Microscopic Polarization Behavior of Cerium Sulfide Inclusions in Stainless Steel
Authors: Nishimoto Masashi , Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: February 2020

Stainless steels suffer from pitting corrosion in chloride environments. Sulfide inclusions such as MnS are known to be preferential sites for pit initiation, and the chemical composition of the inclusions has a significant influence on pitting corrosion behavior. 1–3 CeS inclusions have better pitting corrosion resistance than MnS inclusions.4 However, little is known about the electrochemical properties of CeS inclusions and the relationship between those properties and pitting corrosion resistance. In this study, a microelectrochemical technique was applied to elucidate the electrochemical properties of CeS inclusions.
A Ce-added austenitic stainless steel (0.011%C, 0.47%Si, <0.05%Mn, 0.033%P, 0.017%S, 0.01%Cu, 9.41%Ni, 17.90%Cr, <0.01%Mo, <0.005%Al, 0.091%Ce, 0.0042%N, 0.008%O) was prepared by vacuum induction melting and then was hot rolled. This steel was heat-treated at 1373 K for 30 min and then quenched in water. After heat-treatment, the steel surface was polished by diamond paste down to 1 µm. A scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system was used to ascertain the chemical composition of the inclusion. The SEM image and EDS maps of the inclusion shown in Fig. 1 indicate an enrichment of Ce, Cr, and S, and a small amount of O in the inclusion. The relative atomic percentage of Ce:Cr:S:O was 27:22:45:7, indicating the formation of a mixture of CeS and CrS. This type of inclusions is referred as (Ce,Cr)S in this study.
The microscopic potentiodynamic polarization curve was measured in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The electrode area was ca.70µm × 70µm. All the potentials reported in this study refer to an Ag/AgCl (3.33 M KCl) electrode. As shown in Fig. 2, meta-stable pitting events were generated around 0.95 V, and small pits were observed at the (Ce,Cr)S/steel matrix boundary.
To clarify the dissolution potential range of the (Ce,Cr)S inclusion, microscopic anodic polarization was carried out in naturally aerated 0.1 M Na2SO4 (pH 5.7) at 298 K. The anodic polarization curves are shown in Fig. 2a. The increases in current density observed in the potential range from ca. 0.37 to 0.58 V can be attributed to the dissolution of the (Ce,Cr)S inclusion. Interestingly, the dissolution potential of the (Ce,Cr)S inclusion was significantly lower than the initiation potential of the pitting events at the inclusion. In the case of MnS inclusions 5,6, the pits were initiated in the dissolution potential range of the inclusions. It would appear that the dissolution products such as Ce3+ ions inhibit pit initiation, resulting in the increase of the pit initiation potential.
To ascertain the dissolution products from (Ce,Cr)S, the potential-pH diagrams were calculated. Figure 3 shows the potential-pH diagrams for the CeS-H2O and CrS-H2O systems at 298 K. The standard chemical potentials of CeO2 and Ce(OH)3 were obtained from the literature.7 Those of other species used in the calculations were obtained from the HSC thermo-chemical database.8 The experimental conditions of the microscopic anodic polarization are indicated by the slashed regions, which are located in the stable regions of S2O3
2-, Ce3+, CeO2, HCrO4
-, and Cr2O3. Among these species, Ce3+, CeO2, and Cr2O3 are expected to be the factors of the improvement of pitting corrosion resistance. As shown in Fig. 3b, Ce3+ ions were released from the CeS inclusion at lower potentials of the conditions employed in this experiment. It is possible that the Ce3+ ions inhibit corrosion. At the higher potentials, CeO2 is estimated to be deposited on the CeS inclusion and suppress the dissolution of the inclusion. In the case of the CrS-H2O system, Cr2O3 is expected to be formed on the CrS inclusion surface and act as a barrier against the dissolution of the inclusion in the lower potential region.
References;
1. I. Muto, Y. Izumiyama, and N. Hara, J. Electrochem. Soc., 154, C439 (2007).
2. I. Muto, S. Kurokawa, and N. Hara, J. Electrochem. Soc., 156, C395 (2009).
3. N. Shimahashi, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C262 (2013).
4. B. Baroux, in Corrosion Mechanisms in Theory and Practice (Third Edition), P. Marcus, Editor, p. 443, CRC Press, Boca Raton (2012).
5. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159, C341 (2012).
6. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C511 (2013).
7. S. A. Hayes, P. Yu, T. J. O’Keefe, M. J. O’Keefe, and J. O. Stoffer, J. Electrochem. Soc., 149, C623 (2002).
8. A. Roine, HSC Chemistry Thermo-chemical Database, Version 6.1, Outotec Research Oy, Pori, Finland (2007).

Figure 1