Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses. Three basic mechanisms of Scc have been identified.

Active path dissolution
This process involves accelerated corrosion along a path of higher than normal corrosion susceptibility, with the bulk of the material typically being passive.

The grain boundary is the most common active path, where segregation of impurity elements can make it marginally more difficult for passivation to occur. For example, when an austenitic stainless steel has been sensitised 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.

Hydrogen embrittlement
Hydrogen dissolves in all metals to a moderate extent. It is a very small atom, and fits in between the metal atoms in the crystals of the metal. Consequently it can diffuse much more rapidly than larger atoms. For example, the diffusion coefficient for hydrogen in ferritic steel at room temperature is similar to the diffusion coefficient for salt in water. Hydrogen tends to be attracted to regions of high triaxial tensile stress where the metal structure is dilated. Thus, it is drawn to the regions ahead of cracks or notches that are under stress. The dissolved hydrogen then assists in the fracture of the metal, possibly by making cleavage easier or possibly by assisting in the development of intense local plastic deformation. These effects lead to embrittlement of the metal; cracking may be either inter- or transgranular. Crack growth rates are typically relatively rapid, up to 1 mm/s in the most extreme cases. The bcc (body-centred cubic) crystal structure of ferritic iron has relatively small holes between the metal atoms, but the channels between these holes are relatively wide. Consequently, hydrogen has a relatively low solubility in ferritic iron, but a relatively high diffusion coefficient. In contrast the holes in the fcc (face-centred cubic) austenite lattice are larger, but the channels between them are smaller, so materials such as austenitic stainless steel have a higher hydrogen solubility and a lower diffusion coefficient. Consequently, it usually takes very much longer (years rather than days) for austenitic materials to become embrittled by hydrogen diffusing in from the surface than it does for ferritic materials, and austenitic alloys are often regarded as immune from the effects of hydrogen.

Film-Induced cleavage
If a normally ductile material is coated with a brittle film, then a crack initiated in that film can propagate into the ductile material for a small distance (around 1m) before being arrested by ductile blunting. If the brittle film has been formed by a corrosion process then it can reform on the blunted crack tip and the process can be repeated. The brittle films that are best-established as causing film-induced cleavage are de-alloyed layers (e.g. in brass). The film-induced cleavage process would normally be expected to give a transgranular fracture.

by Dr. R. A. Cottis, Corrosion and Protection Centre, UMIST under contract from NPL for the Department of Trade and Industry