Deposition of cobalt rich Zn–Co alloy coatings of high corrosion resistance

Abstract

Cobalt rich Zn–Co alloy coatings were electrodeposited from an alkaline electrolyte under equilibrium (not anomalous) codeposition conditions. The main part of the study deals with Zn–15Co alloy coatings possessing the best appearance. X-ray diffraction (XRD) measurements ascertained that the alloy structure consisted of a single gamma phase Co5Zn21. Electrochemical impedance spectroscopy (EIS) showed that the corrosion resistance of Zn–15Co at the initial corrosion stages was about three times as high as that of pure Zn and comparable to the corrosion resistance of pure Co. Testing in salt mist revealed the corrosion resistance of Zn–15Co under these conditions to be about three times higher than that of cobalt lean Zn–1Co alloy. The morphological properties and the chemical composition of corrosion products were studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Zinc was found to be in an oxidised state within a corrosion product layer, whereas cobalt was not oxidised. Based on this, development of some cobalt enriched protective layer during corrosion was assumed. The results demonstrated that Zn–15Co coatings, owing to their high corrosion resistance, may be considered as a replacement for chromated coatings (Zn and Co lean alloys) or Zn–Ni.