This steel was nitrided by using a low-frequency direct-current pulsed-plasma technique. A 6h treatment was found to produce a compact surface nitride layer which was composed of Fe4N and Fe2-3N with a thickness of about 2µ. The plasma-nitriding resulted in an appreciable reduction in H permeability, from 2.97 x 10-10 (for the untreated steel) to 7.1 x 10-11molH/ms for electrolytic H-charging via the nitrided surface. When the sample was oriented with the nitrided surface as the H-exit face, the permeability was reduced even further, to 3.9 x 10-13molH/ms.

Pulsed Plasma-Nitrided API 5L X-65 Steel: Hydrogen Permeability and Microstructural Aspects. A.H.Bott, S.P.Brühl, B.Gómez, M.A.Zampronio, P.E.V.Miranda, J.N.Feugeas: Journal of Physics D, 1998, 31[24], 3469-74