Tailoring Polycarboxylate Copolymers as High-Performance Corrosion Inhibitors: The Synergistic Interplay of Functional Groups, Composition, and Counterions

Abstract

Corrosion of steel reinforcement is a critical failure mode in concrete structures. This study aims to tailor polycarboxylate copolymer colloids as high-performance corrosion inhibitors and elaborate on the factors affecting their performance against chloride-induced conditions in simulated concrete pore solution (CPS) containing saturated Ca(OH)₂ and 0.3 M NaCl. The copolymers were synthesized via free-radical polymerization of methacrylic acid (MAA) with acrylamide, 2-acrylamide-2-methylpropane sulfonic acid (AMPS), or hydroxyethyl methacrylate (HEMA) at MAA to comonomer molar ratios of 1:1, 2:1, and 6:1, and neutralized with either NaOH (Na⁺) or monoethanolamine (MEA). The effects of functionality (—COOH, —OH, —CONH₂, and —SO₃H), molar ratios, and counterions on the inhibition efficiency (η%) of copolymers for carbon steel immersed in a CPS were assessed electrochemically through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), alongside surface characterization (SEM, AFM). The key finding was a cation-dependent reversal in η%. When neutralized with Na⁺, the copolymer with sulfonic acid groups (MAA-co-AMPS (1:1)) performed best, achieving η: 90.5 % from polarization and a correspondingly high charge transfer resistance, indicating 92.9 % inhibition via EIS. In contrast, neutralization with MEA favored the hydroxy-functional copolymer (MAA-co-HEMA (1:1)), which showed η: 89 %. This shift is attributed to distinct adsorption mechanisms: chemisorption for Na⁺ salts versus a competitive/cooperative mechanism for MEA. Furthermore, the 1:1 molar ratio was optimal, providing better chain rotation, flexibility, and a high density of functional groups for adsorption. Surface analysis confirmed the formation of a uniform, smooth passive layer by the top-performing MAA-co-AMPS (1:1)/Na⁺ system. This work demonstrates that synergistic control of copolymer composition and neutralizing cation is essential for designing advanced green corrosion inhibitors.