1

1. Introduction:

Cathodic protection (CP) is a technique used to control the

corrosion of a metal surface by making it the cathode of an

electrochemical cell. The sacrificial metal then corrodes instead of the

protected metal. Cathodic protection systems are used to protect a wide

range of metallic structures in various environments.

Common applications are; steel water or fuel pipelines and storage

tanks such as home water heaters; ship and boat hulls; offshore oil

platforms and onshore oil well casings and metal reinforcement bars in

concrete buildings and structures.

2. Cathodic protection history:

The first reported practical use of cathodic protection is enerally

credited to Sir Humphrey Davy in the 1820s. Davy’s advice was sought

by the Royal Navy in investigating the corrosion of copper sheeting used

for cladding the hulls of naval vessels. Davy found that he could preserve

copper in seawater by the attachment of small quantities of iron, zinc or

tin. The copper became, as Davy put it, “cathodically protected”. The

most rapid development of cathodic-protection was made in the United

States of America and by 1945, the method was well established to meet

the requirements of the rapidly expanding oil and natural gas industry,

which wanted to benefit from the advantages of using thin-walled steel

pipes for underground transmission. In the United Kingdom, where low-

pressure, thicker-walled castiron pipes were used extensively, very little

cathodic protection was applied until the early 1950s. The increasing use

of cathodic protection in modern times has arisen, in part, from the initial

success of the method as used from 1952 onwards to protect about 1000

miles of wartime fuel-line network. (Uhlig, 1971 and Gummow, 2000).

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3. Types of cathodic protection:

There are two types of cathodic protection: sacrificial anode

cathodic protection and impressed current cathodic protection.

3.1 Sacrificial anode system:

Sacrificial anode protection is called "sacrificial" because the anode

is thought of as "sacrificing" itself to protect the structure. This type of

protection utilizes a galvanic cell consisting of an anode made from a

more active metal than the structure, so this method is also called

galvanic anode protection. The anode is attached to the structure, either

directly or to permit measurement of the anode output current, through a

test station.

Magnesium and zinc are the most common galvanic anodes for

underground use. In salt water, zinc anodes and aluminum alloy anodes

are commonly used. In fresh water, magnesium is frequently used.

For underground use, magnesium and zinc anodes are packaged in

a backfill consisting of 75 % gypsum, 20 % bentomite and 5 % sodium

sulfate. The purpose of the backfill is to absorb products of corrosion and

to absorb water from the soil to keep the anodes active. Magnesium and

zinc are also available in ribbons and extruded rods.

Sacrificial anodes require no external power. The protective current

comes form the electrochemical cell created by the connection of the

anode material to the more noble or electrically positive metal of the

structure (Kean, 2001 and Uhlig, 2000).

The cathodic protection of a steel pipe with sacrificial anodes is

illustrated in figure (1). Electrons are supplied to the steel pipe, via the

electrical connection, and a corresponding amount of anode material goes

into solution as metal ions, according to the laws of electrolysis. Some

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anode material is lost by self-corrosion, and the anodes are not converted

to electrical energy with 100 % efficiency (Davies, 2001).

Sacrificial anodes must be located close to the structure being

protected. Although almost any piece of zinc etc could provide cathodic

protection over a short period of time, cathodic protection schemes are

usually required to operate over periods of several years. Anodes can lose

their activity and become passivated, developing a non-conducting film

on their surfaces so that they no longer are able to supply current. This

can be avoided by careful control of the concentrations of trace impurities

in the anode materials, and by alloying. For zinc anodes the level of iron,

for example, must be kept below 0.005 % for satisfactory long-term

operation of the anodes. To prevent passivation of aluminum anodes

alloying with, for example, indium has been found to be successful alloy

with mercury is now disliked on environmental grounds (Faulkner and

Menkes, 1983).

