Failure of hydrocracker heat exchanger tubes in an oil refinery by polythionic acid-stress corrosion cracking

摘要

Heat exchanger tubes of a hydrocracker unit in an oil refinery developed cracks at the bent sections after about 48 h of operation at 400 deg C following a period of downtime. The tubes were fabricated from 321 stainless steel. Detailed analysis by various electron-optical techniques and X-ray diffraction showed that the tubes were cracked by stress corrosion cracking induced by polythionic acid. Due to the presence of H_2S in the environment, the inner surface of the tubes was converted into sulfur-bearing scale during operation. This could promote the formation of polythionic acid by aqueous condensates during downtime leading to cracking in the presence of residual internal stresses.