Abstract
The coating film formation and the corrosion resistance of phenolic resin were examined as an alternative for chromate on the galvanizing steel sheet. The effect of cure temperature and phosphoric acid on film formation and corrosion resistance was investigated by thermogravimetry and differential thermal analysis (TG-DTA), surface energy measurement, and the coating film characterization by XPS and FT-IR analysis. The mechanism of corrosion resistance was estimated by the polarization measurement of the coating plate. The resin, which was in an aggregate state at 80°C, gradually became loose by thermal energy as temperature rose, and it self-cured above 130-150°C to make the film that gives good corrosion resistance. Addition of phosphoric acid to the phenolic resin solution has improved the corrosion resistance even at 120°C that is below the original self-cure temperature. Though the mechanism of the effect of phosphoric acid is not always clear, it was found that phosphoric acid promotes the cure of the resin and enhances the insolubility to the salt water. Also, it was confirmed that phosphoric acid itself is incorporated into the coating film. It seems that these facts improve corrosion resistance.
