ASSESSMENT OF CORROSION RATE OF DISSIMILAR WELDED METALS BY AFIGO OGHENEKARO SAMSON

ASSESSMENT OF CORROSION RATE OF DISSIMILAR WELDED

METALS: GRAVIMETRIC ANALYSIS

BY

AFIGO OGHENEKARO SAMSON

CHE/10/1948

A PROJECT SUBMITTED TO

INDUSTRIAL CHEMISTRY DEPARTMENT

SCHOOL OF SCIENCE

FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF BACHELOR

OF TECHNOLOGY (B.TECH) DEGREE IN INDUSTRIAL CHEMISTRY

SEPTEMBER, 2015.

CERTIFICATION

This is to certify that this work was carried out by Mr. AFIGO, Oghenekaro Samson with

matriculation number CHE/10/1948, in the Department of Industrial Chemistry in partial

fulfillment of the requirements for the award of Bachelor’s Degree of Technology (B. Tech.) in

Industrial Chemistry, Federal University of Technology, Akure.

…………………..…………… …………………………….

PROF. AYIESANMI A.F. Date

Supervisor

DEDICATION

This work is dedicated to God Almighty for His grace and mercy upon me throughout my study

in the Federal University of Technology, Akure. Also to my parents, brothers and sisters, and all

my friends who have in one way or the other contributed to my academic progress.

ACKNOWLEDGMENT

All praises and adoration are due to the God Almighty, the most merciful and the most beneficent

for His guidance and protection during my studies in the Federal University of Technology, Akure

(FUTA).

A very much appreciate to the invaluable role of my Supervisors, Prof. Ayiesanmi A. F. in

conjunction with Mr. Shoetan I. O., Chemistry Department FUTA for their guidance and

understanding throughout the duration of this project. I cannot forget Mr. Abe and Mr Ajayi,

Chemistry Department FUTA for their assistance during the practical aspect of my project, so also

the work of Mr. Ajayi cannot be left unappreciated.

It is my radiant sentiment to place on record my best regards, deepest sense of gratitude to Mr.

Amoruwa, Mrs. Kadiri V., Mr. Usman, Mr. Anas of Nigerian Gas Company Limited, Warri Delta

State, for their careful and precious guidance which were extremely valuable for my study both

theoretically and practically.

I am also grateful to my parents Mr. & Mrs. Afigo for the gift of education and their financial

support, spiritual and parental support, also to my uncle in person of Mr James Ikpro and my lovely

sister Mrs. Afigo Joy for her unquantifiable financial and moral support.

ABSTRACT

Assessment of rate of corrosion of mild steel, galvanized steel, stainless steel, zinc and dissimilar

welded metals (mild steel and galvanized steel, mild steel and stainless steel, mild steel and zinc,

galvanized steel and stainless steel, galvanized steel and zinc) immersed in acid (0.4 M HCl) and

base (20% NaOH) test solution. The weight loss, corrosion rate and metal loss were determined.

The results of this study revealed that samples corrosion rate increase with increase in exposure

time and galvanic corrosion depends on the standard electrode potential of the samples. The

corrosion rate of the metal plates and dissimilar welded metals immersed in acid test solution (0.4

M HCl) increases in this order; zinc < galvanised steel < mild steel < mild steel and galvanised

steel < mild steel and zinc < galvanised steel and zinc < galvanised and stainless steel < stainless

steel and zinc < mild steel and stainless steel < stainless. The corrosion rate of the metal plates and

dissimilar welded metals immersed in base test solution (20% NaOH) increase in this order;

stainless steel < galvanised < zinc <mild steel and stainless steel < galvanised and stainless steel <

mild steel and galvanised steel < mild steel < galvanised steel and zinc < mild steel and zinc <

stainless steel and zinc. Accelerated galvanic corrosion occur in welded samples of mild steel and

galvanised steel, mild steel and stainless steel, mild steel and zinc, galvanised steel and stainless,

stainless steel and zinc. No accelerated galvanic corrosion was observed in galvanised steel and

zinc.

TABLE OF CONTENT

Certification i

Dedication ii

Acknowledgment iii

Abstract iv

List of Figures ix

List of Tables xi

List of Schemes xiii

List of Plates xiv

CHAPTER ONE

INTRODUCTION 1

1 Background 1

1.1 Aim and Objectives of the Project 4

CHAPTER TWO

LITERATURE REVIEW 5

2.1 Definition of Corrosion 5

2.2 Economic Impact 6

2.3 Types of Corrosion 8

2.3.1 Galvanic Corrosion 8

2.3.2 Uniform Attack 9

2.3.3 Selective Leaching 9

2.3.4 Erosion Corrosion 10

2.3.5 Stress Corrosion 10

2.3.6 Crevice Corrosion 11

2.3.7 Pitting 11

2.3.8 Intergranular Corrosion 12

2.4 Definition of Welding 12

2.5 Types of Welding 13

2.5.1 Gas Welding 13

2.5.2 Arc Welding 14

2.5.3 Resistance and thermite welding 17

2.5.4 New processes welding 18

2.6 Electrolysis 19

2.7 Electrochemical Series 22

2.8 Conditions Necessary for Galvanic Corrosion 23

2.9 Mechanisms of Galvanic Corrosion 24

2.10 Factors that Effect the Rate of Corrosion 25

2.10.1 Electrode Potential 25

2.10.2 Electrode Efficiency 27

2.10.3 Variable Potential 27

2.10.4 Electrolyte 28

2.10.5 Area Ratio 30

CHAPTER THREE

MATERIALS AND METHODOLOGY 32

3.1 Materials 32

3.2 Apparatus 32

3.3 Reagents 32

3.4 Preparation of Materials 33

3.5 Welding 33

3.6 Preparation of Standard Solutions 34

3.7 Gravimetric analysis 34

3.7.1 Exposure Area 35

3.7.2 Sample Weight 35

3.7.3 Sample Volume 35

3.7.4 Sample Density 36

3.7.5 Weight Loss 36

3.7.6 Corrosion Rate 37

3.7.7 Metal Loss 37

CHAPTER FOUR

RESULT AND DISCUSION 38

4.1 Welded Dissimilar metals 38

4.2 Samples Exposure Area 38

4.3 Measured Sample Weight 39

4.4 Measured Sample Volume 39

4.5 Determined Sample Density 42

4.6 Measured weight loss 43

4.7 Calculated Corrosion Rate 45

4.8 Calculated Metal Loss 48

4.7 Assessment of Accelerated Galvanic Corrosion 50

CHAPTER FIVE

CONCLUSION AND RECOMMENDATION 54

5.1 Conclusion 54

5.2 Recommendation 54

REFERENCES 56

LIST OF FIGURES

Figure 1: Corrosion of zinc metal 5

Figure 2: Galvanic Corrosion 8

Figure 3: Gas welding 14

Figure 4: Shielded metal arc welding equipment 15

Figure 5: Gas-Tungsten Arc equipment 15

Figure 6: Gas metal arc welding equipment 16

Figure 7: Submerged arc welding equipment 17

Figure 8: Resistance welding machine 18

Figure 10: Laser welding 18

Figure 11: Corrosion of mild steel in acid test solution 19

Figure 12: Corrosion of mild steel in base test solution 20

Figure 13: Corrosion of mild steel and galvanised steel in acid test solution 21

Figure 14: Corrosion of mild steel and galvanised steel in base test solution 22

Figure 15: Weight loss of metal plates in acid test solution (0.4 M HCl) 43

Figure 16: Weight loss of metal in base test solution (20% NaOH) 44

Figure 17: Corrosion rate (mpy) in acid media (0.4 M HCl) 46

Figure 18: Corrosion rate (mpy) in base media (20% NaOH) 46

Figure 19: Metal loss (mils) in acid media (0.4 M HCl) 48

Figure 20: Metal loss (mils) in base media (20% NaOH) 49

Figure 21: Corrosion rate of individual dissimilar welded metals in 51

acid test solution (0.4 M HCl)

Figure 22: Corrosion rate of individual dissimilar welded metals in 52

base test solution (20% NaOH)

