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>_x</i>R‘<i>_y</i>(SiO_3/2)₈, (<i>x </i>+ <i>y</i> = 8), bearing 3-(<i>N</i>-(3-triethoxysilylpropyl)ureido)propyl (ureasil - U) and isooctyl (IO) groups (i.e., U₂IO₆ 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 U₂IO₆ POSS precursor was made in two steps:  in the first, an appropriate stoichiometric (2:6) mixture of 3-aminopropyltriethoxysilane (AP₂) and isooctyltrimethoxysilane (IO₆) was autoclaved under basic hydrolysis conditions giving AP₂IO₆(SiO_3/2)₈ cubes, which were reacted in the second step with 3-isocyanatopropyltriethoxysilane (ICPTES), leading to the bis end-capped sol−gel precursor U₂IO₆ POSS having a cube-like structure. Coatings were made from sols catalyzed with acidified water. IR and ²⁹Si NMR spectroscopic studies combined with mass spectrometric measurements were employed to confirm the cube-like structure of AP₂IO₆ and U₂IO₆ POSS. The structure and morphology of the U₂IO₆ 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 U₂IO₆ POSS coatings which were chronocoulometrically charged at different potentials. At potentials more positive than the corrosion potential (<i>E</i>_corr ∼ −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^-1 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.