Role of Niobium and Tantalum on the Diffusivity of Point Defects in the Passive Film of TiZrNbTa Multi-Principal Element Alloys in Sodium Chloride Solution

In this work, the alloying contents of Nb and Ta in TiZrNbTa refractory multi-principal element alloys (RMPEAs) were systematically tailored. Combining multiple electrochemical techniques with X-ray photoelectron spectroscopy (XPS), the passivation behavior and point defect diffusion within the passive film were quantitatively resolved and interpreted within the framework of the point defect model (PDM). The results show that all three alloys exhibit typical self-passivation behavior, with pitting potentials (Epit) exceeding 3.0 VSCE. As the Nb and Ta contents increase, the corrosion rate of the alloys decreases, and their resistance to metastable pitting is enhanced. The passive films exhibit n-type semiconductive characteristics, with their outer layers dominated by high-valent oxides of the constituent elements. Based on the analysis using defect chemistry and the solute–vacancy interaction model (SVIM), the doping of Nb and Ta reduces the vacancy concentration within the film while suppressing the outward diffusion of Ti cations, resulting in pronounced Ti depletion and significant Nb and Ta enrichment in the film. In addition, the incorporation of Nb and Ta weakens chloride ion adsorption and retards point defect diffusion, thereby delaying film dissolution and localized breakdown. On the other hand, Ti and Zr facilitate the rapid growth of passive films and act synergistically with Nb and Ta, thereby conferring the alloys with superior pitting resistance.