*3.1. Electrochemical Measurement*

Potentiodynamic polarization tests were conducted to analyze the difference in corrosion characteristics depending on the Cr content, and the results are shown in Figure 3 and Table 2. In Figure 3, all of the specimens show an active corrosion behavior that increases with increasing potential in Cl−-containing environments without passivation. Additionally, there was no significant difference in the corrosion potential regardless of the Cr content. The corrosion potential (Ecorr) and corrosion current density of 0.5 Cr steel were slightly lower than that of 0 Cr and 0.3 Cr steels, but this is an insignificant difference that can be regarded as an experimental error.

**Figure 3.** Potentiodynamic polarization curves in 3.5 wt.% NaCl solution.

**Table 2.** Potentiodynamic polarization test results in 3.5 wt.% NaCl solution.


In order to obtain a better understanding of the effect of Cr on the corrosion behavior of ACS under aqueous conditions, EIS measurement was performed in a 3.5 wt.% NaCl solution at room temperature. Figure 4 shows the results of EIS measurement in the form of Nyquist and Bode plots under open circuit potential (OCP) according to various immersion times. The Nyquist plots were not perfect semicircles due to dispersion effects that are often caused by the geometrical inhomogeneity or non-uniform current distribution on the electrode surface [11]. The capacitive loops in the high- and low-frequency regions overlapped. The capacitive loop of the high-frequency region showed the resistance of the film, whereas that of the low-frequency region showed the charge transfer resistance [12–14]. In the 0 Cr steel, the size of the semicircle for 1 h was larger than that for 0 h, and then became smaller with respect to immersion time. This means that a thin and weak oxide layer was formed on the steel surface at the initial stage of the corrosion process, and deteriorated with respect to immersion time due to its instability [15]. Similar to 0 Cr steel, the size of the semicircles for the 0.3 Cr and 0.5 Cr steels also increased immediately after immersion, and then decreased with immersion time. However, the size of the overall capacitive semicircles is ordered as 0.5 Cr > 0.3 Cr > 0 Cr. In general, the size of the capacitive semicircle on the Nyquist plot represents corrosion resistance. This means that the Cr alloying element within 0.5 wt.% improves corrosion resistance in an aqueous environment. In the Bode plots, the impedance at a low frequency and the shoulder width at the phase angle were increased and wider immediately after immersion, and then decreased and narrower with immersion time. This result is consistent with the Nyquist impedance interpretation.

**Figure 4.** Nyquist and Bode impedance plots of EIS data of (**a**,**b**) 0 Cr steel, (**c**,**d**) 0.3 Cr steel, and (**e**,**f**) 0.5 Cr steel in 3.5 wt.% NaCl solution.

To determine the optimized values for the resistance and capacitance parameters, the equivalent circuit was used as shown in Figure 5. Rs is the test solution resistance, Rfilm is the oxide film resistance, Rct is the charge transfer resistance, and Rfilm + Rct is total resistance or polarization resistance (Rp), which is proportional to the radius of the capacitive loop in the Nyquist plot. The constant phase element (CPE) is the capacitive response of the system. CPE1 is the capacitive response of the oxide film, and CPE2 is

the capacitive response of the double layer caused by the dissolution of the metal and the charge separation between the metal/electrolyte interface [16–19]. In the equivalent circuit, CPE is defined as below:

$$\mathbf{Z\_{CPE}} = \mathbf{Q\_0^{-1}}(\mathbf{j}\omega)^{-\mathbf{n}} \tag{1}$$

where Z is the impedance, Q0 is the coefficient of proportionality, j is the imaginary number, ω is the angular frequency, and n is the empirical CPE exponent (0 ≤ n ≤ 1) measuring the deviation from the behavior of an ideal electric capacity [20,21]. CPE can represent resistance (n = 0), capacitance (n = 1), inductance (n = −1), or Warburg impedance (n = 0.5) in accordance with n [22].

**Figure 5.** Equivalent circuit for ACS in 3.5 wt.% NaCl solution.

The EIS data were fitted using the ZSimpWin (Princeton Applied Research, Oak Ridge, TN, USA) program and the results are shown in Table 3. The Rp of 0.3 Cr and 0.5 Cr steels was higher than that of 0 Cr steel, and the Rp also increased as the Cr content increased. This indicates that the corrosion resistance is increased as the Cr content increases. This is believed to be due to the bigger grain size of the steel with higher Cr content. Metal with active polarization behavior decreases the corrosion rate with bigger grain size [23]. Therefore, Cr improved the corrosion resistance of the ACS that was immersed in the Cl- -containing aqueous solution.

**Table 3.** Parameters from electrochemical impedance spectroscopy measurements.



**Table 3.** *Cont*.
