Ultrasonication assisted plasma electrolytic oxidation accelerated growth of SiO2/ZrO2 coating on zircaloy-4

Abstract

Ultrasonication is proposed to accelerate the growth of coating formed on a zircaloy-4 by plasma electrolytic oxidation (PEO) at low electric power. PEO was conducted at a DC current density of 20 mA/cm2 and a limiting voltage of 300 V for 5–15 min. The PEO bath contained a balanced ratio of sodium phosphate, sodium silicate, and potassium hydroxide. Surface investigation using optical and electron microscopes on the resulting coatings revealed a significant enhancement of Faradaic efficiency from 15–17 % to 38–46 % due to sonication. Ultrasonication-assisted PEO (UPEO) accelerated the coating growth by lowering the breakdown voltage, generating more plasma discharge, and avoiding the gas shielding effect. Consequently, the sonication did not alter the structure and composition of the coatings. The coatings were composed of amorphous SiO2 as the matrix and reinforced by the crystalline ZrO2, as confirmed by the x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) analyses. The combined effect of composition, porosity, and thickness yielded an insignificant difference in the hardness and wear-rate, and the corrosion resistance between the PEO and UPEO coatings. Electrochemical measurements demonstrated an order magnitude enhancement of corrosion resistance as a result of coating. The inner layer served effectively as the corrosion barrier, while the mediate and outer layers gave significant protection against the mechanical load. The faster growth rate provided by ultrasonication is beneficial for the extensive development of PEO technology.