1. Corrosion is the primary means by which metals deteriorate when exposed to various environments like water, acids, bases, salts, oils, gases, and other chemicals. Understanding metallurgical factors that influence corrosion, like chemical composition, material structure, alloying elements, and heat treatment, is important for controlling corrosion in industries like petroleum and chemicals. 2. Key metallurgical factors that affect corrosion include the chemical composition, material structure, grain boundaries, alloying elements, mechanical properties, heat treatment, surface coating, welding, and manufacturing conditions of metals and alloys. 3. Common forms of corrosion include general corrosion, localized corrosion, intergranular corrosion, stress corrosion cracking, and corrosion

1. 1
Metallurgical Factors Affecting Corrosion in Petroleum and Chemical Industries
Dr. Eng. M. I. Masoud, Associate Professor
Industrial Engineering Department, Faculty of Engineering, Fayoum University
Abstract:
Humans have most likely been trying to understand and control corrosion for as long as
they have been using metal objects. With a few exceptions, metals are unstable in ordinary
aqueous environments. Certain environments offer opportunities for these metals to
combine chemically with elements to form compounds and return to their lower energy
levels. Corrosion is the primary means by which metals deteriorate. Most metals corrode on
contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal
polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to
gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing
gases. The production of oil and gas, its transportation and refining, and its subsequent use
as fuel and raw materials for chemicals constitute a complex and demanding process.
Various problems are encountered in this process, and corrosion is the major one. Since
metals are the principal material suffering corrosive deterioration, it is important to develop
a background in the principles of metallurgy to fully understand corrosion. The control of
corrosion through the use of coatings, metallurgy, nonmetallic materials for constructions
cathodic protection and other methods has evolved into a science in its own right and has
created industries devoted solely to corrosion control. Metallurgical factors that affect
corrosion are chemical composition, material structure, grain boundaries, alloying elements,
mechanical properties, heat treatment, surface coating, welding and manufacturing
conditions. Understanding these factors are of great importance to decrease and control
corrosion problem in many industrial applications.
:
Introduction
.
1
Corrosion is the destructive attack of a material by reaction with its environment. The
serious consequences of the corrosion process have become a problem of worldwide
significance. In addition to our everyday encounters with this form of degradation,
corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of
product, reduction in efficiency, costly maintenance, and expensive over design; it also
jeopardizes safety and inhibits technological progress. Corrosion control is achieved by
recognizing and understanding corrosion mechanisms, by using corrosion- resistant
materials and designs, and by using protective systems, devices, and treatments. Major
corporations, industries, and government agencies have established groups and committees
to look after corrosion-related issues, but in many cases the responsibilities are spread
between the manufacturers or producers of systems and their users. This study will focus
mainly on the metallurgical factors and how it can affect corrosion of materials and alloys.
From a purely technical standpoint, an obvious answer to corrosion problems would be to
use more-resistant materials. In many cases, this approach is an economical alternative to
other corrosion control methods. Corrosion resistance is not the only property to be
considered in making material selections, but it is of major importance in the chemical and
petroleum industries. The choice of a material is the result of several compromises. [1-7]
For example, the technical appraisal of an alloy will generally be a compromise between
corrosion resistance and some other properties such as strength and weldability.

2. 2
The final selection will be a compromise between technical competence and economic
factors. In specifying a material, the task usually requires three stages:
1. Listing the requirements
2. Selecting and evaluating the candidate materials
3. Choosing the most economical material.
Since metals are the principal material suffering corrosive deterioration, it is important to
develop a good background in the principles of metallurgical variables to fully understand
corrosion and its preventions. [5-8]
1.1. Corrosion Theory:
Corrosion specifically refers to any process involving the deterioration or degradation of
metal components. The best known case is that of the rusting of steel.
Corrosion processes are usually electrochemical in nature, having the essential features of a
battery. When metal atoms are exposed to an environment containing water molecules they
can give up electrons, becoming themselves positively charged ions provided an electrical
circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes,
a crack, or it can extend across a wide area to produce general wastage. Localized corrosion
that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive
agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting
corrosion also occurs much faster in areas where microstructural changes have occurred
due to welding operations. Corrosion is the disintegration of metal through an unintentional
chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to
be oxidized, some more easily than others. [8-10]
The corrosion process (anodic reaction) of the metal
dissolving as ions generates some electrons, as shown here,
that are consumed by a secondary process (cathodic
reaction). These two processes have to balance their
charges. The sites hosting these two processes can be
located close to each other on the metal's surface, or far
apart depending on the circumstances. This simple
observation has a major impact in many aspects of
corrosion prevention and control, for designing new
corrosion monitoring techniques to avoiding the most
insidious or localized forms of corrosion.

