*3.3. Electrochemical Measurements*

Figures 2 and 3 show the EIS plots carried out under different immersion times at open circuit potential for 200 and 300 steels, respectively. As can be seen from the Nyquist and Bode plots (Figure 2a,c and Figure 3a,c), the conductivity of the solution was very low. This result is understandable since the tested electrolyte consisted of pure water saturated with CO2 with a conductivity of approximately 190 μS cm−1. However, as time increased, the conductivity of the solution increased, likely due to the release of ions into the bulk solution from the metal surface. To compare the corrosion behavior of both samples, the IR drop was manually compensated. The corrected plots (Figure 2b,d,f, and Figure 3b,d,f) were then fitted with the equivalent circuit displayed in Figure 4 Due to the imperfection of the metal surface, the double-layer capacitance (*C*dl) was simulated using a constant phase element (CPE) [14]. The impedance of the CPE is described by the following equation [5,21]:

$$Z\_{\rm CPE} = \frac{1}{Q(j\omega)^n} \tag{3}$$

where *Q* stands for CPE constant, *n* is the exponent, *j* is the imaginary unit, and *ω* is the angular frequency at which *Z* reaches its maximum value.

**Figure 2.** EIS plots obtained after different immersion times for the 200 steel before (**a**,**c**,**e**) and after (**b**,**d**,**f**) the IR drop correction.

**Figure 3.** EIS plots obtained after different immersion times for the 300 steel before (**a**–**c**) and after (**d**–**f**) the IR drop correction.

**Figure 4.** Equivalent circuit used to fit the EIS plots. Here, *R*<sup>s</sup> is the electrolyte resistance, *Qd*<sup>l</sup> is the constant phase element representing the double charge layer capacitance and *R*ct is the charge transfer resistance. *L* and *R*<sup>L</sup> represent the inductance and inductance resistance, respectively.

As can be seen from figures (Figure 2b,d,f and Figure 3b,d,f), the fitted results were similar to those obtained experimentally and the values of *χ*<sup>2</sup> were very low (Table 4), indicating that the equivalent circuit, employed to simulate the system under investigation, was the most appropriate one. It follows from the Nyquist diagrams (Figures 2c and 3c) that the shape of the curve did not change with the immersion time and exhibited a depressed semicircle in the whole frequency range due to the inherent charge transfer processes controlling the corrosion reactions. An inductive loop is also visible at low frequencies, likely due to the relaxation time of the intermediate adsorbed species.

**Table 4.** Electrochemical impedance parameters of the two steels in the tested solution after the IR drop correction.


The CO2 gas dissolves in the solution forming carbonic acid, which successively dissociates into bicarbonate and carbonate anions, according to the following reactions:

$$\text{CO}\_2 + \text{H}\_2\text{O}\_{(l)} \leftrightarrow \text{H}\_2\text{CO}\_3 \tag{4}$$

$$\text{H}\_2\text{CO}\_3 \leftrightarrow \text{H}^+ + \text{HCO}\_3^- \tag{5}$$

$$\rm{HCO\_3^-} \leftrightarrow \rm{H^+} + 2\rm{CO\_3^{2-}} \tag{6}$$

In the presence of CO2, the process is controlled by the three cathodic reactions [6,22,23]:

$$2\text{H}\_2\text{CO}\_3 + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{HCO}\_3^- \tag{7}$$

$$2\text{HCO}\_3^- + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{CO}\_3^{2-} \tag{8}$$

$$\text{2H}^+ + \text{2e}^- \rightarrow \text{H}\_2 \tag{9}$$

The anodic reaction in a CO2-saturated solution can be summarized by the multi-step dissolution of carbon steel [24]:

$$\text{Fe} + \text{H}\_2\text{O} \rightarrow \text{(FeOH)}\_{\text{ads}} + \text{H}^+ + \text{e}^- \tag{10}$$

$$\text{(FeOH)}\_{\text{ads}} \rightarrow \text{FeOH}^+ + \text{e}^- \tag{11}$$

FeOH++H<sup>+</sup> <sup>→</sup> Fe2++H2O (12)

The inductive loop, observed at low frequencies, is likely due to the adsorption of (FeOH)ads on the metal surface [24].

