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Item Multilayer Zn-Ni-Al2O3 coatings for corrosion protection(Inderscience Publishers, 2014) Ullal, Y.; Hegde, A.C.The paper reports the development of composite coatings of Zn-Ni-Al2O3 by composition modulated multilayer (CMM) technique using m-aminophenol and gelatin as additives. The bath constituents and deposition parameters were optimised by conventional method. The role of gelatin and m-aminophenol in the bath was analysed by cyclic voltammetry (CV) study. Corrosion performance of the monolayer coatings was enhanced further by multilayer technique. The modulation in composition was effected by pulsing the DC in square-wave patterns. The coatings configurations were optimised for peak performance of the coatings against corrosion. It was found that corrosion resistance of CMM coatings increased with number of layers up to certain optimal numbers, and then decreased. Incorporation of Al2O3 particles into metal matrix was confirmed by EDAX. Formation of layered coating and their phase structures were analysed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) study. Reasons responsible for better corrosion resistance of CMM composite coatings are analysed and results are discussed. Copyright © 2014 Inderscience Enterprises Ltd.Item Electrolytic preparation of cyclic multilayer Zn-Ni alloy coating using switching cathode current densities(2010) Venkatakrishna, K.; Hegde, A.C.Cyclic multilayer alloy (CMA) coating of Zn-Ni was developed on mild steel using single bath technique, by proper manipulation of cathode current densities. The thickness and composition of the individual layers were altered precisely and conveniently by cyclic modulation of cathode current densities. Multilayer coatings, having sharp change in compositions were developed using square current pulses. Gelatin and sulphanilic acid (SA) acid were used as additives. Laminar deposits with different configurations were produced, and their corrosion behaviors were studied, in 5% NaCl solution by electrochemical methods. It was observed that the corrosion resistance of CMA coating increased progressively with number of layers (up to certain optimal numbers) and then decreased. Cyclic voltammetry study demonstrated the role of gelatin and SA in multilayer coating. The coating configuration has been optimized for the peak performance against corrosion. The substantial decrease of corrosion rate, in the case of multilayer coatings was attributed to the changed intrinsic electric properties, evidenced by Electrochemical Impedance Spectroscopy (EIS) study. The surface morphology and its roughness were examined by Atomic Force Microscopy (AFM). The surface and cross-sectional view of coatings were examined, using Scanning Electron Microscopy (SEM). X-ray photoelectron spectrum (XPS) study was carried out for surface analysis. The relative performance of pure Zn, monolithic and CMA coatings were compared and discussed. © 2010 Springer Science+Business Media B.V.Item Electrodeposition of high performance multilayer coatings of Zn-Co using triangular current pulses(2010) Yogesha, S.; Hegde, A.C.Compositionally modulated alloy (CMA) coatings of Zn-Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn2+ and Co2+ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn-Co) 2•0/4•0/300, was found to exhibit ?80 times better corrosion resistance compared with monolithic (Zn-Co)3•0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott-Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating. © 2010 Maney Publishing.Item Optimization of bright zinc-nickel alloy bath for better corrosion resistance(2010) Yogesha, S.; Hegde, A.C.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/dm2 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 ?-Phase with composition of Ni5Zn21, 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). © 2010 Indian Institute of Metals.Item Development of nano-structured Zn-Ni multilayers and their corrosion behaviors(2011) Yogesha, S.; Bhat, R.S.; Venkatakrishna, K.; Pavithra, G.P.; Ullal, Y.; Hegde, A.C.Composition modulated multilayer alloy (CMMA) coatings of Zn-Ni was developed using single bath technique (SBT). CMMA coatings were developed galvanostatically using square current pulses. The cyclic cathode current densities (CCCDs) and number of layers were optimized for highest corrosion resistance. Experimental results showed that CMMA coating, developed at 2.0/5.0 A/dm2, having 300 layers is ?29 times higher corrosion resistant than monolithic alloy of same thickness. Tafel and impedance data revealed its good protection ability. The improved corrosion behavior exhibited by multilayers was explained using dielectric spectroscopy. The formation of multilayer and corrosion mechanism was analyzed using scanning electron microscopy (SEM). Copyright © Taylor & Francis Group, LLC.Item Development of nano-structured cyclic multilayer Zn-Ni alloy coatings using triangular current pulses(2011) Bhat, R.S.; Hegde, A.C.Cyclic multilayer alloy (CMA) deposits of Zn-Ni were developed on mild steel from sulphate bath having thiamine hydrochloride (THC) and citric acid (CA) as additives. CMA coatings were developed galvanostatically using triangular current pulses, under different conditions of cyclic cathode current density (CCCD's) and number of layers. The corrosion behaviors of the coatings were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy methods, and were compared with that of monolayer Zn-Ni alloy of same thickness. At optimal configuration, CMA coating represented as, (Zn-Ni)2.0/5.0/300 was found to exhibit ?40 times better