How does rust form?

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Just three ingredients: iron, water and air. “Rust is the result of the reaction between iron and oxygen”, says chemist Jorge Masini, from USP. That is: when these two elements come together, they tend to unite to form a third: iron oxide – or “rust”, in popular parlance. But marriage doesn’t happen like that, out of nowhere. He needs a helping hand from the water. Why? Simple: iron can only bond with oxygen in the air if it can release electrons. When these particles leave the metal, they make room for oxygen to enter. Only the electrons need a force for that. That’s where the water comes in. The liquid helps the electrons to come out of the metal, as if pulling them out. The path is then free for the iron atoms to stick to those of oxygen and rust is born. Of course, you don’t even have to throw water on the iron to create corrosion. The very air of the atmosphere, after all, is already loaded with moisture.

Thus, rust ends up forming on virtually any piece – so much so that it is rare to find iron in nature that is not at least a little bit corroded. But what about stainless steel objects, which do not rust? Steel, after all, is nothing more than iron without some of its natural impurities, such as sulfur and phosphorus. Removing these elements leaves the iron stronger, but not more resistant to rust. The secret of stainless steel, then, lies in a chemical element that is added to iron: chromium. What he does is form a film around the piece, which prevents the metal from coming into direct contact with water and air. As there is no rust without the presence of both, iron is saved.