Corrosion inhibition evaluation of sulphur-doped pomegranate peel waste-derived carbon dots for carbon steel in acidic environment

Sulphur-doped carbon dots (S-PPCDs) were synthesized from pomegranate peel waste through a one-step hydrothermal process and explored as green corrosion inhibitors for carbon steel in 5% hydrochloric acid. The nanomaterials were systematically characterized using TEM, FTIR, UV–Vis, SEM/EDAX, and fluorescence analysis, which confirmed their nanoscale dimensions (∼8.9 nm), spherical morphology, and the presence of oxygen-, nitrogen-, and sulphur-containing functional groups that promote adsorption. Corrosion inhibition performance was evaluated through gravimetric tests, electrochemical methods (EIS, LPR, PDP), and surface characterization (SEM, AFM, optical profilometry). The results reveal a strong concentration dependence, with maximum inhibition efficiency of ∼89.8% at 100 mg l -1 and 30 °C. At higher concentration (150 mg l -1 ), a slight decrease in efficiency was observed, attributed to multilayer formation or competitive adsorption. Elevated temperature (60 °C) reduced protection efficiency to ∼47%, indicating that the adsorption mechanism is predominantly physical. Potentiodynamic polarization results show that S-PPCDs act as a mixed-type inhibitor, reducing both anodic metal dissolution and cathodic hydrogen evolution. Surface analysis confirms the formation of a compact, adherent inhibitor layer that significantly reduced roughness and pitting compared to uninhibited samples. The findings highlight the dual benefits of waste valorization and sustainable corrosion protection, positioning S-PPCDs as an environmentally benign, low-cost, and highly efficient alternative to conventional toxic inhibitors for acidic environments.