The chemial durability of the Li_(0.25)Na_(0.25)Pb_(0.25)PO_3 base glass adn the oxynitride metaphosphate glasses,Li_(0.25)Na_(0.25)Pb_(0.25)PO_(3-3x/2)N_x,has been studied by chemical attack in aqueous solutios at a constant pH in static conditions.The dissolution process of the base galss is explained through surface hydration and the release of hydrated polyphosphate chains into solution.A hydrated corrosion layer is formed from the dissolution products,which limits dissolution over long periods.The dissolution rate of oxynitride glasses decreases as nitrogen content in the glass network increases,showing the greatest decrease for N/P ratios below 0.2.The dissolution kinetics of the oxynitride glasses wth different nitrogen contents results in a progressive change in the dissolution mechanism.Unlike in the base hosphate glass,the dissolution mechanism of the oxynitride ones is explained by taking into account the increased stiffness and covalency of the glass network.
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