Oxidation kinetics parabolic rate equation

Figure 2 shows the relation between the square of the weight gain and the oxidation time for specimens oxidized in air at temperamre between I200 C and ISOO-C. For all temperatures, oxidation kinetics were of the parabolic type. Thus, the oxidation behaviour is governed by the parabolic rate equation ... 

Figure 9. Schematic of the behavior of the parabolic rate constant as a function of oxygen pressure for the oxidation of copper. The rate increases /I/n = I/S, see Equation 20] until the formation of CuO occurs. At that point the chemical potentials of oxygen are fixed both at the Cu-Cu O and at the CugO—CuO interface, and the kinetics become independent of external oxygen pressure (13).

In both of these equations, x is the film thickness, t is the time of the oxidation, and k and are experimentally determined constants. The constant fep is called the parabolic rate constant. A linear rate is usually found when the film is porous or cracked. The parabolic equation is found when the film forms a coherent, impenetrable layer. As the rate of film growth, dx/dt, diminishes with time for the parabolic rate law, this equation is associated with protective kinetics. The parabolic rate law arises when the reaction is controlled by diffusion. The species with the lowest diffusion coefficient plays the most important role in this case. 

In principle, the rate equations for surface reaction kinetics are linear and describe a linearly time-dependent growth of the corrosion layer. However, during this growth the oxygen activity on the surface increases and gradually approaches the value for equilibrium of gas phase and oxide surface. Because of the dependence on ao with a negative exponent, the rate gradually decreases, and several authors have misinterpreted this kinetics as parabolic kinetics (see Sect. G.2.3.2). 

The reaction rate varies with time according to a parabolic law [191], The kinetics are determined by the diffusion of oxygen through the Si02 layer. The temperature dependence of oxidation follows the Arrhenius equation. 

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. 

Like the oxidation of hydrocarbons, the autocatalytic oxidation of polymers is induced by radicals produced by the decomposition of the hydroperoxyl groups. The rate constants of POOH decomposition can be determined from the induction period of polymer-inhibited oxidation, as well as from the kinetics of polymer autoxidation and oxygen uptake. The initial period of polymer oxidation obeys the parabolic equation [12]... 

Equation (65) demonstrates that decreasing rates are to be expected in the transition from linear to parabolic kinetics. Therefore, the kinetics of oxidation, sulfidation, and so on cannot be exactly linear, even in the start of reaction, but the rate must decrease. This decreasing linear rate often has been misinterpreted, for example, in... 

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