Chemical Treatment to Mitigate Polythionic Acid SCC Without a Soda-Ash Wash: Laboratory and Plant Experience

A novel oxidative chemical treatment method is already being used to neutralize pyrophoric metal sulfides present in Hydroprocessing reactor systems. It is hypothesized that this treatment will similarly neutralize the iron sulfides which contribute to the formation of polythionic acids. A simple laboratory test has been developed to test the effectiveness of the chemical treatment. Previous experimental studies into polythionic acid stress corrosion cracking (PTASCC) have typically immersed stainless steel specimens in Saman's solution, made by bubbling gaseous SO₂ and H₂S through the cell at controlled rates to produce a mixture of di-thionic through hexa-thionic acids along with sulfuric and sulfurous acids. In Saman's solution, it can be difficult to obtain cracking even with the standard sensitizing heat treatments suggested in ASTM A262¹. To circumvent these challenges, the current work uses standard U-bends (ASTM G30²), coated with an air-sprayed suspension of iron sulfide powder. Specimens are suspended in saturated air (100% RH) at 50°C. Similarly, slow strain rate testing (SSRT) specimens were produced and tested using similar methods to identify the effect of exposure to FeS and various SCC mitigation and elimination treatments. To date, results indicate that the test method is successful for achieving PTASCC; test exposures of the oxidative chemical treatment are currently in progress. Parallel to laboratory testing, trials the treatment on actual refinery Hydroprocessing equipment are in progress. A hydrotreating reactor and associated process equipment including feed/ effluent exchangers, fired heater and piping systems, previously treated with soda-ash wash during shutdowns was instead treated only with the oxidative chemical treatment for the current outage, and then opened to air. Internal inspections using standard API recommended methods under a complex risk based inspection program to examine for PTASCC did not reveal any evidence of cracking. The authors hope that these positive results, along with future work, may allow this method for PTASCC mitigation to be recognized in NACE SP0170³.