For superhydrophobic surfaces immersed in water, a thin layer of air could be entrapped in the solid/liquid interface. This air may hinder the diffusion of dissolved corrosive species (such as Cl- ions in water) to the metallic substrate and, consequently, protect the metal from corrosion. However, in the dynamic water, the relative motion between the solid and the liquid would labilize the entrapped air and, consequently, decrease the corrosion resistance. In this work, to clarify the role of water flow velocity in such corrosion behavior, a superhydrophobic surface on aluminum substrates coded as Al-HCl-H2O-BT-SA was prepared by sequential treatment with HCl, boiling water, bis-(γ-triethoxysilylpropyl)-tetrasulfide (KH-Si69, BT) and stearic acid (SA). The contrast samples coded as Al-HCl-BT-SA, Al-HCl-H2O-SA, and Al-HCl-SA were also prepared similarly by omitting the treatment in boiling-water, the BT passivation, and the treatment in boiling-water/passivation by BT, respectively. These samples were then immersed into an aqueous solution of NaCl with different flow velocity (0, 0.5, 1.0, 1.5, and 2.0 m s-1), and its dynamic corrosion behavior was investigated. The results showed that, as the flow velocity increased, the corrosion resistance of the Al-HCl-H2O-BT-SA sample indeed deteriorated. However, compared with the contrast samples of Al-HCl-BT-SA, Al-HCl-H2O-SA, and Al-HCl-SA, the deterioration in corrosion resistance for the Al-HCl-H2O-BT-SA sample was much lower, implying that the dynamic corrosion resistance of the superhydrophobic surfaces was closely related with the micro-structures and the organic passivated layers. The present study therefore provided a fundamental understanding for the applications of superhydrophobic samples to prevent the corrosion, especially, for various vessels in dynamic water.
Five functional silanes--3-aminopropyltriethoxysilane (APTES), 3-aminopropyltrimethoxysilane (APTMS), N-(2-aminoethyl)-3-aminopropyltriethoxysilane (AEAPTES), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS), and N-(6-aminohexyl)aminomethyltriethoxysilane (AHAMTES)--were assessed for the preparation of hydrolytically stable amine-functionalized silica substrates. These can be categorized into three groups (G1, G2, and G3) based on the intramolecular coordinating ability of the amine functionality to the silicon center. Silanizations were carried out in anhydrous toluene as well as in the vapor phase at elevated temperatures. Aminosilane-derived layers prepared in solution are multilayers in nature, and those produced in the vapor phase have monolayer characteristics. In general, vapor-phase reactions are much less sensitive to variations in humidity and reagent purity, are more practical than the solution-phase method, and generate more reproducible results. Intramolecular catalysis by the amine functionality is found to be important for both silanization and hydrolysis. The primary amine group in the G1 silanes (APTES and APTMS) can readily catalyze siloxane bond formation and hydrolysis to render their silane layers unstable toward hydrolysis. The amine functionality in the G3 silane (AHAMTES) is incapable of intramolecular catalysis of silanization so that stable siloxane bonds between the silane molecules and surface silanols do not form easily. The secondary amine group in the G2 silanes (AEAPTES and AEAPTMS), on the other hand, can catalyze siloxane bond formation, but the intramolecular catalysis of bond detachment is sterically hindered. The G2 silanes are the best candidates for preparing stable amine-functionalized surfaces. Between the two G2 aminosilanes, AEAPTES results in more reproducible silane layers than AEAPTMS in the vapor phase due to its lower sensitivity to water content in the reaction systems.
Modification with gold black considerably increased the detection current of protruding micropillar electrodes in microfluidic electrochemical enzyme-linked immunosorbent assay (ELISA).
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