Aluminum does not need anodizing for protection, it almost instantaneously oxidizes in air and that oxide layer requires no further treatment to resist most common chemicals.Sometimes it's just a clearcoat paint, really thin but protects the aluminium from oxidizing.
This is why you need to be very careful when cleaning aluminium car rims, you need to use a pH neutral cleaning fluid because the rims are anodised and alkaline/acid based cleaners will stain the aluminium and it can't be removed without destroying the finish. Natural aluminium is quite soft, scratches and dents easily, a fingernail can gouge it.FREQUENTLY ASKED QUESTIONS ABOUT ANODIZING
WHAT IS ANODIZING?
Anodizing is a process for finishing aluminum alloys that uses electrolytic oxidation of the aluminum surface to produce a protective oxide coating. The anodic coating consists of hydrated aluminum oxide and is considered resistant to corrosion and abrasion. Coatings are 0.1 to 1.0 mil thick and are essentially transparent, although they may be colored. Unlike most other finishes, anodizing preserves the natural luster, texture, and beauty of the metal itself. The anodized coating is hard, durable, will never peel, and, under normal conditions, will never wear through.
WHAT IS THE PURPOSE OF ANODIZING?
The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. There are some types of anodizing that produce a porous oxide layer that can be colored with organic dyes or metallic pigments giving the aluminum a decorative and protective finish. In short, the main purposes for anodizing are corrosion resistance, abrasion/wear resistance, and cosmetics.