| Magnesium alloys have been widely applied in many fields,such as the automobile,aerospace,electronics and biomedicine,because of their high strength-to-weight ratio.There are confronting with environmental pollution and resource shortage,magnesium alloys have received more attention.However,magnesium alloys have high chemical activity and easily corroded.The poor corrosion resistance of magnesium alloys greatly limits its further application.In this paper,the zinc phosphate conversion coatings are prepared on the surface of AZ91D magnesium alloy to solve the problem of poor corrosion resistance of magnesium alloy,which effectively improve the corrosion resistance of magnesium alloy.In this paper,the effects of different zinc oxides on the corrosion resistance of zinc phosphate conversion coatings were investigated.The zinc phosphate conversion coatings were prepared by using nano zinc oxide and micron zinc oxide as zinc sources respectively.The microstructure and corrosion resistance of zinc phosphate conversion coatings were investigated by SEM,XRD and electrochemical workstations.EDS analysis were carried out on the conversion coatings of different phosphating time to explore the effect of zinc oxide on the growth of zinc phosphate conversion coatings.The results showed that MACC?micron ZnO-assisted zinc phosphate conversion coatings?is porous and has many micro-cracks on the surface,and the corrosion resistance of magnesium alloy can't be improved obviously.On the contrary,NACC?nano ZnO-assisted zinc phosphate conversion coatings?is uniform and dense,with few cracks on the surface,and exhibits good corrosion resistance,the corrosion potential(Ecorr)is-1.29 V,and the corrosion current density(icorr) is3.43×10-5 A·cm-2.Zinc oxide acts as the core of zinc phosphate crystal and zinc oxide uniformly adsorbed on the grain boundary of magnesium alloy,which reduces the size of zinc phosphate crystal.The orthogonal experimental design was used to investigate the influence of the composition of phosphating solution on the zinc phosphate conversion coatings.The corrosion resistance of the conversion coatings was evaluated by electrochemical workstation.The best corrosion resistance is obtained by the range analysis:1.25g/L NaNO3 3g/L C6H8O7·H2O,2.5g/L NaF,5.5g/L ZnO,12.5mL/L H3PO4,reaction temperature 50?,reaction for 30 minutes.The corrosion resistance of the zinc phosphate conversion coating prepared by this process is further improved,the corrosion potential(Ecorr)is-1.04 V,and the corrosion current density(icorr)is1.1×10-5 A·cm-2.The corrosion resistance and corrosion behavior of zinc phosphate conversion coating were investigated by full immersion corrosion test.SEM,XRD,FT-IR and EDS were used to analyze the morphology and composition of the conversion coatings at different immersion times.At the same time,the corrosion resistance of the samples with different immersion time was investigated by electrochemical workstation.The experimental results showed that the immersion corrosion process of zinc phosphate conversion coating is divided into three stages:the dissolution of conversion coatings,the corrosion of matrix and the deposition of insoluble matter.The average corrosion rate in the first corrosion stage is about 0.355 mg/cm2·h,and the average corrosion rate in the second stage is significantly reduced by about 0.102mg/cm2·h.The corrosion rate in the third stage tends to be stable,and the corrosion rate is about 0.0106 mg/cm2·h.With the prolonged immersion time,a dense flaky Mg?OH?2 layer is formed on the surface of the sample,which hinders the invasion of Cl-and reduces the corrosion rate of the magnesium alloy.The deposited Mg?OH?2 is a two-layer structure.In the initial deposition stage,it is mainly evenly dispersed on the surface of the alloy to form a tightly arranged inner layer.With the extension of immersion time,the polarity of Mg?OH?2 crystals becomes stronger,and the agglomeration takes place and becomes a ball,becoming the outer layer. |
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