Solid-phase adsorption effect in the diffusion kinetics of selective dissolution of alloys

Abstract

The purpose of the paper is to solve the problem of non-stationary diffusion at potentiostatic selective
dissolution of a homogeneous alloy, accompanied by preliminary solid-phase adsorptive accumulation of
a component in the surface layer. The analytical expressions for the concentration field and the transient current
of an electronegative alloy component are obtained analytically by means of integral Laplace-Carson
transformation method. It was shown that the function of equilibrium adsorption isotherm is completely responsible
for the contribution of the pre-polarization segregating the components into the value of the partial
current of selective dissolution controlled by solid-state diffusion in the alloy. Mathematical modeling of the
solid-phase adsorption effect during anodic selective dissolution of a homogeneous alloy in the mode of diffusion
kinetics shows that the change in the surface concentration results in a proportional change in the
slope of the criterion Cottrell transient current. The results of the theoretical analysis were used in the treatment
of experimental data obtained for Ag-Au alloys. Their advantage is that the isotherm of equilibrium
solid-phase adsorption of components of such metallic systems on their boundary with an aqueous solution of
surface-inactive anions is known. It has been established experimentally that taking the effect of solid-phase
adsorption into account in the case of chronoamperometry of selectively dissolving Ag-Au alloys based on
silver in nitrate electrolyte results in an increase in the calculated values of the solid-state diffusion coefficients.
The important result is that the pre-polarization solid-adsorptive accumulation of gold in the surface
layer of these alloys is the less noticeable the higher the content of silver in the bulk alloy.