Hydrogen uptake, diffusivity and trap binding energy were determined for Monel K-500 in several conditions. The total atomic hydrogen concentration increased from 0 to 132ppm[wt] as the hydrogen overpotential decreased to -0.5V in alkaline 3.5%NaCl electrolyte at 23C. The room-temperature H diffusion coefficient ranged from 9 x 10-15 to 3.9 x 10-14m2/s for single-phase solid solution, aged, and cold worked then aged microstructures. The diffusivity was independent of the lattice H concentration but depended weakly upon the metallurgical condition; with slower H diffusion after aging. The apparent activation energy for H diffusion ranged from 29 to 41kJ/mol at the 95% confidence level. The lower value approached almost perfect lattice transport, while the high value was strongly influenced by traps of low to intermediate strength. Atomic hydrogen trapping, at what were believed to be spherical-coherent γ′(Ni3Al) precipitates, was evident in aged as compared with the solution heat-treated + water-quenched condition. Thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy (10.2kJ/mol) at Ni3Al interfaces was significantly less than the activation energy (25.6kJ/mol) for perfect lattice diffusion.

Hydrogen Diffusion and Trapping in a Precipitation-Hardened Nickel-Copper-Aluminum Alloy Monel K-500 (UNS N05500). J.H.Ai, H.M.Ha, R.P.Gangloff, J.R.Scully: Acta Materialia, 2013, 61[9], 3186-99