Corrosion behavior of electrodeposited Zn–Co and Zn–Co–Mo coatings on mild steel in NaCl solution

Abstract

Vehicle body, particularly against chloride-containing environments such as de-icing salts, is a challenging task. The Zn-Co films are a potential and promising one for coated steel sheets, but a third alloying element in the Zn-Co alloys such as molybdenum greatly improves the performance of anticorrosive properties. In this study, the corrosion behavior and surface morphology of electrodeposited binary Zn-Co and ternary Zn-Co-Mo coatings on mild steel were examined. Coatings were electrodeposited from an acidic chloride solution containing sulphanilic acid (brightener) and gelatin (grain refiner). The chemical component of the coatings was verified by ICP-OES and EDS. Corrosion resistance was tested in 3.5 wt% NaCl solutions by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The surface morphology and chemical composition were characterized using scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Results revealed that 1.7 wt% cobalt showed the highest improvement in binary alloy. The incorporation of molybdenum (∼ 1.0 wt%) resulted in a smoother, more uniform, and finer-grained ternary coating. Electrochemical studies resulted in noteworthy decrease in corrosion current density and increase charge transfer resistance for Zn-Co-Mo. The XPS analysis also evidences the presence of stable, molybdenum-enriched passive oxide layers which block chloride ion ingress. The ternary Zn-Co-Mo coating (containing 1.7 wt% Co and 1.0 wt% Mo) shows excellent corrosion resistance in chloride environment by the creation of a dense and stable passive film, as it finds potential application for corrosion protection of car parts in automotive industry.