Effects of self-induced mechanical stress in hydrogen sorption by metals, by EIS

Electrochemical impedance spectroscopy (EIS) approach to the processes of h ydrogen sorption by metals has been extended to account for the effects of mechanical stress induced by the presence of hydrogen in the metal perfect- crystal matrix. The EIS equations for the simplest mechanism of the hydroge n absorption have been modified to account for the effects of self-induced stress. That has been done on the basis of the recently elaborated theory f or the diffusion of hydrogen in metals in the transient breakthrough experi ment, where the so-called 'uphill effect' can be observed. In this paper it is shown that the general character of the EIS data is not influenced by t he stress. However, the apparent diffusion coefficient, derived directly fr om the EIS data, is always larger than the diffusion coefficient of hydroge n. The higher concentration of hydrogen in the metal near its surface, the larger is the difference. For some Pd alloys, the difference can attain one order of magnitude. The rate of hydrogen absorption process is influenced by the stress only as the result of the change of hydrogen concentration in the metal. The effect of stress on the diffusion of hydrogen is discussed in comparison with the effect of hydrogen traps in a real metal matrix. (C) 1999 Elsevier Science Ltd. All rights reserved.