Kinetic study of the corrosion of silicon nitride materials in acids

The leaching behaviour of gas-pressure-sintered silicon nitride materials with different yttria, alumina and silica contents in 1 N sulfuric acid at 60 to 101 deg C was studied. The progress of corrosion was investigated in terms of mass loss and thickness of the corroded layer. Dependencies affecting the progression of the corrosive attack, such as the SiO2 content of the grain boundary phase and the size distribution of the triple junctions, were described. The influence of temperature on the individual stages of the entire process was investigated. First attempts at modelling the complex corrosion behaviour were made. To simplify the mathematical modelling, plates (16 x 16 x 2.5 mm) fulfilling conditions allowing them to be considered as one-dimensional were chosen. Activation energies were calculated for the pure dissolution of the grain boundary phase from mass loss data and from the thickness of the corroded layer. Known solid state reaction models and corrosion models show only a limited ability to describe the investigated system.