Selective dissolution of electrodeposited Zn-Ni alloy coatings

The corrosion of Zn-Ni alloy coatings in pure and NaHCO3 containing NaCl solutions was investigated under open circuit potential conditions. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were applied for the alloy surface characterisation, while direct current plasma emission spectrometry was used for test solution and corrosion product film analysis. It has been established that Zn dissolves preferentially leaving a top layer slightly enriched with Ni. However, this layer does not act as a barrier layer for the further corrosion attack. The presence of NaHCO3 in NaCl solution causes a reduction in the corrosion rates of Zn-Ni alloy and the thickness of corrosion product (oxide) film, as well as affecting the ratio of the soluble and insoluble fractions of ionised Zn and Ni and the development of the surface morphology. In a pure NaCl solution Zn-Ni corrosion occurred with surface roughening, while in the presence of NaHCO3 the process was accompanied by surface smoothing.