ThinLayerAnalysis
Anodised Aluminium
Anodisation
is an electrochemical method of forming a thicker oxide layer on a
metal than would normally form. One of the materials that
benefits from this process is aluminium. Aluminium (Aluminum in
the US) is naturally a very soft metal that is easily extruded and can
be formed into complex shapes. It is lightweight and siutable
for a wide range of applications from aircraft to window frames.
However, it is also an extremely reactive metal and and subject
to corrosion.
Aluminium oxide (alumina) is a very hard material (very close to diamond) and a few nanometres of Al2O3
forms naturally on exposed aluminium surfaces. Anodisation can be
used to increase the thickness of this layer, providing greater
protection for the metal beneath. It should be noted that the
anodised layer is frequently somewhat porous, unlike the naturally
grown oxide, and this permits decorative dyes to be added to the layer.
In this example ball cratering was used to examine the thickness
of the anodised layer on a section of black anodised aluminium angle.
15mm black anodised aluminium angle
Under the microscope the surface appears quite crystalline and this is shown up in the following picture of the crater,
The crater was formed using a 40mm diameter ball and the polished area
of the black anodised layer appears glassey under the microscope.
This is not particularly obvious in the micrograph except in the
lower left hand quadrant of the crater wall where the high reflection
provides some photgraphic evidence for this observation.
Measurement of the crater gives a thickness of approximately
13µm, although the roughness of the surface obviously means that
there is a relatively large uncertainty on this measurement.
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