Corrosion metallurgically influenced

Very pure single crystals have defects that can effect corrosion, but impurities and alloying elements, grain boundaries, second phases, and inclusions often have serious effects. Welded structures invariably corrode first at the welds because of metallurgical heterogeneities that exist in and near welds. The most susceptible site or defect in a metal will be the first to be attacked on exposure to a corrosive environment. Sometimes such attack simply results in innocuous removal of the susceptible material, leaving a surface with improved corrosion resistance. (Frankel)5 

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. 

The various classes of metallic phases that may be encountered in crystalline alloys include substantially pure elements, solid solutions of one element in another and intermetallic compounds. In crystalline form, alloys are subject to the same type of defects as pure metals. Crystalline alloys may consist of a solid solution of one or more elements (solutes) in the major (base) component, or they may contain more than one phase. That is, adjacent grains may have slightly or extremely different compositions and be of identical or disparate crystallographic types. Often, there is one predominant phase, known as the matrix, and other secondary phases, called precipitates. The presence of these kinds of inhomogeneities often results in the alloy having radically different mechanical properties and chemical reactivities from the pure constituent elements. (Noel)5 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27 

Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium 

A new microelectrochemical technique using microcapillaries as electrochemical cells has been developed. Small surface areas a few micrometers or even nanometers in diameter are exposed to the electrolyte. This leads to current resolution of the order of picoamperes. Microelectrochemical techniques combined with statistical approach of the data evaluation may result in greater understanding of the mechanism involved in these processes (29). 

1 Chemical Composition and Microstructure When an alloy composed of various elements corrodes, usually one or more elements dissolve preferentially 

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Metallurgically influenced corrosion is mainly composed of the corrosion due to chemical composition (alloying elements, metalloids and impurities), metallurgical properties (metallic phases, grain joints) and fabrication procedures (thermal treatments, lamination and welding). Figure 6.24 shows weld zone, dealloying, exfoliation and internal modes of attack. 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

R. Steigerwald, Metallurgically Influenced Corrosion, Corrosion, Vol 13, Metals Handbook, 9th ed., ASM International, 1987, p 123-135... 

Figure 1.11 Metallurgically influenced corrosion. (Reproduced with permission of NACE International from Reference 3.)...

Annealing is the only way to correct a sensitized stainless steel. Because different stainless steels require different temperatures, times, and quenching procedures, the user should contact the material supplier for such information. A number of tests can detect sensitization resulting from carbide precipitation in anstenitic and ferritic stainless steels. The most widely used tests are described in ASTM Standards A 262 and A 763. More detailed information on sensitization of stainless steels can be fonnd in varions books on metallurgically influenced corrosion. 

N.D. Greene and B.E. Wilde, Variable Corrosion Resistance of 18 Chromium-8 Nickel Stainless Steels Influence of Environmental and Metallurgical Factors, Corrosion, Vol 26, 1970, p 533-538... 

In recent decades many researchers have performed much work on the corrosion of magnesium alloys. They have studied the corrosion phenomena, the factors influencing corrosivity and the practical applications of magnesium alloys. They have also systematically studied different corrosion cases, compared the corrosion behaviours of different alloys in different environments, proposed some practical designs to avoid serious corrosion, and sununarized various effects of environmental and metallurgical factors on the corrosivity of magnesium alloys. There have, however, been very few mechanistic studies and some fundamental questions remain unanswered. These fundamental problems have potential... 

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