CORROSION RATE INVESTIGATION DUE TO STRAY CURRENT

AHMAD FARIS BIN WAHID

UNIVERSITI TEKNOLOGI MALAYSIA

DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

Author’s full name : AHMAD FARIS BIN WAHID

Date of birth : 20th

NOVEMBER 1987

Title : CORROSION RATE INVESTIGATION DUE TO STRAY CURRENT

Academic Session: 2009/2010-2

I declare that this thesis is classified as :

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1. The thesis is the property of Universiti Teknologi Malaysia.

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the purpose of research only.

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exchange.

Certified by :

SIGNATURE SIGNATURE OF SUPERVISOR

871120-23-5095 Assoc. Prof. Dr. Zainal Zakaria

(NEW IC NO. /PASSPORT NO.) NAME OF SUPERVISOR

Date : 21th April 2010 Date : 21th April 2010

CONFIDENTIAL (Contains confidential information under the

Official Secret Act 1972)*

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the organization where research was done)*

OPEN ACCESS I agree that my thesis to be published as online

open access (full text)

NOTES: * If the thesis is CONFIDENTAL or RESTRICTED, please attach with the letter

from the organization with period and reasons for confidentiality or restriction.

“I hereby declare that I have read this thesis and in my

opinion this thesis is sufficient in terms of scope and quality for the

award of the Degree of Bachelor of Engineering (Chemical-Gas)”

Signature : …………………………..

Supervisor’s Name : Assoc. Prof. Dr. Zainal Zakaria

Date : April 2010

CORROSION RATE INVESTIGATION DUE TO STRAY CURRENT

AHMAD FARIS BIN WAHID

A report submitted in partial fulfillment of the requirements for the award of the

Degree in Bachelor of Engineering (Chemical-Gas)

Faculty of Chemical and Natural Resources Engineering

Universiti Teknologi Malaysia

APRIL 2010

ii

I declare that this thesis entitled “Corrosion Rate Investigation Due to Stray Current”

is the result of my own research except as cited in the references. The thesis has not

been accepted for any degree and not concurrently in candidature of any other degree

Signature : …………………………………….

Name : Ahmad Faris Bin Wahid

Date :

iii

To my beloved father and mother for her love and encouragement.

iv

ACKNOWLEDGEMENT

For the success of my study, it is a great pleasure to acknowledge all lecturers

from Gas Department of Faculty of Chemical and Natural Resources Engineering. I

would like to express my deepest appreciation to my supervisor, Assoc. Prof. Dr.

Zainal Bin Zakaria for his wise words, advices, sacrifices to help me in completing

this research with flying colours.

Though my parents may not have taught me to generate a study, their eternal

love, support, and confidence in their son contributed significantly in making me the

individual I am today. Finally I would like to express my appreciation to every

person who contributed with either inspirational or actual work to this study.

v

ABSTRACT

Failures in gas pipelines due to stray current can have severe environmental

and economic consequences. Gas pipeline can severely affected by stray currents

which travel onto a pipeline from nearby external sources. This study is being

conducted to investigate the corrosion rate under variety of external voltage (stray

current) between several pair of metal and investigate the effect of arrangement

metals in electrochemical series at constant stray current supply. The corrosion of

anode metal being categorized to its classes to see the corrosion level of anode metal

based on its corrosion rate. The study was done using Corrosion Kit. Therefore,

different pair of metals (steel and platinum, steel and steel, zinc and steel, copper and

steel, copper and brass) were examined in term of corrosion rate of anode and

cathode metal due to variation of stray current, the effect of arrangement metals in

electrochemical series at constant stray current supply and to classified the condition

of anode metal. All the samples were examined based on electrolytic cell in a test

solution of pH7.The corrosion rate is proportionally to the external voltage (stray

current) supply. From 0 voltage (V) to 4V, for pair of steel-platinum metal, the

corrosion anode metal is increasing from 0.30 millimeter per year (mm/yr) to 2.26

mm/yr while corrosion rate of cathode metal is increasing from 0 mm/yr to 0.006

mm/yr. At constant stray current supply, the corrosion rate at anode metal is

increasing as the distance between metals in electrochemical series increase where

corrosion rate of anode metal pair of steel-platinum metal at 4V is 2.26 mm/yr but

for pair of zinc-steel metal at 4V is 0.48 mm/yr. The classes of corrosion anode metal

define that at corrosion rate anode metal greater than 1.5 mm/yr, the corrosion rate of

anode metal is not satisfactory. For steel-platinum metal, steel metal having high

corrosion rate anode metal which at 3 V and 4V, where corrosion rate is 1.96 mm/yr

and 2.26 mm/yr.

vi

ABSTRAK

Pada masa kini, kegagalan paip gas disebabkan aliran arus sesat

mengakibatkn kesan terhadap persekitaran dan terhadap ekonomi. Paip gas akan

mengalami kesan pengaratan akibat aliran arus sesat yang mengalir di dalam paip gas

dari sumber aliran arus sesat. Kajian ini dilakukan bagi mengkaji kadar pengaratan

terhadap variasi aliran arus sesat bagi pelbagai pasangan logam dan untuk melihat

kadar pengaratan berdasarkan susunan pasangan logam di dalam siri

keelektromotifan pada aliran tetap arus sesat. Pengaratan bagi logam anod

dikategorikan ke dalam kelasnya untuk melihat tahap pengaratan anod metal

berdasarkan kadar pengaratam. Kajian telah dilakukan menggunakan Kit Pengaratan.

Oleh demikian, beberapa pasangan logam telah dipilih (besi dan platinum, besi dan

besi, zink dan besi, kuprum dan besi, dan besi dan loyang) untuk melihat kadar

pengaratan terhadap variasi aliran arus sesat bagi pelbagai pasangan logam dan

untuk melihat kadar pengaratan berdasarkan susunan pasangan logam di dalam siri

keelektromotifan pada aliran tetap arus sesat. Kesemua pasangan logam diuji

berpandukan sel elektrolitik dimana pH elektrolit adalah 7. Kadar pengaratan

berkadar langsung terhadap variasi arus sesat yang dibekalkan. Dari 0 voltan (V)

hingga 4V, bagi pasangan besi-platinum, kadar pengaratan logam anod meningkat

dari 0.30 milimeter setahun (mm/yr) kepada 2.26 mm/yr, bagi kadar pengaratan

logam katod meningkat dari 0 mm/yr kepada 0.006 mm/yr. Pada kadar aliran arus

yang tetap, kadar pengaratan logam anod meningkat apabila jarak logam dalam siri

keelektromotifan meningkat di mana kadar pengaratan logam anod pasangan besi-

platinum pada 4V adalah 2.26 mm/yr tetapi bagi pasangan zink-besi pada 4V adalah

0.48 mm/yr. Kelas pengaratan logam anod mengkelaskan kadar pengaratan logam

anod lebih dari 1.5 mm/yr, logam anod tersebut tidak memuaskan. Bagi pasangan

besi-platinum, logam besi mengalami kadar pengaratan yang tinggi pada 3V dan 4V

di mana kadar pengaratannya adalah 1.96 mm/yr dan 2.26 mm/yr.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES ix

LIST OF FIGURES x

LIS OF ABBREVIATIONS/SYMBOLS xii

LIST OF APPENDICES xiii

1 INTRODUCTION

1.1 Problem Background 1

1.2 Objectives of the project 3

1.3 Scope of the Research 3

2 LITERATURE REVIEW

2.1 Corrosion Reaction 5

2.2 Types of Corrosion 11

2.2.1 Uniform Corrosion 11

2.2.2 Pitting Corrosion 13

2.2.3 Bimetallic Corrosion 15

2.2.4 Stray Current Corrosion 17

2.3 Types of Stray Current Corrosion 20

viii

2.3.1 Direct Stray Current Corrosion 20

2.3.2 Alternating Stray Current Corrosion 23

2.3.3 Telluric Stray Current Corrosion 24

2.4 Controlling Stray Current Corrosion 25

2.5 Corrosion Rate of Metal 28

3 METHODOLOGY

3.1 Equipment Used 30

3.2 Electrode Material 30

3.3 Experimental Setup 31

3.4 Evaluation of Corrosion Rate 33

4 RESULT AND DISCUSSION

4.1 Effect Stray Current at Anode Metal 34

4.2 Effect Stray Current at Cathode Metal 36

4.3 Effect of Constant Stray Current to the Arrangement

of metal in Electrochemical Series 38

4.4 Classes of Corrosion Anode Metal 39

5 CONCLUSIONS AND RECCOMENDATION

5.1 Conclusions 42

5.2 Recommendation 42

REFERENCES 44

APPENDIX 46

ix

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 List of metals in electromotive series 9

2.2 Galvanic series 10

2.3 Materials failures in Japanese oil refineries 12

and petrochemical industry

4.1 Classes of corrosion rate anode metal 40

x

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Diagrammatic galvanic cell 8

