Application of Wagner's pellet method in investigating the mechanism of formation of heterophasic two-layer scales on alloys

Abstract

Investigations were carried out on a model system Ag-Zn-S and consisted of determinations of kinetic data, and distribution of the chemical potential of silver in the scale, as well as the amount of inward diffusion of sulfur in the formation of two-layer scales on binary alloys. The chemical potential of silver was measured with AgI as a solid electrolyte and the diffusion of sulfur with the aid of the radioactive isotope³⁵S. The inner layer consisted of pellets of Ag₂S+ZnS mixtures having different degrees of dispersion and of mixtures of Ag₂S and powdered foam glass. The investigations were carried out on the following systems: Ag-Ag₂S-S; Ag-Ag₂S+ ZnS-Ag₂S-S; Ag-Ag₂S+ glass-Ag₂S-S; (Ag-Zn)-Ag₂S-S. It was established that the ability of dispersed ZnS to decrease the rate can be explained on the basis of a decreasing cross section for the outward diffusion of silver ions in the inner layer. This phase makes the plastic flow of the scale toward the core very difficult. As a result of this, the secondary dissociation processes destroy the compactness of the scale over the whole cross section and favor the inward diffusion of sulfur. The chemical potential varies linearly across each layer.