Development of anti-corrosive multi-layered coatings of zinc-nickel alloy

Purpose - The purpose of this paper is to develop and optimize anti-corrosive multi-layered coatings of zinc-nickel alloy on carbon steel. Design/methodology/approach - A variety of composition-modulated multi-layer alloy (CM MA) coatings of zinc-nickel were developed on a carbon steel substrate by cyclic changes in cathode current during electrodeposition, coupled with variation of the thicknesses of the individual layers. The corrosion behavior of the coatings was studied in 5 percent NaCl solution by electrochemical methods. Cyclic cathode current densities-(CCCDs) and the number of alloy layers were optimized for highest performance of the coatings against corrosion. The factors responsible for improved corrosion resistance were analyzed in terms of change in the intrinsic electrical properties of the capacitance value at the electrical double layer that was associated with micro/nanometric layering. The formation of the semi-conductive surface film, which was responsible for the improved corrosion resistance, was supported by a Mott-Schottky plot and the cyclic polarization study. The formation of multi-layered deposit and the mechanism of corrosion degradation of the coating were analyzed using scanning electron microscopy. Findings - CMMA coatings with an optimal configuration of (Zn-Ni)(2.0/4).(0/300) showed similar to 35 times better corrosion resistance compared to a monolithic (Zn-Ni)(3.0) alloy coating of the same thickness. The peak performance was attributed to the change in intrinsic electrical properties of the coating and this conclusion was supported by dielectric spectroscopy. Originality/value - The paper describes the optimization of CCCD and the number of deposited layers by development of electrolytic deposition of anti-corrosive multi-layered zinc-nickel coatings from a single plating technique.