Investigation On Co-Zn Interfacial Reaction And Phase Equilibria Of The Zn-Co-Al/B Ternary System

Hot-dip galvanizing is one of common and useful methods of protecting steel fromcorrosion. However, the equipment in the processing of hot-dip galvanizing were alwaysseverely eroded and corroded by molten zinc. The Co-based superalloys possessed excellentperformance on high hardenability, perfect anti-corrosion and resistance of high temperatureoxidation to molten Zn attack, they were usually adopted as the galvanizing pot hardware incontinuous galvanizing line (CGL). While those materials reacted obviously with the moltengalvanizing alloy containing Al. To study the mechanism for the failure of those Co-basedsuperalloys, the ternary interfacial reaction (Zn, Co, Al) in Zinc bathes which varying withdifferent content of Al was investigated systematically by experiment. The polynary phasediagrams and relative thermodynamic information are quite useful for the analysis of thegrowing kinetics of intermetallic compounds and designing the preferable compositions ofanti-corrosion superalloys. For these reasons, this research optimized Zn-Co-Al ternaryphase diagram and created complete thermodynamic database with the means of CALPHADmethod. What’s more, the450°C isothermal section of the Zn-Co-B ternary system has beendetermined experimentally, and it was calculated further based on the experimentalinformation. The research is significant for the study of anti-corrosive Co-based superalloysin Zn-Al bathes, and could be quite important for the analysis of the the effect Co on thecoatings at the same time.The effect of pure zinc bath and with the content of0.3wt%Al,5%wt%Al,55wt%Alon the microstructure and reaction kinetics of intermetallic compounds was studied usingCo/Zn and Co/Zn-Al solid/liquid diffusion couples by means of scanning electronmicroscopy (SEM) and wave dispersive spectrometer (WDS). It was found that themicrostructure, composition and growing rate of the intermetallic compounds in the Co-Zninterface were quite different. The growth of Co-Zn layers were restrained by the Co-Alcompounds in low aluminum bathes. Base on the measured thickness of the intermetalliccompounds at different dipping time, the reaction-diffusion constants of intermetalliccompounds in0.3wt%Al、55%wt%Al Zinc bath were evaluated.There is a disagreement about the solubility of CoAl at low temperature between theevaluated results and the experimental in the Co-Al sub-system. This work re-evaluated theparameters of CoAl of the previous evaluated result. On the basis of the literature data, theZn-Co-Al ternary system has been further optimized. The isothermal section at450°C,600°C,800°C and the liquidus projection are calculated by the software of Thermo-Calc. The450°C isothermal section of the Zn-Co-B ternary system has been determined usingscanning electric microscopy, electron probe microanalysis and x-ray diffraction with themethod of equilibrated alloys. Seven three-phase regions (α-Co+Co2B+β1, Co2B+β1+γ,Co2B+CoB+γ, CoB+γ+γ1，CoB+γ1+γ2, CoB+γ2+L, CoB+L+B) exist in the450°C isothermalsection, and no ternary compound is found. On the basis of the literature data and theexperimental results in this work, the phase diagram calculation for Zn-Co-B ternary systemwas carried out also by means of CALPHAD method.