Water vapor oxidation

Non-oxide ceramics are seldom used in the presence of an atmosphere of diy oxygen. In this case, a passive oxidation takes place with the water vapor according to the equation  

At high temperatures, the water vapor in the air plays a very important role in the kinetics of degradation. The parabolic regime is preserved, but for identical conditions of temperature and oxygen pressure, we observe that the water vapor considerably increases the rate of oxidatioa There is molecular diffusion of H2O, which constitutes an independent oxidant and acceleration of the diffusion of oxygen because of the stractmal modifications of Si02 caused by H2O. 

Non-oxides are used to manufacture parts for uses in aggressive environments (electrolytic baths, heat exchanger, etc.) which contain molten salts. In most cases there is first an oxidation of the ceramic and then an interaction between the oxide 

For example, for SiC in the presence of molten Na2S04, the overall reaction is written  

N02SO4 + SiC + O2 — Na iOs + CO + SO2 Table 7.5 shows corrosion start temperatures in air by a few molten salts. 

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For example, hydrocarbons react with oxygen at high temperatures to form (primarily) carbon dioxide and water vapor. Oxidation that takes place very rapidly, usually at high temperatures, is known as combustion. The combustion of hydrocarbons in petroleum and natural gas has been another major source of energy in human civilization over the past 200 years. 

H. Asteman, J.E. Svenson, L.G. Johnson, M. NoreU, Indication of chromium oxide hydroxide evaporation during oxidation of 304 L at 873 K in the presence of 10% water vapor, Oxid. Met. 52 (1999) 95-111. 

M.P. Brady, K.A. Unocic, M.J. Lance, M.L. Santella, Y. Yamamoto, L.R. Walker, Increasing the upper temperature oxidation hmit of alumina forming austenitic stainless steels in air with water vapor, Oxid. Met. 75 (2011) 337—357. 

Peraldi R and Pint B A (2004), Effect of Cr and Ni Contents on the Oxidation Behavior of Ferritic and Austenitic Model Alloys in Air with Water Vapor, Oxid Met, 61, 463-483. 

Young D J and Pint B A (2006), Chromium Volatilization Rates from Cr203 Scales into Flowing Gases Containing Water Vapor, Oxid Met, 66, 137-153. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

The high fluorine content contributes to resistance to attack by essentially all chemicals and oxidizing agents however, PCTFE does swell slightly ia halogenated compounds, ethers, esters, and selected aromatic solvents. Specific solvents should be tested. PCTFE has the lowest water-vapor transmission rate of any plastic (14,15), is impermeable to gases (see also Barrierpolymers), and does not carbonize or support combustion. 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

Sihcon dioxide layers can be formed using any of several techniques, including thermal oxidation of siUcon, wet anodization, CVD, or plasma oxidation. Thermal oxidation is the dominant procedure used in IC fabrication. The oxidation process selected depends on the thickness and properties of the desired oxide layer. Thin oxides are formed in dry oxygen, whereas thick (>0.5 jim) oxide layers are formed in a water vapor atmosphere (13). 

In a vacuum, uncoated molybdenum metal has an unlimited life at high temperatures. This is also tme under the vacuum-like conditions of outer space. Pure hydrogen, argon, and hehum atmospheres are completely inert to molybdenum at all temperatures, whereas water vapor, sulfur dioxide, and nitrous and nitric oxides have an oxidizing action at elevated temperatures. Molybdenum is relatively inert to carbon dioxide, ammonia, and nitrogen atmospheres up to about 1100°C a superficial nitride film may be formed at higher temperatures in the latter two gases. Hydrocarbons and carbon monoxide may carburize molybdenum at temperatures above 1100°C. 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

Several nonequilihrium forms of aluminum oxides have been observed (11,12) in hydrothermal experiments at low water vapor pressures in the temperature region of 300—500°C. The KI—AI2O2 form, also known as tondite [12043-15-1] AI2O2 I/5H2O, is characterized by a distinct x-ray diffraction pattern. 

Synthesis Gas Preparation Processes. Synthesis gas for ammonia production consists of hydrogen and nitrogen in about a three to one mole ratio, residual methane, argon introduced with the process air, and traces of carbon oxides. There are several processes available for synthesis gas generation and each is characterized by the specific feedstock used. A typical synthesis gas composition by volume is hydrogen, 73.65% nitrogen, 24.55% methane, <1 ppm-0.8% argon, 100 ppm—0.34% carbon oxides, 2—10 ppm and water vapor, 0.1 ppm. 

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