# Why does iron oxide form hydrous iron oxide while chromium oxide does not?

Both Cr2O3 and Fe2O3 share a similar structure, and both can form some sort of passivating layer, but only one of them hydrates, thus turning into rust and providing a pathway for more water and O2 to further oxidize the iron. I believe the same applies to aluminum and nickel as well. iron seems to be the only one to do this, but I cannot figure out why.

• I object. Iron oxide doesn't hydrate either. Whenever you end up with hydroxide, you've never had the Fe2O3 in the first place. Mar 5 at 6:11
• Iron iii oxide doesnt hydrate? I thought that adding H2O molecules was hydration, and I definetly thought that Fe2O3.H2O was a thing, though I may be mistaken Mar 5 at 13:48
• Fe2O3.H2O is a thing all right, but it is not produced by putting together Fe2O3 and H2O. Ditto for Cr2O3.H2O. Mar 5 at 16:53
• hmm, okay, thankyou. It appears you are correct. Is there anyway you can tell me how Cr2O3, and well as Al2O3 are formed then? I was told that Cr2O3 "does not readily hydrate" like Fe2O3... do they form via the same mechanism as Fe2O3 and then loose the H2O? Mar 5 at 18:18
• No, unlike the iron rust, they don't form hydrated. That's where the difference originates. Mar 5 at 19:04

TL;DR: Guignet's green is a green pigment with composition $$\ce{Cr2O3.2H2O}$$.

Guignet's green or $$\ce{Cr2O3.2H2O}$$ is the hydrous form of the oxide which is used for similar purpose as the anhydrous form. This pigment becomes anhydrous when heated at 200 °C, hence not suitable for glass and ceramic industries.
$$\ce{2K2Cr2O7 + 4H2B2O4 -> 2Cr2B2O6 + 2K2B2O4 + 3O2 + 4H2O}$$ $$\ce{Cr2B2O6 + 3H2O -> Cr2O3.2H2O + H2B2O4}$$