Electrochemical Stability of Passive Films on β-TiZrTaNb Alloy in Seawater-Based Electrolytes: Influence of Fluoride, pH, and Scan Rate

The corrosion behavior and passive-film stability of a β-TiZrNbTa (β-TZNT) alloy were thoroughly examined in artificial seawater (ASW), with a focus on the effects of pH, temperature, immersion time, fluoride ion concentration, and potential scan rate. In addi-tion to electrochemical methods such as open-circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS), scanning elec-tron microscopy (SEM) and X-ray diffraction (XRD) were used for surface characteriza-tion. The establishment of a stable and efficient passive layer enriched with Zr-, Nb-, and Ta-oxides was responsible for the β-TZNT alloy's superior corrosion resistance in fluo-ride-free ASW when compared to commercially pure titanium. Reduced passive-film re-sistance resulted from corrosion kinetics being greatly accelerated by decreasing the pH and increasing the temperature. Due to the chemical dissolution of TiO₂ through soluble fluoride complexes, the presence of fluoride ions significantly reduced passivity and in-creased corrosion current densities by more than an order of magnitude. A bilayer pas-sive structure with a compact inner barrier layer and a porous outer layer was identified by EIS analysis. The integrity of this structure gradually decreased as the fluoride con-centration and acidity increased. Over time, passive film degradation predominated in fluoride-free seawater, whereas prolonged immersion encouraged partial re-passivation in fluoride-containing media. Overall, the findings highlight the potential and constraints of β-TZNT alloy for advanced marine and offshore applications by offering new mecha-nistic insights into the synergistic effects of fluoride ions and environmental parameters on corrosion performance.