Fig.1 Principle of cathodic protection with sacrificial anodes (schematic)

(Reberge, 1999)

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3.1.1 Anode materials and performance characteristics:

The composition of anode must be such as to produce the following

properties:

A sufficiently negative potential to ensure cathodic

protection in a particular environment.

Ability to continue to corrode during use and not to develop

a passive or protection film on the surface.

High anode efficiency.

There are some materials which can be used as sacrificial anodes

such as (Shrier (2), 1976, Kenneth, 1985):

1. Zinc: High-duty zinc alloy are widely used for anodes in marine

situations. Since the corrosion of zinc is comparatively low in

seawater, the high efficiency 85 – 95 % of these alloys is

maintained throughout the current density range. Iron is the major

harmful impurity and should be maintained below about 0.0014 –

0.005 %, depending on the alloy used. Small addition of aluminum

and silicon can be used to neutralize the effect of iron.

2. Aluminum: Aluminum depends for its corrosion-resistance on a

protective oxide film. This film is detrimental to the use of

aluminum as an anode material so the alloys used for sacrificial

anodes include mercury or indium, which prevent the occurrence

of the passive film on the anode material. Other elements are also

added, e.g. zinc and tin to make the anode potential more negative.

3. Magnesium: Magnesium has the most negative electrode potential

of the metals used for sacrificial anodes and is particularly useful in

environment of high resistivity where its high current output per

unit weight is useful. Magnesium is not widely used for seawater

applications.

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3.2. Design Calculations (Galvanic Anodes):

Anode Life Expectancy (Galvanic Anodes): Cathodic protection systems

are normally designed to perform for a 15 to 30 year life. In such eases

the anodes are replaced frequently or an impressed current system is used

(if possible). The general formula to calculate the life of a galvanic anode

is:

Life(Years)=Constant × Anode Weight(Kg) × Efficiency × Utilization FactorCurrentOutput (Amps)

The constants vary with the anode material and are based on the

length of time in years that one kilogram of material would last when

discharging one ampere. These constants are:

For Magnesium 0.052

For Zinc 0.0192

For Aluminum 0.0696

The efficiency for each of the anodes is 0.5 (50%) for magnesium,

and 0.95 (95%) for zinc and aluminum.

The utilization factor is based on the assumption that the anode has

reached its useful life at .85 (85%) consumption.

The number of anodes required for the system can be calculated by

dividing the total anode weight to the weight of a single anode.

It should be noted that the actual number of anodes used in a

system may have to be increased to take into consideration other factors

such as non-uniform current distribution and avoid current shielding

effects due to the shape of the structure.

Resistance to Earth-Water of Anode System : The resistance calculations

are required for determining the anode current output of the system.

The formulas that can be used are:

18log3.2522.0

dL

LRv

6

N

sL

dL

LRv 656.0log3.2218log3.2522.0

where:

Rv = Resistance of one vertical anode (ohm)

Rs = Resistance of anode system (ohm)

N = Number of anodes

L = Length of each anode (m)

p = Water/soil resistivity (ohm-m)

d = Diameter of anode (m)

s = Spacing of anodes (m)

Where more than one anode is in the circuit then the resistance

formula (Rs) is used.

Current Output of the Anode System : The current output of the system

can be calculated as follows:

io = V( Driving Voltage )Rs

3.1.2 Advantages and disadvantages:

There are advantages and disadvantages to sacrificial and

impressed current systems. The designer needs to assess the engineering

and economic aspects of each in making the choice of the type of

protection system to use. The following advantages are associated with

sacrificial anode CP systems (Roberge, 1999 and Ted, 2004):

No external power sources required.

Ease of installation (and relatively low installation costs).

Unlikely cathodic interference in other structures.

Low-maintenance systems (assuming low current demand).

System is essentially self-regulating.

Relatively low risk of overprotection.

Relatively uniform potential distributions.

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Unfortunately, these relatively simple systems also have some

limitations such as

Limited current and power output.

High-resistivity environments or large structures may require

excessive number of electrodes. Maximum resistivity of

6000 to 10,000 Ω.cm is generally regarded as the limit,

depending on coating quality.

