Ex situ IR and Raman spectroscopy as a tool for studying the anticorrosion processes in (3-glycidoxypropyl)trimethoxysilane-based sol–gel coatings

Abstract Ex situ IR reflection–absorption (IR RA) and Raman spectroscopic measurements were performed on sol–gel anticorrosion coatings to take advantage of the complementarity of the two vibrational techniques. The coatings were prepared on the basis of (3-glycidoxypropyl)trimethoxysilane, (3-aminopropyl)trimethoxysilane and amino-heptaisooctyl-functionalised polyhedral oligomeric silsesquioxane and their protective efficiency evaluated in terms of the hydrophobic poly(dimethylsiloxane) (PDMS) content. The results of IR absorbance and rheology measurements of sols showed that coatings should be deposited 30 min after the addition of acetic acid sol–gel catalyst and thermally treated at 150 °C. The surface energy values of coatings decreased with increasing amounts of PDMS chains, from 29.6 to 22.6 mJ/m 2 . Potentiodynamic polarisation revealed that the concentration of PDMS chains in coatings should be optimised to depict the lowest corrosion current density, of the order of magnitude 10 −9  A/cm 2 . Extensive ex situ IR RA studies of the degradation of coatings during polarisation indicated that breakage of siloxane bonds occurred through the formation of lower cyclosiloxanes and hydration, while conformational changes of methylene groups were noted in coatings without PDMS chains. Ex situ confocal Raman images of coatings with PDMS recorded after anodic polarisation mainly revealed bands of PDMS chains of decreasing intensity, while no signal was found in areas of pitting corrosion. The stretching ν(Si–O–Si) mode of the condensed network around 476 cm −1 could not support the formation of lower cyclosiloxanes, since it overlapped with the 496 cm −1 siloxane mode of PDMS chains.