EIS and XPS investigations on the corrosion mechanism of ternary Zn–Co–Mo alloy coatings in NaCl solution

Abstract

The changes in composition of the corrosion products of electrodeposited ternary Zn–Co–Mo alloy coatings on AISI 1015 steel during exposure to 0.5moldm−3 NaCl solution were investigated. XPS studies demonstrated that at the initial stage of corrosion on the surface of Zn–Co–Mo coating zinc hydroxide layer is formed. Hydroxyl groups react with chloride and carbonate ions which lead to the formation of zinc hydroxy carbonates and zinc hydroxy chlorides. The share of these compounds in the oxidation products is initially large. However, with time zinc hydroxy compounds slowly changes to zinc oxide, which is more stable corrosion product. It was estimated that after 24h of exposure to NaCl solution nearly 60% of zinc detected on the surface of Zn–Co–Mo coating was present in the ZnO form, 18% in the form of zinc hydroxy chloride, and more than 21% as zinc hydroxy carbonate. XPS analyses revealed that the amount of zinc hydroxy chloride increases as the exposure time lengthens and it is significantly higher than at the surface of binary Zn–Co coating. The presence of crystalline zinc chloride hydroxide as a stable product of corrosion of ternary Zn–Co–Mo alloy coating in a 0.5moldm−3 NaCl solution was confirmed by XRD analysis. According to XRD and FTIR other zinc corrosion products like: ZnO, Zn(OH)2 and Zn5(CO3)2(OH)6 were also present. The results of XPS and EIS measurements allow us to assume that in the presence of Mo in the alloy, on the surface of ternary Zn–Co–Mo alloy (3.4wt.% Co, 2.7wt.% Mo) coating more zinc hydroxy chloride is formed, which favors higher corrosion resistance of this coating.