2.2 Electromotive series behavior 10

2.3 Uniform corrosion behavior 11

2.4 Situation uniform corrosion 12

2.5 Pitting corrosion 15

2.6 Bimetallic corrosion 16

2.7 Stray current electrolysis 19

2.8 Complex stray current exposures 22

2.9 Anodic interference 22

2.10 Cathodic interference 23

2.11 Combined cathodic and anodic interference 26

2.12 Use of drainage bond or electrical bonding to mitigate

stray current discharge from the pipeline 27

2.13 Principle cathodic shield prevent anode interfere to

other pipeline 27

2.14 Use of extra sacrificial anode to mitigate cathodic

interference 56

3.1 Corrosion kit 30

3.2 Pair of steel and platinum metal 31

3.3 Pair of steel and steel metal 32

3.4 Pair of zinc and steel metal 32

xi

3.5 Pair of copper and steel metal 32

3.6 Pair of steel and brass metal 33

4.1 Corrosion rate of anode metal 35

4.2 Corrosion rate of cathode metal 37

4.3 Corrosion rate pair of metal due to arrangement in 38

electrochemical series

xii

LIST OF SYMBOLS

emf - Electromotive force

Erev - Reversible potential

AC - Alternating current

DC - Direct current

M - Metal

n - Electron charge

e - Electron

Cu2+

- Copper ion

Fe3+

- Iron ion

I - Current in ampere, Amp

F - Faraday

Ib - Pound

V - Voltage

g - Weight losses, gram

cm2

- Surface area of metal, cm2

+ - Anode connection

- - Cathode connection

mm/yr - Millimeter per year

ipy - Inches per year

xiii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A No external voltage supply 0V 47

B External voltage supply at 2V 49

C External voltage supply at 3V 50

D External voltage supply at 4V 52

E Effect corrosion rate anode metal to the variation

external voltage 53

F Effect corrosion rate cathode metal to the variation

external voltage 53

G Corrosion rate pair of metal due to arrangement in

electrochemical series 54

H Potential voltage pair of metal based on electrochemical

series 55

xiv

CHAPTER 1

INTRODUCTION

1.1 Problem Background

Corrosion is usually defined as the deterioration of a metal or its properties

caused by a reaction with its environment. Most metals occur naturally in the form of

oxides and are usually chemically stable. When exposed to oxygen and other

oxidizing agents, the refined metal will try to revert to its natural oxide state. In the

case of iron, the oxides will be in the form of ferrous or ferric oxide, commonly

known as rust. This situation will cause the loss of metal by conversion from element

state to chemically state. Metallic corrosion is caused by the flow of direct current

from one part of the metal surface to another. This flow of direct current causes the

loss of metal at the point where current discharges into the environment (oxidation or

anodic reaction).

Protection occurs at the point where current returns to the metal surface

(reduction or cathodic reaction). The corrosion process consists of an anodic and a

cathodic reaction. In the anodic reaction (oxidation) the metal is dissolved and

transferred to the solution as ions. The electrons released by the anodic reaction are

conducted through the metal to the cathodic area where they are consumed in the

cathodic reaction. A necessary condition for such a corrosion process is that the

environment is a conducting liquid (an electrolyte) that is in contact with the metal.

The electrical circuit is closed by ion conduction through the electrolyte. In

accordance with the conditions this dissolution process is called wet corrosion, and

2

the mechanism is typically electrochemical. Corrosion is express and measure in

term of corrosion rate. Normally, there are three main methods that are used to

express the corrosion rate. The corrosion rate is express in term of thickness

reduction of the material in unit time, weight loss per unit area and time and

corrosion current density (Einar, 2003). The most related for this study is

determining the corrosion rate by measuring weight loss per unit are and time.

Stray currents are currents flowing in the electrolyte from external sources,

not directly associated with the cathodic protection system. Any metallic structure,

for example, a pipeline, buried in soil represents a low-resistance current path and is

therefore fundamentally vulnerable to the effects of stray currents. Stray current

tends to enter buried structure in a certain location and leave it in another. It is where

the current leaves the structure that severe corrosion can be expected. Corrosion

damage induced by stray current effects has also been referred to as electrolysis.

Electrolysis process is a production of chemical changes in the electrolyte by the

passage of current through an electrochemical cell. An electrochemical cell can either

drive an external electrical device (load) or be driven by it (power supply), depending

upon the relative electromotive forces applied by the cell and the device. The current

that flows through a cell will produce an electrochemical reaction that follows the

principles of electrochemical stoichiometry. The electromotive force (emf) of the cell

is then called its reversible potential (Erev). An electrochemical cell can be described

as galvanic, reversible, or electrolytic: A galvanic cell is a cell in which current

flows, power is produced, and the cell reaction is proceeding spontaneously. An

electrolytic cell is a cell in which current flows, power is consumed, and the cell

reaction being driven is the reverse of the spontaneous cell reaction. A reversible cell

is a cell in which no current flows. The cell reaction in a reversible cell is neither

spontaneous nor non spontaneous; it is called reversible because an infinitesimal

change in the cell potential can cause it to proceed in either direction. Electromotive

force series (emf series) is a list of elements arranged according to their standard

electrode potentials, with noble metals such as gold being positive and active metals

such as zinc being negative. For the study and understanding of stray current effects

it is important to bear in mind that current flow in a system will not only be restricted

to the lowest-resistance path but will be distributed between paths of varying

3

resistance, as predicted by elementary circuit theory. Naturally, the current levels will

tend to be highest in the paths of least resistance (Roberge, 2000). Besides, for this

study the focus of the study in determines the rate of corrosion due to stray current is

based on electrolytic cell.

Literature and research supports the position that the steel in the gas pipelines

can and does carry electric current, often when the pipeline is within a high power

electric line easement, near an electric station or other source of high voltage Induced

AC current is not grounded adequately, the AC discharge on the pipeline can in the

long term, cause serious metal loss on the pipe wall and leaks. This problem has

become more acute due to the increased tendency to locate pipelines in utility

corridors near high-voltage electric transmission lines. The problem of stray current

and the harm it can cause to a dairy operation has been recognized in the courts. In

1999, a jury awarded a dairy farmer $700,000 after deciding stray current from an

automated feeding system slashed the herd's milk output and increased the death rate

among the Jersey cows (Smart et al, 1999).

1.2 Objective of the Project

Gas pipeline can severely affected by stray currents which travel onto a line

from nearby external sources. The objective of the study is to investigate the rate of

corrosion due to stray current.

1.3 Scope of the Research

The research will focus on the experimental process using SOLTEQ

Corrosion Study Kit. Below are the scopes:

i) To investigate corrosion rate under variety of external voltage (stray current)

between several pair of metals.

4

ii) To investigate effect of constant external voltage (stray current) to the pair of

metals in electrochemical series.

iii) To investigate classes of corrosion rate of anode metal.

CHAPTER 2

LITERATURE REVIEW

The effect of stray current to the pipeline will greatly affect the life span of

the gas pipeline. It may shorten the life period of the pipeline without notice.

Furthermore, this will lead to the severe danger to the surroundings if the pipeline

contains flammable substance. Besides it will give great effect to the economic

consequences. The stray currents represent a serious problem for each electric system

for transportation where the rails are the return conductor of traction circuit.

Especially for the traction systems, the stray currents create or accelerate the

electrolytic corrosion of metallic structures located in the proximity. In particular the

presence of structures in reinforced concrete requires a careful monitoring of the

scaffolds that in presence of stray currents can be damaged by corrosion phenomena

(Bertolini et al, 2006).

2.1 Corrosion Reaction

Corrosion specifically refers to any process involving the deterioration or

degradation of metal components. Corrosion may be defined as any reaction between

a metal and its environment which results in a transformation of the metallic to the

combined state. Nearly, all metals are inherently unstable and it is their tendency to

seek self destruction by reacting with their environment to attain a state of lower

energy by forming a metal compound. This is the state in which the majority of the

metals are found in nature as a mineral, a compound of the metal and a non metal.

6

This is usually accomplished by a reduction process in which energy (either chemical

or electrical) is supplied to the mineral to generate the metallic state. In this process

the metal is pushed up the energy hill into instability. This instability will then

demand that the metal endeavors to return to the combined state as fast as its

environment will allow the haven where its energy is at a minimum. The result of

this reaction is the formation of corrosion product, which is frequently a substance of

very similar decomposition to the original mineral. The basic cause of the corrosion

of metals is their spontaneous tendency to return to their stable state. Except for

noble metals, gold, platinum and possibly silver which have inherent thermodynamic

stability in most environments, practically all metals have intrinsic tendency for

corrosion. An example of this cycle is behavior of iron (steel). Many iron ores

contain iron in oxidized form (oxides and carbonates) which are reduced by carbon

to metallic iron. In the presence of moisture content of the iron, so obtained, will then

oxidized to rust (corrosion product). If the rust is analyzed, it will be found to be an

iron oxide of similar composition to the mineral hematite, though it must be

emphasized that rust is a far more complex substance than Fe2O3 (Sheir, 1968).

