Aluminum oxide defect reactions

The metallurgical characteristics of the aluminum oxide layer also depend on its physical metallurgy, such as defects and metallurgical structure included in the oxide layer. For instance, when intermetallic compound particles as secondary phases are exposed on the surface, a discontinuous oxide film with various defects is often produced at the metal-particle interface. This discontinuous oxide film is weakly or non-protective chemically and physically. Because corrosion is a chemical and electrochemical reaction on the surface, corrosion behavior is readily influenced by surface morphology. The aluminum surface is usually adsorbed or contaminated by water, gases and many kinds of micron-sized substances. Microscopic heterogeneous structures such as vacancies, steps, kinks, and dislocations, and macroscopic heterogeneous structures such as scratches, pits and other superficial blemishes influence the corrosion behavior of aluminum and its alloys to different extents. 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

The use of TiC vapor at 200-500°C seems to be more efficient [54,89- 92]. When dealumination pretreatment is performed on the precursor, usually by acid leaching, structural defects are created in which Ti incorporation presumably takes place. When no pretreatment is made, the reaction with TiCl4 does not lead to reduction of the aluminum content [90] if any substitution occurs, extra-framework aliuninum oxide species are presiunably formed. Evidence of partial incorporation of titaniiun come from the presence of the typical 960 cm IR absorption. 

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