The effect of the ultrasound agitation and source of ceria particles on the morphology and structure of the Zn–Co–CeO2 composite coatings

Abstract

The goal of this research was to develop and analyse novel Zn–Co–CeO2 protective coatings on steel. Two different sources of ceria (CeO2) particles were employed: commercial powder and synthesized stable colloidal dispersion-sol. The plating solution was agitated by ultrasounds (20 and 30 W cm−2) or by magnetic stirrer (300 rpm) during electrodeposition. The CeO2 particles used were characterized by different methods, namely scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to compare the morphology, crystallinity and particle size. The morphology of developed composite coatings was analysed by SEM, the preferred texture was calculated based on XRD results and corrosion behaviour was evaluated by electrochemical impedance spectroscopy (EIS) and polarization measurements. Utilizing ultrasounds during electrodeposition resulted in advanced properties of the nanocomposite coatings compared to magnetic stirring. The particle content in the coatings increased to ~5 wt% when ultrasounds were applied and ceria sol used as a source of particles. Presence of ceria in composite coatings offered favourable action in corrosion behaviour increasing barrier properties of composite coatings, thereby providing a prolonged lifetime of Zn–Co alloy coatings. The benefit was more pronounced in the case of ceria sol, and deposition at 20 W cm−2, when values of impedance modulus at low frequencies reached four times higher values after 4 days exposure to 3 wt% NaCl solution than for coatings deposited with magnetic stirring for both ceria source. The lowest value of corrosion current density (2.15 μA cm−2) was determined for Zn–Co–CeO2 (CeO2 sol) composite coatings deposited under 20 W cm−2 US power.