Chromia, Cr₂O₃, is the protective layer of a large family of metallic alloys denominated as chromia-formers. The chromia structure is particularly complex [1] and merits to be studied if one wants to try to improve its chemical durability.

In this work, oxidation of pure Cr is studied as a function of temperature (from 800 °C to 900 °C) and oxygen partial pressure p(O₂) (from 10^-14 to 10^-12 atm.) Chromia semiconducting properties are investigated by photoelectrochemistry at macroscopic, mesoscopic and microscopic scales, whereas its morphological and structural properties are studied by Transmission Electron Microscopy with orientation mapping tool (called ASTAR). By associating these two techniques -which gives respectively the type of semiconduction and the oxide growth direction- it becomes possible to identify the nature of the major point defects in chromia [2].

In every case, the morphology of the oxide scale is duplex (see Fig. 1). It is revealed that the morphology is linked to the growth direction and not to the nature of the major point defect: anionic (inward) growth leads to equiaxed grains whereas cationic (outward) growth is conducive to columnar grain formation.

Finally by tuning the oxygen partial pressure, chromia semiconducting properties can be controlled. In particular when p(O₂) increases from 10^-14 to 10^-12 atm., the type of semiconduction of the external chromia subscale changes from n-type to p-type ; and, for intermediate conditions, chromia is insulating [3]. The growth of such stoichiometric chromia layer may be helpful for the optimization of chemical durability of chromia forming alloys.

References

[1] A. Holt, P. Kofstad, Electrical conductivity and defect structure of Cr₂O₃. I. High temperatures ( > ~ 1000°C), Solid State Ionics, 69 (1994) 127-136.

[2] L. Latu-Romain, Y. Parsa, S. Mathieu, M. Vilasi, M. Ollivier, A. Galerie, Y. Wouters, Duplex n- and p-type chromia grown on pure chromium: a photoelectrochemical and microscopic study, Oxid. Met. 86(5) (2016) 497-509

[3] L. Latu-Romain, Y. Parsa, S. Mathieu, M. Vilasi, A. Galerie, Y. Wouters, Towards the growth of stoichiometric chromia on pure chromium by the control of temperature and oxygen partial pressure, Corros. Sci. (2017) DOI 10.1016/j.corsci.2017.07.005

Figure 1: ASTAR grain orientation maps recorded with a probe size of 1 nm and a step size of 3 nm (see color codes on the left) of Cr₂O₃ grown on pure Cr (a) at 800 °C during 60 min at a p(O₂) of 10^-14 atm, (b) at 850 °C during 30 min at a p(O₂) of 10^-13 atm, (c) at 900 °C during 30 min at a p(O₂) of 10^-12 atm. From [3].