Textural and morphological studies on zinc–iron alloy electrodeposits

Abstract

Zinc–iron alloy electrodeposits have industrial significance, since they provide better corrosion resistance and with improved mechanical properties when compared to pure zinc coatings. This is due to the unique phase structure of the alloy formed. But this deposition belongs to anomalous deposition, where the electrochemically less noble zinc deposits more preferentially, than the more noble iron. So the industrial process control over the deposition becomes difficult. So, this study correlates the effect of various deposition parameters over the deposition kinetics and deposited alloy characteristics. Transition and partial current densities were computed. Effect of hydrogen overpotential and surface coverage due to the adsorbed intermediates over anomalous deposition were explored. Plausible deposition mechanism and mathematical model was proposed to predict the anomalous electrodeposition characteristics. Textural, morphological and phase structural characteristics of the alloy was investigated. By the substitution of iron in the hcp lattice, c/a ratio was lowered and the lattice geometry was distorted. Intermetallic compounds of variable composition such as FeZn14, Fe5Zn33, Fe3Zn13 and FeZn3 with ‘η’ and ‘Γ’ phase structures were noted. Electrodeposition parameters were optimized and smooth, adherent, strain-free deposits with required iron content and hardness were obtained. The role of charge and mass transfer control on the texture and morphology of zinc–iron alloy electrodeposition was studied. Zinc-rich alloy with distorted ‘η’ hcp phase was predominant under kinetic limitations, by the iron substitution along ‘c’ axis. The ‘Γ’ phase with relatively iron-rich alloy was formed under diffusion control.