The SCC Behavior of E690 Welded Joint in Thin Electrolyte Film Containing Sulfur Dioxide

Abstract

E690 steel, as a newly-developed high strength steel in recent years, is of great potential to be widely used in offshore platforms in the near future. Since the stress corrosion cracking (SCC) susceptibility of high strength steel increases sharply with the strength, E690 may experience risks of stress corrosion cracking (SCC) and therefore catastrophic failures when used in marine environment, especially in polluted marine atmosphere containing SO2. In general, the welded joints have lower mechanical strength compared to the majority of the steel structure and are considered more vulnerable to corrosion and stress corrosion cracking. In this study, slow strain rate tests (SSRT) and electrochemical measurements, combined with metallographic analysis and micro-hardness tests, were employed to investigate the behavior and mechanism of SCC in the E690 welded joint. The metallographic observation revealed at least three different microstructure types in the heat affected zone (HAZ) of E690 welded joint, including coarse grained heat affected zone (CGHAZ), fine grained heat affected zone (FGHAZ) and intercritical heat affected zone (ICHAZ). CGHAZ exhibited the highest micro-hardness. FGHAZ showed a little lower micro-hardness than base metal (BM). And the micro-hardness of ICHAZ was equal to or even lower than that of BM. The results showed that the E690 welded joint had a very high SCC susceptibility in thin electrolyte film containing sulfur dioxide. From the fracture morphologies, distinct characteristics of brittle fracture were observed. The plasticity index in SO2 atmosphere was significantly decreased compared to that in air, showing the loss percentage of elongation and area reduction up to 30.0 % and 36.9 %, respectively. After etching, the morphology of the cross-section indicated that ICHAZ of the welded joint was the most susceptible location to SCC due to its more negative potential and higher corrosion current density from the electrochemical measurements. Fracture morphology showed that stress corrosion cracks were more likely to initiate and extend along the interface of martensite-austenite (M-A) island and adjacent ferrite.