**3. Results and Discussion**

#### *3.1. Metallographic Measurement*

Figure 1 shows the microstructures and the grain size distribution of the tested steels, with the microstructure parameters summarized in Table 2. It is clear from the data that both samples show a similar chemical composition, with very little variation between different elements, but different microstructures. The 300 steel shows a coarse-grained microstructure with an average grain size of *circa* 4265.9 μm2, whereas the 200 steel shows a fine-grained microstructure with an average grain size of *circa* 115.6 μm2.

**Figure 1.** Microstructure and the histogram showing the grain size number distribution of 200 (**a**–**c**) and 300 steels (**d**–**f**).


**Table 2.** Grain size analysis of the steels.

#### *3.2. Gravimetric Measurements*

Table 3 shows the corrosion rates of the samples obtained from the weight loss measurements after 24 h of immersion in the tested solution. It follows from the data that the *CR* of the sample with fine grains is approximately two times higher compared to the sample with coarse grains. The analysis of the chemical composition (Table 1) shows that both specimens have a similar composition, with very little variation in concentration of the alloying elements. It can be inferred that the small differences observed in microalloying do not affect the corrosion rate of the metal and that the difference in the corrosion rate is therefore related to the microstructure. Similar results were also reported in [8,11,12,14–16,18]. Onyeji et al. [8] reported that the corrosion behavior of two X65 steels with identical chemical composition but different microstructures varied. The authors observed that the coarse-grained steel showed higher corrosion resistance compared to the steel with a fine grained microstructure. The authors suggested that this behavior was attributed to the enhanced reactivity of the surface after the grain refinement, which causes a preferential dissolution of the grains.

**Table 3.** Weight loss and corrosion rate of the steels after 24 h of immersion time.

