**1. Introduction**

Steel sheets for automobiles are exposed to various corrosive environments due to climate change. In particular, the increased inundation from heavy rain and the use of salt for snow removal accelerate the corrosion of the automobile steel sheet, which leads to the deterioration of the durability and the collision safety of the vehicle [1]. Furthermore, with the recent rapid development of industry, these corrosive environments are becoming more and more severe. Therefore, it is essential to evaluate the corrosion life of the steel for predicting the durability of automobile parts. Currently, corrosion life is most often evaluated by the salt spray test (SST) and cyclic corrosion test (CCT). The accelerated CCT method for simulating an actual environment is used by many automakers. When an automotive carbon steel (ACS) sheet is evaluated through a CCT, atmospheric corrosion occurs on the test specimen and is greatly affected by environmental factors such as the type of material, humidity, time of wetness (TOW), and temperature [2]. For example, in the case of Cu and Ag, the corrosion rate is most affected by sulfides such as H2S and SO2 in the air, whereas in the case of Fe, acid fumes and fine dust are known to be more important. Furthermore, in coastal cities, the corrosion rate changes depending on the chloride concentration in the air. When the salt particles in the air are adsorbed on the metal surface, water may be condensed on the surface even if the relative humidity is low, which facilitates the formation of a water film [3]. As the TOW lengthens, the corrosion rate increases.

**Citation:** Cho, S.-w.; Ko, S.-J.; Yoo, J.-S.; Yoo, Y.-H.; Song, Y.-K.; Kim, J.-G. Effect of Cr on Aqueous and Atmospheric Corrosion of Automotive Carbon Steel. *Materials* **2021**, *14*, 2444. https://doi.org/ 10.3390/ma14092444

Academic Editor: Marián Palcut

Received: 6 April 2021 Accepted: 5 May 2021 Published: 8 May 2021

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Generally, weathering steel is widely used to prevent atmospheric corrosion and contains alloying elements such as Cr, Cu, and Ni. Particularly, Cr is known as an element that improves the corrosion resistance of low alloy steel in various corrosive environments. When the weathering steel is exposed to a corrosive environment, Fe oxides (porous rust layer) are initially formed under attack from oxygen, similarly to common steel. However, over time, a dense rust layer (protective rust) is formed on the steel surface. This rust layer protects the steel surface, inhibiting corrosion and reducing corrosion rates compared to common steel. In common steel, the oxide rust layer penetrates into the substrate, and corrosion of the substrate continues, while in weathering steel, the amorphous layer enriched with Cu, Cr, and Ni inhibits further corrosion progress. Weathering steel forms a thin rust layer with FeOOH as the main component on the steel surface in the early stages. Then, a Cr-enriched layer (Cr-goethite) with very small particles containing Cr is formed on the steel surface [4,5]. The Cr-goethite layer has cation selectivity, preventing the penetration of corrosion substances such as SO4 <sup>2</sup><sup>−</sup> and Cl<sup>−</sup> from the outside [6–8]. However, unlike these positive effects, negative effects have also been reported. According to Park et al. [9], in the flue gas desulfurization environment, Cr induces localized corrosion when Cr and Cu are added together in low-carbon steel because Cr segregates into the grain boundary, forming a Cr depletion region.

As described above, many research endeavors have been undertaken with respect to Cr's effect on the corrosion of metallic materials; however, only a few studies on the effect of Cr on the corrosion of ACS have been conducted. In this study, the effect of the Cr alloying element on the aqueous corrosion and atmospheric corrosion of ACS was investigated. The aqueous corrosion properties were analyzed using electrochemical measurements in a chloride (Cl−)-containing solution, and atmospheric corrosion properties were analyzed via a CCT.

### **2. Materials and Methods**

The specimens used in the electrochemical test and CCT were ACS containing 0, 0.3, and 0.5 wt.% Cr (produced by POSCO, Gwangyang, Korea), as described in Table 1. The microstructure images of the specimens are shown in Figure 1. In Figure 1, all the steels are composed of ferrite and martensite phases. Most martensite was formed along the grain boundary, with a small amount present inside the ferrite matrix. There is no noticeable difference in the three steels except for the grain size. The higher the Cr content, the bigger the grain size.


**Table 1.** Chemical compositions of the specimens (unit: wt.%).

**Figure 1.** SEM images of the microstructure of (**a**) 0 Cr, (**b**) 0.3 Cr, and (**c**) 0.5 Cr, etched with 2% nital solution.

The specimens were cut to a size of 1.5 cm × 1.5 cm, polished with 600-grit SiC paper, and cleaned with distilled water. Additionally, the solution for electrochemical measurements was 3.5 wt.% NaCl. All of the electrochemical measurements were conducted in a 3-electrode electrochemical cell. The test specimen was used as the working electrode, a carbon rod was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. Potentiodynamic polarization tests were performed with a potential sweep of 0.166 mV/s according to ASTM G5. To establish a stable potential, the scan was initiated after the specimen was stabilized in the solution [10]. Electrochemical impedance spectroscopy (EIS) tests were performed with an amplitude of 10 mV in the frequency range of 100 kHz to 10 mHz. Electrochemical tests were conducted by a potentiostat (BioLogics, VMP-2, Seyssinet-Pariset, France). After the CCT, the test specimens were mounted with epoxy and analyzed by an optical microscope (OM), and the components of the corrosion product were analyzed by an electron probe micro-analyzer (EPMA; JEOL, JXA-8530F, Fukuoka, Japan), X-ray diffraction (XRD; Rigaku, D/max-2500V/PC, Tokyo, Japan), and transmission electron microscopy (TEM; FEI, Tecnai F20 G2, Hillsboro, OR, USA).

The specific CCT process is shown in Figure 2. The specimens used for the CCT were cut to a size of 3 cm × 7 cm, and exposed on only one side to the corrosive environment. The CCT was performed for 10, 20, and 30 cycles, respectively. The salt solution used for the CCT was 5 wt.% NaCl. The length of a CCT cycle was 24 h, consisting of a wet stage for 21 h and a dry stage at 30% of relative humidity and 50 ◦C for 3 h.

**Figure 2.** Specific conditions for the cyclic corrosion test.
