| As the most widely used colored light metal material,aluminum alloy?AA?plays an important role in the development of the national economy.Chemical conversion technology is one of the most widely used and most economical aluminum alloy surface treatment technologies.The chromate conversion coating?CrCC?used in traditional chemical conversion technology isgradually banned due to the high toxicity and carcinogenicity of chromate.In the alternative of CrCC,the research of domestic and foreign scholars found that zirconium-titanium conversion coating is the most promising chromium-free chemical conversion coating to replace CrCC.However,compared with CrCC,there is still a big gap between the corrosion resistance of the zirconium-titanium conversion coating and the good adhesion of the paint film.In this paper,AA6061 was used as the research object to develop a chromium-free chemical conversion coating with better adhesion to the paint film and higher corrosion resistance.The preparation of the coating and the coating formation mechanism were studied.The main research work and results of the thesis are as follows:A Ti-Zr Composite conversion coating?Ti-ZrCC?with high corrosion resistance is obtained on the surface of the aluminum alloy by adding?NaPO3?6 and Ce?NO3?3·6H2O to the main film formers H2TiF6,H2ZrF6.By optimizing the content of different components and process conditions,the optimal film formation process was determined:H2TiF6?50mass%?2ml·L-1,H2ZrF6?40mass%?1ml·L-1,6 partial Sodium phosphate0.5g·L-1,water and cerium nitrate 0.1g·L-1,reaction temperature 30°C,reaction time150 seconds,pH of the solution was 3.5.Through the Polarization Curve and Electrochemical Impedance Spectroscopy?EIS?,the corrosion potential(Ecorr)of the new titanium-zirconium chemical conversion coating is-8.693V,and the corrosion current density(Icorr)is significantly reduced to 0.1148?A·cm-2,The coating resistance?Rct?is 68140?.The corrosion resistance of aluminum alloys has been significantly improved.?2?The surface morphology and elemental composition of the conversion coating were observed by Scanning Electron Microscope?SEM?and Energy Dispersive Spectrometer?EDS?.The variation of aluminum alloy micro-area structure at different coating formation stages was studied by EPMA.The results show that the coating was mainly composed of Al,O,Ti,and P elements,and also contains a small amount of F,Zr elements.It is presumed that the main components are TiO2,ZrO2,Al2O3 and a small amount of phosphate.The film formation begins on the Al?Fe?Si intermetallic compound,and Ce3+is preferentially adsorbed on the intermetallic compound.Its hydrolysis causes the local pH of the solution to decrease,which promotes the dissolved charge transfer of the matrix,and the occurrence of microanode/microcathode reaction promotes the local pH increase of the solution.Na3AlF6/Al2O3 and ZrO2/TiO2 begin to deposit on the surface of the metal substrate and deposit into a film.Sodium hexametaphosphate acts primarily as a stabilizer and dispersant in solution.?3?On the surface of aluminum alloy,salicylic acid and Ce?NO3?3·6H2O were used as assistants to obtain Ti-Zr composite conversion coating completely free of Cr and P.The effects of various processes on the corrosion resistance of the coating during the reaction were studied.The best process for film formation is:H2TiF6?50mass%?4ml·L-1,H2ZrF6?45mass%?0.4ml·L-1,salicylic acid 0.35g·L-1,Ce?NO3?3·6H2O0.14g·L-1,reaction temperature was 30°C,Ph value was 4.0.The Icorr of the coating was0.2072?A·cm-2,and the Ecorr was-0.675V. |
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