Effects of nano-SiO2 and trivalent chromium conversion on corrosion resistance of NiZn-plated steel: towards a multilayer coating for steel protection

Purpose This study aims to enhance to corrosion protection of NiZn-plated steel by electroplating multilayer coating. Design/methodology/approach The multilayer coating consists of three layers on mild steel substrate, such as Cr3+ chromate conversion layer (CCC), electrodeposited nanosilica zinc-nickel composite layer (ZnNiSi) and electrodeposited zinc-nickel alloy layer (ZnNi). Its morphology, composition and corrosion behaviour were investigated by various methods. Findings Polarization curves indicated that polarization resistance and corrosion current density of CCC/ZnNiSi/ZnNi/Fe (6.956 kO.cm2; 2.56 mu A.cm(-2)) were two times higher and five times lower than that of ZnNiSi/ZnNi/Fe (3.42 kO.cm(2); 12.52 mu A.cm(-2)), respectively. From electrochemical impedance spectroscopy data, charge transfer resistances were 1.344, 2.550 and 2.312 kO.cm(2) for ZnNi, ZnNiSi/ZnNi and CCC/ZnNiSi/ZnNi, respectively. Salt spray test indicated that after 48 h, surface of ZnNi and ZnNiSi was covered by white rust, whereas no white rust was observed on surface of CCC/ZnNiSi/ZnNi. After 600 h, there were red rust spots (1% surface coverage) on surface of Zn-Ni, whereas only white rust was observed on both ZnNiSi/ZnNi (100% surface coverage) and CCC/ZnNiSi/ZnNi (10% surface coverage). Originality/value Multilayer coating enhanced significantly the corrosion protection for steel, as compared to the single-layer coating.