The new insights towards smart pH-responsive anticorrosion coating based on HNTs carrying synergistic inhibitors: Experimental and molecular simulation

Corrosion of steel structures in reinforced concrete structures in coastal areas has long been an urgent concern. In this study, a combined corrosion inhibitor, consisting of benzotriazole (BTA) and sodium D-gluconate (SD), was loaded onto halloysite nanotubes (HNTs) using the vacuum negative pressure method. Then, the HNTs were encapsulated with biomass polyelectrolytes sodium alginate (SA) and chitosan (CS) by layer-by-layer self-assembly technique to prepare BTA/SD-HNTs@CS/SA (FHNTs) composite. Furtherly, a functional epoxy coating (FEP) with pH-responsive and long-term service ability was successfully fabricated. It is found that BTA/SD was released in large quantities under alkaline (pH = 11) and acidic (pH = 2) conditions, while essentially no release under neutral condition. The electrochemical impedance spectroscopy (EIS) measurements demonstrate that the corrosion protection ability of FEP was significantly stronger than pure epoxy coating (EP), and the addition of 2 % FHNTs having the most excellent anticorrosion performance with the maximum charge transfer resistance (Rct) value (4.8 x 106 omega cm2) after 4 weeks of immersion, which was 2 orders of magnitude greater than the pure EP coating. Particularly, molecular dynamics (MD) simulations indicate that FEP coating has better barrier properties than EP, while the fractional free volume (FFV) value of H2O and Cl- in EP was 9.1 % and 5.1 %, respectively, whereas this value decreased by about 15 % to 7.7 % and 4.4 % in FEP. The results of density functional theory (DFT) calculations suggest that the released BTA and SD molecules have a synergistic adsorption effect on the surface, which can form a compact film to hinder the invasion of corrosive species. Overall, this study provides new insights for developing smart pH-responsive and superior anticorrosion coatings.