*3.2. Cyclic Corrosion Test Results*

The specimens after the CCT were cut and the cross-section was observed with an OM and the results are shown in Figure 6. After 10 cycles, the rust of all steels was thin and relatively uniform. However, after 20 cycles, brown and black oxides were formed on the inner layer and outer layer, respectively. Especially after 30 cycles, the amount of corrosion product of 0.3 Cr and 0.5 Cr steels was greater than that of 0 Cr steel, and corroded in a more localized way. Furthermore, since the thickness of the oxide layer is proportional to the amount of corrosion of the base metal, a thick oxide layer was locally formed on the 0.3 Cr and 0.5 Cr steels.

**Figure 6.** Cross-section OM images of (**a**) 0 Cr steel, (**b**) 0.3 Cr steel, and (**c**) 0.5 Cr steel after CCT.

To determine the localized corrosion tendency, the pitting factor (PF) with a concept similar to that given in ASTM G46 was used. A PF value of 1 means perfect uniform corrosion, and a higher PF means an increased localized corrosion tendency. The PF for each CCT cycle was derived by the following equation, and the variation of the PF according to CCT cycle is shown in Figure 7.

$$\text{PF} = \frac{\text{P}}{\text{d}} \tag{2}$$

where p is the maximum penetration depth, and d is the average penetration depth.

**Figure 7.** Variation of pitting factor according to CCT cycle.

In the case of 0 Cr steel, the PF was approximately 2 regardless of the cycle, while the PF of 0.3 Cr and 0.5 Cr steels changed depending on the cycle. In all cycles, the PF of the Cr-added steels was higher than that of the 0 Cr steel, but the PF was not proportional to Cr content. This indicates that the Cr alloying element can accelerate localized corrosion, and the presence or absence of Cr greatly affects the localized corrosion, not the Cr content.

The cross-section of the specimen after 10 and 30 cycles was analyzed to determine the chemical composition using EPMA, and the results are shown in Figure 8. The rust layer of 0 Cr steel was composed entirely of porous iron oxide (e.g., γ-FeOOH, γ-Fe2O3, Fe3O4). In addition, Cl− was accumulated at the metal/rust interface and on the inner layer with uniform concentration and distribution. The Cr-added steels had a very dense and uniform Cr-enriched region in the inner rust layer, while the outer rust layer was composed of porous iron oxide, like 0 Cr. Cl− was detected underneath the Cr-enriched layer and at the metal/rust interface, but unlike 0 Cr steel, it was localized and non-uniformly concentrated. The rust layer of the 30-cycle steel was exfoliated from the metal, and the Cl− concentration in the inner rust layer was increased significantly compared to 10 cycles. Therefore, it is considered that the corrosion is accelerated because the protective oxide layer loses its protective property after the rust layer exfoliates.

To summarize the above results, Cl− was concentrated at the metal/rust interface in all of the specimens regardless of Cr content. Generally, since the localized corrosion in an atmospheric environment is caused by Cl− enrichment [4,24], localized corrosion with a PF of approximately 2 or higher occurred in all of the steels, as shown in Figure 7. However, 0.3 Cr and 0.5 Cr steels had higher PFs than 0 Cr steel because Cl- was localized and non-uniformly concentrated as compared with 0 Cr steel.

**Figure 8.** EPMA analysis of (**a**) 0 Cr steel, (**b**) 0.3 Cr steel, and (**c**) 0.5 Cr steel after CCT.
