Abstract
Long-term stability in contact with water of organosilane layers formed by octadecyltrimethoxysilane (ODTMS) on polished aluminum alloy (AA2024) through dip-coating was studied by combining SEM, water contact angle measurements, and X-ray photoelectron spectroscopy. Similar organosilane layers were formed on AA2024 coated with permanganate conversion coating, 1,2-bis(triethoxysilyl)ethane (BTSE) and hydrated SiOx as under-layers, after which their long-term durability was also tested. During immersion in water for about one month, all the samples exhibited a decrease in hydrophobicity, implying the prepared organosilane layer was not stable over time, gradually hydrolyzing and letting water interact with the underlying layer. In parallel, SEM images of one-layer samples taken after immersion showed clear signs of local electrochemical corrosion, while XPS analysis confirmed a loss of silicon from the surface layer. The highest stability over time was demonstrated by a one-layer sample prepared in an ethanol/water bath for 5 min and by a similar ODTMS layer prepared on hydrated MnOx as an under-layer.
Highlights
Hydrophobic and superhydrophobic surfaces have been a subject of active research for several decades as they present interest for both fundamental science and industry [1–17]
Such trenches appear on the surface of AA2024 in the presence of electrolytes or water as a result of electrochemical couples that form between the aluminum matrix and second-phase particles embedded in it [42,49,50]
This work systematically studied how the hydrophobicity of alkylsilane layers formed on aluminum surfaces from octadecyltrimethoxysilane (ODTMS) changed over time after immersion in water
Summary
Hydrophobic and superhydrophobic surfaces have been a subject of active research for several decades as they present interest for both fundamental science and industry [1–17]. The water-repellent properties of materials are known to be either provided by their intrinsically low surface energy or by depositing a thin top-layer with low energy. Increased surface roughness can render material superhydrophobicity, while smooth surfaces are known to demonstrate water contact angle (CA) only as high as ~120◦ [3–10,12,14,15,18–20]. Various organosilanes, including fluorinated alkylsilanes (FASs), have been used as hydrophobizing agents deposited either via immersion in liquid bath [1,8,14,21,26,27,32–35] or from gas phase [13,27]. Apart from water repellency, which is normally expected in case of alkylsilanes, organosilane layers were reported as part of anti-ice (or ice-phobic) surfaces [4,8,14,16,17,36,37], as well as part of anticorrosive coatings [21–23,27,32,33,38–40]
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