LIST OF TABLES

Table 1: Galvanic series 23

Table 2: Galvanic corrosion table 26

Table 3: Electrical conductivity of electrolytes 29

Table 4: Constant K values for CR calculation 36

Table 5: Constant K values for ML calculation 37

Table 6: Exposure area of sample in acid media (0.4 M HCl) 38

Table 7: Exposure area of sample in base media (20% NaOH) 39

Table 8: Sample weight before immersion in acid test solution (0.4 M HCl) 40

Table 9: Sample weight before immersion in base test solution (20% NaOH) 40

Table 10: Sample volume before immersion in acid test solution (0.4 M HCl) 41

Table 11: Sample volume before immersion in base test solution (20% NaOH) 41

Table 12: Sample density of samples before immersion in acid test solution 41

(0.4 M HCl)

Table 13: Sample density samples before immersion in base test solution 41

(20% NaOH)

Table 14: Weight loss of metal plates in acid test solution(0.4 M HCl) 42

Table 15: Weight loss of metal plates in base media (20% NaOH) 43

Table 16: Corrosion rate of metal in acid test solution (0.4 M HCl) 45

Table 17: Corrosion rate of metal in base test solution (20%NaOH) 45

Table 18: Metal loss of metal in acid test solution (0.4 M HCl) 47

Table 19: Metal loss of metal in base test solution (20% NaOH) 48

Table 20: Corrosion rate of individual dissimilar welded metals in acid 50

test solution (0.4 M HCl)

Table 21: Corrosion rate of individual dissimilar welded metals in base 50

test solution (20% NaOH)

LIST OF SCHEMES

Scheme 1: Galvanic corrosion of two dissimilar 3

Scheme 2: Galvanic corrosion of iron and tin connected directly 25

LIST OF PLATES

Plate 1: The collapse silver bridge has seen from ohio side 6

Plate 2: Impact of corrosion on metallic structure 6

Plate 3: Uniform attack of metal 9

Plate 4: Selective leaching of zinc 9

Plate 5: Erosion corrosion of metal 10

Plate 6: Stress corrosion of metal 10

Plate 7: Crevice Corrosion of metal 11

Plate 8: Pitting Corrosion of metal 12

Plate 9: Thermite welding machine 18

Plate 10: Accelerated corrosion of a copper tube attached to a stainless steel tank 31

Plate 11: Metal plates immersed in 0.4 M HCl and 20% NaOH 34

Plate 12: Cleaning of immersed sample 36

Plate 13: Prepared sample for weighing 36

Plate 14: Weighing of cleaned coupons after immersion 36

Plate 15: Welded dissimilar metal sample 38

CHAPTER ONE

INTRODUCTION

1.1 Background

The word corrode is derived from the Latin corrodere, which means “to gnaw to pieces”.

Corrosion is defined as the destruction or deterioration of materials because of the chemical

interaction with their environment. Regardless of all the recent advances in polymer technology

and the development of high strength plastics, industry and civilization would grind to a halt

without the metals. Metals are usually exposed to the action of bases or acids in the industries.

Acid and base being corrosive causes severe damage to properties of metal resulting in sudden

failure of material in service (Mars, 2005; Barbara et al., 2006; Pierre, 2007). The corrosion costs

in most of the countries are in the range of 2 - 4 % of the gross national product (Vedula, 1998).

Consequently, it is needed to investigate the trend of the metal-dissolution on their exposure to

various medium, as this is an important factor in material selection that determines the service life

of the material (Osarolube et al., 2008; Noor and Al - Moubaraki, 2008). The present work has

been focused to study the corrosive tendency of dissimilar welded metals metals viz mild steel,

galvanized steel, stainless steel and zinc in hydrochloric acid and sodium hydroxide medium by

employing weight loss method.

The process of corrosion may be fast, slow or moderate. The corrosion of metals can be considered

as extractive metallurgy. Rusting is a term reserved for steel and iron corrosion. During the process

of corrosion most of metals form their metal oxide. Practically all environments are corrosive. Air,

moisture, fresh distilled water, salt and mine water, rural and urban and industrial atmosphere,

steam and other gases such as ammonia, chlorine, hydrogen sulphide and sulphur dioxide, fuel

gases, inorganic and organic acids, solvents, petroleum oils cause corrosion environment.

Inorganic materials are more corrosive than the organic materials. High temperature and pressure

produce more severe corrosion condition (Mars, 1987; Sedriks and Dudts, 2001).

Corrosion is a destructive phenomenon that, besides its economic effects, is detrimental to

appearance of metals and in some case equipment failure. Nature has endowed each metallic

substance with a certain natural energy level or potential.

When two metals having different energy levels or potentials are coupled together, current will

flow. The direction of positive current flow will be from the metal with the more negative potential

through electrolytes such as soil or water to that which is more positive. Corrosion will occur at

the point where positive current leaves the metal surface.

Current will be generated when two dissimilar metals are electrically connected and immersed in

an electrolyte, one of the metals will corrode. The path of the current will be from the corroding

metal, through the electrolyte to the non-corroding metal and then back through the connection

(conductor) between the two metals. The corroding metal is the one where the current leaves to

enter the electrolyte and is called an anode. The metal that receives the current is called the cathode

(Fontana and Greene, 1967; Uhlig, 1971). The presence of the second metal can actually increase

the rate of corrosion. The values of the standard electrode potentials for tin (E° = −0.14 V) and

iron (E° = −0.45 V) in table 1 show that iron is more easily oxidized than tin. As a result, the more

corrosion-resistant metal (in this case, tin) accelerates the corrosion of iron by acting as the cathode

and providing a large surface area for the reduction of oxygen (scheme 1). This process is seen in

some homes where copper and iron pipes have been directly connected to each other. The less

easily oxidized copper acts as the cathode, causing iron to dissolve rapidly near the connection and

occasionally resulting in a catastrophic plumbing failure (http://chemwiki.usdavis.edu/Analytical-

_Chemistry/ Electrochemistry/Case_Studies/ Corrosion_ Basics_II).

Corrosion rate is the speed at which any metal in a specific environment deteriorates. It can also

be defined as the amount of corrosion loss per year in thickness (Baboian, 1995). Using metal

coupons to assess rate of corrosion is the oldest and simplest method in monitoring of corrosion

(Memon et al., 2011). Coupons are described as small pieces of metal, usually of rectangular shape

or cylindrical, which are inserted in the process stream and removed after a period of time usually

one hour interval (Rajendran et al., 2002; Baboian, 1990).The most common and basic use of

coupons is to determine average corrosion rate over the period of exposure (Abdel-Gaber et al.,

2003). This is accomplished by weighing the degreased coupon before and after immersions

followed by its exposure to various acidic solutions to remove corrosion deposits on the substrate

surface. The difference between the initial and final weights of the coupon, which is the weight

loss. At least two, and preferably more specimens should be exposed for each condition (Memon

et al., 2011).

Scheme 1: Galvanic corrosion of two dissimilar

metals

http://chemwiki.usdavis.edu/Analytical_Chemistry/

http://chemwiki.usdavis.edu/Analytical_Chemistry/

1.2 Aim and Objectives of the Project

The present investigation was aimed to assess the rate of corrosion of dissimilar welded metals.

This project work is centred on obtaining relevant information on the effect of dissimilar welded

metals on the rate of corrosion using weight loss method which may increase or decrease the rate

of corrosion.

To achieve the aim of the research work, the following objectives are set to:

i. cut different metal plates to precise dimensions;

ii. weld the different dissimilar metals using suitable welding techniques;

iii. determine their physical parameters;

iv. prepare test solutions of 0.4 M HCl , 20% NaOH and cleaning solution of SnCl2, SbCl3

and HCl;

v. determine the corrosion rate.

CHAPTER TWO

LITERATURE REVIEW

2.1 Definition of Corrosion

Corrosion is the destructive attack of a metal and its properties by chemical (dry corrosion) or

electrochemical (wet corrosion) reaction with its environment. Non-metals are not included in the

present definition (Uhlig, 1949; Uhlig, 1950). Reaction can be divided into partial reactions of

oxidation and reduction is termed as electrochemical (Barrett and Massalski, 1980). For example,

when zinc is placed in dilute hydrochloric acid, a vigorous reaction occurs; hydrogen gas is evolved

and zinc dissolves, forming a solution of zinc chloride. The reaction is

Zn + 2HCl → ZnCl2 + H2 ----------- (1)

This reaction can be written in the form

Zn + 2H+ → Zn2+ + H2 --------------- (2)

Equation (2) can be divided into two reactions, the oxidation of zinc and the reduction of hydrogen

ions:

Oxidation (anodic reaction):

Zn → Zn2+ + 2e- ----------- (3)

Reduction (cathodic reaction):

2H+ + 2e- → H2 ------------ (4)

Zn2+

H+ H+ H+

H+ H+

e- e-

Zinc

Figure 1: Corrosion of zinc metal

An oxidation is indicated by a production of electrons and reduction is indicated by consumption

of electrons. The rate of electrochemical reaction is limited by physical or chemical factors.

