Anticorrosive pigment for smart and green protective coatings: An alkyd resin coating incorporating lanthanum-loaded hydroxyapatite, derived from a phosphate mining industry by-product

Intelligent anti-corrosive coatings have gained considerable interest as a method to protect metals. This approach offers an effective strategy for mitigating corrosion by providing advanced control over the process. In this research, lanthanum-loaded hydroxyapatite pigment (La-HAp) was successfully synthesized using phosphate sludge (PS) waste. The pigment was incorporated into an alkyd resin coating to provide smart corrosion protection for mild steel exposed to a saline solution. The synthesized pigments were characterized using various analytical techniques, including FTIR, XRD, and SEM−EDS. The inhibitory performance of La-HAp on mild steel submerged in a 3% NaCl solution was evaluated using linear polarization and EIS techniques. The findings demonstrated substantial inhibitory effectiveness, achieving 82% inhibition efficiency. Further investigation compared the protective capabilities of modified (C-La-HAp) and reference (C-Ref) alkyd coatings on carbon steel using EIS. The results indicated significant corrosion protection enhancement with 5% La-HAp, showing a polarization resistance of 19.7 MΩ·cm² after four weeks of immersion in the aggressive solution. This corrosion resistance is primarily attributed to the formation of a barrier against the corrosive environment and the trapping of Cl^- ions by the HAp, accompanied by the release of La, phosphate, and calcium ions. La ions react with OH^- ions produced in the cathodic zones to form a uniform protective layer of Lahydroxides, while iron orthophosphates provide additional protection at anodic sites. This synergistic blend of mechanisms imparts intelligent corrosion-inhibiting properties to the alkyd resin, effectively guarding against the damaging effects of corrosive agents.