*3.3. Rust Constituent Analysis*

The constituent of the rust formed after the CCT was analyzed by XRD, EDS, and TEM, and the results are shown in Figures 9 and 10. As shown in Figure 9, the phase of rust formed by the CCT was almost the same for all steels. The rusts were composed of various oxides and hydroxides such as α-FeOOH (goethite), β-FeOOH (akaganeite), γ-FeOOH (lepidocrocite), γ-Fe2O3 (maghemite), and Fe3O4 (magnetite). Among them, akaganeite always contains Cl− in the lattice because it is stabilized by the Cl− entering the lattice structure. In addition, akaganeite is formed only during the dry stage in an atmospheric environment, and Cl− in the akaganeite is dissolved in water to promote corrosion during the wet stage. That is, the akaganeite acts as a Cl− reservoir. As a result, a large amount of akaganeite is formed inside the pit generated by atmospheric corrosion [3,6,24–26]. Therefore, akaganeite was formed in the inner rust layer, and Cl− was observed at the metal/rust interface, as shown in Figure 8.

**Figure 9.** XRD analysis of the specimen surfaces after 20 cycles of CCT.

The EDS results, TEM images, and diffraction patterns of the inner and outer rust formed on the 0.5 Cr steel were analyzed, and the results are shown in Figure 10. Chlorine was observed in the inner rust particle as an acicular single crystal with a size of about 100 nm, as shown in Figure 10a. This is the major characteristic of akaganeite [27]. In Figure 10b, Cr and Cl− were observed together in the inner rust particle. The particle was polycrystalline and was a spherical agglomeration with a size of several nanometers. As the spherical-shaped rust is the main feature of goethite [28], the particle is Cr-containing nanoscale goethite (Cr-goethite). Since dissolved or enriched Cr suppresses the growth of goethite crystals [7], the size of the Cr-goethite particles is very small. Cr-goethite is so small in size that it acts as a protective film that is densely formed in the inner rust layer. Furthermore, Cr-goethite has cation selectivity so it can inhibit the penetration of aggressive anions such as Cl− and SO4 <sup>2</sup><sup>−</sup> and improve corrosion resistance [7,25,29–31]. In short, Cr-goethite was formed in the inner rust layer of Cr-added steels, which blocked the inflow of additional Cl− from the outside and consequently improved the corrosion resistance. As shown in Figure 10c, Cl− and Cr were not detected in the outer rust particle. Therefore, the outer rust layer is composed of various rusts such as lepidocrocite, maghemite, and magnetite detected from the XRD analysis results. In summary, the inner rust layer consists of akaganeite containing Cl− and Cr-goethite, while the outer rust layer is composed of amorphous iron oxyhydroxide mixed with various types of rust.

**Figure 10.** TEM images and the diffraction patterns of (**a**,**b**) inner rust and (**c**) outer rust formed on the 0.5 Cr steel.
