*3.4. Localized Corrosion Mechanism of Cr-Added Steel under Wet/Dry Conditions*

The Cr alloying element accelerates localized corrosion under Cl-containing wet/dry conditions unlike the immersion condition. The mechanism of localized corrosion of Cradded steel under wet/dry conditions is as follows, and a schematic diagram is shown in Figure 11.

**Figure 11.** Schematic diagram of the mechanism of localized corrosion of Cr-added steel under wet/dry conditions.

Corrosion of steel begins in areas where the inherent oxide film is weak. During the wet stage, Cl− ions existing in the aqueous adsorption layer move to these weak areas, and a Cl-concentrated region (nest) is formed [32]. The Cl− is adsorbed on the steel surface, and then atmospheric corrosion initiates. Thereafter, Fe2+ reacts with H2O to form Fe(OH)2, and with salt or Cl− in the air to form FeCl2.

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^- \tag{3}$$

$$\text{Fe}^{2+} + 2\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_2 + 2\text{H}^+ \tag{4}$$

$$\text{Fe}^{2+} + 2\text{Cl}^- \rightarrow \text{FeCl}\_2 \tag{5}$$

During the dry stage, Fe(OH)2 is transformed into lepidocrocite, and FeCl2 formed in the Cl-concentrated region is transformed into akaganeite. After that, lepidocrocite and akaganeite are reduced to amorphous oxide or magnetite in the wet stage. Next, magnetite is re-oxidized into lepidocrocite.

$$2\text{y-FeOOH} + \text{Fe}^{2+} \rightarrow \text{Fe}\_3\text{O}\_4 + 2\text{H}^+ \tag{6}$$

$$\text{Fe}\_3\text{O}\_4 + 3/2\text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{3y-FeCOH} \tag{7}$$

In the atmospheric rusting process, lepidocrocite on the steel surface transforms into amorphous ferric hydroxide, then it converts to goethite. Cl− may facilitate this reaction and promote goethite formation [33–35].

$$\text{\(\gamma\text{-FeOOH}\rightarrow\text{FeO}\_{\text{x}}\text{(OH)}\text{)}\_{3-2\text{x}} \text{ (amorphous ferric oxyhydroxide)} \rightarrow \text{\(\alpha\text{-FeOOH}\)}\tag{8}$$

$$\text{\textsuperscript{\text{\tiny}}-\text{FeOOH}} \rightarrow \text{FeO}\_{\text{x}}\text{(OH)}\\\text{\textsuperscript{\text{\tiny}}-\text{\_2x}Cl \rightarrow \text{\color{orange}{\text{\tiny}}x-\text{FeOOH}}+\text{HCl}}\tag{9}$$

Cr3+ ions dissolved in the early stages of the corrosion process are more easily deposited as hydroxides near the steel surface compared to the Fe2+ ions since the solubility of Fe2+ ions is higher than that of Cr3+. Additionally, the Cr3+ ions act as nuclei for the growth of Cr-goethite. Finally, an ultrafine Cr-goethite layer is formed in the inner rust layer when the wet/dry process is repeated [6]. Since Cr-goethite has cation selectivity, it suppresses the penetration of aggressive anions and improves corrosion resistance. Then, after the formation of Cr-goethite, the inflow of extra Cl− from the outside is blocked, so that Cl− is locally accumulated underneath the Cr-enriched layer.

During the wet stage, Cl− ions in the akaganeite formed in the inner rust layer are dissolved and eluted in water, resulting in the formation of FeCl2 and CrCl3. The hydrolysis reactions of these Fe and Cr salts occur.

$$\text{FeCl}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{Fe(OH)}\_2 + 2\text{HCl} \tag{10}$$

$$\text{CrCl}\_3 + 3\text{H}\_2\text{O} \rightarrow \text{Cr(OH)}\_3 + 3\text{HCl} \tag{11}$$

The pH of the steel surface is reduced by the hydrolysis reaction of Fe and Cr [36,37]. Since Cr tends to strongly hydrolyze up to pH 1.4, it enhances the susceptibility to localized corrosion compared to the hydrolysis reaction of Fe [38]. That is, FeCl2 and CrCl3 are formed from the Cl- nest developed in the early stage, and the CrCl3-formed regions are locally accelerated. Even if Cr is not added, Cl− may cause localized corrosion. However, when Cr is added, localized corrosion is more accelerated since Cr is strongly hydrolyzed to a very low pH.

Finally, when the wet/dry cycle is continuously repeated, the rust layer is exfoliated and loses its protective property. Then, Cl− easily penetrates into the gap between the separated rust layer and substrate, which accelerates the localized corrosion of ACS.

#### **4. Conclusions**

In this study, the effect of Cr alloying element on the corrosion properties of ACS in aqueous and atmospheric conditions was investigated using electrochemical measurements and a CCT. The conclusions based on the investigations are as follows:


**Author Contributions:** Conceptualization, S.-w.C. and Y.-H.Y.; Data curation, S.-w.C.; Formal analysis, S.-w.C.; Funding acquisition, J.-G.K.; Investigation, S.-w.C., S.-J.K., and J.-G.K.; Methodology, J.-G.K.; Project administration, Y.-H.Y., Y.-K.S., and J.-G.K.; Resources, Y.-H.Y. and Y.-K.S.; Software, S.-J.K. and J.-S.Y.; Validation, Y.-H.Y. and J.-G.K.; Visualization, S.-w.C.; Writing—original draft, S.-w.C.; Writing—review and editing, S.-w.C., S.-J.K., J.-S.Y., Y.-H.Y., Y.-K.S., and J.-G.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by POSCO, grant number 2018Z098.

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

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

