Hi, I need to know why stainless steel is rust proof please tell me its for my science project. :)
Stainless steel is characterized by the presence of chromium. Chromium oxidizes like aluminium does, by passivation, that is, it forms a though oxide layer that sticks to the metal underneath and protects it. On the other hand, iron oxidizes by turning into rust, which is a brittle material that flakes off, exposing the metal under for more oxidation. Now, what happens when you have a metal that is made of both iron and chromium? Any iron on the surface will rust and flake off (at the atomic level, so do not expect to see much rusting) until the surface is a pure chromium layer (again, we are talking about a layer that is a few atoms thick) and that chromium will oxidize but stay put. Then rust then has no way of getting deeper, and the metal will remain clean.
Stainless steel is an alloy of iron and other metals, notably Chromium and vanadium. You do not see it with the naked eye but at the surface of any stainless steel (SS) there are iron atoms and chromium atoms that are exposed. However, the chromium reacts preferentially to form an oxide. In essence, think of chromium sacrificing itself for iron so that iron will not rust. What a nice guy huh! This is called passivation just like most responders said. However, more than that the chromium oxide forms a tight protective layer that forms a physical barrier preventing Oxygen or any oxidant to reach and attack the iron underneath it. Also, the chromium oxide formed is not FLAKY and porous, so it does not mar much of the surface as an iron oxide (rust) would. For science project, you may not make it too technical but you can make a reference or make an ANALOGY of rust prevention to high school or social situations as being there for your friend or classmate But in real life, when the odds are too great, that protection afforded by chromium may not be enough. Because eventually all steel exposed to very oxidizing atmospheres will rust. Just a little break in the surface is enough to start the process of rusting.
Why doesn't stainless steel rust? Nancy Avery, New London, Conn. Metallurgical engineer Michael L. Free of the University of Utah offers this explanation: Stainless steel remains stainless, or does not rust, because of the interaction between its alloying elements and the environment. Stainless steel contains iron, chromium, manganese, silicon, carbon and, in many cases, significant amounts of nickel and molybdenum. These elements react with oxygen from water and air to form a very thin, stable film that consists of such corrosion products as metal oxides and hydroxides.
