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

A novel bifunctional polyhedral oligomeric silsesquioxane (POSS) based silane precursor RxR‘y(SiO3/2)8, (x + y = 8), bearing 3-(N-(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(SiO3/2)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 (Ecorr ∼ −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.