Electrochemically enhanced uniform TiO₂ films on TA1 titanium via stepwise anodizing: Decoupling grain-orientation effects and corrosion-resistance mechanisms

Abstract

This study elucidated the critical roles of grain orientation and microstructural homogeneity in governing the growth, uniformity, and electrochemical corrosion resistance of anodizing TiO₂ films on TA1 titanium. Recrystallized TA1, with a nearly uniform grain structure (median grain-size deviation ≈0.61 μm), exhibited low color difference ($\mathrm{\Delta E}$ ≈ 0.46–0.56) and stable interference colors over 50–110 V, whereas phase-transformed TA1, with a ∼ 20-fold larger median deviation (≈65.71 μm), contained coarse, anisotropic grains that caused pronounced color non-uniformity. Further investigations confirmed that a 2093 mV surface potential difference between $\left(11\overline{2}0\right)\left(0001\right)$ and $\left(0001\right)\left(11\overline{2}0\right)$grains tilted 45° along the TD direction dictated localized discharge dynamics, directly resulting in a 46.7 nm thickness disparity (107.6 nm purple vs. 154.3 nm yellow layers) under 70 V anodizing. This orientation-dependent thickness contrast mechanistically explained the macroscopic color demarcation through interference principles. To eliminate these variations, a stepwise anodizing strategy was developed, in which secondary anodizing was used to redistribute the electric potential and promote preferential growth in thinner-film regions, thereby markedly enhancing TiO₂ film uniformity. Electrochemical tests in seawater showed that such uniform films exhibited higher low-frequency impedance and a higher barrier-layer resistance R_m (457–3248 Ω·cm²), thereby affording superior corrosion resistance. These findings not only clarified the mechanism of anisotropic anodizing behavior but also provided a practical approach to fabricate uniform, highly protective, and functional titanium surfaces for marine and biomedical applications.