Tannic acid loading into polyurethane and epoxy coatings: Active corrosion protection of AA2024 alloy

Abstract

Driven by the recent discovery of tannic acid (TA) as an effective green corrosion inhibitor for AA2024 alloy, this study evaluates its suitability as an additive for epoxy (EP) and polyurethane (PU) coatings, both through direct addition and by immobilization into layered double hydroxides (LDH-TA). LDH-TA was characterized by XRD, FTIR, and UV-visible spectrophotometry. Also, its anticorrosion performance was evaluated using EIS. The results show that the direct addition of TA to EP and PU coatings reduces their barrier properties, whereas incorporating LDH-TA into the polyurethane matrix provides superior corrosion protection compared to the epoxy coating. Therefore, LDH-TA-loaded PU coating (PU-LDH-TA) was further characterized through rheology analysis, surface characterization, release studies, and electrochemical techniques (EIS and SVET). The findings indicate that after 28 days, |Z| at 10⁻² Hz for PU-LDH-TA remains over one order of magnitude higher than PU. Besides, upon coating defects, tannate anions released from the polyurethane coating form a protective Al/tannate-rich hy- droxide layer at the defect site. Thus, PU-LDH-TA demonstrates effective active corrosion protection in saline media while maintaining the structural integrity of the coating