Review and theoretical analysis of ac–av methods for the investigation of hydrogen insertion: Part II. Entry side impedance, transfer function and transfer impedance formalism

The formalism related to alternating current–voltage methods is reviewed, completed and unified for hydrogen insertion in metals and alloys, in relation to permeation cell membranes with potentiometric or potentiostatic detection of hydrogen and electrodes with impermeable boundary conditions. The electrochemical system representation in terms of an electrical dipole or quadripole is first given to introduce the impedance on the entry side, the transfer impedances and the potential and current density transfer functions through the membranes, which can be used experimentally to investigate hydrogen insertion. Next, the surface process is presented, taking the direct (one-step) insertion mechanism into consideration for the sake of simplicity. Based on the surface process and diffusion formalisms, theoretical expressions are derived for the entry side impedance, transfer functions and transfer impedances, depending on the electrode or membrane type and the boundary conditions.