Effect of Surface Hydrogen Concentration on Hydrogen Embrittlement Properties of Stainless Steels and Ni Based Alloys

Hydrogen embrittlement properties of several stainless steels and Ni based alloys under cathodic charge (CHE) were investigated. Hydrogen concentration in the materials was varied by controlling hydrogen charging conditions. Slow strain rate test (SSRT) under cathodic charge in aqueous solution was carried out to evaluate CHE susceptibility. Mechanical degradation by hydrogen was evaluated by relative fracture elongation (relative fracture EI.) against that in air. Critical surface hydrogen concentration (H-C), the maximum hydrogen at which EI. was hard to be decreased, was derived from SSRT results under various hydrogen charging levels. H-C strongly depended on Ni equivalent (Ni-eq), which is a parameter consisting of alloy chemical compositions, reflecting stability of austenitic phase. CHE test results were compared to susceptibilities to hydrogen gas embrittlement (HGE) and internal reversible hydrogen embrittlement (IRHE), which are caused by highly pressurized gaseous hydrogen. Materials accepting higher H-C generally showed higher resistance to HGE and IRHE. Comparing H-C to H-E, concentration of hydrogen absorbing from highly pressurized gaseous hydrogen, enables the risk assessment of hydrogen embrittlement in actual service conditions.