Microstructure evolution for oxide film of anodic aluminum foil used in high voltage electrolytic capacitor

The oxide film of anodic aluminum foil is the main working medium of aluminum electrolytic capacitor, and its quality directly affects the electrical performance of capacitor. The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, including hydration, formation, heat treatment and phosphoric acid treatment. The microstructure evolution for oxide film of anodic aluminum foil during preparation process is studied quantitatively in this paper. The results show that, the pores area on the foil surface keeps decreasing after hydration and subsequent formation. However, after phosphoric acid treatment, the pores area stops decreasing and increases to a local maximum value. The length distribution of 'corn-flake' structures keeps increasing, from the range of 60–120 nm to 100–140 nm. After phosphoric acid treatment, the petal edges become serrated, and many small corrosion holes with the length of 20–30 nm are left inside. The thickness of the outer amorphous layer in the anodized oxide film keeps decreasing from the initial 390 nm to the final 90 nm. In contrast, the thickness of the inner dense layer keeps increasing from the initial 130 nm to the final 550 nm. The decreasing trend of grain size continues until the end of preparation process, and reaches the final value of 10 nm. The analysis of microstructure of oxide film is helpful for obtaining optimal process parameters and improving performance of aluminum foil.