Abstract
The degradation behaviour of silicon nitride in aqueous acidic solutions under steady state conditions was studied at 90 C and different flow rates. Two different compositions were chosen: Si3N4+4 wt.% Al2O3+6% Y2O3 (SN0) and Si3N4+2.4 wt.% MgAl2O4+1.3 wt.% Y2O3 (SN1). Corrosion was monitored by mass change, eluate chemistry, microstructure of the penetrated zone and fracture strength. The main corrosion mechanism is the leaching of the grain boundary phases. The penetrated depths consists of at least two zones with an outer layer, which is characterised by a complete or at least extensive removal of the total grain boundary phase and an inner zone, where pure silica, from which Y and Al have been leached, persists. The rates of dissolution of Al, Y and Si ions decreased with corrosion time. The fracture strength decreased with increasing degree of dissolution of the grain boundary phase. Changes in the chemistry of the grain boundary phases result in strong changes of corrosion resistance. Successful corrosion penetration depth monitoring can be done by polarised reflected light optical microscopy (colour changes), Micro Raman Spectroscopy (luminescence changes) and SEM (porosity changes). The corrosion process and its kinetics are evidenced best by microanalysis of cross sections, Raman spectroscopy and the eluate chemistry changes in addition to mass changes. # 2003 Elsevier Science Ltd. All rights reserved.
Published Version
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