Quick Preparation Of Colored Ti-Zr Conversion Coating On6063Aluminum Alloy And Research Of Coating-forming Mechanism

As we all know, Ti-Zr conversion coating is one of the most promising surface treatmenttechniques instead of chromate conversion coating on aluminum alloy. Our research grouphad developed a technique of colored Ti-Zr conversion coating, but the preparation took along time. On this basis, the research in this thesis was focus on speeding up coatingpreparation by selecting suitable additives and optimizing the formula and process parameters,and finally an optimal quick coating preparation technique was obtained. By this technique,the golden Ti-Zr conversion coating on aluminum alloy with excellent anti-corrosion propertycould be achieved quickly at room temperature. Besides, the coating-forming mechanism wasdiscussed in this thesis.Through the selecting experiments, the kinds of additives were determined. Afterwards,the orthogonal experiments and the single-factor experiments were conducted to optimize thequick preparation technique. The optimal formula and process parameters of quickpreparation technique was as follows: H₂TiF₆7g/L, H₂ZrF₆1g/L, F﹣3g/L, organic colorant7g/L, sulfate5g/L, organic inhibiter2g/L，pH=4.7⁵.1. The preparation could be achieved in10min at room temperature.Homogeneous golden conversion coating could be obtained on Al alloy throughtreatment of the optimal quick preparation technique. The optimal coating showed excellentanti-corrosion property and its corrosion current density was0.11μA/cm2, only1.3%of Alalloy substrate. According to the result of SEM analysis, many particles of different sizespiled up together to form the conversion coating and the thickness of the coating was about5μm.The compositions of the conversion coating were characterized by EDS, XRD and XPS.The result showed that the main elements of conversion coating were F, Al, Na, C and O.These elements existed in the coating in the form of Na₃AlF₆, metallic oxides and metallicorganic complexes.Through the analysis of E-t curves during coating-forming process and the analysis of surface morphology of different time, the coating-forming mechanism was obtained. First, Aldissolved into Al³⁺continually because of the continuous anodic reactions and cathodicreactions; meanwhile, F﹣and Na+existing in the solution reacted with Al³⁺to generateNa₃AlF₆which would precipitate on the surface of the substrate; Afterwards, the hydrolysateof tannic acid reacted with metallic ions to form stable complex compounds which sufferedfrom large steric hindrance effect because of big volume so that precipitated on Na₃AlF₆instead of on the surface of substrate.The promotion mechanism of sulfate was also studied and the result indicated that thereduction reaction of sulfate which could consume e﹣and accelerate the dissolution of Alhappened easily in the solution because sulfate had strong oxidation ability. As a result,sulfate could provide abundant Al³⁺to accelerate the generation and deposition of Na₃AlF₆sothat sped up coating formation and reduced the preparation time.