Corrosion of Si_3N_4-ceramics in aqueous solutions Part 2. Corrosion mechanisms in acids as a function of concentration, temperature and composition

摘要

The corrosion behaviour of Si_3N_4-ceramics with different grain boundary compositions was investigated in H_2SO_4 with different concentrations at 90 deg C and was compared with the corrosion behaviour of glasses. In part I of this paper the influence of temperature was investigated [Schilm, J., Gruner, W., Herrmann, M. and Michel, G. Corrosion of Si_3N_4-ceramics in aqueous solutions, J. Eur. Ceram. Soc., 26 (2006) 3909-3917] The investigation showed that the corrosion rate as well as the corrosion mechanisms strongly depend on the concentration and temperature of the acid and the composition of the grain boundary phase. The observed dependencies allow the prediction of the corrosion behaviour of silicon nitride ceramics in acids. The investigation of the morphology of the corrosion layers by SEM and TEM proves that the observed passivation of the materials during corrosion in H_2SO_4 with concentrations higher than 0.5 mol/1 is caused by the formation of SiO_2-rich layers inside the corroded triple junctions. The experimental results were discussed in terms of corrosion mechanisms allowing a qualitative prediction of the corrosion behaviour at least. The corrosion resistant Si_3N_4-materials can be reached by tailoring the amount and composition of the grain boundary phase.