Chromium oxide diffusion coefficient

A.C.S. Sabioni et al,161 (see also Refs 162-164) showed that the values of self-diffusion coefficients of chromium and oxygen in Cr203 are considerably lower than the previously reported ones and are insufficient to explain the observed growth rate of the chromium oxide layer. 

Relatively little work has been done on the effects of refractory-metal additions on the oxidation of Al203-forming alloys. In superalloys, where the refractory-metal concentration is rarely above 10 weight percent, three effects have been observed. One effect is beneficial and arises since these elements can be considered to be oxygen getters in comparison to the base metals (Ni and Co) and they can, therefore, promote the selective oxidation of aluminium and chromium. The other two effects are deleterious. One occurs because the refractory elements decrease the diffusion coefficients of the elements needed to be selectively oxidized. The other arises because of the relatively poor protectiveness of the oxides of the refractory metals, which makes their presence in the external scale undesirable. Refractory metals... 

Small Cr contents increase the rate of reaction, but at 20% Cr, the reaction rate starts to decrease and exhibits a minimum value at 25%-30% Cr (Figure 20.62). The minimum value depends on the pressure. More chromium is needed to stabilize a protective film since the diffusion coefficient of chromium in cobalt is lower than for chromium in nickel. However, since the adhesion strength of the film on Co-Cr alloys is poorer than on Ni-Cr alloys despite the identical oxidation rate of the Cr-containing Co alloys with Cr203 protective film, the practical oxidation resistance is lower. Other alloying elements, as Figure 20.62 shows, have little influence on scale resistance. 

It can be seen from Tables 1 and 2 that the diffusion coefficients complexes decrease when the chromium oxidation state increases and that the values found by different methods are in a good agreement. 

For very low diffusion coefficients smaller inter-gap spacings are required. Electron and hole hopping diffusion rates in fully shielded mesoporous oxides are low, typically 10 m s V with notable exceptions and therefore sub-micron inter-electrode gaps are necessary for feedback currents to be measurable. A focused ion beam (FIB) machined trench (ca. 400 600 nm wide) in tin-doped indium oxide (ITO) films has been suggested and used for films of mesoporous titania, chromium oxide,and for dye-modified metal-organic framework UMCM-1. The dual-hemisphere... 

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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