AZ31镁合金表面镧转化膜的形成机制

Formation Mechanism of Lanthanum Conversion Coatings on AZ31 Magnesium Alloys

通过化学浸泡方法在AZ31镁合金表面制备了高防护性镧转化膜。X射线光电子能谱(XPS)对膜组成的测试结果表明:镧转化膜主要由氢氧化镧组成,另外还有少量氢氧化镁及碳酸镧、碳酸镁成分。电化学技术测量成膜过程中试样开路电位(OCP)的变化,分为剧增、降低、缓慢平稳增长3个阶段。原子力显微镜(AFM)对膜层形貌的表征结果表明:镧转化膜最初为均匀致密膜,后出现纤维结构,然后纤维结构不断稳定发展。在上述结果基础上讨论了镧转化膜的形成机制:成膜过程分为紧密层形成、纤维层萌生、纤维层稳定生长3个特征阶段,前两个阶段发生非常迅速,后一阶段持续时间较长,是成膜的主要过程。

The lanthanum conversion coatings with strong protective ability were prepared on AZ31 magnesium alloy by chemical immersion treatments. The X-ray photoelectron spectroscopy （XPS） characterization revealed that the conversion coatings mainly consisted of lanthanum hydroxide as well as small amount of magnesium hydroxide and lanthanum/magnesium carbonate. The evolution of OCP （ Open Circuit Potential） during coating formation involved rapid increasing stage, decreasing stage and slowly stable increasing stage. The atomic force microscopy （AFM） characterization of the coating surface morphology showed the initial compact layer and the sequential fibrous feature. Based on the experimental results, the formation mechanism of lanthanum conversion coating was proposed. There were three characteristic stages during coating formation, which were the formation of the compact layer, the germination of the fibrous layer and the stable growth of the fibrous layer. The former two stages occurred quickly whereas the last stage lasted long time, which was the major step during coating formation.