Stress corrosion cracking of steels in industrial process environments

[摘要] The interactions between engineering materials and their environment which give rise to stresscorrosion cracking are reviewed and industrial examples from the petrochemical industry aredescribed. In one of the examples, cracking took place in carbon steel exposed to pressurisedgas containing carbon monoxide, carbon dioxide and water. The crack morphology in thissystem was studied by metallography of samples from industrial gas processing plants and thecrack growth rates were determined using precracked specimens. Constant extension ratetests, U-bend specimens and potentiodynamic studies were used to evaluate alternativematerials and inhibitor additions in CO-C02-H20 environments. Electrochemical noise wasaccessed as a technique to monitor sec on line.It was found that the CO-C02-H20 system was characterised by a time dependent adsorptionof carbon monoxide at anodic and cathodic sites. The adsorption produced a critical balancebetween crack tip corrosion rate and the repassivation process comparable to the behaviour atthe active-passive transition zone in more conventional systems. The anodic passivationexhibited a breakdown potential near to -400 mV (Ag/ AgCI) that defined the zone of secsusceptibility. Inhibition by CO and hence sec was virtually independent of CO partial pressureprovided there was a sufficient reservoir of CO. The addition of commercial film forminginhibitors did not greatly influence the system and sec was still observed in CO-C02-H20environments to which inhibitors had been added. Steels containing alloy additions of morethan 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 notwholly resistant to sec.The morphology of stress corrosion cracks in CO-C02-H20 mixtures was influenced by carbonmonoxide partial pressure. More corrosion was observed on the crack walls at low carbonmonoxide partial pressure and widened cracks resembling •mesa• corrosion were common. Thisincreased corrosion was probably due to difficulty in maintaining passivity in the crevice formedby the growing crack. Similarly, crevicing in precracked specimens appeared to inhibit sec andno crack extension was observed. Electrochemical noise proved to be a useful tool formonitoring.

[发布机构] University of Pretoria