3. 3
1.2. Why Metals Corrode:
The driving force that makes metals corrode is a natural consequence of their temporary
existence in the metallic form. To reach this metallic form their occurrence in nature in the
form of various chemical compounds, called ores, it is necessary for them to absorb and
store up for later return by corrosion, the energy required to release the metals from their
original compounds. The amount of energy required and stored up varies from metal to
metal. It is relatively high for such metals as magnesium, aluminum, and iron and relatively
low for such metals as copper and silver. [11]
Following the different factors affecting
corrosion:
1. Material factor, 2. Environmental Factor, 3. Stress Factor, 4. Geometry Factor
5. Temperature Factor, 6. Time Factor
Stress corrosion cracking of mild steels in nitrate solutions and crack velocity increases by
the effect of high temperature. A lesson hard to learn is that corrosion usually will be
lessened or prevented by avoiding unnecessarily high temperatures, especially if these are
variable over a surface. [11, 12]
1.3. Forms of Corrosion and its Analysis:
Destruction by corrosion takes many forms depending on the nature of the metal or alloy,
the presence of inclusions or other foreign matter at the surface, the homogeneity of its
structure, the nature of the corrosive medium, the incidental environmental factors such as
the presence of oxygen and its uniformity, temperature, velocity of movement, and such
other factors as stress( residual or applied, steady or cyclic), oxide scales ( continuous or
broken), porous or semi-porous deposits on surfaces, built-in crevices, galvanic effects
between dissimilar metals and the occasional presence of “stray” electrical current from
external sources.
Fig. 1. Chart of different corrosion modes in petroleum and chemical industries. [5]

4. 4
Figure (1) represents a chart of failure modes distributions in chemical and petroleum
industries. [2,6]
Corrosion failure analysis involves metallurgical investigations of
components, equipment, metals, alloys, coatings, linings and structures due to corrosion,
environmental degradation and abuse, misapplication of the particular metal and
mechanical failure. [5]
Studies of failure analysis are particularly strong in the chemical
processing, refining and oil & gas industries. Failure mechanisms evaluated usually
include; general corrosion, localized corrosion, intergranular corrosion, weld corrosion,
stress corrosion cracking, fatigue & corrosion fatigue, fretting & wear, hydrogen
embitterment and hydrogen sulfide cracking.
1.4. Human Error as a Factor of Corrosion Failures:
The depth of the analysis into the roots of the failure is the key to accurately unearthing all
of the failure sources. In an effort to better understand the general sources of plant failures a
failure analysis company decided to look at the last three years' projects to determine the
major failure contributors. There were a total of 131 analyses and we list the primary failure
mechanisms below:
23 Corrosion 18%
57 Fatigue 44%
15 Wear 11%
17 Corrosion fatigue 13%
19 Overload 15%
total = 131
Note: In defining these five categories there is the possibility of confusion between
corrosion fatigue and fatigue. The practice was to assign fatigue as the mechanism in those
cases where the component would have eventually failed and corrosion was not needed to
affect the failure. In those situations where the component would not have failed without
the action of the corrosion, i.e., there was cyclic loading but it was not severe enough to
cause cracking without corrosion, the cause was listed as corrosion fatigue. [5, 8, 10]
According to this study, the attribution of responsibility for corrosion failures investigated
by a large US based chemical company was broken down into the following:
Lack of proving: new design, material or process 36
Lack of, or wrong, specifications 16
Bad inspection 10
Human error 12
Poor planning and coordination 14
Other 4
Unforeseeable 8
This data set indicates that only 8% of corrosion failures are unforeseeable. In other words
92 % of the corrosion failures could be preventable!?! [5, 8, 10]
2. Metallurgical Factors Affecting corrosion:
The structure of metals and alloys is of decisive importance in determining their corrosion
characteristics which cannot, therefore, be discussed without the use of metallurgical terms.
For example of these factors, sulfide inclusions in pure iron have a marked tendency to

5. 5
react in even mild corrosive environments. Also heavily deformed material by plastic
deformation at room temperature (cold worked, cold rolled, cold drown, etc.) have a
remarkable effect on the corrosion resistance of the materials. [18]
Figure (2) shows some
metallurgical factors that can affect corrosion failure of metals and alloys.
Fig. 2. Material factors controlling corrosion failure. [9]
In highly deformed metals, the grains are deformed and the grain structure is completely
disrupted. Normally, in this condition the material is somewhat more reactive in
electrochemical environments. In addition, the influence of impurities, inclusions, grain
boundaries and differences in grain orientation also may result significantly different
electrochemical reactivates in many metals and alloys. This will cause high corrosion
possibility. [9]
Some fundamental metallurgical concepts are presented in this section:
2.1. Effect of Crystal Structure Defects and Phases on corrosion:
On a smaller scale the differences in sub-microscopic characteristics of metals must be
considered. The crystal structure assumed to be perfect in three dimensions, but in reality
there are variations in the structure caused by crystalline defects. These defects may be
vacancies caused by the absence of atoms in the crystal, impurity atoms of different sizes,
interstitial atoms and large lattice disturbance called dislocations. Each of the mentioned
imperfections can produce highly localized differences in electrochemical behavior and
corrosion resistance of the material as well. The vacancy, the impurity atom and interstitial
atom are point defects, whereas dislocations are line defects affecting a much greater
volume of the crystal. In corroding environment (such as in petroleum and/or chemical
industries) these areas are usually more anodic than the surrounding matrix. The large
numbers of triangular corrosion pits are a result of electrochemical attack due to the stress

