In Situ Observation of Pitting Corrosion on AA1050 Under Open Circuit Conditions
Authors: Kakinuma Hiroshi , Muto Izumi , Oya Yoshiyuki , Momii Takahiro , Sugawara Yu , Hara Nobuyoshi 
Published: December 2020

Commercial purity aluminum alloys show high formability and high strength, and its application is expanded to infrastructure and battery materials. The aluminum alloys contain various types of intermetallic particles, and it is well known that the intermetallic particles have an important role in improving the mechanical properties. However, some of the particles can be a pit initiation site under corrosion environments. The intermetallic particles, such as Al5Fe2 and Al8Fe2Si, show higher electrode potential than that of the Al matrix. Therefore, these intermetallic particles work as a preferential cathodic reaction site. Due to the generation of the hydroxide ions by O2-reduction reaction, the pH around the intermetallic particles increases. The Al matrix is dissolved by the pH increase, and a trench is formed around the particles. Inside the trench, pH decreases due to the hydrolysis reactions of Al ions, and it triggers the initiation of a crystallographic pit.1) Although the role of the solution chemistry in pitting corrosion has been widely studied, the correlation between the initiation and propagation of pitting and the change in open circuit potential (OCP) is not well known. Because the pitting occurs at the small intermetallic particles, it requires specific technique to measure the OCP and observe the initiation of pitting at the same time. In this research, the pitting behavior under open circuit conditions was analyzed by in situ microscopic observation technique, and the correlation between the dissolution behavior and change in the OCP was elucidated.
AA1050 was used as specimens. The intermetallic particles were characterized by an optical microscope and a field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS). Two types of the intermetallic particles, Al-Fe and Al-Fe-Si, were confirmed to exist.
Before the measurements, the specimen surface was mechanically ground with SiC paper to 4000-grid and polished with a diamond paste (15, 6, 1, and 0.25 µm). Ethanol was used as a lubricant to minimize any water exposure. After the polishing, the specimens were cleaned ultrasonically with ethanol and dried by N2 gas. To evaluate the macroscopic electrochemical behavior of the specimens, the electrode area of 1 mm × 1 mm was fabricated, and the OCP was measured in naturally aerated 0.1 M NaCl (pH 6.0) at 298 K. In this research, all potentials refer to the Ag/AgCl (3.33 M KCl) electrode. To analyze the correlation between the pitting corrosion behavior and the change in OCP, the electrode area of 500 µm × 500 µm, 300 µm × 300 µm, and 100 µm × 100 µm was fabricated and the OCP measurements were conducted. During the OCP measurements, whole electrode area was observed in situ using water immersion objective lens. After the measurements, the specimen surface was observed using an optical microscope and SEM-EDS.
To analyze the corrosion behavior of AA1050, the OCP of the specimen with the electrode area of 1 mm × 1 mm was measured in 0.1 M NaCl for 1 ks. During the measurements, potential oscillations were observed. The initiation of pitting corrosion was confirmed, and the pit initiation site was found to be Al-Fe-Si particle. Neither trenching nor pitting was initiated around the Al-Fe particles. To confirm the smallest size of the electrode area containing a pit initiation site, the OCP measurements were conducted using the electrode areas of 500 µm × 500 µm, 300 µm × 300 µm, and 100 µm × 100 µm. During the measurements with 100 µm × 100 µm, no pitting was confirmed by in situ observation, and almost no oscillation of the OCP was generated. In the case of the measurements with the electrode area of 300 µm × 300 µm, one large pit and some small pits were initiated, suggesting that the distribution density of the initiation sites for pitting is ca. one site per 300 µm × 300 µm. During the measurements with the electrode area of 500 µm × 500 µm, many large pits were initiated. The size of the pits was larger than that observed in the electrode area of 300 µm × 300 µm. The OCP dropped at the moment of pit initiation. During the growth of pitting, the OCP remained at lower potentials. Finally, the stop of the pit growth and the increase in OCP were observed simultaneously. In this research, the microscopic measurements were conducted four times to confirm the reproducibility of the results.

References

1. H. Kakinuma, I. Muto, Y. Oya, Y. Kyo, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 166, C19 (2019).