Breakdown mechanism of MoO3 passive film: Electrochemical measurements and first-principles calculations

The mechanism of passivity breakdown on MoO₃ films is studied. It confirms that the molybdate is generated easily on the surface of Mo immersed into the 3.5 wt% NaCl solution. The diffusion coefficient of oxygen vacancy in the MoO₃ (as the precursor of molybdate) is approximately 10⁻¹⁸–10⁻¹⁹ cm²/s by analyzing the Mott-Schottky curves and electrochemical impedance spectroscopy (EIS). It proves that this value obtained by experiment is corresponding to the calculated result based on the density functional theory (DFT). The diffusion barrier value of oxygen vacancy is 0.27 eV (26.05 kJ/mol) according to the diffusion coefficient at different temperatures. Both the experimental and calculated results demonstrate that the breakdown of MoO₃ film is due to the adsorption of Cl⁻ on the surface, and Cl⁻ could increase the diffusion coefficient of oxygen vacancy, in turn, promoting the transport rate of point defects. A theoretical model was proposed for the breakdown of MoO₃ passive film.