Fabrication And Characterization Of Anticorrosion And Antifouling Conversion Coating On Aluminum Alloys

Aluminum alloys are the most widely used non-ferrous metal materials. But in some corrosive environments, aluminum alloys are prone to corrosion. Besides their surface are easily fouled by organic or inorganic contaminants. Therefore it is necessary to make some surface treatmenst for aluminum alloys in order to improve their corrosion resistance and antifouling performance. In this work a chrome-free conversion process was developed. The yellow conversion coating on 5052 aluminum alloy was prepared by chemical conversion treatment with K2ZrF6 and K2TiF6 as the main salt, KMnO4 as the oxidant and NaF as the accelerant. The antifouling performance was improved by following FAS-17 treatment for the prepared conversion coating on 5052 aluminum alloy. The morphology, composition and thermal stability of the conversion coating were studied by SEM, EDS, FT-IR, XPS and DSC. The anticorrosion and antifouling performances of conversion coating were studied by copper sulfate spot test, salt water immersion, polarization curves, electrochemical impedance (EIS), Fluorescence microscopy analysis and chromatic aberration analysis. The anticorrosion and antifouling mechanism was also discussed.By studying the influences of the process parameters such as pH, conversion time, temperature, solution concentration on the conversion coating, the optimized process parameters were obtained as following:the solution compostions were KMnO4 4g/L, NaF 0.1 g/L, K2ZrF6 0.25g/L, K2TiF6 1.5g/L, the pH value was about 2.0, the treatment temperature and time were 70～80℃ and 5min respectively. The coating was mainly consist of AlF3·3H2O、AlOx/Al、Al2O3、MnO2、TiO2. The corrosion rate of aluminum alloys with conversion coating in 3.5% NaCl solution was decreased from 0.240 g/(m2·h) to 0.046 g/(m2·h) by sealing treatment. The results of polarization curves also showed that the corrosion potential of aluminum alloy with conversion coating in 3.5% NaCl solution increased and the corrosion current density reduced by about two orders of magnitude. EIS results showed that the conversion and sealing treatments improved the corrosion resistance of 5052 aluminum alloys.By FAS-17 modifying, a thin hydrophobic fluorosilanes film was formed on the surface of the conversion coating. This further improved the corrosion resistance of aluminum alloys in 3.5%NaCl solution, its corrosion rate was dropped from 0.240 g/(m2·h) to 0.046 g/(m2·h). The results of electrochemical tests showed that FAS-17 modification made its corrosion potential increase and its corrosion current density reduce, and improved the shielding performance of the coating.The simulated pollutant test was carried out on the FAS-17 modified coating. The fluorescent substance in residue was observed by fluorescence microscope after contaminant was removed from the coating. The chromatic aberration of coating was analyzed before and after the removal of pollutant using colorimeter. The cleaning rate of aluminum alloy sample under different treatment was calculated and compared. The results showed that the coating modified by FAS-17 had the maximum cleaning rate and the best antifouling performance.