Development and characterization of Zn–Ni,Zn–Co and Zn–Ni–Co coatings

Abstract

Acidic sulphate baths having ZnSO₄ 7H₂O, NiSO₄ 7H₂O, CoSO₄ 7H₂O and sulphamic acid (SA) were optimized for the deposition of smooth Zn–Ni, Zn–Co and Zn–Ni–Co coatings on mild steel substrate. Bath compositions and deposition conditions were optimized by Hull cell method for maximum efficiency against depositions in corrosion. Faradic efficacy exceeded 90% as the current density increased; increased wt-% of nobler metal content (Ni and Co) in the deposit. Potentiodynamic polarization and electrochemical impedance spectroscopy indicates that the Zn–Ni–Co coating; corrosion resistance was 28 times higher than the Zn–Ni and 21 times higher than the Zn–Co coating. The good corrosion resistance of the Zn–Ni–Co coating was ascribed to its phase structure and the surface morphology. New cheap sulphate baths have been suggested for bright and uniform coating of Zn–Ni, Zn–Co and Zn–Ni–Co and result have been discussed.