Controllable Preparation And Hydrophobic Modification Of TiO₂ Micro-nanostructures At The Inner Surface Of Slender Titanium Tubes

Metal titanium tubes are widely used in heat exchangers for desalination equipment due to its high specific strength and corrosion resistance.By preparing the superhydrophobic coating with micro-nanostructures at the inner surface of the titanium tube,the purpose of drag reduction and energy efficiency can be achieved,so it possesses important research values and application prospects.In the processes of preparing titanium dioxide?TiO₂?nanotubes by the traditional anodic oxidation method,the arrangement between the substrate and the cathode seriously affects the distribution of the electric field in the solution,which affects the uniformity of the nanotubes.Therefore,it is urgent to develop anodic oxidation method suitable for slender titanium tubes to obtain a uniform and superhydrophobic nanotube coating.In this work,the growth of nanotubes at the inner surface of titanium tube were studied by coaxial anodic oxidation method employing titanium tubes with inner diameter of 3 mm.On the basis of this,a dynamic anodic oxidation method was also developed to fabricate the TiO₂ nanotube arrays at the inner surface of 930 mm titanium tube.After the modification of fluorosilane,the inner surface presents superhydrophobic.The results show that in the static coaxial anodic oxidation method,the TiO₂ nanotubes at the inner surface of the titanium tube reach the growth limit of 500 nm at 1 h,and the surface morphology was almost unchanged with increasing duration.The gas bubbles released during anodization lead to the electrolyte solution expelled out and hence prevent further anodization at the top of titanium tube.A dynamic coaxial anodic oxidation method has been further designed and developed.The results show that the growth of TiO₂ nanotubes at the inner surface of titanium tube is closely related to the flow rate of solution.When the flow rate is 12.2 ?L/s and 64.2 ?L/s,the TiO₂ nanotube arrays grow uniformly at the inner surface of 100 mm and 930 mm titanium tubes,respectively.When the flow rate is 12.2 ?L/s,the nanowires appear at the surface of the TiO₂ nanotubes with the extended time.When the anodic oxidation is 4 h,the inner surface of the titanium tube is completely covered by the uniform cluster nanowires.These nanowires can be completely removed after ultrasonic cleaning for 15 min,leaving a fully opened nanotube arrays.The hierarchical structure at the inner surface of the titanium tube surface including the nanowire and nanotube modified by fluorosilane,the water contact angle can be up to 165°,rolling angle is less than 2°,achieving superhydrophobic characteristics.In contrast,the inner surface only covered by nanotubes have a water contact angle of 120°,indicating that the hierarchical structure is necessary for the preparation of superhydrophobic surfaces.The dynamic coaxial anodic oxidation method developed in this paper has laid a foundation for the uniform preparation of nanotubes?holes?at the inner surface of the tubular matrix of valve metal.The prepared superhydrophobic titanium tubes have important applications in the fields of heat exchangers and drag reduction for fluid interface prospect.