Anodes may have to be replaced frequently under high

current demand.

Anodes can increase structural weight if directly attached to

a structure.

3.2 Impressed current system:

Impressed current protection provides dc from a power source. The

current is delivered to anodes made of a material having a very low or

essentially inert dissolution rate. The anodes serve simply to introduce the

protective current into the electro1yte (Uhlig, 2000).

In contrast to the sacrificial anode systems, the anode consumption

rate is usually much lower. Unless a consumable “scrap” anode is used, a

negligible anode consumption rate is actually a key requirement for long

system life. Impressed current systems typically are favored under high-

current requirements and/or high-resistance electrolytes (Peabody, 2001).

Figure (2) shows schematic diagram of impressed current system.

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Fig.2 Principle of cathodic protection with impressed current (schematic)

(Roberge, 1999)

3.2.1 Power sources:

The most common power source for impressed current protection is

the transformer rectifier. This unit, commonly called simply a rectifier,

reduces incoming ac voltage and rectifies it to dc. There are also solid-

state “switchmode” rectifiers that perform similar functions without the

use of transformers. Rectifiers can be provided with constant voltage,

constant current, or structure-to-electrolyte potential control.

In areas where electrical power is not readily available, solar power

and wind driven generators coupled with storage batteries are used. There

is also some use of thermoelectric cells, in-line turbine generators (in gas

or oil pipelines), and internal combustion engine driven generators

(Willett, 2000).

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3.2.2 Impressed current anodes:

Impressed current anodes do not have to be less noble than the

structure that they are protecting. Although scrap steel is occasionally

used as anode material, these anodes are typically made from highly

corrosion-resistant material to limit their consumption rate. After all,

under conditions of anodic polarization, very high dissolution rates can

potentially be encountered. Anode consumption rates depend on the level

of the applied current density and also on the operating environment

(electrolyte). For example, the dissolution rate of platinized titanium

anodes is significantly higher when buried in soil compared with their use

in seawater. Certain contaminants in seawater may increase the

consumption rate of platinized anodes. 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. (Shrier (2), 1976 and Peabody,

2001).

A variety of materials are used for impressed current anodes.

Among the oldest are high silicon, chromium bearing cast iron, graphite,

and junk steel. Magnetite and lead-silver anodes are also used, with lead-

silver being confined to use in sea water.

Among newer materials are “dimensionally stable anodes,” so-

called because the anode itself consists of a deposit on an inert substrate.

This deposit may be consumed, but the anode shape tends to remain

stable. Included in this category is platinized niobium or titanium and

mixed-metal oxide titanium anodes.

Underground impressed current anodes are usually backfilled in a

carbonaceous material such as metallurgical or calcined petroleum coke.

The purpose of the backfill is to increase the effective size of the anode,

thus reducing its resistance to earth, and also to provide a uniform

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environment around the anode, increasing its life. Anode life is extended

by the large coke backfill column since the current is discharged from the

coke column as opposed to being discharged from only the anode.

Another advantage to increasing the size of the anode is that the resultant

reduction in anode current density reduces acidity in the vicinity of the

anode (Uhlig, 2000).

The properties of an "ideal" impressed current anode material must

have the following (Roberge, 1999):

Low consumption rate, irrespective of environment and

reaction products.

Low polarization levels, irrespective of the different anode

reactions.

High electrical conductivity and low resistance at the anode-

electrolyte interface.

High reliability.

High mechanical integrity to minimize mechanical damage

during installation, maintenance, and service use.

High resistance to abrasion and erosion.

Ease of fabrication into different forms.

Low cost, relative to the overall corrosion protection scheme.

Some of the more common materials used as an auxiliary anodes

for marine purpose are considered below (Shrier (2), 1976 and Kenneth,

1985):

1. Scrap steel and cast iron:

These materials are cheap but bulky and have a limited durability

as anode. They have largely been replaced for marine situations by other

materials, although they may still be used to a limited extent for steel

piling and jetties.

