Definition.

Stress corrosion cracking is the intergranular or transgranular cracking of a material due to the combined action of tensile stress and a specific environment.

Mechanism.

Even after many years of intensive study, the exact mechanism of stress corrosion cracking remains a matter of extensive disagreement and study. It is commonly attributed to the rupture of protective films at the tips of pits or pre-existing cracks due to the applied stress. In many cases, the materials appear to be totally resistant to corrosion in a given environment until stresses are applied. They then crack catastrophically without any sign of other corrosion attack.

Examples.

Many materials, particularly high strength materials, are susceptible to stress corrosion cracking when exposed to a specific environment. For example, cold worked brass, which is found in ammunition cartridges, is susceptible to stress corrosion cracking when exposed to an environment containing ammonia. In chloride containing environments, titanium alloys, aluminum alloys, and high strength stainless steels are susceptible and specific alloys, which are resistant to stress corrosion cracking, should be used. The stresses required to initiate and propagate cracking are often low and many failures occur due to residual stresses rather than applied stress.

Appearance.

Stress corrosion cracking must be evaluated using microscopic examination of the cracked sections. The cracking is often branched. Stress corrosion cracking can occur in the presence of other forms of corrosion attack or without the presence of other visible attack.

Significant Measurements.

In general, alloys known to be susceptible to stress corrosion cracking should be avoided. In some cases, special heat treatments can minimize the susceptibility to stress corrosion cracking. Many tests have been developed to test the susceptibility of metals to stress corrosion cracking. All of these combine mechanical loading, often in the presence of a pre-existing crack, and exposure to the specific environment of interest. For alloys with limited sensitivity to stress corrosion cracking, critical stresses can be defined below which stress corrosion will not occur. The structural analysis and manufacturing processes required to accommodate these critical stress criteria are often very complex and the use of highly resistant materials is recommended.

Source : "Corrosion Control" NAVFAC MO-307 September 1992