Electrochemical diagrams showing equilibrium cathodic phases and their compositions as function of electrolyte composition and type of alloy phase diagram

Abstract

In this paper the dependence of the elemental and phase compositions of the binary A-B cathodic deposits are computed by constructing “Electrochemical Diagrams”, abbreviated as ECDs, which potentially have a high practical significance. An ECD shows phases and compositions of cathodic alloy products as function of electrolyte composition at fixed temperature, pressure and at low current density upon galvano-static electrochemical co-deposition of components A-B from the same electrolyte on an inert cathode. The following rules are established here to construct the ECDs for binary A-B alloys: i). among all possible cathodic phases the one will be electrodeposited that provides the least negative cathodic deposition potential, ii). two or three (but not four or more) phases can be co-deposited on the cathode if their equilibrium cathodic potentials are equal and are the least negative among all possible cathodic phases, iii). the equilibrium compositions of solid and liquid A-B alloys that form on the cathode can be found from the condition of equality of the partial equilibrium cathodic potentials of components A and B. Based on the above three rules, ECDs are constructed for some basic types of binary equilibrium alloy phase diagrams with the composition of the alloy on its y-axes as function of the ionic composition of the electrolyte on its x-axes. In all cases the clear connection is shown between the ECDs and their corresponding alloy phase diagrams. The following geometric features are established for ECDs: i). solution phases are characterized by continuous curves, ii). phases of fixed compositions are characterized by horizontal lines, iii). two-phase mixtures are characterized by vertical lines, iv). three-phase equilibria are characterized by single points along the vertical lines corresponding to the two-phase equilibria of the parent phases of the third phase. It is shown that the restricted phase rule of Gibbs (= the maximum number of coexisting phases = the number of independent components + 1) is valid also for the ECDs. As examples, ECDs are calculated for the Ni-Co system and for the Ti-B system and good agreement is found between these calculated diagrams and the experimental data.