(NaPO3)6对AZ91D镁合金微弧氧化陶瓷层电化学腐蚀特性的影响

在Na2SiO3-KOH电解液体系中添加一定量的(NaPO3)6, 利用微弧氧化(MAO)技术在AZ91D 镁合金表面制备了原位生长的陶瓷层. 采用动电位极化和电化学阻抗谱(EIS)技术研究了添加(NaPO3)6前后, 制备的陶瓷层在3.5%(w) NaCl溶液中的室温电化学行为. 结果表明, 添加(NaPO3)6后, 陶瓷层的自腐蚀电位显著上升, 自腐蚀电流密度明显减小. 这主要是由于(NaPO3)6增加了反应过程中基体镁合金表面的“氧空位”和溶液中PO3-4的含量, 促使元素Mg在金属/膜层(M/F)界面上快速形成相应氧化物, 从而增加了陶瓷层的厚度和致密性. 根据电化学反应体系和陶瓷层的特殊结构, 建立了合理的等效电路, 并结合EIS 数据, 分析了添加(NaPO3)6提高陶瓷层耐电化学腐蚀性能的机理.

Effect of(NaPO3)6 on Electrochemical Corrosion Characteristic of Micro-arc Oxidation Ceramic Coatings Formed on AZ91D Mg Alloy

(NaPO3)6 was added to Na2SiO3-KOH electrolyte system, and in-situ ceramic coatings were prepared on AZ91D magnesium alloy by micro-arc oxidation technique. Room temperature electrochemical behavior of the ceramic coatings, formed in electrolyte with and without (NaPO3)6, in NaCl (w=3.5%) solution was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the ceramic coating formed in electrolyte containing (NaPO3)6 indicated a considerable increase in corrosion potential and a remarkable decrease in corrosion current density compared to the bare AZ91D magnesium alloy. The main cause was that the addition of (NaPO3)6 increased oxygen vacancies on the Mg substrate surface and the content of PO3-4 in the electrolyte. As a result, the (NaPO3)6-addition speeded up the formation of Mg oxide at the interface between metal and ceramic film(coating) (M/F), and increased the thickness and compactness of the ceramic coating. Based on the studied electrochemical reaction system and the special structure of the ceramic coating, a reasonable equivalent circuit was established. Combining with the EIS data, the effect mechanismof (NaPO3)6-addition on improving the electrochemical corrosion resistance of the ceramic coatings was investigated.