| In order to improve the corrosion resistance of aluminum alloy, the surface treatment of aluminum alloy become particularly important. Currently, chromate conversion coating is banned gradually due to its environmental toxicity. As an environmentally friendly treatment technique, rare earth and siloxane conversion film are particularly favored. They are promising to replace the widely used chromate conversion coating. In this paper, we used an ordered deposit manner to make a cerium salt/siloxane composite film, which has better corrosion resistance than the existing methods such as cerium doping siloxane film. At the same time, we prepared a conversion film with high super-hydrophobicity and corrosion resistance, which can meet the high requirement of surface treatment of aluminum alloy in severe environment.Firstly, 3D optical microscope, SEM, EDX and electrochemical testing such as corrosion potential, electrochemical impedance spectroscopy(EIS) and electrochemical noise(ECN) were employed to explore the corrosion behavior of 6063 aluminum alloy in 3% NaCl solution. It was found that pitting corrosion dominated on 6063 aluminum alloy in 3% NaCl, and the intermetallic compounds such as Mg2 Si and AlFeSi are the sources of pitting corrosion. In the first 20 h during the immersion, metastable pits initiate and the corrosion potential fluctuate positively due to the exposure of cathodic phase. Later, the driving force of pit decline because of the stripping of cathodic phase, leading to a lower growth rate of pits. The ECN test shows that most potential noise transients fluctuate negatively and the current noise transients are caused by the nucleation of new metastable pits.Secondly, the influence of the concentration of CeCl3,H2O2 and the deposition time on the corrosion resistance of cerium salt conversion film were optimized. Electrochemical tests indicated that the cerium salt conversion film has protective effect just at the beginning of corrosion, after samples were immersed in 3% NaCl for one week, a lot of pits can be found, which suggests that the long-term corrosion resistance of cerium salt conversion film is unsatisfied.Thirdly, we prepared a cerium salt / siloxane composite film on aluminum alloy. The electrochemical test and corrosion morphology show that the addition of cerium salt can inhibit the nucleation rate of pitting corrosion on aluminum alloy substrate, but it reduces the resistance of silicone coatings and undermine its insulativity. Thanks to the synergism of cerium salt and siloxane, we proposed a sequential deposit method of cerium salt/siloxane(CS) composite film and compared its corrosion resistance with the existing methods, such as cerium doped siloxane method and siloxane / cerium salt(SC) sequential deposit method. The polarization resistance of CS composite film is 2.16×107?, which is higher than the other two methods and its corrosion resistance is the best, mainly because this approach overcomes the shortcomings of doped silicone film and SC film. The cerium salt deposit can enhance the bonding strength of siloxane to aluminum substrate. In addition, silicone film can prevent the stripping of cerium salt.Finally, Bis-1,2-[triethoxysilylpropy]tetrasulfide(BTSPS) as low surface energy compound, was employed to prepare a super hydrophobic surface on aluminum alloy which was etched by 0.08 M H2C2O4 / 3.7% HCl(1:1) for 13 h. In order to improve the healing ability of film after pitting corrosion occurred, a cerium salt / silicone composite film was assembled on the super-hydrophobic surface of aluminum alloy. It was found that this composite film not only exhibit super-hydrophobicity but also good corrosion resistance on pitting corrosion. |
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