Effect of Cr(III) passivation layer on surface modifications of zinc-nickel coatings in chloride solutions

Surface chemical and morphological modifications of as-plated and Cr(III)-passivated monophasic zinc-nickel coatings induced by corrosion in chloride solutions are demonstrated. The passivated samples showed slower anodic dissolution, less significant de-alloying, smaller surface dezincification and lower coating cracking, as demonstrated by Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDX) of the surface and inductively coupled plasma atomic emission spectroscopy solution analysis. Surface characterization by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and SEM-EDS indicated simonkolleite as the main corrosion product for both, as-plated and Cr(III)-passivated coatings. In contrast, only for as-plated coating, which experienced higher cracking, new Ni containing phases (metallic Ni and NiO) were evidenced. The phase transition via selective dissolution of zinc is supposed to increase the concentration of the structural defects and could explain cracking in the non-passivated Zn-Ni coating.