DFT Study of Defects in Paramagnetic Cr2O3
Résumé
Ni-based alloys with high Cr concentrations tend to show good corrosion resistance at high temperatures thanks to the formation of a protective chromia scale on the alloy surface. Growth kinetics of the scale is generally controlled by a diffusion process. This factor depends on the nature, concentration, and diffusivity of the dominant type of point defects in the chromia scale. However, the nature of the dominant point defects, which depends on alloy nature, temperature, and oxidizing atmosphere, remains unclear.
DFT calculation is a powerful technique to predict the nature of dominant point defects by calculating defect formation energies. In the literature, the magnetic state of chromia used for calculating defect formation energies is always the fundamental state at low temperature, which is antiferromagnetic. But chromia formed during oxidations above the Néel temperature (318 K) exhibits the paramagnetic state. The transition of magnetic state near Néel temperature may affect the defect formation in chromia.
The aim of this work is to determine the kind of dominant 0D-defects in chromia scale during high temperature oxidation. For this purpose, defect formation energies were calculated with ab initio calculations in antiferromagnetic and paramagnetic chromia. Brower diagrams at different temperatures were drawn based on the calculated defect formation energies with thermodynamic approximations, in order to study the dependence of temperature and oxygen partial pressure on the nature of dominant point defects.
A previous experimental study suggests that O vacancies are the favourable point defects at low temperature/or low oxygen partial pressure p(O2) while Cr vacancies are dominant at high temperature/or high p(O2) for Ni-30Cr oxidations. In order to compare experimental and simulation results, oxidation tests are carried out in Rhines Pack controlled atmosphere. The grown oxides are characterized by photoelectrochemistry in order to determine their electronic properties and to deduce the dominant type of point defects.