Hydrogen diffusion in plasma ion nitrided steel

An API X-65 type steel was plasma ion nitrided at 500 degrees C during 0.5 h, 16 h and 28 h. Electrochemical tests were performed to obtain potentiodynamic polarization curves and hydrogen permeation curves at 50 degrees C, using a 0.1 N NaOH solution as electrolyte. This enabled the determination of hydrogen permeability, apparent solubility and diffusivity of the substrate, the composite material and the nitrided cases. Microhardness profiles were obtained and then compared to microdeformation profiles determined by X-ray diffraction. The nitriding treatment created two surface layers, an outermost white layer and a diffusion layer. The hydrogen permeability was found to be strongly affected by the nitriding treatment. The apparent hydrogen diffusion coefficient in the diffusion zone was slightly bigger than that in the substrate, but it was markedly smaller in the white layer. Nitriding treatments generating white layers as thick as 5 mu m were very effective to impede the hydrogen contamination of the steel.