Corrosion cell can be described by Figure 2.1. Figure 2.1 shows that

bimetallic galvanic cell. A pair of copper and iron in sodium chloride but it must be

stressed that two different metals are not essential for a corrosion cell to occur, where

corrosion can take place on the exposed surface of a single metal. The fact remains,

however, that the components of corrosion cells are similar to those of the cell now

to be described in which for ease of definition, the electrodes have been made

physically different. If a piece of clean, uncoated pure iron with a homogeneous

surface is immersed in a solution of sodium chloride in contact with oxygen the

corrosion reaction will be relative slow. Similarly, if a piece of clean, pure copper

with a homogeneous surface is placed in an adjacent location in the same solution

but so that the metals are not touching, the immediate corrosion reaction on the

copper will be very slow. If now the two metals are join together by an external

electrical path the corrosion rate of the iron will immediately increase by a very large

factor and corrosion on the copper will be stopped. If Figure 2.1 is consulted it will

be seen that the cell has several well defined component parts. Four of these are

important. The iron electrode which leads the conventional current into the cell. This

7

is the anode and has a negative polarity and is the electrode at which oxidation of the

metal where corrosion takes place. The copper electrode which leads the

conventional current out of the cell. This is the cathode, which has a positive polarity

and is the electrode at which a process of reduction will occur, so that corrosion is

prevented. The saline solution in which both the anode and the cathode are bathed.

This is the electrolyte solution and is the vehicle which carries the internal current

flow in the cell. The external low resistance metallic path which carries the electronic

current which flows in the opposite sense to the conventional current. Corrosion

normally occurs at anodes, though there are certain exception to this rule. The

polarity may be positive or negative depending on the type of cell. Cells may be

either galvanic where current is generated by an electrochemical reaction or

electrolytic where an electrochemical reaction is made to take place by means of

external source of e.m.f. but irrespective of the polarity the electrode which leads the

electrons out of the cell into the external metallic circuit is the anode as shown in

Figure 2.1 and the electrode by which the electrons enter the cell is the cathode

(Rogers, 1968) .The chemical reaction occurs in Figure 2.1 are

Anode reaction:

Fe Fe2+

+ 2e-

Cathode reaction:

1/2O2 + H2O + 2e- 4OH

-

Figure 2.1 Diagrammatic galvanic cell

8

Table 2.1 shows the list of metals in electromotive series and the standard

potential voltage of the metals. Normally this series is used for electrolytic cell. Such

measurements lead to the arrangement of redox (process oxidation and reduction)

couples into an electromotive force (emf) series which allows the prediction of

reaction spontaneity between species or the identification of possible oxidising and

reducing agents in order to determine the corrosion mechanisms (Bertolini, 1993).

Figure 2.3 shows the electromotive series behavior where the upper part is known as

anode due to its electronegativity than lower part as cathode. List of metals whose

order indicates the relative tendency to be oxidized, or to give up electrons. The list

also includes the gas hydrogen. The electromotive series begins with the metal most

easily oxidized, the metal with the greatest electron-donating tendency, and ends

with the metal least easily oxidized. The tendency to be oxidized is not an absolute

quantity; it can only be compared with the tendency of some other substance to be

oxidized. In practice, the tendency to be oxidized, called the oxidation potential and

expressed in volts, is measured relative to a standard hydrogen electrode, which is

arbitrarily assigned an oxidation potential of zero. The oxidation potential measures

the tendency of the half reaction M→M+n

+ne− to occur, in which some metal M loses

n electrons, e−, and acquires a positive charge of +n. The more positive the oxidation

potential, the more readily oxidation takes place. The electromotive series is thus a

list of the metals in the order of their tendency to undergo the half reaction. The

series is also called the replacement series, since it indicates which metals replace, or

are replaced by, other metals (or hydrogen) in compounds. In general, a metal will

replace any other metal lower in the series and will be replaced by any metal higher

in the series. The order of some common metals in the electromotive series, starting

with the most easily oxidized, is: lithium, potassium, calcium, sodium, magnesium,

aluminum, zinc, chromium, iron, cobalt, nickel, lead, hydrogen, copper, mercury,

silver, platinum, and gold. A list arranged according to oxidation potential and

including not only metals but also all other elements and ions is called the

electrochemical series (Sheir, 1968).

Table 2.2 shows the galvanic series where the galvanic series is said

commonly used for galvanic cell where a metal nearer to cathodic end of the series

9

will tend to receive protection and the metal nearer the anodic end will tend to suffer

corrosion. It must be stressed that any galvanic series though it may indicate the

tendency of the more anodic member of the couple to corrode more quickly in terms

of the probable e.m.f(electromotive force) of the corrosion cell formed.Since many

practical corrosion failures are associated with the coupling of metallic areas of

different potential with the consequent formation of a corrosion cell, it is often

thought that the corrosion intensity of a metallic couple might be estimated using

standard electrode potentials that known as electromotive series. Unfortunately this is

not possible. The standard electrode potentials are obtained from thermodynamics

data and although they may be used to predict the tendency of reactive metals to

corrode they cannot be used to predict the rate of corrosion or its location. It goes

same as galvanic series Neither (anode and cathode) can these value be used to

predict with exactitude which electrode couple of a couple will be anode or cathode

in electrolytes which differ from the standard conditions for electromotive series.

Galvanic series do not predict the rate at which attack will occur. This depends on

the current that flows in the cell (Rogers, 1968).

Table 2.1 List of metals in electromotive series

List of metals E° (volts)

-3.03

-2.92

-2.87

-2.71

-2.37

-1.66

-0.76

-0.44

-0.13

0

+0.34

+0.80

+1.50

10

Figure 2.2 Electromotive series behavior

Table 2.2 Galvanic series

Platinum Cathodic

Gold

Graphite

Titanium

Silver

Stainless Steel

Nickel

Copper & its alloy

Tin

Lead

Cast iron

Steel or iron

Aluminum alloys

Steel or iron

Zinc

Magnesium alloys Anodic

2.2 Types of Corrosion

Generally, all corrosion is interrelated. However, it can take many forms.

Corrosion can be uniform or localized. It may also combine with other forms of

attack to produce even more undesirable effects.

11

2.2.1 Uniform Corrosion

This is characterized by an even or equal distribution of corrosion that leaves

the surface clean or coated with corrosion products. In simple meaning uniform

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

surface. This even or equal distribution is due to the movement of the anodic and

cathodic sites on the metal’s surface. With uniform attack, fouling of the metal is

usually a bigger problem than failure. Uniform (or general) corrosion refers to the

relatively uniform reduction of thickness over the surface of a corroding material. It

is relatively easy to measure, predict and design against this type of corrosion

damage. Uniform corrosion is usually controlled by selecting suitable materials,

protective coatings, cathodic protection and corrosion inhibitors (Graver, 1985).

Figure 2.3 shows the uniform corrosion behavior where the thickness of the metals

reduces uniformly. While in Figure 2.4 shows the real picture of the uniform

corrosion for one of the port concrete.

Figure 2.3 Uniform corrosion behavior

Figure 2.4 Situation uniform corrosion

12

Mechanism of uniform corrosion occurs when there are local anodic and

cathodic sites on the surface of the metal. Due to polarization effects, these locations

shift from time to time and a given area on a metal will be act as both an anode and

as a cathode over any extended period of time. The averaging effect of these shifting

local action cells results in a rather uniform attack and general loss of material and

roughening of the surface. Significant measures uniform corrosion is weight loss

approach. The most commonly used method of measuring the corrosion rate of

metals when uniform corrosion occurs. In this method, a test sample is cleaned,

weighed, and its surface area is measured. It is then exposed for a specific period of

time, re-cleaned and re-weighed. The amount of metal loss as measured by the

weight loss is used to calculate the loss in thickness of the metal assuming that the

corrosion was absolutely uniform. In some cases this is further verified by thickness

measurements. These results are commonly expressed in “Mils per Year” or

“Microns per Year.” It must be remembered that these rates are usually calculated

from weight loss rather than thickness loss and are only valid if the attack was

uniform. Table 2.3 shows materials failures in Japanese oil refineries and

petrochemical industry statistic of various type of corrosion. From that figure

uniform corrosion is assumed to be the most common form of corrosion and

particularly responsible for most of the material loss. Traditionally, however, it is not

recognized as a dangerous form of corrosion, because: of the prediction of thickness

reduction rate can be done by means of simple tests. Corresponding corrosion

allowance can be added, taking into account strength requirements and lifetime.

Besides, available protection methods are usually so efficient that the corrosion rate

is reduced to an acceptable level. Actual methods are application of coatings,

cathodic protection or possibly change of environment or material. By use of modern

materials with higher and more precisely defined strength, and higher cost, uniform

corrosion has required increased attention. This is so because the strength data

correspond to thinner and more accurately determined material thickness, and the

material cost makes a thinner corrosion allowance attractive (Einar, 2003).

