Stress corrosion cracking of steels in industrial process environments

Abstract

The interactions between engineering materials and their environment which give rise to stress corrosion cracking are reviewed and industrial examples from the petrochemical industry are described. In one of the examples, cracking took place in carbon steel exposed to pressurised gas containing carbon monoxide, carbon dioxide and water. The crack morphology in this system was studied by metallography of samples from industrial gas processing plants and the crack growth rates were determined using precracked specimens. Constant extension rate tests, U-bend specimens and potentiodynamic studies were used to evaluate alternative materials and inhibitor additions in CO-C02-H20 environments. Electrochemical noise was accessed as a technique to monitor sec on line. It was found that the CO-C02-H20 system was characterised by a time dependent adsorption of carbon monoxide at anodic and cathodic sites. The adsorption produced a critical balance between crack tip corrosion rate and the repassivation process comparable to the behaviour at the active-passive transition zone in more conventional systems. The anodic passivation exhibited a breakdown potential near to -400 mV (Ag/ AgCI) that defined the zone of sec susceptibility. Inhibition by CO and hence sec was virtually independent of CO partial pressure provided there was a sufficient reservoir of CO. The addition of commercial film forming inhibitors did not greatly influence the system and sec was still observed in CO-C02-H20 environments to which inhibitors had been added. Steels containing alloy additions of more than 9 % chromium were found to be resistant to sec but austenitic-ferritic weld joints cracked. A low alloy 3% nickel steel performed well in the constant extension rate tests but was not wholly resistant to sec. The morphology of stress corrosion cracks in CO-C02-H20 mixtures was influenced by carbon monoxide partial pressure. More corrosion was observed on the crack walls at low carbon monoxide partial pressure and widened cracks resembling •mesa• corrosion were common. This increased corrosion was probably due to difficulty in maintaining passivity in the crevice formed by the growing crack. Similarly, crevicing in precracked specimens appeared to inhibit sec and no crack extension was observed. Electrochemical noise proved to be a useful tool for monitoring.