Bionanocomposite Thin Films Based on Eco-Friendly Carbon-Clay Materials for Corrosion Protection of Light Alloys
Authors: Barra Ana , Garcia-Galvan Federico R. , Galvan Juan Carlos , Nunes Cláudia , Ferreira Paula , Ruiz-Hitzky Eduardo 
Published: December 2024

This study aims to develop new environmentally compliant coatings as surface pretreatments for the protection of light alloys. To achieve this goal, chitosan and zein biopolymers were combined with eco-friendly carbon-clay fillers to produce bionanocomposite thin films. Chitosan- and zein-based suspensions were prepared for the synthesis of the bionanocomposite thin films. For the chitosan-based suspension, carbon-sepiolite fillers and/or caffeic acid were dispersed in an acetic acid solution [1]. For the zein suspension, the carbon-sepiolite and/or caffeic acid additives were dispersed in an ethanol/water mixture. The carbon-clay fillers were prepared by impregnating natural sepiolite clay with liquid caramel, followed by thermal treatment under N2 flow at 550ºC for graphitization [2,3]. The resulting suspensions were then deposited on AA2024-T3 aluminium alloy and AZ61 magnesium alloy samples using the dip-coating technique. A post-treatment involving the application of an organic-inorganic hybrid top-coat on a series of bionanocomposite thin films. The purpose was to seal the inherent pores within these layers and enhance their corrosion resistance. The hybrid matrix was prepared by co-hydrolysis and polycondensation of a mixture of γ-methacryloxypropyltrimethoxysilane and tetramethyl orthosilicate by using the sol-gel process. The resulting coatings were characterized using field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis and differential scanning calorimetry (TGA/DSC). The corrosion protection of these coatings was evaluated by open circuit potential (OCP) measurements and electrochemical impedance spectroscopy (EIS) during immersion tests in a 0.06 M NaCl solution. FESEM images acquired on the corrosion test areas confirmed the corrosion protection offered by the bionanocomposite thin films. Electrochemical studies showed that the degradation process of these coatings goes through two stages. Initially, a barrier effect exerted by the sol-gel top-coat predominates. Once this layer has failed, an active protection mechanism against corrosion engaged by the bionanocomposite thin film is activated. As a remarkable conclusion of the study, it should be noted that these bionanocomposite thin films exhibit relevant properties that make them promising green candidates for applications in the automotive and aeronautical industries.

Acknowledgements:

This work was developed within the scope of the projects CICECO UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC) as well as MCIN/AEI/10.13039/501100011033 - PID2019-105479RB-I00 and PID2022-139920OB-I00 projects, Spain. AB and PF are thankful to FCT for grant SFRH/BD/148856/2019 and the Investigator FCT (IF/00300/2015), respectively. CN is grateful to Portuguese national funds (DL 57/2016/CP1482/CT0031).

References:

[1] A. Barra, J.K. Wychowaniec, D. Winning, M.M. Cruz, L.P. Ferreira, B.J. Rodriguez, H. Oliveira, E. Ruiz-Hitzky, C. Nunes, D.F. Brougham, P. Ferreira. Magnetic Chitosan Bionanocomposite Films as a Versatile Platform for Biomedical Hyperthermia.
Advanced Healthcare Materials, 2303861 (2023).

[2] A. Barra, C. Nunes, E. Ruiz-Hitzky, P Ferreira. Green Carbon Nanostructures for Functional Composite Materials.
International Journal of Molecular Sciences, 23 (3), art. no. 1848 (2022).

[3] A. Barra, O. Lazăr, G. Mihai, C. Bratu, C. Ruiz-García, M. Darder, P. Aranda, M. Enăchescu, C. Nunes, P. Ferreira, E. Ruiz-Hitzky, Graphene-like materials supported on sepiolite clay synthesized at relatively low temperature.
Carbon, 218, art. no. 118767, (2024).