Formation and Durability of Hydrated Layers for Several Oxide Glasses
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FORMATION AND DURABILITY OF HYDRATED LAYERS FOR SEVERAL OXIDE GLASSES JUNJI NISHII, TOMOKO AKAI, MASARU YAMASHITA, HIROSHI YAMANAKA AND HAJIMU WAKABAYASHI* Department of Optical Materials, Osaka National Research Institute, AIST 1-8-31 Midorigaoka, Ikeda, Osaka 563, Japan *New Glass Research Center, Yamamura Glass Co.,Ltd. 2-1-18 Naruohama, Nishinomiya, Hyogo 663, Japan
ABSTRACT Formations and durabilities of hydrated layers were compared between a soda-aluminosilicate (NAS), a soda-lime-aluminosilicate(NCAS) and a soda-lime-alumino-borosilicate(NCABS) glasses. The first step of our study was to prepare the optically transparent hydrated layers on the surface of specimens by an autoclave(400 *C, 20 kgf/cm 2 ) treatment. Distributions of OH groups in hydrated layers were analyzed by an etch sectioning and FTIR measurement. The rates of hydration of the glasses were in the order NAS>>NCAS>NCABS. The hydration of the NCABS glass, which is a modified nuclear waste glass, required the treatment longer than those of the NAS and NCAS glasses. In the second step, we investigated the durabilities of hydrated layers by immersing the specimens into a distilled water at 100 *C. The dissolutions of hydrated layers were confirmed for each glass. The dissolution rates of hydrated layers were in the order NCAS>NCABS>,NAS. It has become apparent by an XPS analysis that the highest durability of the hydrated NAS glass was due to the formation of a sodium free A12 0 3-SiO 2 layer on the surface. The hydrated layer of the NCAS glass, while the sodium ions were almost leached out during immersion, dissolved to water most quickly than those of other glasses. In the hydrated layer of the NCABS glass, a half amount of sodium and boron ions remained and inhibited the dissolution of hydrated layer.
INTRODUCTION Corrosion of nuclear waste glasses affects their long-term chemical durability and fracture strength. There are many studies on the extrapolation of their short-term corrosion phenomena in laboratory to long periods of time[I-5]. Corrosion of oxide glasses can be classified at least into two steps, i.e., (1)ion exchange between hydronium or hydrogen ions and alkali ions, followed by the hydration of glass network and (2)dissolution of hydrated layer to water. The other types of corrosions, such as surface layer exfoliation, stable film formation, pitting, etc., can be caused depending upon the glass composition and the conditions of solution[6]. Corrosion mechanism of glasses is usually investigated by placing the specimens in a static or circulated water. Several corrosion processes occur in a restricted narrow region from the surface and overlap with each other. The analysis of the chemical composition including the OH contents becomes consequently difficult. As reported by Bartholomew[7], we believe the treatment of specimens in an autoclave is one of the promising methods to clarify these complicated corrosion behavior. A wide range of water contents and thickness of hydrated layers can be attained easily depending on the treat
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