Layered Coating of Zn–Co Alloys on Mild Steel using Triangular Current Pulses for Better Corrosion Protection

Abstract

Cyclic multilayer alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel (MS) from acid sulphate bath having thiamine hydrochloride and citric acid as additives. Depositions were carried out galvanostatically from a single bath containing Zn+2 and Co+2 ions. The triangular current pulses cycling between two cathode current densities were used for gradual change in composition in each layer. CMA coatings were developed under different sets of cathode current densities and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarization and Electrochemical Impedance Spectroscopy method, and results were compared with that of monolayer Zn–Co alloy coatings. At optimal configuration, CMA coating represented as (Zn–Co)3.0/5.0/300 was found to exhibit ~11 times better corrosion resistance compared to monolayer, (Zn–Co)4.0, deposited for same length of time from same bath. The formation of layered and corrosion mechanism was analyzed using Scanning Electron Microscopy. The corrosion protection of layered coatings of Zn–Co alloy was attributed to the different phase structure of the alloys in alternate layers, evidenced by XRD analysis.