The Application of a Diffusion/Trapping Model for Hydrogen Ingress in High-Strength Alloys

A potentiostatic double-pulse (PDP) technique was applied to three high-strength alloys, AISI(1) 4340 steel, Monel† K500, and MP35N, in 1 M HAc/1 M NaAc (Ac = acetate) with 15 ppm As2O3. In all cases, hydrogen ingress occurs under interface control and not diffusion control. The apparent rate constant for hydrogen trapping (ka) and the hydrogen ingress flux (J) were obtained, and the irreversible trapping constant (k) was evaluated from ka. The values of both k and J are higher for AISI 4340 steel than Monel K500 and MP35N. The density of irreversible traps (Ni) for the steel was determined from k as 2 × 108 m−3, which is in reasonable agreement with the concentration of MnS inclusions, 2 × 109 m−3. However, the values of Ni for the Monel and MP35N were three orders of magnitude smaller than the concentration of sulfur and phosphorous impurities, which were assumed to be the primary irreversible traps. The low values of the calculated trap density are probably primarily caused by sulfur and phosphorus segregated as clusters at grain boundaries. The difference in both the nature of the irreversible traps and the interfacial flux probably contribute substantially to the difference in susceptibility of AISI 4340 steel and the two nickel-containing alloys to hydrogen embrittlement.