NACE International announces NACE Standard RP0170-2004, “Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment,” a revised standard developed by NACE Task Group 173. 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 sulphide 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 oxidisable sulphur species (sulphur, H2S, metal sulphides). 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 desulphurising, 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.
