Corrosion resistance of Zn–Co–Fe alloy coatings on high strength steel

The corrosion properties of electrodeposited zinc–cobalt–iron (Zn–Co–Fe) alloys (up to 40 wt.% Co and 1 wt.% Fe) on steel were studied by using various electrochemical techniques and compared with zinc (Zn) and cadmium (Cd) coatings in 3.5% NaCl solution. It was found that with an increase in Co content in the coating the open circuit potential (OCP) became more positive than that of the zinc coating. For Co contents higher than 30 wt.% the OCP shifted close to that of Cd, but still remained electronegative to the steel substrate. Zn–Co–Fe coatings with ≥30 wt.% Co+1 wt.% Fe are nano-crystalline in nature and show superior corrosion resistance as compared to the Zn, low Co content Zn–Co–Fe and Cd coatings. During longer immersion, Zn–Co–Fe alloys with 34–40 wt.% Co became more noble to steel due to dezincification of the surface but the corresponding corrosion current density decreased. The corrosion resistance determined by the electrochemical techniques are confirmed by salt spray testing showing the superior corrosion resistance for Cd and high Co content Zn–Co–Fe alloys and poor performance of alloys with lower Co contents and pure Zn.