Preparation And Application Of Magnesium Alloy Chemical Conversion Coatings

Magnesium alloys are widely used in automotive,electronics,aerospace and other fields due to their good performance.However,magnesium alloys have poor corrosion resistance because of their active chemical properties.In this paper,chemical and electrochemical methods were used to prepare the conversion coating on the surface of magnesium alloys.The properties were characterized by scanning electron microscopy(SEM),electrochemical workstation test,X-ray diffraction(XRD).laser confocal and droplet test to determine the optimum process for the preparation of magnesium alloy conversion film,the composition of the conversion supplement solution and the formation mechanism of the conversion film.The conversion film for magnesium alloy phosphate was prepared by a chemical method.The effects of sodium pyrophosphate,EDTA,nitrilotris(methylenephosphonic acid)and sodium citrate on the properties of the film were studied to determine the optimum formula of magnesium alloy phosphate conversion solution.The final conclusions were as follows:zinc oxide 2.5 g/L,phosphoric acid]2.0 g/L,sodium fluoride 2.0 g/L,sodium tartrate 4.0 g/L,sodium nitrate 6.0 g/L sodium pyrophosphate 0.5 g/L,EDTA 0.3 g/L,conversion temperature 45℃ and conversion time 25 min.The test results showed that the corrosion current and corrosion potential of the film are 7.152×10-6 A/cm2 and-1.410 V,respectively.The obtained conversion film was uniform and dense,whose surface was covered with spherical particles of uniform size.The main components of the conversion film were Zn3(PO4)2.Zn2Mg(PO4)2,AIPO4.Zn and Mg3(PO4)2.The composition of the supplement solution by the orthogonal and single factor experiments was determined,which was zinc oxide of 1 2.39 g/L,phosphoric acid of 49.55 g/L,sodium fluoride of 11.15 g/L,sodium tartrate of 24.78 g/L,sodium nitrate of 24.78 g/L,sodium pyrophosphate of 2.97 g/L,EDTA of 0.99 g/L,The solution can restore the processing capacity,and the film layer was completely dense when adding supplement solution of 12 mL per 1 dm2 sample.The conversion film for magnesium alloy was prepared by an electrochemical method.The effects of tartaric acid,sodium fluoride and sodium dihydrogen phosphate on the properties of the film were studied and the optimal formulation of the conversion solution was determined:manganese dihydrogen phosphate 30.0 g/L,zinc nitrate 11 0.0 g/L,ammonium molybdate 0.4 g/L,tartaric acid 0.5 g/L,sodium fluoride 0.5 g/L,conversion temperature 25℃ and conversion time of 4 min.The test results showed that the film had the corrosion current of 2.785×10-6 A/cm2,which was 2 orders higher than that of magnesium alloy matrix.The formed film was uniform and dense.The phase compositions of the film layer were Zn3(PO4)24H2O,ZnO,Zn,Mg3(PO4)2 and MnPO4.The orthogonal and single factor experiments were used to determine the supplemental solution:manganese dihydrogen phosphate 234.10 g/L,zinc nitrate 838.86 g/L,ammonium molybdate 3.12 g/L,tartaric acid 3.90 g/L and sodium fluoride 4.68 g/L.The solution can restore the processing capacity and prolong the service life of the treatment liquid when adding the supplementary liquid of 11 mL per 1 dm2 sample.This paper analyzed the film formation process of magnesium alloy phosphate and electrochemical conversion film.The matrix dissolved in the initial stage of the formation of phosphate conversion film.After a short time,the film layer formed rapidly,and the film-forming particles increased.Then the film layer dissolved and grew alternately.The film formation speed was greater than the dissolution rate.Finally,the film layer increased to a certain thickness,which reached the equilibrium stage of deposition/dissolution.At the initial stage of film formation of magnesium alloy electrochemical conversion film,the conversion film was continuously formed on the surface of the substrate,and the gap between the film-forming particles was larger.With the extension of the conversion time,the conversion film was continuously thickened to cover the substrate well.Finally.the open circuit potential gradually became flat and the film layer completely covered the substrate.