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This standard recommended practice provides methods to protect austenitic stainless steels and other austenitic alloys from polythionic acid stress corrosion cracking (PTA SCC) occurring during downtimes and contiguous shutdown and start-up periods. This standard is directed toward preventing stress corrosion cracking (SCC) by polythionic acids that are formed by the reaction of sulfide corrosion products with oxygen and water. For practical purposes, it should be assumed that such acids can be formed by reaction of oxygen and water with oxidizable sulfur species (sulfur, H2S, metal sulfides).

Primary protection methods to prevent polythionic acid formation include appropriate material selection, avoidance of oxygen entry, alkaline washing of surfaces, and the prevention of liquid water formation. Regardless of the protection method selected, appropriate confirmation steps to validate compliance with the requirements of this standard are required by the user to ensure protection is provided.

This standard is intended primarily for petroleum refining industry materials and corrosion engineers as well as inspection, operations, and maintenance personnel. While the focus of this standard is on refining industry units such as desulfurizing, hydrocracking, and hydrotreating in which the incidence of PTA SCC has been comparatively high, it can be applied to other units using austenitic stainless steels and other austenitic alloys, such as crude distillation units and fluid catalytic cracking units, when the user may have a concern for PTA SCC. The user must consider other factors such as the effect of the alkaline chemicals on catalysts, as well as the appropriate means and protective equipment required for handling these chemicals. For the purposes of this standard, the term other austenitic alloys refers to those alloys of nickel, iron, and chromium that may be susceptible to PTA SCC.

The techniques described in this standard are not designed to remove chloride deposits, but should minimize the possibility of chloride SCC (Cl SCC) by the wash solutions.

This standard was originally prepared in 1970 by NACE Task Group T-8-19, revised in 1984 and 1993, and reaffirmed in 1997 by Group Committee T-8. It was revised in 2004 by Task Group (TG) 173 on Polythionic Acid SCC Prevention. TG 173 is administered by Specific Technology Group (STG) 34 on Petroleum Refining and Gas Processing. TG 173 is sponsored by STG 39 on Process Industry - Materials Applications, and STG 60 on Corrosion Mechanisms. This standard is issued by NACE International under the auspices of STG 34.