Nanostructured multilayer alloy or composition modulated multilayer alloy coatings of Zn-Co have been developed, and their corrosion behaviours were studied by potentiodynamic polarisation and electrochemical impedance spectroscopy methods. The coatings were developed galvanostatically using square, triangular and sawtooth current pulses through single bath technique. The cyclic cathode current density and the numbers of layers have been optimised for peak performance of the coatings against corrosion. Under optimal conditions, the coatings developed using square, triangular and sawtooth current pulses were found to be respectively y100, 80 and 90 times more corrosion resistant than monolithic alloy of same thickness. The better corrosion resistances of the composition modulated multilayer alloy coatings were attributed to the dielectric barrier at the interface, as evidenced by dielectric spectroscopy. Surface morphology, multilayer formation and surface after corrosion tests were examined by scanning electron microscopy.
Optimization of an acid chloride bath for electrodeposition of smooth Zn-Ni alloy on to mild steel was studied using thiamine hydrochloride (THC) as brightener. The influence of deposition current density, temperature, composition, and corrosion properties of Zn-Ni alloy coatings was investigated. The effect of bath composition and operating parameters on deposits characters like composition, micro-hardness, thickness and adhesions were tested. Under no conditions of current density employed in the present study, the anomalous type of co-deposition has changed to normal type. Electrolytically deposited Zn-Ni alloys were characterized by electrochemical AC and DC techniques. The experimental results revealed that coatings having ~ 3.16 wt. % Ni at 3.0 A/dm 2 was the most corrosion resistant. The better corrosion resistance at optimal current density was attributed to the formation of n-type semiconductor film at the interface using Mott-Schottky (M-S) analysis. The effect of THC on plating process was investigated through cyclic voltammetry techniques. X-ray diffraction (XRD) studies of the coatings showed the presence of J-Phase with composition of Ni 5 Zn 21, responsible for it extended orrosion resistance. The change in the surface morphology of the coatings, with current density was analyzed using scanning electron microscopy (SEM).
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 (CMMA) 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 ,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.
Micro/nanostructured multilayer coatings of Zn-Co alloy were developed periodically on mild steel from acid chloride bath. Composition modulated multilayer alloy (CMMA) coatings, having gradual change in composition (in each layer) were developed galvanostatically using saw-tooth pulses through single bath technique (SBT). CMMA coatings were developed under different conditions of cyclic cathode current densities (CCCDs) and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) method. Optimal configuration, represented as (Zn-Co) 2⋅0/4.0/300 was found to exhibit ~ 89 times better corrosion resistance compared to monolithic (Zn-Co) 3⋅0 alloy deposited for same time, from same bath. The better corrosion resistance of CMMA coatings was attributed to changed interfacial dielectric properties, evidenced by dielectric spectroscopy. Improved corrosion resistance was attributed to formation of n-type semiconductor film at the interface, supported by the Mott-Schottky plot. Further, the formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy (SEM).
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