrosion progresses, the rust layer formed on the surface is

thought to be protecting the surface but the surface of the base metal becomes coarser as

corrosion loss increases. This is demonstrated by increase in cut off to 2.5mm which

can be explained by the fact that the surface irregularities are more widely spaced and a

larger distance is required to pick up a sufficient amount of irregularities for statisticalcalculations of roughness values.

This phenomenon can be explained by the types of corrosion mechanism involved. At

the onset of exposure, the atmosphere attacks the metal surface as a thin layer of

electrolyte is formed on it. At first, general corrosion occurs whereby there is a gradual


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reduction in thickness of the corroding material and the formation of a rust layer which

increases over time. However this reduction in thickness is not uniform as corrosion

does not proceed at the same rate over the whole metal surface area.

The rust layer is therefore also not uniform as seen from its color variation on the metal

surface from figure 4.1 and there are small variations on the surface which causes the

layer of rust to be porous because of the differential rate of corrosion. With time, there

is a diffusion process whereby atmospheric corrodants such as chloride ions diffuse into

the rust layer and attacks the base metal underneath at specific points. Pitting corrosion

occurs as the rust layer is porous and has cracks. This is a possible explanation for the

numerous pits found on the base metal after the rust layer has been removed during

cleaning.

Fig. 4.2: Formation of rust layer and cracks

4.3.3 Roughness Values Analysis

The results for the surface roughness measurements of the specimens are presented in

graphs below where they have been plotted against corrosion loss. A general trend has

been observed as shown in figures 4.2 to 4.5.

Rust layer

Base Metal (carbon

steel)

Pit

Crack


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Fig. 4.3: Graph of Ra(m)against Corrosion loss (m)

From figure 4.3, it is observed that the rate of Raagainst corrosion loss increases rapidly

initially and then levels down almost to a steady value of about 8m with increasing

corrosion lost. This shows that the value of Ra is extremely affected by corrosion loss

which itself increases as layer of rust increases. Roughness values of about 2m for

small corrosion losses may be due to direct atmospheric attack itself. This explains the

initially high rate of increase of Raand it decreases afterwards as rust layer thickens but

does not immediately die down to zero because of diffusion of atmospheric corrodantsinto the rust layer which attacks the base metal underneath, causing the formation of

pits which grow deeper with corrosion loss.

y = 2.073ln(x) - 2.996

R = 0.556

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60 70 80

Ra

(m)

Corrosion loss (m)

Ra against Corrosion Loss


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Fig. 4.4: Graph of Rq (m)against Corrosion loss (m)

Although Rais a reliable quantity due to its averaging of the surface, it is not influenced

by sudden peaks. But Rqhas a magnifying effect on odd single spikes or valleys as it is

a root mean square deviation as compared to Rawhich is an arithmetic one. Hence it is

expected to be greater than Rasince any unusually high peak or deep valley will push

up the Rqvalue because of the squaring and rooting process in its calculation, making

it more sensitive to peaks and valleys.

Fig. 4.5: Comparison between Raand Rq

y = 2.601ln(x) - 3.495

R = 0.553

0

2

4

6

8

10

0 10 20 30 40 50 60 70 80

Rq

(m)

Corrosion loss (m)

Rq against Corrosion Loss

0

2

4

6

8

10

0 10 20 30 40 50 60 70 80

Roughness

(m)

Corrossion loss (m)

Ra and Rq against Corrosion Loss


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RzDINgives a proper idea of the depth of the pits from peak to valley, which is about ten

times that of Ra and Rq as shown by the graph in figure 4.2. Hence it is gives an

indication of the extent of extremely localized corrosion, giving rise to pits, and the

graph shows that the difference between maximum and minimum penetration increases

with time. It can be as much as 35m in extreme cases, which indicates pits of about

35m in depth beneath the base metal. This is a very serious as they are very difficult to

detect and predict given that they occur inside the base metal and beneath the rust layer

invisible to the naked eye.