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2. Graphite:

Graphite, particularly when impregnated with resin to reduce

porosity, has been widely used in saline situations but it is brittle and may

fracture in seawater if subjected to mechanical shock. It is not particularly

resistant to the velocity affects of moving water, e.g. erosion and

impingement.

3. High-silicon iron:

High-silicon iron is usually denoted "HSI", these contain about

14.5 % Si and have been used for anodes in seawater. However, they tend

to pit and small additions of molybdenum (3 %) or chromium (4-5 %)

considerably improve their performance. The chromium-containing

silicon iron (HSCI) is the preferred alloy for seawater use.

4. Lead alloys:

A number of different lead alloys have been used for marine

conditions, but the Pb-6Sb-1Ag is now generally used. This depends on

the formation of a PbO2 film to provide the long-term performance

achieved with anodes produced from such alloys. For seawater service,

platinum-activated lead alloy anodes have proved to be successful. Small

platinum wires are inserted into the surface and provide microelectrodes

of Pt, which simulate the formation of a stable PbO2.

5. Platinized-type anode:

Platinum has many of the requirements for an ideal anode material.

It is one of the most noble metals and also tends to passivate by forming a

thin electrically conductive film. These properties provide it with a long,

virtually permanent life. It is, however, a very expensive material so is

usually used in the form of an electro-deposited coating on titanium.

Other methods of application of the platinum such as spraying and

cladding are also employed. Titanium and niobium are also used in

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preference to titanium for some systems where high voltage is employed.

Table (1) shows the properties of impressed current anodes.

Table 1. Properties of impressed anodes (Shrier (2), 1994)

Material

Consumption rate or

operating current

density

Notes

Consumable:

Scrap iron ca 9 kg/Ay Cheap; suitable for buried or

immersed use

Cast iron < 9 kg/Ay Cheap; buried or immersed use;

carbon skeleton reduces

consumption

Semi-consumable:

Silicon cast iron

(Fe-14Si-(3 Mo))

5-50 A/m2

(in fresh water or

soil)

Buried or immersed use;

consumption (<1 kg/Ay); Mo

reduces consumption in seawater

Graphite 2.5-10 A/m2 Consumption rate very much less

than steel or cast iron (<1 kg/Ay);

chloride ions reduce consumption

Non-consumable:

Lead alloys:

1. Pb-6sb-1Ag < 50-200 A/m2

(in seawater)

PbO2 film restrains consumption

2. Pt-activated < 50-500 A/m2

(in seawater)

PbO2 film protective

Platinized Ti, Ta or Nb < 1000 A/m2

(consumption)

Discontinuities in Pt coat protected

by oxide film on subtrate; sensitive

(100 Hz) AC ripple on DC. or

negative current spikes causing

electrode consumption; maximum

operating potential with Ti

subtrate: 9 V

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3.2.3 Advantages and disadvantages of impressed current systems:

The following advantage can be cited for impressed current

systems (Roberge, 1999 and Ted, 2004):

High current and power output range

Ability to adjust (“tune”) the protection levels

Large areas of protection

Low number of anodes, even in high-resistivity

environments

May even protect poorly coated structures

While the disadvantages can be cited as follow:

Relatively high risk of causing interference effects.

Lower reliability and higher maintenance requirements.

External power has to be supplied.

Higher risk of overprotection damage.

Risk of incorrect polarity connections (this has happened on

occasion with much embarrassment to the parties

concerned).

Running cost of external power consumption.

More complex and less robust than sacrificial anode systems

in certain applications.

3.2.4 Current requirements:

It was indicated earlier that the cathodic current was a poor

indicator of adequate protection. Whilst, to a first approximation the

protection potential is a function of the metal, the current required for

protection is a function of the environment and, more particularly, of the

cathodic kinetics it entails.

