Abstract

Hydrogen permeation technique is a widely used testing method for the determination of hydrogen diffusion coefficient (D), which is an important parameter considering hydrogen embrittlement. A palladium (Pd) or nickel (Ni) coating is often utilised on the hydrogen detection side of the test specimens. Here, we investigate the effect of Pd and Ni coatings on hydrogen diffusion in a martensitic 500 HBW hardness low-alloy steel in the thickness range of 0.5–0.8 mm using a refined successive transient method and compare against an uncoated reference specimen. Both coatings yield similar average D values (6–6.6 × 10⁻⁷ cm²/s), but the best repeatability is achieved with Pd coating. With Ni coating, D values decrease with the increasing specimen thickness, which is partly caused by a slower hydrogen diffusion in Ni, and therefore a concentration gradient at the specimen-coating interface. The uncoated specimen has a poor transient fit, and significantly lower D (2.1 × 10⁻⁷ cm²/s) due to surface oxidation. With both coatings, the steepness of the last decay transient was highly affected by specimen thickness, and therefore the density of reversible hydrogen traps is only comparable for similar thicknesses.