陈化对AZ91D镁合金上磷化膜的影响及磷化膜退化机制的研究

以磷酸盐化学转化膜为研究体系,采用交流阻抗(EIS)及红外(FT-IR)分析方法及检测手段,研究陈化时间对AZ91D镁合金磷化膜性能的影响及磷化膜的腐蚀退化机制.研究发现(1) EIS图谱和FT-IR图谱对比结果表明陈化时间对AZ91D磷化膜层性能有影响,磷化膜经陈化72h后更稳定.(2)磷化试样在硼酸缓冲液中浸泡实验的EIS测试表明,磷化膜的腐蚀退化机制分3个阶段:浸泡初期,中期和后期.浸泡初期(5～10h),电解质缓慢向磷化膜渗入,且有少量电解质浸入内层界面;浸泡中期(30～70h),随着内层界面腐蚀反应的发生,腐蚀产物开始对外层晶体层产生破坏作用,使缺陷扩大,导致电解质开始大量进入到内层界面;浸泡后期(80～120h),内层腐蚀反应达到平衡,膜层电阻等各项参数不再改变.

Corrosion degradation mechanism of phosphate coating and aging time on the anticorrosive properties of phosphate coating on AZ91D magnesium alloy

The effect of aging time on the anticorrosive properties of phosphate coating in 3.5wt% NaCl solution and corrosion degradation mechanism of phosphate coating in boric acid buffer solution was investigated by electrochemical impedance spectroscopy (EIS),Fourier transform infrared (FT-IR) spectrum.The results showed that,(1) Comparing the results of EIS with that of FT-IR,the properties of phosphate coating was influenced by aging time,and the phosphate coating was more stable when the aging time exceeds 72h.(2) The degradation process of phosphate coating in boric acid buffer could be divided into three stages:the initial immersion stage,the medium-term and the later immersion.At the initial immersion stage (5-10h),the electrolyte infiltrated the crystal outer layer slowly;the structure of outer layer was unspoiled.At the medium-term (30-70h),defects of the crystal outer layer became to expanding for the detrimental effect of corrosion products,anticorrosive properties of the phosphate coating declined rapidly.Later immersion (80-120h),the corrosion reaction on the inner layer achieved balance,the electrochemical parameters such as resistance of coating was changeless.