Posted by David Sherman on Wednesday, September 12, 2007 at 02:04:35 :
In Reply to: The chemistry of heated rust? posted by chriscase on Wednesday, September 12, 2007 at 01:17:49 :
I don't know how many Fes and Os there are, but when heated sufficiently, ordinary red rust which is an iron hydroxide turns into black iron oxide. The black oxide can be picked up with a magnet, unlike regular rust. Presumably it's a different size too, but I suspect that the reason heating a rusty nut loosens it is the thermal expansion and contraction cracking the rust bond, rather than any chemical change from red to black oxide.
I don't know if eight times is right, but rust is definitely a lot bigger than the original iron. Where water and salt can get in through tiny cracks in concrete and cause the re-bar to rust, the expanding rust soon cracks the concrete off of the iron. This is obvious on many old bridges and is the reason why re-bar in new bridges is usually painted.
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