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

Acidic sulphate baths having ZnSO4 7H(2)O, NiSO4 7H(2)O, CoSO4 7H(2)O 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.