Electrodeposition of Zn-Cr Alloys: A Comparison with Pure Zinc

Abstract

The electrodeposition of zinc coatings is one of the most common surface treatments for providing corrosion resistance to metallic components. From the thermodynamic point of view, zinc is located in a sweet spot, with its reduction potential being lower than most other common metals, while at the same time being not too susceptible to atmospheric corrosion; overall these features make zinc the ideal candidate for the electroplating of sacrificial coatings for corrosion protection. Owing it to its low cost and scalability, zinc electroplating has cemented itself as a cornerstone process in a variety of industrial sectors in the last decades. [1]While zinc coatings corrosion resistance has proved to be satisfactory in most cases, it could still be improved to operate even in more aggressive environments. For this purpose, electrodeposition of zinc alloys with transition metals from the iron group (Ni, Co, Fe) has proven to be a successful method to enhance zinc coatings corrosion resistance. In particular, zinc-nickel alloys have found the greater success among them, due to improved corrosion resistance and mechanical properties. [2]Zinc-chromium alloys also present a good opportunity to improve coatings performance, thanks to the passivating effect of Cr. In fact, electrodeposited ZnCr alloys with a Cr content up to 10.4% were found to exhibit a corrosion potential shifted by up 150 mV in the anodic direction with respect to the pure zinc counterpart. [3] This shift led to a significant decrease in the coating dissolution rate but was sufficiently small that the layers could still preserve their sacrificial nature towards the underlying steel.In this work, ZnCr coatings are deposited on low carbon steel from an acidic sulfate formulation containing Cr(III) ions at various concentrations. The properties of ZnCr coatings are explored by varying the most common parameters such as current density, composition, and pH. Furthermore, the resulting coatings are characterized through conventional surface analysis techniques such as SEM, EDS, XRD and XRF. Finally, the corrosion resistance performance of the ZnCr coatings is compared to pure Zn coatings to highlight the benefits of the alloy.Bibliography[1] R. Winand, “Electrodeposition of Zinc and Zinc Alloys,” Mod. Electroplat. Fifth Ed., pp. 285–307, Feb. 2011.[2] N. Lotfi, M. Aliofkhazraei, H. Rahmani, and G. B. Darband, “Zinc–Nickel Alloy Electrodeposition: Characterization, Properties, Multilayers and Composites,” Prot. Met. Phys. Chem. Surfaces, vol. 54, no. 6, pp. 1102–1140, Nov. 2018.[3] V. Chakarova, T. Boiadjieva-Scherzer, D. Kovacheva, H. Kronberger, and M. Monev, “Corrosion behaviour of electrodeposited Zn-Cr alloy coatings,” Corros. Sci., vol. 140, pp. 73–78, Aug. 2018.