Metallurgical Factors Affecting Corrosion

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. 1. Introduction: 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.

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.

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]

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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 57 Fatigue 15 Wear 17 Corrosion fatigue 19 Overload total = 131

18% 44% 11% 13% 15%

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 Lack of, or wrong, specifications Bad inspection Human error Poor planning and coordination Other Unforeseeable

36 16 10 12 14 4 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 metalenvironment 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.

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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

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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]

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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 stresscorrosion 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]

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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]

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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. References: 1. Robert F. Hochman, NACE Basic Corrosion Course, , Houston, Texas, 1970. 2. M. G. Fontana and J. H. Peacock, NACE Basic Corrosion Course, Houston, Texas, 1970. 3. F. L. LaQUE, NACE Basic Corrosion Course, Houston, Texas, 1970.

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4. Scott D. and Willium W. Scott, Jr., Corrosion in Petrochemical Industries, ASM International, 1995. 5. Einar Mattsson, Basic Corrosion Technology, the Institute of Materials, London, 1996. 6. http://www.corrosion-doctor.org. 7. Bogart, L.G. Vande, Technical Paper No. 408, Crane Co. Chicago, Illinois, May 1939. 8. Brasunas, Anton, NACE Basic Corrosion Course, Texas, 1970. 9. Bruce D. Craig, Practical Oil-Field Metallurgy, PennWell Publishing Co., 1984. 10. A. W. Peabody, NACE Basic Corrosion Course, Houston, Texas, 1970. 11. Ellis D. Verink, NACE Basic Corrosion Course, Houston, Texas, 1970. 12. N. E. Hamner, NACE Basic Corrosion Course, Houston, Texas, 1970. 13. W. E. Berry, NACE Basic Corrosion Course, Houston, Texas, 1970. 14. http://www.Sciencedirect.com. 15. N. D. Greene, Jr., NACE Basic Corrosion Course, Houston, Texas, 1970. 16. N. Hackerman, NACE Basic Corrosion Course, Texas, 1970. 17. Anton deS. Brasunas, NACE Basic Corrosion Course, Houston, Texas, 1970. 18. B. W. Lifka, NACE Basic Corrosion Course, Houston, Texas, 1970. 19. K. G. Compton, NACE Basic Corrosion Course, Houston, Texas, 1970. 20. M. E. Parker, NACE Basic Corrosion Course, Houston, Texas, 1970. 21. H. P. Godard, NACE Basic Corrosion Course, Houston, Texas, 1970. 22. Evans, Ulick R., An Introduction to Metallic Corrosion, Edward Arnold, London, UK, 1948. 23. Fontana, Mars G. & Greene, Norbert D., Corrosion Engineering, McGraw-Hill, New York, New York, 1967. 24. LaQue, F.L., May, T.P. & Uhlig, H. H., Corrosion in Action, International Nickel Company of Canada, Toronto, Canada, 1955. 25. McKay, Robert J. & Worthington, Robert, Corrosion Resistance of Metals and Alloys, Reinhold Publishing, New York, 1936. 26. Trethewey, K.R. & Chamberlain, J., Corrosion for Students of Science and Engineering, Longman Scientific & Technical, Burnt Mill, UK, 1988. 27. Gosta Wranglen, An Introduction to Corrosion and Protection of Metals,Chapman and Hill, New York, 1985. 28. B. Arzamasove, Material Science, Mir Publishirs Moscow, 1989 29. William D. Callister, Jr., Materials Science and Engineering an introduction, John Wiley & Sons, Inc., New York, 2003. 30. William F. Smith, Javad Hashemi, Foundation of Materials Science and Engineering, McGrawHill, New York, 2004. 31. Fathy M. Bayoumi, and Wafaa A. Ghanem, Materials letters,Vol. 59, pp. 38063809, 2005. 32. Liu Chenglong, Yang Dazhi, Lin Guoqiang and Qi Min, Materials Letters,Vol.59, pp. 3813-3819, 2005. 33. J. Hu, W. Y. Chu, W. C. Ren, C. K. Yao and L.J. Qiao, The Journal of Science and Engineering, Corrosion, Vol.60, No.2, pp. 181-186, 2004. 34. S.U. Koh, B.Y. Yang, and K.Y. Kim, Journal of Science and Engineering, Corrosion, Vol.60, No.3, pp. 262-274, 2004. 35. S.U. Koh, J.S. Kim, B.Y. Yang, and K.Y. Kim, Journal of Science and Engineering, Corrosion, Vol.60, No.3, pp. 244-253, 2004.