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In short, the current demand for cathodic protection varies

according to the aggressiveness of the corrosive environment. It is for this

reason that cathodic protection finds its greatest application where the pH

is close to neutral. The more acid environments entail a current output

that rapidly becomes uneconomic. The more alkaline environments prove

less aggressive to the structure and therefore often do not justify cathodic

protection (Ashworth and Booker, 1986 and Shrier (2), 1994

3.2.5 Potential requirement:

In practice, the structure-to-electrolyte potentials are measured

using a standard reference electrode based on copper/copper sulphate,

silver/silver chloride or pure zinc. The reference electrode should be very

close to the surface whose potential is being measured. For steel in an

aerobic electrolyte of nearly neutral pH a commonly accepted protection

is -850 mV/Cu-CuSO4 (Ashworth and Booker, 1983 and Faulkner and

Menkes, 1983).

The reference electrode for making this measurement should be

placed as close as possible to the protected structure to avoid and/or

minimize an error caused by IR drop through the electrolyte.

Table 2. Protective potential in sweater for available type of reference

electrode (Trethewey and Chamberlain, 1996)

Reference electrode ElectrolyteProtective potential in 20

Ω cm seawater at 25o C, V

Calomel Saturated KCl -0.800

Ag/AgCl Seawater -0.800

Cu/CuSO4 Saturated CuSO4 -0.850

Zinc (mil-A-18001H) seawater -0.240

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4. Selection of whether impressed current system or sacrificial anode

system

The following factors show whether to use impress current system

or sacrificial anode system in cathodic protection (Kenneth, 1985):

a. Size and geometry of project (impressed current method is

usually used for large projects);

b. Availability of the power supply;

c. Possibility of the interface problems;

d. Necessity for safety from spark hazards and accumulation of

hydrogen in enclosed spaces;

e. Replaceability of sacrificial anodes;

f. Expected economic life of the system.

4.1 Typical basic appreciation of cathodic protection by sacrificial

anodes:

a. Estimate of total current requirements (control densities allowed,

spare capacity, allowance of protective coatings and linings,

assessment of environmental media);

b. Resistivity of water, soil or other electrolyte solutions;

c. Requirements for insulating flanges and bonding to foreign

structures and assessment of extra current allowances;

d. Selection of suitable anode metal (zinc, magnesium, aluminum,

iron, mild-steel or other metals anodic to the protected structures

for equipment) and its alloying composition;

e. Requirements for introduction of current control to limit output

within the optimum parameters;

f.Selection of the suitable of anodes to provide optimum life;

g. Selection of the suitable shape of anodes to secure optimum

spread;

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h. Determination of the total number of anodes required;

i. Anode spacing to give uniform current distribution;

j.Selection of test-point localities;

k. Attachment of anodes (sacrificial anodes should be conductively

attached to the protected metal but their sacrificial mass should

preferably be separated from the protected surfaces).

4.2 Typical basic appreciation of cathodic protection by impressed

current cathodic control:

a. Estimate of the local current requirements;

b. Resistivity of water, soil or other electrolyte solutions;

c. Requirements for insulating flanges and bonding to foreign

structures and assessment of extra current allowances;

d. Selection of suitable ground-bed locations (in low resistivity

soil or media, reasonably near power supply, at points where

there are no interface problems, where beds and cable are

reasonably secure from interference or distribution);

e. Decision on the type of anodes and the design of their

attachment;

f. Decision on whether the anodes (if elongated ones selected)

should be installed vertically or horizontally;

g. Decision on the voltage to be used;

h. Determination of the optimum anode material;

i. Optimum number and size of the anodes;

j. Decision on anode spacing;

k. Type and location of reference electrode;

l. Location of controllers, power supply and transmission (cabling

and installation);

m. Potential hazards of marine and surface traffic;

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n. Wave action and soil instability;

o. Weed fouling and microbiological effect.

