**1. Introduction**

The 2205 duplex stainless steels (DSS), with excellent corrosion performance, as well as good mechanical properties, have been used as engineering alloys for many years and provide wide applications in many industrial fields, especially in aggressive environments [1–6]. The effects of microstructural modifications of 2205 DSS on their mechanical and corrosion resistance were intensively investigated in the past. Few works highlighted the changes in corrosion resistance performances that are caused by the phase proportions of austenite and ferrite [7,8]. Several researchers discussed the passive film properties of the 2205 DSS during electrochemical corrosion [9–12]. Certain studies also indicated that the pitting corrosion of 2205 DSS often initiated from the σ phase due to the depletion of both Cr and Mo [13,14].

By contrast, due to the high contents of alloying elements in 2205 DSS, the secondary phases, such as σ, χ, Cr2N, α , and M23C6 were also easily precipitated in between approximately 300–1000 ◦C, leading to a detrimental effect on both mechanical properties and corrosion resistance behavior [15,16]. The χ phase, as a precursor of the σ phase, gradually disappeared with the σ phase precipitation. The M23C6 phase usually formed on the austenite grain boundaries during isothermal heating at 950 ◦C. The Cr2N were often found in ferrite subsequently to rapid cooling from the higher annealing temperature of 1050–1250 ◦C. When compared to these phases, the σ phase was more easily precipitated and had increased effect on the material properties [17–19]. Certain researchers had focused on the σ

phase precipitation. Chen et al. found that the σ phase was often precipitated at the α/γ interphase boundaries following the sample aging at 900 ◦C for 5 min [20]. Elmer et al. in situ observed the dissolution of the sigma phase in the 2205 duplex stainless steel, while the σ phase could be detected subsequently to aging at 850 ◦C only for 81s, but it was completely dissolved as the temperature increased to 985 ◦C [21]. Sieurin et al. found that the sensitive temperature of the σ phase in the 2205 DSS steel was between 650 ◦C and 920 ◦C, whereas the "nose temperature" was approximately 850 ◦C in the TTP (temperature-time-precipitation) diagram [22].

In fact, it was inevitable that certain treatments, such as welding and other thermal treatments, could cause the precipitates to form in 2205 DSS when it would be exposed to the sensitive temperatures of the precipitates [23–25]. The cold rolling is an industrial technique for alloy hardening, also producing a high amount of deformation and increasing the grain energy, finally affecting the precipitation behavior and the microstructure of the materials [26]. Cho et al. observed that the cold deformation promoted the σ phase precipitation in 2205 DSS, as compared to the non-cold-rolled materials [27]. Breda et al. revealed the strain-induced martensite occurrence in the cold-rolled 2205 DSS [28]. In contrast, the researches regarding the effect of precipitates on the corrosion behavior of the hot-rolled and cold-rolled 2205 DSS steels have rarely been contrasted.

The purpose of the present research was to investigate the effects of hot-rolling or cold-rolling treatments on the microstructures and the corrosion resistance of 2205 DSS steels. The microstructure and the chemical composition of the 2205 DSS was investigated with an optical microscope and a scanning electron microscope, while potentiodynamic polarization and electrochemical impedance spectroscopy were employed to detect the corrosion resistance of the 2205 DSS steels for the corresponding corrosion resistance properties forecasting. Besides, the microstructures prior to and following corrosion resistance testing were contrasted to distinguish the corrosion mechanism of constituent phases. The final purpose of this paper was to provide a scientific basis for the hot working process optimization, the microstructure constituent prediction, and the corrosion resistance prediction.
