Defect site corrosion can occur in practically
any ceramic. If the defects are small, they are not relevant
Aluminium oxides (99.5 %) typically contain small proportions
of Na2O3CaO, Fe2O3 and often of MgO as additives. Na2O and
MgO combine with Al2O3 to form a small number of isolated
crystallites distributed through the material, with the chemical
composition of MgO . Al2O3 (spinel) or Na2O . 11Al2O3 (?-Al2O3).
Corrosive attack is not critical, because the reactions only
take place on the surface itself.
If larger inhomogeneities are dissolved away, significant
damage to the surface occurs. In the final stage this can
lead to deeper damage (in particular if intergranular corrosion
is also taking place), which can in the end result in component