(Wagner and Trand, 1938; Piontelli, 1954). Metals that normally fall victim to corrosion will

sometimes exhibit passivity to corrosion. Passivity is the characteristic of a metal exhibited when

that metal does not become active in the corrosion reaction. Passivity is caused by the buildup of

a stable, tenacious layer of metal oxide on the surface of the metal (Revie and Greene, 1969;

Skaperdas and Uhlig, 1942).

2.2 Economic Impact

Rust is one of the most common causes of bridge accidents. As rust has a much higher volume

than the originating mass of iron, its build-up can also cause failure by forcing apart adjacent parts.

Rust was also an important factor in the Silver Bridge disaster of 1967 in West Virginia, when a

steel suspension bridge collapsed within a minute, killing 46 drivers and passengers on the bridge

at the time (https://en.m.wikipedia.org/wiki/Silver_Bridge).

Plate 2: Impact of corrosion on metallic structure

Source: https://en.m.wikipedia.org/wiki/Corrosion

Plate 1: The collapse silver bridge has seen from ohio

side

Source: https://en.m.wikipedia.org/wiki/Silver_Bridge

http://en.wikipedia.org/wiki/Silver_Bridge

http://en.wikipedia.org/wiki/West_Virginia

http://en.wikipedia.org/wiki/Suspension_bridge

Perhaps most dangerous of all is corrosion that occurs in major industrial plants, such as electrical

power plants or chemical processing plants. Plant shutdowns can and do occur as a result of

corrosion. This is just one of its many direct and indirect consequences. Some consequences are

economic, and cause the following:

i. Replacement of corroded equipment

ii. Overdesign to allow for corrosion

iii. Preventive maintenance, for example, painting

iv. Shutdown of equipment due to corrosion failure

v. Contamination of a product

vi. Loss of efficiency, such as when overdesign and corrosion products decrease the heat-

transfer rate in heat exchangers

vii. Loss of valuable product, for example, from a container that has corroded through

viii. Inability to use otherwise desirable materials

ix. Damage of equipment adjacent to that in which corrosion failure occurs

Still other consequences are social. These can involve the following issues:

i. Safety, for example, sudden failure can cause fire, explosion, release of toxic product,

and construction collapse

ii. Health, for example, pollution due to escaping product from corroded equipment or due

to a corrosion product itself

iii. Depletion of natural resources, including metals and the fuels used to manufacture them

iv. Appearance as when corroded material is unpleasing to the eyes (Craig and Aderson,

1987).

http://en.wikipedia.org/wiki/Corrosion#cite_note-9

http://en.wikipedia.org/wiki/Corrosion#cite_note-9

2.3 Types of Corrosion

Corrosion damage can occur in many ways, for example, in failure by cracking and loss of strength.

Forms of corrosion are galvanic corrosion, uniform attack, selective leaching, erosion corrosion,

stress corrosion, crevice corrosion, pitting and intergranular corrosion (Uhlig, 1948).

2.3.1 Galvanic Corrosion

Galvanic corrosion also known as bimetallic corrosion tends to occur when dissimilar conducting

materials are connected electrically and exposed to an electrolyte (Figure 2). The following

fundamental requirements therefore have to be met for galvanic corrosion:

i. Dissimilar metals

ii. Electrical contact between the dissimilar conducting materials (can be direct contact or a

secondary connection such as a common grounding path).

iii. Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials

(Uhlig, 1948).

Figure 2: Galvanic Corrosion

2.3.2 Uniform Attack

Uniform corrosion is the attack of a metal at essentially the same at all exposed areas of its surface

(Plate 3). At no point is the penetration of the metal by corrosion twice as great as the average rate.

Rusting steel in the atmosphere and the corrosion of copper alloys in seawater are common

examples where uniform corrosion is usually encountered (Uhlig, 1948).

2.3.3 Selective Leaching

It is also known as parting or dealloying is the selective removal of one element from an alloy

leaving an altered residual structure. The most common example is the selective removal of zinc

in brass alloy (dezincification) (Plate 4) (Baboian, 1977).

Plate 3: Uniform attack of metal

Source: Uhlig, 1948

Plate 4: Selective leaching of zinc

Source: Baboian, 1977

2.3.4 Erosion Corrosion

Erosion corrosion is the acceleration or increase in the rate of deterioration or attack on a metal

because of relative movement between a corrosive fluid and the metal surface (Plate 5) (Baboian,

1977).

2.3.5 Stress Corrosion

Stress corrosion cracking (SCC) refers to the cracking caused by the simultaneous presence of

tensile stress and a corrosive environment. The impact of SCC on a material usually falls between

dry cracking and the fatigue threshold of that material (Plate 6) (Baboian, 1977)

Plate 5: Erosion corrosion of metal


Plate 6: Stress corrosion of metal


2.3.6 Crevice Corrosion

Crevice corrosion occurs at narrow openings or spaces between two metal surfaces or between

metals and nonmetal surfaces (Plate 7). A concentration cell forms with the crevice being depleted

of oxygen. This differential aeration between the crevice (microenvironment) and the external

surface (bulk environment) gives the crevice an anodic character. This can contribute to a highly

corrosive condition in the crevice. Some examples of crevices are flanges, deposits, washers, rolled

tube ends, Threaded joints, etc. (Baboian, 1977).

2.3.7 Pitting

Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the

material. Pitting is considered to be more dangerous than uniform corrosion damage because it is

more difficult to detect, predict and design against. Pitting corrosion (plate 8) can produce pits

with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion

products. Pits can be either hemispherical or cup-shaped (Baboian, 1977).

Plate 7: Crevice Corrosion of metal


2.3.8 Intergranular Corrosion

Intergranular corrosion is very relevant for the brewery industry. It consists of a localized attack,

where a narrow path is corroded out preferentially along grain boundaries of a metal. This type of

corrosion may have extreme effects on mechanical properties, resulting in a loss of strength and

ductility (Plate 9) (Baboian, 1977).

2.4 Definition of Welding

Welding is any process in which two or more pieces of metal are joined together by the application

of heat, pressure, or a combination of both. Most of the processes may be grouped into two main

categories: pressure welding, in which the weld is achieved by pressure; and heat welding, in which

Plate 8: Pitting Corrosion of metal


Plate 8: Intergranular Corrosion of metal Source: Baboian, 1977

the weld is achieved by heat. Heat welding is the most common welding process used today.

Brazing and soldering are other means of joining metals.

With the development of new techniques during the first half of the 20th century, welding replaced

bolting and riveting in the construction of many types of structures, including bridges, buildings,

and ships. It is also a basic process in the automotive and aircraft industries and in the manufacture

of machinery. Along with soldering and brazing, it is essential in the production of virtually every

manufactured product involving metals (Samuel, 2009).

2.5 Types of Welding

The welding processes most commonly employed today include:

1. Gas welding

2. Arc welding

3. Resistance and thermite welding

4. New processes welding

2.5.1 Gas Welding

Gas welding is a non-pressure process using heat from a gas flame. The flame is applied directly

to the metal edges to be joined and simultaneously to a filler metal in wire or rod form, called the

welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment

that is portable and does not require an electric power source. The surfaces to be welded and the

welding rod are coated with flux, a fusible material that shields the material from air, which would

result in a defective weld (Samuel, 2009).

2.5.2 Arc Welding

Arc-welding processes, which have become the most important welding processes, particularly for

joining steels, require a continuous supply of either direct or alternating electrical current. This

current is used to create an electric arc, which generates enough heat to melt metal and create a

weld.

Arc welding has several advantages over other welding methods. Arc welding is faster because of

its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain

methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are

shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc (Samuel, 2009).

1. Shielded Metal Arc

In shielded metal-arc welding, a metallic electrode, which conducts electricity, is coated with flux

and connected to a source of electric current. The metal to be welded is connected to the other end

of the same source of current. By touching the tip of the electrode to the metal and then drawing it

away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the

Figure 3: Gas welding

point of the metal electrode, which supplies filler metal for the weld. This process, developed in

the early 20th century, is used primarily for welding steels (Samuel, 2009).

2. Gas-Tungsten Arc

In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in

shielded metal-arc welding. A chemically inert gas, such as argon or helium, is used to shield the

metal from oxidation. The heat from the arc formed between the electrode and the metal melts the

edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point

of the weld (Samuel, 2009).

