**2. Materials and Methods**

The material under study was a 2205 DSS steel in the form of a 4-mm-thick hot-rolled bar that was supplied by the Taiyuan Iron & Steel Company Ltd. (TISCO, Taiyuan, China). It was solution that was annealed at 1050 ◦C and water quenched. The corresponding chemical composition is presented in Table 1. Following solution treatment, the specimens were cold rolled with reductions of 50%. Given the "nose temperature", aging treatments were carried out on the specimens at 850 ◦C for different times ranging from 10 min to 4 h. Subsequently, each specimen was mounted on epoxy resin, mechanically ground with SiC papers down to 3000 grit, polished to mirror finish, as well as cleaned with distilled water and ethanol.


**Table 1.** Chemical composition of 2205 duplex stainless steel (wt %).

Prior to use, all of the samples were etched with a mixed solution of K2S2O5 (0.3 g), HCl (20 mL), and H2O (80 mL). The microstructures of the samples were observed with an optical microscope (OM, DMRM, LEICA, Shanghai Optical Instrument Factory, Shanghai, China). The different phases of the specimens were determined through X-ray diffraction analysis (XRD, X'Pert Powder, PANalytical, Almelo, the Netherlands) and the corresponding phase composition of the alloy was analyzed through electron dispersive X-ray spectroscopy (EDS, Octane SDD, EDAX Inc., Mahwah, NJ, USA) of the scanning electron microscopy (SEM, EVO18, Carl Zeiss Jena, Oberkochen, Germany). The phase proportion was analyzed with statistical methods with the Image-Pro Plus image manipulation software (Image ProPlus 6.0, Media Cybemetics, Rockville, MD, USA, 2006).

The electrochemical experiments were performed with a 3.5 wt % NaCl solution, at room temperature and atmospheric pressure. A three-electrode corrosion cell that was equipped with a saturated calomel electrode (SCE) reference electrode and a platinum foil counter electrode was utilized. The specimen with an exposure area of 1 cm<sup>2</sup> was used as a working electrode. Prior to experimentation, the samples were allowed to stabilize at an open circuit potential for 30 min, until the fluctuation potential reached 10 mV. The polarization curves were recorded potentiodynamically at 0.5 mV·s<sup>−</sup>1, while the potential scanning range was from below 200 mV of the open-circuit potentials to the potential when the current indicated that stable pitting had occurred.

The samples with the σ phase were treated through electrolysis in a 10 wt % KOH aqueous solution with a voltage of 2 V, until the σ phase was completely dissolved. Consequently, the sample surface was treated with alcohol and wascoated with a cyanoacrylate adhesive for the residual σ phase insulation. Following the cyanoacrylate glue complete solidification, the cotton swabs that were dipped in acetone were used to gently wipe the cyanoacrylate glue. Finally, an appropriate polishing processing was required to ensure the surface smoothness. The specific process is presented in Figure 1. It could be observed that the σ phases were dissolved through this method. Also, the second layer of σ phases was not exposed.

**Figure 1.** Process of preparing specimens without σ phase, (**a**) original specimen; (**b**) specimen electrolytically corroded by 10 wt % KOH; and, (**c**) specimen covered by cyanoacrylate glue.
