In Situ Microscopic Observation of Pitting Corrosion Behavior of A1050-O
Authors: Kakinuma Hiroshi , Muto Izumi , Sugawara Yu , Oya Yoshiyuki , Kyo Yoshihiko , Hara Nobuyoshi 
Published: February 2020

Al-Fe alloys are used for containers, cables, and window flashings because of their high formability, high strength/weight ratio, and easy machinability. The mechanical properties of Al-Fe alloys have been improved by controlling the morphology of precipitates, however, there have been reports that some precipitates are the sites for pit initiation. The precipitates are known to have a function as the sites for oxygen reduction reaction which results in local alkalization. Due to the local alkalization, Al-matrix around the precipitates dissolves, and trenches are formed around the precipitates. It is known that pits are readily initiated in the trenches, the corrosion behavior from trench formation to pit initiation is still unclear. In this research, in situ observations on the dissolution behavior around precipitates was performed using a micro-electrochemical system, and the effect of pH and precipitate compositions on pitting were determined.

In this study, A1050-O was used as the specimens. The specimen surface was polished down to 0.25 µm with a diamond paste and cleaned ultrasonically in ethanol before electrochemical　measurements were taken. Two types of the precipitates, AlxFe and AlxFe2Si, were observed in A1050-O.

Anodic polarization curves were measured in 0.1 M NaCl (pH 6.0, naturally aerated). The polarization was started from -0.9 V vs. Ag/AgCl (3.33 M KCl), and the scanning rate was 23 mV / min. A stable pit was initiated at -0.4 V, and it was confirmed that the pit was generated at a precipitate.

Using a micro-electrochemical system, potentiodynamic anodic polarization curves for a small area with each type of the precipitate were measured in 0.1 NaCl (pH 6.0, naturally aerated) at 298 K. The electrode area was ca. 5000 µm2. The surface appearance of the precipitate was observed during the anodic polarization using a water immersion objective lens. A deep trench was formed around the AlxFe2Si precipitate in the potential range from -0.86 to -0.65 V. Around the AlxFe precipitate, no trench was formed. It was found that the pit was initiated in the trench around the AlxFe2Si precipitates.

In order to analyze the corrosion behavior of A1050-O, open circuit potentials were measured in 0.1 M NaCl (pH 6.0, naturally aerated) and 0.1 M NaCl-containing citrate buffer (pH 6.0, naturally aerated) solutions. Fig. 1a shows the time variation of the open circuit potentials of A1050-O in each electrolyte. In 0.1 M NaCl, the potential increased to ca. -0.65 V. On the other hand, in the 0.1 M NaCl-containing citrate buffer solution, the potentials decreased to ca. -0.83 V. Fig. 1b and 1c show the optical micrographs of the AlxFe2Si precipitate after the immersion. Even though a deep trench was formed around the precipitates in 0.1 M NaCl, no trench was formed around the precipitates in 0.1 M NaCl-containing citrate buffer solution.

The depassivation pH of pure Al (99.999%) was measured to elucidate the corrosion behavior from trench formation to pit initiation. Electrolyte was 0.1 M NaCl (naturally aerated), and the pH was adjusted with NaOH solution. The open circuit potential decreased with pH from pH 6 to 13, but the open circuit potential increased at pH 14. After the measurement, the specimen surface was corroded. It is suggested that the surface film on pure Al was stable from pH 6 to 13, and that the open circuit potential decreased due to the pH dependence of the electrode potential of hydrogen generation reaction. At pH 14, the surface film dissolved, and bare Al was exposed to the solution. Because oxygen reduction reaction readily occurs on the bare Al, predominant cathodic reaction changed from hydrogen generation to oxygen reduction. The increase in the open circuit potential in 0.1 M NaCl (Fig. 1a) indicates the exposure of bare Al-matrix. In this research, it was found that the citrate buffer prevent the local alkalization and the trench formation around the precipitates.

Figure 1