Metallurgical Factors Affecting Corrosion

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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.

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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.

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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]

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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

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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

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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]

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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]

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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]

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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.

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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.

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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

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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]

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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.

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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

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

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

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

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

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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Metallurgical Factors Affecting Corrosion