The electrochemical behavior of Fe³⁺ – WO₄²⁻ – Cit³⁻ and Fe³⁺ – MoO₄²⁻ – WO₄²⁻ – Cit³ systems

Abstract

The kinetic parameters of electrochemical behavior of tungsten at the deposition of Fe–W and Fe–Mo–W alloys were determined using linear voltammetry and analyzing polarization relationships. In the presence of citrate ions the cathode process was shown to proceed with the participation of [FeHCitWO₄]⁻ clusters. An optimal concentration ratio of the components in electrolyte required for the Fe—W alloy deposition was defined as c(Fe³⁺):c(Cit³⁻):c(WO₄²⁻) = 1:1.5:0.3. The deviation from this ratio by an increase in the concentration of tungstate ions results in the formation of dimer forms W₂O₇²⁻ and [FeW₂O₇HCit]⁻ clusters; as a result the concentration of electrode active particles [FeWO₄HCit]⁻ diminishes and the cathode process is inhibited. A peculiar feature of the formation of electrolytic alloy Fe–Mo–W is a competitive reduction of molybdates and tungstates. Based on the analysis of the kinetic parameters and characteristic criteria of electrochemical reactions, we proposed the mechanism for the co-deposition of alloy containing iron with molybdenum and tungsten; this mechanism is a sequence of coupled reactions of irreversible reduction of intermediates with slow charge transfer stage and previous chemical step of the ligands release. The data of X-ray phase analysis show that the binary alloys Fe–W are solid solutions of tungsten in iron and ternary alloys Fe-Mo- W are X-ray amorphous.