In Situ Microscope Observation of Pitting Corrosion on Al-Mg Alloy Using Micro Electrochemical Measurement and Effects of pH on Dissolution Behavior
Authors: Kakinuma Hiroshi , Muto Izumi , Sugawara Yu , Oya Yoshiyuki , Kyo Yoshihiko , Hara Nobuyoshi 
Published: February 2020

Al-Mg alloys are well known as high corrosion resistance Al alloys and used for structural materials of aircrafts, automobiles, ships, and pressure containers. However, pitting corrosion is sometimes caused at intermetallic compounds. The mechanism of the pitting is thought to be as follows : (1) oxygen reduction reaction on intermetallic compounds occur, and pH increases on and around the precipitates locally; (2) the surrounding aluminum-matrix dissolves, and trenches are formed at the boundaries between the intermetallic compounds and aluminum-matrix; (3) pH decrease inside the trench causes pitting. Though the pH is the key factor for the initiation of pitting, the detail of the pH change and the effect of bulk solution pH on the pit initiation are still unclear. In this research, micro electrochemical measurements with in situ microscope observation was applied for the investigation on the initiation of pitting on AA5182-O (Al-Mg alloy) and the effects of pH on the pitting and dissolution behavior were discussed.

A commercial Al-Mg alloy (AA5182-O) was used as the specimens in this study. At first, metallographic characterization of intermetallic compounds in the specimens was conducted by an optical microscope and scanning electron microscope. Before the surface observation, the specimen was polished down to 0.25 µm with a diamond paste and cleaned ultrasonically in ethanol. Al-Fe and Mg-Si intermetallic compounds were observed.

Macroscopic anodic polarization curves were measured. The specimens were polished down to 0.25 µm with a diamond paste and ultrasonically cleaned in ethanol. After that, with the exception of the electrode area (1.0 cm × 1.0 cm), the specimen surfaces were masked with an epoxy resin. In order to make clear the effect of pH on pit initiation, the electrochemical measurements were carried out in 0.1 M NaCl and 0.2 M citrate buffer with 0.1 M NaCl solutions. Anodic polarization was started from -1.2 V vs. Ag/AgCl (3.33 M KCl), and the scanning rate was 23 mV / min. A Pt wire was used as the counter electrode, and the reference electrode was a Ag/AgCl. In the citrate buffer with NaCl solution (pH 6.0), pitting potential was about 0.1 V higher than that measured in the NaCl (pH 6.0). After polarization, the specimen surfaces were observed by an optical microscope. No difference was observed in the morphology of pits initiated in the NaCl and the citrate buffer solutions.

Microscopic electrochemical measurements were carried out. The solutions used and electrochemical conditions were the same as the macroscopic polarization, and the electrode area was about 5.0 × 103 µm2. The surface of the electrode area was observed by an optical microscope during electrochemical measurements. Figure 1(a) shows microscopic polarization curves in 0.1 M NaCl and 0.1 M NaCl-0.2 M citrate buffer. Figure 1(b), (c), (d) and (e) show the pictures of specimen surfaces taken by an optical microscope before and after polarization. Figure 1 (b) and (c) show the specimen surfaces in 0.1 M NaCl (pH 6), and Fig. 1 (d) and (e) show the images obtained in 0.1 M NaCl-citrate buffer solution (pH 6). Blue lines in Fig. 1(b) and Fig. 1(c) show electrode area. Black arrows in Fig. 1(b) and (d) indicate Fe-Al and Mg-Si intermetallic compounds, and red arrows shown in Fig. 1(c) and (e) indicate pits. As shown in Fig. 1 (b) and (c), the color of Al-Fe intermetallic compounds and aluminum-matrix around the compounds changed clearly. These color change was observed mainly in cathodic potential range. It was seen that the intermetallic compounds and the aluminum-matrix dissolved and trench was formed by the dissolution of aluminum-matrix. At -0.59 V, pits were observed at the color changed area. On the other hand, in 0.1 M NaCl-citrate buffer solution, only the color change of intermetallic compounds was observed. In this case, small black points were observed on the Al-Fe intermetallic compounds and the number increased in cathodic potential range. Although the pit initiated at the boundary between Al-Fe intermetallic compounds and aluminum-matrix, the color change of aluminum-matrix was not observed. In NaCl-buffer solution, pitting potential was about -0.42 V, but the pit was observed at -0.33 V.

Figure 1