Benefits of Coupling of Electrochemical Technique with Either IR, Raman or AFM Technique in the Corrosion Investigation
Authors: Surca Angelja Kjara , Kreta Ahmed , Mihelčič Mohor , Gaberscek Miran , Rodošek Mirjana 
Published: February 2020

The added value in data that can be obtained by coupling of the electrochemical techniques with others guaranteed a steady development of this field. While the electrochemical measurement gives only a sum of all processes in the cell, coupling with other methods offer information about surfaces of working electrodes, interfaces, reaction intermediates, reaction products, their orientation, dynamic processes in cells etc. Various spectroscopic techniques can be extremely useful, being chosen with regard to the intention of the experiment [1]. For instance, coupling of electrochemical techniques with vibrational ones (infrared (IR), Raman) can offer precious data on mechanisms of different processes and structural changes in materials. In addition, when samples are in the form of coatings, coupling of the electrochemical technique with an atomic force microscopy (AFM) leads to data on morphology changes with changes in potential [2].
In this work we present how coupling of various techniques can contribute to the added value in the field of corrosion of metals/alloys or protective coatings. The protective coatings used herein were prepared via sol-gel route from various organic-inorganic hybrids on aluminium alloy AA 2024. Different trialkoxysilanes, having various functionalization, have been used to introduce multifunctionality to the produced coatings. Trialkoxysilanes were bis end-capped mostly, enabling the sol-gel reactions of hydrolysis and condensation on the both sides of the molecules. Consequently the formed protective coatings were compact and highly crosslinked. The coupled techniques were used to investigate structural (IR, Raman) and morphological (AFM) changes that occur in protective coatings during gradual anodic polarization. Information on bonds that are the most prone to the cleavage gives insight into the weakest points of the coatings. Consequently, iterative upgrading of the coating’s composition to a more effective one is possible.
We applied three different approaches, i.e. in situ Raman spectroelectrochemistry [3], ex situ IR reflection-absorption (IR RA) spectroelectrochemistry [3] and in situ electrochemical AFM. For in situ approaches we manufactured custom made three-electrode cells. These techniques are not new, but they still have enormous potential in investigation of unexploited possibilities, which emerge with the developments in instrumentation and building materials. Namely, due to the diversity of spectroscopic and other instruments, in situ cells need to be manufactured in the way to meet their specific characteristics to obtain the desired and maximized signals, but also adapted according to type of electrodes, their size and shape, their position in the in situ cells, sealing possibilities …

In situ Raman spectroelectrochemical and ex situ IR RA give complementary results. Also the type of samples is different. As coatings for the in situ Raman measurements should be thick enough to obtain the Raman spectra, the ex situ IR RA spectroscopy is only possible for thin coatings deposited on reflective substrates. The measurement is performed with a p polarized radiation at a near grazing incidence angle of 80°. It will be evidenced that the latter technique enables the detection of hydration of coatings during their exposure in electrolytes, but also the cleavage of siloxane bonds and eventual re-formation of silanol bands, changes in C-H stretching vibrations during degradation at anodic potentials. In situRaman spectra did not reveal shifts of bands during degradation, but Raman imaging revealed potential to follow pit formation [3].

In situ electrochemical AFM is appropriate for investigations of corrosion processes at a high lateral resolution [2]. The metals/alloys (herein AA 2024) can be studied with regard to the shape evolutions around intermetallic inclusions, as well to deposition of corrosion precipitates and formation of passivating layers. On the other side, the morphology characteristic of the deposited sol-gel protective coating was quite homogeneous at initial stages of the experiment. After exposure to potential, we could observe dissolution of the intermetallic particle, which was not completely covered.
Finally, the electrochemical impedance spectroscopy (EIS) was used to compare the response in both in situ cells (Raman and AFM) to that obtained in the standard corrosion electrochemical cell. It was found out that the shapes of the spectra and the circuit parameters’ trends support the confidence in the results measured in in situ cells.
[1] R. Greef, R. Peat, L.M. Peter, D. Pletcher, J. Robinson, Instrumental Methods in Electrochemistry, Ellis Horwood Limited, New York, 1990.
[2] S.D. Zhang, Z.W. Liu, Z.M. Wang, J.Q. Wang, Corros. Sci. 83 (2014) 111-123.
[3] M. Rodošek, A. Rauter, L. Slemenik Perše, D. Merl Kek, A. Šurca Vuk, Corros. Sci. 85 (2014) 193-203.