*3.2. Potentiodynamic Polarization Results*

The potentiodynamic polarization curves of the RASP-processed SA106B low-carbon steel with various duration times are shown in Figure 5. The potentiodynamic polarization test has usually been used to characterize the corrosion properties of materials [21]. The corrosion current density (Icorr) can be used to calculate the corrosion rate of metals. The Icorr is determined from the Tafel plot by extrapolating the linear portion of the polarization curve near corrosion potential (Ecorr) [22]. As illustrated by the curve in Figure 5, all the potentiodynamic polarization curves show similar characteristics. When the polarization potential was higher than the corrosion potential, the current of the steel increased sharply with the increase of the polarization potential, indicating that the low-carbon steel exhibited the activation state in the solution. These results were consistent under two different electrolyte solutions, as indicated in Tables 2 and 3. The Icorr values decreased after the RASP, indicating the corrosion resistances for the samples were improved. It can also be seen that the Icorr increased

slightly at a longer RASP time, the increasing corrosion rate can be attributed to the microscopic changes of the samples.

**Figure 5.** Potentiodynamic polarization curves of SA106B low-carbon steel for various RASP-processed periods ranging from 0 min to 20 min: (**a**) 0.05 M H2SO4 + 0.05 M Na2SO4 solution; (**b**) 0.2 M NaCl + 0.05 M Na2SO4 solution at room temperature.

**Table 2.** Electrochemical parameters of SA106B low-carbon steel for various RASP-processed periods ranging from 0 min to 20 min in the 0.05 M H2SO4 + 0.05 M Na2SO4 solution.


**Table 3.** Electrochemical parameters of SA106B low-carbon steel for various RASP-processed periods ranging from 0 min to 20 min in the 0.2 M NaCl + 0.05 M Na2SO4 solution.


The carbon steel which differs from a passive material, like a stainless steel, is a material with active dissolution behavior [23,24]. When we estimate its corrosion resistance, the primary factor parameter is the Icorr, followed by the Ecorr. The corrosion resistance of the 5-min RASP-processed steel exhibited the best corrosion resistance among all of the samples. The original carbon steel had the worst corrosion resistance.

## *3.3. EIS Study*

In studying corrosion and passivation processes, electrochemical impedance is a powerful tool to provide more information about the electrochemical processes. The impedance responses of these systems are given in Figures 6 and 7 in Nyquist and Bode formats, respectively. Figure 6 shows the effect of the RASP-processed time on the Nyquist plots of the SA106B low-carbon steel. One can see that the samples exhibited a high-frequency capacitive reactance arc and low-frequency inductance arc characteristics. At least two time constants are clearly observed in the Nyquist and Bode representations. With the increase of the RASP-processed time, the diameter of the semicircle decreased. The electrochemical impedance spectroscopy diagram shows that the larger the capacitive reactance arc, the better the corrosion resistance. We can conclude that the corrosion resistance of the samples improved after the RASP. The red circle with small capacitive reactance arc or inductive arc in the high-frequency part may be caused by the high-frequency phase shift. For the exact details, please refer to the specific introduction of Mansfeld [25]. A capacitive reactance generally above 10 kHz is essentially not a reflection of the electrochemical process.

**Figure 6.** The effect of the RASP-processed time on the Nyquist plots of the SA106B low-carbon steel measured at open-circuit potential with a sinusoidal potential amplitude of 10 mV, running from 100 kHz to 10 MHz in the (**a**) 0.05 M H2SO4 + 0.05 M Na2SO4 solution and (**b**) 0.2 M NaCl + 0.05 M Na2SO4 solution at room temperature.

**Figure 7.** Bode plots of the RASP-processed SA106B specimens. (**a**) Frequency-impedance relation and (**b**) frequency-phase relation in the 0.05 M H2SO4 + 0.05 M Na2SO4 solution. (**c**) Frequency-impedance relation and (**d**) frequency-phase relation in the 0.2 M NaCl + 0.05 M Na2SO4 solution.