6. 6
field around dislocation. The shape of the pit is related to the orientation of the grain to the
corroded surface. [19-23]
Metals listed as pure or commercially pure, actually contain a variety of impurities and
imperfections. These impurities and imperfections are inherent cause of corrosion in
aggressive environment. It has been shown that, as purity increases, the tendency for a
metal to react in electrochemical environments reduces proportionality. However, high
purity metals generally have low mechanical strength; hence they are rarely used in
engineering applications. It is necessary, therefore to work with metallic materials which
are stronger and which are usually formed from a combination of several elemental metals.
The common alloys are those which have a good combination of mechanical, physical and
fabrication qualities which tend to make them structurally, as well as economically, useful.
Although an alloys resistance to corrosion is very important, this factor is sadly neglected
in the aim to improve the mechanical properties.
Alloying by itself has produced a number of materials which have improved corrosion
resistance compared to high purity metals. These materials are not truly corrosion resistant
under all conditions. In fact, they may be subjected to catastrophic failure as, intergranular
attack of sensitized stainless steel, stress corrosion cracking of brass, stainless steel and
high strength steel, etc. [24-26]
The properties of steel or any multiphase material depend greatly on the relative physical
and structural characteristics (amount, distribution, size, shape and strength) of the various
phases in the alloy. In many cases, multiphase materials present a problem from the
corrosion standpoint because the two phases may have marked differences in
electrochemical characteristics. It is possible that one phase will be selectively attacked in a
corrosive environment. However, some two-phase alloys have small electrochemical
differences and excellent corrosion resistance and others develop a protective film which
results in improved corrosion properties for both phases.
Ferrite
Fig. 3. Typical microstructure of 0.4%C steel, revealed ferrite and pearlite. [14]
In general, the existence of more than one phase in an alloy usually results in poorer
corrosion resistance than the equivalent single phase materials. Figures (3 and 4) shows
multi phase steel alloys of different carbon contents. [14, 27, 28]

7. 7
These are examples of some metallurgical factors affecting corrosion, this information if
used wisely, can be a tremendous help in obtaining the maximum in corrosion resistance
from alloys.
Fig. 4. Cementite precipitation at austenitic boundaries, remaining austenite is
transformed into pearlier (1.3 C %). [14]
2.2. Effect of Castings Composition Inhomogeneity on Corrosion:
It is very important to understand the casting inhomogeneous chemical composition as a
result of non-equilibrium solidification. This inhomogeneity is very important in
determining the corrosion characteristics of many cast alloys. If the cooling of the cast alloy
is rapid, sufficient diffusion cannot occur and non-equilibrium variations results. Consider a
single grain of material rapidly solidified from a liquid. A variation in composition will
exist from the interior to the surface of the grain because diffusion is not fast enough. This
variation in chemical composition from the interior to the exterior of a grain is termed
"coring". Cored structures generally have widely different corrosion characteristics and
actually have a built in electrochemical cell. In some cases it is possible to reduce major
chemical differences in a cored structure by reheating the alloy and holding it for a
relatively long time at temperature just below the solidus line. This allows diffusion to
occur more rapidly and assists in homogenization of the alloy. It is possible also to
determine the approximate temperature for homogenization annealing. It is important to
mention that many cases of intergranular corrosion result from this type of chemical
inhomogeneity. [5, 29-32]
2.3. Effect of Heat Treatment on Corrosion Behavior:
Many of the final mechanical properties and the corrosion resistance of a material can be
related to its heat treatment. That is its metallurgical processing using heat treatment to
transform or anneal the structure of the metal or alloy.
Annealing; is very important heat treatment process usually done to produce one of two
effects affecting corrosion resistance of the material: [5, 33, 34]

8. 8
First, homogenization of cast alloys which may exhibit chemical inhomogeneity (coring);
and second, remove residual stress and cold work effects in deformed metals. This
annealing treatment to a great extend reduces the corrosion tendency of the materials and
alloys. Chemical inhomogeneity of an alloy cause reduction in its mechanical properties, in
addition, reducing in corrosion resistance often occurs. Homogenizing anneal is important
for chemical homogeneity of an alloy by diffusion (movement of atoms in the solid state).
Residual stresses and cold work in deformed metals reduces its resistance to general
corrosion. Deformation decreases corrosion resistance basically because it increases
dislocations in the cold deformed metals; these areas of high dislocation density are usually
subjected to pitting corrosion. Often impurities or atoms of alloying metals migrate to these
imperfections to cause an even greater change in the electrochemical characteristics of
these defects. Thus annealing a cold worked metal may result in a decrease in dislocations
and important improvements in the corrosion resistance. Just as a homogenization anneal is
required to produce more uniform chemical composition, full annealing tends to produce a
more uniform crystal structure, with fewer defects leading to better corrosion resistance. [5,6]
Aluminum, magnesium, nickel, copper and some forms of stainless steel, when alloyed
with certain elements can be strengthened by age hardening or precipitation hardening
treatments. It is well known that pure aluminum forms a thin film of aluminum oxide which
provides a very effective barrier to environmental attack. This explains the extremely good
corrosion resistance of aluminum. Aluminum alloys do not readily form this protective
oxide. Therefore, the corrosion resistance of the important aluminum age hardening alloys
is not as good as pure aluminum with its atmospherically oxidized surface. To develop this
protection on alloys, a thin layer of pure aluminum is often clad to the outer surface. This
layer develops a protective surface oxide. It is also possible to use treatments to develop
thick oxide layers directly on the alloy. [34-37]
Because the cladding is only a few mils thick, little or no loss of the mechanical properties
of the alloy is noted. However, it is possible that deterioration of the resistance to corrosion
of clad material can occur due to improper heat treatment.
In the precipitation hardening alloy, the precipitated phase may cause a so-called denuded
zone adjacent to the grain boundaries that is more subject to corrosion attack. In various
corrosive environments, this results in preferential corrosion adjacent to and at grain
boundaries. This termed intergranular corrosion. [36-38]
2.4. Martensitic Transformation and Corrosion:
Quenching of steel from Gamma region (austenite phase) results in the formation hard and
brittle phase called martensite (hard and brittle phase), best combination of strength and
toughness can be obtained after tempering at intermediate temperature. Generally, an
increase in strength and hardness due to heat treatment is accompanied by decreased
corrosion resistance. If possible hardened steels should be protected in corrosive
environment by some form of surface treatments ranging from simply painting or
lubricating to special plating or coating procedures. A good example of the problem
encountered when using a heat treated unprotected high strength steel in the so-called
sucker rods used in the oil well pumping. When salt water or hydrogen sulfide solutions are
encountered, stress corrosion cracking occurs. [38-40]

