Study On Synchronous Ti/Zr Chemical Conversion Technology And Film Formation Mechanism On Various Metal Surfaces

In recent years,with the rapid increase in domestic automobile production and sales,problems such as fuel consumption,safety,and pollution have followed.The development of new energy vehicles has solved these problems very well.Vehicle lightweighting is an area that cannot be ignored in the development of new energy vehicles.One of the key research directions is the research and development of primer coating processes suitable for various aluminum alloys and galvanized steels.Chromium-free titanium-zirconium chemical conversion coatings are regarded as a good research direction because they meet the requirements of enhancing the corrosion resistance of the substrate and improving the adhesion between the substrate and the surface paint.In this paper,chromium-free titanium-zirconium conversion coatings were prepared by simultaneous chemical transformation on 6061,7075 aluminum alloys and galvanized steel,and the optimal conditions for the formula parameters of the simultaneous transformation process were obtained.The microscopic morphology and composition of the samples were studied by SEM,EDS,EPMA and XPS analysis,and then the film formation mechanism of the multi-metal synchronous chemical conversion coating was discussed.details as follows:The formula process range suitable for the chemical conversion of three metals at the same time was determined through the multi-metal single chemical conversion process experiment:H₂Ti F₆1.6～2.6ml/L,H₂Zr F₆0.8～1.3ml/L,p H 3.5～4.5,temperature30℃～40℃.The conversion time is 90～150s.Through single factor experiment,copper sulfate drip resistance experiment and electrochemical experiment,the optimal formulation process for the simultaneous chemical transformation of three metals of titanium and zirconium was obtained:H₂Ti F₆2.2ml/L,H₂Zr F₆1.0ml/L,p H 3.9,temperature 35℃.The conversion time is 120s.The SEM analysis of the surface of the three metal synchronous chemical conversion coatings shows that the conversion coatings are formed by accumulation of particles with different particle sizes.Conversion coating particles preferentially accumulate around characteristic sites such as intermetallic compounds,segregation phases,and impurities.According to EDS and XPS analysis,6061 aluminum alloy and 7075 aluminum alloy conversion coating exist in the form of Al₂O₃,Ti O₂,Zr O₂,Zr F₄;galvanized steel conversion coating is mainly composed of Ti O₂,Zr O₂,Fe₂O₃,Zn F₂and Zr F₄several substances.Through the analysis of the E-t curve of the chemical conversion film formation process of the three metals and the morphology analysis of the conversion film at different film formation times,it is found that the specific film formation process is divided into three stages:the removal of the natural oxide film,and the nucleation of film particles.As well as growing up,particle stacks are deposited on the surface of the substrate.Based on the above analysis,three metal synchronous chemical transformation film-forming mechanisms can be obtained:for the two aluminum alloys,the reaction of Al dissolution into Al³⁺occurs in the micro-anode region,and the oxygen absorption or hydrogen evolution reaction occurs in the micro-cathode region;OH^-and Al³⁺,Ti F6^2-,Zr F6^2-combine to form Al₂O_3,Ti O₂,Zr O₂,F^-and Zr form Zr F₄;the metal oxides and fluorides formed in the solution gradually deposit on the surface of the substrate.The film formation mechanism of galvanized steel is similar to that of aluminum alloy.The main components of the film layer are:Fe₂O₃,Ti O₂and Zr O₂and other metal oxides and Zn F₂,Zr F₄and other metal fluorides.