If well polished in most environments, generally none required.
Aluminum self-protects: "Aluminum’s unique self-protective properties come from the formation of a thin, highly adherent aluminum oxide (Al2O3) layer. Upon exposure to oxygen, aluminum reacts to form aluminum oxide, creating a dense and impermeable barrier that prevents further oxidation of the underlying aluminum. This oxide layer is self-repairing; if scratched or damaged, the exposed aluminum quickly re-oxidizes, restoring the protective layer."
However, in salty air: "Salt air, containing chloride ions, can significantly affect aluminum surfaces, leading to corrosion. The chloride ions disrupt the protective oxide layer on aluminum, causing localized corrosion such as pitting. This process is electrochemical in nature, involving anodic metal dissolution and cathodic oxygen reduction reactions. Salt exposure can induce several types of corrosion on aluminum, including pitting corrosion, crevice corrosion, and galvanic corrosion. To mitigate salt air corrosion, it is essential to apply protective coatings and ensure proper maintenance of aluminum components exposed to salty air."
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