corrosion of silicon nitride in sulphuric acid aqueous solution

The corrosion of a hot pressed silicon nitride (produced with the addition of 3 wt.% Y2O3+3 wt.% La2O3 as sintering aids) in 1.8 M sulphuric acid aqueous solution was studied in the temperature range of 25-70°C for times up to 400 h. No reaction products were detected on the exposed surfaces. The corrosion follows a progressive chemical dissolution of the La-Y-Si-O-N grain boundary phases: their basic character, due to the presence of Y and La cations, favours the attack by acid aqueous environments. The reaction front enters irregularly into the bulk and depletes an external layer and inner zones of the samples of grain boundary phases. At 25°C up to 100 h the corrosion kinetics is linear: the chemical reaction is rate limiting. At 40°C and 70°C the kinetics fits a three-dimensional diffusion law: the mass transport of the reactants to the reaction sites and/or the removal of products from the reacting interface(s) are rate limiting.