6063铝合金中性无铬转化膜制备及膜层性能分析OA北大核心CSTPCD

To improve the corrosion resistance of aluminum alloy micro devices,the work aims to develop a mild and controllable preparation process of conversion coating.The neutral chromium-free conversion treatment was used to prepare a conversion coating on the surface of 6063 aluminum alloy.By studying the effects of several additives such as NaF,NH4HF2,KMnO4,sodium dodecyl sulfate(SDS),and gallic acid on the appearance and corrosion resistance of the conversion coating,NH4HF2 was determined as the optimal additive.The electrochemical methods were utilized to analyze the corrosion resistance of the coating,SEM and EDS were applied to analyze the surface morphology and elemental composition,and XRD and XPS were used to characterize the crystalline structure and compound composition of the coating.The optimal coating-forming process for the neutral conversion treatment was obtained as:EDTA-2Na 8.0 g/L,tannic acid 1.0 g/L,Na2WO4 6.0 g/L,H2ZrF6 4.0 g/L,NH4HF2 3.0 g/L,pH 6.6 and treatment at 30℃for 15 minutes.The conversion coating prepared by the process owned the dense and uniform appearance in light yellow.The electrochemical test results showed that the conversion coating had good corrosion resistance,and the self-corrosion current density decreased from 16.22 µA/cm2 of the bare aluminum alloy to 0.87 µA/cm2 after conversion treatment.EDS spectroscopy analysis indicated that the coating was mainly composed of Al,C,F,O,and Na elements.The XRD results illustrated that the conversion coating contained Na3AlF6 compound crystals.The XPS analysis exhibited that the coating also contained Al2O3,AlF3,and organic complexes.The formation of the conversion coating might be triggered by the dissolution of aluminum element in the conversion solution.Due to the presence of Fe and Cu elements in 6063 aluminum alloy,the difference of standard electrode potential between these metallic elements and Al,enabled them to form corrosive micro batteries in electrolyte solution.Therefore,Al element was oxidized to Al3+,while the conversion solution contained both Na+ and F-,which could react together with Al3+ to form insoluble Na3AlF6 cryolite and could be deposited on the surface of the aluminum substrate.EDS analysis showed that in the early stage of coating formation,the content of F element in the precipitated particles was 7.88at.%and Na element was 2.01at.%,and the F/Na ratio was greater than 2:1 in Na3AlF6,indicating the presence of other forms of F-containing compound.Based on the result of XPS and the types of ions in the solution,it could be concluded that the F-reacted with Al3+ to form AlF3.The ionization of ZrF62-was multi-stage process,so the concentration of free Zr4+ in the solution might be extremely low,resulting in very few Zr containing complexes.In addition,organic complexes had high steric hindrance and slow deposition.The Zr element detected in the coating might be introduced by adsorption on Na3AlF6 and AlF3 during the course of precipitation.By the neutral conversion treatment,a uniformly colored and corrosion-resistant conversion coating could be prepared on the surface of 6063 aluminum alloy,which was mainly composed of insoluble compounds of aluminum.The neutral chromium-free conversion process in this study can provide some reference for the research on conversion coatings.