ON THE CORROSION MECHANISMS OF Al-Cu-Li ALLOYS: AN INVESTIGATION USING GLOBAL AND LOCAL ELECTROCHEMICAL TECHNIQUES

In this study, the corrosion mechanism of an Al-Cu-Li alloy manufactured by two different treatment routes (T3 and T851) was evaluated by immersion and electrochemical tests in solutions containing chloride ions (Cl-). For both alloys, the formation of cavities on the surface was associated with micrometer-sized intermetallics (IM’s), however, in addition to this attack, the alloy submitted to T851 treatment also presented an attack called severe localized corrosion (SLC), caused by the preferential attack to the nanometric T1 (Al2CuLi) phase. The electrochemical concepts involved in these two types of attacks were discussed. During the IM’s corrosive process, whereas the O2 reduction occurred over the IM’s, the Al dissolution is favored around the particle, forming trenching and cavities (with 2 and 6 μm of depth). On the other hand, the mechanism associated with the SLC is related to the formation of a differential aeration cell followed by the evolution of H2, with greater depth of attack penetration (8 and 35 μm). Additionally, by the use of the Scanning Vibrating Electrode Technique (SVET), it was concluded that the higher anodic currents observed for the T851 temper were related to the relation between the anodic area (Aa) and the cathodic area (Ac).

本研究针对经两种不同工艺（T3与T851）制备的Al-Cu-Li合金，在含氯离子（Cl⁻）的溶液中通过浸泡试验与电化学测试开展腐蚀机制研究。两种合金表面空腔的形成均与微米级金属间化合物（intermetallics，IMs）密切相关；但经T851处理的合金除上述腐蚀损伤外，还出现了一种被称为严重局部腐蚀（SLC）的失效形式，该损伤由对纳米级T1相（Al₂CuLi）的优先腐蚀所引发。本文对上述两类腐蚀过程所涉及的电化学原理进行了讨论。在金属间化合物的腐蚀进程中，氧还原反应发生于金属间化合物表面，而颗粒周边区域更易发生铝溶解，进而形成沟槽与空腔（深度可达2~6 μm）。与之相对，严重局部腐蚀的机制则与差异充气电池的形成及氢气析出相关，其腐蚀穿透深度更大（可达8~35 μm）。此外，通过扫描振动电极技术（SVET）的测试结果可得出结论：T851态合金所观测到的更高阳极电流，与阳极面积（Aa）和阴极面积（Ac）的比值密切相关。