Comparison by SEM, TEM, and EIS of Hydrothermally Sealed and Cold Sealed Aluminum Anodic Oxides

A comparative study is made of the changes caused to the microstructure of anodic films by traditional hydrothermal sealing (HTS) and cold sealing (CS) processes, using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. SEM and TEM make it possible to visualize the different stages of the very complex sealing and aging mechanism of aluminum anodic oxide coatings indirectly determined by other techniques, showing evidence of the hexagonal structure of anodic coatings, plugging of the pore mouth with the formation of acicular pseudobohemite and the intermediate sublayers in HTS. It is also possible to verify, both in the coatings resulting from CS and in those subjected to HTS, the stages of dissolution-precipitation, with prior enlargement of the pore diameter and subsequent plugging of the entire pore length. For its part, electrochemical impedance spectroscopy (EIS) provides precise information on the effects of the aging process of anodic layers and their persistence over years and decades.