9. 9
2.5. Corrosion Properties and Material Selection:
The corrosion engineer is often required to consider one or more properties in addition to
corrosion resistance and strength when selecting a material. The selection criteria used by
materials engineers in choosing from a group of materials includes a list of qualities that are
either desirable or necessary. Unfortunately, the optimum properties associated with each
selection criteria can seldom all be found in a single material, especially when the operating
conditions become aggressive. Thus, compromises must frequently be made to realize the
best performance of the selected material. A wide variety of iron- and nickel-based
materials are used for pressure vessels, piping, fittings, valves, and other equipment in
chemical industries. The most common of these is plain carbon steel. [11-15]
A mechanical property can be defined best as a measure of the ability of a material to
withstand the mechanical forces applied to it. Unfortunately, the ability of a metal or alloy
to withstand mechanical loading often is used as the sole criterion in material selection.
Only if the structure is protected from all environmental effects, is the mechanical property
the most important consideration. The problem of material selection in petroleum and
chemical industries is because; too often the relation of the metal to stress plus environment
is neglected. [32]
Although plan carbon steel often used in applications up to 482 to 516°C, most of its use is
limited to 316 to 343°C due to loss of strength and susceptibility to oxidation and other
forms of corrosion at higher temperature. Ferritic alloys, with additions consisting primarily
of chromium (0.5 to 9%) and molybdenum (0.5 to 1%), are most commonly used at
temperatures up to 650°C. Their comparative cost, higher strength, oxidation and
sulfidation resistance, and particular resistance to hydrogen, for example, result in their
being the material of choice. However, these low-alloy steels have inadequate corrosion
resistance to many other elevated temperature environments for which more highly alloyed;
Ni-Cr-Fe alloys are required. For applications for which carbon or low-alloy steels are not
suitable, the most common choice of material is from within the 18Cr-8Ni austenitic group
of stainless steels. These alloys and the 18Cr-12Ni stainless steels are favored for their
corrosion resistance in many environments and their oxidation resistance at temperatures up
to 816°C. [12-15]
3. Corrosion Control by Application of Metallurgical principals:
It is possible to reduce or prevent corrosion by application of the following metallurgical
principals:
1. Use of high purity material,
2. Use of alloying additions,
3. Effective heat treatments,
4. Use surface coatings,
5. Knowledge of the metallurgical history of the used material
A high purity metal has better general corrosion resistance and a reduced tendency to
pitting. The limits of their mechanical properties greatly reduce their application
possibilities. One of the best examples of improvement of corrosion resistance of a material
is found in results achieved by reducing the sulfur content of plain carbon steels. Corrosion
attack on a steel greatly reduced when its sulfur level is low.
Lead in zinc die cast alloys also has a marked effect on the corrosion characteristics of the
material. If the lead exceeds 0.002%, it precipitates at the grain boundaries and produces an
increased tendency for intergranular corrosion.

