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.
Porous alumina was prepared by anodizing electrochemically deposited aluminum（Al）films in an acidic aqueous solution. The Al films were electrodeposited on copper plates from ionic liquid baths containing Al3+ ions. AlCl3-trimethylphenylammonium chloride (TMPAC), AlCl3- 1-ethyl-3-methylimidazolium chloride (EMIC), and Al3Cl-1-butyl-1-methylpyrrolidium chloride (BPC) were used as ionic liquid baths for Al electroplating. The Al film deposited from the AlCl3-EMIC bath was the thickest among the films obtained from baths. The Al films plated from ionic liquid baths were then anodized at 40 V in 0.8 M oxalic acid aqueous solution at 16 °C for 4 h to form porous alumina films, which showed arrayed pores of the same diameter in arrangements that were mutually parallel and perpendicular to the substrate surface. The average hole period was 99-106 nm.
To develop effects of surface polishing conditions on pitting resistance after passivation treatments, pitting potentials of Type 304 stainless steels finished using different polishing techniques and passivated with HNO3 solutions of different types were measured in deaerated 3.5%NaCl. Steel finished with electropolishing and then passivated in 30%HNO3 showed high pitting potential of 1010 mV (vs. Ag/AgCl (sat. KCl)), which is in a trans-passive potential region. Increased pitting potential of 1053 mV was obtained when 0.1 kmol･m－3Na2MoO4-containing 30%HNO3 was used for passivation. Steel finished with mechanical polishing never showed such high pitting potential. No decrease in pitting potential was observed on steel finished with electropolishing and 30%HNO3 or 0.1 kmol･m－3Na2MoO4-containing 30%HNO3 passivation after accelerated corrosion tests such as long term immersion in aerated 3.5%NaCl, salt spraying with 5%NaCl or steaming at 121 °C. To achieve high pitting resistance, combined surface treatment of electropolishing and 30%HNO3 or 0.1 kmol･m－3Na2MoO4-containing 30%HNO3 passivation is recommended for Type 304 stainless steels.