This recommended practice provides methods to protect austenitic stainless steel from polythionic acid stress corrosion cracking (SCC) found to occur during downtimes and contiguous shutdown and start-up periods. The 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 (H 2 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 sec has been comparatively high. It can be applied to other units using austenitic stainless steel; however, the user must consider other factors such as the effect of the alkaline chemicals on catalysts. For the purposes of this standard, related Ni-Cr-Fe alloys that are susceptible to polythionic acid sec are included in the term "austenitic stainless steel."

The techniques described herein are not designed to remove chloride deposits, but will minimize the possibility of chloride SCC by the wash solutions. Since units subject to polythionic acid SCC may contain chloride deposits, measures should be taken to remove these deposits.

This revision of NACE standard RP0170-70 was prepared by NACE Task Group T-8-6 and is published by NACE under the auspices of Group Committee T-8 on Refining Industry Corrosion. This revised standard updates and supersedes RP0170- 70 entitled

"Protection of Austenitic Stainless Steel in Refineries Against Stress Corrosion Cracking By Use of Neutralizing Solutions During Shut Down," published in 1970.