CHAPTER I INTRODUCTION TO CORROSION -...

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CHAPTER I

INTRODUCTION TO CORROSION

1.1 Preamble

This chapter presents the general definition of corrosion, classification of corrosion,

principles of corrosion, importance of corrosion, types of corrosion in general and concrete

corrosion, in particular. The factors influencing the corrosion will also be discussed.

1.2 Definition

Corrosion is destructive phenomenon that, besides its economic effects, is

detrimental to the appearance of metals and in some cases can cause equipment/structural

component failure. It occurs practically in all environments1,2

. Corrosion of metals takes

several forms. First, an overall surface attack slowly reduces the thickness or the weight of

the metal. Second, instead if an overall surface attack, isolated areas may be affected,

producing the familiar localized corrosion. Third, it also occurs along grain boundaries or

other lines of weakness, because of a difference in resistance to corrosive destruction.

Metals and their alloys tend to enter into chemical union with the components of a

corrosive medium to form stable compounds similar to those found in nature. When metal

loss occurs this way, the compound formed is referred to as the corrosion product. Uses of

corrosion resistant materials, application of protective coatings, or control of the

environment are some of the methods for combating corrosion. The selection of materials

or methods of protection must be determined for each environmental condition and within

prescribed economic limits. Past experience and laboratory testing can serve as a guide in

this selection, but exposure under actual conditions is necessary.

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Since corrosion is the destruction of metal or alloy by chemical or electrochemical

change, it is apparently preceded by a wide variety of processes. Usually this destruction

process is associated with the formation of tarnish or oxide films, when directly combined

with gases or liquids in environment. The mechanisms of corrosion attack have never been

fully understood by researchers in this field. Past experiences have shown several theories

to be reasonable, although without complete answers for all types of corrosion.

Most corrosion of metals is electrochemical in nature. Metals corrode, because they

are used in environments, where they are chemically unstable. Only the precious metals

such as gold, silver, platinum, etc. are found in nature in their metallic state. Most of the

commonly used metals including iron are processed from minerals or ores into metals,

which are inherently unstable in their environments.

1.3 Importance of Corrosion Studies

It is necessary to pay more attention to metallic corrosion than it was done earlier due

to the following reasons:

i. increasing use of metals in all fields of technology.

ii. use of rare and expensive metals, whose protection requires special precaution.

iii. use of new high strength alloys, which are usually more susceptible to certain

types of corrosive attack.

iv. increasing pollution of air and water resulting in a more corrosive environment.

v. strict safety standards of operating equipments, which can fail in a catastrophic

manner due to corrosion.


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1.4 Classification of Corrosion Processes

Other than these corrosion processes, further the corrosion is classified into direct

chemical (or oxidation) and electro chemical corrosion.

1.4.1. Direct chemical corrosion (or) dry corrosion

This type of corrosion occurs mainly through the direct chemical action of

environment/atmospheric gases such as oxygen, halogens, hydrogen sulphide, nitrogen or

anhydrous liquid sulphur dioxide with metal surface in immediate proximity.

Corrosion

Metal

Liquid

Metal gas reactions

Immersed

corrosion

Underground

corrosion

Atmospheric

corrosion

Direct oxidation

finishing

Hydrogen

evolution

type

Oxygen

absorption

type

Aerobic

corrosion

Anaerobic

corrosion

Exposed

to rain

Sheltered

corrosion

Above critical

humidity

Below

critical

humidity

Type of film

a. Linear

b. Parabolic

c. Logarithmic

d. Asymptotic


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The extent of corrosion depends upon the following factors:

a) chemical affinity between the corrosive environment and solid metals

b) ability of reaction product on metal surface to form a protective film

There are three main types of corrosion, namely

i. Oxidation corrosion which is brought about by the direct action of O2 at

low or high temperatures on metals usually in the absence of moisture.

ii. Corrosion by hydrogen: Gases like H2 attack metals at ordinary

temperature. It is known as ‘hydrogen embrittlement’.

iii. Liquid metal corrosion is due to chemical action caused by the flowing

liquid metal at high temperature on solid metal or alloy.

