On the Boundary Conditions of Electrochemical Hydrogen Permeation Through Iron

The constant concentration (CC), constant flux (CF), flux continuity (FC), and hybrid of flux continuity and constant concentration (FCCC) boundary conditions for electrochemical hydrogen permeation through iron or steel samples are reviewed and used to evaluate the hydrogen diffusivity from the permeation tests on fully annealed commercial pure iron samples with different surface treatments and thicknesses. The diffusivities evaluated by the CF model are three times higher than that calculated by the CC model. Both the CF and CC models lead to the diffusivity depending on the sample thickness and surface conditions. The FC model is capable of evaluating the diffusivity, the desorption rate constant, and the absorption parameter simultaneously. Since the FCCC model is a hybrid, it results in the diffusivity changing with the surface treatment and both the desorption rate constant and the absorption parameter varying with the sample thickness. In addition, hydrogen flux at the entry surface is calculated from both the CC and FC models. The entry flux calculated from the CC model is dependent on diffusivity, sample thickness, and time. However, in the FC model, the hydrogen flux at the entry surface is a function of diffusivity, sample thickness, time, and surface conditions. Also, the hydrogen concentration on the entry surface is compared between the CF and FC models. © 1999 The Electrochemical Society. All rights reserved.