The approach used in the present work involves investigating the corrosion protection properties of mixed inhibitors for copper and aluminium substrates in chloride-containing solutions, which serve as a benchmark for studies of the alloy AA2024, with Cu and Al being the main culprits for localized corrosion. A synergistic mixture of inhibitors could find potential applications in novel blending combinations, such as in cooling water as an inhibitor in closed systems or incorporated in various protective coatings as additives, nano-containers, etc. If possible, the protective inhibitor film should show irreversibility of inhibition which can be defined as the ability to, once formed, retains its protective properties when the concentration of the corrosion inhibitor decreases. This irreversibility of the protective properties is essential for long-term protection.

Inhibitory action of organic molecules, 2-mercaptobenzimidazole (MBI) and octylphosphonic acid (OPA) and their binary combinations on aluminium, copper and aluminium alloy 2024-T3 was investigated in chloride environments by conventional electrochemical methods and surface-analytical techniques [1,2]. In addition, the influence of pre-treatment of the metal surface and the choice of solvent for liquid-phase deposition on adsorption of MBI and OPA was studied on individual metals, Al and Cu [3]. Although OPA is not an inhibitor for Cu, it can synergistically boost corrosion inhibition of copper when added to MBI. In contrast, a synergistic effect between MBI and OPA as corrosion inhibitors was not observed for AA2024-T3. The mechanism was proposed where the thickness and structure of the surface layer are dependent on the pH. For the sample exposed to MBI at pH 5.5, where the Cu₂O is stable, a thin Cu(I)MBI film is formed.

In contrast, when exposed to the mixture of MBI and OPA at a pH of 4, the amount of produced Cu⁺ ions is boosted, and a much thicker Cu(I)MBI film forms by dissolution-precipitation mechanism. This layer exhibits high inhibitory effectiveness on copper substrate. At even lower pH, the thick compact Cu(I)MBI film does not form due to intensive dissolution of the Cu₂O underlayer, resulting in a voluminous product. The postulated mechanism is confirmed by electrochemical data and composition of the layer by X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy and focused-ion beam scanning electron microscopy with chemical analysis.

Immersion of AA2024 in an OPA-containing solution caused significant localized corrosion, while no local electrochemical activity on AA2024 was detected in an MBI-containing solution, indicating that the MBI inhibitor was very effective against pitting corrosion. Figure shows FIB/SEM (cross-section) analysis of local corrosion induced by Al₂CuMg phase after 24 h immersion of AA2024 in 3 wt.% NaCl containing 1 mM MBI. The chemical analysis employed at the cross-section (yellow rectangles) of the Al₂CuMg revealed that the MBI layer reduces the dissolution rate of dealloying of this phase and the rate of oxygen reduction on the copper remnant sites.

This study shows that the behaviour of each combination of inhibitor and metal substrate is unique and cannot be translated to the more complex system such as alloy. Therefore, a profound understanding of the inhibition mechanism of individual metals is a prerequisite for further investigation of the corrosion inhibition of aluminium alloys.

Acknowledgements: The financial support of the project by the Slovenian Research Agency is acknowledged (grants No. P1-0134, P2-0393 and BI-US/22-24-140) is acknowledged. Barbara Kapun, BSc, is acknowledged for FIB-SEM-EDS analysis.

References:

[1] D.K. Kozlica, A. Kokalj, and I. Milošev, Corros. Sci., 182 (2021) 109082

[2] D.K. Kozlica, J. Ekar, J. Kovač, and I. Milošev, J. Electrochem. Soc., 168 (2021) 031504

[3] D.K. Kozlica, and I. Milošev, to be submitted.