The Preparation And Pretreatment On Cerium-manganese Conversion Film Of 8079 Aluminum Alloy

Aluminum alloy is a very common metal material,it has various advantages in energy,automobile and construction.Because the surface of the aluminum alloy can form a porous natural oxidation layer,it can be used in the atmosphere for a long time without protection.However,the corrosion resistance of the natural oxide layer is not strong,and the aluminum alloy can not be effectively protected under special conditions.Because of the chromium conversion coating's toxicity,it is not suitable for being widely used.In order to make the conversion coating meet the "green" concept,chromium-free aluminum alloy conversion coating process is particularly important.In this paper,the preparation and corresponding pretreatment methods on cerium-manganese conversion coating of 8079 aluminum alloy were studied.The mechanism of coating's preparation was studied by orthogonal test,anti-corrosion time test,electrochemical test,metallographic microscope,X-ray photoelectron spectroscopy(XPS),scanning electron microscope(SEM),contact angle test,salt spray test and adhesion test.Then the properties of the coating was tested.The main conclusions of the study are as follows:1.Six pretreatment methods were studied by metallographic microscope,electrochemical method,scanning electron microscope(SEM)and salt spray test.Then the mechanism was analyzed.The study found that adding NaF in the alkaline solution can effectively expose the cathode site on the 8079 aluminum alloy substrate,which is conducive to the improvement of the quality of the cerium-manganese conversion coating.Finally,a pretreatment process suitable for 8079 aluminum alloy cerium-manganese conversion coating was selected.The specific process:Water sandpaper polishing(from#400 to#800);Alkali etching(50 g/L NaOH + 6 g/L NaF),50?,7min;Acid etching(200 g/L HN03),35?,90s.2.The cerium-manganese treatment solution was optimized by orthogonal test methods.The effect of the components,pH,treatment temperature and treatment time were studied by the single-factor test methods.Finally,a cerium-manganese conversion coating formula suitable for 8079 aluminum alloy was determined.3.The mechanism of coating formation was analyzed.The results showed that there were many microscopic regions with different surface states on the surface after the pretreatment.These different microscopic regions contributed to the occurrence of tiny galvanic reactions,causing the resulting metal oxide/hydroxide to deposit on the aluminum alloy surface.NaF played a promoting role in the coating formation process.As the reaction progresses,the coating gradually covered the surface of the aluminum alloy.When the coating completely covered the surface of the aluminum alloy,the coating formation reaction stoped.Measured the performance of the coating,the results showed that the cerium-manganese conversion coating had the ability of corrosion resistance,and its corrosion current density was reduced from 1.29E-3 mA·cm-2 of the blank to 7.78E-5 mA·cm-2.The contact angle of the conversion coating was increased to 138°.This showed that the hydrophobicity of the membrane was enhanced.The adhesion test was performed after applying the primer to the surface of the conversion coating,and the results showed that the adhesion of the primer was improved after the conversion film was made.