11931Aluminum and Its Alloys

The corrosion resistance of Al is reduced on alloying, and therefore pure Al has a higher corrosion resistance than its alloy counterparts. This decrease in corrosion resistance is offset by an increase in the mechanical properties. Al and its alloys gain their corrosion resistance from the natural formation of a protective oxide film (Al2O3). Corrosion resistance of the alloys is related to the thickness of this film, and recognition of this has led to the process of anodizing, during which the surface film is thickened to produce a hard, compact, and tightly adherent layer. Al and Al alloys are amphoteric in nature and are therefore susceptible to corrosion attack in both strong acids and strong alkalis. An exception to this is where the acids are strongly oxidizing, that is, nitric acid above 82%.

Typically Al alloys are divided into two groups, namely cast and wrought alloys, the latter group having two categories: those that can be hardened by heat treatment and nonhardenable types, which may only be strengthened by cold working. A compromise in mechanical properties and corrosion resistance can be gained by cladding a high-strength Al alloy with pure Al, this is known as Alclad. Table 11.15 provides a summary of the types and uses of common Al alloys.

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