Micro-Electrochemical in Situ Observation of Pit Initiation at Precipitates in AA5182 Al-Mg Alloy in 0.1 M NaCl

Abstract

Al-Mg alloys such as AA5182 offer the advantages of superior mechanical properties and corrosion resistance. They are widely used in transportation industry and architectural applications. Understanding their precise corrosion mechanisms is required to assess the durability and ensure the safety. It is known that the precipitates (e.g. Fe3Al, FeAl, Al2Cu, and so on) in aluminum alloys act as initiation sites for pitting. Oxygen reduction reaction proceeds on the precipitates preferentially, which causes the alkalization. Trenches (micro-crevices) are formed at the boundary between the precipitates and the aluminum matrix because aluminum readily dissolves in alkaline solutions. Pits are thought to be initiated in the trenches 1, but the successive processes from the alkalization to pit initiation is not fully understood. In this study, in situobservation of pit initiation at the precipitates in Al-Mg alloy was performed in 0.1 M NaCl. A commercial AA5182-O sheet was used as specimens, and the surfaces were polished down to 0.25 μm with a diamond paste. The morphology and composition of precipitates in the alloy were analyzed by a field emission scanning electron microscope equipped with an X-ray energy-dispersive spectrometer, and it was concluded that Al-Fe-Mn, Al-Fe-Mn-Si, and Mg2Si intermetallics existed as the precipitates in the alloy. Macro-scale potentiodynamic polarization was carried out to ascertain the pitting potential of AA5182-O in naturally aerated 0.1 M NaCl. The size of the electrode area was 10 mm × 10 mm. It was confirmed that pitting potentials were located around -0.6 V (vs. Ag/AgCl, 3.33 M KCl). In order to make clear the pit initiation site, a micro-electrochemical in situ observation system similar to that developed by Chiba et al. was used. 2 Figure 1a shows the micro-scale anodic polarization curves for a small area with one or several precipitates of Al-Fe-Mn, Al-Fe-Mn-Si, or Mg2Si in naturally aerated 0.1 M NaCl solution at 298 K. The size of the electrode area was around 50 µm × 50 µm. In the case of the area with Al-Fe-Mn-Si or Mg2Si, crevice corrosion was generated at the periphery of the electrode area, but no pit was initiated at the precipitates. On the other hand, a stable pit was initiation at Al-Fe-Mn. Figure 1b shows the surface appearances of the precipitates after the polarization shown in Fig. 1a. The trench formation was clearly observed around Al-Fe-Mn and Al-Fe-Mn-Si precipitates. During the anodic polarization, the video was recorded, and digital image processing was applied to estimate the change in the area of the corroded region (trench) with electrode potential. Figure 1c shows the area ratio of the trench (corroded region) to the precipitate. The corrosion damage around Al-Fe-Mn was severe compared to that around Al-Fe-Mn-Si. The reason for this is related to the difference in the activity of the oxygen reduction reaction on the precipitates. One possible explanation is that silicon in the precipitates suppresses the oxygen reduction rate and the alkalization around the precipitates. The order of pitting corrosion resistance is as follows: Mg2Si > Al-Fe-Mn-Si > Al-Fe-Mn. 1) J. O. Park, C. H. Paik, Y. H. Huang, and R. C. Alkire, J. Electrochem. Soc., 146(1999), 517. 2) A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159 (2012), C341. Figure 1