Hydrogen Ingress in High-Strength Alloys Exposed to Different Electrolytes

A diffusion/trapping model for hydrogen ingress was applied to three high-strength alloys, 4340 (UNS⁽¹⁾ G43400) steel, Monel⁽²⁾ K-500 (UNS N05500), and MP35N⁽³⁾ (UNS R30035), in electrolytes ranging in pH from 2.9 to 9.0. Hydrogen ingress was found to occur under interface control, although in some cases, the data were affected by corrosion or by a change in the surface film on the alloy. The apparent rate constants for hydrogen trapping (k_a) were self-consistent for the steel in an acetate buffer, but anomalies were observed with other electrolytes due to significant metal dissolution or film formation. In contrast, the interface control model was applicable to alloy K-500 in various electrolytes. The value of k_a for alloy K-500 was independent of the electrolyte, indicating that the model adequately represents hydrogen ingress in this alloy over a range of test conditions. The interface control model can be fitted also to the data for alloy MP35N in different electrolytes, and k_a was determined in each case, although the value of k_a appears to depend on pH.