硼酸对镁合金达克罗涂层耐蚀性能的影响

目的提升镁合金的耐腐蚀性能。方法采用电化学测试、盐雾试验、XRD及SEM等方法,研究了不同硼酸添加量对AZ31镁合金达克罗涂层组织与耐蚀性能的影响,并分析其成膜机理。结果镁合金达克罗涂层主要由Zn、Mg、MgCrO_4、ZnO、MgO、Cr_2O_3、CrO_3组成,添加硼酸后,涂层中出现B_2O_3。MgCrO_4、MgO含量随着硼酸含量的增加而降低,而Cr_2O_3和CrO_3含量增加,并在硼酸加入量(质量分数)为2%的涂层中达到极限值。未添加硼酸的达克罗涂层表面存在微孔及微裂纹,添加2%硼酸的涂层的致密性能得到提高,当进一步增加硼酸含量时,涂层的致密性再次降低。添加2%硼酸涂层的腐蚀电流密度为5.068×10~(-5)A/cm~2,比未添加硼酸涂层时降低了1个数量级。涂层的电化学阻抗谱容抗弧半径和阻抗值,均在2%硼酸添加量时达到最大值,此时涂层耐蚀性能最好。结论硼酸具有促进成膜的作用,添加2%硼酸能够增加涂层的致密性,提高涂层的耐蚀性能。

Effects of Boron Acid on Corrosion Resistance of Dacromet Coating on Magnesium Alloys

The work aims to improve corrosion resistance of magnesium alloy. Effects of different content of boric acid content on microstructure and corrosion resistance of Dacromet coating on AZ31 magnesium alloy were studied in methods including electrochemical test, salt spray test, X-ray diffractometer (XRD) and scanning electron microscope (SEM). The Dacromet coating on magnesium alloy consisted of Zn, Mg, MgCrO4, ZnO, MgO, Cr2O3 and CrO3. B2O3 appeared on the coating as boric acid was added. The content of MgCrO4 and MgO decreased while the content of Cr2O3 and CrO3 increased (reached the limit value in the coating containing 2% boric acid) as the addition of boric acid increased. Micropore and micro-cracks were present on the surface of the boric acid-free Dacromet coating. Density of the coating containing 2% boric acid could be improved. The coating density was reduced again with the further increase of boric acid. Corrosion current density of the coating with 2% boric acid was 5.068×10-5 A/cm2, 1 order of magnitude lower than that of the boric acid-from coating. Capacitive arc radius and impedance of the coating on electrochemical impedance spectroscopy reached the maximum as 2% boric acid was added, and the corrosion resistance was the best. Boric acid promotes film formation. Adding 2% boric acid can increase the coating density and improve corrosion resistance of the coating.