The combined mechanism of hydrogen embrittlement and anodic dissolution on the fracture toughness degradation of X65 weld joint in H2S

The embrittlement mechanisms of the X65 weld joint (WJ) in H 2 S were investigated using in situ crack tip opening displacement (CTOD) test, electrochemical measurement, weight loss test, and hydrogen permeation test. The weld metal (WM), coarse-grained heat-affected zone (CGHAZ), and base metal (BM) exhibited significant degradation of fracture toughness in H 2 S, with embrittlement factors ( EF ) of 89.5 %, 83 %, and 80 %, respectively. The WJ exhibited the lowest corrosion potential in H 2 S compared to the isolated WM and BM. Notably, the weight loss test revealed a corrosion step along the fusion line (FL), indicating the galvanic corrosion in the CGHAZ. This led to different embrittlement mechanisms associated with hydrogen embrittlement (HE) and anodic dissolution (AD). Hydrogen-assisted plastic strain localization in grain boundary ferrite (GBF) resulted in HE-driven cleavage cracking in the WM. In contrast, as galvanic corrosion was promoted, the fracture toughness degradation of CGHAZ was governed by the combined effects of HE and AD. The crack propagation followed a cyclic process of cleavage-blunting-reactivation, corresponding to a fracture morphology of alternating cleavage planes and "trench-like" ductile tears. The HE accelerated the cleavage cracking along the {110} plane, while AD inhibited crack tip blunting through preferential dissolution along the {100}, {110}, and {112} planes.