4.4 CHANGES IN DEPTH OF CARBON STEEL AFTER EXPOSURE

Using the weight loss analysis technique to calculate the corrosion loss via the mass loss

of the specimens only gives an average of the corrosion penetration. This method does

not detect rapid changes in corrosion or localized spots where corrosion is extremelysevere. However these localized spots, also called pits or holes, are extremely

dangerous.

A single of these holes causing material loss can cause the failure of a whole

engineering system and it is even more critical that it is difficult to detect since they are

usually covered by layers of corrosion products.

Measuring the actual thickness of the carbon steel test specimens gives an idea of the

real penetration of corrosion. A Nikon Microscope ME600 equipped with a stage

micrometer giving a precision of up to 0.001mm as shown in figure 3.10 has been used

for the measurement. Each specimen taken was processed into a small metal strip and

measured under the microscope at ten different points along its longitudinal length with

each reading being done at 5mm from the previous one and a typical set of results is

shown in table 4.4 for a random sample. Only the deepest penetration was measured

each time and an average was found.

2.852mm 2.766mm 2.99mm 2.784mm 2.888mm

2.652mm 2.994mm 3.186mm 2.734mm 2.85mm

Table 4.4 Set of measurements of specimen thickness


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All the results for each specimen is found in the appendix and further details in the

methodology adopted for this process are also mentioned.

Figure 4.6 shows a graph where the change in thickness of each specimen is plottedagainst corrosion loss; it can also be viewed as a graph of actual penetration v/s average

penetration.

Fig. 4.6: Graph of Change in Thickness (m)against Corrosion Loss (m)

A logarithmic trendline can be incorporated into the set of points of graph 4.6 with R2

value of 0.58. This relatively low value suggests that these points fit onto a logarithmic

pattern only partially. However there are many factors and variables which could not

and had not been fixed such as exposure site, weather conditions, cosine error in

measuring depth on micrometer stage or damage to surface during machining. But it

provides a general trend for the actual pitting underneath the rust layer during corrosion.

Localized corrosion can give rise to pits of about 400mm which is about times themaximum of RzDINand, although at a decreasing rate, the pits keep growing with time.

A single of these cracks into a structural engineering component bearing loads or

undergoing rapid motion can cause premature failure and even the loss of lives.

y = 154.8ln(x) - 241.5

R = 0.58

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80ChangeinThickn

ess(m)

Corrosion loss (m)

Change in Thickness


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4.4 CONCLUSION

Low and medium carbon steel test coupons were exposed to the atmosphere and

subjected to atmospheric corrosion. Afterwards they were removed from the

atmosphere and analyzed to assess the effects caused by atmospheric corrosion.

A rapid visual inspection showed a non-uniform rust layer and, upon cleaning, a

considerable number of pits on the base metals surface underneath the rust layer. It is

therefore evident that although the metal surface was covered by the rust layer, it was

attacked by the atmosphere. This evidence was backed up by the surface roughness

measurements which were carried out post-exposure.

The changes in Ra, Rqand RzDINvalues with corrosion loss showed a relatively accuratemodeling with a simple logarithmic regression analysis, having a R2-value of about 0.6.

This indicates an initially high rate of corrosion which decreases overtime and corrosion

settles down to a nearly constant value afterwards.

The measurement of changes in depth revealed that at several localized spots, the pits

present were much deeper than the average surface. This also demonstrates a

logarithmic regression behavior. It is concluded that the pits formed grow deeper over

time, even while the metal surface is being covered by the rust layer.


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CHAPTER 5

GENERAL DISCUSSION,

CONCLUSION &

RECOMMENDATION


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5.0 GENERAL DISCUSSION AND CONCLUSION

Knowledge of atmospheric corrosion in Mauritian context is largely non-existent as

very few studies have been carried out on the issue for Mauritius. Although many

corrosion studies have been carried through out by many international renowned

researchers and scientists such as The effect of environmental variables onatmospheric corrosion of carbon steel in Shenyang by WANG Chuan, WANG Zhen

Yao & KE Wei, the results and findings are irrelevant in Mauritius because local

features and geographical conditions influence greatly the corrosive action. Hence if

engineers and scientists in Mauritius are to predict and prevent atmospheric corrosion of

materials in the environment of Mauritius, the behavior of metals in local conditions is

required and this project provided some data in this respect.

