In our previous studies, suppression of crevice corrosion for Type 316L stainless steel was confirmed to occur when the solution outside of the crevice was changed from 1 M NaCl to 1 M NaCl-1 M NaNO₃¹. The repassivation inside crevice was thought to be caused by the active to passive transition produced by the increase in pH. The inhibition effect of NO₃^- ions was expected to be attributed to increase in pH inside the crevice induced by reduction reaction of NO₃^- ions. In this reduction process, there is possibility of the generation of NO₂^- and/or NH₄⁺ ions inside the crevice. In this study, the role of NO₃^- ions in the repassivation process for Type 316L was investigated.

A commercial Type 316L stainless steel was used as specimens. The specimens were heat-treated at 1373 K for 3.6 ks and then water-quenched. The specimens were polished down to 1 µm with a diamond paste. After that, ultrasonic cleaning was conducted in ethanol. Anodic polarization measurements were conducted in deaerated acidic 1 M NaCl, 1 M NaCl-0.01 M NaNO₂ or 1 M NaCl-0.01 M NH₄Cl at 298 K. The pH of the solutions was adjusted to 0.2 with HCl to simulate the solution inside the crevice. The surfaces of the specimen were coated with an epoxy resin, with the exception of the electrode area (ca. 10 × 10 mm). The potential scan rate was 3.8 × 10^-4 V s^-1 (23 mV min^-1). The reference electrode was an Ag/AgCl (3.33 M KCl) electrode. All the potentials cited in this work refer to the Ag/AgCl (3.33 M KCl) electrode.

Figure 1 shows the anodic polarization curves measured in deaerated acidic 1 M NaCl, 1 M NaCl-0.01 M NaNO₂ or 1 M NaCl-0.01 M NH₄Cl at 298 K. In all the solutions, active and passive regions were observed. In this work, the potential range of which current density indicated over 1.0 A m^-2 was defined as an active region. In 1 M NaCl, the active region was from -0.28 to -0.12 V, and the peak current density of the active region was 6.6 A m^-2. In 1 M NaCl-0.01 M NaNO₂ and 1 M NaCl-0.01 M NH₄Cl, the active regions were from -0.26 to -0.20 V and -0.24 to -0.17 V, respectively. These results suggest the active region was narrowed by NO₂^- or NH₄⁺ ions. The negative loop of current in 1 M NaCl-0.01 M NaNO₂ was assumed to originate from narrowing the active region. In addition, the anodic peak current densities in 1 M NaCl-0.01 M NaNO₂ (1.7 A m^-2) and 1 M NaCl-0.01 M NH₄Cl (1.3 A m^-2) were lower than that in 1 M NaCl.

In addition to above results, in situ observations inside the crevice were performed, and the role of NO₃^- ions in the repassivation process for Type 316L was determined.

Reference;

1. T. Aoyama, I. Muto, Y. Sugawara, and N. Hara, ECS Trans., 80(10), 519-526 (2017).