磷化液pH值对磷酸盐转化膜在镁合金AZ91D表面成型及耐蚀性的影响

Effect of the pH Value of Phosphating Solution on the Formation and Corrosion Resistance of Phosphate Conversion Coating on the Surface of AZ91D Magnesium Alloy

为进一步揭示酸碱度对镁合金表面转化膜成型的影响,研究了磷酸盐型化学转化膜在镁合金AZ91D表面成型的机理,重点探讨了转化膜的形貌及耐蚀性受磷化液p H值的影响;对比了转化膜的动电位极化测试数据、电化学阻抗谱,通过浸泡试验对不同pH值条件下制备的磷酸盐的转化膜在3.5%的NaCl溶液中的耐蚀情况进行了分析。结果表明:磷酸盐转化成膜的关键在于磷酸二氢锰[Mn(H_(2)PO_(4))_(2)]的水解平衡过程和偏钒酸铵(NH_(4) VO_(3))的水解缩合过程的共同作用。转化膜受磷化液p H值影响较大,随着pH值增加耐腐蚀性能逐渐提升,但测得膜层呈先增厚、达到极值后再减薄的趋势。当磷化液pH值为4.0时,转化膜层的质量最好,具有最好的抗腐蚀性能。

For further revealing the effect of p H value on the formation of conversion coating on magnesium alloy surface,the formation mechanism of phosphate chemical conversion coating on AZ91D magnesium alloy surface was studied,and the influence of the p H value of phosphating solution on the morphology and corrosion resistance of the conversion coating was mainly discussed.The potentiodynamic polarization test data and electrochemical impedance spectroscopy of the conversion coatings were compared,and the corrosion resistance of phosphate conversion coatings prepared in 3.5%Na Cl solution under different p H conditions was analyzed by soaking test.Results showed that the key to the film formation of phosphate was the combined action of the hydrolysis equilibrium process of manganous dihydrogen phosphate[Mn(H_(2)PO_(4))_(2)]and the hydrolysis condensation process of ammonium metavanadate(NH_(4)VO_(3)).The conversion coating was greatly affected by the p H value of the phosphating solution.With the increase of p H value,the corrosion resistance of the conversion coating was gradually improved,but the thickness showed a trend of increasing first and then decreasing after reaching the extreme value.When the p H value of phosphating solution was 4.0,the coating had the best quality and the optimal corrosion resistance.