The work aims to reduce the risk of hydrogen embrittlement caused by the corrosion behavior of titanium alloy in the marine environment and improve the service life of marine equipment by improving the hydrophobicity of the titanium alloy surface to enhance its corrosion resistance. In this study, a three-step method is used to construct a micro-nano hierarchical structure on the surface of titanium alloy. Firstly, by chemical etching, the polished titanium alloy substrate is etched using a mixed solution containing a strong acid. By precisely controlling the concentration and reaction time of the etching solution, a uniform micron-scale pyramid-like convex structure is successfully prepared on the surface, which provides an ideal substrate for subsequent nanostructure growth. Secondly, by anodic oxidation, electrochemical oxidation is carried out in ethylene glycol electrolyte containing ammonium fluoride with etched rough surface as anode. By optimizing the oxidation voltage and time, highly ordered and vertically oriented titanium dioxide nanotube arrays are grown in situ on the micron pyramid. This step critically forms a hierarchical micro-nano composite structure. Finally, the sample is subject to heat treatment in an air environment. This process not only removes the adsorbed water and some hydroxyl groups on the surface, but also promotes the transformation of amorphous titanium dioxide into a more stable anatase phase. The surface morphology, phase composition, wettability and roughness before and after modification are systematically characterized by scanning electron microscope, X-ray diffractometer, contact angle measuring instrument and surface profiler. The open circuit potential, potentiodynamic polarization curve and electrochemical impedance spectroscopy of the samples are tested in simulated seawater environment by electrochemical workstation to quantitatively evaluate their corrosion resistance. The formation mechanism of the micro-structured surface during chemical etching can be elucidated as follows: insoluble reaction products initially deposit on the titanium alloy surface. However, the formation and escape of hydrogen bubbles disrupt the uniformity of this deposited layer. This disruption results in preferential etching, where ravines rapidly form in the weakened or exposed areas, while the regions protected by more sediment remain elevated as protrusions. This cyclic process ultimately constructs a uniform pyramid morphology on the surface. The formation mechanism of the microstructured surface during anodic oxidation can be elucidated as follows: at the bottom of the nanopores, a dynamic equilibrium is achieved between the continuous electrochemical oxidation of titanium and the chemical dissolution of TiO2 by fluoride ions, driving the vertical growth of the tubes. Meanwhile, the tube walls are relatively "protected", resulting in a slower dissolution rate of the oxide, which allows them to remain intact. Ultimately, a highly ordered nanotube array is formed. The surface of the original titanium alloy is smooth, and the contact angle is about 70 °, which is hydrophilic. After chemical etching, a micron-scale rough pyramid structure is formed. The surface roughness is significantly increased from 0.06 μm to 1.97 μm, and the contact angle increases. Heat treatment is the key to achieve superhydrophobicity. After treatment, the surface contact angle increases to 156°, showing excellent superhydrophobic properties. In the electrochemical test, compared with the original titanium alloy, the open circuit potential of the superhydrophobic sample shifts positively to 0.09 V, indicating that its thermodynamic stability is higher. More significantly, the corrosion current density decreases by three orders of magnitude, from 5.38 × 10-5 A/cm2 to 4.39 × 10-8 A/cm2, indicating that the corrosion is greatly inhibited. Electrochemical impedance spectroscopy analysis further confirms this point: after anodic oxidation, the charge transfer resistance increases to 1.42 × 106 Ω·cm2, and after heat treatment to form a superhydrophobic surface, the resistance further soar to 2.46 × 107 Ω·cm2. This huge impedance increase is attributed to the synergistic effect of "physical barrier" and "chemical barrier". On the one hand, the stable air layer captured by the superhydrophobic surface prevents the direct contact between the electrolyte and the surface; on the other hand, the anatase TiO2 nanotube layer with higher crystallinity is itself a dense and stable protective film, which can effectively block the transmission of charge and corrosive media. In conclusion, the method of "chemical etching-anodic oxidation-heat treatment" is used to successfully prepare the surface of titanium alloy with superhydrophobic properties without modification of low surface energy materials, which achieves the purpose of improving the corrosion resistance of titanium alloy surface and provides conditions for marine corrosion protection of complex parts.
Key words
corrosion resistance /
superhydrophobic /
titanium dioxide nanotubes /
anodic oxidation /
heat treatment
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Funding
National Natural Science Foundation of China (51875299)
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