Towards Sustainable Corrosion Protection: An Investigation into Ecofriendly Sol-Gel Conversion Coatings for AZ61 Magnesium Alloy
Authors: Garcia-Galvan Federico R. , López Sánchez Jesús , Barranco Violeta , Llorente Irene , Fajardo Santiago , Galvan Juan Carlos , Carmona Noemi 
Published: December 2024

The sustainability and corrosion protection are two critical factors in the field of materials for mobility. Lightweight alloys, such as the AZ61 magnesium alloy, are gaining recognition for their potential applications in the aerospace, automotive, and marine industries. The use of these alloys not only contributes to weight reduction and consequent fuel and CO2 emission savings, but also opens up new possibilities for innovative design and performance enhancements [1].
In search for sustainable corrosion protection, we have focused our research towards the development of environmentally acceptable sol-gel coatings enriched with green corrosion inhibitors. These coatings not only prolong the lifespan of materials made with the AZ61 magnesium alloy but also align with the commitment to environmental stewardship, offering promising prospects for effective corrosion protection [2].
The sol-gel coatings were synthesized using tetraethyl orthosilicate (TEOS) and 3-(trimethoxysilyl)propyl methacrylate (MAPTMS) as precursors. Four different organic inhibitors - L-cysteine (CYS), N-acetyl-cysteine (N-A-CYS), curcumin (CUR), and methylene blue (MB) - that are environmentally friendly, non-toxic, inexpensive and contain S, N heteroatoms, O and/or OH groups and/or conjugated double bonds - were selected for the study. These compounds were incorporated into the sol as dopants. A set of sols was doped with a common corrosion inhibitor, benzotriazole (BTA), for comparison purposes. The resulting sols were processed and deposited on AZ61 substrates using the dip-coating technique.
The thickness of the coatings was determined using the interference fringe method in the ultraviolet-visible and near-infrared ranges [2]. Transmission spectra of coated glass samples were also obtained to determine the wavelength of each coating at 50% transmission. The hydrophilic character of the coatings was characterized by measuring the contact angle.
To study the corrosion behaviour of the coated surfaces, weathering tests, based on a variation of the ISO 11130 [3], were conducted. The samples were weighed before and after the corrosion test to examine weight variations. Optical microscopy provided insights into the surface of the samples both before and after the corrosion test. Detailed observations of the surface morphologies of the samples were made using SEM, and EDX analyses were performed to verify the compositions of the compounds found. Confocal Raman microscopy was also employed to provide accurate compositional information at a local microscale level, shedding light on the compounds formed considering the chemical strategies developed in the protective sol-gel coating [4,5]. The study was concluded by applying global and localized electrochemical impedance spectroscopies (EIS, LEIS) to study the corrosion protection behaviour of the sol-gel coatings during immersion tests in 0.006 M and 0.6 M NaCl aqueous solutions.
As a concluding remark, it is noteworthy that in several of the tested sol-gel coatings, micro-cracks and defects formed during weathering tests self-seal, thanks to oxy-hydroxides adhering to the Mg alloy substrate. Additionally, in response to the corrosive anions of the aqueous solution, due to the release of the organic inhibitors that were nanoencapsulated in the sol-gel matrix, an active corrosion protection is initiated beneath the coatings. In conclusion, the tested eco-friendly coatings, applicable by dip-coating or spray at room temperature, offer potential for broad industrial use and economic feasibility, suggesting industrial scale-up feasibility.

Funding Sources

This work has been supported by the Projects PID2022-139920OB-I00, PID2021-126323OA-I00 and TED2021-129688-CT21 (Ministry of Science, Innovation and Universities, MICINN, Spain). The team extends its acknowledgements to Miguel Romero Martín of the ECORR Group for his help with impedance measurements and result analysis.

References

1. Kumar, D., Phanden, R.K., Thakur, L. A review on environment friendly and lightweight Magnesium-Based metal matrix composites and alloys.
Materials Today: Proceedings, 38, pp. 359-364 (2020).
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Gels 10 (3), art. no. 168 (2024).

3. ISO

Standard 11130

; Corrosion of Metals and Alloys – Alternate Immersion Test in Salt Solution. Available online: https://www.iso.org/standard/71867.html (accessed on 18 April 2023).
4. Aparicio, M., Mosa, J., Rodriguez, G., Guzman, J., Picard, Q., Klein, L.C., Jitianu, A. Consolidated Melting Gel Coatings on AZ31 Magnesium Alloy with Excellent Corrosion Resistance in NaCl Solutions: An Interface Study. ACS Applied Materials and Interfaces 11 (3), pp. 3493-3505 (2019)
5. López-Sánchez, J., Serrano, A., del Campo, A., Muñoz-Noval, Á., Salas-Colera, E., Cabero, M., Varela, M., Abuín, M.,Castro, G.R., Rubio-Zuazo, J., Rodríguez de la Fuente, O., Carmona, N. A combined micro-Raman, X-ray absorption and magnetic study to follow the glycerol-assisted growth of epsilon-iron oxide sol-gel coatings.
Journal of Alloys and Compounds 892, art. no. 162061 (2022).