Electrodeposition, Microstructural Characterization and Anticorrosive Properties of Zn-Mn Alloy Coatings

Abstract

Zinc alloy coatings are widely used for sacrificial corrosion protection of ferrous materials, in particular in the automotive industry. Zinc-Manganese (Zn-Mn) alloys were reported by Boshkov et al. to provide higher corrosion protection than pure zinc coatings due to a double protective effect involving corrosion products with low dissolution rates as well as higher cathodic protection than pure zinc due to the use of Mn [1-5]. In the framework of this work, various chloride baths with and without additives were investigated by means of voltammetric and Electrochemical Quartz Crystal Microbalance (EQCM) studies in order to define optimal deposition parameters such as deposition potential, plating bath composition and additives in regards with the Mn content in the final coatings. The composition, morphology and crystallographic structure of the coatings were studied using Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). The aim of this study is to obtain compact, smooth and monophasic Zn-Mn alloy coatings with a high Mn content. A transition from 11 wt.% Mn to 17 wt.% Mn was observed in the case of coatings deposited in presence of additives. Coatings deposited from the additive free electrolyte were rough with a dendritic morphology. By contrast, the 17 µm-thick coatings obtained from the bath with additives consisted of a compact and smooth monophasic single ε-Zn-Mn hexagonal close-packed (HCP) phase with a highly textured crystallography along the <110> direction. The corrosion behavior of the Zn-Mn alloys and pure zinc deposits was investigated by means of potentiodynamic polarization curves and polarization resistance (Rp) measurements. Higher corrosion protection was obtained for coatings deposited in presence of additives compared to pure Zn coatings. Zn-Mn coatings deposited in absence of additives found to provide lower protective ability than Zn coatings. The results clearly show that the alloying of Zn with Mn is not a sufficient condition to provide higher corrosion protection than that of pure zinc layers. In fact, coating morphology and crystallographic orientation have a significant influence on the anticorrosive properties of Zn-Mn alloys. 1. Boshkov, N., et al., Galvanic alloys Zn–Mn—composition of the corrosion products and their protective ability in sulfate containing medium. Surface and Coatings Technology, 2005. 194(2-3): p. 276-282. 2. Boshkov, N., S. Vitkova, and K. Petrov, Corrosion Products of Zinc-Manganese Coatings: Part I - Investigations Using Microprobe Analysis and X-Ray Diffraction. Metal Finishing, 2001. 99(9): p. 56-60. 3. Boshkov, N., Galvanic Zn–Mn alloys—electrodeposition, phase composition, corrosion behaviour and protective ability. Surface and Coatings Technology, 2003. 172(2-3): p. 217-226. 4. Boshkov, N., et al., Influence of the alloying component on the protective ability of some zinc galvanic coatings. Electrochimica Acta, 2005. 51(1): p. 77-84. 5. Boshkov, N., K. Petrov, and G. Raichevski, Corrosion behavior and protective ability of multilayer Galvanic coatings of Zn and Zn–Mn alloys in sulfate containing medium. Surface and Coatings Technology, 2006. 200(20-21): p. 5995-6001.