Hydrogen-Induced Hydroxylation and Defect-Controlled Weakening of Ti--6Al--4V Passive Films in Artificial Seawater
Abstract
Ti--6Al--4V alloy is selected for marine and offshore structures due to its high specific strength in conjunction with fast formation of a nanometric titanium-oxide passive film. At the same time, cathodic polarization in presence of chloride ions in seawater leads to generation of atomic hydrogen at the metal/electrolyte interface, its incorporation into the near-surface alloy structure, and change of the passive-layer chemistry and electronic defects population. This work investigates the influence of hydrogen charging on the state of the passive film on the Ti--6Al--4V alloy in artificial seawater for 0, 1, 4, 8, and 12 h. Composition of the alloy was 6.03 wt.\% Al, 4.14 wt.\% V, 0.01 wt.\% C, 0.08 wt.\% O, balance Ti, and the charging condition was pH $8.2 \pm 0.1$, $25 \pm 1\,^{\circ}$C, cathodic current density of 20 mA cm$^{-2}$. The electrochemical impedance spectroscopy, potentiodynamic polarization, Mott-Schottky analysis of donor density, thickness of the passive film, and TiO$_2$/TiOOH surface fractions were used to analyze passivity deterioration during hydrogen formation. Polarization resistance decreased from $1.93 \times 10^6$ to $2.86 \times 10^5\,\Omega\,\mathrm{cm}^2$ after 12 h, whereas the corrosion current density increased from $4.62 \times 10^{-8}$ to $9.19 \times 10^{-8}$ A cm$^{-2}$. Donor density increased from $4.22 \times 10^{19}$ to $6.10 \times 10^{19}$ cm$^{-3}$, passive-film thickness decreased from 1.81 to 1.12 nm, TiO$_2$ content decreased from 93.86\% to 86.18\%, and TiOOH-related fraction increased from 6.14\% to 13.82\%. Passive-film integrity index decreased from 1.000 to 0.086, whereas barrier-disorder amplification factor increased from 1.000 to 38.996. This study shows that hydrogen charging influences passivation not only by thinning of the oxide film. It results in conversion of the dense TiO$_2$-dominated barrier into the hydroxylated, donor-defective, electrochemically heterogeneous film, which stays passive but allows more charge transfer.