Figure 4: Shielded metal arc welding equipment

Figure 5: Gas-Tungsten Arc equipment

3. Gas-Metal Arc

In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or

carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc,

and melts off in droplets to enter the liquid metal that forms the weld. Most common metals can

be joined by this process (Samuel, 2009).

4. Submerged Arc

Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to

shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular,

Figure 6: Gas metal arc welding equipment

fusible material formulated to produce a proper weld. This process is very efficient but is generally

only used with steels (Samuel, 2009).

2.5.3 Resistance and Thermite Welding

In resistance welding, heat is obtained from the resistance of metal to the flow of an electric

current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to

great pressure, and a heavy current is applied briefly. The point where the two metals meet creates

resistance to the flow of current. This resistance causes heat, which melts the metals and creates

the weld. Resistance welding is extensively employed in many fields of sheet metal or wire

manufacturing and is particularly adaptable to repetitive welds made by automatic or

semiautomatic machines.

In thermite welding, heat is generated by the chemical reaction that results when a mixture of

aluminium powder and iron oxide, known as thermite, is ignited. The aluminium unites with the

oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler

Figure 7: Submerged arc welding equipment

metal for the weld. Thermite welding is employed chiefly in welding breaks or seams in heavy

iron and steel sections. It is also used in the welding of rail for railroad tracks (Samuel, 2009).

2.5.4 New Processes Welding

The use of electron beams and lasers for welding has grown during the second half of the 20th

century. These methods produce high-quality welded products at a rapid rate. Laser welding and

electron-beam welding have valuable applications in the automotive and aerospace industries

(Samuel, 2009).

Plate 9: Thermite welding machine

Source: Samuel, 2009 Figure 8: Resistance welding machine

Figure 10: Laser welding

2.6 Electrolysis

Electrolysis, process by which electric current is passed through a substance to effect a chemical

change. The chemical change is one in which the substance loses or gains an electron (oxidation

or reduction). The process is carried out in an electrolytic cell, an apparatus consisting of positive

and negative electrodes held apart and dipped into a solution containing positively and negatively

charged ions. The substance to be transformed may form the electrode, may constitute the solution,

or may be dissolved in the solution. Electric current (i.e. electrons) enters through the negatively

charged electrode (cathode); positively charged components of the solution travel to this electrode,

combine with the electrons, and are transformed to neutral elements or molecules. The negatively

charged components of the solution travel to the other electrode (anode), give up their electrons,

and are transformed into neutral elements or molecules. If the substance to be transformed is the

electrode, the reaction is generally one in which the electrode dissolves by giving up electrons

(http://www.britannica.com/science/electrolysis)

For instance mild steel immersed in acid test solution (0.4 M HCl), the reaction is as follows:

Fe + 2HCl → FeCl2 + H2

Fe2

H+ H+ H+

H+ H+

e- e-

Figure 11: Corrosion of mild steel in acid test solution

Mild steel

http://www.britannica.com/science/electrolysis


Fe + 2H+ → Fe2+ + H2

The equation can be divided into two reactions, the oxidation of mild steel and the reduction of

hydrogen ions:


Fe → Fe2+ + 2e-


2H+ + 2e- → H2

While mild steel immersed in base test solution (20% NaOH), the reaction is as follows:

Fe + 2NaOH → Fe(OH)2 + Na2


Fe + 2Na+ → Fe2+ + Na2


hydrogen ions:


Fe → Fe2+ + 2e-

Fe2+

Na+ Na+ Na+

Na+ Na+

e- e-

Figure 12: Corrosion of mild steel in base test solution

Mild steel


2Na+ + 2e- → Na2

When dissimilar welded metal in electrical contact with each other immersed in test solution. The

metal with the most negative standard potential (E0) value (table 1) undergo oxidation.

For instance, mild steel and galvanized steel immersed in acid test solution (0.4 M HCl), the

reaction is as follows:

Zn + 2HCl → CrCl2 + H2


Zn + 2H+ → Cr2+ + H2


hydrogen ions:


Zn → Zn2+ + 2e-


2H+ + 2e- → H2

Zn2+

H+ H+ H+

H+ H+

Galvanised

steel Mild steel e- e-

Figure 13: Corrosion of mild steel and galvanised steel in

acid test solution

While mild steel and galvanized steel immersed in base test solution (20% NaOH), the reaction is

as follows:

Zn + 2NaOH → Zn(OH)2 + Na2


Zn + 2Na+ → Cr2+ + Na2


hydrogen ions:


Zn → Zn2+ + 2e-


2Na+ + 2e- → Na2

The reaction for samples used in this study are all documented in the appendix.

2.7 Electrochemical Series

The electrochemical series is built up by arranging various redox equilibria in order of their

standard electrode potentials (redox potentials). The most negative standard potential (E0) values

Zn2+

Na+ Na+

Na+

Na+ Na+

Galvanised

steel Mild steel e- e-

Figure 14: Corrosion of mild steel and galvanised steel in

base test solution

are placed at the top of the electrochemical series, and the most positive at the bottom

(htttp://chemwiki.ucdavis.edu/physical_Chemistry/Equilibria/Redox_Equiliria/The_Electrochem

ical_Series)

Table 1: Galvanic series

Element Redox Equilibria Standard Potential (E0)

Mg Mg2+(aq) + 2e- 𝑀𝑔(𝑠)←

→ -2.37

Aluminum Al3+(aq) + 3e- 𝐴𝑙(𝑠)←

→ -1.66

Zinc Zn2+(aq) + 2e- 𝑍𝑛(𝑠)←

→ -0.763

Chromium Cr3+(aq) + 3e- 𝐶𝑟(𝑠)←

→ -0.74

Iron Fe2+(aq) + 2e- 𝐹𝑒(𝑠)←

→ -0.44

Cadmium Cd2+(aq) + 2e- 𝐶𝑑(𝑠)←

→ -0.403

Tin Sn2+(aq) + 2e- 𝑆𝑛(𝑠)←

→ -0.14

Lead Pb2+(aq) + 2e- 𝑃𝑏(𝑠)←

→ -0.126

Hydrogen 2H+(aq) + 2e- 𝐻2(𝑔)←

→ 0

Copper Cu2+(aq) + 2e- 𝐶𝑢(𝑠)←

→ + 0.337

2.8 Conditions Necessary for Galvanic Corrosion

The basic requirements necessary to cause galvanic corrosion are:

1. An electrolyte bridging the two metals which may not always be aggressive to the

individual metals when they are not coupled.

2. Electrical connection between the metals. This usually involves direct physical contact but

it can also arise where electrical continuity is established between two metals, for example,

by an insulation-coated conductor, by welding, by structural metal work or electrical

earthing. It is not necessary for the metal junction to be immersed in the electrolyte.

3. A sufficient difference in potential between the two metals to provide a significant galvanic

current.

4. A sustained cathodic reaction on the more noble of the two metals by one of the

mechanisms described below; in most practical situations this is the consumption of

dissolved oxygen (Bardal, 1984).

2.9 Mechanisms of Galvanic Corrosion

Different metals and alloys have different electrochemical potentials (or corrosion potentials) in

the same electrolyte. The potential difference (i.e., the voltage) between two dissimilar metals is

the driving force for the destructive attack on the active metal (anode). Current flows through the

electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The

conductivity of electrolyte will also affect the degree of attack. The cathode to anode area ratio is

directly proportional to the acceleration factor.

For instance, if iron is in contact with a more corrosion-resistant metal such as tin, copper, or lead,

the other metal can act as a large cathode that greatly increases the rate of reduction of oxygen.

