The effects of phosphoric acid in a 3-amino-propyltriethoxy silane (3-APS) coating system on electro-galvanized steel were examined in terms of the phosphoric acid effect on corrosion resistance. Acetic acid and 3-glycidoxy-propyltriethoxy silane (3-GPS) were also used for comparison of corrosion protection performance to that of phosphoric acid and 3-APS. The corrosion resistance mechanism was analyzed using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).
Adding phosphoric acid to the solution of 3-APS improved corrosion protection when the coating was immersed in 0.6 mol/dm3 NaCl. Results show that the addition of phosphoric acid had the greatest anti-corrosion effect and that it suppressed electrolyte uptake effectively. The roles of phosphoric acid were explained by the formation of protonated amine group (-NH3+) and Si-O-Si bonds in the coating. These coating changes are expected to produce the superior corrosion resistance. Acetic acid had no effect on the corrosion resistance compared to phosphoric acid. Results show that 3-GPS mixed with phosphoric acid, which has no amine group in the molecule, was inferior to the 3-APS and phosphoric acid composite coating.