5.6.2.1 Liquid
phase corrosion
This is of the greatest importance for applications
in machine and plant construction.
5.6.2.1.1 Acids
Laboratory porcelain, similar to silicate
and borosilicate glasses (laboratory glass), is highly resistant
to corrosion at moderate temperatures (up to approx. 100 °C).
Aluminium oxides (where the Al2O3 content is between 92 %
and 99.5 %) are very resistant to mineral acids if the secondary
phase (the phase between the grain boundaries) is
acid resistant.
After extremely pure aluminium oxides, extremely pure non-oxide
materials have the lowest corrosion rates. This applies above
all to the S and HP materials which, in contrast to the LPS
materials, do not contain an inter-granular phase.
SISIC has a special position, due to the infiltrated, acid
resistant, silicon phase.
Hydrofluoric acid (HF) and its mixtures
with concentrated H2SO4 and HNO3 attack all materials that
contain SiO2. Only extremely pure aluminium oxide (purity
> 99.9 %) and SSIC or HPSIC are resistant.
Generally speaking, dilute acids are more corrosive than concentrated
acids (due to the effect of the pH value).
5.6.2.1.2 Alkaline Solutions and Molten
Alkalis
Like laboratory porcelain, aluminium oxides containing low
proportions of Al2O3 only react slightly with alkaline solutions,
but are heavily corroded by molten alkalis. Highly pure oxides
are more resistant, but do react significantly with melts.
The aluminium oxide is then dissolved forming, for example,
sodium aluminate (Na2Al2O4).
Non-oxide ceramics are less heavily attacked by alkaline solutions
than Al2O3. The silicon phase in SISIC, however, is significantly
attacked, and resistance to corrosion from molten alkalis
is poor.
5.6.2.1.3 Water
Hydrothermal corrosion, i.e. corrosion in water or water vapour
at temperatures > 100°C and increased pressure, is
a special case. With the exception of SSIC, where weak corrosive
attack can be detected in distilled water, all the other materials
show a significant loss of mass that rises at higher temperatures
and increasingly dissolves the inter-grain phase. In the case
of Al2O3 at 220 °C and above the Al2O3 matrix also starts
to dissolve. Drinking water and dilute solutions of salts
attack ceramic materials significantly less than distilled
water.
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