Because the reduction of oxygen is coupled to the oxidation of iron, this can result in a dramatic

increase in the rate at which iron is oxidized at the anode. Galvanic corrosion is likely to occur

whenever two dissimilar metals are connected directly, allowing electrons to be transferred from

one to the other (http://chemwiki.usdavis.edu /Analytical_Chemistry/ Electrochemistry /

Case_Studies/ Corrosion_Basics_II).

http://www.corrosionclinic.com/different_types_of_corrosion.htm

At cathode: O2(g) + 4H+(aq) + 4e−→ 2H2O(l)

At anode: Fe(s) → Fe2+(aq) + 2e−

The iron (II) ions react with hydrogen ions and oxygen to produce iron (iii) ions:

4Fe2+(aq) + 4H+(aq) + O2(aq) → 4Fe3+(aq) + 2H2O(l)

The iron(III) ions react with hydroxide ions and oxygen to produce hydrated iron (iii) oxides (also

known as iron(III) hydroxides:

Fe3+(aq) + 3OH-→ Fe(OH)3(s)

Iron (iii) hydroxide Fe(OH)3 then dehydrates to produce rust (Fe2O3.nH2O):

Fe(OH)3(s) Fe2O3.nH2O

2.10 Factors that Effect the Rate of Corrosion

2.10.1 Electrode Potential

The value of the potential for any alloy, even in sea water, can be changed by a variety of factors

such as temperature, velocity, biocide treatment etc. However, the relative ranking of alloys

remains largely unchanged by these factors. A simplified version of the galvanic series is shown

in table 1. Alloys which are coupled to metals which have more electropositive potentials are the

ones which may suffer bimetallic corrosion. Stainless steels, such as 316, have a thin, protective

Scheme 2: Galvanic corrosion of iron

and tin connected directly

dehydrates

film and while this is intact corrosion rates are very low and the alloy is said to be in its passive

state. When breakdown of the film occurs, usually localised, such as crevice corrosion, then the

stainless steel is said to be active.

However, the magnitude of the potential difference alone is not sufficient to predict the risk of

bimetallic corrosion. For instance, metals with a potential difference of only 50 mV have shown

bimetallic corrosion problems, while other metals with a potential difference of 800 mV have been

successfully coupled together. The potential difference between the metals provides no

information on the kinetics of bimetallic corrosion which depends upon the current flowing

between them. There is a wide range of factors which affect the severity of bimetallic corrosion

such as area ratio, temperature, flow rate, composition of the electrolyte etc. The important ones

are discussed below. A general idea of the qualitative risk of bimetallic corrosion can be gained

by putting these effects together into a form such as table 2. This is not meant to be rigorous but

gives a guide to acceptable and potentially dangerous combinations (Johnsen and Oslen, 1992;

Crum, 1984)

Table 2: Galvanic corrosion table

Zinc Galvaniased

steel

Aluminium Cast

iron

Lead Mild

steel

Tin Copper Stainless

steel

Zinc A A B C C C C C C

Galvaniased

steel

A A B B B C C C C

Aluminium B B A B B B B C C

Cast iron C B B A A A A B B

Lead C B B A A A A B B

Mild steel C C B A A A A B B

Tin C C B A A A A B B

Copper C C C B B B B A A

Stainless

steel

C C C B B B B A A

Where A is galvanic action insignificant, B is galvanic action may occur and C is galvanic action

will occur.

2.10.2 Electrode Efficiency

Current flow between the two metals in contact is accompanied by a shift in the potential of the

anode to a more electropositive value and in the potential of the cathode to a more electronegative

value. These changes, called polarisation, result in the two metals approaching the same potential,

any potential difference between them being equal to the product of the current and the resistance

through the electrolyte. The extent of polarisation depends upon both the metal and the

environment. In neutral electrolytes the cathode is almost always polarised much more than the

anode, which accounts for the fact that a small area of sacrificial anode will effectively provide

protection to a relatively large cathodic area. The extent of polarisation will determine how

effective any particular metal may be as a cathode (Davis et al., 1986).

2.10.3 Variable Potential

Changes in the corrosion potential of individual metals and ,therefore, in the polarity of a couple,

from commonly encountered values can result from the presence of complexing ions, from

changes in pH and temperature, and from intense aeration which enhances film formation on the

anodic metal. Large differences in area ratio, especially with metals of nearly the same corrosion

potential, can also influence the relative polarity of metals by causing intense polarisation. Tin is

usually cathodic to iron in neutral salt solution, but in the presence of the complexing organic acids

found, for instance, in fruit preservation, it is anodic and therefore an effective sacrificial coating

for tin cans. Lead, tin, and their solder alloys form soluble corrosion products in alkaline solutions

and are anodic to iron, whereas in acid solutions they are usually cathodic to iron.

Reversals of significant practical importance are encountered with zinc and aluminium, which are

usually anodic to iron in natural waters. At temperatures above 600C iron becomes anodic to zinc,

which explains why problems are sometimes encountered in galvanised steel hot-water systems.

In fresh waters, polarity reversal is enhanced by the presence of bicarbonate and decreased by

sulphate and chloride. The aluminium/iron couple undergoes polarity reversal at elevated

temperatures in fresh water and in sea water (about 95oC) and the aluminium/zinc couple exhibits

polarity reversal in seawater at about 90oC (Davis et al., 1986).

2.10.4 Electrolyte

Electrolyte factors that have a major influence on bimetallic corrosion are composition, pH and,

in particular, electrical conductivity, which affects both the intensity and distribution of corrosion.

The severity of corrosion often increases with increasing electrical conductivity of the electrolyte

because, in practice, high conductivity is often caused by the presence of aggressive ions such as

chloride, or by acid or alkali.

The electrical conductivity of electrolytes can vary widely typical values are as follows:

Table 3: Electrical conductivity of electrolytes

Electrolytes Electrical

conductivity

Distilled water 0.5 – 2 µS/cm

Supply water 50 – 1,500 µS/cm

Saturated sodium

Chloride solution

2500,000 µS/cm

Sulphuric acid Up to 8,000,000 µS/cm

Molten salts 20,000 - 107 µS/cm

In common with local-cell corrosion, bimetallic corrosion is sensitive to the presence of

constituents in the electrolyte that effect stability of the corroded-metal ions. Thus, the presence of

bicarbonate, silicate and sulphate in water enhances formation of insoluble and adherent corrosion

products. In some cases insoluble corrosion products from the anodic metal can deposit upon, and

induce crevice corrosion of, the cathodic metal of a couple; stainless steels are susceptible to this

type of corrosion, especially when in contact with aluminum or mild steel. In some cases the

composition of the environment can be modified by adding inhibitors to control both local-cell

and bimetallic corrosion.

This practice has proved effective in the treatment of waters in engine cooling systems and

industrial cooling plants. Bicarbonate, cyanide and tartrate ions form soluble complexes with

copper and zinc, and thus their presence increases the rate of corrosion.

The effect of pH is even more far-reaching and depends upon the composition of both metals

forming the couple. In the case of magnesium and its alloys there is a possibility of a significant

amount of hydrogen evolution at the cathode in neutral solutions. Enhancement of the corrosion

rate starts as pH is decreased to about 4-5 for aluminum and ferrous alloys, and at about pH 6 for

zinc. At pH 1, hydrogen evolution is the predominant reaction on the cathode of most coupled

metals and bimetallic corrosion rates may be increased by two to three orders of magnitude

compared with those in neutral solutions. In sulphuric acid, however, much smaller increases in

corrosion rate are encountered with lead because of the formation of films of insoluble lead

sulphate.

Hence, care is required if hydrogen evolution is occurring and the cathode material is susceptible

to hydrogen embrittlement. Low pHs can also occur between two tightly coupled metals when one

undergoes crevice corrosion (e.g. 316 and superaustenitic stainless steel in sea water). The pH can

become so low (<1) that corrosion initiates on the cathode within the crevice (Davis et al., 1986).

2.10.5 Area Ratio

The area ratio is very important in the consideration of the likelihood of bimetallic corrosion. The

larger the cathode compared to the anode, the more oxygen reduction can occur and, hence, the

greater the galvanic current and, therefore, corrosion.

Under static conditions, where the bimetallic corrosion current is often dependent upon the rate of

diffusion of dissolved oxygen to the cathode, the amount of bimetallic corrosion is independent of

the size of the anode and is proportional to the area of the cathodic metal surface. This is sometimes

known as the catchment area principle, and has important implications in designing to minimise

the risk of bimetallic corrosion. Thus, for a constant area of cathode metal the amount of corrosion

of the anode metal is constant, but the intensity of corrosion is increased as the area of the anodic

metal is decreased.

Under immersed conditions in many supply waters, which have a relatively low electrical

conductivity, adverse effects are uncommon if the contacting metals adjacent to the junction are

of similar area. Thus, galvanized steel pipes can be used with brass or gunmetal connectors, but

serious corrosion to the pipe end is likely to result if the contact is directly to a large area of copper,

such as a tank or cylinder. Similarly, stainless steel and copper tubes can usually be joined without

problems resulting, but accelerated corrosion of the copper tube is likely to occur if it is attached

to a stainless steel tank (see Plate) (Davis et al., 1986).

