Recently a friend with a rusty car asked me ‘why don’t my cooking utensils corrode like this?’ Well, I explained. they are made of stainless steel.
Stainless steel is an alloy containing other things besides iron. For example, stainless steel contains at least ten percent chromium, together with various amounts of elements like manganese, silicon, carbon, and nickel. These elements form very stable, hard, impervious oxides when exposed to water and air.
These oxides stay on the surface as a very thin film which acts as a rust-barrier. Oxygen and water can’t contact the metal underneath, so rust can’t develop. Since the oxide film is only a few atoms thick it’s invisible, and the steel still looks shiny.
The big disadvantage of pure iron is that it rusts rather easily, forming flaky brown iron oxide when exposed to moist air. The iron oxide grows thicker the longer it’s exposed to water and air because it’s porous. It soon flakes off, exposing more metal to be attacked by oxygen. So the iron continues to rust.
Your aluminum utensils resist corrosion for a similar reason. Aluminum is a rather reactive metal. In moist air it quickly forms
aluminum oxide on the surface. But unlike iron oxide this oxide forms a tough continuous skin over the surface of the metal, which protects it from further reaction with oxygen.
So next time you clean that saucepan, be thankful for that tough oxide layer.