Towards establishing reliable approaches for measurement of hydrogen diffusion characteristics using the electrochemical permeation technique

Abstract

The diffusion behaviour of hydrogen in high strength steels is of interest as they are highly vulnerable to hydrogen induced degradation. The electrochemical permeation technique based on Devanathan and Stachurski is a reliable approach to evaluate the effective diffusion coefficient (Deff) of hydrogen in steels. Determining the Deff by the time lag method requires the establishment of a steady-state hydrogen permeation flux which, however, in many reported works shows atypical behaviour. This work reveals the origin of unwanted artefacts that may affect the steady-state hydrogen (H)-flux during permeation by investigating the effects of the steel microstructure, H-charging current density, steel specimen thickness, H-charging side substrate surface roughness and thickness of the electrocatalytic Ni coating on the H-detection side. A dual phase ferrite-martensite (DP600) and an interstitial free (IF) ferritic steel were studied for their H-permeation behaviour in 0.1 M NaOH under galvanostatic H-charging conditions. For DP600, it was identified that severe (-10 mA/cm2) H-charging current density lead to extensive corrosion product formation on the charging side of the steels, although according to the Pourbaix diagram it was expected to be well in the immunity region, as well as a drastic change in the geometrically necessary dislocation (GND) density in the bulk, whereas low (below −250 μA/cm2) condition did not lead to such effects. In comparison, for IF steel even at low (below −250 μA/cm2) lead to corrosion product formation. The formed corrosion products strongly determined the hydrogen activity on the H-charging side and hence the establishment of the steady-state.