Effect of Na2SO4 on structure and corrosion resistance of ceramics coatings containing zirconium oxide on Ti-6Al-4V alloy

A micro-plasma oxidation (MPO) technique has been developed in recent years, by which ceramic coatings are reported to possess improved properties and promising application prospects in many fields. The aim of this work was to study the effect of sodium sulfate, as an additive in the zirconate system, on the structure and corrosion resistance of ceramics coatings containing zirconium oxide grown on Ti-6Al-4V by MPO process. The phase composition, morphology, and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy, and energy distribution spectroscopy. Meanwhile, the corrosion resistance of the coated samples was examined by polarizing curves and potentiodynamic anodic curves in 3.5% NaCl solution. The results show that ceramic coatings were composed of m-ZrO2, t-ZrO2, and KZr2(PO4)(3). The Ti content in the coating near the substrate decreased sharply, and then remained at 5 wt% or so through the coating, while the Zr content near the substrate increased greatly, and then remained at about 55 wt% through the coating. The addition of sodium sulfate did not change the composition of the coatings, but increased the relative proportion of zirconium oxide to KZr2(PO4)(3) in the coating. Sodium sulfate decreased the thickness of the coating, while improving the density of the coatings. Moreover, the addition of the sodium sulfate improved the corrosion resistance of the coated samples in a 3.5% NaCl solution, whether considering localized pitting corrosion resistance or uniform corrosion resistance.