Potentiodynamical deposition and corrosion behavior of thin Zn-Sn coatings with layered structure and varied composition from deep eutectic solvent

Abstract

A novel Zn-Sn coating with a thickness of ~1μm was potentiodynamically deposited on the Fe substrate from a deep eutectic solvent-based electrolyte without any additives. The Zn-Sn coating is a mixture of pure Zn and pure Sn single-phases. The Zn content is ~26.5wt.% in the coating, but varies on the coating thickness. Cross-sectional transmission electron microscopy shows a double-layered structure in the coating, which contains a Sn-riched inner layer and a Zn-Sn alloy outer layer. Potentiodynamic polarization measurements indicate that the thin Zn-Sn coating shows obvious fluctuations of open circuit potential and significant passivations in the 3.5wt.% NaCl aqueous solution. The formation of Zn hydroxides on the surface is responsible for the passivation of the thin coating, as revealed by X-ray photoelectron spectroscopy. The layered structure with varied compositions in the Zn-Sn coating is believed to depress the pitting corrosion and produce multiple passivations, thus improving the corrosion resistance of the coatings.