corrosion resistance compared to monolayer alloy, (Zn-Ni)3.0. Cyclic voltammetry study demonstrated that THC and CA have improved the appearance of the deposit by complexation with metal ions. The corrosion protection efficacy of CMA coatings was attributed to the difference in phase structure of the alloy in successive layers, evidenced by XRD analysis. The formation of multilayer and corrosion mechanism was analyzed by Scanning Electron Microscopy (SEM) study. © 2011 Allerton Press, Inc.Item Effect of current density on deposit characters of Zn-Co alloy and their corrosion behaviors(2011) Yogesha, S.; Bhat, U.; Hegde, A.C.Electrodeposition of Zn-Co alloy onmild steel has been studied, using thiamine hydrochloride (THC) as an additive. The effect of current density on deposit characters, such as chemical composition, thickness, hardness, phase structure, and surface morphology has been investigated. The electrochemical corrosion study revealed that the coating with 1.69 wt. % Co, deposited at 3.0 A dm-2 exhibited the best corrosion stability. Scanning electron microscopy (SEM), energy depressive spectroscopy (EDX), and X-ray diffraction (XRD) analyses were used to study the surface morphology, chemical composition, and phase structure of the coatings. The results were discussed focusing the effect of THC on coating characters. Copyright © Taylor & Francis Group, LLC.Item Corrosion behavior of electrodeposited Zn-Ni, Zn-Co and Zn-Ni-Co alloys(2011) Bhat, R.S.; Bhat, U.; Hegde, A.C.Zn-Ni, Zn-Co and Zn-Ni-Co alloy coatings were electrodeposited galvanostatically using sulphate bath, having THC as additive. The bath composition and operating parameters have been optimized by standard Hull cell method. The effects of current density (c.d.), pH on composition, thickness, hardness of the deposit were studied. Under all conditions of deposition, the bath followed anomalous type of codeposition with preferential deposition of less noble metal. Corrosion resistances of the coatings were measured by potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) method showed that under optimal conditions, the corrosion resistance of Zn-Ni-Co alloy coatings is approximately 20 times and 18 times better than Zn-Ni and Zn-Co alloys of same thickness. The Zn-Ni-Co coating 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 the possibility of a synergistic catalytic effect of Co on Fe and vice versa. X-ray diffraction study clearly indicates that improved corrosion resistance of ternary alloy is due to the change in the phase structure of the coatings, compared to binary alloys. Surface morphology and composition of the coatings were examined by using Scanning Electron Microscopy (SEM), interfaced with EDX facility, respectively. The ternary Zn-Ni-Co coating may thus replace the conventional Zn-Ni and Zn-Co coatings in a variety of applications. © 2011 by CEE.Item Optimization of deposition conditions for development of high corrosion resistant Zn-Fe multilayer coatings(2011) Yogesha, S.; Hegde, A.C.Composition modulated multilayer alloy (CMMA) coating of Zn-Fe was developed galvanostatically on mild steel through single bath technique (SBT), using thiamine hydrochloride as additive. Electrodeposits with different coating matrices were developed, using square current pulses. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were used to assess the corrosion performance of the coatings. The cyclic cathode current densities (CCCDs) and number of layers were optimized, for highest corrosion resistance. Experimental results showed that CMMA Zn-Fe coating, developed at 2.0-4.0 A/dm2, having 300 layers is ?30 times higher corrosion resistant than corresponding monolithic alloy of same thickness. The corrosion resistance increased with number of layers up to a certain number of layers; and then decreased. The better corrosion resistance was attributed to the dielectric barrier at the interface, evidenced by dielectric spectroscopy. The formation of multilayer and corrosion mechanism was analyzed using scanning electron microscopy (SEM). © 2011 Elsevier B.V. All rights reserved.Item Effect of heat treatment on structure and properties of multilayer zn-ni alloy coatings(International Association of Physical Chemists, 2013) Rao, V.R.; Hegde, A.C.; Udaya Bhat, K.Composition modulated multilayer alloy (CMMA) coatings of Zn-Ni were electrodeposited galvanostatically on mild steel (MS) for enhanced corrosion protection using single bath technique. Successive layers of Zn-Ni alloys, having alternately different composition were obtained in nanometer scale by making the cathode current to cycle between two values, called cyclic cathode current densities (CCCD’s). The coatings configuration, in terms of compositions and thicknesses were optimized, and their corrosion performances were evaluated in 5 % NaCl by electrochemical methods. The corrosion rates (CR)’s of multilayer alloy coatings were found to decrease drastically (35 times) with increase in number of layers (only up to 300 layers), compared to monolayer alloy deposited from the same bath. Surface study was carried with SEM, while XRD was used to determine metal lattice parameters, texture and phase composition of the coatings. The effect of heat treatment on surface morphology, thickness, hardness and corrosion behaviour of multilayer Zn-Ni alloy coatings were studied. The significant structural modification due to heat treatment is not accompanied by any decrease in corrosion rate. This effect is related to the formation of a less disordered lattice for multilayer Zn-Ni alloy coatings. © 2013 by the authors; licensee IAPC, Zagreb, Croatia.