1.4.2 Electrochemical (or) wet corrosion

It occurs due to the existence of separate ‘anodic’ and ‘cathodic’ areas / peaks

between which current flows through the conducting solution.

This type of corrosion occurs,

a) where a conducting liquid is in contact with metal (or)

b) when two dissimilar metals or alloys are either immersed or dipped partially in a

solution.

Different types of electrochemical reactions depending upon the chemical nature of

the environment are given below:

Neutral media

Anode : M Mn+

+ ne-

(Anodic reaction) Mn+

+ nOH- M(OH)n corrosion product (rust)

Cathode

(Cathodic reaction) :O2 + 2H2O + 4e- 4OH

-


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Acid media

Anode : M Mn+

+ ne-

Cathode : 2H+ + 2e

- H2

1.5 Principles of Corrosion

The corrosion depends upon the following principles:

i. Thermodynamic principles

Thermodynamic principles can indicate the spontaneity of a chemical reaction.

They are used to determine whether corrosion is theoretically possible.

ii. Electrochemical Principles

They are extensively used to determine the corrosion behavior of the material. The

corrosion reaction can be represented by partial reactions such as metal oxidation and

reduction of some reducible species of the environment both occurring simultaneously at

equal rates at the mixed potential of the reaction1. Corrosion reaction mainly occurs at the

metal – environment interface.

This type of corrosion can be illustrated by the attack on iron in hydrochloric acid.

When the iron is dipped in acid, a vigorous reaction occurs, as a result, hydrogen gas is

evolved and iron gets dissolved.

Hence, the overall corrosion reaction is

The above reaction can be divided in to two partial reactions

Fe Fe2+

+ 2e- [Oxidation reaction]

2H+ + 2e

- H2 [Reduction reaction]

Fe + 2H+ Fe

2+ + H2


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1.6 Types of Corrosion

Form of corrosion Illustration

a) Uniform Corrosion

This is also called general corrosion. The surface effect produced

by most direct chemical attacks (e.g., as by an acid) is a uniform

etching of the metal. The use of chemical-resistant protective

coatings or more resistant materials will control this type of

corrosion.

b) Galvanic Corrosion

Galvanic corrosion is an electrochemical action of two dissimilar

metals in the presence of an electrolyte and an electron conductive

path. It occurs when dissimilar metals are in contact. Control of

galvanic corrosion is achieved by using metals closer to each other

in the galvanic series or by electrically isolating metals from each

other. Cathodic protection can also be used to control galvanic

corrosion effects.

c) Concentration Cell Corrosion

Concentration cell corrosion occurs when two or more areas of a

metal surface are in contact with different concentrations of the

same solution. Concentration cell corrosion is associated with

gaskets, joints, scale, debris, loose protective films, etc.

Corrosion attack is accelerated, where the oxygen

concentration is least. Metal at the area of low oxygen

availability becomes anodic to other areas. As the cathodic

area is large compared to the anodic area, the intensity of

attack is usually more severe on surrounding areas of the


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same surface. This condition can be eliminated by sealing the

faying surfaces in a manner to exclude moisture. Proper protective

coating application with inorganic zinc primers is also effective in

reducing faying surface corrosion.

d) Pitting Corrosion

Pitting corrosion is localized corrosion that occurs at microscopic

defects on a metal surface. The pits are often found underneath

surface deposits caused by corrosion product accumulation.

Pitting corrosion can lead to unexpected catastrophic system

failure. Sometimes pitting corrosion can be quite small on the

surface and very large below the surface. Methods that can be

used to control pitting include maintaining clean surfaces,

application of a protective coating, and use of inhibitors or

cathodic protection. Molybdenum additions to stainless steel (e.g.

in 316 stainless) are intended to reduce pitting corrosion.

e) Crevice Corrosion

Crevice or contact corrosion is the corrosion produced at the

region of contact of metals with metals or metals with nonmetals.

