The intergranular corrosion of 2000 series Al alloys is related to the depletion of Cu at and around grain boundaries due to precipitation of Cu-containing intermetallic particles (IMPs) such as Al₂Cu. It is also known that the intergranular corrosion resistance is improved by Mg addition. In general, IMPs play an important role in localized corrosion of Al alloys [1-3]. In this study, to clarify the role of IMPs in the initiation of the intergranular corrosion, in situ observations during micro-electrochemical measurements were performed.

Al-4.5Cu and Al-4.5Cu-1.5Mg alloys were prepared as specimens. Solution treatment and artificial aging treatment were conducted. After heat-treatment, the specimen surfaces were grinded and polished down to 1 µm using a diamond paste. SEM/EDS analysis indicated that Al₂Cu particles were generated in both Al-4.5Cu and Al-4.5Cu-1.5Mg alloys. Furthermore, in the case of Al-4.5Cu-1.5Mg, Al₂CuMg was found to be formed in addition to Al₂Cu.

Immersion test was carried out in 0.1 M NaCl for 5 h, and the number and length of intergranular corrosion damages were analyzed. The results show that Mg accelerates the initiation of intergranular corrosion but prevents the growth of intergranular corrosion. For both alloys, in situ observations indicated that pitting corrosion was generated at IMP before the initiation of intergranular corrosion. Acidic solutions inside the pits are likely to cause intergranular corrosion. For the Al-4.5Cu alloy, partial dissolution of Al₂Cu intermetallic particles was observed as the preceding event of pitting. Moreover, the initiation sites of the partial dissolution on Al2Cu particles were found to be Cu2FeAl7.

For the Al-4.5Cu-1.5Mg alloy, pitting corrosion was found to be initiated at discolored intermetallic particles. From the results of SEM/EDS analysis of the specimen after short-time immersion tests, Al₂CuMg was found to discolor more readily than Al₂Cu. The increase in the number of intergranular corrosion damages with Mg addition is determined to be due to the formation of Al₂CuMg particles. It is considered that the decrease in the growth rate of intergranular corrosion can be attributed to Mg in solid solution.

References:

[1] Takumi Kosaba, Izumi Muto, Yu Sugawara, Galvanic Corrosion of AA5083/Fe in Diluted Synthetic Seawater: Effect of Anodizing on Local Electrochemistry on and around Al₆(Fe,Mn) on Al-Matrix, Journal of the Electrochemical Society, 169 (2022), 020550.

[2] Hiroshi Kakinuma, Izumi Muto, Yoshiyuki Oya, Takahiro Momii, Yu Sugawara, Nobuyoshi Hara, Morphological Change and Open-circuit Potential of Single Metastable Pit on AA1050 Aluminum in NaCl Solution, Journal of the Electrochemical Society, 168 (2021), 021504.

[3] Hiroshi Kakinuma, Izumi Muto, Yoshiyuki Oya, Yoshihiko Kyo, Yu Sugawara, Nobuyoshi Hara, Mechanism for the Morphological Change from Trenching to Pitting around Intermetallic Particles in AA1050 Aluminum, Journal of The Electrochemical Society, 166 (2019), C19-C32.