*Metals* **2019**, *9*, 872

The fitted result for the impedance spectrum measured after potentiodynamic polarization in the 0.2 M NaCl + 0.05 M Na2SO4 solution with equivalent circuits in the inset of Figure 8a, as a representative example, is shown in Figure 8. The impedance data were fitted with ZSimpWin 3.50 version software (Ann Arbor, Michigan, MI, USA) using an equivalent circuit. It can be seen that the fitted and measured results match quite well in both Nyquist and Bode plots. The shrinkage of the Nyquist plots at the real parts are usually interpreted as the typical corrosive pitting appearing during the test [26]. It shows that intermediate products appear in the electrode reaction, producing a surface-adsorbing complex with the surface of the metal electrode. The oxidation film on the surface is slightly damaged and has not changed the original surface properties. Tables 4 and 5 list the fitted electrochemical parameters. Rs is the resistance of the solution affecting the charge transfer process, and Rct is the charge transfer resistance of the surface electrode reaction of the low-carbon steel, which can reflect the surface of the electrode due to electricity generation. RL is the inductance resistance and L is the inductance. C1 is the interfacial capacitance and Cad is the adsorption capacitance. Q is the constant phase element (CPE) which represents the capacitance of double layer. The CPE, which has non-integer power dependence on the frequency, is used to represent the capacitances of double layer to account for the deviation from the ideal capacitive behavior due to surface inhomogeneity, roughness and adsorption effects. The impedance of CPE is described by the expression

$$\mathbf{Z}\_{\rm CPE} = \mathbf{Y}\_0^{-1} \left( \mathbf{j} \mathbf{w} \right)^{-\alpha}$$

where Y0 is a proportional factor, j is the imaginary unit, w is the angular frequency, and α is the phase shift, which is a measure of the capacitance dispersion. For α = 0, Q represents a resistance with R = Y0 <sup>−</sup>1; for α = 1, it represents a capacitance with C = Y0; for α = 0.5, it represents a Warburg element and for α = −1, it represents an inductance with L = Y0 <sup>−</sup>1. The Rct value continuously decreased with the increase of the RASP-processed time (Tables 4 and 5), indicating that the corrosion resistance decreased, which may be related to the changes in the thickness, homogeneity, and composition of the oxidation film.

**Figure 8.** The fitted electrochemical impedance spectroscopy (EIS) of original specimen after potentiostatic polarization in the 0.2 M NaCl + 0.05 M Na2SO4 solution: (**a**) Nyquist plots and (**b**) Bode plots. The inset of (**a**) shows the equivalent electrical circuit for the specimen.

**Table 4.** Equivalent-circuit element values for EIS data corresponding to the SA106B low-carbon steel treated for different times in 0.05 M H2SO4 + 0.05 M Na2SO4 solution.



**Table 5.** Equivalent-circuit element values for EIS data corresponding to the SA106B low-carbon steel treated for different times 0.2 M NaCl + 0.05 M Na2SO4 solution.

## *3.4. SEM Photomicrographs*

Figure 9 illustrates the dissolution morphologies of the SA106B low-carbon steel after the polarization tests. Some pitting pits were found in both ferrite and pearlite. The pearlite consists of ferrite and cementite phases. The cementite phases as the cathode have a higher potential compared to the ferrite phases, leading to the severe galvanic corrosion which can induce and worsen pitting [27,28]. With the prolongation of holding time, pits numbers gradually increased. Moreover, their size enlarges and the depth increases due to the micro-cracks effect and the roughness of carbon steel specimens treated by the peening. Thus, the corrosion resistance of the RASP-processed samples decreased. Among them, the sample RASP-processed for 5 min exhibited the best corrosion resistance. This is consistent with the results of the polarization curves and the impedance spectrum, as shown in Figures 5 and 6.

**Figure 9.** SEM micrographs for the samples RASP-processed for (**a**) 5 min, (**b**) 10 min and (**c**)15 min after potentiodynamic polarization in the 0.2 M NaCl + 0.05 M Na2SO4 solution.
