Enhanced corrosion resistance of zinc phosphate coatings on mild steel through incorporation of nanocrystalline CeO2 and CeO2–CuO nanocomposite

Abstract

Herein, 5–10 nm-sized rod-like nanocrystalline CeO₂ and CeO₂–CuO nanocomposite were synthesized and incorporated into zinc phosphate coatings as corrosion inhibition additives. The formulated coatings were studied thoroughly on mild steel in 3.5% NaCl solution and compared with unmodified coating. The concentration of the additive was varied from 0.3 g L⁻¹ to 0.9 g L⁻¹ with three step-ups. X-ray and surface analysis results revealed that both additives favoured the formation of more compact coatings with densely packed zinc phosphate crystals and reduced iron content. Compared with the unmodified coating, the metal oxide incorporated coatings showed low corrosion current density (I_corr) and corrosion rate and high polarisation resistance (R_p) and charge transfer resistance (R_ct), confirming that these nanomaterials act as corrosion inhibitors. Among the three concentrations used, nanocrystalline CeO₂ performed well at 0.6 g L⁻¹ and CeO₂–CuO nanocomposite performed well even at low concentration of 0.3 g L⁻¹, with a more than fifty-one times lower corrosion rate and twenty times higher polarisation resistance (R_p) compared with unmodified coating. Nanocrystalline CeO₂ increased the R_ct from 114 Ω cm² to 427.5 Ω cm² at 0.6 g L⁻¹, whereas the CeO₂–CuO nanocomposite increased this value up to 3645 Ω cm² at 0.3 g L⁻¹. Enhanced corrosion resistance of these coatings was confirmed with an accelerated salt-spray test.