Insights into energy-efficient and eco-friendly sealing of anodic aluminum oxide film holes with alkaline earth metal salts

Abstract

In this study, the impact of various sealing conditions on the quality of the sealed anodic aluminum oxide (AAO) film is systematically investigated, which leads us to gain insights into sealing the AAO film. To this end, magnesium acetate and calcium acetate are employed as the main compositions of the sealant. It is demonstrated that the temperature exerts the greatest influence on the quality of the sealed film based on the weight loss and antistaining adsorption evaluations among the other examined conditions, followed by pH, mixed salts, surfactants, and complexants. For the optimized sample, the minima of the staining level and weight-loss ratio can reach 0 and 13.4 mg dm⁻², respectively, meeting the requirement of the ISO standard. This work also realizes an energy-efficient sealing of the AAO film at intermediate temperatures (75–85 °C). The potentiodynamic scanning and electrochemical impedance spectroscopy measurements reveal that the AAO film sealed by the alkaline earth metal salt possesses a superior corrosion resistance as compared to the one sealed by the conventional nickel salt. Additionally, the potential can be stable in a faster way at higher temperatures over the range of 70–85 °C, as demonstrated in the E–t curve. On the other hand, the potential stabilization is more readily achieved at higher pH values within the range of 5–7, whereas it cannot be realized at pH values above 7. We also note that the appropriate addition of surfactant, complexant and pH buffer can assist in stabilizing the potential. In the absence of any surfactants and complexants, the potential stabilization cannot be reached. Furthermore, the present study discusses the mechanism underlying the effective sealing, thereby paving the way for the future exploration of the energy-efficient and eco-friendly sealing technologies for many practical AAO-based applications.