Corrosion resistance of NbTaₓTiV alloys: Dependence on Ta-modified microstructure and passivation

This study systematically investigated the corrosion behavior of refractory medium-entropy alloys (RMEAs) composed of NbTa_xTiV ( x = 1, 1.25, 1.5, 1.75) in a 3.5 wt% NaCl solution. Combined analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron back-scattered diffraction (EBSD) revealed that moderate addition of tantalum (Ta) effectively suppressed micro-segregation of alloy constituents and inhibited grain coarsening. However, excessive Ta addition increased both elemental segregation and grain size. This change in microstructure provided more active sites and pathways for the corrosion process. To quantitatively evaluate corrosion resistance, open-circuit potential (OCP), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and Mott-Schottky (M-S) tests were conducted on four alloys in simulated seawater. The NbTa_1.5TiV alloy exhibited excellent comprehensive corrosion resistance: current density decreased by 86.7%, charge transfer resistance significantly increased, passive film stability improved, and defect density reduced. XPS analysis confirmed that this performance enhancement resulted from the formation of a dense passive film dominated by high-valent oxides (Nb₂O₅/V₂O₅) on the alloy surface. To further validate the superior corrosion resistance of NbTa_xTiV, immersion tests were conducted. The results indicated that the degree of element segregation and grain size exerted a critical influence on the corrosion resistance of the alloys: moderate element distribution uniformity and fine grain structure facilitate the formation of a dense, stable passive film, thereby enhancing the alloys’ corrosion resistance. Conversely, increased element segregation and coarse grain structure compromise the integrity of the passive film, reducing the alloys’ corrosion resistance.