Plate 10: Accelerated corrosion of a copper tube

attached to a stainless steel tank

CHAPTER THREE

MATERIALS AND METHODOLOGY

3.1 Materials

The mild steel plate, galvanised steel plate, and stainless steel plate of thickness 0.2 cm, 0.15 cm

and 0.1 cm respectively used for this study were purchased at Agricultural Engineering Workshop

(AGE), Federal University of Technology Akure (FUTA) and were cut into 2 × 2 cm. Zinc plate

of thickness 0.1 cm used for this study was purchased from Pascal Scientific Limited Akure, Ondo

State and was cut into 2 × 2 cm.

3.2 Apparatus

Apparatus used in this research include:

i. 100-mL graduated measuring cylinder

ii. 250ml graduated glass beakers

iii. 250ml Volumetric flask

iv. Electronic weighing balance

v. Spatula

vi. Stop watch

vii. Metric ruler

3.3 Reagents

The list of reagents used in this corrosion study include:

i. Ethanol

ii. Acetone

iii. Sodium hydroxide (NaOH)

iv. Concentrated Hydrochloric acid (HCl)

v. Distilled water

vi. Tin chloride (SnCl2)

vii. Antimony chloride (SbCl3)

3.4 Preparation of Materials

The metal surface was treated by degreasing in absolute ethanol and drying in acetone. The

samples were then stored in a moisture-free desiccator to avoid contamination before their use in

the corrosion studies

3.5 Welding

Mild steel and galvanized steel, mild steel and stainless steel, galvanized steel and stainless steel

were welded using arc welding technique. While zinc and mild steel, zinc and galvanized steel,

zinc and stainless steel plates were welded using gas welding technique.

3.8 Preparation of Standard Solutions

The acid test solution (0.4 M HCl) was prepared by measuring 9 ml of concentrated hydrochloric

acid (HCl) and making it up with distilled water up to the 250 mark volumetric flask. The base test

solution (20% NaOH) was prepared by measuring 50 g of concentrated sodium hydroxide and

dilute with 200 ml of distilled water. The cleaning solution was prepared by dissolving 5 g of

SnCl2, 2g SbCl3 in 100ml of distill water and 5ml of concentrated hydrochloric acid (HCl).

3.9 Gravimetric analysis

A known weight of various metals plate (mild steel, galvanised steel, stainless steel and zinc) and

dissimilar welded metals (mild steel and galvanised steel, mild steel and stainless steel, mild steel

and zinc, galvanised steel and stainless steel, galvanised steel and zinc and stainless steel and

zinc) were immersed in the test solution for a total exposure time of 24 hours, the weight loss was

measured at an interval of 1 hour, and the corrosion rate and metal loss were calculated.

3.7.1 Exposure Area

The samples length and breadth were measured using metric ruler and the thickness were

measured using venier calliper. Exposure area of the sample was calculated as follows:

Surface Area (A) = 2 x [(LxB) + (BxT) + (LxT)]

Were L is the length of sample, B is the breath of sample and T is the thickness of sample

3.7.2 Sample Weight

The metal plates were measured using an electronic balance, in grams (g) of 0.001g readability.

Plate 11: Metal plates immersed in 0.4M HCl and 20% NaOH

3.7.3 Sample Volume

Volume of the metal plates were measured indirectly using the technique of water displacement

(also known as Archimedes Principle). This is because the volume of water that the solid displaces

when it is immersed in the water is the same as the volume of the solid itself.

The volume of the samples were calculated as follows:

Volume of sample = V2 – V1

Where V1 is the initial volume before immersion and V2 is the final volume after immersion.

3.7.4 Sample Density

Density of the metal plates were determined from weight and volume measurements as follows:

𝜌 =𝑀

𝑉

Where 𝜌 = density, M = weight of sample and V = volume of sample

3.7.5 Weight Loss

The weight loss (g) was obtained by finding the difference between the weight of the sample before

and after the sample was completely immersed in the test solution in an open glass beaker for a

reasonable time interval. The weight loss was calculated as follows:

Weight loss (g) = 𝑊1 − 𝑊2

Where W1 is the weight of the sample before immersion in test sample and W2 is the weight of the

sample after immersion in test sample

In order to determine accurately the amount of material lost to corrosion, the metal plates were

cleaned after immersion, dried and weighed. Chemical cleaning was used in this corrosion study,

the metal plate after immersion in test solution for a precise time was soaked in cleaning solution

until all corrosion product has be removed.

3.7.6 Corrosion Rate

The weight loss was converted to a corrosion rate (CR), as follows:

Corrosion Rate (CR) = Wieght loss(g) × K

𝑆𝑎𝑚𝑝𝑙𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 ( 𝑔

𝑐𝑚2) × 𝐸𝑥𝑝𝑜𝑠𝑒𝑑 𝐴𝑟𝑒𝑎 (𝐴) × 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑇𝑖𝑚𝑒 (ℎ𝑟)

Table 4: Constant K values for CR calculation

Corrosion Rate Unit (CR) Area Unit (A) K-Factor

mils/year (mpy) in2 5.34x105

mils/year (mpy) cm2 3.45x106

Millimeters/year (mmy) cm2 8.75x104

Plate 14: Weighing of

cleaned coupons after

immersion

Plate 13: Prepared

sample for weighing Plate 12: Cleaning of

immersed sample

3.7.7 Metal Loss

The metal loss (ML) was determined, as follows;

Metal Loss (ML) = Wieght loss(g) × K

𝑆𝑎𝑚𝑝𝑙𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝑔

𝑐𝑚2) ×𝐸𝑥𝑝𝑜𝑠𝑒𝑑 𝐴𝑟𝑒𝑎 (𝐴)

Table 5: Constant K values for ML calculation

Metal Loss (CR) Area Unit (A) K-Factor

Mils in2 61.02

Mils cm2 393.7

Millimeters cm2 10.0

CHAPTER FOUR

RESULTS AND DISCUSION

4.1 Welded Dissimilar metals

The dissimilar welded were observed to be in contact with each other before gravimetric

analysis were carried out (plate 11).

4.2 Samples Exposure Area

The calculated exposure area of the metal plates used in this corrosion study are tabulated in

table 6 and table 7.

Table 6: Exposure area of sample in acid media (0.4 M HCl)

SAMPLE LENGTH

(cm)

BREADTH

(cm)

THICKNESS

(cm)

AREA

(cm2)

Mild steel 2.1 2.1 0.2 10.5

Galvanised steel 2 2 0.15 9.2

Stainless steel 2.2 2.1 0.1 10.1

Plate 15: Welded dissimilar metal sample

Table 7: Exposure area of sample in base media (20% NaOH)

SAMPLE LENGTH

(cm)

BREADTH

(cm)

THICKNESS

(cm)

AREA (cm2)

Mild steel 2.2 2.0 0.2 10.48

Galvanised steel 2 2 0.15 9.2

Stainless steel 2.1 2.0 0.1 9.22

Zinc 2.4 1.3 0.1 6.98

Mild steel and galvanised

steel

M = 2.2

G = 2.0

M = 2.0

G = 2.0

M = 0.2

G = 0.15

19.68

Mild steel and stainless steel M = 2.2

S = 2.1

M = 2.0

S = 2.0

M = 0.2

S = 0.1

19.70

Galvanised steel and

stainless steel

G = 2.0

S = 2.1

G = 2.0

S = 2.0

G = 0.15

S = 0.1

18.84

Mild steel and zinc M = 2.0

Z = 2.4

M = 2.0

Z = 2.0

M = 0.2

Z = 0.1

20.08

Galvanised steeland zinc G = 2.1

Z = 2.4

G = 2.0

Z = 2.0

G = 0.15

Z = 0.1

20.11

Stainless steel and zinc S = 2.0

Z = 2.5

S = 1.8

Z = 1.2

S = 0.1

Z = 0.1

14.7

Zinc 2.4 1.2 0.1 6.48

Mild steel and galvanised steel M = 2.2

G = 2.0

M = 2.1

G = 2.0

M = 0.2

G = 0.15 20.16

Mild steel and stainless steel M = 2.1

S = 2.3

M = 2.0

S = 2.1

M = 0.2

S = 0.1 21.34

Galvanised steel and stainless steel G = 2.0

S = 2.2

G = 2.0

S = 2.0

G = 0.15

S = 0.1 18.84

Mild steel and zinc M = 2.1

Z = 2.4

M = 2.0

Z = 2.0

M = 0.2

Z = 0.1 20.52

Galvanised steel and zinc G = 2.1

Z = 2.4

G = 2.0

Z = 2.0

G = 0.15

Z = 0.1 20.11

Stainless steel and zinc S = 2.2

Z = 2.4

S = 2.0

Z = 2.0

S = 0.1

Z = 0.1 20.12

Where M is mild steel, G is galvanised steel, S is stainless steel and Z is zinc.

