Alloyed Cu is known to inhibit the growth of pitting corrosion on stainless steels after pitting initiation. The Cu²⁺ dissolved from the stainless steel matrix acts as an inhibitor in acidic chloride environments which is formed in pits. The Cu²⁺ suppresses active dissolution rate inside the pits¹. This suppression effect is supposed to be also effective for active dissolution which promotes crevice corrosion. Therefore, if it is possible to introduce Cu²⁺ to the inside of a crevice from the outside, introduced Cu²⁺ is supposed to inhibit crevice corrosion on stainless steel. However, diffusion of ions between inside and outside of the crevice is restricted by its geometry, and Cu²⁺ does not migrate to inside of a crevice according to the electroneutrality principle. [Cu (EDTA (ethylenediaminetetraacetic acid))] ²⁻ is a chelated Cu²⁺ which has negative charge, and is expected to migrate to inside of a crevice from the outside by electrochemical migration. In addition, it is reported that Cu²⁺ in [Cu(EDTA)]²⁻ could be easily substituted by Fe²⁺ in low pH². Therefore, Cu²⁺ is considered to be introduced to the inside of the crevice as [Cu(EDTA)]²⁻, and to affect crevice corrosion of stainless steel. In this study, in situ observation of an inside of the crevice³ was performed to analyze the effect of [Cu(EDTA)]²⁻ on crevice corrosion.

Type 316L stainless steel was used as specimen. The crevice corrosion tests were conducted in a flow cell³. 0.1 M NaCl or 0.1 M NaCl-10 mM [Cu(EDTA)]²⁻ was flowed through the flow cell. The crevice was made by pressing the glass plate on the specimen surface. The specimen surface was perfused with 0.1 M NaCl before the glass plate was pressed. Therefore, inside of the crevice was filled with 0.1 M NaCl. The specimen was polarized at 0.35 V (vs. Ag/AgCl, Sat. KCl). After the crevice corrosion initiation, the different propagation morphology was observed between 2 conditions. In the case of 0.1 M NaCl, the gas bubbles were observed with corrosion propagation. On the other hand, little gas bubbles were observed in the case of 0.1 M NaCl-10 mM [Cu(EDTA)]²⁻. The gas bubbles observed in 0.1 M NaCl were considered to be H₂ generated by the H⁺ reduction reaction inside the crevice. It was reported that the reduction reaction inside the crevice was changed by migrated anions which reacted with H⁺ inside the crevice³. Therefore, the results indicate [Cu(EDTA)]²⁻ migrated to the inside of the crevice from the outside, and affected the crevice corrosion morphology.

References

1. T. Sourisseau, E. Chauveu, and B. Baroux, Corros. Sci., 47, 1097-1117(2005).
2. W. Guan, B. Zhang, S. Tian, and X. Zhao, Appl. Catal. B, 227, 252-257 (2018).
3. T. Aoyama, Y. Sugawara, I. Muto, and N. Hara, J. Electrochem. Soc., 166, C250-C260 (2019).