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 aq ueous solution was studied in the temperature range of 25-70 degreesC for t imes up to 400 h. No reaction products were detected on the exposed surface s. The corrosion follows a progressive chemical dissolution of the La-Y-Si- O-N grain boundary phases: their basic character, clue to the presence of Y and La cations, favours the attack by acid aqueous environments. The react ion front enters irregularly into the bulk and depletes an external layer a nd inner zones of the samples of grain boundary phases. At 25 degreesC up t o 100 h the corrosion kinetics is linear: the chemical reaction is rate lim iting. At 40 degreesC and 70 degreesC the kinetics fits a three-dimensional diffusion law: the mass transport of the reactants to the reaction sites a nd/or the removal of products from the reacting interface(s) are rate limit ing. (C) 2001 Elsevier Science Ltd. All rights reserved.