Study On The Calcium Phosphating Process And The Corrosion Resistance Properties Of AZ31Magnesium Alloys

Chemical conversion is one simple and low cost surface treatment method forimproving of the corrosion resistance properties of magnesium alloys. The traditionalchromate conversion film of magnesium alloys has good corrosion resistance properties,but Cr6+generated in the chromate conversion of magnesium alloys is poisonous toanimal and result in the environment pollution. So chromium-free conversion coatingtechnology, especially phosphate conversion technology, has been paid more and moreattention.The calsium phosphate conversion treatment and the corrosion resisitance propertiesof AZ31magnesium alloys have been investigated in this dissertation. Throughorthogonal designing method deciding the optimal calsium phosphaing process, with dripand immersion test as the asseement standard for the corrosion resistance properties ofthe phosphating film. Phosphating time and temperature have been qualified based on theresults of orthogonal experiment, the final optimal experimental process is as follows:pH=3.0, NaNO3/inter-nitrobenzene sulfonate=10/1.0g/L, tartrate=0.5g/L， sodiummolybdate=0g/L, phosphating time t=20min, phosphating tempeture T=50℃.By means of scanning electron microscopy(SEM), energy distributionspectrum(EDS), X-ray diffraction(XRD), the morphology, compositon and structure ofphosphate conversion film have been studied．The results show that the phosphateconversion film prepared under the final optimal process formed on all substrate ofmagnesium alloys and the phosphate film was uniform, compact and mainly consists ofCa, P, O, H. The phosphate conversion film has better corrosion resisitance by drop,immersion and electrochemical polarization curve test.Through sealing treatment with5%Na2SiO3solution, the corrosion resistance ofphosphating conversion film of magnesium alloys has been improved, it is because ofthat the sealing reagent fill the film voids directly or react with magnesium alloy to formsilicon and magnesium salts. There mainly occurred pitting corrosion on AZ31magnesium alloy. The corrosionof calsium phosphate conversion films occurred from there that the film isinhomogeneity or uneven.The formation of calcium phosphating conversion film on the surafce of AZ31magnesium alloys is under the condition of electrochemistry corrosion of the substrate,the formation mechanism of film is chemical and electrochemical reaction jointmechanism.The formation of film is consists of three stages: early film formed in1~5min, thereis a very thin film formed on the substrate; film growth period is in5~10min, phosphateconversion film covered all the substrate in this step and distributed nonuniformly; filmgrowth balance period is in10~20min, flowerlike phosphating conversion coating formedin this period is uniform and compact, and can cover all the substrate, and the filmformation reach an equilibrium state.