A high resolution scanning transmission electron microscope (STEM), operated in the SEM mode, has been used to examine the morphology of the oxide anodically formed on a surface of pure aluminium, using different electrolyte solutions (tartaric, oxalic, boric, phosphoric and chromic acids). The microstructural changes of these films following their hydration in hot water showed degradation of the oxide and conversion to an oxy-hydroxide phase (pseudo-boehmite). These observations are in agreement with previous EXAFS measurements of the Al-O bondlength which indicated the formation of crystalline boehmite (AlOOH). Observation of the oxide and oxy-hydroxide derived from anodisation in phosphoric acid electrolyte indicates that the oxy-hydroxide is derived from the original oxide. Using x-ray absorption spectrosocpy, phosphorus was detected in the oxide films and also in the oxy-hydroxide films. This confirms that the oxy-hydroxide film is derived from the oxide and is not the result of the hydrolysis of the underlying aluminium. Further, the observation that the silicon impurity incorporated in the oxide formed in tartaric acid and chromic acid electrolytes is removed during the hydration indicates that the oxide is dissolved and re-precipitated in the form of boehmite during the hydration process, rather than being formed by a solid transformation.
El-Mashri, S. M.
"Morphological Study of the Anodic-Oxide and Hydrated-Oxide Films on Pure Aluminium,"
Scanning Electron Microscopy: Vol. 1985
, Article 15.
Available at: https://digitalcommons.usu.edu/electron/vol1985/iss1/15