It may occur at washers, under barnacles, at sand grains, under

applied protective films, and at pockets formed by threaded joints.

Cleanliness, the proper use of sealants and protective

coatings are effective means of controlling this corrosion.

Molybdenum-containing grades of stainless steel (e.g. 316

and 316L) have increased crevice corrosion resistance.


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f) Filiform Corrosion

This type of corrosion occurs on painted or plated surfaces when

moisture permeates the coating. Long branching filaments of

corrosion product extend out from the original corrosion pit and

cause degradation of the protective coating. Filiform corrosion is

minimized by careful surface preparation prior to coating, by the

use of coatings that are resistant to this form of corrosion and by

careful inspection of coatings to ensure that the holes, in the

coating are minimized.

g) Intergranular Corrosion

Intergranular corrosion is an attack on or adjacent to the grain

boundaries of a metal or alloy. The most effective means of

prevention is the proper selection of alloy and/or suitable heat

treatment.

h) Stress Corrosion Cracking

Stress corrosion cracking (SCC) is caused by the simultaneous

effects of tensile stress and a specific corrosive environment.

Stresses may be due to applied loads, residual stresses from the

manufacturing process, or a combination of both. The stress

corrosion cracking can be avoided by using appropriate heat

treatment, selecting the proper alloy for a given environment,

putting the equipment in service in a stress free condition, or using

protective coatings.

i) Corrosion Fatigue

Corrosion fatigue is a special case of stress corrosion caused by the

combined effects of cyclic stress and corrosion. No metal is


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immune from some reduction of its resistance to cyclic stressing if

the metal is in a corrosive environment. Control of corrosion

fatigue can be accomplished by either lowering the cyclic

stresses or by corrosion control by using protection methods.

j) Fretting Corrosion

The rapid corrosion that occurs at the interface between contacting

highly loaded metal surfaces when subjected to slight vibratory

motions is known as fretting corrosion. Fretting corrosion is

greatly retarded when the contacting surfaces can be well

lubricated as in machinery-bearing surfaces so as to exclude direct

contact with air.

k) Erosion Corrosion

Erosion corrosion is the result of a combination of an aggressive

chemical environment and high fluid-surface velocities. Erosion

corrosion can be controlled by the use of harder alloys (including

flame-sprayed or welded hard facings) or by using a more

corrosion resistant alloy.

l) Dealloying

Dealloying is a rare form of corrosion found in copper alloys, gray

cast iron, and some other alloys. Dealloying occurs when the alloy

loses the active component of the metal and retains the more

corrosion resistant component in a porous "sponge" on the metal

surface. Control is achieved by the use of more resistant

alloys-inhibited brasses and malleable or nodular cast iron.


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m) Hydrogen Damage

Hydrogen embrittlement is a problem with high-strength steels,

titanium, and some other metals. Control is effected by eliminating

hydrogen from the environment or by the use of resistant alloys.

n) Microbial Corrosion

Microbial corrosion (also called microbiologically -influenced

corrosion or MIC) is corrosion that is caused by the presence and

activities of microbes. This corrosion can take many forms and can

be controlled by biocides or by conventional corrosion control

methods.

o) Concrete Corrosion or Rebar Corrosion

Concrete is a widely-used structural material that is frequently

reinforced with carbon steel reinforcing rods, post-tensioning cable

or prestressing wires. The steel is necessary to maintain the

strength of the structure, but it is subject to corrosion. This form of

corrosion is discussed at length in Section 1.9.

1.7 Theories of Corrosion

i. Homogeneous theory

A corroding metal irrespective of the presence or absence on its surface of any

micro heterogeneity can be regarded as single electrode on which reactions take place.

Metal becomes unstable due to the charge transfer reaction taking place at the interface.

Hence, it is necessary that the potential difference across the interface be more negative

than the equilibrium potential for the metal dissolution (anodic) reaction or more positive

than the equilibrium potential for the electronation (cathodic) reaction.


