A type 304 re-sulfurized stainless steel (0.05%C, 0.39%Si, 1.51%Mn, 0.04%P, 0.02%S, 0.35%Cu, 8.3%Ni, 18.3%Cr, 0.21%Mo, 0.002%Al, 0.080%N, 0.002%O) was prepared by vacuum induction melting and then was hot-rolled. The specimens were heat-treated at 1373 K for 30 min and quenched in water. After heat-treatment, the specimens were polished with a diamond paste down to 1 µm. Potentiodynamic anodic polarization curves were measured in naturally aerated 3 M NaCl (pH 5.0) and 2.97 M NaCl-10 mM CeCl3 (pH 5.0) at 298 K. The electrode area was ca. 100 µm × 100 µm. All the potentials reported in this study refer to an Ag/AgCl (3.33 M KCl) electrode. The potential scan rate was 3.8 × 10–4 V/s (23 mV/min). A scanning electron microscope (SEM) and a focused ion beam system (FIB) were used to observe the electrode surfaces and the cross-sections of the MnS inclusions after polarization.
Chiba et al. demonstrated that the trench formation at MnS/steel boundaries caused by the MnS dissolution products and Cl– ions act as a precursor of pits initiation. 6 To analyze the effect of Ce3+ on the trench formation at MnS/steel boundaries, the anodic polarization curves of a small area with the MnS inclusion were measured in 3 M NaCl and 2.97 M NaCl-10 mM CeCl3 (Fig. 1). The experiments were started at –0.2 V and stopped at the same potential. A stable pit occurred in the Ce3+-free solution, and no stable pit was initiated in the Ce3+-containing solution. After polarization, the electrode surfaces and the cross-sections of the MnS inclusions were observed. Figure 2 shows the SEM images of the MnS inclusion after polarization measured in Ce3+-free solution. The boundaries between MnS and steel matrix dissolved selectively, and the deep trenches are formed. On the other hand, in the case of Ce3+-containing solution (Fig. 3), little trench formation was observed. This indicates that Ce3+ ions inhibited the trench formation at the MnS/steel boundaries.
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