包铝的7075和2024合金在海洋大气环境中的点蚀演化机制

EVOLUTION MECHANISM OF PITTING OF Al CLAD 7075 AND 2024 ALUMINIUM ALLOY IN COASTAL ENVIBONMENT

通过带包铝层的7075和2024合金在海洋大气环境中长期现场暴露和室内加速模拟试验,用扫描电镜(SEM)、能谱仪(EDS)、电化学测试系统和扫描Kelvin探针等研究了高强铝合金包铝层中点蚀演化机制。结果表明,在海洋大气环境中暴露20 a后,带包铝7075和2024的点蚀都未穿透表面包铝层;在腐蚀严重的部位,蚀坑底部仍然保留10μm厚的内层包铝。内层包铝含有Al,Zn和Mg元素,外层包铝含有Al和Zn元素。少量Mg元素的存在使得内层包铝在25℃的0.6 mol/L NaCl溶液中腐蚀电位和空气中Kelvin电位都相对外层包铝较正,内层包铝具有较高的耐蚀性。在现场暴露和室内加速模拟试验中,点蚀坑在包铝层中倾向于沿横向扩展而不是向纵深方向发展,最终形成宽而浅的平底状点蚀坑。

Evolution mechanism of pitting of A1 clad 7075 and 2024 aluminium alloy in coastal environment was investigated by long-term field testing and laboratory-accelerated test. Corrosion morphologies, elemental distribution and corrosion potential were observed and analyzed by SEM, EDS and electrochemical analysis system. The result of EDS spectrum showed that the outer cladding layer only revealed the presence of A1 and Zn, while the inner cladding layer still showed certain Mg content besides A1 and Zn. A small quantity of Mg enhanced corrosion resistance of the inner cladding layer, which results that the cladding has not been penetrated by pitting after 20 years exposure in coastal environment. Moreover, the shape of those pits in coastal environment was wide and shallow in field testing and laboratory-accelerated test.