Electrochemical Evaluation of Octacalcium Phosphate as a Green Corrosion Inhibitor in 1.0 M HCl

Calcium phosphates represent a promising class of materials for environmental and industrial applications due to their biocompatibility, low toxicity, and chemical stability. Although numerous organic and inorganic green corrosion inhibitors have been reported, the use of synthetic octacalcium phosphate (OCP) as an environmentally friendly inorganic inhibitor for carbon steel in acidic media remains largely unexplored. In this context, the present work investigates the corrosion inhibition performance of OCP synthesized by the coprecipitation method for protecting carbon steel degradation in a 1.0 M HCl solution, a medium commonly employed in industrial pickling and cleaning processes. The synthesized material was characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), and chemical analysis, confirming the successful formation of phase-pure OCP without detectable secondary phases. The anticorrosive performance of OCP was evaluated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Electrochemical measurements, performed under reproductible experimental conditions, demonstrated that OCP significantly reduced the corrosion rate of carbon steel in acidic medium, achieving a maximum inhibition efficiency of 85.8% at an optimal concentration of 50 ppm. Polarization results revealed that OCP acts as a mixed-type inhibitor by suppressing both anodic metal dissolution and cathodic hydrogen evolution. Furthermore, the increase in charge transfer resistance, together with the higher apparent activation energy, suggest that the inhibition mechanism involves the adsorption of OCP on the carbon steel surface through interactions of its oxygen-containing phosphate groups, promoting the formation of a protective layer that enhances the corrosion resistance of the substrate. Owing to its composition based on abundant, non-toxic calcium phosphate compounds, OCP represents a sustainable, efficient, and environmentally benign alternative to conventional corrosion inhibitors for carbon steel protection in acidic industrial environments.