Study On Anti-corrosion Performance Of Organic-inorganic Duplex Protecting Coating On The Surface Of Magnesium

The present work aims at fabricating the duplex protecting coatings on the surface of the magnesium metal by using organosilane such asγ-aminopropyltrimethoxysilane (γ-APS),γ-aminopropyltriethoxysilane (KH550) as precursors in this paper. Silane solutions of different hydrogen ion index were performed. The organic-inorganic duplex protecting coatings were carried out on magnesium metal by means of dip-coating method using these silane solutions. In order to investigate the kinetics of the coating deposition process a series of experiments was conducted at different pH levels and deposition times. The optimum pH value of the silane solution hydrolysis was characterized by the change of conductance rate of these silane solutions with time by using conductivity detector. The surface topography of the organic-inorganic hybrid coatings were observed by metallographic microscope. Potentiodynamic polarization curves measurement and the figuration of metallographic microscope after measurements were used on the organic-inorganic duplex protecting coatings to evaluate its corrosion performance, and were compared with the corrosion performance of the chromate film.The results indicate that the hydrolysis performance of the hydrophilic KH550 is than the hydrophobicγ-APS, but the hydrolysis performance of the KH550 is unstable. As glacial acetic acid wasn't added into silane solution, the hydrolysis is very slow, when glacial acetic acid was added into silane solution, the conductance rate of all organic silane solution tend to the biggest immediately, then some of the silane reacted condensation reaction in silane solution, hydrolysis and condensation tends to dynamic balance at the end. And the smaller the pH value, the better the hydrolysis performance ofγ-APS silane solution, but the impact on hydrolysis of the pH value is not very great. The corrosion performance of organic-inorganic duplex coating is equal to the naked sample when the hydrogen ion index is less than 7, the corrosion performance of organic-inorganic duplex coating have big improve when the hydrogen ion index is more than 7, and the corrosion performance is the best when the hydrogen ion index is 10. If the deposition time is less than 60 second, the longer the deposition time, the denser and better the protective-coatings. Deposition time has a minor effect on the thickness and the electrochemical behavior of the protective coatings when the deposition time is more than 60 second. But the potentiodynamic polarization curves measurement and the figuration of metallographic microscope after measurements indicate that the corrosion performance of organic-inorganic duplex coating is best when the deposition time is 60 second. The results indicate that with increase the dipping number, the poorer the corrosion performance of organic-inorganic duplex coating. The corrosion performance is best when the dipping number is 1 and solidifying time is 10min. And compared with corrosion voltage of chromium film, corrosion voltage of the organic-inorganic duplex protecting coatings is bigger and corrosion current is smaller. Compared with chromium film, the organic-inorganic duplex protecting coatings is smooth and adhesive, and possesses the better electrochemical performance. The organic-inorganic duplex protecting coatings is environmentally friendly. So this process can replace the conventional chromate process effectively.After the rare-earth metal salt add intoγ-APS solution, the corrosion performance of organic-inorganic composite duplex coating is best when the hydrogen ion index is 10 and deposition time is 20 to 30 min.