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Pretreatments of Al-2024 T3 alloy for BTSE silane deposition Lee, Michael Che-Chiang

Abstract

Work in this thesis studied different alkaline etching procedures performed on Al-2024 alloy prior to deposition of bis-triethoxysilylethane (BTSE). X-ray photoelectron spectroscopy and scanning electron microscopy were used to study the chemistry and morphology of the surfaces formed after the various pretreatments and coatings have been applied. The corrosion performance of these samples were assessed using polarisation curve measurements and immersion testing. Sonicating tests were performed to determine, semi-quantitatively, the adhesion of BTSE to Al-2024 panels after application of the various pretreatments. Alkaline etching of the coated and uncoated Al-2024 panels was performed using 10% NaOH in deionised water, varying the duration of the treatment from 1 minute to 10 minutes. It is shown on uncoated Al-2024 that longer etching times increase surface roughness of the substrate, but also reduce corrosion resistance, cause copper enrichment at the surface, and form loosely-bonded etching products which affect the subsequent silane deposition steps. BTSE solutions varying from 1% to 10% (v/v) BTSE were used to coat the pretreated panels. It is found that BTSE-coated samples perform better than their uncoated counterparts in corrosion testing, and that more-concentrated BTSE solutions on identically prepared panels produce thicker coatings, with better passivation. It is also found that, while increasing the alkaline etching time may lead to conditions where more BTSE is deposited to the surface, this "enhanced" silane layer is not as securely-bonded to the surface and will not aid in the passivation of the substrate. Finally, the effects of the HNO₃ desmutting step were studied by varying the treatment times that the Al-2024 panels received in 10% FINO₃ after 1 minute of alkaline etching. It is found that for up to 30 minutes of treatment, FINO₃ is beneficial to silane deposition as it removes the etching product created during the NaOH treatment, builds up the oxide layer and creates more sites for the silane bonding. However, with longer exposure, HNO₃ acts as a corrosion agent removing previously-built oxide layer which reduces the amount of securely-bonded BTSE.

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