Stainless steel:"
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 chromium.
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 air.
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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