Stress Corrosion Cracking:

SCC (Stress corrosion cracking) is a kind of intergranular attack corrosion which occurs at the grain boundaries under tensile stress. SCC occurs in susceptible alloys while the alloy is exposed to an exacting that specific environment if the alloy is in a stressed condition. A stress corrosion cracking appears to be associative independent of common uniform corrosion processes. Therefore, the extent of common corrosion could be necessary nil, and stress cracking could still occur.  Most pure metals are immune to these categories of attack.

According to the most hugely accepted theory, stress corrosion cracking is caused through a process known as chemisorption.  Dissimilar associatively weak physical absorption, like as hydrogen gas on platinum metal, chemisorption might be by of as the formation of a compound among the metal atoms on the surface as a monomolecular layer of the chemisorbed substance, like as Cl^-, OH^-, Br^-, and a few other ions.  The structure of this chemisorbed layer greatly reduces the attraction between neighboring metal atoms. An imperfection initially present after that grows as the metal atoms separate under stress, more chemisorption occurs, and the process continues.  In extremely severe cases, a time required for this cracking to occur is only a matter of minutes.

Several stainless steels are susceptible to stress corrosion cracking.  Stainless steels holding 18 percent chromium and 8 percent nickel are susceptible to cracking within environments containing chloride ions and in concentrated caustic environments (that is, in environments whereas the hydroxyl ion concentration is high). Another, these types of stainless steels do not exhibit any tendency to crack while they are exposed to water environments holding nitrate (NO₃ ^-), sulfite (SO₃--), and ammonium (NH₃⁺) ions.

SCC is of great concern since it could readily crack metal of appreciable thickness.  If the environment is severe sufficient, cracking could occur in an extremely short period of time. The crack could then lead to a serious failure of the elements, or the system, and all the attendant results (for instance, loss of coolant, contamination, and loss of pressure).

The most effectual means for avoiding SCC are proper design, decreasing stress, removing critical environmental contributors (for instance, chlorides, hydroxides, and oxygen), and preventing stagnant areas and crevices in heat exchangers whereas chlorides and hydroxides may become concentrated. Short alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water holding chloride ions.  Nickel based alloys are not affected through chloride or hydroxide ions.