Zinc Coatings on Steel by Electrodeposition from Complexing Alkaline Ammoniacal Baths

Abstract

Zinc coatings on steel substrates were produced by electrodeposition from alkaline ammoniacal baths (pH 9.5). The influence of bath composition, pH, current density and temperature on the voltammetric behavior during zinc deposition was investigated. An optimum bath was established which contained: 60 g/L ZnSO4·7H2O, 50 g/L (NH4)2SO4, 30 g/L Na2SO4 and 80 mL/L NH3. The buffering properties of the ammonia/ammonium couple limited the local change in pH in the vicinity of the electrode surface caused by simultaneous hydrogen evolution. The cathodic current efficiency of the deposition process at 25°C was very high (≈ 100%). Moreover, the bath was characterized by good throwing power (TP = 35.6%) in addition to the high polarization during the deposition process. Analysis of the results from cyclic voltammetry proved that the deposition process involved a nucleation mechanism controlled by mass transfer. The corrosion resistance of the zinc-coated steel was studied by using an anodic potentiodynamic polarization technique and compared with that of uncoated steel. The surface morphology of the as-deposited zinc was investigated by using scanning electron microscopy (SEM).