Intergranular corrosion

Intergranular corrosion is preferential dissolution of the grain-boundary phases or the zones immediately adiacent to them, usually with slight or negligible attack on the main body of the grains. Grain boundaries are generally slightly more active chemically than the grains themselves, because they are areas of mismatch between the orderly and stable crystal lattice structure within the grains.

The preferential attack is enhanced by the segregation of specific elements or compounds, or enrichment of one of the alloying elements, in the grain boundaries, or by the depletion of an element necessary for corrosion resistance in the grain-boundary areas. Susceptibility io intergranular corrosion is usually related io thermal processing, such as welding or stress relieving, and can be corrected by a solution heat treatment, which redistributes alloying elements more uniformly; modification of the alloy; or the use of a completely different alloy.

When the attack is severe, entire grains may be dislodged because of the complete deterioration of their boundaries. In the presence of residual or applied stress, failure by SCC can occur before substantial intergranular attack has occurred. 
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