Aluminum oxide amphoteric nature

Activated alumina is a porous high-area form of aluminum oxide, prepared either directly from bauxite (AI2O3 3H2O) or from the monohydrate by dehydration and recrystallization at elevated temperature. The surface is more strongly polar than that of silica gel and has both acidic and basic character, reflecting the amphoteric nature of the metal. 

The presence of a protective oxide layer on aluminum is the main reason why aluminum alloys are so broadly used with success in indoor and outdoor environments provided they fall within the passivation potential/pH boundaries shown in Fig. 4.14. The aluminum susceptibility to corrode in both acidic and basic environments is referred to as an amphoteric behavior. While the aluminum oxide will form naturally on aluminum, it is common practice to produce this oxide in a controlled process called anodization. As described in Chap. 5, the quahty and properties of the protective oxide can thus be greatly enhanced, providing various finishes for a multitude of applications. 

The concept that by carrying out a surface analysis on a model specimen one can obtain direct access to the interphase chemistry is a potentially very exciting and rewarding idea. The interaction of PMMA with various metals (as exemplified hyO Fig. 10.14 above) has been studied in some detail using high resolution XPS (Leadley and Watts 1997). In this work the manner in which PMMA formed specific interactions with oxidized metal surfaces was studied by the nature of the fine structure in the XPS spectrum. In this manner, it was possible to show that the polymer formed hydrogen bonds with oxidised silicon (an acidic substrate), a bidentate structure with oxidised aluminum (an amphoteric substrate) and would undergo acyl nucleophilic attack with oxidised nickel (a basic substrate). 

Corrosion of A1 is an electrochemical process that involves the dissolution of metal atoms, only taking place once the oxide him has been dissolved or damaged. A1 is amphoteric in nature, its oxide film being stable in neutral conditions, but soluble in acidic and alkaline environments. The thermodynamic stability of aluminum oxide films is expressed by the potenhal versus pH diagram seen in Figure 16.3 [35]. 

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