Oxidation mechanism of metallic chromium at room temperature

[Display omitted] •Metallic Cr exposed to air does not oxide through the formation of an oxide layer.•Cr0, Cr3+ & Cr6+ coexist throughout the entire XPS probing-depth upon air exposure.•A surface region of pure metallic Cr remains after exposure to air for two days.•Air-exposed Cr oxidizes by forming surface regions of Cr2O3, CrO3 and metallic Cr.•The Block-Approach allows for discriminating metal from oxide signals in Cr 2p. A quantitative study of the surface composition of clean metallic and oxidized chromium exposed to atmospheric conditions was carried out employing X-ray photoelectron spectroscopy (XPS). Advanced tools were used for the fitting process and surface composition analysis. We found that, in contrast with most metals, the oxidation does not come about through the formation of an oxide layer. State of the art analysis showed that, even at long exposures, a fraction of the chromium surface retains its metallic nature even at the top layers. Depth profile of XPS spectra shows no shape modulation from angle to angle which strongly suggest both Cr3+ and Cr6+ species coexist with the metallic species throughout the probing depth of XPS. The oxidation mechanism of metallic chromium exposed to ambient is through the partial oxidation of surface regions.