Internal Reactions in Nonmetallic Systems

In this particular case, there is no transport of component B towards the surface. BO is homogeneously precipitated in the region F, and the BO fraction corresponds to the concentration of B in the initially homogeneous alloy. Although the BO fraction is spatially constant in this case, the size of the BO particles is not. The increase in supersaturation becomes slower as the reaction front F advances. Thus, the number of precipitating particles becomes smaller with increasing time and, consequently, their volumes become larger since the local product of number times volume remains constant. 

Even if the transport product cB-DB of component B in the alloy (A,B) cannot be neglected in comparison to that of oxygen, internal oxidation may still occur. The amount of BO precipitates will then be enhanced toward the alloy surface. In this way, a transition from internal to external oxidation becomes more and more likely. This transition (/ ., the formation of a dense external BO layer) is expected to occur if 

In many investigations, F (= width of the oxidation zone) has been measured and the results have been compared with theoretical reaction rates [E. Verfurth, R. A. Rapp (1964)]. In technical applications, the internal oxidation zone sometimes forms below an external oxide scale. Analytical solutions for these cases are also available [C. Wagner (1968)]. 

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