Bionanocomposite coatings incorporating carbon-sepiolite within biopolymer matrices for corrosion protection of aluminium alloys: An electrochemical study

This study reports the development of novel, sustainable bionanocomposite coatings for AA2024-T3 aluminium alloy, achieved by integrating eco-friendly carbon–sepiolite nanofillers within chitosan and zein biopolymer matrices. The carbon–clay filler was prepared by impregnating a suspension containing multiwalled carbon nanotubes and liquid caramel into sepiolite clay, being the graphitization of caramel performed by hydrothermal treatment (180 °C for 18 h) followed by pyrolysis (550 °C for 1 h). The resultant carbon-sepiolite filler was blended with chitosan or zein matrices to obtain bionanocomposite suspensions. Thin bionanocomposite layers were deposited by dip-coating on the metal aluminium alloy surfaces, and selected samples were sealed with a hybrid organic–inorganic sol–gel topcoat to eliminate inherent porosity and boost passive corrosion resistance. The hybrid coating was prepared from a mixture of tetramethyl orthosilicate (TMOS) and γ-methacryloxypropyltrimethoxysilane (MAPTMS), used as alkoxysilane precursors. Electrochemical impedance spectroscopy (EIS) as a rigorous, non-destructive methodology for assessing the coatings' protective performance against corrosion. EIS analysis revealed a clear two-stage degradation and protection mechanism: initially, the sol–gel layer provides a robust passive barrier, and upon its gradual deterioration, the underlying bionanocomposite film activates active corrosion inhibition via physical obstruction and controlled release of functional additives. Field-emission scanning electron microscopy confirmed the integrity, uniformity, and adherence of both monolayer and bilayer systems before and after immersion tests. Electrochemical studies revealed that the degradation process of these coatings occurs in two distinct stages. In the initial stage, the top sol-gel coating primarily provides a barrier effect. Once this layer deteriorates, an active corrosion protection mechanism from the bionanocomposite film is activated. These results demonstrate that the synergy between advanced material design and electrochemical characterization can yield high-performance, environmentally responsible coatings. The innovative carbon–sepiolite bionanocomposite approach, validated by EIS, offers superior corrosion resistance and represents a promising alternative to conventional chromate-based inhibitors in demanding engineering applications.