Influence of zinc (II) ion concentration on Ni–Zn–P coatings deposited onto aluminum and their corrosion behavior

Various Ni–Zn–P alloy coatings were deposited onto aluminum substrates using electroless plating from a sulphate bath containing varying amounts of zinc (II) ions. Different techniques, such as scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, were employed to characterize the morphology, composition and structural properties of the resulting films. The effect of varying Zn content on the corrosion properties of the coatings was tested in 3 wt.% NaCl media, by means of current–potential curves, potential transients and electrochemical impedance spectroscopy measurements. It was found that increasing ZnSO concentration from 5 to 25 g l in the plating bath, increases the Zn content in the film from 9.90 wt.% to 12.7 wt.%, reduces the film thickness, modifies the surface morphology and significantly reduces the corrosion rate.