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ii) Heterogeneous theory

According to this theory, corrosion is caused by local galvanic elements that arise

on the surface of the corroding metal as a result of the chemical structure heterogeneity.

Corroding metal consists of

a. an electron sink area, where de-electronation reaction occurs.

b. an electron source area, where electronation reaction occurs.

c. an ionic conductor to keep the current flowing.

1.8 Corrosion Mechanism

Although several mechanisms have been proposed for the corrosion process, corrosion

is mainly electrochemical in nature and hence in this chapter, the electrochemical

mechanistic aspects have been discussed. According to this approach, the corrosion

reaction can be considered as taking place by two simultaneous reactions; the oxidation of

a metal at an anode (a corroded end releasing electrons) and the reduction of a substance

at a cathode (a protected end receiving electrons). In order for the reaction to occur, the

following conditions must exist:

a. A chemical potential difference must exist between adjacent sites on a metal surface

(or between alloys of a different composition).

b. An electrolyte must be present to provide solution conductivity and act as a source of

material to be reduced at the cathode.

c. An electrical path through the metal or between metals must be available to permit

electron flow.

Fig. 1.1 illustrates the typical electrochemical corrosion of iron in contact with water.


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Fig 1.1 Electrolytic corrosion cell

In a nearly neutral or slightly acid environment, the water is dissociated into

hydrogen ions )H( and hydroxyl ions )OH( as:

When metal is placed in contact with a liquid, surface ionization occurs, because of

the electric charge difference at the solid-liquid interface. For example, iron dissolves in

water in the form of positively charged ferrous ions )Fe(

.

Electrochemically, a chemical substance is “oxidized”, when it loses electrons to a

second substance. The electrode at which oxidation takes place is called as the “anode”. A

chemical substance is “reduced”, when it acquires electrons. The electrode at which

reduction takes place is called the “cathode”. Hence, oxidation reaction results in the

formation of positive charge ferrous ions at the anode. Ferrous ions moving away from the

metal surface are further oxidized to ferric ions )Fe(

as follows:

Corrosion

products

Cat

ion

s

An

ion

s

Cathode

Anode

Electron

migration

H2O H+ + OH

-

Fe Fe++

+ 2e-

Fe++

Fe+++

+ e-


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The positively charged ferric ions are attracted to the negatively charged hydroxyl

ions and form the corrosion product 3)OH(Fe

The rust consists of iron hydroxide or iron oxide hydrates in various states,

depending on the degree of oxidation and dehydration.

The reduction reaction at the cathode must take place concurrently in order to

continue the corrosion process. Several reactions are possible and the one that occurs is

determined by the environment. Without the presence of air or oxygen, hydrogen ions can

be reduced by the excess of electrons at the cathode surface and evolve as molecular

hydrogen by

If hydrogen is not removed from the surface, the cathodic reaction decreases and

the corrosion rate is reduced. With the presence of air, the more likely reaction is the

reduction of oxygen. Two possible reactions occur:

Hydrogen evolution or oxygen reduction with the formation of water is likely to

occur in acid media. On the other hand, oxygen reduction with the formation of hydroxyl

ions is more dominant in a neutral or alkaline environment. In either case, there is an

increase in the alkalinity of the solution at the cathode.

In summary, corrosion occurs when metal atoms detach themselves from the metal

surface at the anode and enter the solution as ions, leaving behind electrons in the metal.

The electrons flow through the metal to the cathode and neutralize positively charged

hydrogen ions that collect at the surface. The neutral hydrogen atoms combine to form

Fe+++

+ 3(OH)- Fe(OH)3

2H+ + 2e

- H2

O2 + 4H+ + 4e

- 2H2O

O2 + 2H2O + 4e- 4(OH)

-


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hydrogen gas. In solutions, where hydrogen tends to evolve too slowly, oxygen is reduced

and combines with hydrogen ions or water to form hydroxyl ions.

1.9 Concrete Corrosion or Rebar Corrosion

As the present study deals with the investigation of concrete corrosion and its control, a

detailed account of this type of corrosion is discussed below.