13

Table 2.3 Materials failures in Japanese oil refineries and

petrochemical industry

2.2.2 Pitting Corrosion

Pitting corrosion (sometimes only called pitting) occurs on more or less

passivated metals and alloys in environments containing chloride, bromide, iodide or

perchlorate ions when the electrode potential exceeds a critical value, the pitting

potential, which depends on various conditions. The pitting potential is not a

thermodynamically defined potential and depends for one thing upon the rate of

potential increase when the polarization curve is recorded. As in Figure 2.5 shows

pitting corrosion where the small hole form of corrosion is characterized by narrow

pits with a radius of the same order of magnitude as, or less than, the depth. The pits

may be of different shape, but a common feature is the sharp boundary. Pitting is a

dangerous form of corrosion since the material in many cases may be penetrated

without a clear warning (because the pits often are narrow and covered) and the pit

growth is difficult to predict. This is connected to the fact that the extent and the

intensity of pitting corrosion is difficult to measure because the number and size of

14

pits (diameter and depth) vary from region to region and within each region. Short-

term testing in the laboratory for determination of pit growth is also problematic

because, under realistic conditions, it may take long time, such as many months,

before the pits become visible. Another problem is that the critical size, such as. The

maximum pit depth, increases with increasing surface area.

Factors of general significance for pitting are pH and chloride concentration.

The pitting potential and pitting resistance normally increase with increasing pH and

decreasing chloride concentration. Besides, it depends on flow velocity. When pitting

is initiated, an aggressive environment is established within the pit, and this is

combined with an increased pH at the adjacent free surface. The increased pH gives

an increased resistance to initiation of new pits around the first one formed. If there

is no movement of the liquid, these conditions will be conserved, and the result is

large and few pits. Liquid flow gives a higher probability for washing away the

aggressive environment in the pit, and at the same time it increases the transport of

oxygen to the active area, so that the pit may be passivated before it gets the chance

to grow to a considerable size. The alkaline layer around an active pit is also washed

away to a higher extent, and pit initiation in the neighbourhood of an active pit

occurs more easily. This means that increased flow velocity results in smaller but

more numerous pits, which makes pitting corrosion less serious. Next, the gravity

force. Horizontal top surfaces are often heavily pitted, while underside surfaces are

hardly, or not at all, attacked. Vertical surfaces are intermediate as to the extent of

pitting. The reason is that the aggressive environment in the pits has a higher density

than pure water. Cu2+

and Fe3+

ions favour pit initiation and accelerate pit growth

because they lead to increased potential (they are oxidizers).The Cu ions have a

double effect because Cu is precipitated on the material surface and forms efficient

cathodes. For metallurgical properties as a factor influence the pitting corrosion.

Impurities and inclusions are important such as AlFe secondary phase, which

contributes to the localization of pits on aluminium. It is because of the oxide film on

top of and around the inclusion is weak, thin and stressed and such inclusions are

efficient cathodes. Pitting of this type will usually last only until the pits are so large

that the inclusions are falling out. Next, is about the insulating ability of the oxide. If

the oxide insulates efficiently, the surface is inactive as a cathode. This is to a certain

15

extent the case for aluminium in seawater; which is the direct reason for the relative

slow growth of pits in this case. For Surface roughness, the main trend is that smooth

surfaces get few, large pits while rough surfaces get numerous smaller pits. For

temperature factor, increasing temperature gives usually decreasing pitting potential

and increasing liability to pitting corrosion Lastly, galvanic contact with a more

noble material increases the tendency to and the rate of pitting corrosion (the

corrosion potential is lifted (Einar, 2003).

Figure 2.5 Pitting Corrosion

2.2.3 Bimetallic Corrosion

Bimetallic corrosion occurs when two metals, with different potentials, are in

electrical contact while immersed in an electrically conducting corrosive liquid.

Figure 2.6 shows the bimetallic corrosion process where the metals have different

natural potentials in the liquid, a current will flow from the anode (more

electronegative) metal to the cathode (more electropositive), which will increase the

corrosion on the anode. The effect of coupling the two metals together increases the

corrosion rate of the anode and reduces or even suppresses corrosion of the cathode.

This additional corrosion is bimetallic corrosion. It is also referred to as a galvanic

corrosion, dissimilar metal corrosion or contact corrosion. In general, the reactions

which occur are similar to those that would occur on single, uncoupled metal, but the

rate of attack is increased, sometimes dramatically. With some metal combinations

the change in the electrode potential in the couple potential can induce corrosion

16

which would not have occured in the uncoupled state (like pitting corrosion). In

some environments the change in potential of the cathode in the couple can also

introduce problems such as hydrogen embrittlement. Hydrogen embrittlement is the

process by which various metals, most importantly high-strength steel, become

brittle and crack following exposure to hydrogen. The basic requirements necessary

to cause bimetallic corrosion are an electrolyte bridging the two metals which may

not always be aggressive to the individual metals when they are not coupled, and

may be in the form of a bulk volume of solution, a condensed film, or a damp solid

such as soil, salt deposits, or corrosion products. Besides, 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 structural metal work or electrical earthing. It is not

necessary for the metal junction to be immersed in the electrolyte. Furthermore, a

sufficient difference in potential between the two metals to provide a significant

galvanic current (Rogers, 1968).

Figure 2.6 Bimetallic corrosion

2.2.4 Stray Current

Stray currents are currents flowing in the electrolyte from external sources.

Any metallic structure, for example a pipe line, buried in soil represents a low

resistance current path and is therefore fundamentally vulnerable to the effects of

stray currents. Stray current tends to enter a buried structure in a certain location and

17

leave it in another. It is where the current leaves the structure that severe corrosion

expected. Overprotection might also occur at a location where the high current

densities of stray current enter a structure. There are a number of source of

undesirable stray currents, including foreign cathodic protection installations, dc

transit systems such as electrified railways, subway systems, and streetcars, welding

operations, and electrical power transmission systems.

If an electric current flowing along a metallic conductor leaves the metal for a

water path it will cause ionization of the metal and an area of rapid corrosion (area of

high anodic potential will occur). In ships the most common cause of stray current is

the use of hull as an earth. This procedure is unobjectionable only as long as the

return current does not flow wholly or in part along a sea water path. For example, if

a return current flowing along a piping system carrying sea water meets some

electrical resistance which it jumps by flowing through the sea water a perforation of

the pipe wall can confidently be expected. The phenomenon is most likely to arise if

the supply is direct current but as most alternating current contains some direct

current component. One of the most frequent causes of stray current is welding

machines incorrectly earthed when used on ships alongside jetties, especially if two

ships welded from the same generator set. Corrosion by stray currents is usually

related directly to Faraday’s Law. If current(I) amps passes, the amount of corrosion

suffered will be I/F gram-equivalents per second, the Faraday (F) is 96500 coulombs

per gram equivalent, thus 1 ampere flowing for one year will corrode approximately

20Ib of iron, 22Ib of copper or 75Ib of lead (Graver, 1985).

Cathodic protection was an offshoot of the studies on interference corrosion

caused by the many electric railways and trolley lines which spread over the United

States in the early part of this century. The direct current discharged into the ground

caused many failures of nearby piping systems. Some of the electrical currents which

corrode pipe lines are those which arise from galvanic potential differences between

various parts of the structure in contact with the earth. Others, however, are the result

of the leakage of current from some electrical system so that part of the current path

is through the earth. Whenever a pipe line lies within such as current path, there is an

opportunity for current to enter and leave the line; at the points where it leaves, it will

18

corrode the pipe. Because of the inherently accidental or unintentional nature of such

currents, they are usually known as stray currents, and the damage they do is known

as stray-current electrolysis. By far the greatest source of stray current is the electric

railway, or its urban counterpart, the street car (Einar, 2003).

Figure 2.7 shows stray current electrolysis, return current from streetcar

divides, part going back to substation along rails, and part leaking off rails onto

pipeline. Near the substation this current flows from the pipe line through the soil to

the rail system, causing corrosion of the pipe. Installation of a metallic bond from the

pipe to the negative bus at the substation will avert the damage in simplified form the

route taken by the current. It should be noted that the rail is supposed to provide the

return path; it might be assumed that, if the rail joints were adequately bonded, no

trouble would result. This, however, is not true. The electric current does not follow

the path of least resistance. At least it does not all follow that path. Electric current,

when offered two or more parallel paths, divides itself between them inversely as the

resistance. If the rail path has one-tenth the resistance of the earth path, then it will

carry 10 times as much current but the remaining portion, the one eleventh of the

total current which flows through the earth, is sufficient to do a great deal of damage.

The situation is not always as simple as that shown in the diagram. Sometimes the

affected line may lie at a considerable distance from the tracks; perhaps the point of

attack is near the crossing of two lines where some current leaves one line and enters

the other. Figure 2.8 shows a complex stray current exposure, the point at which

damage occurs to pipe line "A" is remote from the offending street railway tracks.

Such a condition may be detected by fluctuating potentials or currents in pipe line

"A" and located by a line current survey. a case which proved to be very difficult to

detect, in view of the distance from the nearest track to the point of damage; yet the

relation is clear, once it has been traced out and diagrammed. There are other

possible sources of stray current in addition to electric railways. Almost any DC

power network is capable of causing damage in this way, although most of them are

innocent. Mine railways, cranes, and other machinery using DC should be suspected.

Frequently, there are severe exposures in and near chemical plants using electrolytic

processes. Welding equipment, particularly when employed in production work, is a

common source of trouble, although rarely is the damage done at any great distance

19

from the equipment; this makes it much easier to locate. Finally, it is unfortunately

true that the direct currents introduced deliberately into the earth for the purpose of

applying cathodic protection to one structure, are capable of doing great damage to

other structures which occupy the same earth. This is not strictly stray current; the

damaging effect is altogether accidental, but the presence of the current in the earth is

not accidental (Marshal, 1954).

