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.
This study examined chemical resistance of electroless-deposited nickel-tin-boron （Ni-Sn-B）, Ni-Sn-phosphorus （Ni-Sn-P）, and Ni-P layers on steel plate - cold-rolled commercial （SPCC） against an aqueous solution of sodium hypochlorite （NaClO）. Compared to the Ni-P layer, Ni-Sn-B and Ni-Sn-P layers exhibited higher chemical resistance against 0.2 g L－1 NaClO aqueous solution. After immersion of the Ni-P layer for 72 h to the NaClO aqueous solution, the Ni-P layer surface showed a clearly corroded surface. An image obtained using scanning electron microscopy revealed that the surface was dissolved and that it had nibbling patterns on its surface. However, surface corrosion was not observed in Ni-Sn-B and Ni-Sn-P layers. Results of compositional analysis with X-ray photoelectron spectroscopy indicate that B and P contained in the Ni-Sn-B and Ni-Sn-P layers were dissolved in the NaClO aqueous solution and that, from comparison of Ni/Sn atomic ratios before and after immersion of the Ni-Sn-B and Ni-Sn-P layers, Ni atoms were also dissolved in tiny amounts. The Ni-Sn-B, which had high Sn content, inhibited the dissolution of Ni from Ni-Sn surfaces.