Pulsed-plasma-nitrided API 5L X-65 steel: hydrogen permeability and microstructural aspects

Nitriding of an API 5L X-65 steel was performed using a low-frequency do pu lsed-plasma technique. A 6 h treatment was found to produce a compact surfa ce nitride layer composed of gamma' (Fe4N) and epsilon (Fe2-3N) nitrides wi th a thickness of around 2 mu m. The plasma-nitriding treatment resulted in a significant reduction in hydrogen permeability, from 2.97 x 10(-10) mol H m(-1) s(-1) (for the untreated steel) to 7.1 x 10(-11) mol H m(-1) s(-1) for electrolytic hydrogen charging via the nitrided surface. When the sampl e was oriented with the nitrided surface as the hydrogen-exit face, the per meability was reduced even further, to 3.9 x 10(-13) mol H m(-1) s(-1).