Figure 2.7 Stray current electrolysis

Figure 2.8 Complex stray-current exposures.

2.3 Types of Stray Current

There are a number of sources of undesirable stray currents, including foreign

cathodic protection installations, dc transit systems such as electrified railways,

subway systems, and streetcars, welding operations, and electrical power

20

transmission systems. Stray currents can be classified into three categories. There are

direct stray current corrosion, alternating stray current corrosion and telluric stray

current corrosion. Stray current control is essential in direct current (DC) mass transit

system where the rail insulation is not sufficient quality to prevent corrosion risk to

the rails, supporting and third party infrastructure. For a transit system place above

ground, the primary corrosion is again to the rail and the fixings as well as to any

nearby utility cables and pipes (Cotton et al, 2005). Effects of stray current on the

corrosion behavior of steel in concrete have been studied, with regards to both

corrosion initiation and propagation. Results showed that DC current can induce

corrosion initiation on the reinforcement in the anodic zone only after it has

circulated for a certain time, which depends on the anodic current density, the

presence of chloride in the concrete and interruptions in the current. AC current

proved to be much less dangerous than DC, although it can influence the corrosion

rate of steel in chloride-contaminated concrete and stimulate macrocouples

(Bertolini, 2006).

2.3.1 Direct Stray Current Corrosion

Typically, direct stray currents come from cathodic protection systems, transit

systems, and dc high-voltage transmission lines. A distinction can be made between

anodic interference, cathodic interference, and combined interference. Anodic

interference is found in relatively close proximity to a buried anode, under the

influence of potential gradients surrounding the anode. Figure 2.9 shows a pipeline

will pick up current close to the anode. This current will be discharged at a distance

farther away from the anode. In the current pickup region, the potential of the

pipeline subject to the stray current will shift in the negative direction in essence it

receives a boost of cathodic protection current locally. This local current boost will

not necessarily be beneficial, because a state of overprotection could be created.

Furthermore, the excess of alkaline species generated can be harmful to aluminium

and lead alloys. Conversely, in areas where the stray current is discharged, its

potential will rise to more positive values. It is in the areas of current discharge that

anodic dissolution is the most severe. Cathodic interference is produced in relatively

21

close proximity to a polarized cathode. Figure 2.10 shows the current will flow away

from the structure in the region in close proximity to the cathode. The potential will

shift in the positive direction where current leaves this structure and this area

represents the highest corrosion damage risk. Current will flow onto the structure

over a larger area, at further distances from the cathode, again with possible

damaging over protection effects. An example of the combined anodic and cathodic

interference. In Figure 2.11 shows current pickup occurs close to an anode, and

current discharge occurs close to a cathodically polarized structure. The degree of

damage of the combined stray current effects is greater than in the case of anodic or

cathodic interference acting alone. The effects are most pronounced if the current

pickup and discharge areas are in close proximity. Correspondingly, the damage in

both the current pickup (overprotection effects) and discharge regions (corrosion)

will be greater (Roberge, 2000). In the case of DC, a cathodic reaction example

oxygen reduction or hydrogen evolution takes place where the current enters the

buried structure, while an anodic reaction (e.g., metal dissolution) occurs where the

current returns to the original path, through the soil. Metal loss results at the anodic

points, where the current leaves the structure; usually, the attack is extremely

localised and can have dramatic consequences especially on pipelines (Bertolini et

al, 2006).

Figure 2.9 Anodic interference

22

Figure 2.10 Cathodic interference

Figure 2.11 Combined cathodic and anodic interference

2.3.2 Alternating Stray Current Corrosion

There is an increasing trend for pipelines and overhead power lines to use the

same right of way. Alternating stray current effects arise from the proximity of

buried structures to high-voltage overhead power transmission lines. There are two

dominant mechanisms by which these stray currents can be produced in buried

23

pipelines: electromagnetic induction and transmission line faults. In the

electromagnetic coupling mechanism, a voltage is induced in a buried structure under

the influence of the alternating electromagnetic field surrounding the overhead

transmission line. This effect is similar to the coupling in a transformer, with the

overhead transmission line acting as the primary transformer coil and the buried

structure acting as the secondary coil. The magnitude of the induced voltage depends

on factors such as the separation distance from the power line, the relative position of

the structure to the power lines, the proximity to other buried structures, and the

coating quality. Such induced voltages can be hazardous to anyone who comes in

contact with the pipeline or its accessories. The second mechanism is one of resistive

coupling, whereby ac currents are directly transmitted to earth during transmission

line faults. Causes of such faults include grounding of an overhead conductor,

lightning strikes, and major load imbalances in the conductors. Usually such faults

are of very short duration, but due to the high currents involved, substantial physical

damage to coated structures is possible. Ancillary equipment such as motorized

valves, sensors, and cathodic protection stations could also be damaged. These faults

represent a major threat to human and animal life, even if no contact is made with the

pipeline (Roberge, 2000). Effects of stray AC currents are more complex; AC can

influence the anodic behaviour of steel and thus may increase the corrosion rate of

steel, as well as galvanic effects. Nevertheless, steel in soil is usually under cathodic

protection and it was shown that stray AC current can induce corrosion only in those

particular circumstances where very high currents are picked up by the buried

structure, so that the current density in the points where the current enters and leaves

the structure is extremely high. A threshold ranging between 20 and 100A/m2 has

been proposed (Bertolini, 2006).

2.3.3 Telluric Stray Current Corrosion

These stray currents are induced by transient geomagnetic activity. The

potential and current distribution of buried structures can be influenced by such

disturbances in the earth’s magnetic field. Such effects, often assumed to be of

greatest significance in closer proximity to the poles, have been observed to be most

24

intense during periods of intensified sun spot activity. In general, harmful influences

on structures are of limited duration and do not remain highly localized to specific

current pickup and discharge areas. Major corrosion problems as a direct result of

telluric effects are therefore relatively rare. The geomagnetic field varies with time,

generating electric fields and currents in power transmission systems and pipelines

(oil, gas, water). Geomagnetically induced currents (GIC) and their effects on

technological systems have previously been studied at high latitudes where the

effects of geomagnetic disturbances can be large. There have been few reports of

GIC effects in pipelines at low latitudes. Australia has the majority of its population

distributed along the coastal regions. Pipelines are essential for transporting

resources to these centres. Enertrade is responsible for the gas pipeline in north

Queensland which runs from Townsville, south east to Moranbah which is located

south west of Mackay.

The pipeline has a cathode protection system that maintains the pipe to soil

voltage in order to inhibit corrosion. Pipeline voltage measurements have been

compared with the magnetometer data located at Townsville. In this study the

pipeline data were provided at a sample interval of 60sec. Two data intervals are

presented. The first is for geomagnetic quiet conditions while the second is for data

recorded during a geomagnetic disturbed interval. Following a similar approach to

high latitude data, found a moderate correlation between the time varying magnetic

field and the pipeline voltage (correlation coefficient ~0.4). However, that there is a

diurnal variation in the pipeline voltage data which is in phase with the

magnetometer signature of the Sq currents that are routinely detected at low latitudes.

The Australian Standard AS2885.1-1997 Section 5.6.5.3 states that the potential of

structures subject to telluric variations shall not be less negative than -0.85 volts for

more than 10% of the test period and the average potential over the test period must

be more negative than -0.85 volts. Electromagnetic coupling between currents in

space (the ionosphere) and pipelines cause currents and associated voltages in the

pipeline. Estimating the relationship between the space currents and pipeline voltage

variations can be divided in to two parts. Determining the geo electric field at the

surface of the earth and computing the associated currents and voltages using Ohm

and Kirchhoff Laws in the pipeline (Waters, 2006).

25

2.4 Controlling Stray Current

Stray current need to be control in a way to prevent the pipeline or others

metal instrument being corroded. Way to control stray current is by removing the

stray current source. But this method is not preferable or significant because the

needs of the source to evaluate other material such as cathodic protection as the

source of the stray current. Indeed the cathodic protection is use to protect the target

pipeline. By removing the cathodic protection it may danger the protected pipeline.

Using electrical bonding is the establishment of an electrical connection between the

interfering and interfered-with structure is a common remedial measures. Figure 2.9

shows how the interference problem presented in Figure 2.12is mitigated by an

electrical bond created between the two structures a variable resistance may be used

in the bonded connection. Also-called forced drainage bond imposes an additional

external potential on the bond to assist stray current drainage through the bond

(Roberge, 2000).

Figure 2.12 Use of drainage bond or electrical bonding to mitigate stray

current discharge from the pipeline.

In cathodic shielding the aim is to minimize the amount of stray current

reaching the structure at risk. A metallic barrier (or “shield”) that is polarized

cathodically is positioned in the path of the stray current as shown in Figure 2.13.

The shield represents a low-resistance preferred path for the stray current, thereby

minimizing the flow of stray current onto the interfered-with structure. If the shield is

connected to the negative terminal of the power supply of the interfering structure, its

26

effects on the protection levels of the interfering structure have to be considered;

these will obviously be reduced for a given rectifier output.

