To activate aluminum by the usual alloying elements (Zn, In, Sn, or Hg), the following three- step mechanism is proposed: (Step 1)Al(M)→Al3++Mn+ The dissolution of the anode caused by the galvanic couple oxidizes aluminum as well as the alloying elements in solid solution with aluminum. Step 1 produces cations in the electrolyte. (Step 2)Al+Mn+→Al3++M Because alloying elements are cathodic to aluminum, their cations will plate out on the aluminum surface (Step 2). In Step 3, which occurs simultaneously with Step 2, the aluminum oxide film is locally separated. This shifts the anode potential toward the potential of bare aluminum, which is more active (−2.3 V SCE at pH 8).
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Reboul et al. (1984) studied this question.
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