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Abstract: Green biosynthesis of ZnO nanoparticles from Suaeda corniculata extract as a bioactive additive for corrosion-resistant zinc electrodeposition

Corrosion of metallic structures remains a major economic and industrial burden worldwide, causing production interruptions, structural failures, and costly replacement of degraded equipment. Zinc electrodeposition is one of the most widely used methods for protecting mild steel, and coating performance can be substantially improved by incorporating additives into the electrolytic bath. However, most additives used commercially are synthetic organic compounds whose toxicity limits their broader application, motivating a shift toward natural, biodegradable, and environmentally friendly alternatives that can play the same functional role without the associated hazards. This work reports the green biosynthesis of zinc oxide nanoparticles (ZnO NPs) using the aqueous extract of Suaeda corniculata, a salt-tolerant halophyte plant rich in polyphenolic compounds (18.45 ± 0.03 mg GAE/g), which act as natural reducing and stabilizing agents during nanoparticle formation, avoiding the need for hazardous chemical reagents. The biosynthesized ZnO NPs were characterized using four complementary techniques. UV-visible spectroscopy revealed a characteristic absorption band at 370-390 nm, while Fourier-transform infrared spectroscopy confirmed a Zn-O vibration band near 438 cm⁻¹. Scanning electron microscopy showed irregular agglomerates of nanometric grains with spherical and hexagonal morphologies. Energy-dispersive X-ray spectroscopy confirmed Zn and O as the principal elemental constituents, with the Zn:O mass ratio increasing from 41.5:58.5 to 66.6:33.4 after calcination. These biosynthesized nanoparticles were then evaluated as additives in the zinc electrodeposition bath used to coat E24-2 mild steel. Coating parameters, including current density, bath pH, and NPs-ZnO concentration, were systematically optimized, with the best deposit quality obtained at pH 4.5 and 0.09 A. Corrosion resistance of the coated steel was assessed under realistic conditions in natural seawater, using open-circuit potential monitoring, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. The addition of ZnO NPs markedly improved deposit adhesion, thickness, and brightness compared with the additive-free bath, and shifted corrosion potentials toward more cathodic values. EIS measurements showed a maximum inhibition efficiency of 87.20% at a concentration of 1.2 g/L, closely corroborated by potentiodynamic polarization data, which indicated 86.39% inhibition efficiency at the same concentration, together with a marked reduction in corrosion current density and coating porosity. These findings demonstrate that plant-mediated ZnO nanoparticles, obtained through an inexpensive and eco-friendly biosynthesis route, act as effective multifunctional additives for zinc electroplating baths, simultaneously enhancing coating morphology and corrosion resistance. This approach offers a sustainable, low-cost alternative to toxic synthetic additives, and opens promising perspectives for the metal-coating and marine-protection industries, connecting green nanotechnology with the biochemical valorization of underexploited halophyte flora from arid and saline environments such as those of Algeria.