| As a prominent lightweight material in structural alloys,the proportion of magnesium alloys in automobiles has been increasing in recent years.It can reduce the weight of parts without reducing the strength of parts.At the same time,it has high recycling value and can effectively reduce the consumption of resources.Corrosion protection is an important subject in the application field of magnesium alloys.How to design and construct a high corrosion resistance,green and environmental protection magnesium alloy surface coating to meet the requirements of various complex environments in the driving process has been an important challenge in the application of magnesium alloys in the automotive field.In this experiment,AZ31B die-casting magnesium alloy was used as the matrix.First,the vanadate coating on magnesium alloy surface was prepared by chemical conversion process under hydrothermal conditions.The effects of pH,reaction temperature,reaction time and vanadate concentration on the properties of the coating were studied by the combination of orthogonal experiment and single factor experiment.Corrosion potential was used as evaluation criterion and a series of characterization tests were carried out.The experimental results show that the optimum preparation process of magnesium alloy vanadate coating is solution pH=10,reaction temperature 120 C,reaction time 3h,and sodium metavanadate concentration 15g/L.The corrosion potential of the vanadate film prepared under the optimum process is up to-1.441V,which is 153mV higher than that of the blank sample.The corrosion current density can reach 5.97×10-7A·cm-2,which is an order of magnitude lower than the blank sample of magnesium alloy,and the coating has a certain self-healing ability.In order to further improve the adhesion between the modified coating and the subsequent coatings and enhance the overall corrosion resistance,the silane/vanadate hybrid coatings on magnesium alloys were successfully prepared by combining chemical conversion process with silanization process.Four silane coupling agents,namely,gamma-aminopropyl trimethoxysilane(KH540),gamma-aminopropyl triethoxysilane(KH550)and 3-aminopropyl trimethoxysilane(KH-540),were used to further impregnate the coating to form silane/vanadate hybrid coating.The experimental results show that the effect of KH540 treatment at pH=11 is the best,and the open circuit potential is about-1.574V,After the formation of silane/vanadate hybrid coatings by two-step method,the environmentally friendly low-temperature baking water-borne acrylic resin topcoat was selected and coated,and the adhesion and corrosion resistance of the whole coatings were tested.The overall coatings showed excellent corrosion resistance.The corrosion potential increase by 1031 mV compared with blank sample,the corrosion current density decreases by two orders of magnitude,and the samples could still reach 8 levels after 96 hours in the salt fog box.The pull-out test shows that the film has good adhesion,up to 4.88 MPa.The wettability test shows that the film has a certain hydrophobic property and can effectively resist the invasion of corrosive media.The coating is green and environmentally friendly,and has excellent corrosion resistance.It is a promising new surface treatment process for magnesium alloys to replace the traditional chromate coating. |
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