The Study of Sol-Gel Protective Coatings Using Combined Analytical Tools
Authors: Surca Angelja Kjara , Gaberscek Miran , Rodošek Mirjana 
Published: February 2020

Trialkoxysilane organic-inorganic hybrids give versatile options to design various functional materials on a molecular level [1]. Among possible effective products there are also protective coatings for different metals/alloys. Versatility of approaches reflects in this work through application of: (i) short bis end-capped trialkoxysilanes, which enable formation of compact coatings; (ii) bis end-capped trialkoxysilanes with long poly(dimethylsiloxane) chain in-between the terminal trialkoxysilyl groups and (iii) cyclic tetrasiloxane composed of siloxane ring that is functionalized with trimethoxysilyl groups. All described precursors enabled formation of crosslinked structures through sol-gel reactions of hydrolysis and condensation. In the case of bis end-capped trialkoxysilane-based coatings, the effect of addition of polyhedral oligomeric silsesquioxanes (POSS) on the protective efficiency is also regarded. POSS have ordered structures composed of hard silica core rounded by organic shell [2]. The functional groups can be reactive (alkoxysilyl, vinyl, amine, …) or just offer a desired property, for example, introduce hydrophobicity (alkyl). Through reactive functional groups POSS can firmly bond into the crosslinked structure of the coating. It is already confirmed that POSS can influence chemical, scratching, mechanical, thermal and electrochemical performance of coatings. Despite quite some reports on behavior of POSS in sol-gel and other polymeric materials exist, the application of cyclic siloxane-based precursors have been highly overlooked [3].
The protective coatings prepared from the above described precursors are dip-coated on aluminum alloy AA 2024. The coatings are first characterized using standard techniques like infrared (IR) and Raman spectroscopy, contact angle measurements, surface energy value, scanning electron (SEM) and atomic force (AFM) microscopy. Potentiodynamic polarization, impedance spectroscopy or exposure in salt spray chamber is used to test the protective efficiency of coatings. Moreover, there are applied also advanced analytical techniques that combine electrochemical technique with either vibrational spectroscopic (IR, Raman) [4] or AFM technique [5]. It is evident that solely electrochemical measurement gives a sum of all processes that occur in the electrochemical cell. The coupling of another technique gives additional data. For instance, on identification of reaction intermediates or products, interfaces, type of adsorbed species and their orientation, intercalation mechanisms in the intercalation compounds, etc. In case of protective coatings, in this way one may obtain information on processes that occur in coatings during forced polarization to high anodic potentials. IR or Raman spectroscopy serves to identify the bonds in coatings that are the most prone to cleavage, detect hydration of coatings during their exposure to electrolyte, eventually even follow the breaking of Al-O-Si bonds that serves to adhesion of protective coating to the alloy substrate. AFM, on the other hand, records the changes in morphology after application of potential pulses.
Accordingly, there are applied three different combined techniques for the study of corrosion processes on alkoxysilane-based coatings described above. These techniques are ex situ IR reflection-absorption (IR RA) and in situ Raman spectroelectrochemistries, and in situ electrochemical AFM. Listed techniques have a potential to upgrade the value of obtained results compared to sole techniques. Each of them, however, is subjected to special characteristics of the coupled spectroscopic or microscopic technique. Consequently, the geometries of the in situ cells are different with regard to usual electrochemical cells. Namely, the in situ cells should enable the approach of IR radiation, laser beam or cantilever and also to obtain the maximized signals. The sealing of in situ cells may require a certain level of skill and materials’ knowledge. The in situ cells were herein designed as three-electrode cells. Anyhow, the inevitable developments in instrumentation and building materials will further enlarge the enormous potential of coupled techniques. In order to characterize the functioning of the in situ cells, the electrochemical impedance spectroscopy can be applied. The impedance measurement of equal alkoxysilane-based protective coatings in various in situ cells should give answer to query if various geometry designs influence their electrochemical response.

References:

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