Thermodynamics oxide formation

The solid phase analyses by QXRD show that the main phase in the residues is magnetite. Also, magnetite concentration increases with temperature. Thermodynamically, oxide formation is favored by temperature [14]. Magnetite formation could explain the low recoveries and losses of cobalt and manganese. There are numerous studies showing that cobalt can co-precipitate with iron and/or manganese oxides because it can replace these cations in the spinel, and that this phenomenon is favored by increased temperatures [7, 18]. 

For all three halates (in the absence of disproportionation) the preferred mode of decomposition depends, again, on both thermodynamic and kinetic considerations. Oxide formation tends to be favoured by the presence of a strongly polarizing cation (e.g. magnesium, transition-metal and lanthanide halates), whereas halide formation is observed for alkali-metal, alkaline- earth and silver halates. 

Certainly a thermodynamically stable oxide layer is more likely to generate passivity. However, the existence of the metastable passive state implies that an oxide him may (and in many cases does) still form in solutions in which the oxides are very soluble. This occurs for example, on nickel, aluminium and stainless steel, although the passive corrosion rate in some systems can be quite high. What is required for passivity is the rapid formation of the oxide him and its slow dissolution, or at least the slow dissolution of metal ions through the him. The potential must, of course be high enough for oxide formation to be thermodynamically possible. With these criteria, it is easily understood that a low passive current density requires a low conductivity of ions (but not necessarily of electrons) within the oxide. 

Consider Ni exposed to Oj/HjO vapour mixtures. Possible oxidation products are NiO and Ni (OH)2, but the large molar volume of Ni (OH)2, (24 cm compared with that of Ni, 6.6 cm ) means that the hydroxide is not likely to form as a continuous film. From thermodynamic data, Ni (OH)2 is the stable species in pure water vapour, and in all Oj/HjO vapour mixtures in which O2 is present in measurable quantities, and certainly if the partial pressure of O2 is greater than the dissociation pressure of NiO. But the actual reaction product is determined by kinetics, not by thermodynamics, and because the mechanism of hydroxide formation is more complex than oxide formation, Ni (OH)2 is only expected to form in the later stages of the oxidation at the NiO/gas interface. As it does so, cation vacancies are formed in the oxide according to... 

In summary for non-metal transfer situations chemical thermodynamics is a useful guide to probable behaviour. The transfer of a non-metal, X, dissolved in a molten metal, M to another metal M", will depend on the relative free energies of formation of M X and M X (see Section 7.6). Thus sodium will give up oxygen to Zr, Nb, Ti and U, as the free energy of oxide formation of these metals is greater than that for sodium on the other hand, sodium will remove oxygen from oxides of Fe, Mo and Cu unless double oxides are formed. 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

If results are required at very high temperatures, as in experiments related to steel making, even short-term survival makes severe demands on the construction of the cell (Komarek and Ipser 1984). However, oxygen concentration cells have been employed with molten ionic slags to determine the thermodynamics of oxide formation in iron between 1500-1600°C (Kay 1979). Other applications include the use of YSZ for studies of semiconducting systems (Sears and Anderson 1989, Lee et al. 1992). 

The formation of an oxide layer is thermodynamically favourable and kinetically rapid at room temperature, but as the temperature rises, the free energy of oxide formation (originally negative) increases to the point where the metal, oxide and oxygen are in equilibrium. At temperatures above this equilibrium value, and if the oxygen partial pressure is low enough, the oxide can decompose. 

The surface of the electrode must remain constant as the potential is changed in a series of potentiostat, steady-state measurements. Apart from difficulties connected with impurity adsorption, which are reduced if the experiments are carried out sufficiently quickly (Cliapter 8), it may be that thermodynamically the most stable state of the surface changes with potential most commonly by means of oxide formation. If this is suspected, it is helpful to keep observing the electrode surface during the potential measurements. The methods used must be in situ spectroscopic ones (see Section 7.5.15) FTIR or ellipsometiy are the most readily applied (see Sections 7.5.15.2 and 7.5.16, respectively). 

