This study investigated synergic effects of phosphoric acid（H3PO4）, amino-propyltriethoxysilane（3-APS）and ammonium metavanadate（NH4VO3）in a conversion coating on electro-galvanized steel（EG）to clarify EG corrosion suppression by these substances.
Results showed that the ratio of white rust during salt spray testing（SST）was lessened when a coating was formed with 3-APS and H3PO4. The phenomenon was explained: adding 3-APS into H3PO4 causes the decrease of corrosion resistance by suppressing salt water uptake in the coating because the coating itself became compact. Moreover, a coating formed with NH4VO3, 3-APS, and H3PO4 exhibited higher corrosion resistance in SST than a coating with 3-APS and H3PO4. Results show that NH4VO3 changed more into tetravalent or trivalent vanadium by oxidation reaction of zinc when the coating was formed together with 3-APS and H3PO4 on EG. These vanadium compounds in the coating were estimated as dissolved easily in the case in which salt water permeated into coating. The compounds were then deposited at the corroded area to reduce EG corrosion. These findings suggest that NH4VO3, 3-APS, and H3PO4 in the conversion coating contributed both to the high barrier and to self-protective effects of the coating.