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目的：通过Mo-Mn无铬转化膜提高AZ91 D镁合金的表面耐蚀性。方法采用正交实验法,研究不同浓度的NaMoO4和KMnO4以及温度对转化膜的影响。优选实验参数后,考察时间对转化膜的影响。利用SEM及EDS研究转化膜的微观形貌及成分变化,测试转化膜在3.5%NaCl溶液中的极化曲线和交流阻抗谱。结果当NaMoO4和KMnO4的质量浓度分别为10,6 g/L,pH=5,温度为50℃,转化时间为40 min时,转化膜颜色较为均匀,微观裂纹相对较少,自腐蚀电位比镁基体大约提高0.075 V,自腐蚀电流密度比镁基体降低近1个数量级。当NaMoO4和KMnO4的质量浓度分别为20,8 g/L,pH=5,温度为50℃,转化时间为40 min时,转化膜颜色最为均匀,微观裂纹相对最少,自腐蚀电位比镁基体提高大约0.047 V,自腐蚀电流密度比镁基体降低2个数量级。交流阻抗谱图显示,后一种转化膜试样的极化电阻为1450.2Ω,而镁基体的极化电阻为806.4Ω。结论 Mo-Mn无铬转化膜可以显著提高AZ91D镁合金的表面耐蚀性。

ABSTRACT:Objective To improve the surface corrosion resistance of AZ91D magnesium alloy by Mo-Mn chromium-free conver-sion coating. Methods Effects of different concentrations of NaMoO4 and KMnO4 and temperature on the transformed sample film were studied by orthogonal experimental method. After optimizing experiment parameters, the effects of the time on the conversion film was also studied. The morphology and component of conversion coatings were researched by SEM and EDS. Moreover, the po-larization curves and electrochemical impedance spectroscopy of conversion coating were tested in 3. 5%NaCl aqueous solution. Results The results showed that when the condition was 10 g/L NaMoO4 , 6 g/L KMnO4 at 50 ℃ and pH 5 for 40 min, where the morphology of the sample was relatively even and the amount of cracks was relatively low. Compared to the corrosion resistance of magnesium substrate, the corrosion potential of conversion film was increased by about 0. 075 V ,and the corrosion current density decreased by nearly 1 order of magnitude. When the condition was 20 g/L NaMoO4 , 8 g/L KMnO4 at 50 ℃ and pH 5 for 40 min, the morphology of the sample was the most even and the amount of cracks was the least. Compared to the corrosion resistance of magnesium substrate, the corrosion potential of conversion film was increased by about 0. 047 V ,and the corrosion current density decreased by nearly 2 orders of magnitude. EIS showed that the polarization resistance of the latter conversion film sample was 1450. 2 Ω, while the polarization resistance of magnesium matrix was 806. 4 Ω. Conclusion Mo-Mn chromium-free conversion coating could obviously improve the surface corrosion resistance of AZ91D magnesium alloy.