Dual-functional nitrogen-doped carbon dots for protection and fluorescence monitoring of steel corrosion

Carbon steel is extensively employed in industrial systems, yet it suffers severe degradation in aggressive acidic environments. Conventional corrosion inhibitors often suffer from environmental hazards, limited stability, and single-function protection, creating an urgent demand for greener, multifunctional alternatives. Carbon dots (CDs), with superior water dispersibility, abundant surface functional groups, and intrinsic fluorescence, offer unique advantages for simultaneous corrosion inhibition and monitoring. However, most existing works focus only on inhibition or only on sensing, and even dual-functional systems seldom establish a reliable quantitative relationship between fluorescence signals and actual corrosion degree. Herein, nitrogen-doped carbon dots (NCDs) were synthesized via a hydrothermal method using 4-aminosalicylic acid and o-phenylenediamine as precursors at different molar ratios. Structural and optical characterizations confirmed the successful preparation of NCDs with strong photoluminescence. Corrosion inhibition performance of NCDs for Q235 carbon steel in 1 mol/L HCl was systematically evaluated via electrochemical measurements, mass-loss tests, surface analysis, and an adsorption isotherm model. Notably, the as-prepared NCDs, as a mixed-type corrosion inhibitor, exhibit outstanding long-term corrosion protection (a corrosion inhibition efficiency of up to 98% during 240 h exposure) owing to their strong physical and chemical adsorption. Moreover, NCDs possess dual fluorescence responses towards H + and Fe 3+ , and exhibit excellent sensing performance for Fe 3+ with a detection limit of 0.3707 μmol/L, enabling early-stage corrosion monitoring. This study highlights that NCDs can serve as a new class of eco-friendly dual-functional inhibitors integrating efficient corrosion inhibition and corrosion sensing, offering promising prospects for advanced anticorrosion applications in aggressive environments.