Modification Of Conversion Coating On AZ31B Magnesium Alloy Surface

Because of its low density, high specific strength, high specific stiffness, goodmachining and excellent damping performance, magnesium alloys have been widelyused in automobiles, military,3C electronics industry and have become "the21stcentury green engineering material". However, active properties and poor corrosionresistance limit their further application. In order to improve magnesium alloy corrosionresistance, chromate conversion coating technology is widely used because of its lowcost and excellent corrosion resistance. But because of carcinogenicity, the country first"heavy metal pollution prevention12thfive-year plan" limits chromium use in2011. Sochromium-free conversion coating processing technology is urgently needed.Based on the previous research, this paper respectively discusses modificationeffect of the calcium chloride and nitrate cerium on V-Si composite conversion coatingin order to improve Mg alloys’ corrosion resistance. The further study was done forsolution composition and the reaction conditions. Results showed that: the conversioncoating was formed on AZ31B magnesium alloy surface under the condition of2g/Lcalcium chloride,2g/L ammonium metavanadate,0.12g/L fluorosilicate,45℃, pH1.8and12min, neutral salt spray time was up to56h; the conversion coating obtained undercondition of4.0g/L nitrate cerium,1.2g/L ammonium metavanadate,0.4g/Lfluorosilicate,50℃, pH2.5and25min, neutral salt spray time was up to72h. Neutralsalt spray time showed that both conversion coating are more than8h industryapplication standard of magnesium alloy. Both coating corrosion resistance wereimproved. scanning electron microscope, EDS energy spectrum analysis and corrosionresistance results showed that: both coating were morphous structure; the compositionof conversion coating modified with nitrate cerium were O, F, Mg, Al, Si and V; thecomposition of conversion coating modified with calcium chloride was O, Mg, V andCe. Compared with the blank sample, polarization curve of modified samples showedthat the corrosion potential more positive and current density down more than one orderof magnitude, so the corrosion resistance was significantly improved.