Evaluation of corrosion resistance and self-healing performance of phytic acid-vanadate composite coating on Q235 steel

The chemical conversion coating can effectively retard the corrosion rate of the metal substrate and improve the adhesion of subsequent coatings. However, when the coating is damaged, the metal substrate must be re-treated to apply a new coating. Herein, two different coating-forming materials-phytic acid (PA) and vanadate (V) with distinct self-healing mechanisms were used to prepare phytic acid-vanadate (PA-V) composite conversion coating on a Q235 steel surface. The electrochemical characterization of the coating on the Q235 steel was examined using electrochemical impedance spectroscopy (EIS), polarization, and open circuit potential (OCP) methods in 3.5 % NaCl solution. Corrosion resistance measurement results exhibited that under the protection of the phytic acid-vanadate composite conversion coating, the corrosion current density of steel was significantly reduced, which was about one-sixth that of steel coated solely with phytic acid. Furthermore, the PA-V4 coating exhibited the best corrosion resistance, providing 94.3 % anti-corrosion efficiency. The analysis results of the microstructure, composition, and elemental chemical states of different samples indicated that PA-V composite conversion coating with self-healing property was precipitated on the steel surface. Particularly, SEM and EIS studies have indicated that the self-healing property of the PA-V conversion coating was attributed to the synergistic effect of the chelation reaction between phytate ions and iron ions, as well as the release and migration of vanadate compounds.