Posibilities of ex situ IR and in situ Raman spectroscopies for investigation of sol-gel anticorrosion coatings during anodic polarisation

摘要

Organic-inorganic hybrid materials have been extensively investigated during recent years for various applications, among them also coatings for corrosion protection of various metals. In this study we prepared anticorrosion coatings from hydrophobic precursor 3-glycidoxypropyl terminated poly(dimethylsiloxane) (EP-PDMS-EP) and crosslinker (3-aminopropyl)trimethoxysilane (APTMS) for aluminium alloy AA 2024. To influence the mechanical hardness, thermal stability, chemical and scratching resistance the aminopropyl-heptaisooctyl-polyhedral oligomeric silsesquioxane was incorporated into the structure of the coatings trough the reaction between amino and epoxy groups. The effect of POSS on thermal stability will also be evaluated. In order to obtain information about structural changes in material during their forced electrochemical treatment in situ and ex situ vibrational techniques were used. Namely the combination of vibrational spectroscopies, i.e., infrared reflection-absorption (IR RA) and Raman, provided complementary information. IR RA spectroscopy could give information about hydration of the coatings and changes in the siloxane and ν(C-H) spectral region. Ex situ IR RA measurements were performed under P-polarisation with near-grazing incidence angle 80°, giving the longitudinal optical (LO) modes as the result, which are blue-shifted with regard to the transversal optical (TO) modes recorded with absorbance IR technique. The LO-TO splitting results from long-range Coulombic interactions among vibrating dipoles.With the aim of compering ex situ IR RA measurements with the findings obtained from another vibrational technique a special in situ Raman spectroelectrochemical cell was constructed. The Pt grid and Ag/AgCl electrodes were used as counter and reference electrodes, while the anticorrosion coating on AA 2024 was mounted in horizontal position and charged chronocoulometrically in steps of +0.1 V from -0.8 V to 0.2 V for at last 240 s at each potential. Changes in Raman images revealed the lowering of the band intensities at the sites, at which the pitting corrosion was noted.rnThe anticorrosion properties of coatings were also evaluated using standard techniques like potentiodynamic polarisation technique, electrochemical impedance spectroscopy and exposure in salt chamber. SEM was used for evaluating the morphology of the coatings.