Functionally graded coating (Cu/Zn/Sn) to improve steel bar performance using a modified spray technique: Experimental and simulation study

Abstract

This study presents a novel functionally graded (FGCs) coating materials designed to improve the corrosion resistance of steel rebars embedded in concrete exposed to aggressive chloride environments. A triple-layered Cu/Zn/Sn coating at the nanoscale is applied using a modified spray coating technique that integrates manufacturing efficiency with metallurgical bonding. Reinforced concrete specimens are exposed to a 3.5 wt.% NaCl solution for 28, 56, and 90 days at three different temperatures (298, 308, and 318 K) to assess long-term corrosion performance under simulated marine conditions. Comprehensive surface characterization is performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and microhardness testing. The results confirmed the formation of a dense, well-adhered protective layer with improved surface uniformity. Electrochemical tests, including Tafel polarization and electrochemical impedance spectroscopy (EIS), demonstrated a substantial improvement in corrosion resistance. At 298 K after 28 days of immersion, the coated samples exhibited a corrosion current density of 0.299 × 10⁻⁷ A/cm² and achieved a maximum inhibition efficiency of 97.92%. Additionally, the porosity percentage is extremely low at 1.29 × 10⁻⁷%. The EIS analysis supported these findings, revealing that the highest solution resistance is 21,521 Ω·cm². Furthermore, the coated steel exhibited enhanced mechanical properties, with surface hardness increasing from 195.8 HV (uncoated) to 221.4 HV. These results indicate that this FGM coating provides durable protection against chloride-induced corrosion, offering a practical and cost-effective solution for prolonging the life span and performance of reinforced concrete structures exposed to marine and de-icing environments. Notably, these findings aligned well with DFT-based simulations, which confirmed the coating’s thermal stability and chloride adsorption behavior at the atomic level.