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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.

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    $\begingroup$ I object. Iron oxide doesn't hydrate either. Whenever you end up with hydroxide, you've never had the Fe2O3 in the first place. $\endgroup$ Mar 5 at 6:11
  • $\begingroup$ 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 $\endgroup$
    – Mr.Black
    Mar 5 at 13:48
  • $\begingroup$ Fe2O3.H2O is a thing all right, but it is not produced by putting together Fe2O3 and H2O. Ditto for Cr2O3.H2O. $\endgroup$ Mar 5 at 16:53
  • $\begingroup$ 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? $\endgroup$
    – Mr.Black
    Mar 5 at 18:18
  • $\begingroup$ No, unlike the iron rust, they don't form hydrated. That's where the difference originates. $\endgroup$ Mar 5 at 19:04
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TL;DR: Guignet's green is a green pigment with composition $\ce{Cr2O3.2H2O}$.

Long answer

Chromium oxide is largely used as a green pigment in various industrial and commercial purpose as it is the only green pigment which can withstand both acidic as well as alkaline environment. It is also used as permanent paints for cements, distempers and paints. It can also withstand high temperatures, hence used for coloring glasses, enamels and pottery glazes.

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.

It is prepared by mixing 3 parts of boric acid and 1 parts of potassium dichromate which is heated in reverberatory furnace. It becomes a dull red mass initially but swells up releasing oxygen to give a nice green color. It is then boiled with water whereby chromium borate decomposes to hydrated chromium oxide and boric acid. The precipitate is dried and separated while the mixture is washed with mother liquor to evaporate the boric acid.

$$\ce{2K2Cr2O7 + 4H2B2O4 -> 2Cr2B2O6 + 2K2B2O4 + 3O2 + 4H2O}$$ $$\ce{Cr2B2O6 + 3H2O -> Cr2O3.2H2O + H2B2O4}$$

Reference

  1. S. P. Das Gupta & H. N. Bose (1948) A Note on Preparation of Chromic Oxide Green, Transactions of the Indian Ceramic Society, 7:2, 60-63, DOI: 10.1080/0371750X.1948.10877850
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