Linear-parabolic oxidation kinetic

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

The aforementioned inconsistencies between the paralinear model and actual observations point to the possibility that there is a different mechanism altogether. The common feature of these metals, and their distinction from cerium, is their facility for dissolving oxygen. The relationship between this process and an oxidation rate which changes from parabolic to a linear value was first established by Wallwork and Jenkins from work on the oxidation of titanium. These authors were able to determine the oxygen distribution in the metal phase by microhardness traverses across metallographic sections comparison of the results with the oxidation kinetics showed that the rate became linear when the metal surface reached oxygen... 

Oxide Thickness Versus Time. Silicon oxidation has been modeled by using the linear-parabolic macroscopic formulation of Deal and Grove (69). As a starting point for the study of this model, the kinetics of oxidation... 

Numerous other models have been proposed to explain the deviation of dry oxidation from linear-parabolic kinetics. For example, field-assisted oxidant diffiision during the oxidation of metals was proposed by Cabrera and Mott (75) and used by Deal and Grove (69) to explain the results for thin oxides. Ghez and van der Meulen (76) proposed the dissociation of molecular oxygen into atomic oxygen at the Si-Si02 interface and the re-... 

The oxidation kinetics when liquid oxides are formed often exhibit two stages an initial period of rapid parabolic oxidation followed by linear kinetics. The rapid parabolic period has been modeled by assuming diffusion of metal and/or oxygen through liquid channels, which surround islands of solid oxide. 

In graphical form, the parabolic oxidation is represented by a horizontal parabola as shown in Fig. 2-7 a. The smaller kp is, the lower is the course of this parabola and the more protective is the situation for oxidation. For the experimental determination of kp, i.e., for the oxidation rate constant, the data measured as oxide scale thickness or mass gain as a function of time are plotted in a parabolic way. That is, the square of the scale thickness or the mass gain measured is plotted on the ordinate while time is plotted in a linear form on the abscissa. Fig. 2-7 b. In such a plot the oxidation kinetics... 

The high temperature oxidation of pure Si3N4 has been studied by Hirai et al. (1980) as well as Narushima et al. (1993 a). As for SiC, as the temperature is increased toward the melting point of silica, the scales formed on Si3N4 become more amorphous and the oxidation kinetics change from parabolic to linear. 

Figure 5. Effects of SiC and TaB incorporation on oxidation behaviour ofZrB ceramics at 130(FC. It can be clearly seen that incorporation of SiC changes the oxidation kinetics from severe linear to mild parabolic [Talmy, 2001].

For pure iron, the oxidation rates in the low oxygen (0.9%) atmospheres followed a coupled linear-parabolic kinetics, exhibiting a slow transition from linear kinetics controlled by surface reactions to parabolic kinetics controlled by solid phase diffusion. Increasing H2O content from 10% to 20% increased the iron oxidation rate when the oxygen content was low, especially at temperatures above lOOO C. In the atmospheres containing higher levels of oxygen (3.6-5.4%), the oxidation of pure iron followed the parabolic law and the effect of steam content became very small. 

Assuming that the contribution made by the free oxygen was controlled by boundary layer diffusion [35] whereas those made by carbon dioxide and water vapour were controlled by the rates of surface reactions [57], the authors derived separate equations to calculate the components on the right-hand side of Equation (8.8). Based on laboratory examination results, the authors believed that when the steel was oxidized in dilute O2-N2 atmospheres, the oxidation rate followed a linear kinetics law until the scale thickness was 400-500 microns. Thereafter, the oxidation kinetics gradually changed from linear to parabolic. 

The contributions of H2O and CO2 components to the overall oxidation kinetics revealed were also supported by previous studies on iron and steel oxidation in pure CO2 [33,65-69], pure H2O [33,69], CO2-CO mixtures [65, 67-68,70-73] and H2O-H2 [74] mixtures. It had been observed that iron and steel oxidation in pure CO2 and CO2/CO initially followed the linear kinetics law as the oxidation kinetics were controlled by surface reaction or the rate of gas phase dissociation. At the later stage, the reaction followed the parabolic law as diffusion of iron through the wustite layer became the controlling step. Similar patterns of oxidation kinetics were also observed for oxidation in H2O [33] and H2O-H2 gas mixtures [74] but the rates were generally... 

After the initial linear period, the oxidation kinetics became parabolic if the scale remains adherent. The oxidation rate became independent of the oxygen concentration and concentrations of other gas species at the parabolic oxidation stage. 

In principle, there is a critical scale thickness at which the oxidation kinetics change from linear to parabolic. Such a critical scale thickness has not been determined. 

The oxidation of aluminium at room temperature is reported to conform to an inverse logarithmic equation for growth periods up to 5 years duration. At elevated temperatures, oxidation studies over shorter periods illustrate conformity to parabolic, linear and logarithmic relationships according to time and temperature. These kinetic variations are attributed to different mechanisms of film formation . ... 

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