5. Over protection:

The main disadvantages of over protection are waste of electric

power and increased consumption of auxiliary anodes. In the extreme,

additional disadvantages result, if so much hydrogen is generated at the

protected structure that blistering of organic coatings, hydrogen

embrittlement of the steel (loss of ductility through absorption of

hydrogen) or hydrogen cracking caused. Damage to steel by hydrogen

absorption is particularly apt to occur in environments containing

sulfides. In the case of amphoteric metals (e.g. aluminum, zinc, lead, tin)

excess alkalies generated at the surface of over protected systems damage

the metals by causing increased attack rather reduction of corrosion. In

addition, the large currents associated with more negative potentials

produce high local concentrations of hydroxyl ion, which may cause

excessive chalking or damage any barrier coating such paint (Trethewey

and Chamberlain, 1996; Wiliams, 1999; Batt and Robinson, 2004 and

Farwest, 2006).

6. Controlled potential cathodic protection:

A constant applied direct voltage is used in power-impressed

cathodic protection systems and provides a relatively constant protective

current. However, as conditions change, the required protective current

may vary widely and the structure may be under or over protected much

of the time.

A recent innovation is the use of automatic potential control (APC)

rectifiers. This employs control circuitry to maintain the structure

potential constant with respect to a reference electrode permitting the

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current to vary continuously so suit the needs of the system (Francis,

1975 and Shrier (2), 1976).

The surface was protected by a barrier coating or paint film, the

currents observed would be much lower, if the protective coating is

damaged, causing an increase of effective surface area, then the current

density would fall, this would in a change of potential back towards the

free corrosion potential (figure 2.4), together with a corresponding

increase in corrosion rate. (Trethewey and Chamberlain, 1996).

7. Coating and cathodic protection

Large structures, even in near-neutral pH environments, require a

considerable current for cathodic protection. As a result structure coatings

are an almost mandatory requirement when cathodic protection is

contemplated. The coating then provides the major part of the protection

and the cathodic protection provides the protection at the coating defects.

A coating deteriorates chemically and mechanically during its

lifetime. This leads to a progressive increase in both the number of

defects and the current required to protect the steel as they occur

(Roberge, 1999).

The cost of cathodic protection falls as the surface is coated

because less anode material is needed or less current demand. So the

means of protecting steel structure.

There is however, one important exception, the underwater steel

work of the majority of offshore platforms used in the North Sea (and in

the other areas of the world) is not protected by coatings. There are two

reasons for this (Kenneth, 1985):

These structures are almost all protected using sacrificial

anodes and it is easer to design for a more less constant

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current demand than for an increasing current demand in

these systems.

The construction of the platform is made against a tight

schedule; and weather conditions that prevented completion

of coating would involve launching a platform with an

incomplete coating and insufficient anodes to protect the

new partially coated structure.

The approximate costs and weight arising from the coated and

uncoated options are shown in table (3).

Table 3. Cost and weight variations due to coating submerged steel

(notional deep water structure). Aluminum sacrificial cathodic protection

system (Kenneth, 1985)

ParameterBare steel

option value

Coated steel

option value

Anode net weight, kg 230 115

Anode gross weight, kg 292 157

Anode quantity (total) 5530 5132

Budget price per anode, £ 300 160

Budget installation price per anode, £ 300 250

Budget coating cost per m2 (63000 m2), £ - 20

Total budget cost, £ 3,318,000 3,364,120

Total estimated weight, ton 1615 806 + paint weight

8. Time effect:

the rate of dissolution increases with increasing time and this is

normal case. But this increasing is not equally with time, where the

dissolution rate in the first hour is more than second hour and so on. The

reasons of that are attributed to continuous growth of the corrosion

products layer with time, which affects the transport of oxygen to the

metal surface and the activity of the surface and hence the corrosion rate.

