Discussion on the corrosion of polysulfuric acid in petrochemical plants

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The stress corrosion induced by polyoxosulfuric acid in austenitic stainless steels mainly occurs in hydrofining, hydrocracking, and catalytic reforming units. Polysulfate stress corrosion cracking occurs during shutdowns and maintenance periods. It does not occur during normal operation. Austenitic stainless steels are most prone to this, especially those that have been welded or \"sensitized\" in the range of 427–816°C. The occurrence of polyoxosulfate stress corrosion cracking is often closely related to intergranular corrosion in austenitic stainless steels. This is a special form of stress corrosion fracture. It is actually an accelerated form of intergranular corrosion; such fracture occurs when sensitized stainless steel is exposed to pyrosulfuric acid at room temperature or slightly higher temperatures. This stress corrosion fracture can occur with only moderate tensile stress. Generally speaking, stress corrosion in polyoxysulfates is intergranular. Unless appropriate preventive measures are taken, austenitic stainless steels that have been exposed to sensitization temperatures in the range of 427–816°C are prone to this type of fracture. If austenitic stainless steels are sensitized only due to welding or heat treatment, they are prone to stress corrosion cracking caused by polyoxysulfuric acid when continuously exposed to working environments below 427°C; in such cases, the occurrence of cracking can be prevented by using low-carbon or stabilized steel grades. For operating temperatures above 427°C, if operation in such an environment is prolonged, materials capable of withstanding harsh conditions must be carefully selected. The National Association of Corrosion Engineers (NACE) has issued the RP01 standard, \"Prevention of Pulsating Sulfate Stress Corrosion Cracking in Austenitic Stainless Steel Equipment During Refinery Shutdowns,\" specifically to prevent such corrosion. This standard recommends the following preventive measures: 1) Using dry nitrogen for purging and sealing process equipment is a method to isolate it from oxygen (air). 2) Clean the surfaces of all equipment with an alkaline solution to neutralize any polyoxysulfates that may have formed. Stainless steel containing stabilizing elements should be used.