How fast a surface oxide layer is formed on bulk chromium?

If a chromium piece is cut in half, how long would it take for an oxide layer to form on the new surface (room temperature)? i.e. is it a matter of seconds/minutes/hours/days/weeks?


The closest study I found is by [G. Thurner and P.H. Holloway (1992)](http://przyrbwn.icm.edu.pl/APP/PDF/81/a081z2p12.pdf), but I can't really tell from their results what would happen at ambient conditions:
[![enter image description here](https://i.stack.imgur.com/TKbwk.png)](https://i.stack.imgur.com/TKbwk.png)


(I [found](http://pubs.rsc.org/en/content/articlepdf/1981/F1/F19817702621) [several](http://jes.ecsdl.org/content/104/6/334.full.pdf) [publications](http://lappd.uchicago.edu/materials/chromium_oxide.pdf) about high temperature oxidation, but none for surface oxidation at ambient conditions)




First, a definition of the Langmuir unit, used in the graph in the question, which is not often run across except by vacuum (particularly UHV) folks. One Langmuir is the exposure of a surface to a gas pressure of $10^{-6}$ Torr for one second. It roughly corresponds to one monolayer equivalent of gas hitting the surface. At normal atmospheric pressure, with 20% oxygen in air, 1 Langmuir of oxygen hits the surface in about 6.6 nanoseconds (yes, nanoseconds).


Another paper that could be of interest is [C. Palacio et al.](http://www.sciencedirect.com/science/article/pii/0039602887900872), "AES, XPS and EELS study of the initial oxidation of polycrystalline chromium", Surface Science 182(1-2) 41-55 (1987). They see chemisorption of oxygen in the first 1-2 L, nucleation and lateral growth of oxide islands up to 10 L, and continued growth of an oxide film above 10 L.


Overall, there is not a huge difference between the two papers. By ~10 L of exposure to oxygen there is a continuous oxide on the surface at 300K. Following from the numbers in the first paragraph, that occurs in less than 100 nanoseconds. Basically, it oxidizes as it is cut.


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In learning about different steels, I've picked up some information that can hopefully help you. 


I have been told by metal vendors and fabricators that if you have some pure chromium, it will passivate (form a chromium oxide layer) in ambient conditions. This layer is only a few molecules thick so I would hazard a guess that within a day you should achieve >90% thickness of this chromium oxide layer.


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