When austenitic stainless steels are heated at around 900 K, sensitization occurs and Cr-depleted areas are formed at grain boundaries. As a result of sensitization, the localized corrosion resistance of the stainless steels decreases¹. The mechanism of intergranular corrosion of sensitized stainless steels was well understood with the results of immersion tests in acidic solutions, but the mechanism of pitting corrosion and the pit initiation sites for sensitized stainless steels in near-neutral solutions have remained unclear.

Micro-electrochemical measurements are promising techniques for investigating the pit initiation process. Chiba et al. developed a micro-electrochemical system for in situ high-resolution optical microscopy². In this study, we applied this technique to investigate the pit initiation of sensitized stainless steel. In addition to this, the electrochemical properties of single grain boundary were measured.

A commercial 18Cr-8Ni stainless steel sheet (0.06 %C, 0.39 %Si, 1.1 %Mn, 0.003 %S, 8.0 %Ni, 18.0 %Cr, 0.13 %Mo, 0.22 %Cu, 0.002 %Al, 0.04 %N, and 0.003 % O) was used as specimens. The steel was solution-treated at 1373 K for 0.5 h and was quenched in water. As a sensitization-treatment, the steel was heat-treated at 923 K for 2 h and then quenched in water. After that, the steel surface was polished down to 1 μm by a diamond paste. Potentiodynamic anodic polarization was carried out in naturally aerated 0.1 M NaCl at 298 K. The electrode areas were ca. 1 cm × 1 cm (macro-scale) and 100 μm × 100 μm (micro-scale). The solution was naturally aerated 0.1 M NaCl. All potential were measured against a Ag/AgCl(3.33 M KCl) electrode. In anodic polarization, electrode potential was scanned at 23 mV s^-1.

The results of macro-scale anodic polarization for the solution-treated and sensitized specimens are shown in Fig. 1. For the sensitized specimen, pitting occurred at around 0.4 V, and the morphology of the pit was different from that formed on the solution- treated specimen. Grain boundary corrosion and pitting were generated.

Micro-scale polarization (electrode area: ca.100 μm × 100 μm) was conducted to elucidate electrochemical properties of individual grain-boundaries of the sensitized stainless steels. Figure 2 shows the preparation procedure for the electrode area with a single grain-boundary. To know the location of grain boundaries, electrolytic etching in oxalic acid was performed for 10 s, and the indentations with a Vickers hardness tester were formed on the both sides of the selected grain boundary segment. To prevent the overlapping of grain boundary and masking, both sides of the grain boundary were solution-treated using a micro spot TIG welder (Fig. 2a). Then specimen was mirror polished again and masked (Fig. 2b).

The anodic polarization behavior of a single grain-boundary in the sensitized stainless steel is shown in Fig. 3. Figure 3b is the composite image of the as-polished electrode area and the etched surface. This figure indicates that the electrode area contained single grain-boundary. Figures 3c and 3d are the optical micrographs of the electrode area before and after polarization. These results indicate no pit was initiated at the grain boundary, and this means that not all grain boundaries act as a pit initiation site even in the case of the sensitized stainless steels.

References;

1. U. Kamachi Mudali, R. K. Dayal, J. B. Gnanamoorthy, and P. Rodriguez, ISIJ Int., 36, 799-806 (1996)
2. Aya Chiba, Izumi Muto, Yu Sugawara, and Nobuyoshi Hara, J. Electrochem. Soc., 159, C341 (2012)