I believe that many are confusing Aluminum Oxide (a chemical compound) and Aluminum Corrosion (exposure to atmospherical impacts). Aluminum Oxide is either a manmade compound - or a natural occurring phenomena over many millennia.
I use Amerimax .014 Roofing material for most of my Gas Checks. Despite many, many, many thousands of rounds I have not seen any affects from the Aluminum Checks.
ALUMINUM OXIDE
(or Aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al2O3 is significant in its use to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.
CORROSION BEHAVIOR OF ALUMINUM IN ATMOSPHERE
From the extensive weight-loss studies conducted around the world on aluminum corrosion rates of 0.03 to 4 µm/year have been reported. The corrosivity of a location would depend upon the distance the airborne salt travels, the direction and velocity of the wind, the frequency of the prevailing wind and topography of the coast and the expanse of seawater over which the wind has come. The salt content drops rapidly with increasing distance from the sea.
Loss of tensile strength can occur due to atmospheric corrosion. In rural area's atmosphere, the corrosion rate averages 0.03 µm/year (0.001 mils/year). In industrial locations, the corrosion rates average 0.8–0.28 µm/year (0.03–0.11 mils/year). In certain polluted environments, a higher corrosion rate of 13 µm year (0.52 mils/year) was obtained. Table 10.10 shows the corrosion rates of aluminum and selected aluminum alloys in different environments.