Several carbon steel test coupons were exposed to the atmosphere across six differentsites in the island, which are representative of the different categories of corrosivity of

the different types of environment according to ISO 9223. When they were removed, it

was observed that the rust layer was not uniform as shown by the variation in color of it.

This implied that the corrosion process was not uniform across the metal surface. The

samples were then cleaned and it was observed that there were many pits on the base

metal. This means that the base metal was attacked underneath the rust layer during the

exposure time.

The corrosion of the sample specimens were monitored using a weight loss method. It

was observed that, as expected, mass of the specimens were increasing with increasing

days of exposure. From these values, the respective corrosion was calculated and used

as a reference for further tests.

Surface roughness measurements were then taken on the specimens to verify whether

there is a correlation with corrosion loss. At first it was observed that the required cut

off for each specimen was not the same for every specimen and that it changed from

0.8mm to 2.5mm with increasing corrosion loss. It is concluded that with time, themetal surface changed from a fine surface with low roughness to a coarser one with

increased roughness values.

Therefore roughness increases with time and this can be confirmed by the graphs of

roughness values against corrosion loss. Roughness values and corrosion loss are


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observed to be related by an approximate logarithmic correlation. Roughness values

increase rapidly with time initially because of direct attack by the atmosphere and slow

down because of partial protection by the rust layer.

The increase in roughness is explained because of the formation of localized valleys on

the base metal surface and beneath the rust layer which grow deeper with time. Thegraph of RzDIN shows clearly that the maximum peak to valley heights increase with

corrosion loss. Formation of localized pits is the phenomenon explaining this. Initially

as the metal surface was attacked directly by the atmosphere, the rate was high because

the whole surface was attacked by a general corrosion which proceeded uniformly and

caused the formation of rust layer. With time, the layer grew thicker and shielded

partially the base metal surface but cracks started to form in the rust layer which,

coupled with the fact that corrosion in not uniform now, caused it to become porous.

Atmospheric corrodants diffused into the rust layer and attacked the base metal surface

at localized spots. It proceeded by a general corrosion mechanism which was not

uniform as well as pitting corrosion mechanism.

The weight loss method and surface roughness measurements are averaging methods

only and do not give values for the actual corrosion loss. From the graph of change in

thickness against corrosion it is seen that at some localized spots, the valleys grow so

deep so as to produce a corrosion loss of about 500m whereas the same specimen gave

an average corrosion loss of 75m. Hence there is a net difference between the average

corrosion loss and maximum corrosion loss and this difference increases with corrosion

loss. It is concluded that the valleys (pits) grow deeper with time at some localizedspots.

5.1 RECOMMENDATION

The relationship between corrosion loss and roughness values is a single regression with

corrosion loss being the independent variable and roughness values being the dependent

variable. The resulting logarithmic correlations of the different surface parameters in

chapter 4 resulted in a model with R2-value of about 0.6. This means that the variability

of the roughness values around the regression line is about 1 - 0.6 times the original

variance. Ideally the target is to explain all the original variance, i.e. a R2-value of 1.

The value of about 0.6 for the R2-value is because there are many other factors affecting

atmospheric corrosion which have not been plugged in the regression model such as


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concentration of specific atmospheric corrodants such as chlorides, temperature, wind

velocity or time of wetness. Even if some of these factors were accounted for in the

regression model, only a few of them might have turned out to be significant as many of

them are intermittent or simply very difficult to measure to the point that it can said that

plugging in those factors is a bet that might work or not.

Multiple regression can certainly be used to improve the correlation of the data with the

model provided the multiple dependent variables used are pertinent ones and can be

easily obtained. This will increase the R-value closer to 1, indicating a much better

correlation.