4.3 Measured Sample Weight

The measured weight of the metal plates are tabulated in table 8 and table 9. Mild steel, welded

mild steel and galvanised steel was observed to have the highest weight which correspond with

the high density calculated in table 12 and 13 while zinc, galvanised steel and zinc has the lowest

weight which correspond with the low density calculated in table 12 and 13.

4.4 Measured Sample Volume

The volume of the metal plates were measured using volume displacement method were tabulated

in table 10 and table 11.

Table 8: Sample weight before immersion in acid

test solution (0.4 M HCl)

SAMPLE WEIGHT W1

(g)

Mild steel 9.361

Galvanised steel 6.065

Stainless steel 4.427

Zinc 1.283

Mild steel and galvanised steel 17.285

Mild steel and stainless steel 14.897

Galvanised steel and stainless steel 9.943

Mild steel and zinc 9.177

Galvanised steel and zinc 7.5

Stainless steel and zinc 8.848

Table 9: Sample weight before immersion in


SAMPLE WEIGHT

W1 (g)

Mild steel 7.264



Zinc 1.338







4.4 Determined Sample Density

The samples density calculated from the measured weight and volume of the samples are tabulated

in table 12 and table 13. The calculated density of mild steel in table 12 and table 13 correspond

with standard density (7.85g/cm3) of mild steel (http://en.m.wkipedia.org/wiki/ Carbon-steel).

Table 10: Sample volume before immersion in

acid test solution (0.4 M HCl)

SAMPLE VOLUME

(cm3)

Mild steel 1


Stainless steel 1

Zinc 0.1


Mild steel and stainless steel 2



Galvanised steel and zinc 1


Table 11: Sample volume before immersion in


SAMPLE VOLUME

(cm3)

Mild steel 1


Stainless steel 1

Zinc 0.1


Mild steel and stainless steel 2



Galvanised steel and zinc 1


Table 12: Sample density of samples before

immersion in acid test solution (0.4 M HCl)

SAMPLE Density

(g/cm3)

Mild steel 9.361



Zinc 12.83




Table 13: Sample density samples before

immersion in base test solution (20% NaOH)

SAMPLE Density

(g/cm3)

Mild steel 7.264



Zinc 13.38




http://en.m.wkipedia.org/wiki/%20Carbon

4.6 Measured weight loss

The calculated weight loss from weight of the sample before and after immersion in test solution

are tabulated in table 14 and table 15, and represented in figure 15 and figure 16. The results in

table 16 and 17 revealed that the weight loss increases with time. Stainless steel, welded mild steel

and stainless steel were observed to have the highest weight loss while zinc, welded mild steel and

zinc has the lowest weight loss in acid test solution. Mild steel, welded galvanised steel and zinc

were observed to have the highest weight loss while stainless steel, welded mild steel and stainless

steel has the lowest weight loss in base test solution.







Table 14: Weight loss of metal plates in acid test solution(0.4 M HCl)

SAMPLE 1h 2h 3h 4h 5h

Mild steel 0.021 0.037 0.041 0.086 0.091

Galvanised steel 0.045 0.046 0.05 0.064 0.069

Stainless steel 0.036 0.076 0.107 0.131 0.131

Zinc 0.028 0.028 0.028 0.03 0.033

Mild steel and galvanised steel 0.117 0.139 0.146 0.218 0.223

Mild steel and stainless steel 0.113 0.204 0.246 0.336 0.396

Galvanised steel and stainless

steel 0.094 0.141 0.165 0.236 0.281

Mild steel and zinc 0.153 0.165 0.182 0.191 0.203

Galvanised steel and zinc 0.21 0.221 0.225 0.24 0.244

Stainless steel and zinc 0.151 0.187 0.221 0.246 0.272

Table 15: Weight loss of metal plates in base media (20% NaOH)

SAMPLE 1h 2h 3h 4h 5h

Mild steel 0.033 0.051 0.055 0.06 0.066


Stainless steel 0.001 0.002 0.002 0.002 0.002

Zinc 0.004 0.007 0.016 0.021 0.024


Mild steel and stainless steel 0.018 0.026 0.037 0.038 0.057

Galvanised steel and stainless

steel 0.045 0.048 0.05 0.051 0.058


Galvanised steel and zinc 0.169 0.187 0.198 0.206 0.21

Stainless steel and zinc 0.105 0.12 0.126 0.176 0.193

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Mild steel Galvanisedsteel

Stainless Zinc Mild steel +galvanised

steel

Mild steel +stainless

Galvanisedsteel +

stainless

Mild steel +zinc

Galvanisedsteel + zinc

Stainless +zinc

Figure 15: Weight loss of metal plates in acid test solution (0.4 M HCl)

1h 2h 3h 4h 5h

4.7 Calculated Corrosion Rate

The calculated corrosion rate from the weight loss in test solutions are tabulated in table 16

and table 17 and, are represented in figure 17 and figure 18. The results in table 16 revealed

that the corrosion rate increases in this order; zinc < galvanised steel < mild steel < mild steel

and galvanised steel < mild steel and zinc < galvanised steel and zinc < galvanised and stainless

steel < stainless steel and zinc < mild steel and stainless steel < stainless. While the result in

table 17 revealed that the corrosion rate increases in this order; stainless steel < galvanised <

zinc <mild steel and stainless steel < galvanised and stainless steel < mild steel and galvanised

steel < mild steel < galvanised steel and zinc < mild steel and zinc < stainless steel and zinc.

0

0.05

0.1

0.15

0.2

0.25



steel


Galvanisedsteel +

stainless

Mild steel +zinc


Stainless +zinc

Figure 16: Weight loss of metal in base test solution (20% NaOH)

1h 2h 3h 4h 5h

Table 16: Corrosion rate of metal in acid test solution (0.4 M HCl)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3)

DENSIT

Y (g/cm3)

AREA

(cm2)

EXPOSURE

TIME (hrs)

CORROSIO

N RATE

(MPY)

Mild steel 0.091 1 9.361 10.5 5 638.82

Galvanised steel 0.069 0.5 12.13 9.2 5 426.63

Stainless steel 0.131 1 4.427 10.1 5 2021.57

Zinc 0.033 0.1 12.83 6.48 5 273.88

Mild steel and

galvanised steel 0.223 2.5 6.914 20.16 5 1103.91

Mild steel and

stainless steel 0.396 2 7.449 21.34 5 1803.41

Galvanised steel

and stainless

steel

0.281 1.5 6.629 18.84 5 1552.48

Mild steel and

zinc 0.203 1.5 6.118 20.52 5 1115.72

Galvanised steel

and zinc 0.244 1 7.5 20.11 5 1116.26

Stainless steel

and zinc 0.272 1.5 5.899 20.12 5 1581.29

Table 17: Corrosion rate of metal in base test solution (20%NaOH)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3) DENSITY (g/cm3)

AREA

(cm2)

EXPOSURE

TIME (hrs)

CORROSION

RATE (MPY)

Mild steel 0.066 1 7.264 10.48 5 598.21

Galvanised steel 0.025 0.5 11.554 9.2 5 162.28

Stainless steel 0.002 1 4.05 9.22 5 36.96

Zinc 0.024 0.1 13.38 6.78 5 182.55

Mild steel and

galvanised steel 0.086 2 8.571 19.68 5 351.8

Mild steel and

stainless steel 0.057 2 7.72 19.7 5 258.61

Galvanised steel

and stainless steel 0.058 1.5 6.737 18.42 5 372.54

Mild steel and zinc 0.201 1.5 6.918 20.08 5 998.39

Galvanised steel

and zinc 0.21 1 7.779 20.11 5 926.26

Stainless steel and

zinc

0.193 1.5 5.975 14.7 5 1516.18

0

200

400

600

800

1000

1200

1400

1600

Figure 18: Corrosion rate (mpy) in base media (20% NaOH)

0

500

1000

1500

2000

2500

Figure 17: Corrosion rate (mpy) in acid media (0.4 M HCl)

4.8 Calculated Metal Loss

The calculated metal loss from the weight loss in test solutions are tabulated in table 18 and

table 19, and are represented in figure 19 and figure 20. The results in table 18 revealed that

the metal loss increases in this order; zinc < galvanised steel < mild steel < mild steel and

galvanised steel < mild steel and zinc < galvanised steel and zinc < galvanised and stainless

steel < stainless steel and zinc < mild steel and stainless steel < stainless. While the result in

table 19 revealed that the metal loss increases in this order; stainless steel < galvanised < zinc

<mild steel and stainless steel < galvanised and stainless steel < mild steel and galvanised steel

< mild steel < galvanised steel and zinc < mild steel and zinc < stainless steel and zinc.

