| Magnesium(Mg)alloy is lower in density,which is only 1.7 g/cm3.Therefore,it was widely used in the aerospace,automotive,electronics and other fileds which have high requirements on the use of light weight metal materials.However,poor corrosion resistance as a bottleneck has been restricting its development and application.In order to improve the corrosion resistance of Mg alloy,the design of new type Mg alloy and surface treatment are the main methods used currently.The properties of die casting,fatigue properties,compressibility and strength of the Mg alloy were improved when added with proper content of aluminum(Al).However,the research on the effect of A1 content on the microstructure and corrosion resistance of Mg alloy and plasma eletrolytic oxidation(PEO)coating was less studied in recent years.In this dissertation,the influences of Al content on the microstructure and corrosion resistance of Mg alloy and its PEO coatings were investigated systemically.The results were summarized as follows:(?)The corrosion behavior of three Mg alloy including AZ91,AZ111 and AZ141(near the eutectic point)(hypoeutectic and hypereutectic structure)in 3.5 wt.%NaCl aqueous solution was studied.The analysis of microstructure indicated that the Al content could affected the composition and distribution of?phase and eutectic phase.The corrosion behavior of Mg alloy were depended on its microstructure.First,the increasing volume fraction of eutectic phase promoted the corrosion of?phase.Second,the content and distribution of?phase in Mg alloy were the main factors on the corrosion resistance of Mg alloy.The corrosion degree was affected by the content of?phase,and the corrosion process was affected by the distribution of?phase.With the increasing Al content,the volume fraction of?phase increased,and accordingly the acceleration effect of galvanic corrosion played a leading role.At the same time,in the corrosion process,the network distribution of?phase presented a certain blocking effect.However,due to the discontinuity of the distribution of?phase,the corrosion could be proceed to a phase along the(3 phase in discontinuous place.(?)The relationships between the microstructure of the substrate Mg alloy,the microstructure of the PEO coating and the corrosion resistance of the three PEO coatings were investigated.The four Mg alloys with various Al content were selected as test specimens.The PEO coatings were produced on these four substrates,the components,defects and corrosion behavior of these coatings were systemically investigated.Results indicated that the volume fraction of ? phase and interfaces of?/eutectic phase(?+?)increased with the increasing A1 content,which have strong influences on the microstructure,i.e.,the components and the porosity level of PEO coatings.It could be attributed to that the differences in plasma discharge intensity and plasma dischage type in PEO process.It was found that the corrosion initial site occurred at the microstructural defects of different PEO coatings.(?)The corrosion initial site of the PEO coatings of Mg alloys preferentially initiated above the interface of different phases,which was attributed to that unstable discharge in two phase alloy in the PEO process could resulted in the dielectric barrier layer have microstructural defects.Moreover,the formation of the dielectric barrier was a necessary condition for the the ignition of arcing in PEO process.Furthermore,the quality of the dielectric barrier has a direct effect on the discharge quality,which affected the stability of the plasma discharge in PEO process and the quality of subsequent coating formation.Therefore,the pretreatment method was adopted to make the electrochemical properties of the surface of the Mg alloy uniform.In this section,the effects of hydrofluoric acid(HF)pretreatment on the microstructure and corrosion resistance of PEO coatings deposited on AZ91 Mg alloys were investigated.The results showed that thin fluoride film was formed on AZ91 Mg alloy when AZ91 Mg alloy was dipped in HF solution with or without ultrasound(US).PEO coatings deposited on the pretreated AZ91 became denser with decreased porosity level and have no structural defects.The inner layer(IL)of PEO coating of HF treated AZ91 with US was more intact and contained more F than those obtained without US and pretreatment.Therefore,PEO coating deposited on AZ91 pretreated with(HF+US)offered a better corrosion resistance. |
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