Effect of Sulfate Ions on Repassivation Behavior of Crevice Corrosion on Type 316L Stainless Steel

Abstract

Crevice corrosion is known to be a serious problem of stainless steel structures in Cl- containing environments. The growth and repassivation behavior of crevice corrosion are significantly influenced by the solution composition of corrosion environments. To clarify the repassivation behavior of crevice corrosion, an in situ observation is a promising technique. This is because the quantitative relationship between current values and corrosion morphology could be analyzed. The current distribution inside the crevice can be estimated from the spreading of crevice corrosion depth. In this study, an in situ observation technique was applied to the study of the crevice corrosion of stainless steel. In addition, to clarify the effect of the solution composition outside the crevice on repassivation behavior, an electrochemical flow cell was newly designed. Type 316L stainless steel was used as specimens. The specimens were heat-treated at 1373 K for 3.6 ks and then water-quenched. An electrochemical flow cell was fabricated (Fig. 1). The crevice was made by pressing a glass sheet to the specimen surface, and the specimen was polarized at 0.25 V (vs. Ag/AgCl, 3.33 M KCl). Crevice corrosion tests were initially performed in a flowing 1 M NaCl solution (298 K). During the corrosion tests, the corrosion behavior of the specimen surface inside the crevice was observed using an optical microscope through the glass sheet, and then the time-lapse video was produced by capturing one frame per 10 seconds. In some cases, the flowing solution was changed from 1.0 M NaCl to 0.88 M Na2SO4, after the initiation of the crevice corrosion. The solution was changed when the current reached a value of 100 µA. Figure 2a shows the time variations of the current during the crevice corrosion tests. In the case of test A, the solution was not changed. In test B, the solution was changed from NaCl to Na2SO4 when the current reached the value of 100 µA. In both cases, the current first decreased to ca. 5 µA with the lapse of time and at about 400 s, it became to increase rapidly due to the initiation of crevice corrosion. In test A, the current increased up to ca. 5 mA. On the other hand, in test B where the solution was changed, the current decreased after reaching a maximum value of 3 mA at ca. 2.5 ks. The results of in situ observation of the specimen surface are shown in Fig. 2b. In test A, the corroded area propagated along the crevice mouth (Fig. 2b). At the beginning of crevice corrosion in test B, its propergation was similar to that of test A. When the current began to decrease at ca. 2.5 ks, the direction of corrosion propagation changed toward the inside of crevice (Fig. 2c). Finally, when the current decreased to ca. 1.2 µA, no lateral propagation of the corroded area was observed, suggesting that the repassivation inside the crevice was completed. Figure 1