Quaternary Ammonium–Alkyl Functionalized Biomass-Derived Carbon Dots as Green Corrosion Inhibitors for N80 Steel in Acidic Medium

Carbon steel is highly susceptible to severe corrosion in acidic environments, making the development of efficient and environmentally friendly corrosion inhibitors essential for equipment protection and service life extension. In this work, nitrogen-doped carbon dots (CP-NCDs) were synthesized from natural citrus peel via a hydrothermal doping method. Hydrophobic surface modification was further achieved by introducing quaternary ammonium groups and C14 alkyl chains, yielding CP-NCDs-Q14. Characterization results confirmed the successful incorporation of quaternary nitrogen and long hydrophobic chains through this structural modulation strategy. The corrosion inhibition performance of the two carbon dots in 1 M HCl was systematically evaluated using weight loss and electrochemical measurements. CP-NCDs-Q14 exhibited superior performance, with maximum inhibition efficiencies of 97.52% at 303[Formula: see text]K and 94.58% at 333[Formula: see text]K, outperforming the unmodified CP-NCDs. Surface morphology and elemental distribution analyses confirmed the formation of a compact protective film on the steel surface, effectively preventing corrosion. Langmuir isotherm fitting indicated spontaneous monolayer adsorption for both carbon dots, involving combined physical and chemical interactions. Mechanistic studies revealed that nitrogen-containing groups coordinate with Fe atoms, while the quaternary ammonium–alkyl structure enhances hydrophobic barrier effects. These synergistic features significantly improve corrosion resistance. This work provides new insights into the structural design of biomass-derived carbon dots and offers a promising strategy for the green development of high-efficiency corrosion inhibitors.