Alumina scales coefficients

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

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]. 

Oxygen and cation diffusion coefficients in alumina scales... 

Oxygen-18 diffusion experiments led to the determination of oxygen diffusion coefficients. In the temperature range from 1050 to 1200°C, the results are shown in Table 10.3 [82]. Comparison with the literature is difficult, since very few experiments have been performed on thermally grown alumina scale [82-86] (Fig. 10.7), whereas numerous works have concerned the determination of diffusion coefficients in synthetic alumina [87-98]. 

Chromium is used as an isotopic marker for aluminium. Taking into account that both chromium and aluminium cations diffuse in the same way, the results obtained on chromium diffusion coefficients in alumina scales can be applied to aluminium cation diffusion coefficients in the same scale. Indeed, it is easier to use a chromium isotope as a diffusive species than to use aluminium, which has no natural isotope one can only use an artificial aluminium isotope, Al, which is radioactive and very expensive. Very few works have used this radioactive tracer [90,101]. 

Comparison of oxygen and chromium diffusion coefficients in thermally grown alumina scales doped with RE. 

We have argued that molecular-scale understanding of the structure and composition of mineral surface-aqueous solution sorption complexes is vital to development of robust reactive contaminant transport models. This argument suggests that significant error may be expected in predicted environmental behavior in the absence of such knowledge. In an attempt to test this claim, we have used the results of our XAFS studies of Co(II) sorbed by alumina to refine a quasi-thermodynamic uptake model, then used the model to evaluate the sensitivity of predicted Co(II) partition coefficients to the choice of reactions included in the sorption model. 

The reduction of the coefficient of variation of 54 %. was a promising result and indicated a potential for improving the stability of alumina as a raw material. Based on this result a go ahead for an installation in a full scale silo battery consisting of two silos of 6000 tons was given. 

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