Q. Describe Polythionic Acid Corrosion?

Although it is not typically thought of as high temperature corrosion, polythionic acid stress corrosion cracking occurs in areas where high temperatures are present (e.g. in furnaces and at the flare tip). The presence of polythionic acid is dependent upon formation of a sulphide film that forms when H₂S is burned. During shutdown periods this film may react with oxygen and water to form polythionic acids. Polythionic acids may cause SCC in furnace tubes, heat exchangers, thermowells and vessel linings.