After 24 h of immersion the 300 steel showed a higher capacitive semicircle compared to the 200 steel (Figure 5) The EIS findings are in agreement with the gravimetric results. Since the corrosion resistance of a given metal is a function of the size of the capacitive loop, it follows from the figure that the 300 steel, with a wider Nyquist curve capacitive loop, shows a higher corrosion resistance than the 200 steel. The SEM-EDS and XPS analyses (Section 3.4) indicate that the higher corrosion resistance of the 300 steel was likely ascribed to formation of a more stable and/or compact protective layer comprised of Al2O3, SiO2, Fe2O3 and traces of FeCO3. Since both materials had a similar chemical composition this behavior can be ascribed to different microstructure. Di Schino et al. also observed a similar result [9,10]. They studied the effects of the grain size on the corrosion behavior of refined austenitic stainless steel in a 5% H2SO4 boiling solution after 10 h of immersion. The authors reported that the corrosion rate decreased with increasing the grain size. They suggested that an increase of the grain boundary surface area due to grain refining, caused the passive film to become less stable, due to the defects concentrated in the grain boundaries. In their study, the layer formed on the coarse-grained steel was more stable and could thus provide more protection to the steel substrate by a blocking effect, thereby reducing the diffusion of the aggressive substances from the bulk solution to the metal surface.

**Figure 5.** EIS plots comparing the two steels after 24 h of immersion and after the IR drop correction in the tested solution. (**a**) Nyquist, (**b**) Bode and (**c**) Phase angle.

Figure 6 and Table 5 show the potentiodynamic polarization measurements and corrosion kinetic parameters observed after 24 h of immersion in the tested solution, respectively. It is evident from the data that the corrosion current density of the steel with coarse grains was significantly lower compared to steel with fine grains. The corrosion rate of the metal with coarse grains was found to be 0.15 mm y−<sup>1</sup> against 0.28 mm y−<sup>1</sup> for the metal with fine grains, in agreement with the results observed with the gravimetric measurements. Furthermore, both the anodic and cathodic branches of the polarization curves were shifted towards the lower current densities for the steel with coarser grains. Li et al. [12] also reported a similar result. The authors suggested that the higher anodic current density observed for the steel with finer grains was related to the high energies that the atoms have at the grain boundaries. These atoms are the first to take part in the reaction. An increase in grain refinement leads to an increase in the volume fraction of intercrystalline areas such as grain boundaries and triple junctions [13]. Therefore, as the volume fraction of the grain boundary increases, the amount of the active atoms on the steel surface also increases, which in turn leads to an increase in the anodic current density. The result suggests that the stable layer, formed on the coarse grain size metal surface, hinders both the rate of the cathodic reaction (Equations (7)–(9)) and anodic dissolution (Equations (10)–(12)), by either covering part of the metal surface or blocking the active corrosion sites on the steel surface [5,6].

**Figure 6.** Potentiodynamic polarization curves obtained after 24 h of immersion in the tested solution.

**Table 5.** Potentiodynamic polarization parameters in the tested solution obtained after 24 h of immersion.


To confirm the correctness of the results obtained in pure water, the electrochemical experiments were also carried out in a 3.5 wt.% NaCl aqueous solution saturated with CO2. The EIS and PDP results are displayed in Figures 7 and 8 and Tables 6 and 7, respectively. The results are in agreement with ones observed in pure water saturated with CO2, which shows that the coarse-grained steel still displays a better corrosion resistance compared to the fine grained steel. In particular, it can be seen from the potentiodynamic experiments (Figure 8) that the 300 steel exhibits a pseudo-passive region, only, extends to a small range of potential (i.e., −612 to −586 mV vs. SCE). This result confirms that the coarse-grained steel tends to form a more stable corrosion product layer on the surface, which led to an increase in its corrosion resistance.

**Figure 7.** EIS plots comparing the two different samples after 24 h of immersion in a 3.5 wt.% NaCl saturated with CO2. (**a**) Nyquist, (**b**) Bode, and (**c**) Phase angle.

**Figure 8.** Potentiodynamic polarization curves obtained after 24 h of immersion in a 3.5 wt.% NaCl aqueous solution saturated with CO2.


**Table 6.** Electrochemical impedance parameters of the steels after 24 h of immersion in a 3.5 wt.% NaCl aqueous solution saturated with CO2.

**Table 7.** Potentiodynamic polarization parameters obtained after 24 h of immersion in a 3.5 wt.% NaCl aqueous solution saturated with CO2.


