Glass corrosion mechanisms: A multiscale analysis

Ancient to modern glass used for stained glass windows, representative of the main composition types (Ca-rich, Na- and Ca-rich and K-rich) were treated with boiling concentrated H2SO4 (338 degrees C) and/or in molten NaOH (320 degrees C, with or without water addition). Subsequent annealings in air or in molten KNO3 (334 degrees C) were also performed. Microstructures and corrosion habits were analysed by optical microscopy. Modifications of the silicate network and corrosion products were studied by Raman and IR spectroscopies, dilatometry, microhardness, and TGA. Only K-rich silicate glasses are drastically modified by concentrated sulphuric acid. The alkali leaching takes place in three steps: i) K+-H+ (not H3O+) exchange, partly reversible, inducing glass contraction and cracking, ii) a complete, irreversible H+/K+ ion exchange, iii) a (small) hydration of the formed porous, multicracked lixiviated silica-rich solid crust. The thermal treatment of the Crust induces a water loss (similar to 0.6 wt.% at <200 degrees C) and then the loss of H+ (similar to 2.8 wt.% at 400-600 degrees C), which do not form only hydroxyl ions. Comparison with samples taken from middle-age stained glass windows shows that the treatment with sulphuric acid mimics well the atmospheric weathering. The NaOH treatment involves a rapid dissolution of the upper surface layer but no significant modifications of the remaining bulk glass. Ion diffusion and phase transformations are enhanced both by temperature and time. The high microhardness of the lixiviated glass (similar to 63% of that of the pristine glass) Proves the corrosion layer is not a gel, as claimed by many authors. The interest of K-rich glass as model for the understanding of long term corrosion of glasses used as confinement matrices for nuclear waste is discussed. (C) 2008 Elsevier B.V. All rights reserved.