10. 10
Stainless steel 300L has a maximum limit of 0.03% carbon. This reduces the possibility of
sensitization as a result of welding or/and heat treatment. [18-20, 41-44]
Alloy additions: It is possible to improve the corrosion resistance of certain alloys by
specific alloying additions. The most notable is the addition of chromium to iron in
amounts of 12% or more. At or above this concentration, a passive film o chromium-iron
oxide is formed on the surfaces, which are the basics of stainless steel corrosion resistance.
Corrosion resistance also may be improved by changing the electrochemical potential of the
second phase in an alloy. For example, the addition of magnesium or chromium to
aluminum alloys. In these alloys, the intermetallic compound, FeAl3, greatly affects pitting
and corrosion resistance. The addition of magnesium or chromium changes the FeAl3 to a
complex Al-Fe-Mn or Al-Fe-Cr compound whose potential now approaches that of the
aluminum itself. Consequently, the pitting tendency is greatly reduced. [6, 47-49]
The intergranular sensitization of stainless steels due to the formation of chromium carbide
at grain boundaries can be prevented using alloying additions. In these alloys, it is possible
to add columbium or titanium which selectively ties up the carbon as carbides, eliminating
chromium carbide formation and the sensitization of the alloys as well.
The addition of chromium, particularly to nickel and iron base materials increases their
high temperature oxidation resistance. [6, 50]
Heat treatment: It has been discussed later that it is possible to modify the structure of
metals and alloys in many ways through heat treatment. Age-hardening heat treatments also
have great effect on the corrosion resistance of alloys. Many alloys have a temperature
range for aging in which they are not susceptible to intergranular corrosion. However, the
aging temperature for optimum mechanical properties of an alloy may reduce susceptibility
to intergranular corrosion. Often it is important to sacrifice some mechanical properties in
order to improve corrosion resistance. [45]
It has been shown recently that stress relief greatly improves the resistance of 300 Series
stainless steels to stress corrosion. Of course, the stress relieving must be below the
temperature of sensitization, 427°C. High zinc brasses, susceptible to stress corrosion in the
cold working state, also have been protected effectively by stress relief.
Heat treatment of the surfaces to increase surface hardness of the material or improve the
stability of surface films is important in resisting all types of corrosion but is particularly
useful in improving fretting and erosion-corrosion resistance. [50]
Surface coatings for corrosion control: Because the corrosion reactions occur at metal-
environment interface, it is logical that the interpositions of barriers between the substrate
and the environment would influence the corrosion rate. Various types of barriers are
commonly used for corrosion control including a wide range of metals, inorganic and
organic materials.
Relation of metallurgical history and corrosion: Nearly all forms of metal deterioration
are dependant upon the metallurgical history of the material. Impurities retained from the
original extractive processes, the inclusions and imperfections introduced in casting and
forming, plus structure variations due to heat treatment all alter the corrosion stability of
metals and/or alloys. Thus a thorough knowledge of the background of the material is
important. For example:
1. When steels were made in acid open hearth, higher sulfur resulted and, of course,
corrosion resistance will be lower.
2. Hot rolling steel results in scale formation which, if not properly removed by
pickling, serves to produce corrosion pit initiation sites.

11. 11
3. Slow cooling of regular 300 Series stainless steel through the 760°C to 427°C
temperature range sensitizes it to intergranular corrosion. Rapid cooling or
quenching through this range avoids sensitization. [51, 52]
4. Residual stresses due to cold working of an alloy or unequal cooling of welded
structures can lead to stress corrosion.
Thus knowledge of the basic principles of metallurgy and an understanding of the
metallurgical history of the material is extremely important to the final behavior of the
metal in service especially from the corrosion standpoint of view. [51, 52]
4. Steel Corrosion:
Iron and steel, the most commonly used metals, corrode in many media including most
outdoor atmospheres. Usually they are selected not for their corrosion resistance but for
such properties as strength, ease of fabrication, and cost. These differences show up in the
rate of metal lost due to rusting. All steels and low-alloy steels rust in moist atmospheres. In
some circumstances, the addition of 0.3% copper to carbon steel can reduce the rate of
rusting by one quarter or even by one half. The elements copper, phosphorus, chromium
and nickel have all been shown to improve resistance to atmospheric corrosion. Formation
of a dense, tightly adhering rust scale is a factor in lowering the rate of attack. The
improvement may be sufficient to encourage use without protection, and can also extend
paint life by decreasing the amount of corrosion underneath the paint. The rate of rusting
will usually be higher in the first year of atmospheric exposure than in subsequent years,
and will increase significantly with the degree of pollution and moisture in the air. [6, 53]
During hot rolling and forging the steel surface is oxidized by air and the scale produced,
usually termed millscale. In air, the presence of millscale on the steel may reduce the
corrosion rate over comparatively short periods, but over longer periods the rate tends to
rise. In water, severe pitting of the steel may occur if large amounts of millscale are present
on the surface. [6, 53]
5. Stainless Steel Corrosion:
Stainless and heat resisting steels possess unusual resistance to attack by corrosive media at
atmospheric and elevated temperatures, and are produced to cover a wide range of
mechanical and physical properties for particular applications. Stainless steels are mainly
used in wet environments, as oil filed and chemical industries. With increasing chromium
and molybdenum contents, the steels become increasingly resistant to aggressive solutions.
The higher nickel content reduces the risk of stress corrosion cracking (SCC). Austenitic
steels are more or less resistant to general corrosion, crevice corrosion and pitting,
depending on the quantity of alloying elements. Resistance to pitting and crevice corrosion
is very important if the steel is to be used in chloride containing environments. Resistance
to pitting and crevice corrosion typically increases with increasing contents of chromium,
molybdenum and nitrogen. [54]
Corrosion resistance of stainless steels is a function not only of composition, but also of
heat treatment, surface condition, and fabrication procedures, all of which may change the
thermodynamic activity of the surface and thus dramatically affect the corrosion resistance.
It is not necessary to chemically treat stainless steels to achieve passivity. The passive film
forms spontaneously in the presence of oxygen. Most frequently, when steels are treated to