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36. R.Chu, W. Chen, S.H. Wang, T.R. Jack, and R.R. Fesser, Journal of Science and Engineering, Corrosion, Vol.60, No.3, pp. 275-283, 2004. 37. Q. meng, G.S. Frankel, H.O. Colijn, and S.H. Goss, Journal of Science and Engineering, Corrosion, Vol.60, No.4, pp. 346-355, 2004. 38. F. Zucchi, G. Trabanelli, V. Grassi, and A. Frignani, Journal of Science and Engineering, Corrosion, Vol.60, No.3, pp.363-368, 2004. 39. S. Wang and R.C. Newman, Journal of Science and Engineering, Corrosion, Vol.60, No.5, pp. 448-4454, 2004. 40. J.R. Kish, M.B. Lves, and J.R. Rodds, Journal of Science and Engineering, Corrosion, Vol.60, No.6, pp. 523-537, 2004. 41. D.A. Moreno, B. Molina, C. Ranninger, F. Montero, and J. Izquierdo, Journal of Science and Engineering, Corrosion, Vol.60, No.6, pp. 573-583, 2004. 42. Z. Zeng, K. Natesan, and M. Grimsditch, Journal of Science and Engineering, Corrosion, Vol.60, No.7, pp. 632-642, 2004. 43. I.G. Chamritski, G.R. Burns, B.J. Webster, and N.J. Laycock, Journal of Science and Engineering, Corrosion, Vol.60, No.7, pp. 658-669, 2004. 44. M. Yamamoto, J. Kuniya, and S. Uchida, Journal of Science and Engineering, Corrosion, Vol.60, No.7, pp. 681-688, 2004. 45. K.M. Ismail, A.M. Fathi, and W.A. Badawy, Journal of Science and Engineering, Corrosion, Vol.60, No.9, pp. 795-803, 2004. 46. D.K. Lysogorski, and W.H. Hartt, Journal of Science and Engineering, Corrosion, Vol.60, No.9, pp. 815-823, 2004. 47. R.E. Melchers, Journal of Science and Engineering, Corrosion, Vol.60, No.9, pp. 824-836, 2004. 48. P.M. Singh, J.J. Perdomo, J.E. Oteng, and J. Mahmood, Vol.60, No.9, pp. 852-861, 2004. 49. T. Anita, H. Shaikh, H.S. Khatak, and G. Amarendra, Journal of Science and Engineering, Corrosion, Vol.60, No.9, pp. 873-882, 2004. 50. Y. Li, f. Wang, and G. Liu, Journal of Science and Engineering, Corrosion, Vol.60, No.10, pp. 391-396, 2004. 51. A. Igual Munoz, J.G. Anton, J.L.Guinen, and V.P. Herranz, Journal of Science and Engineering, Corrosion, Vol.60, No.10, pp. 982-995, 2004. 52. Y.Y. Chen, L.H. Wang, J.C. Oung, and H.C. Shin, Journal of Science and Engineering, Corrosion, Vol.61, No.3, pp. 273-284, 2005. 53. G. hinds, j. Zhao, A.J. Griffiths, and A. Turnbull, Journal of Science and Engineering, Corrosion, Vol.61, No.4, pp. 348-354, 2005. 54. R.E. Melchers, Journal of Science and Engineering, Corrosion, Vol.61, No.4, pp. 355-363, 2005. 55. Y.S. Choi, J.J. Shim, and J.G.Kim, Journal of Science and Engineering, Corrosion, Vol.61, No.5, pp. 490-497, 2005. 56. C.F.Chen, M.X. Lu, D.B. Sun, Z.H. Zhang, and W. Chang, Journal of Science and Engineering, Corrosion, Vol.61, No.6, pp. 594-601, 2005.

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