| The application of magnesium alloy was limited due to its poor corrosion resistance.In this paper,the AZ91D magnesium alloy was used as the substrate for surface treatment.Firstly,the surface was pretreated with stannate,phytic acid chemical conversion film,then electroless Ni-P double layer plating and electroless Ni-W-P ternary coating were prepared respectively.The effects of conversion film and heat treatment on the microstructure,phase composition and corrosion resistance of electroless Ni-P double layer coating and Ni-W-P ternary coating were studied in order to improve the corrosion resistance of magnesium alloy and expand the application field of magnesium alloy.The experimental contents and results are as follows:(1)Citric acid was used as main complexing agent.When the concentration of citric acid was 18g/L,the surface morphology of the coating was uniform and compact,and the corrosion potential reached-1.265V,which increased by 0.385V compared with the matrix.The capacitive arc radius has reached the maximum indicating that the Ni-P coating had the best corrosion resistance.(2)Citric acid with concentration of 18g/L as the main complexing agent in this experiment.The electroless Ni-P coating prepared by this experiment was heat treated.It was found that the adhesion between the sample and the matrix was better when the heat treatment temperature was 200 ? after 1 hours of heat preservation.And the corrosion resistance of the sample was higher than that of other temperature heat treatment.The polarization curve results showed that the corrosion potential has reached-1.049V,which moved 0.2V more than that of the untreated sample.The arc radius of capacitive reactance is the largest,and the corrosion rate of immersion test results is also the smallest.(3)Magnesium alloys were pretreated with stannate,phytic acid conversion film,and then electroless Ni-P double coating with low phosphorus in the inner layer and high phosphorus in the outer layer were prepared.The results showed that the coating surface is homogeneous and compact,which is a typical cellular structure.The corrosion potential of the Ni-P coating after treated with phytic acid conversion film reached-0.377V,and the arc radius of capacitive reactance is the largest in the impedance spectrum.The corrosion rate of the Ni-P coating after treated by phytic acid conversion film was the least,which was 0.0268 g/m-h.Thus it was proved that the corrosion resistance of the Ni-P double layer coating after treated with phytic acid conversion film was further improved when the coating was heat treated at 200?.The adhesion between the coating and magnesium alloy substrate was improved.The corrosion potential of the coating increased to-0.335V,the corrosion rate decreased to 0.0213 g/m2·h.(4)Magnesium alloys were pretreated with stannate;phytic acid conversion film,and then electroless plating Ni-W-P three element coating was prepared.The results showed that the surface of the Ni-W-P three element coating was homogeneous and compact,and it was a typical cellular structure,all of which were amorphous structure.The corrosion potential of Ni-W-P coating after phytic acid conversion treatment was the most positive,which was-0.323V.The radius of the resistance arc in the impedance spectrum reached the maximum which indicates that the corrosion resistance of the Ni-W-P coating was the best.The results of soaking experiment also showed that the corrosion rate of Ni-W-P coating after treated with phytic acid conversion film was the lowest,which was 0.0373 g/m-h.Thus it was proved that the corrosion resistance of the Ni-W-P coating after treated with phytic acid conversion film was further improved when the coating was heat treated at 200?,the adhesion between the coating and magnesium alloy substrate was improved,the corrosion potential increased to-0.295V,the corrosion rate decreased to 0.0345 g/m-h,and the corrosion resistance of the coating was further improved. |
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