Aluminum has an equilibrium potential of -1.90 V vs. SCE and therefore thermodynamically it is a very reactive metal. However, in most natural environments aluminum has been found to be stable due to the formation of a thin protective oxide.
This film has a high electrical resistance and thus renders pure aluminum useless as a sacrificial anode. Even when galvanically coupled to a more active metal in a chloride containing environment aluminum corrodes by pitting rather than uniform dissolution. But in the light of aluminum other favorable properties, including a large electrochemical equivalent, relatively low cost and low density, attempts have been made to modify this oxide film.
To disrupt the physical integrity of the protective film several aluminum alloys have been developed. Alloying elements which were found to induce activation included Hg, In, Sn, Ga, Bi, Zn, Cd, Mg and Ba. Commercially successful aluminum anodes mainly contain zinc (0.5-4%) with either indium (0.01-0.05%) or mercury (0.030-0.050%). The current capacity of such alloys is in the region of 2500 A hr/kg and anode efficiencies can be greater than 90%.
The role played by the zinc in the enhancement of aluminum activation is thought to be due to a modification of the passive film on aluminum. For mercury, it is believed that Hg2+ ions dissolved near the anode interface are plated scale (as Hg) at the weak points. The amalgamated areas formed may either allow direct corrosion through the amalgam or enhance the adsorption of Cl- ions and cause film breakdown. Similarly, indium ions are also thought to plate at film defects and thus prevent repassivation by enhancing chloride adsorption. Further since mercury and indium are poor cathodes for hydrogen evolution, this results in higher anode efficiencies.
http://www.corrosion-doctors.org/CP/an-aluminum.htm
