Chloride-induced corrosion protection of cement mortar-embedded steel reinforcement through temperature-optimized innovative coatings

Chloride-induced corrosion of steel reinforcement bars represents a critical durability concern for concrete infrastructure in marine environments and structures exposed to de-icing salts, necessitating effective protective coating systems for construction materials applications. While conventional hard chromium coatings provide protection, their environmental toxicity and application limitations have created demand for sustainable alternatives. This study developed and optimized temperature-controlled electroless nickel-phosphorus (Ni-P) and nickel-phosphorus-tungsten (Ni-P-W) coating systems for steel reinforcement bars embedded in cement mortar under aggressive chloride exposure conditions. Coatings were deposited via autocatalytic (electroless) chemical reduction from acidic nickel sulfate baths (pH 4) using sodium hypophosphite as reducing agent, producing 17 to 25 μm thick uniform coatings on ASTM Grade 40 deformed steel reinforcement bars. Coating deposition was systematically optimized across 75°C, 80°C, and 85°C temperatures using acidic electroless baths, followed by embedment in standard 1:3:0.45 cement mortar and exposure to 3.5 wt.% NaCl solution simulating marine construction environments. Comprehensive materials characterization through X-ray diffraction, scanning electron microscopy, and surface analysis confirmed efficient coating formation with amorphous Ni 3 P and crystalline NiW 2 P 3 phases. Electrochemical evaluation via potentiodynamic polarization and linear polarization resistance demonstrated exceptional corrosion protection, with Ni-P coatings achieving 99% corrosion rate reduction by LPR (98% by PDP) and Ni-P-W systems delivering 96% improvement by LPR (95% by PDP) compared to uncoated reinforcement bars. These findings demonstrate electroless coating technologies as environmentally sustainable and highly effective alternatives for enhancing concrete infrastructure durability in aggressive chloride environments, with direct applications for marine construction and transportation infrastructure systems.