The synthesis of chromium carbide required a high temperature owing to the competition between carbon and oxygen. Figure 14.1 shows that, under standard conditions, Cr203 is thermodynamically favoured at low temperatures. The oxide formation is probably due to water impurities present in the gas phase. The oxide layer is thicker than the usual passivation layer and this oxidation process at low oxygen pressure has already been explained earlier by Moreau and Benard.14... 

Thermodynamic parameters for the benzene oxide-oxepine system are calculated at MP4(SDQ)/6-31+G //HF/ 6-31G level of theory. The effect of solvent polarity on the above equilibrium is studied using the isodensity polarized continuum method. Low polar solvents favor the oxepine formation, whereas medium to high polar solvents lead to benzene oxide formation. The transition state for the tautomerization is fully characterized and the activation energies for the forward and reverse reaction are estimated to be ca. 9.5 and 11.0 kcal mol-1, respectively. The solvent polarity exerts a reasonable effect decreasing the activation energies up to 4 kcal mol-1 <2001MI471>. 

It is well known that redox properties of oxides can be modified by the formation of mixed oxides (12). Effect of mixed oxide formation on AG° of M-0 dissociation is illustrated in Figure 1. The free energy changes of the following reactions, AGA° and AGp°, are calculated from thermodynamic data (13, 14). 

It is important to realize that corrosion rates may be controlled by any of several thermodynamic or kinetic properties of the alloy-scale-environment system and not just by surface or interface reactions. The three stages of high temperature oxidation of a metal, shown schematically in Fig. 1, serve as an example (7). The first or transient stage includes initial gas adsorption, two-dimensional oxide nucleation, initial three-dimensional oxide formation and finally, formation of the dominant oxide that will control the oxidation rate in Stage II. Various portions of Stage I have been widely studied using surface analytical techniques, but its duration can be very short and it is usually assumed (not always correctly) that Stage I has little impact on ultimate corrosion properties of the material. 

Thermodynamic calculation results are shown in Table 4.1. For reaction (5), the main parameters are the following free energy variation 5165 kJ, equilibrium constant at 600 °C 3.4 10-3 and the reagent conversion to reaction products is negligibly low. Much less favorable is the equilibrium state in the reaction (6). Therefore, both reactions are not practically executed. Reaction (6) described in the monograph by Zeldovich el al. [39] and in the article by Anbar [40] runs at a temperature above 1273 K with nitric oxide formation by the mechanism, which includes elementary stages with atomic oxygen participation. However, atomic... 

When the pressure of C02 in a carbonate-oxide system is equal to the equilibrium pressure pe, no net reaction occurs. When p < pe, the thermodynamic driving force favors oxide formation conversely, when p > pe, carbonate formation is favored. In the actual system the favored reaction may not occur, however, because kinetic factors prevent it. Particularly when p is not far from pe, the reaction may not proceed because some rate-limiting process, such as nucleus formation, is proceeding too slowly. The resulting spurious equilibria15 give rise to hysteresis effects, i.e., decomposition stops for some p < pe, recombination stops for some p > pe. It is for this reason that this work relies largely on thermodynamic methods for the calculation of equilibrium pressures. 

The silver-silver Ion electrode. Of the reversible metal electrodes, silver has been most often employed. There is only one stable oxidation state of silver above 300°C there is no danger of oxide formation because Ag20 is unstable.57 The metal has no observable tendency to dissolve in molten silver salts and is highly reversible in mixed chloride and nitrate eutectics. The Ag(I) ion can be introduced into the melt by either adding silver nitrate to a nitrate melt (AgCl to a chloride melt) or by anodizing a silver electrode. The potentials of silver nitrate concentration cells show ideal thermodynamic behavior up to 0.5 mol % in (Na,K)N03 eutectic and in NaN03.58... 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

Theory of electric circuits (Kirchhoff), 7 Thermal nitrogen oxides, formation of, 55 Thermodynamic Lyapunov functions, 3... 

Table VII. Thermodynamics of Formation of Other Lanthanide Oxides at 298.15...

Enthalpies of sublimation based on Knudsen effusion measurements have been determined for LaBe by Gordienko, et al. (57). The thermodynamics of formation of lanthanide hexaborides from oxides have been deduced by Portnoi, et al. (162) vaporization and stabilities have been studied by Smith (168). 

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