Mª. J. Bartolomé

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.

XPS characterization of porous and sealed anodic films on aluminum alloys

The present work uses X-ray photoelectron spectroscopy (XPS) to compare the chemical composition of the outer surface of porous and sealed anodic films on commercially pure Al and on Al-Cu, Al-Si-Mg, and Al-Mg alloy substrates. The study also considers some inner surfaces, obtained by mechanical abrasion and argon ion bombardment, in the bulk of porous and sealed anodic films on the Al-Mg alloy. The anodic films seem to be constituted mainly by pseudobohemite, with a uniform distribution of sulfate ions throughout the coating. Especially high O/A1 ratio values have been determined on the outer surface of hydrothermally sealed films, which is interpreted as being due to the extra fixing of water and hydroxyl groups on the pseudobohemite.