Cellulose nanocrystals-reinforced waterborne epoxy coatings with enhanced corrosion resistance for steel

The practical applications of waterborne epoxy coatings are limited due to their poor barrier properties caused by the formation of numerous micropores and defects during the curing process. Herein, cellulose nanocrystals (CNCs)-reinforced waterborne epoxy coatings were fabricated by the direct addition of 0.2-1.0 wt% CNCs to waterborne epoxy emulsion followed by amine curing agent addition and spray coating. The incorporation of 0.2-0.5 wt% CNCs had no discernible impact on the stability of the waterborne epoxy emulsion. This led to the uniform dispersion of CNCs in the cured coating matrix, as evidenced by differential scanning calorimetry analysis. Because of the good compatibility, 0.2-0.5 wt% CNCs-reinforced epoxy coatings exhibited superior corrosion protection performance for steels. The impedance modulus of these coatings remained at 10⁸ Ω cm² after being immersed in a 3.5 wt% NaCl solution for 21 d. The hydroxyl groups present on the CNC surface undergo a reaction with the epoxy group, enhancing intermolecular interaction and leading to the formation of a defect-free dense structure that improves coating barrier properties. However, the incorporation of an excessive amount of CNCs (i.e., 0.8 and 1.0 wt%) significantly compromised the corrosion resistance of epoxy coatings due to aggregation-induced coating defects. Overall, this study provides a facile and green strategy for corrosion resistance improvement of waterborne epoxy coatings.