Or ionic? :) On a boat, Aluminum "corrodes" and forms an oxide layer (also known as sapphire), which then protects it from further corrosion. But, said layer i

Or ionic? :) On a boat, Aluminum "corrodes" and forms an oxide layer (also known as sapphire), which then protects it from further corrosion. But, said layer is not electrically conductive (or in this case does not allow Al ions to flow into the electrolyte), so you need something to prevent the protective layer from forming.