| Due to the advantage of smaller density, higher intensity, cheaper price, easy machining and excellent surface decorative performance, aluminum and alloys have been widely used in aviations, architecture, vehicles, ornamental and household applications et al. But, they also have some shortcomings, such as low hardness, poor wear resistance and susceptible to corrosion, which effect the scope of their application and lifetime. It is very important to improve the corrosion resistance of aluminum and alloys by suitable surface treatments. Anodizing process is a common and effective surface treatment method for aluminum and alloys. Anodizing process is simple and the cost is lower in contras to other surface treatment methods. In a harsh environment with Cl-, anodic aluminum oxide (AAO) has poor corrosion resistance, decrease the work time of aluminum and alloys. The researchers have found that it was one of the most simple and easy methods by adding additives in aluminum anodizing process to improve corrosion resistance of AAO. So, combined with the current investigation on additives, the influence of inorganic salts and organics additives and their composites were studied on improving the corrosion resistance of AAO.In the paper, five single additives such as citric acid, sodium gluconate, SDBS, Ce(SO4)2 and boric acid were added into sulfuric acid electrolyte respectively. And the AAO were fabricated in these electrolytes at constant voltages by the traditional anodizing technology. The impedance change and Tafel curve in 3.5% NaCl solution of AAO were studied by electrochemical methods. The concentrations of single additives were analyzed for corrosion resistance of AAO by combing with current-time curves. Besides, the corrosion resistance of composite additives were studied, the optimal single additive as main additive, and analyzed the influences on corrosion resistance of AAO by boiling water sealing.The results of experiments indicated: the single additives such as citric acid, sodium gluconate, SDBS, Ce(SO4)2 and boric acid all can improve the corrosion resistance of AAO, and the optimal concentration is 0.10 mol/L, 0.10 mol/L, 0.050 mol/L, 2.0 mmol/L, 0.050 mol/L respectively. But Ce(SO4)2 was selected as the optimal single additive. The electrochemical impedance spectroscopy (EIS) showed that: the impedance value of anodic aluminum oxide films which were prepared in sulfuric acid electrolyte by adding composite additives of Ce(SO4)2 + citric acid,Ce(SO4)2 + sodium gluconate and Ce(SO4)2 + SDBS was respectively 4710 K?,4150 K?,3510 K?, the impedance value was bigger with the composite additives than without additives and the performance of composite additive (Ce(SO4)2 + citric acid) was the optimal. Subsequently, the property of the three types of AAO films by boiling water sealing, which were formed in the electrolytes of sulfuric acid, sulfuric acid + Ce(SO4)2, sulfuric acid + Ce(SO4)2 + citric acid respectively were compared. The results indicated that the corrosion resistance of AAO films by sealing processing was improved greatly, the impedance values of porous layers reached 10 K?, and the property of the AAO film by boiling water sealing, which was formed in the electrolytes of sulfuric acid + Ce(SO4)2 + citric acid was optimal. |
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