The steel that resides within the concrete structure is the place where the corrosion

starts. Concrete is a widely-used structural material that is frequently reinforced with

carbon steel reinforcing rods, post-tensioning cable or prestressing wires. The steel is

necessary to maintain the strength of the structure. The cracking associated with corrosion

in concrete is a major concern in areas with marine environments and in areas which use

deicing salts. There are two theories on how corrosion in concrete occurs:

a) Salts and other chemicals enter the concrete and cause corrosion. Corrosion of the

metal leads to expansive forces that cause cracking of the concrete structure.

b) Cracks in the concrete allow moisture and salts to reach the metal surface and cause

corrosion.

Both possibilities have their advocates, and it is also possible that corrosion in concrete

can occur either way. The mechanism isn't truly important, the corrosion leads to damage

and the damage must be controlled.

Under certain conditions steel is passive, where the corrosion rate for the metal is

relatively low. Iron is considered an active-passive metal and therefore steel behaves

similarly3. Passivity, defined simply, refers to a loss of chemical reactivity under certain

conditions. Steel achieves this by having a passive film form along its surface.


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1.9.1 Corrosion Factors

The cement paste in concrete is alkaline with a pH typically between 12 and 14.

This paste surrounds reinforcing steel in concrete. This alkaline environment facilitates the

protective passive film around the steel. The passive film is not invulnerable, though, it can

be damaged both chemically and mechanically. Some examples of chemical damage are

carbonation and chloride ingress.

Carbonation is the result of the reaction of atmospheric carbon dioxide and

hydroxides in the cement paste. Through this reaction carbonates and water are formed.

The carbonates that are formed from this reaction consume the hydroxides present and

therefore, can lower the pH of the concrete below the value of 8. This action causes the

steel to depassivate, leaving it susceptible to attack from corrosives. The likelihood of this

occurrence is relative to the impermeability characteristics of the concrete. Adequate

depths of the concrete cover for the bars and the use of good quality concrete mixes have

greatly reduced the concern for carbonation and its effect on corrosion.

Ingress of chlorides, on the other hand, is far more destructive to the steel. These

damaging chlorides are common in concrete environments. They are mainly present in

marine environments and in deicing salts, however they can also be due to admixtures

containing chlorides and chloride contaminated cements, aggregates and batch water. The

chlorides that migrate through the concrete, as opposed to those already present in it, are

the most destructive. The presence of chlorides also causes the depassivation of reinforcing

steel. The exact cause is not yet fully understood. Even at pH levels, where the concrete

should be passivated, chlorides allow corrosion.

Concrete is a permeable material and thus will absorb moisture. Different mixtures

of concrete have different rates of moisture infusion. Along with moisture, oxygen and

chlorides also diffuse through the uncracked concrete to the surface of the steel. A cathodic


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reaction is induced by the presence of these elements. The moisture and chlorides act as an

electrolyte, which facilitates the flow of ionic current. The chlorides initiate corrosion and

oxygen fuels the reaction. It is reasonable to expect that the lower the absorption and

permeability of the concrete, the longer for the initiation of corrosion.

The reasons for the initiation of corrosion are due to poor construction and an

unfavorable environment4 such as environments that favor impact, abrasion, chemical

attack and freeze-thaw cycles. Fig. 1.24 represents the nature of current flow during

corrosion.

Fig. 1.2 Flow of currents during corrosion4

The following are the principal factors4 that control the rate of corrosion:

i. Availability of dissolved oxygen and moisture at cathode: In order for the cathodic

reaction to occur, both oxygen and moisture are necessary. Due to the concrete

cover, both these elements have to reach the steel surface by diffusion. This slow

diffusion produces a significant reduction in the potential difference between the

anodic and cathodic areas. This phenomenon is called “concentration polarisation’.

ii. Resistivity of the medium (concrete and its pore solution): The flow of ions has to

occur through the medium of concrete and the pore solution. Thus, the resistivity of

the concrete can have a significant bearing upon the easy flow of ions.


