Originally Posted by
Duragrouch
(above) Aluminum low cog, makes complete sense. More difficult to plate I think, although nickel or chrome plating would be of more benefit in wear than on steel, though not as critical for corrosion as with steel, if using a proper aluminum alloy. It may be hard-anodized, aluminum oxide is hard.
Seems counterintuitive to me. Alloy for larger cogs and steel for small is more intuitive.