Figure 2.13 Principle cathodic shield prevent anode interfere to other

pipeline.

Sacrificial anodes can be installed at the current discharge areas of interfered-

with structures to mitigate stray current corrosion. This mitigation method is most

applicable to relatively low levels of stray currents. As shown in Figure 2.14 the

current is discharged from these anodes rather than from the structure at risk. The

importance of placing the anodes close to the interfering structure is apparent: to

minimize the resistance to current flowing from the anodes. The galvanically less

noble anodes will generate a cathodic protection current, thereby compensating for

small amounts of residual stray currents that continue to be discharged from the

interfered-with structure (Roberge, 2000).

Figure 2.14 Use of extra sacrificial anode to mitigate cathodic interference

27

AC induced stray current effects can be reduced by locating buried structures

sufficiently far away from power lines. Non metallic pipeline construction materials

such as high-density polyethylene can be used in some cases, where operating

pressures are low. The highest potential shifts occur on metallic structures that have

high-integrity coatings. With high-quality coatings, grounding of the pipeline is

clearly limited. Obviously, it is generally not desirable to sacrifice coating quality to

reduce the magnitude of these effects. A similar reduction of the induced potential

shifts can be achieved with distributed sacrificial anodes attached to the structure.

These anodes provide cathodic protection current and reduce the resistance to

ground, which is fundamentally desirable to minimizing the AC-induced voltage

shifts. The use of such anodes will also tend to mitigate the influence of telluric

effects (Sheir, 1968).

2.5 Corrosion rate of metal

There are many ways of recording the extent or progress of corrosion. There

are however, two ways in which the corrosion behaviour of a metal can be evaluated

so that the results may be meaningful for corrosion controls studies, although,

unfortunately, both methods suffer from lack of precision. One approach is to

measure the change in weight of a test specimen with time. If specimens are weighed

while free corroding the change in weight can be either a gain or a loss depending on

the adherence or otherwise corrosion product. In the presence of films or scales

quantitative estimation of the metal lost is difficult, owing to variations in the

chemical compositions of the residues and the possibility that the corrosion

production are mixtures of several compounds, with a degree of hydration depending

on the conditions. For these reasons it is much more be accurate to determine the

amount of the metal removed by the corrosion reaction by weighing what is left of

the specimen the removal of all the adherent corrosion products. The results are then

expressed as weight loss (milligrams) per surface area (square decimetres) per

standard time (day of 24 hours), usually written as mdd. The second approach which

indicated corrosion measurement in which the attack is indicated on some scale

which relates depth of attack to time, for example inches per year and mils per year.

In general, the rate of corrosion can be calculated using Equation (2.1)

28

Corrosion rate = Average initial weight metal – average final weight metal (2.1)

Day between inspections

Where,

Corrosion rate = weight loss/surface area and time (g/cm2.day)

Average weight metal = weight loss/surface area (g/ cm2)

Day between inspections = period of inspection (day)

CHAPTER 3

METHODOLOGY

3.1 Equipment Used

Corrosion study kit as shown in Figure 3.1 is the best way to investigate the

corrosion rate due to effect of stray current. This equipment used by numerous of

people to evaluate corrosion behavior. The weight loss of the metal due to stray

current corrosion is evaluated.

Figure 3.1 Corrosion kit

3.2 Electrode Materials

There are five set of electrodes being used for this experiment. The first set is

between steel and platinum. The second set is between steel and steel. The third set is

between zinc and copper. The fourth set is between copper and steel. The last set is

30

between steel and brass. This all 5 set applied on different parameters relatively

based on the scope stated.

3.3 Experimental Setup

This experiment is tested for different pair of metal regarding to the define

scope. The positive sign shows where the red wire shall be connected while the

negative sign shows where the black wire shall be connected Figure 3.2 shows the

pair of metal where steel as anode platinum as cathode. For Figure 3.3 where steel is

anode while another same type of steel is cathode. While Figure 3.4 where zinc is

anode while steel is cathode. Figure 3.5 where copper is anode while steel is cathode.

Lastly, Figure 3.6 where steel is anode while brass is cathode.

Figure 3.2 Pair of metal steel and platinum

31

Figure 3.3 Pair of metal steel and steel

Figure 3.4 Pair of metal zinc and steel

32

Figure 3.5 Pair of metal copper and steel

Figure 3.6 Pair of metal steel and brass

3.4 Evaluation of Corrosion Rate

From all data obtain, the rate of corrosion of stray current is calculated using

weight loss of the steel by taking the average reading for 3 days. From Equation

(2.1), then

Average initial weight = First day + second day + third day reading (3.1)

Three days

Average final weight = First day + second day + third day reading (3.2)

Three days

CHAPTER 4

RESULT AND DISCUSSION

4.1 Effect of External Voltage (Stray Current) at Anode Metal

The effect of external voltage at anode metal in the electrolytic corrosion

normally induces largest affect of corrosion. Stray currents are currents flowing in

the electrolyte from external sources, not directly associated with the cathodic

protection system. Any metallic structure, for example, a gas pipeline, buried in soil

represents a low-resistance current path and is therefore fundamentally vulnerable to

the effects of stray currents. To measure metal corrosion it is necessary to make

direct physical contact with the specimen or component acting as the working

electrode (Andrade et al, 2008) .Corrosion is express and measure in term of

corrosion rate of millimeter per year (mm/yr). The metal loss of anode metal will be

rapidly increased as the stray voltage increased that surely increased the corrosion

rate.

By referring to Figure 4.1, there are five pairs of metal used which are steel-

platinum, steel-steel, zinc-steel, copper-steel, and copper-brass. The arrangement is

anode-cathode. There are four corrosion rate measured based on variation of voltage.

The voltage used is 0V, 2V, 3V, and 4V. The area of the metals are used are

approximately 10 cm2

except platinum metal which is 1.26 cm2.The corrosion rate is

measured with difference pair of metal and variation of external voltage.

34

Figure 4.1 Corrosion rate of anode metal

Corrosion rate anode metal (mm/yr) is proportionally to the external voltage

supply. The trends of the results are polynomial. As mentioned before, oxidation

reaction will happen in anode metal and where the corrosion happens here. The

highest the external voltage, the highest will be the corrosion rate of anode metal.

Steel-platinum metal has the highest corrosion rate. At 4V, the corrosion rate of

steel-platinum at anode metal is 2.26 mm/yr follow by copper-copper 1.2 mm/yr,

copper-brass 0.94 mm/yr, copper-steel 0.58 mm/yr, and zinc-steel 0.4 8mm/yr. It is

due to the potential difference between the pair of metal. Steel-platinum metal has

the highest potential difference +0.75V, follow by copper-copper 0V, copper-brass -

0.01V, copper-steel -0.11V and zinc-steel -1.21V. When the stray voltage is supply,

it adds more voltage to the potential difference and hence increases the corrosion rate

anode metal. Before the stray current is picked up by the reinforcement such as gas

pipeline a significant potential difference has to be produced between the point

0

0.5

1

1.5

2

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Co

rro

sio

n R

ate

(m

m/y

r)

Voltage (V)Steel-Steel Zinc - Steel Copper - Steel Copper - Brass

35

where the current enters the reinforcement such as underground pipeline system at

cathodic site and the point where the current returns to the concrete back at anodic

site (Hernándeza et al, 2008). The driving force required to produce the circulation of

an appreciable current density in the anodic area greater than 2mA/m2 is at least

500mV (Bertolini et al, 1993).

The relationship between corrosion rate of anode and voltage supplied is not

linear where the relationship between discharge current and anode consumption rate

is not of the simple linear variety; the consumption rate can increase by a higher

percentage for a certain percentage increase in current (Roberge, 2003).

4.2 Effect of External Voltage (Stray Current) at Cathode Metal

Figure 4.2 shows there are five pairs of metal used which are steel-platinum,

steel-steel, zinc-steel, copper-steel, and copper-brass. The arrangement is anode-

cathode. There are four corrosion rate measured based on variation of voltage. The

voltage used is 0V, 2V, 3V, and 4V. The areas of metals used are approximately 10

cm2

except platinum metal which is 1.26 cm2.The corrosion rate is measured with

difference pair of metal and variation of external voltage.

Corrosion rate cathode metal (mm/yr) is proportionally to the external voltage

supply. The trends of the result are polynomial. Basically, cathode will be protected

which mean will be having less corrosion due to cathodic reaction. The highest the

external voltage, the highest will be the corrosion rate. The zinc-steel has the highest

corrosion rate. At 4V, the corrosion cathode rate of zinc-steel is 0.28 mm/yr follow

copper-steel 0.23 mm/yr, copper-brass 0.19 mm/yr, steel-steel 0.01 mm/yr and steel-

platinum 0.006 mm/yr. It is due to the potential difference between the pair of metal.

Steel-platinum metal has the highest potential difference +0.75V, follow by copper-

copper 0V, copper-brass -0.01V, copper-steel -0.11V and zinc-steel -1.21V. For

zinc-steel metal, steel as the cathode has the highest corrosion cathode because of the

pair of metal has the negative potential difference and the ability for the anode metal

36

to protect cathode become less. Detection of stray current interactions is usually

carried out on the basis of potential measurements. The structure potential value and

its variability are adopted as criteria of the electrolytic corrosion hazard (Darowicki

and Zakowski, 2003).

