Coupling mechanism of Mo-Nb-polarization on the reversal phenomenon of corrosion resistance in TC4 and TC27

This work aims to clarify the coupling mechanism among Mo/Nb alloying, phase constitution, potentiostatic polarization and passive-film evolution that governs the corrosion resistance reversal of TC4 and TC27 titanium alloys in simulated polluted seawater. Electrochemical tests combined with microstructural and compositional analysis reveal a reversal in corrosion resistance depending on surface condition. Under natural immersion, TC27 exhibits superior corrosion resistance, attributed to its higher β-phase content and enrichment of β-stabilizing elements (Mo, Nb). TC27, alloyed with Mo and Nb, exhibits higher β-phase content and superior corrosion resistance under natural immersion, with its polarization resistance (R_p) approximately 86% higher than that of TC4. However, after potentiostatic polarization at 1 V (vs. SCE), the protective performance reverses: TC4 demonstrates a higher breakdown potential, lower donor density, and greater polarization resistance than TC27. Mott-Schottky analysis reveals a lower charge carrier concentration in TC4’s passive film, associated with fewer defects. The performance reversal stems from compositional and semiconductor property differences of the passive films: TC4’s film is enriched with protective Ti(OH)_x, while TC27’s contains more defect-inducing low-valence oxides, leading to a higher oxygen vacancy concentration. The reversal mechanism is linked to the differing stability of passive films formed under applied potential, where alloying elements in TC27 promote defect formation upon polarization, ultimately degrading its protective quality compared to TC4.