SCC has been one of – if not the – most widely studied forms of corrosion. This is because it is can occur in so many different applications and it is an insidious type of attack that often leads to metal failure before the process of cracking is detected.

The mechanism of SCC combines corrosion and mechanical cracking such that each accelerates the other. Initiation generally occurs on a susceptible metal’s surface due to corrosion of that particular alloy by a particular corrosive medium. A small crack then starts. If aggressive conditions remain in effect, the crack grows and progresses through the metal until too little sound material remains to resist the stresses acting and final failure occurs. The rate of crack growth depends on several factors including the level of tensile stress and the values of other service conditions. The latter include service temperature, the level of concentration of aggressive ions in the corrosive or its pH and the time of exposure to the specific conditions. 

A unique feature of SCC is that it occurs only for particular combinations of metal or alloy, specific corrosive media and other service conditions. For example, copper alloys when exposed to ammonia compounds have long been recognized as likely to produce SCC if tensile stress is also present. Similarly, common austenitic grades of stainless steels, e.g., 304 and 316, in corrosive media with high concentrations of chloride ions and high temperatures will readily crack if tensile stress is present. However these same stainless steels will not crack in ammonia solutions at least up to about 212 F. Carbon steel is not subject to SCC in seawater but it will crack in caustic (NaOH) solutions at certain temperatures and NaOH concentrations. There are many susceptible combinations of alloys, corrosive media and service conditions that can cause cracking and more susceptible combinations are regularly being discovered. Corrosion handbooks provide data on the several combinations that are known to produce SCC.  

Tensile stresses are essential to the incidence and rate of SCC growth. They act to pry open the microscopic-scale tip of a crack and expose “new” metal to the corrosive medium. The corrosive then can attack this uncovered metal so that crack growth is promoted. Frequently corrosion pits on a metal surface will be the initiation points for SCC because they can act to concentrate and magnify tensile stresses. A crack will often begin and grow from the bottom of a corrosion pit. Other stress concentration areas on a susceptible metal surface can also promote crack initiation, e.g., at sharp corners or small radii on a stepped shaft or from undercuts or lack of penetration at a weld. 

Applied tensile stresses often are not the primary source of stress that causes SCC. Instead residual tensile stresses, i.e., those left in the metal after some manufacturing process such as welding or due to plastic deformation caused by cold working the metal, frequently are the more important sources. The designer may overlook residual stresses. These stresses can be essentially eliminated or reduced to minimal levels if a sufficient stress relief heat treatment is accomplished after the offending manufacturing process is completed.  

A metal that that has become sensitized and susceptible to another form of corrosion – intergranular attack (IGA) – is much more likely to experience SCC. Sensitization is a metallurgical degradation process in which areas nearby the grain boundaries of the metal become deficient in an alloying element that is essential to its corrosion resistance. Thus these deficient grain boundary areas become much more susceptible to the initiation and growth of a SCC crack that will follow this intergranular path through the metal.  

The basic control measure for SCC is to be aware of and avoid the susceptible combinations of alloy, specific corrosion medium and service conditions. Another approach is to minimize applied stresses and be particularly careful to avoid (or stress relieve) the often-overlooked residual stresses that may be present in the metal. Other measures include minimizing stress concentration features (both geometric and those caused by pitting) on the metal surface, preventing sensitization of the alloy, avoiding thermal insulation that can leach out and concentrate aggressive ions onto to metal it covers or inputting helpful compressive stresses to the metal surface via shot peening or cold rolling to counteract harmful tensile stresses. Finally, as in any corrosion process, minimize conditions that can accelerate attack, e.g., high temperatures, very low or very high pH (depending on the given metal) and where possible avoid high concentrations of aggressive ions, such as chlorides and fluorides, in the bulk corrosive medium. SCC cracking can initiate at crevices in metallic equipment because the partially closed geometric features that crevices create concentrate aggressive ions to much higher levels than in the bulk liquid. Thus eliminating all possible crevices is important.             

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