For each specific anodic reaction a characteristic number of electrons are

produced in the reaction of one metal ion. Thus, all other things being equal, the

metal loss is proportional to the number of electrons that are produced. As the

electrons produced migrate to cathodic areas through the electron path, the metal loss

is proportional to the current flow. In cases where more positively charged ions are

produced, more electrons flow for a given number of corroding metal atoms but the

current flow remains proportional to the metal loss (Rogers, 1968).

Figure 4.2 Corrosion rate of cathode metal

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5

Co

rro

sio

n R

ate

(m

m/y

r)

Voltage (V)

Steel-Platinum Steel-Steel Zinc - Steel

Copper - Steel Copper - Brass

37

4.3 Effect of Constant External Voltage (Stray Current) to the Arrangement

of metal in Electrochemical Series

The electrochemical series is built up by arranging various redox (reduction

and oxidation process) equilibrium in order of their standard electrode potentials.

The most negative electrode potential values are placed at the top of the

electrochemical series, and the most positive at the bottom. The electromotive series

begins with the metal most easily oxidized, the metal with the greatest electron-

donating tendency, and ends with the metal least easily oxidized. The tendency to be

oxidized is not an absolute quantity; it can only be compared with the tendency of

some other substance to be oxidized. The electromotive series is thus a list of the

metals in the order of their tendency to undergo the half reaction. The overall

electrochemical cell reaction can be written as two half-equations, one equation for

the reduction reaction and one equation for the oxidation reaction.

Figure 4.3 Corrosion rate pair of metal due to arrangement in electrochemical

series

2.26

0.48

0.01

0.51

1.01

1.51

2.01

Co

rro

sio

n R

ate

(m

m/y

r)

Steel Anode Platinum Cathode Zinc Anode Steel Cathode

38

Figure 4.3 shows the rate of corrosion of the pair of metal with supply

constant external voltage at 4V due to arrangement in electrochemical series. The

pair of steel-platinum metal gives the highest corrosion rate at anode metal (2.26

mm/yr) than the pair of zinc-steel metal (0.48 mm/yr). This is because of the distance

between the pair of metal in electrochemical series. Besides, the potential difference

pair of steel-platinum is +0.75v while the zinc-steel is -1.21V.The pair of zinc-steel

metal need supply of 1.21V for electrolytic reaction to happen. The larger the

distance in electrochemical series, the cathode metal will be more protective and

anode metal will be more corrosive. In electrochemical series, the top of the series

easily oxidized, and the bottom part easily reduced (Roberge, 2000).The results can

be apply in real case where the stray current flow exit the metal (anode part) where

corrosion happen badly if the metal at the top of the electrochemical series which

easy to oxidized.

4.3 Classes of Corrosion of Anode Metal

The National Bureau of Standards in the USA has investigated corrosion of

various materials in a large number of environments (Fontana and Greene, 1986).

For corrosion rate below than 0.005 inches per year (ipy) or equivalent to 0.15

millimeter per year (mm/y).Metals in this category have good corrosion resistance

and can be used for critical parts. Besides, corrosion rate between 0.005 to 0.05 ipy

(0.15 mm/y to 1.5 mm/y) .Metals in this group are satisfactory if a higher rate of

corrosion can be tolerated. For corrosion rate greater than 0.05 ipy (1.5 mm/y)

usually not satisfactory for good resistance (Kabir, 2008). These findings can be

related to the stray current phenomena where the current leave the metal (anode part),

corrosion will occur. At good corrosion anode metal means that the anode metal is

having low corrosion rate as the external voltage is increasingly supply while at not

satisfactory corrosion anode metal the anode metal is having high corrosion rate as

the external voltage is increasingly supply.

39

Table 4.1 Classes of corrosion rate anode metal

Pair of Metal External Voltage (Volt)

2 3 4

Steel - Platinum Satisfactory

corrosion

anode metal

Not

satisfactory

corrosion

anode metal

Not satisfactory corrosion

anode metal

Steel - Steel Satisfactory

corrosion

anode metal

Satisfactory

corrosion

anode metal

Satisfactory corrosion anode

metal

Zinc - Steel Good

corrosion

anode metal

Good

corrosion

anode metal

Satisfactory corrosion anode

metal

Copper - Steel Good

corrosion

anode metal

Satisfactory

corrosion

anode metal

Satisfactory corrosion anode

metal

Copper - Brass Good

corrosion

anode metal

Satisfactory

corrosion

anode metal

Satisfactory corrosion anode

metal

Table 4.1 shows the classes of corrosion for pair of steel-platinum, steel-steel,

zinc-steel, copper-steel, and copper-brass metal. For anode steel in steel-platinum

metal, at 2V it has satisfactory corrosion anode metal but for 3V and 4V have not

satisfactory corrosion anode metal where the corrosion rate at anode very high. For

anode steel in steel-steel metal, at 2V, 3V and 4V it has moderate corrosion anode

metal. For anode zinc in zinc-steel metal, at 2V and 3V it has good corrosion anode

metal but 4V it has moderate corrosion anode metal. For anode copper in copper-

steel metal, at 2V and 3V it has good corrosion anode metal but for 4V it has

moderate corrosion anode metal. For anode copper in copper-brass, at 2V it has good

corrosion anode metal but 3V and 4V it has moderate corrosion anode metal. Those

classes of corrosion anode metal can be related to the effect of stray current enter the

metal where the protection happen (cathode part) but where the part the current exits

the metal (anode part) where corrosion will occur.

CHAPTER 5

CONCLUSION AND RECCOMENDATION

5.1 Conclusion

As conclusion, the corrosion rate is proportionally to the external voltage

(stray current) supply. From 0V to 4V, pair of steel-platinum metal, the corrosion

anode metal is increasing from 0.30 mm/yr to 2.26 mm/yr while corrosion rate of

cathode metal is increasing from 0 mm/yr to 0.006 mm/yr.

At constant stray current supply, the corrosion rate at anode metal is

increasing as the distance between metals in electrochemical series increase where

corrosion rate of anode metal pair of steel-platinum metal at 4V is 2.26 mm/yr but

for pair of zinc-steel metal at 4V is 0.48 mm/yr.

The classes of corrosion anode metal define that at corrosion rate anode metal

greater than 1.5 mm/yr, the corrosion rate of anode metal is not satisfactory. For

steel-platinum metal, steel metal having high corrosion rate anode metal which at 3

V and 4V, the corrosion rate is 1.96 mm/yr and 2.26 mm/yr.

5.2 Recommendation

Further research shall be made for this research on different parameters such

as effect of pH electrolyte to the corrosion rate of stray current and the prevention of

41

stray current phenomena. These parameters affect the corrosion rate investigation

due to stray current.

REFERENCES

Andrade, C., Martinez, F., and Castellote, M. (2008). Feasibility of Determining

Corrosion Rates by Means of Stray Current-Induced Polarization. Journal

Application Electrochemical.

Bertolini Luca, Maddalena Carsana, and Pedeferri Pietro (2006). Corrosion

Behaviour of Steel In Concrete In The Presence Of Stray Current. Corrosion

Science 49 (2007) 1056–1068.

Bertolini, L., Bolzoni, F., Pastore, and T., Pedeferri, P. (1993). Stray Current Induced

Corrosion in Reinforced Concrete Structures. Institute of Materials, London.

Cotton, Member, IEEE, Charalambos Charalambous, Pete Aylott, and Petra Ernst

(2005). Stray Current Control in DC Mass Transit Systems. IEEE Transactions

On Vehicular Technology,Vol.54,No.2,March 2005.

Darowicki, K., and Zakowski, K. (2003). A New Time–Frequency Detection Method

of Stray Current Field Interference on Metal Structures. Department of

Anticorrosion Technology, Chemical Faculty, Gdansk University of Technology

Einar Bardal (2003). Corrosion and Protection. Springer.

Fontana, M. G., and Greene, N. D. (1986). Corrosion Engineering. McGraw-Hill.

43

Gaillard-Allemand, B., Littner*, A., Podor, R., and Rapin, C. (2003).

Characterisation of Molten Glass Corrosion Behaviour by Use of

Electrochemical Techniques. Journal Corrosion Science and Engineering,

Volume 6 Paper HD24.

Graver, D.L. (1985). Corrosion Data Survey-Metals Section. NACE International,

Houston.

Kabir (2008). Corrosion Mechanism. Bangladesh University of Engineering and

Technology.

Marshal Parker, E. (1954). Pipe Line Corrosion and Cathodic Protection . Gulf

Publishing Company.

Roberge, P. R. (2000). Handbook of Corrosion Engineering. McGraw-Hill.

Robles Hernándeza, F.C., Plascenciab, G., and Kevin Kocha (2008). Rail Base

Corrosion Problem for North American Transit Systems. Engineering Failure

Analysis 16(2009)281–294.

Rogers, T., (1968), Marine Corrosion, William Clowes And Sons

Sheir, L.L. (1968), Corrosion Science and Technology. Newnes International

Monographs.