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iii. Passivation of steel: In an alkaline environment, the surface atoms of the steel get

oxidized to form a thin oxide layer (thickness of about 10 nm). This film is stable at

the highly alkaline environment of concrete. The stability of the film is enhanced,

when the steel contains a large amount of alloys. This phenomenon of the

formation of a protective layer around the steel is called ‘passivation’, and is made

possible by the high concentration of OH- present in concrete pore solution. The

level of OH- required to maintain passivation is not a constant value, but depends

on the presence of other ions, especially Cl-. The ratio of OH

- to Cl

- is very

important. Depassivation can occur by a number of mechanisms: (1) Consumption

of OH- by carbonation and other reactions; when the pH falls below 11.5, the film

is no longer stable ; (2) Presence of a high concentration of Cl-, in addition lowers

the pH value. Due to ionic balance with OH-, Cl

- can react with oxide films of

Fe(OH)2 (that have not been converted to the stable oxide film because of lack of

availability of oxygen) to form iron chlorides. This results in pitting corrosion. A

threshold concentration of Cl- has to be exceeded before corrosion can take

place, and this concentration is a function of the OH- concentration or pH. Limits

on Cl- concentration have been stipulated in various codes.

Corrosion of steel in reinforced concrete is initiated when layer of passivating film

on the surface of the steel (composed of FeO) breaks down at low pH levels. With the

availability of moisture and oxygen, the corrosion reaction proceeds and results in the

formation of various rust products. Corrosion inhibitors added to concrete can affect this

process in various ways, such as:

i. Oxidizing or non-oxidizing passivators of steel

ii. Oxygen scavengers

iii. Film forming compounds (adsorption)

iv. Cathodic effects: paste can be made hydrophobic


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1.9.2 Corrosion due to carbonation

Steel corrosion problems are common in reinforced concrete structures around the

world. While chloride-induced corrosion is generally more pernicious and expensive to

repair, carbonation induced corrosion of reinforcement will affect a far wider range of

reinforced concrete structures. Concrete is alkaline in nature with a pore solution pH of

12–13 that naturally passivates embedded reinforcement. The passivation of steel is broken

down by the presence of chloride ions or a reduction in alkalinity of concrete caused by

carbonation. Carbonation takes place as a result of the interaction of carbon dioxide with

the calcium hydroxide in concrete. The carbon dioxide gas dissolves in water to form

carbonic acid that reacts with calcium hydroxide and precipitates mainly as calcium

carbonate that lines the pores5. Depletion of hydroxyl ions lowers the pore water pH from

above 12.5 to below 9.0, where the passive layer becomes unstable, allowing general

corrosion to occur if sufficient oxygen and water are present in the vicinity of the bar.

Carbonation-induced corrosion causes problems aesthetically and structurally due to

expansive corrosion products that cause cracking, delamination and spalling of the

surrounding concrete. A cost-effective means of controlling reinforcing steel corrosion and

extending the service life of corrosion-affected structures is an essential requirement. Many

different approaches to concrete repair and protection exist and provide varying degrees of

long-term success. Protection systems include the following: use of epoxy-coated

reinforcement; protective coatings and membranes; cathodic protection; low permeability

concrete; desalination/realkalization; and admixed corrosion inhibitors. A further option is

a repair and protection system based on treatment with penetrating corrosion inhibitors.

Such inhibitors have the advantage that they are active primarily in the zone of concern

(i.e., the cover zone adjacent to steel reinforcing) and they can be applied at some point

after construction to either delay the onset of corrosion or retard further corrosion.


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1.9.3 Corrosion parameters

Atmospheric corrosion of carbon steel proceeds at rates upto 0.1 mm per year in

environments free of strong chemical splash, spillage or fumes. This corrosion results from

condensing moisture, sulphur dioxide from fuel combustion, dust bearing corrosives, and

the remoteness of structures and equipment from the washing effects of rain water. There

is often rapid and severe corrosion that cannot be stopped, even when inhibitors are used.

