Compositionally Modulated Multilayered Zn-Co Deposits for Better Corrosion Resistance
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JMEPEG https://doi.org/10.1007/s11665-020-05137-6
Compositionally Modulated Multilayered Zn-Co Deposits for Better Corrosion Resistance Ramesh S. Bhat, K. Venkatakrishna, Janardhana Nayak, and A. Chitharanjan Hegde (Submitted July 6, 2020; in revised form August 25, 2020) Zn-Co compositionally modulated multilayer alloy (CMMA) deposits have been developed onto mild steel using single bath technique. Multilayer alloy coatings have been galvanostatically produced using square current pulses. The switched cathode current density and number of layers have been designed for improved corrosion resistance. Experimental data revealed that multilayer coating with 120 layers at 10/ 30 mA/cm2 demonstrated 125 times higher resistance to corrosion than monolayer alloy coating of the same thickness. The improved corrosion resistance of multilayer coatings is due to small changes in the wt.% cobalt, leading to change in the phase structure of deposit in alternate layers. The defects and failures occurring in a single layer in the deposition process are covered by the alternatively deposited coating layers. Therefore, the direction of the corrosive agent is extended or blocked. Further, the better corrosion resistances afforded by Zn-Co CMMA coatings were explained through changes in electronic properties at the interface, supported by Mott-SchottkyÕs plot. However, the decrease of corrosion resistance at a high degree of layering is attributed to the less relaxation time for redistribution of solutes in the diffusion layer, during plating. Potentiodynamic polarization and electrochemical impedance data showed its good protection ability. The enhanced corrosion resistance of multilayered deposits is due to small change in cobalt content, leading to alter the phase structure of the alternate-layers of the deposits. The structural morphology and the topographical structure of the coating were analyzed by scanning electron microscopy and atomic force microscopy. Evaluation of the chemical composition of the alloy coatings was carried out by xray photoelectron spectroscopy. Keywords
corrosion resistance, cyclic voltammeter, hardness, multiple layer coatings, single layer, switching cathode current density
1. Introduction Electrodeposition is a method of electrode positioning to create a thick, uniform and adherent coating, typically of metal or alloys, by the act of electrical current on a surface. Usually, the coating produced is for decorative or protective purposes, or to enhance specific surface properties. The surface can be conductors like metals, or non-conductors like plastics. In many industries, electroplating materials are commonly used, such as the automotive, ship, air space, machinery, electronics, jewellery, defense, and toy industry (Ref 1). Various attempts have been made in recent decades to generate coatings that are corrosion-resistant (C.R). The production of metal alloys from the Zn-Fe group has been of great interest for steel sheet protection (Ref 2-6). Zn-Ni alloy was widely studied and used in the producti
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