| In this paper, CeCl3·7H2O/30 wt% H2O2 system is chosen for cerium conversion treatment on magnesium alloys. The bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) silane as an addition is added into the cerium conversion solution, obtaining the cerium/silane conversion coating. The improved cerium/silane conversion coating is more homogenous and has higher resistance to water uptake, better adhesion to substrate and better anticorrosion properties. The optimum parameters of the single cerium conversion coating and the silane modified cerium conversion coating have been studied. The growth kinetics, microstructure, composition and corrosion resistance of the both coatings are systematically investigated by SEM, EDS, XPS, AFM, electrochemical polarization, electrochemical impedance and immersion tests. The two film forming mechanisms and the silane improved mechanism are deeply discussed. Moreover, the corrosion behaviors and the anti-corrosion mechanisms of the two coatings are studied in 0.05 M NaCl solutions with the static immersion corrosion experiment.The optimal parameters of the single cerium conversion coating are: CeCl3·7H2O 0.1 mol·L-1,30 wt.% H2O2 5 ml·L-1, treatment time 5 min at room temperature. The growth of the single cerium conversion coating has a nucleation, rapid film, steady growth stage. As the treatment time prolonged, the morphology of the cerium conversion coating presents a "spherical"-"honeycomb-like"-"cauliflower-like" process. Mg, Ce and O elements are detected on the coating surface. Growth equilibrium is achieved at around 5 min. The coating has a time-dependant composition. In the initial stage, the coating is composed of magnesium oxide/hydroxide. Treating for more than 5 min, magnesium/cerium oxide/hydroxides co-exist and Ce(Ⅳ) is in dominant. The results of electrochemical experiments indicate that the best anticorrosion property is at the deposition of 5 min. The film formation mechanism points that once magnesium alloys immersing in the cerium conversion solution with pH value ～6, it will immediately dissolve. The generation of OH- causes the local interface alkaline and cerium ions precipitate on the top of magnesium. The cerium compounds become richer and gather to form a film gradually.In view of the silane specific structure, this paper introduces BTESPT to the traditional CeCl3·7H2O/30 wt.% H2O2 transformation system, using a one-step chemical immersion technology to construct a new cerium/silane conversion coating. First the Si-O-Si network structure could control the particle deposition; on the other hand, Si-O-Mg chemical bond will promote the coating/substrate interface bonding. Through the microphology observation, hydrophobic properties and electrochemical performance tests, the optimal concentration of the silane additives to the cerium conversion solution is 25 ml·L-1. The dynamic research of the cerium/silane conversion coating shows that the silane addition decreases the film growth rate. The distributed particles are uniform in the processing of 5 min. Mg, Ce, O and Si elements are detected on the coating surface. With the extension of the treatment time, Mg and Si contents decrease, while the Ce content increases. The best hydrophobic performance and anticorrosion property are at the deposition of 5 min. XPS sputter presents that in addition to magnesium/cerium oxide/hydroxide, the Si-O-Si, Si-O-Mg and Si-O-Ce chemical bonds also exist. Mg and Si contents rise and the Ce content reduces from the outside to the inside of the film. The external Ce(Ⅳ) content is relatively higher than the inside.Compared with the single cerium conversion coating, the cerium/silane conversion coating has a change. After the silane addition, the deposition of the cerium particles is in order. The coating becomes more homogenous and compact. Si-O-Si, Si-O-Mg and Si-O-Ce chemical bonds are detected in the coating. The surface contact angle increases form 52.5° to 112.9°. The adhesion performance between the coating and the substrate improves. The corrosion potential increases from-1.487 V to-1.352 V. Corrosion current density is from 6.966E-6 A·cm-2 down to 1.476E-6 A·cm-2. Electrochemical impendence is from 1271 Ω·cm2 up to 1690 Ω·cm2. The silane modified mechanism points that the fully hydrolysis silane and matrix surface condense, generating Si-O-Mg bond. The excess silanols condense each other, solidifing to Si-O-Si mesh film. Cerium is in the form of particles distributed in the reticulate structure, playing a "hole sealing effect"; on the other hand, Si-O-Ce bonds may generate. The excess cerium oxide/hydroxides may deposit on the Si-O-Si film surface. The silane addition reduces the film forming rate, controlling the growth of the crystallization. On the other hand, Si-O-Si network structure improves the deposition of cerium particles. The generation of Si-O-Mg chemical bond improves the interface binding force between the coating and the substrate. In a word the addition of the silane increases the physical barrier, slowing down the corrosion rate and improving the electrochemical performance of the film.Study on the corrosion behaviors of the films show that for long time static immersion corrosion, the corrosion rate of the cerium/silane conversion coating is lower than that of the single cerium conversion coating. With the extension of immersion time the corrosion potential and film impedance increase at first and then decrease, while the corrosion current density has a change in the opposite derection. The cerium conversion coating is in the turning point at 4 h immersion and the cerium/silane conversion coating in dipping 24 h turning point. Considering the mesh structure of the cerium/silane conversion coating and the "hole sealing effect", thus the ability to corrosion resistance improves compared with the single cerium conversion coating. |
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