*2.1. Material*

Type 304 austenitic stainless steel was used. Table 1 shows the chemical composition of the steel used. The steel was heated at 1100 ◦C for 1 h for solutionizing. The average grain diameter of the steel was 119 μm, measured based on Japanese Industry Standard (JIS) G 0551.

**Table 1.** Chemical composition of type 304 austenitic stainless steel (mass %).


Specimens of 14 × 14 × 5 mm were machined from heat-treated bulk material, as shown in Figure 2. To change the DOS of the specimens, additional heat treatments were applied to the solutionized steel. Figure 3 shows the heating schedule (700 ◦C for 2 h and 500 ◦C for 24 h, furnace cooling) used in the previous studies [21–26]. The electrochemical potentiokinetic reactivation (EPR) ratio of the steel after the full heating schedule as a measure of DOS was 29.3%, measured by the EPR technique (JIS G 0580). In this study, the heating schedule was terminated after various heating periods to decrease the EPR ratio of the specimen. After terminating the heating schedule, each specimen was furnace-cooled. The EPR ratios of the heat-treated specimens are shown in Figure 3, demonstrating that the EPR ratio was lower for a shorter heating time.

**Figure 2.** Dimensions of specimens (in units of mm) and preparation for corrosion testing.

After the heat treatment, the surface of each specimen was ground with emery paper up to grade #2000 and polished with diamond paste of 3 m grain diameter and colloidal silica (OP-U, Struers, Nagoya, Japan). Then, the surface was ion-milled by argon gas (EM RES101, Leica Microsystems, Wetzlar, Germany) to obtain a smooth and clean surface for EBSD analysis.

**Figure 3.** Heating schedule for sensitization and change in EPR ratio of a specimen.
