釩酸根對 AZ31 和 AZ91 鎂合金硝酸鈰化成皮膜結構與性質之影響

Abstract

Magnesium-based alloys have attracted many attentions these years because of their outstanding properties. However, the inadequate corrosion resistance limits the application of magnesium and its alloys. Many studies have established the feasibility of the conversion coating of rare earth metal salts on Mg alloys. Especially, the characteristic of inhibition of metal corrosion by cerium salt extends the research interest in the field of conversion coating. However, a long period of conversion time is necesssary to from a cerium conversion coating. Although adding H2O2 accelerates the conversion rate, the excess hydrogen evolution leads to the blisters on the coating, which has a blister area of 58 %. The present study adds metavanadate into cerium-based conversion solution in order to obtain a conversion coating with dense structure for improved adhesion and corrosion resistance. The analyses of microstructure observations, EIS and salt spray test are conducted to investigate the conversion coating on AZ31 and AZ91. The results indicate the conversion solution containing metavanadate produces a thin bilayered coating without blisters on Mg alloys. The corrosion resistance is improved significantly. Moreover, increasing the concentration of metavanadate promotes the influence of vanadate on the properties of the conversion coating. Furthermore, this work adjusts the pH value of metavanadate solution prior to the addition to the conversion solution, for improving the stability of conversion solution. Adding the solution of metavanadate with pH 2.8-2.9, the conversion coating exhibits good corrosion resistance within short conversion times. Meanwhile, the formation of defect is inhibited. The surface defects area can be reduced to 36 %, and the size of each blister becomes much smaller. The present study compares the different cerium-based conversion coatings with adding metavanadate. Finally, the mechanisms of conversion and defect formation are discussed in details.