Synthesis and electrochemical properties of electroactive aniline-dimer-based benzoxazines for advanced corrosion-resistant coatings

The first high-performance electroactive benzoxazine with an aniline dimer in its structure has been synthesized from 4-aminodiphenylamine (aniline dimer), paraformaldehyde, and phenol. FT-IR and ¹H NMR are used to characterize the molecular structure of the electroactive benzoxazine monomer (PH-BA). The reversible one-electron transfer between the neutral form and the radical-cation form takes place in the redox reaction of PH-BA and its corresponding polymer (pPH-BA). The pPH-BA exhibits oxidation and reduction currents of 0.122 and 0.128 mA cm⁻², respectively, which are higher than the published benzoxazine/aniline oliogomer blend. The current value of the redox peaks of pPH-BA doubles when compared with PH-BA, which might be attributed to the formation of the electron-donating hydroxyl groups during the thermally induced ring-opening polymerization and the hydrogen bonding interactions between phenolic-OH groups and aniline units in pPH-BA network. The pPH-BA coating exhibits outstanding corrosion resistance properties with a corrosion rate of 0.0091 mm per year for carbon steel Q235, which is significantly lower than that of non-electroactive polybenzoxazines. pPH-BA can further facilitate the formation of a metal oxide layer composed of Fe₂O₃ on the steel surface and was characterized by SEM, Raman, and XPS.