# Which form of chromium oxide is formed naturally in the atmosphere?

I'm trying to figure out which form of chromium oxide forms naturally in our atmosphere, i.e., if you take a piece of elemental $\ce{Cr}$ from vacuum to a normal, room temperature environment with air, does it form $\ce{CrO}$ on the surface? Or $\ce{Cr2O3}$? Etc.

This site says:

Chromium (III) oxide, $\ce{Cr2O3}$ is the main oxide of chromium.

So it seems like it might be that one, but "main" is pretty vague. Does anyone have a more definitive answer, with a source?

thank you!

An investigation of the phase diagram reveals that the only stable chrome oxide near room temperature is $\ce{Cr2O3}$. $\ce{Cr3O4}$ appears, but only near 2000K. There is no stable chrome analogue to $\ce{FeO}$ on the phase diagram.
Thus, the one stable oxide that will form under normal atmospheric conditions is $\ce{Cr2O3}$. However, note that it is easy to find mixed metal-oxygen compounds with high mutual solubility.
If you are interested in the thermodynamics of the $\ce{Cr-O}$ system, you could start with M. Kowalski and P.J. Spencer, Calphad 19(3) 229-243 (1995), which covers the $\ce{Cr-O, Fe-O}$ and $\ce{Ni-O}$ systems
Drawing a parallel to the formation of rust (oxides of iron), in which the following reactions: $$\ce{Fe<=>Fe^2+ +2e-}$$ $$\ce{4Fe^2+ +O2<=>4Fe^3+ +2O^2-}$$ form both of the oxidation states of iron, reacting to eventually form both $\ce{Fe2O3}$ and $\ce{FeO}$, the two most common forms of rust. The more common of the two forms is $\ce{Fe2O3}$, which is likely because $\ce{Fe^3+}$ has an electron configuration of $[\ce{Ar}] 3d^5$, which is more stable than the electron configuration of $\ce{Fe^2+}$, which is $[\ce{Ar}] 3d^6$.
It is likely then that both $\ce{CrO}$ and $\ce{Cr2O3}$ form when $\ce{Cr}$ is exposed to the atmosphere, and also that $\ce{Cr^3+}$ has a more stable electron configuration than $\ce{Cr^2+}$, though this is not intuitive since neither half an empty or half-filled 3d orbital.