An investigation of diffusivity and solubility of hydrogen in both annealed and as-cold-rolled specimens of an austenitic Type 304 steel has been carried out at room temperature by means of an electrochemical permeation method. Especially, the effect of formation of the “foreign phases”, such as hydride phases (γ_H, ε_H) and hydrogen-induced martensitic transformation ones (ε+α′) in the cathodic surface on the hydrogen permeation behavior is discussed. Results obtained were as follows:
(1) The linear relation of log(t^1⁄2·J_t) vs 1⁄t in the hydrogen permeation transients was valid at the beginning of the permeation process. The results were explained by a model in which the rate-determining step of the permeation is a hydrogen-diffusion process in the remaining γ-phase matrix of annealed specimen or in the remaining (γ+α′) phase matrix in the case of as-cold-rolled specimen, accompanying the phase transition of the γ or (γ+α′) phase matrices to the γ_H, ε_H hydride phases and/or hydrogen-induced transformation (ε+α′) martensite phases during permeation.
(2) The apparent diffusion coefficient of hydrogen determined from the theoretical permeation transient derived under the condition of the homogeneous medium throughout the specimen is equivalent to the true diffusion coefficient in the remaining matrix phase. However, the apparent hydrogen concentration directly beneath the cathodic surface determined by the theoretical solution of a homogeneous medium corresponds to the phase equilibrium hydrogen concentration of the matrix phase in the phase boundary related to the moving rate parameter of the phase boundary. Therefore, in order to determine strictly the concentration of hydrogen in the matrix phase at the interface of the foreign/matrix phase, especially in the case of higher cathodic current density for hydrogen introduction, it should be desired to use the heterogeneous medium solution related to the moving rate parameter of the interface.
(3) The diffusivity (D) and apparent solubility (C) of hydrogen in the annealed specimen obtained in the temperature range from 311 to 332 K, at i_c=50 A·m⁻² can be described as follows:
D(m²·s⁻¹)=(4.41±0.92)×10⁻⁷exp[−53510±750(J·mol⁻¹)⁄RT] and C(mol·m⁻³)=(6.11±2.17)×10⁵exp[−10170±1050(J·mol⁻¹)⁄RT].
These values were in agreement with those previously obtained from the gas [phase method at high temperature. While, compared with the annealed specimen, the as-cold-rolled one had the higher diffusivity and the low activation energy for hydrogen diffusion, due to the pre-existence of cold worked α′-martensite. The apparent solubility of hydrogen in the as-cold-rolled specimen was higher than that in the annealed one and the hydrogen absorption occurred by the exothermic reaction.