1
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
2
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
3
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
4
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
-
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
5
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
6
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
7
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
8
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
9
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
10
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
11
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
12
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-
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
13
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)
-
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
14
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
15
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
16
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
17
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
18
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
19
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
20
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
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
21
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,
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
22
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
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.
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.
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).
Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark.