Silica formers

At temperatures close to 20 20°, water is a suitable liquid for a thermostat. At higher temperatures it is more convenient to use a fluid such as silicone oil, or above 200°, molten metals. A typical furnace is shown in Fig. 5. A silica former is wound with nichrome wire such that the pitch decreases from one end to the centre and then increases again to the other end. Since heat losses are greatest at the ends this provides a rough correction. An inconel tube also evens out the tempera-... 

There are no structural alloys which rely on silica formation for protection primarily because Si tends to embrittle most alloys. Some intermetallic compounds (e.g., MoSi2), silicide coatings, and ceramics (e.g., SiC) are silica formers. The following is a brief discussion of the oxidation of model Fe-Si alloys. 

But even under steady-state conditions there is a profound influence of physical boundary conditions on corrosion behavior. The most widely known example of this is the boundary between active and passive oxidation of silica-formers. The classic modeling has been done by Wagner [11] for silicon. 

If reaction (1) is fast, a concentration gradient of O2 inwards and SiO outwards must exist. Then the interdiffusion coefficients and the effective thiekness of the boundary layer will control whether the eritical i (SiO) of the condensation reaction (3) is reached or not. From tables and estimations of those physieal constants we can relate back to the oxygen pressure of the bulk gas necessary to induce the critical P(SiO) on the surface. The calculation 3nelded reasonable results for the active-passive boundary of Si in streaming atmospheres with low oxygen contents and accordingly the theory was later applied to other silica-formers [14,15]. 

Under the conditions of active B2O3 vaporization the diffusion of impurities apparently does not exert great influence on the oxidation process, unlike in the case of oxidation of silica formers. One may suppose that up to 1200°C the oxidation rate of B4C is controlled by the oxygen diffusion through the B2O3 layer. 

Figure 14. Schematic model for the hot corrosion kinetics of silica-formers after [74],...

See, for example, E.J. Opila, Oxidation and volatilization of silica formers in water vapor. Journal of the American Ceramic Society 86 1238-1248,2003. 

E.J. Opila, N.S. Jacobson, D.L. Myers, and E.H. Copland, Predicting oxide stability in high-temperature water vapor, Journal of the Minerals, Metals, and Materials Society 58 22-28, 2006 I. Kvernes, M. Oliveira, and P. Kofstad, High temperature oxidation of Fe-13Cr xAl alloys in air/water vapor mixtures, Corrosion Science 17 237-52, 1977 H. Asteman, J.-E. Svensson, M. Norrell, and L.-G. Johansson, Influence of water vapor and flow rate on the high-temperature oxidation of 304L Effect of chromium oxide hydroxide evaporation. Oxidation of Metals 54 11-26,2000 J.M. Rakowski and B.A. Pint, Observations on the effect of water vapor on the elevated temperature oxidation of austenitic stainless steel foil. Proceedings of Corrosion 2000, NACE Paper 00-517, NACE International, Houston, Texas, 2000 E. Essuman, G.H. Meier, J. Zurek, M. Hansel, and W.J. Quadakkers, The effect of water vapor on selective oxidation of Fe-Cr Alloys, Oxidation of Metals 69 143-162,2008 E.J. Opda, Oxidation and volatilization of silica formers in water vapor. Journal of the American Ceramic Society 86(8) 1238-1248,2003. 

Effect of Oxidation on Fracture Strength of Silica-Formers.353... 

To begin this section on the oxidation of silica-formers, SiC and Si3N4, some of the properties of silica are briefly reviewed. Next, the oxidation of silicon is considered. While silicon is not a ceramic, its oxidation behavior nevertheless forms the basis for discussion of other silica-formers. The section then proceeds from the oxidation mechanisms and rates obtained under the most ideal, clean conditions in pure oxygen to those conditions closer to actual application... 

As structural ceramics find more applications in high temperature systems, oxidation and corrosion at high temperatures becomes an important field of study. In this chapter, the critical issues in this field have been surveyed. Ceramics have been classified according to the type of protective oxide they form. These include silica formers, alumina formers, boria formers, and transition metal oxide formers. Most of the literature covers silica formers since there are a number of near-term applications for these materials. Basic oxidation mechanisms, water vapor interactions, volatilization routes, and salt-induced corrosion were discussed for these materials. Less information is available on alumina-forming ceramics. However the rapid oxidation rate in water vapor appears to be a major problem. Boria formers show rapid oxidation rates due to the formation of a liquid oxide film and are volatile in the presence of water vapor due to highly stable Hx-By-Oz(g) species formation. Transition metal carbides and nitrides also show rapid oxidation rates due to rapid transport in the oxide scale and cracking of that scale. 

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