| Magnesium alloys which are compared as the 21st Green Engineering Material possess a unique set of properties such as light weight, high strength-to-weight ratio, high thermal conductivity, dimensional stability and damping characteristics, good electromagnetic shielding and machinability as well as recyclability. However, magnesium alloys have a number of undesirable properties including high chemical reactivity and poor corrosion resistance that have hindered its widespread use in many applications. Therefore, it is of great significance to study the surface treatment technology of magnesium alloy.Based on the review of corrosion mechanism of magnesium alloys and the current study advancement in the whole world, ceria film, zirconia coating, silane organic coating and Si-Ce hybrid coating were directly or indirectly prepared on the magnesium alloy surface via sol-gel method. Meanwhile, the primary research of healing for Si sol-gel coating was also carried out due to the lack of self-healing functionality. Electrochemistry impedance spectroscopy (EIS), potentiodynamic polarization tests, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR) spectrum analysis, UV-Visible spectrum analysis and viscosity measurements were employed to character the properties of the prepared coatings including the corrosion resistance, surface morphology and composition.It is not possible to apply the coating directly on the magnesium alloy surface for some sol-gel coating because the high chemical activities may cause the poor adhesion and subsequent corrosion of substrate. Therefore, a fluoride coating as an interlayer has been used before preparing the ceria film on the magnesium alloy. The cerium film was prepared on the fluorinated magnesium alloy via sol-gel method employing cerium nitrate hexahydrate as the precursor and celloidin as the additive. The tests results showed that the main composition of the film was ceria, magnesium fluoride and magnesium oxide, especially, ceria played a key role in corrosion resistance. The results of electrochemical measurements in 3.5 wt% NaCl solution demonstrated that the corrosion resistance of AZ91D magnesium alloy was greatly improved by the ceria film.Meanwhile, a novel anti-corrosion zirconia coating was prepared via the sol-gel method for fluorinated AZ91D magnesium alloy using zirconium nitrate hydrate as a precursor modified with acetylacetone (AcAc). Zirconium nitrate reacted with AcAc forming a net structure in the coating. This coating possessed many excellent properties such as simple, inexpensive and environmental friendly. The electrochemical measurements results demonstrated that zinconia coating could obviously improve the corrosion resistance of magnesium alloy AZ91D.For the first time, this work investigated the effect of cerium concentration on microstructure, morphology and corrosion resistance of cerium-silica hybrid coatings on AZ91D magnesium alloy. The cerium-silica hybrid coatings were prepared via sol-gel method employing Vinyltriethoxysilane (VETO) and y-glycidoxypropyltrimethoxysilane (GPTMS) as precursors, cerium nitrate hexahydrate as addititive. The results showed that small amount of cerium nitrate could accelerate the hydrolysis of the hybrid sol which was good to increase the corrosion resistance of the coating; however, a large amount of cerium nitrate could destroy the structure of the coating, leading to poor corrosion resistance of zirconia coating. The optimal concentration of cerium nitrate for increasing the corrosion resistance was 0.01 M.As soon as even small defects appear in the sol-gel coating system and the corrosive agents penetrate to the metal surface the coating system can not stop the corrosion process. The development of an environmental friendly approach which can stop the development of corrosion process and heal the corroded areas becomes an important issue for many industries where an adequate corrosion protection is needed. Present work demonstrated the possibility to use zinc nitrate as healing agent to repair the organic silane coatings in NaCl solution. The tests results demonstrated that introduction of zinc nitrate to the electrolyte could stop the development of corrosion process of the coating system and a remarkable recovery on corrosion resistance could be obtained. This effect may be attributed to the formation of zinc oxide/hydroxide on the defective areas, hindering the corrosion activities. At the same time, the nitrate ions might also play an important role in this healing process. |
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