Surface free energies have been evaluated from Young's moduli and atomic spacing for typical aluminum alloys containing 5% zinc with 0.25% tin, with and without heat treatment, and compared with indium-activated aluminum-alloy anodes. It has been shown that the higher the anode efficiency of the alloy, the lower is its surface free energy, leading to a film with reduced thickness and reduced metal/oxide bond strength which, in turn, results in better cathodic protection properties of the alloy.