This recommended practice provides methods to protect austenitic stainless steel and other austenitic alloys from polythionic acid stress corrosion cracking (SCC) occurring during downtimes and contiguous shutdown and start-up periods. This recommended practice is directed toward preventing SCC by polythionic acids formed by the reaction of sulfide corrosion products with oxygen (air) and water. For practical purposes, it should be assumed that such acids can be formed by reaction of air and water with oxidizable sulfur compounds (H2 S, metal sulfides, S).

Basic protection methods include: avoidance of oxygen (air) entry, prevention of liquid water formation, and alkaline wash of surfaces to be exposed to prevent polythionic acid formation. This recommended practice is intended primarily for desulfurizing, hydrocracking, and reforming processes where the incidence of polythionic acid SCC has been comparatively high. It can be applied to other units using austenitic stainless steel and other austenitic alloys; however, the user must consider other factors such as the effect of the alkaline chemicals on catalysts. For the purposes of this recommended practice, the term "other austenitic alloys" refers to those alloys of nickel, iron, and chromium, which may be susceptible to polythionic acid stress corrosion cracking. Historical Document 1993