Abstract
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 °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.
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