Research On Corrosion Resistance Of A Silane/Mg?OH?₂-based Composite Layer On Magnesium Alloy

Magnesium alloys have been the potential implant biomaterials because of their good mechanical properties and perfect biocompatibility. However, the applications of magnesium alloys are limited seriously because of the poor corrosion resistances, especially in the medical field. In order to solve this problem, an in-situ Mg?OH?₂ layer was fabricated on AZ31 magnesium alloy by an hydrothermal method. And the effect of pH value and hydrothermal treatment time on corrosion resisting properties of Mg?OH?₂/AZ31 magnesium alloy system was studied. Then an silane film was constructed on the in-situ Mg?OH?₂ layer by quantizing process, after that the corrosion process of silane/Mg?OH?₂/AZ31 magnesium alloy system and the influence of the curing temperature of the silane on the corrosion resistance of this system were studied. Finally the corrosion mechanism of the oxide film/AZ31 magnesium alloy, Mg?OH?₂/AZ31 magnesium alloy and silane/ Mg?OH?₂/AZ31 magnesium alloy system were studied, which include the following three parts:The first section mainly focused on the effects of pH value of the hydrotherm and reaction time on the growth of Mg?OH?₂ film in order to control the chemical constituents, micro-morphology and corrosion resisting properties. The results shown that the amount of Mg?OH?₂ is increasing with the enhancement of the pH value. At first the amount of Mg?OH?₂ is increasing rapidly with the reaction time increasing, then slowly. These results shown that the Mg?OH?₂ films obtained at pH=10, 4h are compact over the AZ31 substrate with the least defects. Based on the analysis of potentiodynamic polarization and electrochemical impedance, the reason for in-situ Mg?OH?₂ layer improving the corrosion resistance of Mg?OH?₂/AZ31 magnesium alloy system is that Mg?OH?₂ film enhanced corrosion potential and impedance.The second section mainly focused on the preparation of silane/Mg?OH?₂-based composite layer using the quantizing process and the influence of curing temperature on corrosion resisting performance of the composite layer. The results shown that for the long-term corrosion protection of silane/Mg?OH?₂/AZ31 magnesium alloy system, the increase of the curing temperature improve the impedance of the composited system when the curing temperature is lower than 130 °? with the amount increasing of chemical bond?Si-O-Mg?. However, the impedance of the composited system decease when the curing temperature is more than 130 °? due to the carbonization of the silane layer. Otherwise, the composite system provided excellent corrosion resistance compared with the Mg?OH?₂ layer alone and very good adhesion due to the presence of chemical bonding between the Mg?OH?₂ layer and substrate, and the existence of Si-O-Mg bond between Mg?OH?₂ layer and silane layer.The last section mainly researched on corrosion mechanism of AZ31 magnesium alloy, Mg?OH?₂/AZ31 magnesium alloy and silane/Mg?OH?₂/AZ31 magnesium alloy system using equivalent electrical circuits, and discused the role of Mg?OH?₂ and silane/Mg?OH?₂ in corrosion process. The results indicated that AZ31 magnesium surface can form some protective film, the film is not stable in corrosive environment. The formation of in-situ Mg?OH?₂ on AZ31 magnesium alloy not only can enhance the impedance of the composited system, but also prolong markedly the time of electrolyte penetration into the substrate, providing the long-term corrosion protection. However, the construction of silane on in-situ Mg?OH?₂ film can improve the effect of corrosion protection 10 times than that of Mg?OH?₂ film alone.