5.6.4 Concluding
Comments
If only one method of measurement is used
the results of corrosion tests will often be inadequate (if
not even false).
The measured corrosion figure (e.g. the loss of mass) cannot
simply be converted to find the changed properties of the
corroded component. Because of the different types of corrosion,
this must normally be measured directly.
The most important corrosion mechanism in ceramic materials
is inter-granular corrosion. The rate of corrosion is dependent
on the rate at which the medium diffuses through the ceramic.
As a consequence, the level of corrosion can be expected to
follow a  t
law. The steepness of the corrosion parabola is determined
by the solubility product.
If, under external corrosion conditions that remain constant,
the curve of measured properties against time is not continuous
but shows an inflection, then this is an indication either
that secondary processes are playing a part or, in general,
that the corrosion conditions in the component are changing
(perhaps because of a layered structure). This may need additional
investigation.
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