Long-lasting anti-corrosion of waterborne epoxy resin synergistic modified with g-C3N4 and pH-responsive polypyrrole nanotubes

Abstract

A self-healing coating containing g-C₃N₄-PPynts(BTA)@PDA (CPBP) hybrids was developed to inhibit corrosion in steel. Lamellar g-C₃N₄ and polypyrrole nanotubes were synthesized through direct pyrolysis and the methyl orange-iron chloride reactive self-degradation template method, respectively. Subsequently, benzotriazole (BTA) was incorporated into PPy nanotubes (PPynts) using the vacuum negative pressure technique. PDA (polydopamine) was then polymerized on the nanotube surface acting as a ‘gatekeeper’. Finally, g-C₃N₄-PPynts(BTA)@PDA hybrids were formed by attaching PPynts(BTA)@PDA(PBP) nanocontainers to g-C₃N₄ nanosheets. The synthesized polypyrrole nanotubes exhibited superior loading capacity because of their larger size and could release encapsulated inhibitors in response to pH changes in the microenvironment. In addition, the variations in |Z|_0.01Hz values of CPBP/WEC after 40 days of immersion in 3.5 wt% NaCl solution were evaluated using the EIS test, and the results showed that the failure time of CPBP/WEC was prolonged by 15 days compared with that of WEC. Moreover, the EIS test results at different pH conditions showed that the |Z|_0.01Hz values decreased from 2.601 × 10⁹ Ω cm² to 4.9 × 10⁷ Ω cm² after 40 days of immersion at pH = 3, and decreased from 3.168 × 10⁹ Ω cm² to 7.7 × 10⁷ Ω cm² at pH = 11. The |Z|_{0.01 Hz} values were both two orders of magnitude lower compared to pH = 7, but still three orders of magnitude higher than the WEC at pH = 7, which exhibited high impedance values. Meanwhile, CPBP/WEC were found to have significant self-healing properties using EIS analysis of the scratch coatings, salt spray testing, SEM and EDS analysis of the corroded areas. The experimental results indicate that the incorporation of CPBP provides a new method to improve the service life of epoxy resin.