Iron oxide diffusion coefficient

Provided the mole fraction of A does not fall below N, then the oxide AO will be formed exclusively. The important criterion is the ratio of the oxidation parabolic rate constant to that of the diffusion coefficient of For A1 in Fe, the parabolic rate constant is very low, whilst the diffusion coefficient is relatively high, whereas the diffusion coefficient of Cr is much lower. Hence, the bulk alloy composition of A1 in iron required for the exclusive formation of AI2O3 at any given temperature is lower than the Cr concentration required for the exclusive formation of CrjOj. 

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

A number of metals, such as copper, cobalt and iron, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on the relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer... 

Opaque minerals like iron oxides are frequently examined in the reflectance mode - and usually give diffuse reflectance spectra. Reflectance spectra provide information about the scattering and absorption coefficients of the samples and hence their optical properties. The parameters of reflectance spectra may be described in four different ways (1) by the tristimulus values of the CIE system (see 7.3.3) (2) by the Kubelka-Munk theory and (3) by using the derivative of the reflectance or remission function (Kosmas et al., 1984 Malengreau et ak, 1994 1996 Scheinost et al. 1998) and, (4) more precisely, by band fitting (Scheinost et al. 1999). 

Figure 5. Absorption coefficient K for diffuse illumination as a function of the pigment volume concentration for three red iron oxide pigments...

Z. Jurasz, M. Daniliewski, R. Filipek. Calculations of iron diffusion coefficients -high-temperature oxidation in air in a cylyndrical geometry // Defect Diffusion Forum.-2001.- V.194-199.- P.1719-1724. 

The sedimentation diffusion model, when applied to the iron oxide system, gave solid settling velocities in agreement with theory. Solid dispersion coefficients were in the range predicted hy the Kato correlation, hut showed considerable experimental scatter. 

In the case of a sediment devoid of iron oxide, such that none of the reduced sulfate is incorporated in solid phases such as FeS, the concentration of total dissolved HoS equals (SO )q - iS04) providing the diffusion coefficients for H2S and SO4 are equivalent. 

The diffusion coefficient of aluminium in a ferritic iron-aluminium matrix is some orders of magnitude higher than in austenitic Ni3Al. Because of the low diffusion coefficient, diffusion in nickel aluminides is slow and aluminium depletion beneath the alumina scale and formation of nonprotective nickel oxides has been observed in Ni3Al. [4]. 

Fig. 10 Current density versus concentration overpotential for the anodic oxidation of iron(ll) from an equimolar solution of ferrous and ferric ions. The diffusion coefficients Dp,...

A two-compartment electrochemical cell is used to measure the diffusion coefficient of hydrogen in iron. The solubility of hydrogen in the metal under the conditions of the experiment is equal to 1.62 cm (25 °C, 1 bar) per gram of Fe. At the anode, a current density of 80 /tA cm is measured, which is attributed to the oxidation of hydrogen. The thickness of the iron sheet is 20 jim. Calculate the diffusion coefficient of hydrogen in iron. 

The kinetic constants of the Tedmon law are obtained by adjustment of experimental results. The external magnetite layer is assumed to dissolve in the liquid metal and that its dissolution is mass transfer-controlled. Its rate thus depends on the dissolved oxygen and dissolved iron concentrations, on the diffusion coefficient and the solubility of iron in the liquid metal or alloy. The code calculates the dissolved iron concentration on each point of the circuit and takes into account the influence of the fluid velocity on the dissolution of the magnetite layer. However, the erosion or the spallations of the oxide layer are not taken into account by the MATLIM code. 

Several studies showed that CO2 pressure has a very low impact on the oxidation rate but increases the carburization rate [48,62]. Therefore, after exposure under CO2 at 550°C for 1000 h, the amount of carbon transferred into T91 steel grade was multiplied by 1.5 if CO2 pressure was increased from 1 to 250 bar [48]. A deterministic kinetic model for oxidation based on the oxidation model presented previously was developed and allowed to predict flie low influence of CO2 pressure [36,56]. This kinetic model does not rely on any fitted parameters but only on thermodynamic data such as the equilibrium oxygen pressures between the inner oxide and iron and between magnetite and haematite and kinetic data such as the diffusion coefficient of iron through the oxide layer. 

The mole fractions of aluminium and chromium, as well as the molar volumes, can easily be determined. The diffusion coefficients have been chosen from the literature as given in Table 4.1. Only the solubility of oxygen in iron has not yet been measured without uncertainty [21], because even small levels of impurities lead to adulteration of the results by oxide formation. Thus in this work another method has been chosen. The critical value h has been set to 1/3. From [22] the lowest aluminium contents for the formation of a closed alumina layer on Fe-Al binary alloys have been chosen and the oxygen solubility is then calculated. The result is plotted in Fig. 4.1. The three data points are connected by the best fit straight line Xq = ll%e—kt— Compared... 

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