Why is Stainless Steel Stainless?
You may have wondered why stainless steel doesn’t rust compared to some other steel. How does it protect itself from rust? Generally, metals usually corrode when they are exposed to water or oxygen. Not stainless steel. This has made it the preferred type of steel for items such as bank vaults, kitchen sinks and utensils.
Here’s why it doesn’t rust.
In the year leading to the first world war, metallurgist Harry Brearley made an accidental discovery while he was trying to find ways to reduce degree of rust in rifle barrels. He found that an alloy of chromium and low carbon steel develops resistance to stain.
Stainless steel is an alloy containing the following metals in varying proportions: iron, silicon, manganese, carbon and most importantly, chromium which takes up 12% of the composition of stainless steel. Nickel, niobium, titanium and molybdenum have been added in modern times.
Why is chromium important? It easily reacts with oxygen to form a passive thin film of metal oxides and hydroxides that are not visible. Interestingly, the film is so thin that the wavelength of visible light is considered to be thicker. They help protect the steel from further corrosion by preventing more oxygen from reacting with the steel.
There is similarity between the sizes of the chromium atoms and that of its oxides. This makes them neatly packed together and ensures stability. For other metals, the oxide and hydroxide films created continue to grow thicker as the metal is exposed to more oxygen. Iron atoms, for example, are much smaller than that of iron oxides. This makes them loosely packed and rust more readily.
Stainless steel, however, needs a sufficient supply of oxygen to retain its stainless feature. Low oxygen areas make it difficult for the passive film to repair itself when the metal is scratched or cut.
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Images - Gordon Williams Wikipedia