Active corrosion protection of epoxy coating filled with surface-engineered cellulose nanocrystals with polyaniline

Cellulose nanocrystal (CNC) grafted from polyaniline (PANI) doped with phosphate (PO₄^3-) anion, designated here by C-pP, was employed as an active anticorrosive nanoparticle in epoxy (EP) coating at a concentration of 1 wt% for the mild-steel substrate. The presence of the phosphate agents on the backbone of PANI was confirmed by Fourier transform infrared spectroscopy (FT-IR) and further supported by thermogravimetric analysis (TGA). Electrochemical impedance spectroscopy (EIS) measurements for coatings with an artificial scratch revealed that EP/C-pP coating can provide an inhibition efficiency of up to ~69 %, about twice that of EP/PANI coating. Furthermore, the adhesion of the optimal sample was 3 MPa, more than those of EP/PANI and EP coatings. Additionally, the presence of C-pP drastically improved the hydrophobicity of the epoxy coating with an increase in its contact angle from 42.4 ̊ to 92.4 ̊. The superior anticorrosion property of EP/C-pP was attributed to the presence of CNC, which promoted the dispersion of PANI molecules within the EP matrix (confirmed by FE-SEM); resulting in 1) improved barrier effect against corrosive agents, and 2) self-healing property arising from the smart release of phosphate dopants from the PANI backbone under basic conditions and forming protective complexes at the infected areas.