Stainless steels sometimes suffer from pitting corrosion in chloride environments. The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. It is well known that addition of Mo enhances the pitting corrosion resistance of stainless steels, and the role of Mo has been studied extensively. However, it is still debated how Mo inhibits pitting corrosion of stainless steels. In this study, a microelectrochemical technique was applied to clarify the role of Mo addition in the inhibition of pit initiation at MnS inclusions in stainless steels.

Two re-sulfurized stainless steels were prepared by vacuum induction melting (50 kg ingot): one was Mo-free (18Cr-10Ni-0.8Mn-0.03S), and the other was Mo-added (18Cr-10Ni-2.3Mo-0.8Mn-0.03S). The ingots were hot-rolled. In these steels, S was added to form MnS inclusions. To reduce the delta-ferrite phase in the steels, they were heat-treated at 1273 K for 96 h, followed by hot-rolling at 1473 K from 20 mm down to about 12 mm in thickness. Additional heat-treatment was conducted at 1273 K for 96 h and at 1323 K at 0.5 h, and the steels were finally water-quenched. After heat-treatment, the steels were cut into specimens with dimensions of 15 mm × 25 mm × 5 mm. The specimens were abraded with a series of SiC papers up to 1500 grit and then were polished by a diamond paste down to 1 µm. Potentiodynamic polarization curves were measured in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The electrode area was ca. 100 µm × 100 µm. All the potentials reported in this study refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). The potential scan rate was 3.8 × 10^–4 V/s (23 mV/min).

To analyze the effect of Mo on the pit initiation at the MnS inclusion, the anodic polarization curves were measured in 0.1 M NaCl. Figure 1a shows the polarization curves of a small area with the MnS inclusions in Mo-free and Mo-added specimens. Figures 1b and 1c show the SEM images of the inclusions in Mo-free and Mo-added specimens after polarization. The experiments were started at –0.2 V. The gradual increases in the current densities of both specimens above ca. 0.1 V are due to the anodic dissolution of the MnS inclusions. The large increase in the current density of the Mo-free specimen at 0.4 V was attributed to the stable pit initiation at the inclusion. As shown in Fig. 1b, a pit was initiated at the MnS/steel matrix boundary. On the other hand, the current density of the Mo-added specimen decreased to the level of the passive state of stainless steel above ca. 0.45 V. In Fig. 1c, no pit was observed in the Mo-added specimen even though the bare steel surface was exposed by the dissolution of the MnS inclusion. Mo species were estimated to be released from the exposed bare steel surface. This suggests that Mo species inhibited pit initiation at the MnS/steel boundary.