Entry, transport and absorption of hydrogen in low-temperature plasma nitrided austenitic stainless steel

Plasma nitriding of ASTM 316Ti steel at 425°C for up to 30 h produced nitrogen expanded austenite phase with up to 39 at.% N. The 40-μm thick membranes of nitrided and unnitrided steel were charged with hydrogen by galvanostatic cathodic polarisation in 0.1 M NaOH at 25°C and the hydrogen permeation and desorption were studied by the electrochemical technique. Nitriding decreased the hydrogen permeation rate. Diffusivity of hydrogen in the nitrided layer was higher than that in the unnitrided steel, and this layer trapped hydrogen more intensely. However, the concentration of the diffusible hydrogen in the nitrided layer was lower than that in the unnitrided steel. As a result, the absorption of hydrogen by the steel substrate was diminished.