Study On Microstructure And Properties Of Zn-Ti-Ce Hot-dip Galvanized Alloy Coatings

The hot-dip galvanized coatings of Zn-Tiand Zn-0.04wt%Ti-Ce were gained by adding asmall amount of Ti and Ce in the zinc bath in order to get the desirable thickness, corrosionand oxidation resisitance. The microstructure of Zn-Ti and Zn-Ce maste alloys, Zn-Ti andZn-0.04wt%Ti-Ce galvanized coatings with different Ti and Ce content respectively, and theoxidized coantings were analyzed by SEM, EDS,XRD and XPS. The effects of Ti and Ce onthe corrosion resistance were examined by electrochemical impedance spectroscopy（EIS）,electrochemical polarization, neutral salt spray tests（NSS） and immersion corrosion.Otherwise, the isothermal oxidation behaviours and oxidation mechanism of the Zn、Zn-0.04wt%Tiand Zn-0.04wt%Ti-0.02wt%Cegalvanized coatings were studied at380℃.The analysis on the master alloys shows that Zn-3.3wt%Ti is composed of Zn-based andZn15Tiphase, and Zn-2.14wt%Ce is made up of zinc solid solution and CeZn11phase.The study on the microstructure of Zn-Ti and Zn-0.04wt%Ti-Ce coatings cross-sectionreveals that the thickness of ζ phase in the Zn-Ti coating reduces gradually with the adding ofTi, and the Γ₂phase comes out on the top surface η phase when the content of Ti in the zincbath reaches0.06wt%. However, the adding of Ce less than0.03wt%to the Zn-0.04wt%Tibath can not inhibit the growth of ζ phase effectively.The tests on the corrosion resistance of Zn-Ti coating show that the corrosion resistanceof Zn-Ti coating can be improved within the content of0.06wt%Ti, while the Ti content is0.06wt%, the corrosion resisitance of Zn-Ti coating gets poor as the Γ₂phase appears. Whenthe adding of Ce to the Zn-0.04wt%Ti bath is less than0.03wt%, the corrosion resisitance ofZn-0.04wt%Ti-Ce coatings shows further better.To a certain extent, the adding of Ti and Cein the zinc bath can suppress the anodic and cathodic processes of the corrosion reaction,delay the formation of white rusts, enhance the corrosion resistance of galvanized coatings.Among all the coatings attained, the Zn-0.04wt%Ti-0.02wt%Cecoating has the best corrosionresistance.The results on the surface analysis of Zn, Zn-0.04wt%Ti and Zn-0.04wt%Ti-Ce coatingsindicate that the oxide films of ZnO, TiO₂and CeO₂exist on the surface. The presence of Tiand Ce may result in fine, uniform and defect free morphology,and adding of Ti and Ce does not affect the nature of the corrosion products forming on the surface.The research on the immersion corrosion experiment of Zn-0.04wt%Ti-0.02wt%Cecoating shows that there are three stages for the corrosion process. During the initial corrosionprocess, the oxide films dissolve little by little. As it comes to the mid-term corrosion process,the corrosion products play a role in retarding the aggressive medium. With the immersiontime extending, more and more corrosion products overlay on the surface, slowing down therate of corrosion reaction.The study on isothermal oxidation behaviours of hot-dip galvanized coatings ofZn-0.04wt%Ti and Zn-0.04wt%Ti-0.02wt%Ce at380℃indicates that the oxidation scalesand δ（FeZn8.87） phase exist on the surface after oxidized for100h. The isothermal oxidationkinetic curves of the coatings follow parabolic law. The Zn-0.04wt%Ti-0.02wt%Ce coatingreveals the best oxidation resistance not only for its dense oxide films, but also the TiO₂andCeO₂dopped in the ZnO. The holes in the δ phase of the Zn-0.04wt%Ti-0.02wt%Ce coatinglead the structuer loose.