| AZ91D die-casting magnesium alloy have the advantages of high specific strength,excellent mechanical properties and good thermal conductivity.However,high electrochemical activity leads to poor corrosion resistance,Chemical conversion coatings are often prepared on the surface of the magnesium alloy to enhance its corrosion resistance.Since magnesium alloys are extremely susceptible to mechanical damage during the die casting process,there are uneven scratches,dents,pores and other defects.Therefore,polishing treatment is often performed before the material is used,so that the subsequent film layer can obtain better adhesion and corrosion resistance.This article researched the influence of magnesium alloy polishing process on the polishing effect,determined the best polishing solution formulation and discussed its polishing mechanism.On this basis,the preparation process of molybdenum conversion coating and calcium phosphating coating were studied respectively,and the growth mechanism of the coating was discussed.The research used laser confocal microscope,electrochemical workstation,SEM,XRD,spot test,salt spray test and other methods to determine the surface roughness,electrochemical performance,morphology and phase composition.The effect of polishing process on polishing quality was researched,and the optimal composition and process conditions of the polishing liquid were determined as phosphoric acid 625.00 ml·L-1,copper sulfate 0.60 g·L-1,triethanolamine 0.40 g·L-1,Sodium dodecylbenzene sulfonate 0.35 g·L-1 and temperature 45?,the weight loss rate of the polished sample was 0.48 mg·cm-2,the gloss could reach up to 93%,the surface state was uniform,and the mirror-brightness effect had been achieved.The corresponding surface appearance grade was level 1.The surface roughness Ra value was 0.086?m.The molybdenum conversion film was prepared by a chemical conversion method,and the best formula and process were optimized through orthogonal experiments:sodium molybdate 45.0 g·L-1,sodium acetate 8.0 g·L-1,trisodium phosphate 17.0 g·L-1,Triethanolamine 1.2 g·L-1,sodium dodecylbenzene sulfonate 0.4 g·L-1 and sodium citrate 5.0 g·L-1,temperature 60?,conversion time 45 min.The film layer was distributed in layers and the surface was uniform.The main components of the film are Mg3(PO4)2,MoO2,MoO3,Na2MoO4 and a small amount of AlPO4.The corrosion resistance of the sample was greatly improved.The film growth process could be divided into the magnesium alloy dissolution and film formation preparation stage,the initial film formation stage,the film growth stage,and the film formation reaction equilibrium stage.Calcium-based phosphating film was prepared by chemical conversion method,and the best formula and technology were optimized through orthogonal experiment:calcium nitrate 35.0 g·L-1,marif salt 16.0 g·L-1,phosphoric acid 28.0 g·L-1,sodium citrate 2.0 g·L-1,Sodium pyrophosphate 1.8 g·L-1,sodium dodecylbenzene sulfonate 0.6g·L-1 and temperature 55?,reaction time 45 min.The surface of the film had a flower-like structure,and the conversion film was uniform and dense.The corrosion current of the calcium conversion coating was 3 orders of magnitude lower than that of the magnesium substrate,and the maximum arc impedance reached 8913.7?·cm-2,and the corrosion resistance was improved.The surface of the film was mainly Ca3(PO4)2,Ca2P2O7,AlPO4,Mg3(PO4)2 and other phases.The film formation process could be divided into three stages:the dissolution of magnesium alloy and the initial process of film formation,the film growth process,and the film thickening process.After the Na2MoO4 modification,the surface of the film layer was found to be a block structure,densely packed,and the crack diameter decreased.After the molybdenum modification,the corrosion current of the calcium-based phosphating film was reduced to 2.449×10-7A·cm-2,and the resistance value was 10413.8?·cm-2.The corrosion resistance of the sample was improved... |
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