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Structural studies of anodic films on pure aluminium
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Almajdub, Musbah (1998) Structural studies of anodic films on pure aluminium. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b1659620~S1
Abstract
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.
Item Type: | Thesis (PhD) |
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Subjects: | Q Science > QC Physics |
Library of Congress Subject Headings (LCSH): | Metals -- Anodic oxidation, Aluminum |
Official Date: | February 1998 |
Institution: | University of Warwick |
Theses Department: | Department of Physics |
Thesis Type: | PhD |
Publication Status: | Unpublished |
Supervisor(s)/Advisor: | Robinson, Jim |
Sponsors: | Libya ; Saudi Arabia |
Extent: | xv, 120 leaves |
Language: | eng |
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