5083铝合金环氧涂层盐水浸泡失效研究

Study on Failure Process of Epoxy Coating on 5083 Aluminum Alloy under Salt Immersing Condition

目的研究铝合金基体上环氧涂层在盐水浸泡环境中的电化学阻抗谱变化规律,揭示涂层失效的原因和机制。方法采用电化学阻抗谱技术、红外测试、扫描电镜及能谱分析等方法,研究涂覆在5083铝合金基体上的环氧涂层在盐水浸泡过程中的裂化过程和失效机制。结果在3.5%(质量分数)Na Cl溶液的连续浸泡下,涂层电阻明显下降、电容明显增大,相应的涂层孔隙率和吸水体积百分数均逐渐增加;长期浸泡后,涂层孔隙率和吸水体积百分数均趋于稳定。随着浸泡时间的增加,涂层表面孔洞等缺陷增多,保护作用减弱;涂层内氧元素含量逐渐增加,碳元素含量逐渐减小;涂层内有羟基生成和C—O键的断裂发生。结论环氧涂层中环氧官能基团在电解质溶液中发生水解,水解形成羟基和氨基等亲水基团以及涂层中存在的孔洞等缺陷,进一步促进电解液的渗透,加速涂层的劣化。金属基体表面腐蚀反应会促进涂层与基体的剥离,腐蚀产物在涂层内的累积,也会导致涂层内孔隙和缺陷增多,促进涂层劣化。

Objective To study EIS evolution of 5083 aluminum alloy with epoxy coating under salt immersing condition, and to discuss the failure reason and mechanism of the coating. Methods The degradation process and the failure mechanism of an epoxy primer coating on 5083 aluminum alloy in the 3.5% （ mass fraction） NaC1 solution were investigated by EiS, SEM, EDS and FTIR.Results Under continuous immersion condition in the 3.5% （mass fraction）NaC1 solution, the coating resistance decreased and the coating capacitance increased along with immersion time. The porosity and water absorptivity of coating also increased with time, but after a long immersion, they showed little change. With increased immersion time, the pores in coating increased, and the coating protection decreased. With increased immersion time, oxygen content in coating increased, while carbon content declined. Due to hydrolysis of epoxy groups in coating, the group -OH was produced and the C-O bond was partly broken. Conclusion The degradation of the coating under salt immersion condition was mainly due to the hydrolysis of epoxy groups in electrolyte solu- tion. The hydrophilic groups such as --OH and NH2- from the hydrolysis of epoxy groups, as well as the defects of coating such as micro-pores, promoted the electrolyte solution penetration and accelerated the. degradation process. Moreover, corrosion of metal substrate can result in disbonding of coating/substrate interface, the accumulation of corrosion products in the coatings also can lead to increase of pores and defects and degradation of the coating.