Eco-Friendly Sustainable Composites: Polyaspartic Acid–Nitrogen-Doped Carbon Dots for Corrosion Inhibition of Carbon Steel in Saline Acidic Media

Developing high-performance corrosion inhibitors from sustainable and biodegradable sources is crucial for green metal protection strategies, particularly in aggressive saline and acidic environments. In this work, we report a hybrid corrosion inhibitor based on polyaspartic acid grafted with nitrogen-doped carbon dots (PAA/N-CQDs) derived from banana peel waste. The N-CQDs were synthesized via a hydrothermal route and covalently integrated into a polyaspartic acid backbone, yielding a nanostructured composite with abundant amino and carboxylate functionalities. The structure and physicochemical properties of the components and hybrid were confirmed by UV−vis spectroscopy, photoluminescence, XRD, TEM, TGA, FT-IR, and 1H NMR analysis. Electrochemical measurements (weight loss, LPR, PDP, and EIS) in 1 M HCl containing 3.5 wt % NaCl demonstrated excellent inhibition performance toward C1018 carbon steel, with a maximum efficiency of 94.4% at only a 30 ppm inhibitor concentration. EIS and adsorption studies showed that the hybrid inhibitor forms a compact, strongly adherent film that follows Langmuir adsorption behavior and involves a mixed physisorption and chemisorption mechanism. SEM observations corroborated the formation of a protective layer that markedly suppresses surface damage. By combining a low inhibitor dosage, a waste-derived carbon nanomaterial, and high efficiency in a highly corrosive saline acidic medium, this study proposes a scalable and environmentally friendly strategy for carbon steel protection that aligns with circular economy principles.