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Effects of Heat Treatment on Mechanically Plated Zn-Sn Coating and its Corrosion Performance

Paper Number: 54
Author/s: P P Chung, P Cook, Y Durandet1 & J Wang
Organisation: Swinburne University of Technology1, Faculty of Science, Engineering and Technology, Department of Mechanical and Product Design Engineering, Melbourne, Australia & Kwik-Coat Pty Ltd2 , Melbourne, Australia

Abstract

Mechanical plating is commonly used to coat high strength steel fasteners because the process does not induce hydrogen embrittlement. The process uses kinetic energy to deposit metallic coatings onto small metallic work pieces such as fasteners, bolts, washers, and nails at room temperature. Although mechanical plating can evenly coat all surfaces and features of the small components, the deposits tend to be porous, which can affect their corrosion performance in the long term. Recent work indicates that heat treatment after and/or during the mechanical plating process may be beneficial, but the reasons for this are unclear. In the present work, the effects of heat treatment on the microstructure evolution and corrosion resistance of mechanically plated Zn-Sn coating on steel fasteners were studied. The microstructures, compositions and crystallographic structures have been characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-Ray Diffractometry (XRD). After heat treatment, an interfacial layer was formed in between the substrate and the coating. The interfacial layer was Fe-Zn rich and the outer coating was Sn rich. Multi-phase structures were observed within the interfacial layer. XRD indicated that the phases formed were ζ (FeZn13) and δ (FeZn7). Cyclic salt spray test was carried out on coatings with and without heat treatment. The results showed that heat-treated coating exhibited better corrosion resistance than that of without heat treatment. Such significant improvement on the corrosion resistance of heat-treated Zn-Sn coating was believed to be related to the Fe-Zn intermetallic layer formed. The intermetallic layer acted as an ultimate barrier to the exposure of substrate because the dissolution of Zn from Fe-Zn intermetallic layer was slower due to its more positive potential than the pure zinc.