Hans Roggendorf
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Prof. Dr.-Ing. Hans Roggendorf

1972 – 1980  University of Cologne, Subject: Mineralogy, diploma
1980 – 1992  Fraunhofer-Institute for Silicate Research, Würzburg, Germany
1986  PhD in Materials Science at Technical University Berlin
1992 – 1995   R&D department (ProMineral/Sicowa), Essen, Germany
1995  Martin-Luther-University Halle-Wittenberg, Germany

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Corrosion of sodium silicate glasses: formation of reaction layers and their characterization
1M. Dathe, 1W. Lebek, 2E. Koslowski, 3T. Bräuniger, 1T. Pfeiffer, 4D. Enke und *H. Roggendorf
1 and*Martin-Luther-University Halle-Wittenberg, Institute of Physics,
2Martin-Luther-University Halle-Wittenberg, Institute of Chemistry
3Ludwig-Maximilians-University München, Department of Chemistry
4Universität Leipzig, Institut of Technical Chemistry

The chemical durability of sodium silicate is a topic of glass science as shown by the classical studies of El-Shamy, Lewins and Douglas [1], which were further developed by Paul [2]. The corrosion and dissolution of Na2O·xSiO2 glasses (x = 2.0, 2.6 and 3.3) was investigated here by static and dynamic corrosion tests at pH values between 7 and 14. The corrosion temperatures were 30 and 50 °C. The investigated glass compositions are close to those used for water glass production. Several types of leachants with varied pH and SiO2 concentrations have been applied. Influences of pH and of glass or leachant composition were investigated. Under specific conditions either massive reaction layers are formed on top of the corroding glass or not. This layer formation is explained by pH gradients. Selected corroded glass samples have been investigated by optical as well as scanning electron microscopy, Raman microscopy, IR spectroscopy, 29Si-MAS-NMR spectroscopy, and Hg intrusion porosimetry. Especially glass samples corroded in deionized H2O and 0.1 M NaOH were submitted to these surface characterizations. Optical microscopy allows the detection of reaction layers and the measurement of their thickness. Scanning electron microscopy combined with energy-dispersive X-ray analysis allows the identification of ion exchange processes or congruent network dissolution (Figs. 1). By Raman microscopy, the incorporation of water into the glass as well as the structural rearrangement of silicate structure as a function of sample depth were investigated (Fig. 2). 29Si NMR spectroscopy was applied to characterize the silicate structure of reaction layers in comparison to the starting glass. Both structural methods showed a kind of “disproportionation reaction” of Q groups, e. g. Q3 ® Q4 + Q2, towards the surface of the reaction layers. IR spectroscopy also allowed further insights to H2O incorporation and stressed the dominance of molecular water within the reaction layer. Some samples were dried under hypercritical conditions after solvent exchange H2O against CO2. This allowed the characterization of the pore structure of reactions layers without introducing severe drying cracks. Pore sizes between of about 6 to 200 nm were found. The results are discussed with respect to influences of pH and leachant composition on corrosion process.

Figure 1.  SEM micrographs of fracture surfaces of corroded sodium silicate glasses; scale bar in picture legend; after 4 d of dynamic dynamic corrosion, other conditions: a: Na2O·3.3SiO2, corroded in 0.1 M NaOH at 50 °C; b: Na2O·3.3SiO2, corroded in sodium water glass (2 wt. % SiO2)
at 50 °C;

Figure 2.  Raman spectra of reaction layer on top of Na2O·3.3SiO2 corroded 4 d in deionized H2O at 50 °C; line parameter is sample depth

[1] El-Shamy, T. M., Lewins, J., and Douglas, R. W. The dependence on the pH of the decomposition of glasses by aqueous solutions. Glass technol. 1972, 13, 81-87.
[2] Paul, A. Chemical durability of glasses; a thermodynamic approach. Mater. Sci., 1977, 12, 2246-2268.