Inhibition of Corrosion of Aluminum and 2024 Alloy : DFT Experimental approaches – ICAADE

The phenomena will be described thanks to:
- A modelling using quantum physico-chemistry methods, which provide a high degree of precision in the description of the metal systems and aim at including/understanding the physical and chemical interactions between systems (atomic scale),
- Electrochemical techniques (polarization curves, local and global electrochemical impedance measurements) will be used to assess the corrosion inhibitor efficiency for different experimental conditions (chloride concentration, pH of the solution),
- Surface analysis (more specifically XPS, AFM/KFM and raman microscopy) will complement the electrochemical data regarding the adsorption of the corrosion inhibitor.

Simulation (Density Functional studies) and experimental (surface analysis and electrochemical tests) approaches were combined.
The modeling at the atomic scale of the interactions between the organic molecules and aluminum constituted the major part of this work and concern the 8-hydroxyquinoline (8-HQ) as corrosion inhibitor. We ran calculations in the framework of DFT and we opted for a correction to the DFT energy by a term simulating the part of the weak interaction (D: Dispersed). Studies were made for several surface coverage, from one molecule to a total coverage.
Electronic exchanges with the metal were studied in details to describe the inhibitors/metal bonding (Bader population analysis, plot of electronic densities). The Van der Waals energy at the metal/molecules interface and between the molecules in the organic layer were calculated. We demonstrated that the organic layers are stable and strongly chemisorbed on the aluminum surface. Interactions are of covalent type on the one hand but dispersive interactions also take part in the adsorption process (representing 20 to 50% of the energy of interaction molecules/metal). We have also shown that a compact layer of the dehydrogenated species of the inhibitor can limit the reduction of dissolved oxygen on the aluminum surface. Studies taking into account the presence of an hydroxylated alumina layer on the aluminum surface are currently under progress.

In parallel, electrochemical techniques (polarization curves plot and impedance measurements) and XPS surface analysis were used first to determine the efficiency of the 8-hydroxyquinoline and its adsorption on the bare aluminum surface and, secondly to characterize the Al/molecules interface. Electrochemical studies and XPS surface analysis were performed for different pH values of the solution in contact with the aluminum surface.

The understanding of the corrosion inhibition mechanisms will help to guide the search of new species and new processes that could provide corrosion protection close to the corrosion protection afforded by chromate species.

The results obtained during this project led to 2 already published publications, 1 submitted publication and 2 publications are in preparation. Focus has been on a strong scientific dissemination in international congresses (3 oral presentations (1 invited) and 2 poster presentations) and national conferences (3 oral presentations and 3 poster presentations), enabling young scientists (Fatah Chiter Ph.D. and Marie-Laure Bonnet postdoctoral student) to greatly expand their network.

Since the beginning of the 1990s, the high toxicity associated with chromates has imposed restrictions on their use in industrial applications. As a consequence, intense research efforts are being undertaken to find new environmentally friendly compounds as corrosion inhibitors of aluminum and aluminum alloys.
This project aims to investigate, at the atomic scale, the corrosion inhibition mechanisms affecting aluminum and aluminum alloys, in particular the 2024 aluminum alloy currently used for aerospace applications. The efficiency of corrosion inhibitors is generally based on electrochemical tests (polarization curves, electrochemical impedance measurements). We propose to determine the mode of action of efficient corrosion inhibitors (8-hydroxyquinoline and one carboxylate) by using ab initio calculation. We want to correlate the theoretical approach to experimental approaches (electrochemical data and surface analysis) in order to extract the factors which are favourable to the inhibition processes.