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.