LY12铝合金环氧涂层在NaCl溶液中的吸水与失效

WATER UPTAKE AND FAILURE OF EPOXY COATINGS ON LY12 ALUMINUM ALLOY IN NaCl SOLUTION

对LY12铝合金/环氧电极在NaCl溶液中于不同浸泡阶段的阻抗参数进行了解析.根据涂层电容值(Cc)随时间的变化看出涂层中水的扩散是通过“两段吸收”来进行的,通过浸泡初期lnCc-t0.5曲线上的线性段求得了水在涂层中的表观扩散系数值.根据电化学阻抗谱中Cl-参与成膜阻抗出现时间求得Cl-在环氧涂层中的扩散系数值为4.67×10^(-12)·cm2s-1.受涂层吸水的影响,涂层电容上升与涂层电阻下降主要发生在浸泡初期,而涂层电极开路电位(Eoc)值的变化贯穿整个浸泡过程.开路电位值经历4个变化阶段:在前期Eoc急剧上升,随后上升速度变缓,在中后期Eoc上升速率再次增大,在浸泡后期Eoc值开始下降,金属基体发生活化,涂层失效.实验表明,含Cl-盐膜的形成、生长及溶解等过程与Eoc间存在密切关系,在LY12铝合金/环氧体系的失效过程中起到关键作用.

Impedance parameters of LY12 Al alloy epoxy electrodes immersed in NaCl solution for different times were calculated by using proper equivalent circuit (EC). A two-stage sorption for water in epoxy coatings was detected from the time dependence of coating capacitance, CC. The apparent diffusion coefficient of water was obtained from the linear part of InCc-t(0.5) curve at the early immersion stage. The diffusion coefficient of Cl-, calculated from the occurrence time of impedance components of Cl- participating salt film in EIS, is 4.67x10(-12) cm(2) . s(-1). Due to the water uptake the increase of coating capacitance and the decrease of the coating resistance mainly take place in the early immersion time, whilst the open-circuit potentials, E-oc, changes throughout the whole immersion time. The variation of E-oc display four stages: a sharp increase in the early time, a slow increase in the second stage, then dramatic increase again in the intermediate stage and a decrease in the final stage with an activation of alloy base and a failure of coatings. The experimental results show a close relationship between the behavior of Cl- containing salt film (the formation, growth and dissolution etc.) and the change of E-oc, indicating a major role of the salt film on the degradation of LY12 Al alloy/epoxy system.