Silica formers, corrosion

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. 

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

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. 

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. 

In combined corrosion by solntions that contain simultaneously chlorides and sulfates, the surface deterioration and loss of strength are greater in mixes made with blended cements that contain blast furnace slag or silica fume, than in those made with plain Portland or Portland-fly ash cemenL However, the corrosion of steel reinforcement is lower in the former cements (Al-Amoudi, 1993). 

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