Caustic Stress Corrosion Cracking:

Caustic embrittlement, or Caustic stress corrosion, is another form of intergranular corrosion cracking. The mechanism is same to in which of chloride stress corrosion. Mild steels (steels along with low carbon and low alloy content) and stainless steels will crack if those are exposed to concentrated caustic (high pH) environments along with the metal under a tensile stress. Within stress cracking which is induced through a caustic environment, a presence of dissolved oxygen is not essential for the cracking to occur.

Caustic stress corrosion cracking was first encountered within the operation of riveted steam boilers. Those boilers were found to fail on occasion along riveted seams.  Breakdown was attributed to caustic-induced cracking at the highly stressed regions near and under the rivets. Boiler water could simply flow within the crevices that existed under the rivets.

Radiative heating would cause the water within the crevices to boil.  As steam was established, it would escape from the crevice. More boiler water would then flow within the boil, crevice, and pass from the crevice as steam.  The net output of this continuing procedure was focusing of caustic under the rivet. The mixture of high stress and high caustic concentrations finally led to destructive cracking of the boiler vessel.

Whereas the rate of steam generation (boiling) is high, it is harder to eliminate the problem of solute concentration in regions of the boiler.  Caustic stress corrosion might concentrate in like regions as the water evaporates rapidly, but enough concentration of caustic through such a mechanism to induce stress cracking is considered unlikely.

Available data denotes in which caustic concentrations greater than 10,000 ppm, and possibly up to 50,000 ppm, are needed to induce caustic stress cracking (40,000 ppm NaOH is equal to 40 grams per liter or 1 mole per liter). The pH of like a solution is on the order of 14.  An alkaline environment is generates and controlled through use of a solution having a few properties of a buffer, that is, one which tends to retard or slow a reaction or tends to force it in one direction or the other.