(13b) Polythionic Acid Stress Corrosion Cracking

The most common fluid that causes Inter Granular Stress Corrosion Cracking (IGSCC) in petrochemical plants and refineries is polythionic acid. Polythionic acid is formed in the presence of sulfur, moisture and oxygen. Sulfur can come from feed stock, additives or fuels. This failure mechanism is possible from both inside and outside of a tube, depending on the sulfur source. IGSCC is most common in sensitized 300 series stainless steel and higher nickel base austenitic alloys.

This paper will describe the details of polythionic acid formation, case histories of failures and its mitigation.