(Keynote) Local Electrochemical Impedance Spectroscopy: A Powerful Tool for Studying Corrosion Inhibition Mechanisms on 2024 Aluminum Alloy
Authors: Pebere Nadine , Lebon Emilie 
Published: February 2020

Corrosion phenomena on aluminum alloy such as AA2024 are mainly linked to the local galvanic coupling between the intermetallic particles and the surrounding matrix [1]. To have a better understanding of the corrosion associated with copper-rich intermetallics, a simple system consisting of pure aluminum/pure copper (Al/Cu couple) has been previously considered [2]. From the beginning of immersion, increased cathodic activity related to oxygen reduction was observed at the Al/Cu interface. It was underlined that the local alkalinization of the electrolyte at the Al/Cu interface plays a key role in the degradation of the system.
In a recent work devoted to the corrosion inhibition of the 2024 aluminum alloy, it was shown that mixing 8-hydroxyquinoline (8-HQ) and benzotriazole (BTA) led to a synergistic effect for the corrosion protection of the alloy [3]. This synergy was observed by conventional electrochemical measurements (current-voltage curves and global impedance spectroscopy). However, the electrode response corresponds to a surface-averaged measurement ascribable to the behavior of the whole electrode surface. Thus, local impedance measurements were performed on the model Al/Cu couple to analyze the role of the inhibitors on the galvanic coupling. It was shown that the galvanic coupling responsible for the corrosion process is strongly limited by mixing the two compounds which is not the case when the inhibitors are used separately. The synergistic effect was explained by the two different mechanisms by which BTA or 8-HQ inhibit corrosion: the action of BTA is mainly limited to the copper-rich intermetallic particles; 8-HQ acts on the aluminum matrix due to its chelating properties.
In the present work, the same methodology was proposed to evaluate and to compare different compounds known for their effectiveness as corrosion inhibitors for the 2024 aluminum alloy [4-11]. The electrochemical measurements were carried out in a neutral NaCl solution, in presence of 8-hydroxyquinoline (8-HQ), benzotriazole (BTA), mercaptobenzothiazole (MBT) and a cerium salt. The compounds were used alone or as inhibitive mixtures and for various concentrations. Current-voltage curves and impedance measurements confirmed that the compounds are effective inhibitors for the aluminum alloy. Performing local electrochemical impedance measurements on the Al/Cu model allow to better understand the action of each compound on the galvanic coupling. The protective properties of the corrosion inhibitors were compared and discussed.

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