Structural studies of anodic films on pure aluminium
The morphology and composition of anodically formed alumina films on pure aluminium are well characterised, but their structure is not well known due to the long range order. Two systems have been looked at in this study; the nature of the structure of tungsten incorporated from tungstate electrolyte during polarisation of aluminium at 100 V, and the other system is the thin passivating layers formed anodically on aluminium surface by polarisation within the range -2.2 - +2.5 V in aqueous solutions at various pH's. Even though other techniques, such as, electron microscopy and XPS have been used to confirm the presence of anion species in alumina films only EXAFS can yield direct information about the structure of incorporated species. In this study, EXAFS has been used for the first time to resolve the structure of tungsten incorporated into anodic alumina. Accurate determination of structural parameters require the establishment of reliable phase shift. By fitting the EXAFS of tungsten foil and potassium tungstate, the reliability of the calculated phase shifts for oxygen and tungsten was established. Based on phase shift transferability, the phase shifts calculated for 0 and W were used in the fitting of fluorescence EXAFS of tungsten in alumina oxide films formed on aluminium in tungstate solution at 100V. The experimental EXAFS function was fitted to a single oxygen shell with four atoms at a distance of 1.79 ± 0.01 A. From the structural parameters for incorporated tungsten it is clear that it is in a W04 2- even though this can not be the entity which is outwardly mobile. It is suggested that the tungstate may be linked in some way to the alumina and that because aluminium cations are outwardly mobile so tungstate is dragged along. The other kind of anodic films on aluminium are those produced by polarising aluminium in aqueous solutions at various pH's to various voltages within the range -2.2 - + 2.5 V. In some instances, passivating films grow on aluminium causing aluminium protection against corrosion. The structure of these films is not defined and because of their existence on aluminium, and their small thickness, so their structure is not easy to be studied. Electron microscope and XPS techniques have been used to determine their morphology and composition. Some work on indirectly extracting the structure of these films from XPS data was reported and it was claimed that these films have a pseudoboehmite like structure. XPS results of these films reported in this study have confirmed that under open circuit conditions an oxide film forms, but to provide good corrosion protection aluminium is polarised at different potentials. The oxide film which forms in these conditions has been found to have both 4 and 6 coordiation. The Al K-edge EXAFS of these oxide films was fitted in the same way as those for the incorporated tungsten. In this case, the EXAFS of aluminium foil and alpha alumina was fitted by the use of phase shifts calculated in EXCURV92 for aluminium and oxygen. The oxygen and aluminium phase shifts calculated in EXCURV92 were reliable, so they were used to fit the EXAFS of the passivating films on aluminium of unknown structure. The EXAFS results in this study have confirmed that these oxides do not have a single phase crystalline structure like boehmite or any others. Their structure is of a short range order and it is characterised in terms of co-ordination number and shell distances. All of the results reported in the EXAFS section of this work indicate that these oxide films are relatively thin and that is indicated by a metallic aluminium signal present most of the time. The sampling depth of the x-ray photons is much greater than the thickness of the oxide film and for this reason a metallic signal was recorded. By comparing the bond length value to the relation between the bond length and co-ordination number, it is obvious that these oxides films have a mixture of 4 and 6 co-ordination. In some cases, like aluminium polarised to + 0.9 V in pH 10, most of the co-ordination of the oxide is 6 except a little of it in the 4 co-ordination. XANES spectra for the model compounds and those for oxide films on aluminium and their derivative have agreed with the EXAFS results about the structure of thin films formed on aluminium. Also these findings were complemented by the XPS data where the Al 2p of pseudoboehmite has a binding energy of 80.0 eV whilst that of the oxide films in general were - 74 eV. So, it is clear that the oxide films structure is not a psuedoboehmite, but it is a local one with different values for the bond length and as a consequence a mixture of different (4 and 6) co-ordination.