I’ve seen a number of videos recently of people restoring very rusted objects, like toys and an anceint historical knife, where the rust seems to have eaten most of the metal. However, after restoration, the object seems good as new and don’t seem to have lost any significant amount of mass even though you’d expect there to be nothing left. How is this even possible?
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Rust is iron oxide; that is, iron atoms combine with oxygen atoms from the air. Furthermore, the iron oxide molecules can bind with several water molecules to form hydrated iron oxide. Also, rust is usually flaky and porous; a lot of its volume is just empty space–but microscopic empty space. So most of the volume you see didn’t come from the iron or steel object itself; it came from the atmosphere.
One point to note, in many cases, rust can also become an external protective barrier that stops further oxidation and keeps the underlying steel protected. ‘Weathing Steel’ is a type engineered specifically to quickly form a solid, protective rust coating. Useful in external structures and gives quite an attractive red finish.
There are several types of rust actually, and they don’t al penetrante the metal in the same way. Therefore, some steel types just seem to disintegrate while others remain mostly intact. It all has to do with the alloy and surface treatment or annealing it underwent.
Lots of those restoration videos are faked/artfully edited. Depends on the video. The real art is when people make something intentionally cruddy/mud caked or rusted looking so they can make a fake restoration video.
Mass is definitely being lost, but the way rust flakes off it looks like way more is being lost than actually is. I’ve seen pipes that I’d swear were completely compromised and after the rust is cleaned it turns out on 3% of the thickness is actually lost.