Effect of Oxalate Anions on Zinc Electrodeposition from an Acidic Sulphate Bath

Abstract

Zinc coatings were electrodeposited on steel substrate. The effect of oxalate anions as chelating agents on nucleation and growth mechanism, surface morphology, predominant texture and corrosion behaviour of zinc electrodeposits was investigated. Evaluation of nucleation mode of zinc in the presence and absence of oxalate anions showed that the instantaneous nucleation is predominant in both cases. From the AFM images, the nuclei densities for zinc deposition were 3.5 × 10₉ and 2 × 10₉ cm₋₂ in the absence and presence of oxalate anions, respectively. These are one order of magnitude higher than those obtained by analysis. Electrochemical impedance spectroscopy revealed that in the bath containing oxalate anions, the growth of deposit is semi-infinite diffusion controlled. While in the absence of oxalate anions, the charge transfer control was governed. In the absence of oxalate anions, the texture components of zinc coating were mostly basal and low angle pyramidal planes, whereas in the presence of oxalate anions, the high angle pyramidal and prism planes were dominant. Furthermore, oxalate anions changed the zinc surface morphology from packets of hexagonal platelets to a granular nature. In the presence of oxalate anions, the zinc coating produced had a better corrosion resistance than that deposited from bath without oxalate anions.