Preparation Of Corrosion Resistant Coating On Magnesium Alloys And Study Of Their Properties

Magnesium alloy is an ideal engineering material because of its low density,high specific strength,strong ductility and abundant storage capacity.However,magnesium alloys are easy to corrode,which limits their application scope.Surface treatment is one of the most economical and effective means to improve the corrosion resistance of magnesium alloys.However,the high cost and the production of harmful substances in the preparation process have gradually failed to meet the needs of current development.Therefore,it is urgent to find a more economical,efficient and environmentally friendly preparation method of magnesium alloy corrosion-resistant film.In this paper,AZ91D magnesium alloy was used as the matrix material,and the film was prepared by in-situ growth,hydrothermal synthesis and alkali-thermal modification,and the corrosion resistance of the film was analyzed and discussed.The main research contents are as follows:（1）Taking ammonium carbonate and calcium gluconate as raw materials,mixing with deionized water,heating to prepare a reaction solution,placing a sample in the reaction solution,and in-situ growing on AZ91D magnesium alloy to prepare an ammonium magnesium carbonate tetrahydrate/calcium carbonate composite film.The film was characterized by SEM,XRD and XPS,the surface morphology was studied,the composition of the film was determined and the forming mechanism of the film was analyzed.The prepared film has good corrosion resistance to Mg substrate.In electrochemical test,the corrosion current density(I_corr)of the best sample is 1.125×10^–8 A cm^–2,which is about 3orders of magnitude lower than that of Mg substrate,and the corrosion inhibition efficiency reaches 99.96%.Cross-cutting experiments show that the film has good adhesion.The method adopts a one-step process,and the preparation process is environmentally friendly,pollution-free and low in price.（2）Using deionized water as solvent,nano-SiO₂,KOH and Ca（OH）₂ as solutes,EDTA as complexing agent,and reaction kettle as container,the multi-structure composite film was prepared on AZ91D magnesium alloy by hydrothermal synthesis.The surface morphology of the film was observed by SEM,and the film was composed of a porous microstructure surface film and a dense lower film.XRD,XPS and FTIR confirmed that the main components of the film were Mg（OH）₂ and K₂Al₂Si₄O₁₂·x H₂O.The film modified by stearic acid shows stable hydrophobicity and excellent corrosion resistance.The static water contact angle of the sample is stable at 125°,and the corrosion current density(I_corr)in electrochemical test is 1.4086×10^–8 A cm^–2,and the corrosion inhibition efficiency reaches99.96%,showing excellent corrosion resistance.（3）Nano-SiO₂,KOH and CaCl₂ were mixed with deionized water to prepare a reaction solution,which was put into a crucible together with magnesium alloy and heated in a Muffle furnace.A large number of hydroxyl groups were introduced into the surface of magnesium alloy by alkali heating process to generate Mg（OH）₂,which was stabilized by SiO₂.Finally,the SiO₂/Mg（OH）₂ composite film was prepared on the surface of AZ91D magnesium alloy.The surface morphology of the sample was observed by microscope and SEM,and the main components of the film were identified as Mg（OH）₂ and SiO₂ by EDS,XPS and XRD.Electrochemical test shows that the surface of the sample is non-conductive,and the diameter of the impedance semicircle is about 6 orders of magnitude higher than that of the Mg substrate.The immersion test results show that the film has excellent long-term corrosion resistance.The results of cross-cutting experiment show that the adhesion of the film is excellent.This method does not use expensive drugs and complicated instruments,nor does it need pretreatment.