Microstructure evolution of a Zn-Al coating co-deposited on low-carbon steel by pack cementation

Abstract

In this study, Zn–Al coatings were co-deposited on AISI 1020 steel by pack cementation. The as-prepared coatings were characterised by scanning electron microscopy, energy–dispersive X–ray spectroscopy, and X–ray diffraction. The evolution mechanism of the coating was modelled and analysed in detail. Furthermore, the corrosion resistance of the as-received coatings was investigated by a potentiodynamic polarisation test in a 3.5% NaCl solution. We observed that the relatively loose and porous structure of those coatings was a function of the Al content in the packed powders. The regions with a natural porous structure evidently increase with an increase in Al concentration. The Zn-rich layer was formed because of the dominant inward diffusion of Zn with a small diffusion of Fe in the substrate. Thus, the Zn-rich layer/substrate interface moved into the ferrous substrate. As the reaction continued, the deposition of Al dominated. The interdiffusion of Al and Zn in the Zn-rich layer and the diffusion of Fe into the Zn-rich layer towards the Al-rich layer led to the nucleation and growth of the Fe–Al phases and the consumption of the Zn-rich layer. As a consequence, the Al-rich layer/Zn-rich layer interface moved inside the Zn-rich layer. At higher deposition temperatures, when the heat was released because of the Fe–Al phase formation, the evaporation of Zn resulted in void formation in the Al-rich layer. The potentiodynamic polarisation test results show that both Zn and Zn–Al coatings significantly enhance the corrosion resistance of a ferrous substrate, but Zn–Al coatings seem to be more efficient.