Hydrogen diffusion and trapping in a precipitation-hardened nickel-copper-aluminum alloy Monel K-500 (UNS N05500)

Hydrogen uptake, diffusivity and trap binding energy were determined for the nickel-copper-aluminum alloy Monel K-500 (UNS N05500) in several conditions. The total atomic hydrogen (H) concentration increased from 0 to 132 wppm as the hydrogen overpotential decreased to -0.5 V in alkaline 3.5% NaCl electrolyte at 23 degrees C. The room-temperature H diffusion coefficient ranged from 0.9 to 3.9 x 10(-14) m(2) s(-1) for single-phase solid solution, aged, and cold worked then aged microstructures. Diffusivity was independent of lattice H concentration but depended weakly on metallurgical condition, with slower H diffusion after aging. The apparent activation energy for H diffusion was in the range of 29-41 +/- 1.5 kJ mol(-1) at the 95% confidence level. The lower value approached nearly perfect lattice transport, while the high value was strongly influenced by traps of low-to-intermediate strength. Atomic hydrogen trapping at metallurgical sites, strongly suggested to be spherical-coherent gamma' (Ni3Al) precipitates, was evident in the aged compared to the solution heat treated + water-quenched condition. Both thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy interpreted as Ni3Al (10.2 +/- 4.6 kJ mol(-1)) interfaces was significantly less than the activation energy for perfect lattice diffusion (25.6 +/- 0.5 kJ mol-1) in this nickel-based alloy system. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.