Electrochemical Performance and Corrosion Protection of Zinc–Carbon Quantum Dot (Zn–CQD) Nanocomposite Coatings on Mild Steel

Abstract

Corrosion of mild steel in chloride-containing environments remains a major challenge for industrial infrastructure, necessitating the development of advanced protective coatings with enhanced electrochemical stability and durability. This study investigated the electrochemical performance and corrosion protection behavior of electrodeposited zinc–carbon quantum dot (Zn–CQD) nanocomposite coatings on mild steel. Carbon quantum dots (CQDs) were incorporated into a zinc sulfate electrolyte at concentrations of 0.0, 0.5, 1.0, 1.5, and 2.0 g L⁻¹, and the resulting coatings were evaluated using open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), microhardness, atomic force microscopy (AFM), water contact angle measurements, and ASTM B117 salt spray testing. Electrochemical results revealed a progressive reduction in corrosion current density from 18.6 μA cm⁻² for bare mild steel to 1.12 μA cm⁻² for the Zn–CQD-1.5 coating, while the charge-transfer resistance increased from 438 Ω cm² to 5215 Ω cm². Similarly, polarization resistance increased from 420 Ω cm² to 4978 Ω cm², accompanied by a reduction in corrosion rate from 0.214 to 0.013 mm year⁻¹, representing approximately 94% corrosion protection efficiency. Surface characterization demonstrated that CQD incorporation refined the coating microstructure, reducing average surface roughness from 198 nm for bare steel to 38 nm, while increasing Vickers microhardness from 182 HV₀.₁ to 291 HV₀.₁. The water contact angle increased from 61° to 108°, indicating enhanced hydrophobicity, whereas the corroded area after 500 h salt spray exposure decreased from 88% for bare steel to only 2.5% for Zn–CQD-1.5. However, excessive CQD loading (2.0 g L⁻¹) produced slight nanoparticle agglomeration that marginally reduced corrosion resistance. Overall, the Zn–CQD-1.5 nanocomposite coating exhibited the optimum combination of electrochemical stability, mechanical performance, and long-term corrosion protection, demonstrating the considerable potential of carbon quantum dots as sustainable nanoscale reinforcements for high-performance zinc protective coatings on mild steel