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Appendix A

Corrosion occurs in different forms and each form has its own mechanism of action

which depends on the exact nature of the environment in which corrosion takes place.

The four main mechanisms by which atmospheric corrosion occurs are detailed below.

A.1 UNIFORM CORROSION

According to tonnage wasted, this is the most severe form of corrosion and it results in

a fairly uniform penetration (or thinning) over the entire exposed metal surface. The

general attack results from local corrosion-cell action; that is, multiple anodes andcathodes are operating on the metal surface at any given time. The location of the

anodic and cathodic areas continues to move through out on the surface, resulting in

uniform corrosion. Uniform corrosion often results from atmospheric exposure,

especially polluted industrial environments (Davis, 2001).

Also known as generalized corrosion, it is a well distributed and low level attack against

the entire metal surface with little or no localized penetration. It is the least damaging of

all forms of corrosion. Generalized corrosion usually occurs in environments in which

the corrosion rate is inherently low or well controlled.

It is the only form of corrosion whereby weight loss or metal loss data from corrosion

coupons or ultrasonic testing can be used to accurately and reliably estimate corrosion

rates (Common Types of Corrosion, 2000).

A.2 GALVANIC CORROSION

Galvanic corrosion occurs when two metals with different electrochemical potentials or

with different tendencies to corrode are in metal-to-metal contact in a corrosive

electrolyte.


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When two metals with different potentials are joined, such as copper (+0.334 V) and

iron (-0.440 V), a galvanic cell is formed. A cell, in which the chemical change is the

source of energy, is called a galvanic cell. The corrosion which is caused due to the

formation of the galvanic cell is, therefore, called galvanic corrosion. The driving force

or corrosion is a potential difference between different materials (Ahmad, 2006).

A.3 PITTING CORROSION

It is a form of localized corrosion of a metal surface where small areas corrode

preferentially leading to the formation of cavities or pits, and the bulk of the surface

remains unattacked. Metals which form passive films, such as aluminum and steels, are

more susceptible to this form of corrosion. It is the most insidious form of corrosion. It

causes failure by penetration with only a small percent weight-loss of the entire

structure. It is a major type of failure in chemical processing industry. The destructivenature of pitting is illustrated by the fact that usually the entire system must be replaced

(Ahmad, 2006).

A.4 CREVICE CORROSION

Crevice corrosion is a form of localized corrosion that occurs in zones of restricted flow

where a metallic material surface is in contact with a small volume of confined,

stagnant liquid whereas most of the material surface is exposed to the bulk environment.

Crevice zones may result from the design of the component or from the formation ofdeposits during service, shutdown, or even fabrication. These deposits may come from

suspended solids in the environment, corrosion products, or biological activity. Low-

flow areas are prone to the formation of such deposits. Crevice corrosion occurs mainly

(but not exclusively) on passive materials. The most important problem is the crevice

corrosion of stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in

aerated chloride environments, particularly in sea or brackish water, but also in

environments found in chemical, food, and oil industries. Other cases of crevice

corrosion are also known such as the so-called corrosion by differential aeration of

carbon steels, which does not require the presence of chloride in the environment

(Combrade, 2002)


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Appendix BThere are a number of surface texture parameters available nowadays to describe a

surface. They can be grouped into roughness, waviness, spacing and hybrid. Eachfamily of surface texture parameters has its own definition of their components and

some are calculated over the sampling length only while others are calculated over the

whole assessment length (Zygo Corporation, 2005)

B.1 ROUGHNESS

They are parameters which measure the vertical deviations of the surface characteristics

and are finer irregularities inherent to the production processes. Roughness includes

parameters such as Ra, Rq, Ry, Rp, Rvand so on.

Fig. B.1: Definition of Rpand Rv

Figure A.1 shows the graphical definition of two parameters which are Rpand Rv. Rpis

the highest peak. It is the maximum distance between the mean line and the highest

point within the samplin

jean emmanuel 2011

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