Hydrogen trapping in precipitation-hardened alloys

Abstract

The ingress of hydrogen in three precipitation-hardened alloys (Inconel 718, Incoloy 925, and 18 Ni maraging steel) exposed to an acetate electrolyte (1 mol L −1 HAc/1 mol L −1 NaAc where Ac = acetate) was studied using a potentiostatic pulse technique. The data were shown to fit a diffusion/trapping model under interface control, and values were determined for the irreversible trapping constants ( k) and the flux of hydrogen into the alloys. The density of irreversible trap defects in Inconel 718 and Incoloy 925 was calculated from k and found to be in excellent agreement with the concentration of NbTi(CN) and TiC particles, respectively. The maraging steel was characterized by two trapping constants; one is associated with quasi-irreversible traps that saturate, leaving only irreversible traps thought to be TiC/Ti(CN) particles. The irreversible trapping constants for these alloys are consistent with their relative susceptibilities to hydrogen embrittlement. Moreover, a comparison of the trapping constants with those for AISI 4340 steel and two other nickel-base alloys (Monel K-500 and MP35N) indicates that a strong correlation exists between hydrogen embrittlement susceptibility and trapping capability over all the alloys.