Smart, Osstendorp and Wood (1999). Journal Induced AC Creates Problems For

Pipe Lines In Utility Corridors. Pipe Line & Gas. Industry, Vol. 8.

Waters, C.L. (2006). Telluric Currents. Space Physics Group Newcastle Australia.

APPENDIX

Table A No External Voltage Supply 0V

Sample (Steel-

Platinum)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Densit

y

(g/cm3

)

Corrosion Rate

(mm/d)

Corrosion

Rate (mm/y)

Steel (anode) 22.809 22.611 0.198 0.066 10 7.86 0.000839695 0.30648855

Platinum (cathode) 3.553 3.553 0 0 1.26 21.45 0 0

Sample (Zinc-Zinc)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Densit

y

(g/cm3

)

Corrosion Rate

(mm/d)

Corrosion

Rate (mm/y)

Zinc(Anode) 26.112 25.892 0.22

0.07333333

3 10 7.86 0.000932994 0.340542833

Zinc (cathode) 26.12 26.118 0.002

0.00066666

7 10 7.86 8.48176E-06 0.003095844

Sample (Zinc-Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Densit

y

(g/cm3

)

Corrosion Rate

(mm/d)

Corrosion

Rate (mm/y)

Zinc (anode) 23.87 23.865 0.005

0.00166666

7 10 7.15 2.331E-05 0.008508159

Steel (cathode) 22.742 22.739 0.003 0.001 10 7.86 1.27226E-05 0.004643766

Sample (Copper-

Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Densit

y

(g/cm3

)

Corrosion Rate

(mm/d)

Corrosion

Rate (mm/y)

Copper (anode) 26.113 26.1 0.013

0.00433333

3 10 8.96 4.83631E-05 0.01765253

Steel (cathode) 23.008 23.005 0.003 0.001 10 7.86 1.27226E-05 0.004643766

Sample (Copper-

Brass)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Densit

y

(g/cm3

)

Corrosion Rate

(mm/d)

Corrosion

Rate (mm/y)

Copper (anode) 26.095 26.079 0.016

0.00533333

3 10 8.96 5.95238E-05 0.02172619

Brass (cathode) 24.269 24.266 0.003 0.001 10 8.47 1.18064E-05 0.004309327

Table B External Voltage at 2V

Sample (Steel-

Platinum)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Steel (anode) 23.173 22.813 0.36 0.12 10 7.86 0.001526718 0.557251908

Platinum

(cathode) 3.553 3.553 0 0 1.26 21.45 0 0

Sample (Zinc-

Zinc)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Zinc(Anode) 26.24 25.955 0.285 0.095 10 7.86 0.001208651 0.441157761

Zinc (cathode) 26.12 26.116 0.004 0.001333333 10 7.86 1.69635E-05 0.006191688

Sample (Zinc-

Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Zinc (anode) 24.24 24.185 0.055 0.018333333 10 7.15 0.00025641 0.093589744

Steel (cathode) 23.01 22.99 0.02 0.006666667 10 7.86 8.48176E-05 0.030958439

Sample (Copper-

Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Copper (anode) 26.11 26 0.11 0.036666667 10 8.96 0.000409226 0.14936756

Steel (cathode) 23.6 23.582 0.018 0.006 10 7.86 7.63359E-05 0.027862595

Sample (Copper-

Brass)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Copper (anode) 26.095 25.922 0.173 0.057666667 10 8.96 0.000643601 0.234914435

Brass (cathode) 24.123 24.119 0.004 0.001333333 10 8.47 1.57418E-05 0.005745769

Table C External Voltage at 3V

Sample (Steel-

Platinum)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Steel (anode) 22.978 20.864 2.114 0.4228 10 7.86 0.005379135 1.96338422

Platinum

(cathode) 3.553 3.552 0.001 0.0002 1.26 21.45 7.40001E-06 0.002701

Sample (Zinc-

Zinc)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Zinc(Anode) 26.154 25.129 1.025 0.205 10 7.86 0.002608142 0.95197201

Zinc (cathode) 26.189 26.182 0.007 0.0014 10 7.86 1.78117E-05 0.00650127

Sample (Zinc-

Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Zinc (anode) 23.842 23.62 0.222 0.0444 10 7.15 0.000620979 0.22665734

Steel (cathode) 23.02 22.9 0.12 0.024 10 7.86 0.000305344 0.11145038

Sample (Copper-

Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Copper (anode) 26.401 25.741 0.66 0.132 10 8.96 0.001473214 0.53772321

Steel (cathode) 22.768 22.692 0.076 0.0152 10 7.86 0.000193384 0.07058524

Sample (Copper-

Brass)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day) Area(cm2)

Density

(g/cm3)

Corrosion

Rate (mm/d)

Corrosion Rate

(mm/y)

Copper (anode) 23.731 22.764 0.967 0.1934 10 8.96 0.002158482 0.78784598

Brass (cathode) 24.292 24.22 0.072 0.0144 10 8.47 0.000170012 0.06205431

Table D External Voltage at 4V

Sample (Steel-Platinum)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day)

Area(cm2

)

Density

(g/cm3

)

Corrosion

Rate (mm/d)

Corrosion

Rate (mm/y)

Steel (anode) 22.299 20.348 1.951 0.48775 10 7.86

0.00620547

1

2.26499681

9

Platinum (cathode) 3.576 3.574 0.002 0.0005 1.26 21.45 1.85E-05

0.00675250

7

Sample (Zinc-Zinc)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day)

Area(cm2

)

Density

(g/cm3

)

Corrosion

Rate (mm/d)

Corrosion

Rate (mm/y)

Zinc(Anode) 26.188 25.152 1.036 0.259 10 7.86

0.00329516

5

1.20273536

9

Zinc (cathode) 26.189 26.18 0.009 0.00225 10 7.86 2.8626E-05

0.01044847

3

Sample (Zinc-Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day)

Area(cm2

)

Density

(g/cm3

)

Corrosion

Rate (mm/d)

Corrosion

Rate (mm/y)

Zinc (anode) 23.83 23.45 0.38 0.095 10 7.15

0.00132867

1

0.48496503

5

Steel (cathode) 23.222 22.98 0.242 0.0605 10 7.86 0.00076972

0.28094783

7

Sample (Copper-Steel)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day)

Area(cm2

)

Density

(g/cm3

)

Corrosion

Rate (mm/d)

Corrosion

Rate (mm/y)

Copper (anode) 26.161 25.582 0.579 0.14475 10 8.96

0.00161551

3

0.58966238

8

Steel (cathode) 23.737 23.583 0.154

0.05133333

3 10 7.86

0.00065309

6

0.23837998

3

Sample (Copper-Brass)

Initial

weight(g)

Final

weight(g)

Weight

loss(g)

Corrosion

Rate(g/day)

Area(cm2

)

Density

(g/cm3

)

Corrosion

Rate (mm/d)

Corrosion

Rate (mm/y)

Copper (anode) 23.048 22.12 0.928 0.232 10 8.96

0.00258928

6

0.94508928

6

Brass (cathode) 24.279 24.1 0.179 0.04475 10 8.47

0.00052833

5

0.19284238

5

Table E Effect Corrosion Rate Anode Metal to the Variation External Voltage

Corrosion Rate (mm/yr)

Pair of Metal /

Volt 0 2 3 4

Steel-Platinum 0.306489 0.557252 1.963384 2.264997

Steel-Steel 0.340543 0.441158 0.951972 1.202735

Zinc - Steel 0.008508 0.09359 0.226657 0.484965

Copper - Steel 0.017653 0.149368 0.537723 0.589662

Copper - Brass 0.021726 0.234914 0.787846 0.945089

Table F Effect corrosion rate cathode metal to the external voltage

Corrosion Rate (mm/yr)

Pair of Metal /

Volt 0 2 3 4

Steel-Platinum 0 0 0.002701 0.006753

Steel-Steel 0.003096 0.006192 0.006501 0.010448

Zinc - Steel 0.004644 0.030958 0.11145 0.280948

Copper - Steel 0.004644 0.027863 0.070585 0.23838

Copper - Brass 0.004309 0.005746 0.062054 0.192842

Table G Corrosion rate pair of metal at constant external voltage due to arrangement in electrochemical series

Anode

Corrosion Rate (mm/yr)

Pair of Metal / Volt 0 2 3 4

Steel-Platinum 0.306489 0.557252 1.963384 2.264997

Zinc - Steel 0.008508 0.09359 0.226657 0.484965

Cathode

Corrosion Rate (mm/yr)

Pair of Metal / Volt 0 2 3 4

Steel-Platinum 0 0 0.002701 0.006753

Zinc - Steel 0.004644 0.030958 0.11145 0.280948

Table H Potential voltage pair of metal based on electrochemical series

Pair of Metal / Volt

E

cathode

E

anode Potential Voltage (E cathode- E anode) in Volt

Steel-Platinum 1.2 0.45 0.75

Steel-Steel 0.34 0.34 0

Zinc - Steel -0.45 0.76 -1.21

Copper - Steel -0.45 -0.34 -0.11

Copper - Brass -0.35 -0.34 -0.01