Porous oxides growth kinetics

Data on anion incorporation into a growing porous oxide were obtained Fukuda and Fukushima.165,166 Their study was the first to demonstrate a correlation between the kinetics of accumulation of oxalate165 or sulfate166 anions and the change of porous oxide growth stages. The results of galvanostatic and potentiostatic... 

When the results for oxide growth and anion incorporation172,160 are compared with the kinetics of space charge accumulation in barrier and porous alumina films [see Section IV(1)], it can be concluded that anion incorporation modifies the electrostatics of the external oxide interface, thus influencing oxide dissolution and pore formation.172... 

Fig. 1.90 Kinetic interpertation of paralinear oxidation. Curves a and b correspond to the growth of the inner compact layer and the outer porous layer, respectively curve c represents the total weight and is the algebraic sum of curves a and b. Note that as oxidation proceeds, y tends to a limiting value y, (curve a) and the overall rate of oxidation tends to a constant...

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. 

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