20

9. Temperature effect:

The increase in the rate of dissolution with increasing seawater

temperature (may be explained in terms of the following effects:

1. A temperature increase usually increases the reaction rate

which is the corrosion rate and according to the Freundlich

equation (Shrier (2), 1976):nO2

Ck rate Corrosion …1.1

Where k is rate constant of reaction, CO2 (concentration of

oxygen) and n is order of reaction. The rate constant (k)

varying with temperature according to Arrhenius equation

(Shrier (2), 1976):

RTE

oekk …1.2

Where ko is constant, E is activation energy, R is universal

gas constant and T is temperature. Then from this formula

( RTE

oekk ) indicates that the k is increased with increasing

temperature and then the corrosion rate which is leads to

increasing the rate of zinc dissolutions.

2. Increasing seawater temperature leads to decreasing seawater

viscosity with a consequent increase in oxygen diffusivity

according to stokes-Einstein equation (Cussler, 1984 and

Konsowa and El-Shazly, 2002):

constantTD …1.3

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Where μ is the seawater viscosity and D is the diffusivity of

the dissolved oxygen. As a result of increasing the diffusivity

of dissolved oxygen, the rate of mass transfer of dissolved

oxygen to the cathode surface increases according to the

following equation (Konsowa and El-Shazly, 2002):

22 Od

Od CDCkJ

…1.4

With a consequence increase in the rate of zinc dissolution.

Where kd is mass transfer coefficient and J is mole flux of

oxygen.

3. The decreases in seawater viscosity with increasing

temperature improve the seawater conductivity with a

consequent increase in corrosion current and the rate of

corrosion.

10. Flow rate effect:

the dissolution rate of increases with increasing the flow rate. This

may be attributed to the decrease in the thickness of hydrodynamic

boundary layer and diffusion layer across which dissolved oxygen

diffuses to the tube wall of steel with consequent increase in the rate of

oxygen diffusion .but The flow rate of seawater may also caused erosion

which combined with electrochemical attack.

11. pH effect:

the rat of dissolution increases with decreasing of pH (particularly

at range of pH 5 to 2). Within the range of about 5 to 12 the corrosion rate

of steel, where it depends almost on how rapidly oxygen to the metal

surface. Although it was expected that at very high of pH value (12), the

22

dissolution rate of zinc is much reducing because the steel becomes

increasingly passive in present of alkalies and dissolved oxygen, but the

nature of electrolyte (seawater) prevent that where chloride ions

depassivate iron even at high pH. Within the acidic region (pH<5) the

ferrous oxide film (resulting from corrosion) is dissolved, the surface pH

falls and steel is more direct contact with environment. The increased rate

of reaction (corrosion) is then the sum of both an appreciable rate of

hydrogen evolution and oxygen depolarization.

23

References :

Kean, R. L. (2001), "Corrosion servies", Eur. Ing.

Uhlig, H. H. (1971), "Corrosion and corrosion control, an

introduction to corrosion science and engineering", John-Wiley

and Sons.

Uhlig, H. H. (2000), "Corrosion handbook", edited by Winston R.

R., 2nd edition, John-Wiley and Sons.

Gummow, A. R. (2000), "Corrosion control municipal

infrastructure using cathodic protection", Published in "Material

performance", 39 (2).

Davies, K. G. (2003), "Cathodic protection in practice". (internet

site: www.npl.co.uk)

Faulkner, L. L and Menkes, S. B. (1983), "Corrosion and

corrosion protection handbook", Marcel Dekker.

Roberge, R. P. (1999), "Handbook of corrosion engineering".

Shrier (2), L. L. (1976), "Corrosion 2, corrosion control", Newnes-

Butterworth.

Kenneth, A. C. (1985), "Marine and offshore corrosion",

Butterworth and Co.

Roberge, R. P. (1999), "Handbook of corrosion engineering".

Ted, H. (2004), "Cathodic protection for port facilities", Matcor

Inc.

Clive, H. H., Mathew, R. S. and Steven, P. C. (2002), Zinc Link, 4

(2).

Peabody. A. E. (2001), "Control of pipeline corrosion", 2nd edition.

Whitten, D. P. (2000), "General chemistry", 6th edition.