Superhydrophobic Surface On Aluminum Alloy And Its Corrosion Resistance

In this thesis,superhydrophobic surface on the aluminum alloy was constructed via a multi-step process of hydrochloric acid etching,boiling-water treating,and surface modifying successively by bis-[3-（triethoxy）siliconpropyl]tetrasulfide（BTESPT）and stearic acid（STA）.The so-obtained superhydrophobic sample was coded as Al-HCl-H₂O-BTE-STA.The surface microstructure,surface composition,surface wettability,and stability exposing to air and to ethanol under ultrasonication of the so-obtained superhydrophobic sample were characterized.The corrosion resistance in the static and dynamic NaCl aqueous solution,and the neutral salt spray was also studied.The main results are as follows:（1）Surface morphology,chemistry,and wettability.After being etched in HCl,micro-steps were generated on the Al alloy,which were further covered by densely packed nanoplates of AlOOH after treated by boiling water.After successive surface modification by BTESPT and STA,the static contact angle（SCA）and the sliding angle（SA）of the sample was 167.0 ± 2.3 ° was 3.5 ± 0.8 °,respectively.（2）Stability.The Al-HCl-H₂O-BTE-STA sample exposing to the air and to the ethanol under ultrasonication possessed good stability.Specifically,after exposing to the atmospheric environment（20-30 ℃,50-60 RH %）for 100 days,the SCA decreased to 163.7 ± 1.8° and the SA increased to 10.0 ± 1.1°.After exposing to the ethanol under ultrasonication（40 KHz,100 W）for 60 min,the SCA decreased to159.9 ± 2.5° and the SA increased to 20.0 ± 1.2°.（3）Corrosion resistance.After the corrosion test,the variation in the surface morphology,surface composition and surface wettability was monitored.（i）Corrosion in the static 3.5 wt.% NaCl aqueous solution for 18 days.The change in the surface microstructures was small;the content ratio of aluminum to oxygen,viz.,[O]/[Al],increased from 0.84 to 0.98;the SCA and SA changed to152.4 ± 2.1° and 49.0 ± 1.2°,respectively.（ii）Corrosion in the dynamic 3.5 wt.% NaCl aqueous solution for 50 h.As the flow rate of the NaCl aqueous solution increased from 0.5 m/s to 1.0 m/s and finally to 2.0 m/s,the SCA / SA changed to 165.2 ± 1.5° / 15.0 ± 1.2°,162.2 ± 2.4° /18.7 ± 2.3°,155.6 ± 1.7° / 36.2 ± 2.5°,respectively.Under the highest flow rate of 2.0m/s,the change in the surface microstructures was small and the content ratio of aluminum to oxygen,viz.,[O]/[Al],increased from 0.84 to 0.91.（iii）Corrosion in the neutural salt spray for 48 h.After corrosion,the change in the surface microstructures was small and the content ratio of aluminum to oxygen,viz.,[O]/[Al],increased from 0.84 to 0.96.The SCA and SA changed to 159.5 ± 2.0°and 24.0 ± 1.1°,respectively.In particular,it should be pointed out that under the same experimental conditions,the superhydrophobic Al-HCl-H₂O-BTE-STA sample possessed much better corrosion resistance and stability as compared with the control samples,such as the Al-HCl-BTE-STA sample and the Al-HCl-H₂O-STA sample.This is mainly due to the following reasons:（1）An oxide layer was generated on the Al surface by boiling-water treating.On the one hand,this barrier oxide layer could delay the occurrence of corrosion.On the other hand,the hydroxyl groups on the surface of the oxide layer could improve the bonding between the BTESPT film and the substrate and consequently enhance the film stability.（2）BTESPT / STA bilayer could improve the corrosion resistance of the sample.BTESPT molecules possessed a good corrosion resistance properties,and,after heat treatment,BTESPT on the solid Al substrate is hydrophobic.However,the superhydrophobicity for the heat-treated sample with the BTESPT as the low-surface-energy outerlayer could be further improved by grafting with STA.