The corrosion behaviors of 1420 and 2195 Al-Li alloys under 308 and 490 MPa tensile stress respectively in neutral 3.5% NaCl solution were investigated using electrochemical impedance spectroscopy(EIS) and scanning electron microscope(SEM). It is found that the unstressed 1420 alloy is featured with large and discrete pits, while general corrosion and localized corrosion including intergranular corrosion and pitting corrosion occur on the unstressed 2195 alloy. As stress is applied to 1420 alloy, the pit becomes denser and its size is decreased. While, for the stressed 2195 alloy, intergranular corrosion is greatly aggravated and severe general corrosion is developed from connected pits. The EIS analysis shows that more severe general corrosion and localized corrosion occur on the stressed 2195 Al-Li alloy than on 1420 Al-Li alloy. It is suggested that tensile stress has greater effect on the corrosion of 2195 Al-Li alloy than on 1420 Al-Li alloy.
The effects of aging on mechanical properties, intergranular corrosion and exfoliation corrosion behaviors of a 2197 type Al-Li alloy were investigated, and the mechanisms were studied through microstructure observation and electrochemical measurement of simulated bulk phase. The main strengthening precipitates of the alloy aged at 175 ℃ and 160 ℃ are δ′ and T1. T1 precipitation in the alloy aged at 160 ℃ is delayed, which results in its slower age strengthening and over-aging behavior than the alloy aged at 175 ℃. Meanwhile, aging temperature of 160 ℃ causes more uniform distribution and finer size of T1, resulting in its better strengthening effect. As aging time and aging temperature are increased, the size of T1 at grain boundaries and the width of PFZ along grain boundaries are increased, leading to an increase in the susceptibility to intergranular corrosion and exfoliation corrosion. It is suggested that better comprehensive properties can be obtained when the alloy is aged at 160 ℃.
To clarify the localized corrosion mechanism associated with precipitates containing Mg in Al alloys, the simulated bulk precipitates of S and β were synthesized through melting and casting. Their electrochemical behaviors and coupling behaviors with α(Al) in NaCl solution were measured. Meanwhile, simulated Al alloys containing S and β particles were prepared and their corrosion morphologies were observed. It’s found that there exist two kinds of corrosion mechanisms associated with precipitates containing Mg. The precipitate of β is anodic to the alloy base, resulting in its anodic dissolution and corrosion during the whole corrosion process. While, there exists a corrosion conversion mechanism associated with the S precipitate, which contains active element Mg and noble element Cu simultaneously. At an initial stage, S is anodic to the alloy matrix at its periphery and the corrosion occurs on its surface. However, during its corrosion process, Mg is preferentially dissolved and noble Cu is enriched in the remnants. This makes S become cathodic to α(Al) and leads to anodic dissolution and corrosion on the alloy base at its periphery at a later stage.
铝合金中通常含有一些时效析出相及杂质相等第二相粒子,这些第二相粒子对铝合金局部腐蚀行为产生重大影响。概括了析出相θ(Al_2Cu)、η(MgZn_2)、β(Mg_3Al_2)及含 Fe 杂质相 FeAl_3等在铝合金局部腐蚀中的作用机制,重点介绍了 S 相(Al_2CuMg)和 T1相(Al_2CuLi)对铝合金局部腐蚀作用机制的不同观点。
To clarify the corrosion mechanism associated with the precipitate of T1(Al2CuLi)in Al-Li alloys,the simulated bulk precipitate of T1 was fabricated through melting and casting.Its electrochemical behavior and coupling behavior with α(Al)in 3.5% NaCl solution were investigated.Meanwhile,the simulated Al alloy containing T1 particle was prepared and its corrosion morphology was observed.The results show that there exists a dynamic conversion corrosion mechanism associated with the precipitate of T1.At the beginning,the precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface.However,during its corrosion process,its potential moves to a positive direction with immersion time increasing,due to the preferential dissolution of Li and the enrichment of Cu.As a result,the corroded T1 becomes cathodic to the alloy base at a later stage,leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery.It is suggested that the localized corrosion associated with the precipitate of T1 in Al-Li alloys is caused by the alternate anodic dissolution of the T1 precipitate and the alloy base at its adjacent periphery.