#### *3.4. Morphological Analysis*

Figure 9 shows the surface morphology after 24 h of immersion in pure water saturated with CO2. It is clear from figures that the surface morphology of the tested samples differs significantly. The severity of the corrosion process revealed the microstructure of the 200 steel (Figure 9a). By contrast, the surface of the 300 steel appears much smoother. The EDS analysis listed in Table 8 confirmed formation of a protective corrosion product layer, mainly composed of aluminum, oxygen, and other alloying elements. However, the data shows that the surface of the 300 steel was covered by a more homogeneous layer with the concentration of the abovementioned elements uniformly distributed over the entire surface. On the other hand, the surface of the 200 steel shows the presence of darker areas (e.g., red square 2 in Figure 9a), where the concentration of Al and O was higher compared to lighter areas (e.g., red square 1 from Figure 9a) and much similar to that found on the surface of the 300 steel. These results confirmed the observations reported in this study and are in agreement with the literature [9–16]. The results suggest that the better corrosion resistance performance observed of the coarse-grained steel was ascribed to its ability to form a more homogeneous and stable layer over the entire surface, which shields the material from the aggressive solution. Moreover, the morphological analysis also shows the presence of carbon on both surfaces. The presence of carbon was likely due to the precipitation of FeCO3. Iron carbonate can form when the concentration of Fe2+ and CO2<sup>−</sup> 3 ions exceeds its solubility product (i.e., super-saturation):

$$\text{Fe}^{2+} + \text{CO}\_3^{2-} \rightarrow \text{FeCO}\_3 \tag{13}$$

**Figure 9.** SEM analysis on the sample (**a**) 200 and (**b**) 300, after 24 h of immersion. (The red square area corresponds to the area of the EDS analysis).


**Table 8.** EDS analysis of the tested samples.

The precipitation of FeCO3 depends not only on the concentration of Fe2+ and CO2<sup>−</sup> 3 ions, but is also affected by other factors such as temperature, CO2 partial pressure, and pH. Among the abovementioned factors, the pH of the solution can be regarded as one of the most influential factors [25]. Dugstad [25] reported that an increase in pH of the solution significantly reduced the concentration of Fe2+ ions required to exceed the FeCO3 solubility product and therefore promoteing its precipitation. In this study, the pH increased as the immersion time increased, going from 4.12, at the beginning of the experiment, to 5.61 after 24 h of immersion. As such, making the precipitation of FeCO3 more probable. Moreover, Dugstad [25] also reported that the concentration of Fe2+ ions is higher at the surface/solution interface compared to the bulk solution. Consequently, the concentration of Fe2+ ions required to promote the formation of FeCO3 on the metal surface is lower and thus, increasing the likelihood of having FeCO3 on surface. However, only small traces of FeCO3 could be found as confirmed by the XPS analysis (Figure 10). Previous studies reported that FeCO3 begins to decompose at temperatures below 100 ◦C according to the following reaction [6,23,26]:

$$\text{FeCO}\_3 \rightarrow \text{FeO} + \text{CO}\_2 \tag{14}$$

In the presence of CO2 or water vapor, FeO transforms into Fe3O4 [23,26].

$$\text{C3FeO} + \text{CO}\_2 \rightarrow \text{Fe}\_3\text{O}\_4 + \text{CO} \tag{15}$$

$$\text{3FeO} + \text{H}\_2\text{O} \rightarrow \text{Fe}\_3\text{O}\_4 + \text{H}\_2\tag{16}$$

In the presence of oxygen, FeO and Fe3O4 transform into Fe2O3 [23,26].

$$\text{4FeO} + \text{O}\_2 \rightarrow \text{2Fe}\_2\text{O}\_3\tag{17}$$

$$\text{In the air}: \ 4\text{Fe}\_3\text{O}\_4 + \text{O}\_2 \rightarrow \ 6\text{Fe}\_2\text{O}\_3\tag{18}$$

