SCC Behaviors Of E690 Steel And Welded Joint In Thin Electrolyte Layer Containing Sulfur Dioxide

In this work, stress corrosion cracking (SCC) behaviors and mechanism of E690 steel and its welded joint in thin electrolyte layer containing sulfur dioxide (SO2) were investigated. HAZ-simulated microstructures were prepared according to real HAZ microstructures, and a comparative study of the SCC behaviors of base metal and their HAZ-simulated microstructures was conducted by U-bend specimen and slow strain rate tensile (SSRT) method in this environment. Then, effect of SO2 content and pre-deformation on SCC behavior of E690 steel in this environment was investigated. In the end, SSRT method, combined with mechanical and electrochemical measurements of various microstructures in the welded joint, was employed to investigate the SCC behavior and mechanism of E690 welded joint in simulated marine atmosphere containing SO2.Results showed that corrosion products of E690 steel in marine atmosphere containing SO2 was composed of a-FeOOH, y-FeOOH, β-FeOOH, Fe3O4 and FeOCl. The content of a-FeOOH was gradually increased with exposure time; meanwhile, Cr and Ni were enriched in inner rust layer, which resulted in the densification of the rust layer. Then, Cl- was concentrated in the bottom of the rust layer, leading to the initiation of corrosion pits and microcracks. Therefore, E690 steel had a very high SCC susceptibility in this environment, and the SCC mechanism was the combination of anodic dissolution (AD) and hydrogen embrittlement (HE) with a transgranular mode (TG-SCC).HAZ-simulated microstructures prepared with heat treatment can well simulate the typical HAZ microstructures of E690 steel. The base metal and its HAZ-simulated microstructures were all highly susceptible to SCC in a simulated SO2-polluted marine atmosphere. The SCC susceptibility gradually increased in the following order:BM, FGHAZ, CGHAZ, and ICHAZ. The cracking mode of BM and CGHAZ was transgranular (TG-SCC) whereas that of FGHAZ and ICHAZ was intergranular (IG-SCC).SO2 in marine atmosphere can facilitate the densification of inner rust layer by promoting the formation of α-FeOOH and enrichment of Ni and Cr in the inner rust layer, leading to the concentration of Cl- under the rust layer, which then significantly promoted the initiation and propagation of SCC cracks and consequently the SCC susceptibility was enhanced. The results of U-bend specimen after different periods of dry/wet cyclic corrosion indicated that SO2 had a dual effect on SCC:facilitating the initiation of SCC cracks in the initial stage, whereas suppressing further corrosion and cracks propagation in the later period.Elastic and plastic pre-deformation can significantly accelerate corrosion and SCC process by promoting both the anodic reaction and cathodic hydrogen evolution reaction. SCC microcracks can be initiated along prior austenite grain boundaries after exposure in thin electrolyte layer containing SO2 for four days under a certain extent of elastic and plastic pre-deformation. After exposure in this environment for four days, the SCC susceptibility increased gradually with the increase of pre-deformation, especially in the plastic deformation stage.The welded joint of E690 steel had a very high SCC susceptibility in simulated marine atmosphere that contains sulfur dioxide with a combined SCC mechanism of AD and HE. ICHAZ in the welded joint was the most vulnerable zone to SCC because of its less strength, more negative potential, higher corrosion current density and presence of M-A islands. The M-A islands in ICHAZ had a detrimental effect on SCC behavior through the synergistic effect of stress concentration, micro-galvanic effect, and hydrogen embrittlement, leading to the cracking along the interface of M-A islands and adjacent ferrite matrix.