Such corrosion may be caused by biological organisms. Solution acidity, oxidizing agents,

temperature, film deposition, dissolved salts, fluid velocity and impurities are some of the

major corrosion contributors. The seven major corrosion parameters are a) solution acidity,

b) oxidizing agents, c) temperature, d) films, e) dissolved salts, f) fluid velocity and g)

impurities. Each parameter is explained as follows:

(a) Solution acidity

Solution acidity is represented by the concentration of hydrogen ions with the

relation:

pH = - log [H+]

Since the discharge of hydrogen ions takes places in most corrosion reactions, acidity of a

solution is one of the most important factors in corrosion combating. A survey of 944

cases6 involving carbon steel showed that 71 cases are related to corrosive acids. As a

general rule, acid (pH<7) solutions are more corrosive than neutral (pH = 7) or alkaline

(pH>7) solutions. In the case of ordinary iron or steel, the dividing line between rapid

corrosion in neutral or alkaline solutions occurs at about pH range of 4 to 10 with a rate of

about 0.3 mm per year. In the acidic environment (HCl addition), whereof pH = 2.9, the

corrosion rate is above 0.8 mm per year. In an alkaline environment (pH>10), the corrosion

of carbon steel is below 0.3 mm per year. Exceptions are the amphoteric metals such as


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aluminum and zinc in highly alkaline solutions, which cause even more corrosion than acid

solutions.

(b) Oxidizing agents

In some corrosion processes, such as the dissolution of zinc in hydrochloric acid,

hydrogen evolves as gas. In the dissolution of copper in sodium chloride, the removal of

hydrogen occurs by the reaction with some oxidizing chemical, such as oxygen, to form

water. For this reason, oxidizing agents are often powerful accelerators of corrosion. In

many cases, the oxidizing power of a solution is its most important single property.

Oxidizing agents can accelerate the corrosion of one class of materials and retard

the corrosion of another class. In the latter case, the behavior of the material usually

represents the surface formation of layers of oxides using absorbed oxygen, which make

the material more resistant to further chemical attacks. It is this property of chromium

which is responsible for the principal corrosion –resisting characteristics of stainless steel.

At room temperature, the corrosion rates of carbon steel in a slowly moving, air-

saturated (dissolved oxygen 6 ml/l ) water containing 165 parts per million (ppm) of CaCl2

range between a negligible amount to about 0.5 mm per year. The corrosion rate is almost

linearly proportional to the concentration of dissolved oxygen. Destructive effects of high

oxygen levels justify deaeration to lessen the rate of corrosion. In general, the expected rate

of attack for air-saturated water at low fluid velocities and ambient temperature is about

0.3 mm per year.

(c) Temperature

Rate of corrosion tends to increase with the increase in temperature. High

temperatures accelerate the diffusion of oxygen through cathodic layers of protective oxide

film. Temperature also has a secondary effect through its influence on the solubility of air

(or oxygen). Experimental results indicate that temperature rise of 18 to 20o C will double


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the corrosion rate. In a closed system, where the oxygen cannot escape, the corrosion rate

increases with temperature, until all the oxygen is consumed. In an open system where the

oxygen is free to escape the corrosion rate decreases with increase in T. This is due to a

drop in the oxygen solubility in water above 80oC. On the other hand, the corrosion rate of

stainless steel will increase considerably through the loss of the oxidizing substance

(dissolved oxygen), which is essential to maintain its protective film.

(d) Films

There are films made of metal oxide, oil and grease that can protect a material from

direct contact with corrosive substances. Such oil films can be applied intentionally or

occur naturally as in the case of metals submerged in sewage or equipment used for the

processing of oily substances. Once corrosion starts, its further progress often is controlled

by the nature of films that can form or accumulate on the metallic surface. One common

example is PbSO4 film on the lead container in contact with sulfuric acid. Another example

is the thin oxide film that forms on stainless steel surface.

