Application of ex situ IR reflection-absorption, in situ Raman and in situ electrochemical AFM in the study of sol-gel protective coatings

摘要

Organic-inorganic hybrid materials can be excellent solution for primers in protective coatings for metals/alloys. Various functionalizations of these hybrids have been tested in order to examine their influence on multifunctionality of the final coatings. In the case of protective coatings most often hybrid molecules with hydrophobic character have been chosen. In this study, however, we have not focused on the improvement of the performance of coatings, but on the development and application of various ex situ an in situ approaches for investigation of sol-gel-based protective coatings. First, the combination of vibrational spectroscopies, i.e., infrared reflection-absorption (IR RA) and Raman, provided complementary information. The electrochemical treatment served to introduce high anodic potentials into the system, consequently inducing the beginning of degradation of the coatings. Ex situ IR RA technique can be performed when coating is deposited on highly reflective substrate, consequently aluminium alloy AA 2024 has been used as a model substrate. Moreover, the IR light should be p polarised and should fall on the sample under near grazing incidence angle conditions of 80°. Consequently, the longithudinal optical (LO) modes appear in the measured spectra. IR RA spectra can give important information on hydration of the coatings, eventual breakage of siloxane bonds and changes in the C-H stretching region. The LO-TO splitting results from long-range Coulombic interactions among vibrating dipoles. In situ Raman spectroelectrochemical measurements are performed in a custom-made three-electrode teflon cell. Pt grid is used as a counter electrode, while Ag/AgCl serves as a reference electrode. The working electrode is the sol-gel coating deposited on the A A 2024. In addition to single Raman spectra on the specified site, Raman imaging can also been performed. For either ex situ IR RA or in situ Raman experiments, the coating is charged chronocoulometrically in steps of+0.1 V from-0.8 V to 0.2 V for at last 240 s at each potential. After each potential step the concerned spectrum is recorded. The third technique that was applied is in situ electrochemical atomic force microscopy (AFM). This technique enables the detection of morphology characteristics of the coatings. The measurements have not only been performed on initial coatings, but also on coatings exposed to certain potentials. This was enabled to us by the construction of custom-made in situ electrochemical AFM cell, in which the coating on AA 2024 was introduced in the middle of the cell and with the contact from the rear side. Pt grid and Ag/AgCl were positioned beside the sample and served as counter and reference electrode.