Effects of adding aluminium in zinc bath on Co-Zn interfacial reaction

Abstract

Effects of adding 0.3 wt.% Al in Zn bath on the microstructure and reaction kinetics of intermetallic compounds have been studied using Co/Zn and Co/Zn-Al solid/liquid diffusion couples by means of scanning electron microscopy (SEM) and wave dispersive spectrometry (WDS). The intermetallic compounds in the Co-Zn interface have been identified and the diffusion process of Al in zinc bath has been analyzed. The diffusion constants of intermetallic compounds have been evaluated. The chemical potential of Al and standard Gibbs free energy of intermetallic compounds have been calculated using the Co content as a variable based on the calculation of phase diagram (CALPHAD) method. The results show that the chemical potential of Al decreases with increasing Co. At the solid/liquid interface, the Co content is high, the chemical potential of Al atoms is lower than that in other areas, uphill diffusion of Al atoms occurs, and an Al-rich metastable phase forms. The Gibbs free energy of the CoAl phase is lower than that of Co-Zn compounds; therefore, the Co atoms diffuse through the γ2 layer into the Al-rich area and nucleate to form a shape-stable CoAl layer at the solid/liquid interface and significantly inhibit the Co-Zn interfacial reaction.