Kinetics and mechanism of gallic acid as an ecofriendly corrosion inhibitor for steel rebars in mortar

Chloride-induced corrosion of steel rebars embedded in mortar was effectively controlled by blending of gallic acid in wet mixture. Mixing of optimized concentration of gallic acid (GA) inhibitor (0.125%) in mortars considerably increased the charge transfer resistance of embedded rebars (80.11 Ω cm²) in comparison to that noted for the control mortars (3.36 Ω cm²) after 37 cycles of wet/dry treatments. The corrosion inhibition efficiency after this cycle of exposure was of the order of 95%. The studied inhibitor also improved the workability (% flow of wet mix) from 25% for control to 43% at its optimum dosing. It is suggested that GA transforms into negatively charged gallate anions in an alkaline-pore solution of concrete and electrostatically attracted to the positively charged calcium ions, forming a complex with a partial positive charge. This complex interacts with the negatively charged maghemite film formed on rebar surface and forms a protective film of FeO⁻ Ca⁺⁺– GA anion, isolating it from the aggressive electrolyte. The experimental results showed that Ca⁺⁺ ion of the concrete had a strong synergistic effect in boosting the inhibitive performance of the GA. The inhibition efficiency increased from 84.1% in the presence of GA only to 97.9% for the mixture of GA and calcium ions.