Oxide scales silica

Aluminium is obtained on a large scale by the electrolysis of the oxide, dissolved in fused cryolite The oxide, occurring naturally as bauxite, AI2O3.2H2O, usually contains silica and iron(III) oxide as impurities. These must be removed first, since aluminium, once prepared, cannot be freed of other metals (which will be deposited on electrolysis) by refining it. The crude oxide is dissolved under pressure in caustic soda solution the aluminium oxide and silica dissolve and the ironflll) oxide is left ... 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

Scaling up of the deposition-precipitation procedure does not present any difficulty with urea or sodium nitrite. The preparation of tin oxide on silica catalysts has been scaled up from 11 to a vessel of 2 m3 without any difference in the final catalyst. The injection procedure has been scaled up by recirculating the suspension of the support from a large vessel through a small vessel in which the alkaline solution was injected into the vigorously agitated suspension. Provided the flow of the fluid being injected was kept slow and continuous, excellent results have thus been obtained. 

In conclusion, according to the results, P-Si AlONs could be severely corroded in sew than in air and steam atmosphere. Dense and protective oxide scale could not be formed due to the oxide product of silica being flowed out by the water. Therefore, the reaction mechanism of SiAlON materials has been proposed, and the results in this study could present a valuable example for the exploitation of high corrosion resistance materials. 

These values suggest that not only the inward difliision of oxygen through the silica-based surface oxide scale have to be considered as rate-controlling mechanism, but also the interfacial reaction between Si02 and AI2O3, oxidation products of SiC and AIN, which led to the formation of mullite ... 

Dehydration of alcohols is an acid-catalyzed elimination reaction. Experimental evidence shows that alcohols react in the order tertiary (3°) > secondary (2°) > primary (1°) this reactivity relates directly to the stability of the carbocation intermediate formed in the reaction. Generally, sulfuric or phosphoric acid is used as the catalyst in the research laboratory. A Lewis acid, such as aluminum oxide or silica gel, is usually the catalyst of choice at the fairly high temperatures used in industrial scale reactions. 

Finally, moderate amounts of alkali impurities have been intentionally introduced into silica scales growing on both SiC and Si3N4 by vapor techniques (Pareek and Shores, 1991 McNallan et al., 1994 Sun et al., 1994). Alkali halides or alkali salts are vaporized in one portion of a furnace and transported with a carrier gas to the test specimen in another zone of the furnace. Depending on the activity of the alkali vapor species in the test, the oxide scale composition varied from 0.4 (Pareek and Shores, 1991) to 30 (Sun et al., 1994) mole percent alkaline oxide. Oxidation kinetics for the lowest levels of alkali impurity in the scale were parabolic, but elevated over rates ob-... 

Sintering aids are often added to SiC and Si3N4 to promote densification of these materials. Sintering aids have several noteworthy effects on the oxidation of silica forming materials including increased oxidation rates, change in rate controlling mechanism, and alteration of the oxide scale structure. 

From Fig. 7-1 it can be seen that after silica forming materials, those materials with the next most protective oxide scale would be alumina formers. Since alumina scales are less sensitive to impurities, AIN and AI4C3 appear, at first glance, to be ideal high temperature oxidation resistant materials. However, the recession rates in this figure are based on those observed for alumina forming metal alloys. Oxidation of both of these ceramic materials results in the formation of gaseous products which can alter the protective qualities of the alumina scale. 

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. 

Figure 19.2 Biogenic oxides, (a) Silica in diatom frustule (scale bar = 1 (im). (Reproduced with permission from Ref. [33]. Copyright 2003, Wiley-VCH Verlag GmbH, Weinheim.) (b) Iron oxide in magnetotactic bacteria (scale bar = 100 nm). (Reproduced with permission from...

Water-side deposits are of many types. Hardness (calcium and magnesium)-based deposits and iron oxide are the most common water-side deposits and often affect boUers and cooling systems. Process and oil leaks can foul boilers and cooling systems. BiofouUng, mud, and debris are often found in cooling systems. Treatment chemicals, if not properly controlled, can add to deposits and scales. Silica can form hard, adherent deposits in boUers, steam turbines, and cooling systems. Corrosion products can add to deposits. 

More pure PC products can be produced by using column chromatography with adsorbents. On a commercial plant scale oil-free lecithin or its ethanol-soluble PC fraction is treated in a chromatographic column with aluminium oxide or silica gel adsorbents. The PC is concentrated at 70-95% purity [23, 24]. 

Because calcium oxide comprises about 65% of Pordand cement, these plants are frequendy situated near the source of their calcareous material. The requisite silica and alumina may be derived from a clay, shale, or overburden from a limestone quarry. Such materials usually contain some of the required iron oxide, but many plants need to supplement the iron with mill scale, pyrite cinders, or iron ore. Silica may be supplemented by adding sand to the raw mix, whereas alumina can be furnished by bauxites and Al202-rich flint clays. 

---