A Structural and Corrosion Study of Triethoxysilyl Functionalized POSS Coatings on AA 2024 Alloy

A novel bifunctional polyhedral oligomeric silsesquioxane (POSS) based silane precursor R<i><sub>x</sub></i>R‘<i><sub>y</sub></i>(SiO<sub>3/2</sub>)<sub>8</sub>, (<i>x </i>+ <i>y</i> = 8), bearing 3-(<i>N</i>-(3-triethoxysilylpropyl)ureido)propyl (ureasil - U) and isooctyl (IO) groups (i.e., U<sub>2</sub>IO<sub>6</sub> POSS) was synthesized, and the corresponding coatings, prepared under the acid hydrolysis conditions, were studied in order to assess their corrosion inhibition of the AA 2024-T3 alloy. The U<sub>2</sub>IO<sub>6</sub> POSS precursor was made in two steps:  in the first, an appropriate stoichiometric (2:6) mixture of 3-aminopropyltriethoxysilane (AP<sub>2</sub>) and isooctyltrimethoxysilane (IO<sub>6</sub>) was autoclaved under basic hydrolysis conditions giving AP<sub>2</sub>IO<sub>6</sub>(SiO<sub>3/2</sub>)<sub>8</sub> cubes, which were reacted in the second step with 3-isocyanatopropyltriethoxysilane (ICPTES), leading to the bis end-capped sol−gel precursor U<sub>2</sub>IO<sub>6</sub> POSS having a cube-like structure. Coatings were made from sols catalyzed with acidified water. IR and <sup>29</sup>Si NMR spectroscopic studies combined with mass spectrometric measurements were employed to confirm the cube-like structure of AP<sub>2</sub>IO<sub>6</sub> and U<sub>2</sub>IO<sub>6</sub> POSS. The structure and morphology of the U<sub>2</sub>IO<sub>6</sub> POSS coatings were studied with the help of infrared reflection−absorption (IR RA) spectroscopic measurements combined with XPS and AFM measurements, providing information about the formation of partially self-assembled coatings. The degree of corrosion inhibition was assessed from the potentiodynamic measurements showing around 10 times smaller current densities for the coatings only 30−40 nm thick. Ex situ IR RA spectroelectrochemical measurements were performed by consecutive measurements of the IR RA spectra of U<sub>2</sub>IO<sub>6</sub> POSS coatings which were chronocoulometrically charged at different potentials. At potentials more positive than the corrosion potential (<i>E</i><sub>corr</sub> ∼ −0.5 V), the amide I bands shifted, indicating the formation of new urea−urea aggregations and associations, with the newly formed strong band at 1680−1690 cm<sup>-1</sup> suggesting the formation of amidonium ions. These results showed that the urea groups represented the weakest part of the coatings due to their tendency to protonation.