In searching for the underlying causes that trigger SCC, three basic mechanisms have been identified. (reference)
This process involves accelerated corrosion along a path of higher than normal corrosion susceptibility, with the bulk of the material typically being passive. The most common active path is the grain boundary, where segregation of impurity elements can make it marginally more difficult for passivation to occur. For example, when an austenitic stainless steel has been sensitized by precipitation of chromium carbide along the grain boundary, the local chromium concentration at the grain boundary will be reduced, and this region will be slightly less easily passivated.
Consequently, a form of crevice corrosion can occur, whereby the grain boundary corrodes, with the specimen surface and the crack walls remaining passive. This process can occur in the absence of stress, giving rise to intergranular corrosion that is uniformly distributed over the specimen. The effect of the applied stress is probably mainly to open up the cracks, thereby allowing easier diffusion of corrosion products away from the crack tip and allowing the crack tip to corrode faster. Active path corrosion processes are inherently limited by the rate of corrosion of the metal at the crack tip, which limits the maximum crack growth rate to around 10-2mm/s, and crack growth rates are often much lower, down to around 10-8 mm/s (about 1 mm in 3 years) or less.