Plasma electrolytic oxidation of zircaloy-4 in a mixed alkaline electrolyte

Characteristics of coating resulted from plasma electrolytic oxidation (PEO) of zircaloy-4 in an electrolyte containing phosphate, silicate, and hydroxide were investigated. The mixed electrolyte is proposed to reduce power consumption. PEO was conducted in a DC mode at a current density of 30, 50, and 70 mA/cm². Plasma was generated at a relatively low breakdown voltage of 150 V, independent of the current density. The resulting coating was composed of amorphous SiO₂ and crystalline monoclinic (m) and tetragonal (t) ZrO₂ phases. XRD analysis revealed that the porous outer layer developed at 30 mA/cm² was dominated by the m phase. The percentage of the m phase decreased while the t phase increased with increasing current density. Therefore, the coating formed at 70 mA/cm² showed the highest abrasive wear resistance, three times higher than the substrate. Similarly, the polarization curve and electrochemical impedance spectroscopy showed that the highest corrosion resistance, more than two orders of magnitude higher than the substrate, was obtained in the coated-70 mA/cm² specimen. The use of low power and a mixed electrolyte is beneficial in producing PEO coating with high wear resistance and excellent anti-corrosion characteristic.