Investigation On Preparation Technology And Film Formation Mechanism Of Rare Earth Conversion Coating For Aluminum Alloy

This thesis is aimed at the preparation technology of REM (rare earth metal) conversion coating on the aluminum and its alloy. The mechanism of the growth of spontaneous deposited conversion coatings and the formation of cracks on the surface of the coatings were discussed. For purpose of reducing the cracks on the conversion films and improve the corrosion resistance of these films, phosphate post-treatment process was developed. The theory of the post-treatment was studied. In this paper, characterization methods used mainly included:The microstructure, element composition and thickness have been examined by scanning electron microscopy (SEM), energy spectrometer (EDS) and coating thickness gauge. The corrosion behavior and corrosion mechanism of the substrate with conversion coating were evaluated by polarization curves, electrochemical impedance spectroscopy (EIS) and corrosion test.The technical parameter of spontaneous deposited conversion coating was optimized by orthogonal tests. The analysis on the relationship between the thickness, mass per unit area, the micro-morphology, the element composition of the conversion coatings and the deposited time, further research on the effects of technical parameters (concentrations of rare earth salts, concentrations of sodium perchlorate, concentrations of oxidizer H2O2, immersion time and pH value of solution). The spontaneous depositing process theory about the formation of conversion films and crack initiation was proposed. At first, the deposition mechanism involves both the oxidation of aluminum and the reduction of hydrogen peroxide in the reactions. Then, the generation of hydroxide ions results in a pH increased near the surface of the substrate, which causes the formation of cerium oxide conversion coating by a series of reactions. At last, after the formation of the film, some cracks induced by dehydration of the coating will appear.The optimum technical parameter of phosphate post-treatment was obtained by orthogonal tests. The effects of technical parameters (concentrations of sodium phosphate, temperature, treating time and pH value of solution) in post-treatment process were studied in detail. The experimental results showed that after spontaneous deposited Rare Earth conversion coating was post-processed, converted the structure of deposited coatings into hydrated cerium phosphate structure, the number of cracks was reduced and corrosion resistance was improved a lot.