| Magnesium alloy material has the characteristics of high specific strength,strong plasticity and light weight,and has been widely used in military industry,aerospace,large-scale transportation and other fields.However,magnesium alloy is easy to cause galvanic corrosion and poor corrosion resistance,which greatly limits its application in industry.Treatment of magnesium alloy enhances corrosion resistance,which is beneficial to industrial use of magnesium alloy.At the same time,the corrosion resistance and electrical conductivity of magnesium alloys can make magnesium alloys widely used in electronic communication and other fields.The main purpose of this paper is to prepare two kinds of manganese phosphating films on the surface of AZ91D magnesium alloy by chemical transformation method.Firstly,different additives are added to prepare the phosphating film with excellent corrosion resistance,and then the better phosphating solution is determined to prepare the corrosion-resistance conductive phosphating film.The corrosion resistance of the film was tested by electrochemical test,potassium permanganate drop test and neutral salt spray test.Digital multimeter detects the conductivity of the film.SEM,EDS and XRD were used to characterize the film morphology and material composition.The main research work is as follows:1.Optimize the basic process formula of phosphating solution through orthogonal experiment and single factor experiment.Select strontium nitrate as a suitable additive,optimize the concentration of strontium nitrate and study its effect on the performance of phosphating film.The optimized process formula of basic phosphating film:35 g/L Mn(H2PO4)2,3 g/L Na NO3,3 g/L EDTA-4Na,reaction time 10 min,reaction temperature 60°C,p H 3?3.5(phosphoric acid adjust).The results show that the basic phosphating solution added with 0.5 g/L strontium nitrate has the best corrosion resistance.The icorr of the film is 7.28×10-7A/cm2 and the Ecorr is-1.485 V,which is about 1 order of magnitude smaller than the basic phosphate film's icorr,and about 2.5 orders of magnitude smaller than the substrate's icorr;the film is uniform and dense,and the main substances are Mg HPO4and Mn HPO4.The SEM image shows that strontium nitrate can reduce the grain size,increase the grain nucleation density,and then increase the density of the film,and improve the corrosion resistance of the film.2.The corrosion-resistant conductive phosphating film on the surface of AZ91D magnesium alloy was prepared,and the influence of the concentration of each component of the phosphating solution and the reaction conditions on the corrosion resistance and conductivity of the film was studied.Through orthogonal experiment and single factor experiment optimization,the process formula of magnesium alloy corrosion-resistant conductive phosphating film is:NH4H2PO4 20 g/L,C2H3Na O2 15g/L,Na NO3 65 g/L,C6H8O7 5 g/L,Mn SO4 6 g/L,Ca(NO3)2 2 g/L,Na F 2 g/L,OP-10 3ml/L,p H 3?3.5(adjusted by phosphoric acid),reaction temperature 60°C,reaction time5 min.The prepared phosphating film's icorr is 2.33×10-6 A/cm2 and Ecorr is-1.536 V,which is 1.5 orders of magnitude smaller than the substrate's icorr.The film grade is 7after 48 h of neutral salt spray test.The surface contact resistance of the phosphating film is 0.18?,that is,the phosphating film has good corrosion resistance and conductivity,and can be used as a corrosion-resistant conductive phosphating film.The test results show that the film is uniform and dense,with a film thickness of about 4?m.The film is mainly composed of Mg,O,P,Mn,Ca and other elements,and the main material of the film is MgHPO4. |
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