Contribution of electron microscopy to the study of sealing processes in oxalic anodised aluminium

Abstract

Transmission electron microscopy (TEM) of anodic coatings reveals a substructure in which three regions are clearly differentiated: a very thin band at the hexagonal cell boundaries; the cell walls contaminated with anions from the anodising bath; and the hydrated alumina that fills the pores with a certain amount of retained water. Scanning electron microscopy (SEM) appears to be a highly suitable tool to observe, at the surface and at more internal levels, the structural changes associated with each sealing degree. Both types of electron microscopy allow the possibility to visualise the different stages of the very complex sealing and ageing mechanisms indirectly determined by gravimetric and electrochemical techniques. KeywordsOxalic anodised aluminium. Sealing. Electron microscopy.Contribución de la microscopía electrónica al estudio de los procesos de sellado en el aluminio anodizado en ácido oxálico

FESEM and EIS Study of Sealed AA2024 T3 Anodized in Sulfuric Acid Electrolytes: Influence of Tartaric Acid

“…13,16,[33][34][35][36][37][38][39] The formation of petalshaped crystals on the surface, so-called smudge, is the result of aluminum salt precipitation. 2,14,15,36,37,39 According to Wefers, the chemical nature of the crystals seems to be close to pseudoboehmite or boehmite and their morphology reflects the chemical and thermal conditions prevailing during the sealing treatment. 36,37 The various transformations of the porous structure occurring during the sealing process were evaluated by impedance spectroscopy for both types of anodic film.…”

“…FESEM was found to be a powerful tool to visualize changes in oxide film morphology after anodizing and sealing treatments. 5,9,[13][14][15] Electrochemical impedance spectroscopy ͑EIS͒ was carried out to characterize the properties of the sealed and unsealed anodic films. The sensitivity of EIS makes it a successful technique for assessing the sealing quality and corrosion resistance of the anodic films.…”