The XPS was employed to characterize the composition of the thin corrosion layer formed after 24 h of immersion at 25 ◦C (Figure 10). The high-resolution spectra of aluminum (Al2p), silicon (Si2p), oxygen (O1s), carbon (C1s), and iron (Fe2p) are presented in Figure 10 and the binding energies and the corresponding quantification (%) of each peak are presented in Table 9. The deconvolution of the Al2p spectrum (Figure 10b,c) shows two peaks located at 74.7 and ~77.5 eV that can be attributed to Al2O3 and anhydrous Al2O3, respectively [27]. The high-resolution XPS spectrum of Si2p (Figure 10d,e) shows two main peaks at around 102 and 102.5 eV corresponding to Si-O/Si-O-C bond (Silicon oxide/Silicon oxycarbide). The silicon oxycarbide phase might have been formed on the surface as part of the protective layer when the material was exposed to the atmosphere. The O1s spectrum (Figure 10f,g) is fitted into three distinct peaks namely, 530, 531.6, and 533 eV. The peak observed at 530 eV was ascribed to O2<sup>−</sup> and could be related to oxygen atoms bonded to the metal, (i.e., Al2O3, Fe2O3, and SiO2 oxides) [6], whereas the peaks at 531.6 and 533 eV are associated with single bonded and double-bonded oxygen in FeCO3 [6,28]. A fourth peak was observed at 354.6 eV for the 300 steel and can be attributed to the COO− of FeCO3. The C1s spectrum (Figure 10h,i) corroborated with

the data observed for the O1s spectrum, which shows three peaks at 284.8, ~286.3, and ~288.5 eV. The 284.6 eV peak may correspond to secondary carbon [28], whereas the peaks at ~286.3 are ~288.5 eV are attributed to the C–O and C=O bonds of FeCO3 [6]. As in the case of the O1S spectrum, for the 300 steel, a fourth peak was observed at 289.8 eV, which is characteristic of FeCO3 [29]. The deconvoluted Fe2p3/2 peaks (Figure 10j,k) could be attributed to α-Fe2O3 or/and γ- Fe2O3 oxides [6,30], likely due to the partial decomposition of iron carbonate (i.e., Equations (14)–(18)).

**Figure 10.** *Cont*.

**Figure 10.** XPS analysis carried out after 24 h of immersion. Survey (**a**), 200 steel (**b**,**d**,**f**,**h**,**j**) and 300 steel (**c**,**e**,**g**,**i**,**k**).


**Table 9.** XPS analysis of sample surfaces after 24 h of immersion.

The XPS analysis is in agreement with the EDS analysis (Table 8), indicating that the corrosion products formed on both samples were mainly composed of Al2O3 and SiO2, with traces of FeCO3/Fe2O3.

To study the micro-galvanic activities occurring at the grain boundaries and triple junction on the metal surface, AM-KPFM measurements were carried out. AM-KPFM is a powerful technique for assessing the Volta potential (ΔΨ) of a metal surface. The ΔΨ is a characteristic property of the metal surface and can provide an insight into the local electrochemical activities on the metal surface [31]. Figure 11 the topography and the corresponding Volta potential maps of the two tested steels. The Volta potential maps show darker color representing the anodic regions, whereas the lighter color representing cathodic regions. The ΔΨ mapping clearly shows potential differences at grain boundaries and triple junctions, indicating higher electrochemical activity in these regions. The grain boundaries and triple junctions are characterized in the maps as lighter zones, thus representing the cathodic areas and displaying a relative ΔΨ difference of *circa* +30 mV with respect to the adjacent matrix. The results confirmed that these regions are more active compared to the adjacent matrix, which makes them more susceptible to corrosion attack during the exposure to electrolytes. Therefore, the grain refinement enhances the reactivity of the surface, which could promote a preferential dissolution of the grains [7,8,11,12,14–16,18].

**Figure 11.** AM-KPFM analysis of the steels with the topography and the corresponding Volta potential map, respectively. Steel 200 (**a**,**b**); 300 (**c**,**d**).

### **4. Conclusions**

The effect of the grain size on the corrosion behavior of two electric steels in pure water saturated by CO2 can be summarized as follows:


**Author Contributions:** G.P. conceived and designed this study. G.P. performed the electrochemical experiments, analyzed the experimental data, wrote and edited the manuscript; D.D. performed the gravimetric and SEM-EDS experiments; R.W. performed the XPS analysis; T.M. performed the AM-KPFM analysis; U.L.-B. prepared the samples; K.W. supplied materials, supervision, and funding acquisition; J.B. supervision and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** Part of this work was supported by AGH University of Science and Technology, Faculty of Foundry Engineering, Department of Chemistry and Corrosion of Metals, project no. 16.16.170.654. R.W. has been partly supported by the EU Project POWR.03.02.00-00-I004/16.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
