Effect of Curing Temperature and Phosphoric Acid on Corrosion Resistance of Mannich-modified Phenolic Resin on Galvanized Steel Surface

Abstract

Phenolic resin modified by Mannich reaction with an amine was investigated as an alternative to chromate on galvanized steel surface. The corrosion resistance of the resin coating was improved with increasing of the curing temperature, furthermore also improved by addition of phosphoric acid into the resin solution. Surface free energy measurement, infrared absorption analysis, GDS analysis, XPS analysis, etc. were carried out in order to clarify the effect of curing temperature and the addition of phosphoric acid; as the result some findings were obtained. The coating film became more hydrophobic, because cross-linking reaction proceeded with the elimination of the diethanolamine, which is the amine for modification, in curing over 160. It was revealed that the phosphoric acid included in the resin coating has been eliminated together with diethanolamine during the cross-linking reaction and it has diffused through the resin coating to form an insoluble phosphate layer at the interface between resin coating and galvanized steel. The electrochemical measurement clarified that the insoluble phosphate layer suppressed cathode reaction reduction reaction of dissolved oxygen .