12. 12
improve passivity (passivation treatment), surface contaminants are removed by pickling to
allow the passive film to reform in air, which it does almost immediately. [54]
5.1. Pickling and Passivation of Stainless Steel:
Stainless steel can corrode in service if there is contamination of the surface. Both pickling
and passivation are chemical treatments applied to the surface of stainless steel to remove
contaminants and assist the formation of a continuous chromium-oxide, passive film.
Pickling and passivation are both acid treatments and neither will remove grease or oil. If
the fabrication is dirty, it may be necessary to use a detergent or alkaline clean before
pickling or passivation. [56]
5.2. Stainless Steel Weld Decay:
This type of intergranular corrosion can occur in the heat-affected zone of welded
components and also in cast components of stainless steel due to precipitation, during
cooling, of chromium carbides at the grain boundaries (and hence loss of chromium in the
immediately-adjacent zone). The local loss in corrosion resistance arises because the
chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of
stainless steels. The susceptibility to weld decay can be counteracted by carrying out a
suitable post-weld heat treatment to restore a uniform composition at the grain boundaries
but this is clearly often not a practicable proposition. Consequently the usual strategy in
combating weld decay is by the choice of stainless steel with either of the two following
features:
a. Specification of a stainless steel containing a small amount of either titanium or
niobium; which have a higher affinity than does chromium for carbon: hence
carbides of these elements tend to form instead of chromium carbides, thus
avoiding the Cr-depletion problem: such steels are usually termed “stabilised
stainless steels”
b. Specification of a stainless steel with low carbon content (< 0.03%); this will
clearly decrease the likelihood of carbide formation in the steel. Such low-carbon
grades of stainless steel are often designated by a “L” in their code; for instance
the “316” grade of steel (18%Cr/10Ni/2.5Mo) is designated as “316L” when its
carbon content has been limited in this way. [6, 54]
5.3. Problems of Welding Stainless Steel:
To fabricate complex equipments and structures for modern industries it is necessary to
produce structurally sound joint using different welding procedures.
Through the application of heat, welding may:
1. Induce phase transformations,
2. Cause secondary precipitation,
3. Produce high stress in the adjacent to weld which greatly reduce corrosion
resistance in these zones.
For example, welding can cause intergranular sensitization of stainless steels. In the heat
affected zone to welds (HAZ), sensitization can occur in austenitic stainless steels and this
can lead to rapid deterioration and destruction especially in high corrosive atmosphere like
petroleum and chemical industries. [44]

13. 13
During welding, a large variation of thermal expansion occurs between the solidifying
molten metal pool and the base metal. Upon solidification, this often can result in high
tensile stresses. Under these conditions, the highly stressed areas are subjected to stress
corrosion cracking in corrosive environment. It is important to stress relieve welds exposed
to environments that could cause stress corrosion cracking.
The other basic metallurgical considerations discussed in this survey should be considered
in welding. For example, for optimum corrosion resistance, it is important to maintain
homogeneity between the weld and base metal, so weld filler-metals should be used which
are chemically and electrochemically similar to the base metals. In addition, any phase
transformations which could occur in welding must be considered in estimating the
corrosion stability of welded structures. [45]
5.4. Problems Involved in Heat Treatment of Stainless Steel:
Stainless steels, particularly the 300 Series are subject to a heat treating effect called
"sensitization". These alloys when heated in 427 to 760°C form chromium carbide, these
carbides forms only at the grain boundary. Thus the chromium near the grain boundaries is
tied up as carbide and no longer can act as a deterrent to corrosion. The grain boundaries
are susceptible to intergranular corrosion and are anodic to the surrounding grains. [41-44]
Fig. 5. Intergranular corrosion occurs in stainless steel. [36]
Sensitized stainless steels can deteriorate completely in the specific hours in strongly acid
solutions. The principals of heat treatment applicable to each material must be considered
to produce the more effective combination of mechanical properties and environmental
stability. Figure (5) is schematic illustration of stainless steel intergranular corrosion. [44]
6. Stress Corrosion Cracking (SCC):
Stress corrosion cracking (SCC) is the cracking induced from the combined influence of
tensile stress and a corrosive environment. The required tensile stresses may be in the form
of directly applied stresses or in the form of residual stresses. The problem itself can be
quite complex. The situation with buried pipelines is a good example of such complexity.

14. 14
Fig. 6. Intergranular SCC occurs in a heat exchanger (X500). [6]
Cold deformation and forming, welding, heat treatment, machining and grinding can
introduce residual stresses. The magnitude and importance of such stresses is often
underestimated. The residual stresses set up as a result of welding operations tend to
approach the yield strength. Stress corrosion cracking usually occurs in certain specific
alloy-environment-stress combinations. [6]
Fig. 7. SCC of 316 stainless steel chemical processing piping system (X300). [6]
Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a
catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and
the damage not easily predicted. Figure (6) micrograph illustrates intergranular SCC of an
heat exchanger tube with the crack following the grain boundaries.
The micrograph of SCC in a 316 stainless steel chemical processing piping system was
shown in Fig. (7). Chloride stress corrosion cracking in austenitic stainless steel is
characterized by the multi-branched "lightning bolt" transgranular crack pattern.

