Construction And Properties Of Self-cleaning Functional Aluminum-based Superhydrophobic Surfaces Via Anodization Method

Aluminum(Al)and its alloy are important engineering materials in modern industry due to their numerous advantages,such as abundance in nature,good ductility,high specific strength and excellent electrical conductivity.They have been widely used in various fields,including aerospace,marine,civilian industries and so on.However,they can be easily corroded and contaminated under harsh environment conditions,particularly in marine environment when exposed to salty water or moisture or damaged after exposure in outdoor environments for quite a long time or accumulated by ice in tough freezing weather,which can adversely affect their aesthetic appearance and desired functionalities or even cause economic losses and serious accidents.To prevent such problems from happening,it can be achieved by transforming the hydrophilic nature of Al and its alloy surfaces to be superhydrophobic to form a corrosion and contamination resistant as well as anti-icing surface protective layer.In this study,the self-cleaning functional Al-based superhydrophobic surfaces with hierarchical micro-nanostructures were fabricated successfully via a facile and cost effective method using anodization followed by surface modification with myristic acid and 1H,1H,2H,2H-perfluorodecyltriethoxysilane(FAS).The surface morphology and chemical composition were characterized by an atomic force microscope(AFM)and a field emission scanning electron microscope(FESEM)with attached energy dispersive X-ray spectrum(EDS).The surface wettability,mechanical and chemical stabilities,weathering resistance,corrosion resistance,anti-icing as well as self-cleaning performance of the Al-based superhydrophobic surfaces were systematically investigated.The main results are as follows:(1)The superhydrophobic Al alloy surface was obtained with a static water contact angle(CA)of 155.6 ± 1.0° and a sliding angle(SA)of 5.7 ± 0.2° by anodization under voltage of 16 V and surface modification with molten myristic acid.For the bare Al substrate,the as-prepared optimal superhydrophobic Al surface displays a CA as high as 156.0 ± 0.7° and a SA as low as 2.5 ± 1.4° at the voltage of 20 V followed by modification with FAS.The FESEM images show that three types of hierarchical micro-nanostructures,i.e.,mico-nanowire structure,mico-nanopore structure and mico-nanowire pyramid structure were constructed on the Al-based superhydrophobic surfaces and the evolved formation mechanism from anodized alumina nanopore structure to anodized alumina nanowire structure was discussed in detail.The static and dynamic behaviors of water droplets on the Al-based superhydrophobic surfaces were investigated by contact angle measuring system and the IV surface wettability was also theoretically analyzed based on the classical Cassie-Baxter equation.The corresponding results showed that the air/water interface occupies 87.5% and 82.2% of the apparent area when the water droplets contact superhydrophobic Al alloy surface modified by myristic acid and the superhydrophobic Al surface modified by FAS,respectively.(2)The superhydrophobic Al alloy surface was proven to function well after 9 months indoor exposure at room temperature and immersion in common solvents including deionized(DI)water,hot DI water(80 °C)and absolute ethanol for 36 h,showing good long-term stability in air and chemical stability.Besides,the optimal superhydrophobic Al surface is found to have good mechanical stability against sand erosion and chemical stability in a series of solutions: it retained a CA as high as 151.8 ± 1.1° after mechanical sandblasting for 90 s and was stable after immersion in the common solvents and hot DI water(80 °C)for 7 days as well as after dripping test using both acidic and alkaline solutions.In addition,after exposure to UV/water condensation cycles for 7 days,the optimal superhydrophobic Al surface remained superhydrophobic,indicating excellent weathering resistance.(3)Electrochemical measurement results showed that the corrosion current density(Jcorr)of superhydrophobic Al alloy surface was significantly reduced by nearly 4 orders of magnitude and the corrosion potential(Ecorr)shifted from-0.838 V to 0.403 V as compared to that of the bare Al alloy substrate,indicating excellent corrosion resistance.The optimal superhydrophobic Al surface demonstrated a decrease of about 4 orders of magnitude in the Jcorr and considerably positive shift of 0.93 V in the Ecorr compared with the bare Al.The corresponding protection efficiency(P%)can reach up to 99.99% for Al substrate.(4)The superhydrophobic Al surfaces showed a delay in icing temperature and icing time as compared to bare Al substrate.The water droplets on the bare Al surface start to freeze at-12.3 °C and the ice growth process lasts 2 s while the corresponding icing temperature and icing time are-24.0 °C and 23 s for the optimal superhydrophobic Al surface.The ice adhesion strength of four types of surfaces with different extent of wettability,namely,superhydrophilic anodic Al oxide(AAO),hydrophilic bare Al,hydrophobic Al modified by myristic acid and FAS and superhydrophobic Al surfaces were comparatively investigated.The ice adhesion of the optimal superhydrophobic Al surface was decreased more obviously than others and is as low as 0.036 ± 0.022 MPa,which is about 3.5% that of the bare Al surface(1.024 ± 0.283 MPa).Both lower icing temperature and extremely low ice adhesion strength indicate that the as-prepared superhydrophobic Al surface has a robust anti-icing property,which is much better than that of modified hydrophobic Al surface,and far better than that of bare Al and AAO surfaces.(5)The carbon powder as contaminant can be easily carried away when the water droplets slide off from the Al-based superhydrophobic surfaces.Meanwhile,the optimal superhydrophobic Al surface shows the lowest extent of dirt accumulation only about 0.18% compared with bare Al surface,AAO surface and modified hydrophobic Al surface,indicating its outstanding self-cleaning efficiency.Moreover,the superhydrophobic Al surfaces can be able to retain the self-cleaning functionality even after sandblasting treatment,weathering resistance,electrochemical corrosion-resistant measurement as well as multiple icing/deicing cycles test.