For many years (about 80) it has been known
that adding chromium and nickel (the nickel is not
essential but the chromium is) to iron makes a
steel that does not rust (it is stainless). The
chromium in the steel forms a very thin layer of
oxide that prevents the iron from rusting.
A similar mechanism works with aluminum.
When aluminum is exposed to air and water, a thin,
invisible coating of oxide forms that prevents
further corrosion. Rusting is the process of iron
being oxidized in the presence of water to a
mixture of iron oxides and hydroxides.
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. Chromium plays a dominant role in
reacting with oxygen to form this corrosion
product film. In fact, all stainless
steels by definition contain at least 10 percent
The presence of the stable film prevents
additional corrosion by acting as a barrier that
limits oxygen and water access to the underlying
metal surface. Because the film forms so readily
and tightly, even only a few atomic layers reduce
the rate of corrosion to very low levels. The fact
that the film is much thinner than the wavelength
of light makes it difficult to see without the aid
of modern instruments. Thus, although the steel
is corroded on the atomic level, it appears
stainless. Common inexpensive steel, in
contrast, reacts with oxygen from water to form a
relatively unstable iron oxide/hydroxide film that
continues to grow with time and exposure to water
and air. As such, this film, otherwise known as
rust, achieves sufficient thickness to make it
easily observable soon after exposure to water and
In summary, stainless steel does not rust
because it is sufficiently reactive to protect
itself from further attack by forming a passive
corrosion product layer. (Other important
metals such as titanium and aluminum also rely on
passive film formation for their corrosion
resistance.) Because of its durability and
aesthetic appeal, stainless steel is used in a
wide variety of products, ranging from eating
utensils to bank vaults to kitchen sinks.
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