15. 15
6.1. Stress Corrosion Crack Propagation Rate:
Stress corrosion cracks (shown in Fig. (8)) propagate over a range of velocities from about
10-3
to 10 mm/h, depending upon the combination of alloy and environment involved. Their
geometry is such that if they grow to appropriate lengths they may reach a critical size that
result in a transition from the relatively slow crack growth rates associated with stress
corrosion to the fast crack propagation rates associated with purely mechanical failure. [6]
6.2. Pipeline Stress Corrosion Cracking:
Over 98% of pipelines are buried. No matter how well these pipelines are designed,
constructed and protected, once in place they are subjected to environmental abuse, external
damage, coating disbandment, inherent mill defects, soil movements/instability and third
party damage. In pipelines SCC occurs due to a combination of appropriate environment,
stresses and material. Environment is a critical causal factor in SCC. High-pH SCC failures
of underground pipelines have occurred in a wide variety of soils, covering a range in color,
texture, and pH. No single characteristic has been found to be common to all of the soil
samples. Coating types such as coal tar, asphalt and polyethylene tapes have demonstrated
susceptibility to SCC. Fusion bonded epoxy hasn't shown susceptibility to SCC. Propagated
SCC of pipeline is shown in Fig. (8). [6, 45]
Figure 8 Stress corrosion crack propagation. [6]
Loading is the next most important parameter on SCC. Cyclic loading is considered a very
important factor; or the crack tip strain rate defines the extent of corrosion or hydrogen
ingress into the material. [6]
6.3. Control of Stress Corrosion Cracking:
The most effective means of preventing SCC are: 1) design properly with the right
materials; 2) reduce stresses; 3) remove critical environmental species such as hydroxides,
chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat exchangers where
chloride and hydroxide might become concentrated. Low alloy steels are less susceptible
than high alloy steels, but they are subject to SCC in water containing chloride ions. [5-7]
In an ideal world a stress corrosion cracking control strategy will start operating at the
design stage, and will focus on the selection of material, the limitation of stress and the

16. 16
control of the environment. The skill of the engineer then lies in selecting the strategy that
delivers the required performance at minimum cost: [6]
6.3.1. Selection and Control of Material:
The first line of defense in controlling stress corrosion cracking is to be aware of the
possibility at the design and construction stages. By choosing a material that is not
susceptible to SCC in the service environment and by processing and fabricating it
correctly, subsequent SCC problems can be avoided. Unfortunately, it is not always quite
that simple. Some environments, such as high temperature water are very aggressive, and
will cause SCC of most materials. Mechanical requirements, such as high yield strength,
can be very difficult to reconcile with SCC resistance (especially where hydrogen
embrittlement is involved). Finally, of course the materials that are resistant to SCC will
almost inevitably be the most expensive. [6, 25]
6.3.2. Control of Material Stress:
As one of the requirements for stress corrosion cracking is the presence of stresses in the
components, one method of control is to eliminate that stresses, or at least reduce it below
the threshold stress for SCC. This is not usually feasible for working stresses, but it may be
possible where the stress causing cracking is a residual stress introduced during welding or
forming. Residual stresses can be relieved by stress-relief annealing, and this is widely used
for carbon steels. [12]
For large structures, for which full stress-relief annealing is difficult or impossible, partial
stress relief around welds and other critical areas may be of value. However, this must be
done in a controlled way to avoid creating new regions of high residual stress, and expert
advice is advisable if this approach is adopted. [17]
6.3.3. Control of Environment:
One of the most direct ways of controlling SCC through control of the environment is to
remove or replace the component of the environment that is responsible for the problem.
Unfortunately, it is relatively rare for this approach to be applicable. [23]
7. Intergranular Corrosion:
The microstructure of metals and alloys is made up of grains, separated by grain
boundaries. Intergranular corrosion is localized attack along the grain boundaries, or
immediately adjacent to grain boundaries, while the bulk of the grains remain largely
unaffected. This form of corrosion is usually associated with chemical segregation effects
(impurities have a tendency to be enriched at grain boundaries) or specific phases
precipitated on the grain boundaries. The attack is usually related to the segregation of
specific elements or the formation of a compound in the boundary. Corrosion then occurs
by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost
an element necessary for adequate corrosion resistance, thus making the grain boundary
zone anodic relative to the remainder of the surface. The attack usually progresses along a
narrow path along the grain boundary and, in a severe case of grain-boundary corrosion;
entire grains may be dislodged due to complete deterioration of their boundaries. In any
case the mechanical properties of the structure will be seriously affected. [32]

