Effects of heat treatment on microstructure evolution and corrosion performance of mechanically plated zinc coatings

Abstract

Mechanical plating has been widely used as an alternative to electroplating on hardened steel parts due to its unique ability to overcome hydrogen embrittlement. Heat treatment after electroplating of high strength steel parts is required to minimize the risk of hydrogen embrittlement, while it is usually not required after mechanical plating. In this study, the effects of heat treatment on the microstructure evolution of mechanically plated zinc coating on steel fasteners were studied. The mechanically plated zinc coatings were heated to 340 ± 10 °C for 15 and 30 min. The coatings were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-Ray Diffractometry (XRD). An interfacial layer was formed between the substrate and the coating after heat treatment. XRD peaks confirmed the presence of ζ-FeZn13, δ-Fe13Zn126 and δ-FeZn7 phases. EDS analysis of the iron‑zinc (Fe-Zn) intermetallic layer revealed the presence of a phase with Fe: Zn ratio of 1:3 near the steel substrate, identified as Γ-Fe3Zn10 phase. Results showed that the Fe-Zn intermetallic phases formed during heat treatment by counter diffusion of Fe and Zn atoms, but Zn was the principal diffusing species. Potentiodynamic polarisation tests indicated that Zn coatings with post plating heat treatment had better corrosion resistance than those without. This was related to the occurrence of the Fe-Zn intermetallic layer in heat-treated Zn coatings which provided an additional and ultimate barrier to the corrosion of the underlying steel substrate.