A recent article in Compressed Air Best Practices magazine highlights the important difference between iron oxide and aluminum oxide.
Iron oxide, you may know it as rust, is often found in black iron pipe and leads to inefficiencies in compressed air systems as the rust builds up and slows down the flow of air, causes pressure drop, and leaks.
Aluminum oxide, on the other hand, protects the surface from further corrosion.
"Oxidization of aluminum forms a protective coating chemically identical to a sapphire. It has a transparent to milky white color, is impervious to air and protects the surface from further corrosion. Once a microscopically thin layer has formed, the corrosion stops. This nearly invisible barrier forms so quickly that aluminum will only oxidize on the surface, providing immediate protection from continued corrosion."
Todd Zarins, Technical & Product Manager, Applied System Technologies™ — Compressed Air Best Practices magazine