Parabolic kinetics causes

The existence or nonexistence of a residual layer has been studied using surface chemistry techniques such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) and solution chemistry calculations. Nickel (1973) calculated the thickness of a residual layer on albite from the mass of dissolved alkalis and alkaline earths released during laboratory weathering. The surface area was also measured, and the thickness of the residual layer was found to range from 0.8 to 8 nm. These results suggested a very thin layer, which would not cause parabolic kinetics. 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

Solution Composition. Changing solution composition can also cause apparent or true parabolic dissolution kinetics through the influence of changing pH and C02 equilibria or through an effect on chemical affinity and reverse rate (Helgeson et al., 1984). 

Oxidation is the main cause of the rapid destruction of PP during its service and processing. The oxidation of polyolefins proceeds by the radical chain mechanism with degenerate branching on hydroperoxide [15], It is known that, at the initial stage of uninhibited oxidation of polyolefins, the oxygen uptake kinetics is described by the parabolic law ... 

At high temperatures, the water vapor in the air plays a very important role in the kinetics of degradation. The parabolic regime is preserved, but for identical conditions of temperature and oxygen pressure, we observe that the water vapor considerably increases the rate of oxidatioa There is molecular diffusion of H2O, which constitutes an independent oxidant and acceleration of the diffusion of oxygen because of the stractmal modifications of Si02 caused by H2O. 

Oxidation kinetics obeyed the parabolic law because the formation of the continuous SiOj-rich layer on the surface of the specimen caused the oxidation rate of the specimen to be mainly controlled by the diffusion rate of oxygen through the SiOj-rich layer (Kohfahl et al., 2007). The thickening rate, h, of the oxide layer was thus given as ... 

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