Table 18: Metal loss of metal in acid test solution (0.4 M HCl)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3)

DENSITY

(g/cm3)

AREA

(cm2)

METAL

LOSS (mils)

Mild steel 0.091 1 9.361 10.5 0.36


Stainless 0.131 1 4.427 10.1 1.15

Zinc 0.033 0.1 12.83 6.48 0.16


Mild steel and stainless 0.396 2 7.449 21.34 0.98

Galvanised steel and stainless 0.281 1.5 6.629 18.84 0.89


Galvanised steel and zinc 0.244 1 7.5 20.11 0.6369

Stainless and zinc 0.272 1.5 5.899 20.12 0.9

Table 19: Metal loss of metal in base test solution (20% NaOH)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3)

DENSITY

(g/cm3)

AREA

(cm2)

METAL

LOSS (mils)

Mild steel 0.066 1 7.264 10.48 0.34


Stainless 0.002 1 4.05 9.22 0.02

Zinc 0.024 0.1 13.38 6.78 0.1

Mild steel + galvanised steel 0.086 2 8.571 19.68 0.2

Mild steel + stainless 0.057 2 7.72 19.7 0.15

Galvanised steel + stainless 0.058 1.5 6.737 18.42 0.18

Mild steel + zinc 0.201 1.5 6.918 20.08 0.57

Galvanised steel + zinc 0.21 1 7.779 20.11 0.53

Stainless + zinc 0.193 1.5 5.975 14.7 0.86

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9



steel


Galvanisedsteel +

stainless

Mild steel +zinc


Stainless +zinc

Figure 20: Metal loss (mils) in base media (20% NaOH)

4.8 Assessment of Accelerated Galvanic Corrosion

The calculated corrosion rate of the individual dissimilar welded metals are tabulated in table

20 and table 21, and are represented in figure 21 and figure 22.

The result in table 21 and 22 revealed that accelerated galvanic corrosion occur in welded

samples of mild steel and galvanised steel, mild steel and stainless steel, mild steel and zinc,

galvanised steel and stainless, stainless steel and zinc. No accelerated galvanic corrosion was

observed in galvanised steel and zinc.

0

0.2

0.4

0.6

0.8

1

1.2


Stainless Zinc Mild steel+

galvanisedsteel

Mild steel+ stainless

Galvanisedsteel +

stainless

Mild steel+ zinc


Stainless +zinc

Figure 19: Metal loss (mils) in acid media (0.4 M HCl)

Where M is mild steel, G is galvanised steel, S is stainless steel and Z is zinc.

Table 20: Corrosion rate of individual dissimilar welded metals in acid test solution (0.4 M HCl)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3)

DENSITY

(g/cm3) AREA (cm2)

EXPOSURE

TIME (hrs)

CORROSION

RATE (MPY)

Mild steel and

galvanised steel

M=0.034

G =1.094

M = 1

G = 0.5

M=10.818

G =12.93

M=10.96

G=9.2 24

M=41

G=1317

Mild steel and

stainless steel

M=0.559

S=1.425

M = 1

S = 1

M=9.978

S=4.912

M=10.04

S=11.3 24

M=802

S=3691

Mild steel and

zinc

M=0.02

Z=0.506

M = 1

Z = 0.5

M=6.164

Z=6.026

M=10.04

Z=10.48 24

M=46

Z=1152

Galvanised steel

and stainless

steel

G=0.02

S=1.712

G = 0.5

S = 1

G=9.432

S=4.791

G=9.2

S=9.64 24

G=33

S=5329

Galvanised steel

and zinc

G=0.335

Z=0.3

G = 0.5

Z = 0.5

G=9.432

Z=5.508

G=9.63

Z=10.48 24

G=530

Z=739

Stainless steel

and zinc

S=2.075

Z=0.396

S = 1

Z =0.5

S=4.566

Z=8.864

S=9.64

Z=10.48 24

S=6791

Z=634

Table 21: Corrosion rate of individual dissimilar welded metals in base test solution (20% NaOH)

SAMPLE WEIGHT

LOSS (g)

VOLUME

(cm3)

DENSITY

(g/cm3) AREA (cm2)

EXPOSURE

TIME (hrs)

CORROSION

RATE (MPY)

Mild steel and

galvanised steel

M=0.028

G =0.659

M = 1

G = 0.5

M=10.966

G =12.35

M=10.48

G=9.2 24

M=35

G=834

Mild steel and

stainless steel

M=0.583

S=0.007

M = 1

S = 1

M=11.566

S=3.847

M=10.48

S=9.22 24

M=691

S=28

Mild steel and

zinc

M=0.111

Z=0.402

M = 1

Z = 0.5

M=7.381

Z=5.992

M=9.6

Z=10.48 24

M=225

Z=920

Galvanised steel

and stainless

steel

G=0.493

S=0.004

G = 0.5

S = 1

G=12.304

S=3.953

G=9.2

S=9.22 24

G=626.07

S=15.78

Galvanised steel

and zinc

G=0.356

Z=0.296

G = 0.5

Z = 0.5

G=9.984

Z=5.574

G=9.63

Z=10.48 24

G=532

Z=728

Stainless steel

and zinc

S=0.006

Z=0.897

S = 1

Z =0.5

S=4.791

Z=8.344

S=7.96

Z=6.74 24

S=22

Z=2293

0

1000

2000

3000

4000

5000

6000

7000

8000

Figure 21: Corrosion rate of individual dissimilar welded metals in acid test

solution (0.4 M HCl)

0

500

1000

1500

2000

2500

Figure 22: Corrosion rate of individual dissimilar welded metals in base test

solution (20% NaOH)

Un-welded metal plate Dissimilar welded metal plate

CHAPTER FIVE

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

Summarily it can be concluded that, the rate of corrosion of metal plates and dissimilar welded

metals is dependent on both the media and chemical properties of the steel. Within the limits of

experimental error, conclusion drawn based on observations and result of the research are:

In corroding media like used in this research, corrosion of metal depends the exposure time and

the standard electrode potential. The closer the standard electrode potential of dissimilar metals

the lesser the corrosion rate.

The corrosion rate of the metal plates and dissimilar welded metals immersed in acid test solution

(0.4 M HCl) increases in this order; zinc < galvanised steel < mild steel < mild steel and galvanised

steel < mild steel and zinc < galvanised steel and zinc < galvanised and stainless steel < stainless

steel and zinc < mild steel and stainless steel < stainless. The corrosion rate of the metal plates and

dissimilar welded metals immersed in base test solution (20% NaOH) increase in this order;

stainless steel < galvanised < zinc <mild steel and stainless steel < galvanised and stainless steel <

mild steel and galvanised steel < mild steel < galvanised steel and zinc < mild steel and zinc <

stainless steel and zinc.

Accelerated galvanic corrosion occur in welded samples of mild steel and galvanised steel, mild

steel and stainless steel, mild steel and zinc, galvanised steel and stainless, stainless steel and zinc.

No accelerated galvanic corrosion was observed in galvanised steel and zinc.

5.2 Recommendation

A satisfactory performance against galvanic corrosion can only be achieved if the following

recommendations and methods to control corrosion are followed:

i. Select metals/alloys as close together as possible in the galvanic series.

ii. Avoid unfavourable area effect of a small anode and large cathode.

iii. Insulate dissimilar metals wherever practical.

iv. Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good

repair on the anode.

v. Avoid threaded joints for materials far apart in the galvanic series

Suggestions for Future Work

i. A better and modern method of checking corrosion rate asides weight loss method should

be employed in future investigations.

ii. It is suggested that future researchers should try developing a model to better interpret the

effect of bimetallic welding on rate of corrosion.

iii. Future researchers are also encouraged to use the existing data in developing a model

capable of predicting time of failure of metals found in environments as investigated.

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ASSESSMENT OF CORROSION RATE OF DISSIMILAR WELDED METALS BY AFIGO OGHENEKARO SAMSON

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