Study On Formation Mechanism And Performance Improvement Of Rare Earth Conversion Coatings On Aluminum Alloy

This thesis is aimed at the depositing process of Rare Earth (RE) conversion coating on the 3A21 aluminum alloy. The mechanism of the growth of electrolytic and spontaneous deposited conversion coatings and the formation of micro-cracks on the surface of the coatings were discussed. In order to seal the micro-cracks on the conversion coatings and to improve the corrosion resistance of these conversion coatings, some post-treatment methods were carried out. An efficacious phosphate post-treatment was developed, and the theory of this post-treatment was studied. A primary study was carried out to make the electrolytic deposited solution more efficient.By investigating the relationship between cathodic voltage, cathodic polarization potential, mass per unit area, the appearance, the composition of the coatings and the deposited time, a three-step depositing process theory about the formation of conversion coatings was proposed, which includes: firstly, an aluminum oxide film with several nanometers is formed quickly on the surface of the aluminum alloy; then, the compound containing Ce is precipitated over the micro-cathodes on the surface of matrix and then extends on the whole aluminum alloy untill an integrated film is formed; lastly, the thickness of the film will be uniformly increased with time. After the formation of the conversion coating, some micro-cracks induced by dehydration of the coating will appear. A mechanism about how the cracks arising and expanding was raised. The influence of deposited time on the surface morphology and the composition of the spontaneous deposited film were studied. The effect of pre-treatment and additives on the precipitation process was investigated also. Based on the above study, the theory of the spontaneous deposited conversion coating was proposed.The phosphate post-treatment of electrolytic deposit Rare Earth (RE) conversion coating was optimized by orthogonal tests. The results showed that after the electrolytic deposit Rare Earth conversion coating was post-treated, the cracking was effectively improved as well as the corrosion resistance. XPS analysis indicated that a mass of volatile crystal water was removed during post-treatment; therefore, the number of cracks in the coating was reduced. The similar phosphate post-treatment was also applied to the spontaneous deposited conversion coating and the number of cracks was reduces and corrosion resistance was improved. Besides Phosphate post-treatment, the Multi-deposited of conversion coating, Boehmite treatment, Nickel acetate post-treatment, Ambient nickel fluoride post-treatment and sol-gel treatment were studied. Nickel acetate post-treatment and ambient nickel fluoride post-treatment showed a great improvement on the corrosion resistance of the conversion coatings. The boehmite treatment also improved the corrosion resistance of the coatings. But the multi-deposited and sol-gel treatment did not displaying a positive effect.In order to make the conversion solution more efficient, the changes of components of electrolytic depositon solution were studied. UV absorbance spectrum was used to determine the concentration of Ce in solution. While the components of electrolytic deposited solution were supplied periodically or not, the performances of sequential deposited conversion coatings were checked. The results showed that the solution without supplementaries became invalid after 3 times deposited, but the solution would keep its efficacity until the tenth deposited while the H₂O₂ was supplied periodically.