Breviary Technical Ceramics



 Liquid phase corrosion

This is of the greatest importance for applications in machine and plant construction. 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). 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. 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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