高强铝合金在大气环境下剥蚀机理的原位研究

High strength aluminum is extensively used in the aeronautic and aerospace industry. However, both 2024-T4 and 7075-T6 suffered exfoliation when they are exposed in atmospheric environment for different periods of time. In this proposed research, the exfoliation mechanism of 2024 and 7075 will be investigated using X-ray microtomography and a new laboratory-accelerated testing technique, the 3-demension information about the internal microstructural changes will be otained. The proposed research covers three following aspects: Firstly, the initial stage of exfoliation will be studied by the electrochemical method, including the effects of breakdown potential and pitting potential on the formation of anodic corrosion path, and the preferential dissolution mechanism of precipitated phases and precipitated free zone (PFZ). Subsequently, the optimized electrochemical pretreatment will be used to initiate intergranular corrosion on the specimen. Secondly, the exfoliation processes in the atmospheric environments will be in-situ monitored. The effects of the environmental factors including relative humidity (RH), temperature, chloride ions, sulphate ions and pH value on the exfoliation kinetics will be investigated. Meanwhile, the role of corrosion product in the mechanical loading at exfoliation crack tips will be studied. Thirdly, the influence of aging process on exfoliation mechanism will be studied, including the distribution of precipitated phases, the character of PFZ and grain size. The proposed research would be very useful to the interpret the exfoliation mechanism of high strength aluminum alloy.

高强铝合金是航空航天领域不可或缺的结构材料，然而前期实验结果表明2024-T4和7075-T6在大气环境中暴露一定年限后均发生了剥蚀。本课题将采用X 射线显微断层成像技术，并结合新型模拟加速剥蚀试验等技术，从而获取2024和7075铝合金在大气剥蚀过程中材料内部的三维腐蚀信息。本课题将重点研究以下三个方面；（1）剥蚀初级阶段的电化学预制备。主要包括击穿和孔蚀等电位下阳极溶解通道形成过程、晶界析出相和晶界无沉淀带等选择性溶解机理。（2）原位研究大气环境因素对2024和7075剥蚀的影响机理。包括大气环境单因素和多因素作用下的剥蚀动力学、各环境因素的临界值、腐蚀产物对剥蚀裂纹尖端扩展作用机制等。（3）原位研究不同时效工艺时显微组织对2024和7075在大气环境下剥蚀的影响机理。主要包括析出相分布、无沉淀析出带特征和晶粒尺寸等。本课题的实施有利于深入理解高强铝合金在大气环境中的失效机制。

高强铝合金是航空航天领域不可或缺的材料，然而2024和7075在万宁、琼海、青岛和江津试验站暴露先后发生剥蚀，这影响了飞行器的服役安全。现有的EXCO等剥蚀模拟加速实验方法与现场暴露试验结果相差较大，且难以研究大气环境因素作用下高强铝合金的剥蚀机理。本课题利用新型的剥蚀模拟加速实验方法和同步辐射X射线显微断层扫描技术，研究了大气环境因素下高强铝合金的剥蚀机理。主要研究内容包括三个方面：①剥蚀初级阶段的电化学预制备；②大气环境因素对2024-T4和7075-T6铝合金剥蚀的影响机理；③时效工艺对2024和7075铝合金在大气环境下的剥蚀影响机理。研究发现：①2024-T4和7075-T6在击穿电位下发生晶间腐蚀（剥蚀初级阶段），2024-T4晶间腐蚀制备方法为：在恒电流密度0.83 mA/cm2下预极化32 h；7075-T6晶间腐蚀制备方法为：在恒电位-710 mV下预极化4 h。②在模拟海洋大气环境中，2024-T4预极化试样剥蚀的临界相对湿度（CRH）为75%，且湿度高于CRH后剥蚀速率与RH、温度和Cl-浓度正相关，但当RH达到96%后剥蚀速率又开始减缓。在模拟海洋-工业大气环境中，2024-T4预极化试样剥蚀的CRH为89%，且剥蚀孕育期比模拟海洋大气环境中的更长，当环境湿度高于CRH时剥蚀速率与SO42-浓度负相关，且RH达到96%后剥蚀速率开始减缓。在模拟工业和无污染大气环境中，2024-T4预极化试样经两个月的加速试验后未发生剥蚀。SO42-对剥蚀的发生有抑制作用。这些结果能够解释2024-T4在现场的暴露实验结果。③2024-T4和7075-T6挤压型材粗晶环下呈现细晶区，且晶粒被拉的越长剥蚀也越严重。2024-T4在腐蚀微电池作用下晶界区的Al2CuMg、贫铜区和Al2Cu相继发生溶解将引起2024-T4剥落腐蚀。7075-T6在腐蚀微电池作用下晶界区域的MgZn2、Al2CuMg和贫铜区的相继溶解将同样引起剥落腐蚀。此外，2024过时效热处理有利于抑制合金的剥蚀敏感性。④同步辐射X射线断层扫描结果表明粗晶区和细晶区交界区易发生剥蚀；剥蚀腐蚀产物并没有出现剥蚀裂纹的尖端，剥蚀裂纹的扩展和繁殖主要是靠腐蚀产物生成过程中的“楔形力”引起的。这些研究结果为在大气环境中预防高强铝合金发生剥落腐蚀，以及研发新型耐剥落腐蚀高强铝合金奠定了坚实的理论基础。