Corrosion Behavior of Electrodeposited Zn-Ni, Zn-Co and Zn-Ni-Co Alloys

Zn-ni, zn-co and zn-ni-co alloy coatings were electrodeposited galvanostaticallyusing sulphate bath, having thc as additive. the bath composition and operating parametershave been optimized by standard hull cell method. the effects of current density (c.d.), phon composition, thickness, hardness of the deposit were studied. under all conditions ofdeposition, the bath followed anomalous type of codeposition with preferential deposition ofless noble metal. corrosion resistances of the coatings were measured by potentiodynamicpolarization and electrochemical impedance spectroscopy (eis) method showed that underoptimal conditions, the corrosion resistance of zn-ni-co alloy coatings is approximately 20times and 18 times better than zn-ni and zn-co alloys of same thickness. the zn-ni-cocoating under optimal c.d. (3.0 a dm-2) was found due to its inherent high dielectric barrier,evidenced impedance signals. high partial c.d. for zinc in zn-ni-co alloy system supports thepossibility of a synergistic catalytic effect of co on fe and vice versa. x-ray diffraction studyclearly indicates that improved corrosion resistance of ternary alloy is due to the change inthe phase structure of the coatings, compared to binary alloys. surface morphology andcomposition of the coatings were examined by using scanning electron microscopy (sem),interfaced with edx facility, respectively. the ternary zn-ni-co coating may thus replacethe conventional zn-ni and zn-co coatings in a variety of applications.