Intergranular

The appearance of stress corrosion cracking may be either intergranular or transgranular in nature. 

Cihal V 1984 Intergranular Corrosion of Steels and Alloys (Amsterdam Elsevier)... 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Proceedings 1987 EPRI Workshop on Mechanisms of Primary Water Intergranular Stress Corrosion Crackiag," Report NP-5987M, Electric Power Research Institute, Palo Alto, Calif., Sept. 1988. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Copper increases tensile strength and hardness and offers some protection against elements that promote intergranular corrosion. However, copper reduces impact strength and dimensional stabiUty owing to aging and is therefore kept at 1.25% max. 

These nnstabdized grades of stainless steel have an increasing tendency to intergranular carbide precipitation as the caikon content increases above 0.03 percent. 

Intergranular Corrosion Selec tive corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

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