Elucidating the loading rate dependence of hydrogen environment-assisted cracking in a Ni-Cu superalloy

Abstract

The effect of loading rate (dK/dt) on the hydrogen environment-assisted cracking (HEAC) behavior of Monel K-500 immersed in 0.6 M NaCl and polarized to −950 and −1200 mVSCE was assessed via a slow-rising stress intensity (K) testing framework at dK/dt ranging from 0.2 to 20 MPa√m/hr. Comparative testing on Monel K-500 in an inert environment (dry N2 with RH < 5%) at dK/dt ranging from 0.1 to 4 MPa√m/hr revealed a clear dK/dt-dependence of local plasticity effects on the direct current potential difference (dcPD) signal. A method for decoupling this “false” signal caused by local plasticity from the ‘true’ crack growth kinetics is presented. Analysis of the corrected data showed that the threshold stress intensity for the onset of HEAC (KTH) was nominally invariant with dK/dt. The HEAC growth rates (da/dt) were only mildly dependent on dK/dt under severe environmental conditions (−1200 mVSCE). However, for moderate conditions (−950 mVSCE), a strong influence of dK/dt on HEAC growth rates (da/dt) was observed. For this latter testing environment, two characteristic regimes of behavior were noted depending on the applied dK/dt: (1) a ‘plateau’ regime where da/dt is independent of dK/dt was observed for dK/dt > 8 MPa√m/hr and (2) a ‘linear’ regime where log(da/dt) linearly scales with log(dK/dt) was observed for dK/dt < 8 MPa√m/hr. These trends are interpreted using resistance curves calculated from the measured da/dt vs. KJ data. The results of this analysis demonstrate that diffusion-mediated crack growth is only observed for dK/dt > 10 MPa√m/hr at −950 mVSCE and all evaluated dK/dt at −1200 mVSCE, while mechanics-based factors govern the da/dt response for dK/dt < 10 MPa√m/hr at −950 mVSCE. The influence of dK/dt is considered in the context of current testing standards for evaluating HEAC susceptibility; specifically, rising K-based frameworks consistently yield conservative HEAC metrics.