Rust
(→What is rust?) |
(→What is rust?) |
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==What is rust?== | ==What is rust?== | ||
Rust is a kind of corrosion. Corrosion is the decay or breakdown of a material in reaction to its environment. | Rust is a kind of corrosion. Corrosion is the decay or breakdown of a material in reaction to its environment. | ||
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| + | For rust to occur it needs an anode (a piece of metal that gives up electrons), an electrolyte (a liquid that helps transport electrons), and a cathode (a piece of metal that accepts electrons). | ||
Rust is an electro-chemical corrosion that occurs in the presence of iron, oxygen, and water. Oxygen and water are available in the open air, and iron is the principal ingredient in steel. | Rust is an electro-chemical corrosion that occurs in the presence of iron, oxygen, and water. Oxygen and water are available in the open air, and iron is the principal ingredient in steel. | ||
| − | + | Iron, with the chemical symbol of Fe, combines with oxygen, O, to form Fe<sub>2</sub>O<sub>3</sub>, the chemical formula for common reddish-brown rust. In its mineral form, this is known as hematite. | |
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| − | + | Rust is commonly referred to as ''iron oxide''. An oxide is a type of chemical compound that contains oxygen and other elements (like nitrous oxide, N<sub>2</sub>O. Oxides are typically formed when elements come into contact with air, through a process known as oxidation. Oxides are common, forming most of the earth's crust. Elements can have multiple oxides. For example, there are sixteen known iron oxides. Thus, it's more accurate to say that rust is '''an''' iron oxide. | |
| − | + | Other metals besides iron oxidize, but their oxidation is not referred to as rust. For example, when aluminum oxidates, it forms a protective, corrosion-resistant coating that looks a lot like un-oxidized aluminum. Zinc's oxide is similar, which is why it is used for galvanizing. However, only the oxidation of iron and steel is known as "rust". Why? Because iron oxide takes up more physical space than iron. It's bigger. So, when iron pulls oxygen atoms from the air, to make iron oxide, it "puffs out", exposing even more of the original steel to the elements. And, it has a distinct reddish-brown color, that is very different from steel: it's reddish-brown. | |
| − | + | There is another iron oxide also known as "rust". It's Fe<sub>3</sub>O<sub>4</sub>. This is also known as "black oxide", or, in mineral form, magnetite. This is the stuff that's left over after using certain products to "convert" rust. Black oxide takes up the same amount of space as iron, so it doesn't bulge it out, like reddish-brown rust, Fe<sub>2</sub>O<sub>3</sub>. Black oxide itself doesn't offer much protection against corrosion, so it's typically covered up (often with [[epoxy primer]]). Black oxide ''(also known as blueing?)'' is an example of a conversion coating, a metal coating where part of the surface is converted into the coating itself, by a chemical or electrochemical process. | |
| − | + | Rust happens more quickly with salt in the air, because salted air is a better electrolyte. | |
The notion that, after you "encapsulate" rust, it still "lives underneath", is generally correct. There is mobile oxygen in the metal, living beneath the surface, causing more rust. | The notion that, after you "encapsulate" rust, it still "lives underneath", is generally correct. There is mobile oxygen in the metal, living beneath the surface, causing more rust. | ||
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