Ab Initio Insights to Metal Passivation—A Study of Diffusion and Defect Formation in Zirconia and Alumina

First principles calculations based on density functional theory have enabled digital workflows to drive materials discovery forward; however, these workflows still have many limitations. One issue is that current workflows are unable to account for metal passivation. Metal passivation refers to the process by which metals form oxide layers that protect the material and prevent corrosion; this process is necessary for materials to be stable in air. Whether an oxide layer forms can be determined thermodynamically with Pourbaix diagrams, but the kinetic characteristics of the oxide, such as how fast it will grow and what thickness it will reach, are not well characterized with first principles. To work toward a first principles understanding of metal passivation, we look to the Point Defect Model [1]. The model offers atomic level insights into passivation and suggests that defect formation and diffusion are responsible for the growth of passive films. To test this idea in zirconium and aluminum, we calculated defect formation energies and diffusion in amorphous oxides for both metals. Since the materials are modelled as amorphous, defect calculations were performed for several different atomic sites and the formation energy varied by as much as 4 eV. Using defect formation energies and diffusion, we calculated defect conductivity for oxygen vacancies in the structures and compared this to oxide thickness measurements found from a literature review. Defect conductivity was correlated with oxide thickness, such that a slower defect conductivity was associated with a thinner oxide forming. These results are consistent with our expectation that growth of the passive film will be affected by how much energy it takes to form a defect and how quickly species can move through the film. This knowledge offers a potential path forward for incorporating passivation into digital materials discovery.<br/><br/>[1] D. D. Macdonald, S. R. Biaggio, H. Song, Journal of The Electrochemical Society <b>139</b>, 170 (1992).