Study on Failure Behavior of Alloy 625 in a Hydrogen-Containing Environment

Ni-base Alloy 625 (UNS N06625) for hydrogenation heat exchangers was studied to investigate its failure behavior in a hydrogen-containing environment. Hydrogen was introduced into Alloy 625, and the effect of hydrogen on pitting behavior was studied by using electrochemical tests, chemical immersion experiments, and other methods, and the initiation and propagation behavior of pitting corrosion and the fracture morphology before and after hydrogen charging were observed with microscopic characterization methods such as scanning electron microscopy and energy dispersive x-ray spectroscopy. In addition, a slow strain rate tensile test was performed to observe the stress-strain curve of Alloy 625 and analyze its strength and plasticity. The findings indicated that hydrogen charging reduced the corrosion resistance of Alloy 625, but did not significantly impact pitting corrosion, with little change in pitting potential and no significant change in the pitting nucleation rate observed. Pitting only propagated inside the precipitated phase, without propagating towards the matrix after the precipitated phase was fully dissolved. Even though the enrichment locations of hydrogen in Alloy 625 did not always coincide with initiation locations of pits and hydrogen had no significant impact on pitting behavior, hydrogen charging resulted in a stronger repassivation tendency. The sensitivity of Alloy 625 to hydrogen embrittlement was significantly increased by hydrogen charging. After hydrogen charging, fracture morphology showed an intergranular fracture feature and cleavages and secondary cracks also appeared.