Effect of Zinc–Nickel Alloy Gradient Coating Microstructure on the Corrosion Resistance of Steel

Zn-Ni alloy gradient coating with current density gradients of 1-5 A/dm² was prepared for a steel substrate and then applied to its surface to improve its corrosion resistance. The morphology, chemical composition, phase structure, surface roughness, and corrosion products of these coatings were determined using field emission scanning electron microscopy (FE-SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS), respectively. In addition, the corrosion resistance of several Zn-Ni coatings was evaluated through electrochemical methods and neutral salt spray tests. The results showed that the Zn-Ni alloy with a gradient coating (Zn-Ni-FG) that was prepared with a gradually decreasing current density from the substrate to the surface, exhibited a corrosion current density of 1.402 µA·cm² and that 1749 h elapsed before 5% of the surface had red rust; these results were 0.28 and 1.4 times the corrosion current density and time to 5% red rust, respectively, of the monolayer coating. The superior corrosion resistance of the Zn-Ni-FG coating compared with the monolayer coating (Zn-Ni-M) was attributed to the gradient of the nickel content in the Zn-Ni-FG coating with a gradual positive shift of the corrosion potential from the surface to the substrate.