Plasma nitrided ferritic stainless steel surfaces as hydrogen permeation barriers

Hydrogen has the potential to replace fossil fuels in certain sectors where decarbonization presents significant challenges. However, components manufactured in metallic alloys that come into contact with hydrogen are susceptible to hydrogen induced embrittlement (HE) to varying degrees. Plasma based surface treatments might provide a barrier to hydrogen diffusion, a prerequisite for HE. This study aims at investigating the performance as hydrogen diffusion barrier of active screen plasma nitrided treatments on a ferritic stainless steel (X6Cr17). The research has focused on the nitriding parameters (mainly processing temperature), as well as the thickness and microstructure of the steel. A variety of techniques, including X-ray diffraction spectroscopy, microscopy, indentation and hydrogen permeation tests were employed throughout the study on different nitrided surfaces. The findings of the study indicate that plasma nitrided surfaces act as effective hydrogen permeation barriers. Results show a reduction of the hydrogen permeation flow by up to two orders of magnitude compared to the same untreated steel alloy (2.0 × 10⁻⁹ vs. 4.2 × 10⁻⁷ Pa.m³/s). This is accompanied by a delay in the hydrogen permeation uptake of >25 times compared to the same untreated steel alloy. However, the findings also indicate that the surface treatment effectiveness is influenced by both the presence of surface defects and the depths and microstructure of the nitrided surfaces.