An aging treatment of UNS S32750 super duplex stainless steel at 1173 K for 1.0 ks produced σ and secondary austenite (γ2) phases in the α and γ phase boundary regions. Small amount of Mn-Cr oxide and MnS complex inclusions were present in the steel. No pitting was observed during the potentiodynamic polarization of a small area (200 × 200 µm) without inclusions in 1 M MgCl2 at 348 K. Because the electrode area included α, γ, σ, and γ2 phases, these phases and their boundaries alone could not act as initiation sites for pitting. The electrode area size was gradually reduced from 3 × 3 mm to 1 × 1 mm, and pitting corrosion was observed to occur at the Mn-Cr oxide and MnS complex inclusions in a region that appeared to be the γ2 phase near the σ phase.

Stainless steel is a material with high corrosion resistance, but pitting corrosion is a problem. Pitting corrosion is thought to be caused by the active dissolution of the matrix due to the synergistic effect of chemical species and chloride ions generated by the dissolution of MnS inclusions1, 2). If we can develop an inhibitor that blocks this synergistic effect of sulfur and chloride ions, pitting corrosion of stainless steel can be effectively supressed. Sodium nitrate is one of the inhibitors of pitting corrosion. However, there are few studies on the effect of nitrate ions on pitting corrosion initiation from MnS. By the way, solution-side manipulations such as switching of solution composition are effective. Therefore, in this study, we made a flow cell microelectrochemical system to elucidate the mechanism of corrosion protection of nitrogen-based inhibitors. In this study, re-sulfurized SUS304 stainless steel was used for solution treatment (1100°C, 30 min, water cooling). Anodic polarization measurements were carried out with an electrode area of 1 cm2 to investigate the effect of inhibitor on the initiation of pitting corrosion. In this experiment, 0.1 M NaCl solution and 0.1 M NaCl with 10 mM inhibitor (sodium nitrate (NaNO3), sodium nitrite (NaNO2), ammonium chloride (NH4Cl)) were used. For microelectrochemical measurements, a microelectrochemical system with In situ observation was used. The anodic polarization curve was measured by preparing a specimen with the electrode surface covered with a 100 μm square. 0.1 M NaCl solution and 0.1 M NaCl - 10 mM NaNO3 solution were used as solutions. Light microscopy, scanning electron microscopy (SEM) and energy dispersive analysis (EDS) were used to observe the specimens after polarization. For the measurements using the flow-cell microelectrochemical system, a flow channel with a width of about 1 mm was fabricated on the coated specimen leaving a 100 μm square, and a platinum counter electrode and a reference electrode were inserted to perform the measurements using the three-electrode method. In the polarization measurement in 0.1 M NaCl solution, pitting corrosion initiated from MnS inclusions. When the polarization measurement was performed in 0.1 M NaCl solution containing inhibitor under the same conditions, no pitting corrosion was observed in sodium nitrate. On the other hand, pitting corrosion occurred in the solution containing sodium nitrite and ammonium chloride. This indicates that only sodium nitrate has a strong inhibitory effect on pitting corrosion. When the polarization measurement was performed in a flow-cell microelectrochemical system while pumping the solution into the flow channel, pitting corrosion occurred at the same potential as in the polarization measurement without the flow cell. In addition, the measurements generated no large or intermittent noises that would obscure the measured values, and in situ observation was also successful. Reference; 1) Chiba, I. Muto, Y Sugawara, and N Hara, J. Electrochem. Soc., 159, C341 (2012). 2) Chiba, I. Muto, Y Sugawara, and N Hara, J. Electrochem. Soc., 160, C511 (2013).