COBALT-RICH ZN–CO ALLOYS: ELECTROCHEMICAL DEPOSITION, STRUCTURE AND CORROSION RESISTANCE

Abstract

Cobalt-rich Zn–Co alloy coatings (e. g., Zn–15Co, Zn–18Co) were electrodeposited from an alkaline electrolyte under anomalous (at high current densities) and non-anomalous (at low current densities) co-deposition conditions. The fibre nanostructures consisting of ca 94% of Co were deposited under certain conditions. X-ray diffraction (XRD) measurements confirmed that the alloy structure consisted of single gamma-phase Co 5 Zn 21 . Electrochemical impedance spectroscopy (EIS) showed that corrosion resistance of the cobalt-rich alloys 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. Tests in salt fog (until red rust) revealed up to four times higher corrosion resistance of cobalt-rich alloys as compared to that of the conventionally used 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 oxidized state within a corrosion product layer, whereas cobalt was not oxidized. Based on this, a cobalt-enriched protective layer forming during the corrosion process was assumed. The results demonstrated that Co-rich coatings, owing to their high corrosion resistance, may be considered as a replacement for chromated coatings (Zn, Co-poor alloys) or Zn–Ni.