Effect of Nitrate Ions on Repassivation Behavior of Crevice Corrosion on Type 316L Stainless Steel
Authors: Aoyama Takahito , Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: February 2020

Once crevice corrosion is initiated on stainless steels in chloride environments, the hydrolysis of dissolved metal ions causes severe acidification inside the crevice, and Cl- ions migrate into the crevice from the bulk solution. This electromigration of Cl- ions promotes subsequent active dissolution inside the crevice. Therefore, neutralization of the pH inside the crevice is expected to be the predominant factor affecting the repassivation of crevice corrosion.To clarify the repassivation behavior of crevice corrosion, in situ observations were performed, and the quantitative relationship between current values and corrosion morphology was analyzed. In addition, to elucidate the effect of the solution composition on the repassivation behavior, an electrochemical flow cell was used. In this study, the effect of NO3
- ions on the repassivation behavior of Type 316L stainless steel was investigated.

Type 316L stainless steel was used as specimens. The specimens were heat-treated at 1373 K for 3.6 ks and then water-quenched. A schematic illustration of the electrochemical flow cell is shown in Figure 1. An artificial crevice was formed between a glass plate and the specimen surface. The specimen was polarized at 0.25 V (vs. Ag/AgCl, 3.33 M KCl). Crevice corrosion tests were performed in a flowing 1 M NaCl solution at 298 K. In the case of the solution change experiment, the inlet solution was changed from 1 M NaCl to 1 M NaCl-1 M NaNO3. The solution was changed when the current exceeded 100 μA. The flow rate of the solution was 24 mL h-1 during the crevice corrosion tests. Since the volume of the cell was ca. 24 mL, it took about 1 h to change the solutions completely. During the crevice corrosion tests, the crevice corrosion behavior of the specimen surface inside the crevice was observed using an optical microscope, and the images were taken every 100 s.

Figure 2a shows the time variations of the currents in the crevice corrosion test in 1 M NaCl, and Figure 2b shows the results of in situ observations of the specimen surface.　From the beginning of the crevice corrosion test to ca. 250 s, the current decreased to ca. 4 μA with time. Around ca. 300 s, the current increased rapidly. This current increase was due to the initiation of crevice corrosion. At this time, two small dark spots were observed inside the crevice (Figure 2b1). These spots were the initiation sites of the crevice corrosion. As shown in Figures 2b1-b3, the crevice corrosion grew toward the crevice mouth. After that, the corroded areas proceeded along the crevice mouth (Figures 2b4-b7). Once the entire crevice mouth was corroded, the crevice corrosion propagated toward the inside of the crevice, and the current increased up to ca. 5 mA at ca. 10 ks. From the results shown in Figure 2, it was confirmed that the crevice corrosion was readily initiated in 1 M NaCl, and no spontaneous repassivation was generated in this solution.

Figure 3a shows the time variations of the current in the solution change experiment from 1 M NaCl to 1 M NaCl-1 M NaNO3, and Figure 3b shows the results of in situ observations of the specimen surface. Around 300 s, the current increased rapidly due to the initiation of crevice corrosion. At 406 s, the current exceeded 100 μA, and the inlet solution was then changed to 1 M NaCl-1 M NaNO3. From 406 s to 3.75 ks the current increased to 2.9 mA. In this period, the corroded areas mainly grew along the crevice mouth as shown in Figures 3b2-b5. At 3.75 ks (Figure 3b6), the current started decreasing. Almost at the same time, the growth of the crevice corrosion along the crevice mouth was stopped. From this time, the crevice corrosion propagated toward the inside of the crevice. Afterwards, the current decreased to ca. 150 μA. After 26.1 ks, the current began decreasing again. As shown in Figure 3b8, no propagation of the corroded area was observed after 43.0 ks. This suggested that the surface of the corroded area was completely repassivated. This result indicated that NO3
- bring about the repassivation.

Figure 1