| Anodic Aluminum Oxide (AAO) Films had a good prospect of application in architecture, decoration, aviation, et al. Combined with the current investigation on additives, the influence of rare earth salts and organics additives and their composites were studied on improving the properties of AAO films, especially on their corrosion resistance.In this paper, five single additives such as Ce(SO4)2, La2(SO4)3, oxalic acid, citric acid and glycerol were added into the sulfuric acid electrolyte respectively. And the porous AAO films were fabricated in these electrolytes by the traditional anodizing technology. The thickness, hardness and anti-corrosion property of the obtained AAO films were examined. Ce(SO4)2 was selected as the optimal single additive and its best adding quantity was 1mmol/L, according to the corrosion resistance tests. Besides, the optimal oxidation time was determined as 1h by the rules of the thickness, hardness and anti-corrosion property of AAO films, formed in sulfuric acid solution with and without the cerium salt additive.Three typical organic additives, such as oxalic acid, citric acid and glycerol were added into the sulfuric acid electrolyte containing cerium salt, respectively. The thickness, hardness and corrosion resistance of the obtained AAO films were examined, and the citric acid was selected as the optimal organic additive, which had the best synergistic effect with cerium salt, and its best adding quantity was 0.6mol/L, according to the corrosion resistance tests. The composite additives of Ce(SO4)2 and organics were better than single additives in improving the properties of AAO films. Besides, current-time curves of anodizing process and the electrochemical impedance spectroscopy (EIS) indicated that: both single and composite additives couldn't change the anodizing processes, but they increased the thickness of barrier layer, and accordingly improved the anti-corrosion property of AAO films. However, after composite additives, the corrosion mechanism of AAO films changed in NaCl solution. The macromolecule complexes blocked up the pores, which improved the anti-corrosion property of AAO films furtherly.Subsequently, the property of the three types of AAO films, which were formed in the electrolytes of sulfuric acid, sulfuric acid + cerium salt, sulfuric acid + cerium salt + citric acid respectively were compared. Through chemical etching method, the corrosion resistance tests of the three AAO films were made in neutral, acidic and alkaline solutions respectively, and the results were neutral＞acidic＞alkaline in turn. The surface morphology, structure and composition of three AAO films were studied by the testing techniques such as TEM, XRD and EDS, and the results indicated that the adding of the additives reduced the pore size and porosity, and increased the wall thickness of pores. But they couldn't change amorphous structure and composition of AAO films.Finally, the working mechanism of additives was explored by analyzing the ionic forms in three electrolytes, current-time curves and the results of EIS. Because of the high valence number and standard redox potential of Ce4+ ion, it played a catalysis-like role to some intermediate reactions in anodizing process, which accelerated the forming speed and change the structure and accordingly improved the properties of AAO films. However, for the composite additives, it was because the stable complexes formed by the synergistic effect of cerium salt and citric acid. These complexes had some influences on the anodizing process, which changed the thickness of barrier layer, the pore size and the porosity, and accordingly improved the properties of AAO films. |
