In chloride containing solutions like atmospheric environments, an interface between intermetallic particles (IMPs) and Al-matrix is mainly one of the initiation sites in localized corrosion of Al alloys. It has been widely reported that Al-matrix around IMPs containing Fe and/or Cu was dissolved like trenching due to the oxygen reduction reaction on the IMPs and the subsequent alkalization on and around the IMPs. The trenching is precursory stage of localized corrosion on Al alloys. In the case of AA5083, Al₆(Fe, Mn) was known to become the initiation site of localized corrosion¹⁾. In addition to Al₆(Fe, Mn), Mg₂Si is precipitated in AA5083 to increase the mechanical strength. Mg in Mg₂Si is easily dissolved to solutions immediately after immersion. Therefore, Mg₂Si is less possible to become the initiation site of localized corrosion compared with Al₆(Fe, Mn). For the reason, few previous research has clearly shown the effect of the Mg dissolution in Mg₂Si on the trenching formation around Al₆(Fe, Mn). In this study, the effect of Mg₂Si on trenching formation around Al₆(Fe, Mn) by immersion tests of microelectrode containing single or double IMPs.

A5083 (Al-Mg alloy) were prepared as specimen in this research. The four types of AA5083 electrodes (1. only Al₆(Fe, Mn), 2. only Mg₂Si, 3. both of Al₆(Fe, Mn) and Mg₂Si, 4. neither Al₆(Fe, Mn) nor Mg₂Si (Al-matrix on AA5083)) were prepared. Diluted synthetic seawater (200 mg/L Cl^-, pH 8.2) was used as the electrolyte for immersion tests. Ag/AgCl (3.33 M KCl) was used as the reference electrode. The immersion time was 1 h.

After the immersion tests, trenching was generated around Al₆(Fe, Mn) on microelectrode containing both of Al₆(Fe, Mn) and Mg₂Si. On the other hand, no trenching was observed around Al₆(Fe, Mn) on microelectrode containing only Al₆(Fe, Mn). The results suggested that the formation of trenching around Al₆(Fe, Mn) was affected to Mg₂Si.

1) T. Kosaba, I. Muto, and Y. Sugawara, J. Electrochem. Soc., 169, 020550 (2022). https://doi.org/10.1149/1945-7111/ac5301