Oxide scale migration

Equation (3.32) can be used to describe the flux of cations, anions, or electrons through the oxide layer. Due to their different mobilities, different species would tend to move at different rates, however, this would set up electric fields tending to oppose this independence. In fact, the three species migrate at rates that are defined by the necessity of maintaining electroneutrality throughout the scale, i.e, such that there is no net charge across the oxide scale. This condition is usually achieved due to the very high mobility of electrons or electronic defects. 

The mechanical properties of the oxide film, in particular, the formabiUty and state of stress, determine the occurrence of cracks. If the formability of the film is poor, the contact between metal and oxide is lost as long as ion migration of the metal is from the inside to the outside. Oxygen migration from the outside to the metal-oxide interface leads to formation of a new oxide scale, which propagates into the metal. 

Improvement in oxidation resistance of iron by alloying with aluminum or chromium probably results from a marked enrichment of the innermost oxide scale with respect to aluminum or chromium. The middle oxide scales are known, from chemical analysis, to be so enriched, and electron-microprobe analyses confirm marked enrichment of chromium in the oxide adjacent to the metal phase in the case of chromium-iron alloys [52]. These inner oxides resist ion and electron migration better than does FeO. For chromium-iron alloys, the enriched oxide scale is accompanied by depletion of chromium in the alloy surface immediately below the scale. This situation accounts for occasional rusting and otherwise poor corrosion resistance of hot-rolled stainless steels that have not been adequately pickled following high-temperature oxidation. 

As shown in the previous section, stress-free growth of an oxide scale on a metallic substrate can occur only if there is no obstacle to the free migration... 

This condition of local equilibrium implies that point defects associated with mass transport within an oxide scale must be created and/or annihilated at gas/scale and/or scale/substrate interfaces. Therefore, the interface reactions must be expressed to account for this required interface action of point defect creation/annihilation. But this interface action depends also on the structure of the interface. A description of the interface structure is thus needed before considering how interface action and interface structure interact to simultaneously achieve, or fail to achieve, the relative displacement of the metal and/or oxide lattices and the migration of the scale/substrate interface. 

The possibility of application of the NEMCA effect in conventional flow reactors and of its extension to oxide catalysts may be of great importance in the future, though both the nature of the migrating, spillover species and their effect on the molecular-scale mechanism require further studies B. Grzybowska-SwierkoszandJ. Haber, Annual Reports on Chemistry, 1994)... 

P.R.171 is used in plastics and in paints. Its lightfastness in PVC equals step 7 to step 8 on the Blue Scale, depending on the exact composition of the tested system, the pigment concentration, and the Ti02 content. Incorporated in plasticized PVC, P.O.171 is migration resistant and heat stable up to 180°C. It is used in conjunction with organic yellow pigments, frequently also with iron oxides, to produce shades of brown. Shades of bordeaux are accessible in deep transparent colorations. 

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