How is iron black, yet steel light grey?
In: Chemistry
Iron itself is light grey. Iron oxide (ie: rust) is a redish brown. Other colors are due to the iron being coated or painted, most likely to protect it against corrosion.
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Grey cast iron is almost black on the outside because of the slag (surface impurities) that cling when they are cast. When you machine it, it is a dark grey.
There is also white cast iron, where iron and carbon form iron carbides, which give is a whitish colour.
Steel can be dark grey to silvery, depending on what is alloyed into it. High Speed Steel is silvery when surface machined, but when it breaks (it is quite brittle) it is also dark grey.
Iron and typical iron alloys like steel and cast iron are silvery grey. Higher carbon alloys like cast iron tend to be lighter coloured than those without much carbon like wrought iron with steel being in the middle.
However, iron and its alloys react with oxygen to form rust. Iron gets bigger when it becomes rust, which causes the rust to flake off exposing more iron to oxygen so it can rust. Obviously, we don’t want this to happen.
We solve this problem by coating it with some sort of protection.
Stainless steel contains additives that oxidizes to form a thin, transparent layer of protection.
In other cases we coat the iron in a layer of paint or oil. This is why swords and fancy kitchen knives need to be oiled.
We also coat iron with other metals that are more corrosion resistant like zinc, chromium, or nickle.
When iron is hot, it it oxidizes more rapidly, but instead of rust it forms hematite which is bluish-black and doesn’t flake like rust does. This can serve as a protective layer against oxidation that causes rust. The hematite is somewhat fragile so it is normally protected with a layer of oil. This is known as “bluing”, particularly when done to firearms, and “seasoning” a pan is similar except that the oil is heated to smoke point which causes it to harden into a sort of plastic. Bluing and seasoning can get quite a bit more complicated.
So, iron and its alloys are all light grey with a bit of variation, but most can be covered by black hematite as part of how we protect them from rust.
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