Stainless steels sometimes 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 In our previous work, ⁵ microelectrochemical measurements and thermodynamic calculations were conducted to elucidate the electrochemical properties of CeS inclusions, and the relationship between those properties and pitting corrosion resistance was discussed. The Ce³⁺ ions released from CeS inclusions was thought to improve pitting corrosion resistance. In this study, we analyzed the role of Ce³⁺ ions for improving pitting corrosion resistance of sulfide inclusions in stainless steels.

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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