On the hydrogen environment-assisted cracking resistance of a compositionally complex Co-Ni-Cr-Fe-Mo-Ti alloy

The hydrogen environment-assisted cracking (HEAC) behavior of a compositionally complex Co-Ni-Cr-Mo-Fe-Ti-Al alloy (MP98T) proposed for marine fastener applications was evaluated via slow-rising stress intensity testing while immersed in 0.6 M NaCl at applied potentials ranging from 1.0 to 1.2 V-SCE (vs. saturated calomel electrode). Results indicate that MP98T is resistant to HEAC at applied potentials representative of cathodic protection. In particular, minimal crack growth was observed across the tested applied potential range, which represented conditions known to promote aggressive HEAC in legacy Ni-based fastener alloys, such as Monel K-500. Measurement of the diffusible hydrogen concentration (C-H,C-Diff) of MP98T as a function of applied potential in 0.6 M NaCl suggests this improved susceptibility may be linked to a reduced efficacy for hydrogen uptake in MP98T at hydrogen overpotentials where Monel K-500 is susceptible. Specifically, the C-H,C-Diff of MP98T at a given applied potential was consistently observed to be less than 50% of that measured for Monel K-500.