Failure of metals by cracking under combined action of corrosion and stress, residual or applied. In brazing, the term applies to the cracking of stressed base metal due to the presence of a liquid filler metal. Stress-corrosion cracking was once and is still occasionally called chloride ion cracking, halogen-induced stress cracking, or halide contamination cracking.
There are believed to be two different sets of conditions present in a material in which stress corrosion cracking occurs: active path corrosion and hydrogen embrittlement. In active path corrosion, cracking is caused by localized corrosion at the crack tip, which then proceeds along a path which is electrochemically active with respect to the surrounding metal. In hydrogen embrittlement, cracking results from the entry of hydrogen into the metal, which reduces the capability of the metal to deform plastically.
The following conditions promote corrosion cracking: (1) a susceptible metal, (2) a specific environment, and (3) a tensile stress. Metal susceptibility and environment specificity depend on the particular metal-environment combination. A metal may be susceptible to stress coirosion cracking in only a few specific environments, and conversely, a particular environment may induce cracking in only certain metals. Usually, the tensile stress must exceed a specific level, depending on the particular metal-environment combination, to produce stress corrosion cracking.
The sequence of events generally leading to failure of a metal by stress corrosion cracking begins with localized chemical attack of the metal surface. A crack then initiates at a sharp intrusion produced by this attack, and grows slowly. When the crack reaches a size at which the metal can no longer support the load, rapid fracture occurs. If a crack-like flaw is already present in the metal surface, localized attack is not necessarily the cause; slow crack growth proceeds from the flaw.
The process of stress corrosion cracking involves a complex interaction of metallurgical, chemical, and mechanical factors. Since these three factors correspond to the three conditions which produce stress corrosion cracking, the role of each factor in this phenomenon must be considered.
Stress Corrosion Cracking in Stainless Steel- Experimental data indicates that as little as 5 ppm of available chloride is sufficient to cause stress corrosion cracking in stainless steel. To minimize the possibility, every precaution must be taken to ensure that all possible sources of halogen contamination are kept away from the base metal. Materials used in purging dams and related items should be only those known to be free of halogens. Several types of commercially available metal marking pens are known to contain significant quantities of available halogens and are capable of causing cracking. Perspiration from hand prints sometimes contains sufficient available chloride to cause stress corrosion cracking, so clean cotton gloves should be used in handling stainless steel.