Experimental Examination of the Influence of the Interface Condition on the Determination of the Diffusion Coefficient of Hydrogen in Iron Alloys.

The surfaces of cylindrical specimens of iron alloys with Cr, Mn, Co, and Ni were polished or electroplated with nickel or palladium. After hydrogen charging, the hydrogen effusion was studied by measuring the pressure increase with time in closed evacuated quartz tubes. The results were evaluated by computer to obtain the temperature and concentration dependence of the hydrogen diffusion coefficients. Previously developed experimental methods for the determination of the true diffusion coefficients of hydrogen in iron alloys could be successfully examined. At room temperature, higher diffusion coefficients than with the merely polished specimens were obtained for surface coatings of nickel or palladium. At about 300 C the diffusion coefficients were less influenced by the various surface treatments, because the heat of chemisorption of hydrogen on metals decreases with increasing temperature. Dirty surfaces with oxygen or oxide films, had apparent diffusion coefficients which were lower than the diffusion coefficient of volume diffusion. However, this retarding influence can be kept so low by hydrogen charging at around 400 degree C that it can be neglected.