The Coloration And Performance Optimization Of Ti/Zr Conversion Coating On Aluminum Alloy

A golden conversion coating on the surface of aluminum alloy which has excellentcorrosion resisting property was prepared by using the treatment solution containing H2TiF6and H2ZrF6as essential component, tannic acid as the coloring agent, Mn²⁺as thecoating-forming accelerators. The corrosion resistance, coating-forming mechanism andcolored reason had been investigated deeply. The related results were as follows:(1) The golden Ti/Zr conversion coating on the surface of aluminum alloy was obtainedthrough the treatment solutioin with H2TiF6and H2ZrF6as the main composites accompaniedwith the coloring agent and coating-forming accelerators. Powder spray experiment of thealuminium alloy after the treatment showed that the current prepared conversion coating metthe requirement of Part4of National Standard GB5237.4-2004of powder coating profiles,which means that the conversion coating technigue can be candidate for industrial application.(2) The corrosion current density of the optimized conversion coating decreased to0.083uA·cm-2from5.894uA·cm-2of the substrate alloy. The surface of conversion coating was stillintact after160h salt spray test and the corrosion area about60%after360h. The resistancecorrosion of this conversion coating was better than that of chromate conversion coating. Theelectrochemical impedance spectroscopy research showed that conversion coating was muchmore compact and uniform and the corrosion resistance wss significantly increased when thecoating-forming accelerator Mn²⁺was introduced into the treatment solution.(3) Through the characterization to main components of the conversion coating, it wasshown that the main components of both conversion coating got in these two optimizedformulas was Na₃AlF₆, followed by the metal-organic complex, a little of Al₂O₃·3H₂O, TiO₂and so on. A double-layer structure was evidenced in the conversion coating. The outer layerconsists of the metal-organic complex which were formed by the reaction of gallic with Ti4+and Al³⁺, the inner layer was mainly made up of Na₃AlF₆.(4)The formation mechanism of the conversion coating can be divided into three stages:the nucleation and the growth of Na₃AlF₆crystals and the formation of metal-organic complex.First, Na₃AlF₆crystals preferentially nucleate on the surface of the substrate. The Na₃AlF₆crystals present as ellipsoidal globules with different size. Then, the Na₃AlF₆crystallinegrains grow in sizes and nearly cover all of the substrate surfaces. At last, the metal-organiccomplex formed and covered the Na₃AlF₆crystals which made the conversion coating golden.(5) The effect of Al³⁺and F-had been analyzed. The Al³⁺dissolved from the surface of thesubstrate and reacted with F-to form AlF₆^3-which prevented the precipitation of Al(OH)3. Mn²⁺changed the morphology of conversion coating and accelerated the coating-forming andimproved the corrosion resistance further. However, the formation mechanism and coatingstructure was not changed. Mn²⁺had a strong reducibility and was readily to be oxidized toMnOOH by O2in the solution,and the MnOOH deposited on the substrate surface as thenucleation for the Na₃AlF₆deposition, which accelerated the growth speed of thecoating-forming. MnOOH nucleation may change the orientation of Na₃AlF₆crystal and thuschange the morphology of conversion coating.