Controlled Synthesis of Hydroxyapatite Coating with Crystal Defects for Enhanced Corrosion Protection

This study aims to address the issue of metal corrosion by employing crystal engineering principles. By synthesizing distinct hydroxyapatite (HAP) crystalline phases through precise control of nucleation and growth conditions, the corrosion resistance of HAP‐based coating is enhanced, with the hexagonal phase Ca_9.868(PO₄)_5.586(OH)_4.006 demonstrating superior protection against corrosion. In addition to the passivation protection mechanisms common to phosphates, the high content of hydroxyl groups in the hexagonal phase of HAP facilitates ion exchange with chloride ions (Cl⁻) from the corrosive environment, leading to the formation of a stable chlorapatite protective layer that significantly enhances the coating's corrosion resistance. Furthermore, the increased crystalline defects in the hexagonal phase can act as nucleation points for the transformation of HAP into chlorapatite, thereby enhancing the stability of the chlorapatite protective layer. Moreover, the increase in crystalline defects and hydroxyl polarization can effectively capture oxygen, reducing electrochemical corrosion processes. The results of electrochemical impedance spectroscopy and polarization curve analysis confirm that the coating with Ca_9.868(PO₄)_5.586(OH)_4.006 achieves the highest impedance, which reflects its optimal corrosion resistance. This study establishes a direct correlation between the crystalline structure of HAP and its anticorrosive properties, providing new insights for applying HAP‐based coatings in metal corrosion protection.