17. 17
A classic example is the sensitization of stainless steels or weld decay. Chromium-rich
grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these
precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes.
Reheating a welded component during multi-pass welding is a common cause of this
problem. In austenitic stainless steels, titanium or niobium can react with carbon to form
carbides in the heat affected zone (HAZ) causing a specific type of intergranular corrosion
known as knife-line attack. These carbides build up next to the weld bead where they
cannot diffuse due to rapid cooling of the weld metal. The problem of knife-line attack can
be corrected by reheating the welded metal to allow diffusion to occur. [6, 12]
8. Corrosion Fatigue:
Corrosion-fatigue is the result of the combined action of an alternating or cycling stresses
and a corrosive environment. The fatigue process is thought to cause rupture of the
protective passive film, upon which corrosion is accelerated. If the metal is simultaneously
exposed to a corrosive environment, the failure can take place at even lower loads and after
shorter time. In a corrosive environment the stress level at which it could be assumed a
material has infinite life is lowered or removed completely. Contrary to a pure mechanical
fatigue, there is no fatigue limit load in corrosion-assisted fatigue. Much lower failure
stresses and much shorter failure times can occur in a corrosive environment compared to
the situation where the alternating stress is in a non-corrosive environment as shown in Fig.
(9) [6]
Fig. 9. Effect of corrosion atmosphere on the fatigue failure stress. [6]
The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-
corrosion cracking, but not branched. The picture shown in Fig. (10) reveals a primary
corrosion-fatigue crack that in part has been widened by a secondary corrosion reaction.
The corrosive environment can cause a faster crack growth and/or crack growth at a lower
tension level than in dry air. Even relatively mild corrosive atmospheres can reduce the
fatigue strength of aluminum structures considerably, down to 75 to 25% of the fatigue
strength in dry air. [11]
No metal is 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 various corrosion control
measures. [6, 11]

18. 18
Fig. 10. Effect of corrosion atmosphere on the fatigue failure stress. [6]
9. Hydrogen Embrittlement:
This is a type of deterioration which can be linked to corrosion and corrosion-control
processes. It involves the ingress of hydrogen into a component, an event that can seriously
reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle
failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement
occurs in a number of forms but the common features are an applied tensile stress and
hydrogen dissolved in the metal. Examples of hydrogen embrittlement are cracking of
weldments or hardened steels when exposed to conditions which inject hydrogen into the
component. Presently this phenomenon is not completely understood and hydrogen
embrittlement detection, in particular, seems to be one of the most difficult aspects of the
problem. Hydrogen embrittlement does not affect all metallic materials equally. [6, 44]
9.1. Sources of Hydrogen:
Sources of hydrogen causing embrittlement have been encountered in the making of steel,
in processing parts, in welding, in storage or containment of hydrogen gas, and related to
hydrogen as a contaminant in the environment that is often a by-product of general
corrosion. Hydrogen may be produced by corrosion reactions such as rusting, cathodic
protection, and electroplating. Hydrogen entry, the obvious pre-requisite of embrittlement
can be facilitated in a number of ways summarized below: [6, 17]
a. By some manufacturing operations such as welding, electroplating, and pickling; if
a material subject to such operations is susceptible to hydrogen embrittlement then a
final, baking heat treatment to expel any hydrogen is employed
b. As a by-product of corrosion reaction such as in circumstances when the hydrogen
production reaction acts as the cathodic reaction since some of the hydrogen
produced may enter the metal in atomic form rather than be all evolved as a gas into
the surrounding environment. In this situation, cracking failures can often be
thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide
causes entry of hydrogen into the component, the cracking phenomenon is often
termed “sulphide stress cracking (SSC)”. [6, 17]

19. 19
9.2. Hydrogen Embrittlement of Stainless Steel:
Hydrogen diffuses along the grain boundaries and combines with the carbon, which is
alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in
small voids along the grain boundaries where it builds up enormous pressures that initiate
cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at
a neutral or basic pH in plants without aluminum components. [6, 37]
If the metal is under a high tensile stress, brittle failure can occur. At normal room
temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through
the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking
under these conditions, the path is transgranular. At high temperatures, the absorbed
hydrogen tends to gather in the grain boundaries and stress-induced cracking is then
intergranular. The cracking of martensitic and precipitation hardened steel alloys is
believed to be a form of hydrogen stress corrosion cracking that results from the entry into
the metal of a portion of the atomic hydrogen. [6, 17]
Summary:
Corrosion control is very important in all around us for many industrial applications as
well. There are several methods for corrosion control first, proper material selection and
design, metal coating, cathodic protection, corrosion inhibitors, using non-metallic
materials, etc. It is concluded from this study that metallurgical factors is the milestone of
the right way for minimizing corrosion to be as less as possible in the most corrosive
environment industries. There are many metallurgical factors that affect corrosion as,
chemical composition, material structure, material structure imperfections and defects,
grain boundaries, alloying elements, mechanical properties, heat treatment, surface coating,
welding and manufacturing conditions and stresses (residual or applied). Understanding
these factors is of great importance to minimize and control corrosion problem in many
industrial applications. Since the environment play an important role in materials corrosion,
petroleum and chemical industries revealed many corrosion problems as, pitting corrosion,
stress corrosion cracking, corrosion fatigue, intergranular corrosion, etc. This because the
very corrosive atmosphere in these kind of industries. All these corrosion failures are to
great extent depend on the abovementioned metallurgical factors. Thus, the corrosion
engineer must understand the basics and fundamentals of metallurgy very well. Many of the
corrosion failure problems can be prevented by a proper attention from the early stage of
material manufacturing, processing, treatment and machining. Because corrosion of metal
is so deeply involved in the basic principals of metallurgy, a few pertinent references which
give additional details on the various metallurgical phenomena are cited in the
Bibliography. Hopefully, this study can help the corrosion engineer to have enough
knowledge and background of metallurgical concepts that help for corrosion prevention and
control. It is believed that, this work needs more study and investigations in the future.
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