Insoluble corrosion products may be completely impervious to the corroding

environment, hence completely protective, or they may be permeable and allow local or

general corrosion to proceed unhindered. Nonuniform or discontinuous film tends to

localize corrosion at certain points by initiating electrolytic effects of the concentration –

cell type. Films tend to retain or absorb moisture and thus, by delaying the time of drying,

increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive

vapors. It is generally agreed that the rust films formed on low-alloyed steels are more

protective than those formed on unalloyed steels.

(e) Dissolved Salts

A survey of 180 inorganic materials6 indicated that 51% of the salts are corrosive to

carbon steel at rates greater than 1.3 mm per year. Acid salts, such as aluminum chloride,


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ferrous chloride, ammonium chloride, etc., hydrolyze to form acid solutions. Acids salts

have a low pH, which will accelerate corrosion simply because of their acidic nature.

Alkaline salts hydrolyze to increase solution pH that can sometimes act as

corrosion inhibitors. Examples of these salts are trisodium phosphate, sodium tetraborate,

sodium silicate and sodium carbonate.

Oxidizing salts such as ferric chloride, cupric chloride, and sodium hypochlorite are

especially corrosive to carbon steel. Examples of oxidizing salts that are inhibitors include

Na2CrO4, NaNO2, and KMnO4.

Hard water is less corrosive than soft water. Deposition of calcium carbonate

provides a protective film, which retards corrosion by shielding oxygen from the cathodic

areas. However, protection by CaCO3 precipitation will be undesirable or unfeasible, since

it can clog equipment or reduce heat transfer.

In summary, the presence of acid or neutral salts will increase the corrosion rates,

whereas the presence of alkaline salts will decrease the corrosion rate.

(f) Fluid Velocity

An increase in the relative velocity between a corrosive fluid and a metallic surface

tends to accelerate the corrosion rate. This effect is due to the higher rate at which

corrosive chemicals, including oxidizing substances such as air, are brought to the

corroding surface. Whereas, corrosion resistance results from the accumulation of layers of

insoluble corrosion product on the metallic surface, the effect of high velocity will be

either to prevent their normal formation, or to remove them after they are formed. The

higher the velocity, the thinner will be the films through which corroding substances must

penetrate, and through which soluble corrosion products must diffuse. Either effect allows

corrosion to proceed unhindered. Similar effects are associated with cavitation-erosion

corrosion.


23

(g) Impurities

Impurities in a corrodent can be good or bad. The chloride ion is a good example;

the presence of a small amount of chloride in a fluid can break down the passive oxide film

on stainless steel. Some impurities can act as inhibitors to retard corrosion. For instance,

inorganic oxidizers such as chromates are used as corrosion inhibitors in cooling water

systems. However, if the impurity is removed, a marked increase in corrosion rates will

result. The effects of impurities are varied and complex. One should be aware of the type,

quantity causes and location before implementation.

1.10 Summary

This chapter presented the details on general definition of corrosion, classification

of corrosion, principles of corrosion, importance of corrosion, types of corrosion, in

general and concrete corrosion, in particular. Also the factors influencing the corrosion

have been discussed. The basic information to understand the overall problem related to

corrosion and to extend the research on corrosion has been explained.


24

1.11 References

1. M.G.Fontana and N.D.Green,

Corrosion Engineering, Mc Graw Hill, New York, Ed., (1978).

2. H.H.Uhlig,

Corrosion and Corrosion Control, 2nd

ed., John Wiley & Sons, New York, (1967).

3. A.Bentur, S. Diamond, and N.S. Berke,

Corrosion of Steel in Reinforced Concrete, E&FN Spon, London, (1996).

4. M.Santhanam,

Conc. Matt., 1(3) (2011) 1.

5. L.J.Parrott,

A review of carbonation in reinforced concrete. Rep., Cement and Concrete

Association/Building Research Establishment, Wexham Springs, U.K, 1986.

6. Corrosion Data survey – Metals Sections, 5th

ed., National Association of Corrosion

Engineers, Houston, Texas, (1974).


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