| Mg alloys have been widely used in many high-tech industries like energy, electronics industry, aerospace, military industries and so on due to their attractive properties like low density, high strength/weight ratio etc. However, during the engineering applications, it is greatly demanded to improve the corrosion resistance of Mg alloys through different surface treatments. The high electrochemical activity of Mg makes it so difficult to perform surface treatments of Mg alloys. It has become an important topic in the research area of surface science and technology of reactive metals to look for new medium suitable for the surface treatments of reactive metals and to study the related mechanisms. Ionic liquid is a kind of new green solvents besides water and organic solvents, which shows its unique advantages in the surface engineering of reactive metals.In the present paper, anodic behavior of Mg in 2:1 acidic AICl3-1-ethyl-3-methyl-imidazolium chloride (AICl3-EMIC) ionic liquid were studied by using a three-electrode cell. The formation of viscous layer in ionic liquid was firstly observed and reported. The model of the formation process of viscous layer was discussed. Secondly, the replacement reaction of Mg and AICl3-EMIC was studied and the electroless deposition of Al on Mg was further performed. The mechanism of replacement reaction was discussed. Thirdly, studies on the anodic behaviors of AZ91D Mg alloy were also performed and the electrodeposition of high-adhesion Al coatings on Mg alloys after electrolytic etching pretreatments was carried out. The mechanisms for the function of viscous layer in anodic etching pretreatments and for the improvement of coating adhesion were discussed. In addition, combining with Raman spectroscopy, ab initio calculation using Gaussian09W was carried out. The ion species and structure of viscous layer were studied and the mechanism of the formation of viscous layer was further discussed.The results showed that the oxide film on the surface of Mg showed great effects on the anodic behavior of Mg in AICl3-EMIC ionic liquid. A thick oxide film on the surface inhibited the dissolution of Mg in the ionic liquid. But with a thin oxide film, dissolution of Mg took place after the breakdown of oxide film under anodic polarization. With a large anodic current applied in the anodic process (≥ 5 mA/cm2), a viscous layer formed at the interface Mg/ionic liquid. The formation of viscous layer was resulted from the accumulation of viscous Mg dissolved ionic liquid at the interface. By the "activation-passivation" of viscous layer, the Mg anode was homogeneously dissolved into the ionic liquid and the oxide film on Mg was completely removed. With a small anodic current applied in the anodic process (≤ 2 mA/cm2), no viscous layer formed at the interface, the Mg anode was heterogeneously etched, leaving a pitting morphology on the surface, and the oxide film on Mg cannot be removed effectively. After the surface oxide film was removed by anodic electrolytic etching pretreatment, there is a replacement reaction between Mg and AICl3-EMIC ionic liquid, forming Al nana-crystals on the surface of Mg.AZ91D Mg alloy dissolved in the AICl3-EMIC ionic liquid. With a large anodic current density≥ 10 mA/cm2 applied, a viscous layer formed at the interface of Mg alloy/ionic liquid. Because of the "activation-passivation" mechanism of viscous layer, the oxide film on the surface and the Mg solid solution phase and intermetallic phase Mg17Al12 dissolved homogeneously into the ionic liquid without any Mg17Al12 adhering on the surface. With a small anodic current density applied, no viscous layer formed. The oxide film cannot be removed effectively and the two phases of Mg alloy dissolved heterogeneously, resulting in a rough etching surface with many etching pits. With the etching time prolonged, the etching pits became deeper and deeper with the fallen down of Mg17Al12 and subsequently adhering on the surface.After electrolytic etching pretreatments at large current densities (≥ 10 mA/cm2), the electrodeposited Al coating on the surface of Mg alloy showed a high adhesion larger than 4 MPa. When the current density of electrolytic etching was ≤ 5 mA/cm2, the coating adhesion of electrodeposited Al was only 0.5-1 MPa. The high adhesion of electrodeposited Al coating can be attributed to the facts that the oxide film on the surface of Mg alloy was completely removed and the Mg alloy dissolved homogeneously during the pretreatment without any Mg17Al12 adhering on the surface. Besides, the electroless replacement of Al might also help to improve the coating adhesion.The anodic electrolytic etching pretreatment of Mg and Mg alloy had great effects on the electroless deposition of Al and the electrodeposition of Al on their surfaces. After the complete removal of oxide film on the surface of Mg by anodic electrolytic etching pretreatment, the electroless deposited Al crystals were nano crystalline. With the complete removal of oxide film on the surface of Mg alloy by electrolytic etching pretreatment at the current density larger than 10 mA/cm2, the electrodeposited Al coating on Mg alloy showed a high adhesion. Besides the complete removal of oxide film, the high adhesion of Al coating was also attribute to the homogeneous dissolution of the two phases of Mg alloy during the pretreatment.Ab initio calculation showed that the dissolved product of Mg in AICl3-EMIC ionic liquid was Mg(AlCl4)3-, rather than MgCl3- or MgCl42-. The three Al of Mg(AlCl4)3- located at the center of three tetrahedrons, where the atoms at the four tessarace were Cl. Two Cl atoms of every tetrahedron formed an octahedron, where the Mg located at center of the octahedron. The formation of viscous layer can be attributed to the accumulation of Mg(AlCl4)3- at the interface of electrode/ionic liquid. The strong interaction of Mg(AlCl4)3- with EMI+ and Al2Cl7- resulted in the formation of a huge networking structure in the ionic liquid, making the viscosity of ionic liquid increase. |
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