**1. Introduction**

Electrical steels, also referred to as silicon steels (i.e., Si is the major additive element), are used as soft magnetic materials for construction of stators and rotors due to their magnetic properties and low cost [1,2]. The magnetic properties of the electrical steel are influenced by different parameters, such as sheet thickness, chemical composition and microstructure, and particularly by the grain size. Previous studies reported that large grain sizes are desired to improve the soft magnetic properties of the electrical steel [3,4]. Lee et al. [4] analyzed the magnetic properties of electrical steel as function of the grain size and found that samples with finer grains exhibited approximately 15% higher core loss W with little effect on the magnetic flux density B, compared to samples with larger grains. It should be noted that electrical steels with a low core loss and high magnetic flux density are preferable for electrical machinery cores from the magnetic properties point of view [4].

Stators and rotors are composed of hundreds of thin electrical steel sheets and used in the cores of electromagnetic devices [2]. Although this equipment is designed to avoid the

**Citation:** Palumbo, G.; Dunikowski, D.; Wirecka, R.; Mazur, T.; Lelek-Borkowska, U.; Wawer, K.; Bana´s, J. Effect of Grain Size on the Corrosion Behavior of Fe-3wt.%Si-1wt.%Al Electrical Steels in Pure Water Saturated with CO2. *Materials* **2021**, *14*, 5084. https:// doi.org/10.3390/ma14175084

Academic Editor: Marián Palcut

Received: 27 July 2021 Accepted: 1 September 2021 Published: 5 September 2021

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introduction of any liquids, they usually operate in severe working environments, such as high pressure, high temperature, and presence of aggressive gases (e.g., CO2, H2S), which can easily compromise their mechanical integrity. With time, the steam condenses into droplets of liquid inside this equipment. The CO2 corrosion in the oil and gas industry is one of the greatest challenges [5,6]. The gaseous CO2 dissolves in the condensed water, forming carbonic acid, which successively dissociates into bicarbonate and carbonate anions [5,6]. The combination of liquid water and CO2 creates aggressive conditions, which may lead to severe corrosion attack, leading to their performance degradation and hence, compromising the functionality of the plant over time. The grain size plays an important role in the design of electrical steel. From the magnetic properties point of view, large grains are more benefical. Grain size has also a strong effect on the mechanical and corrosion properties of the steel [7–11]. The relationship between the grain size and mechanical properties of the steel is well defined by the Hall-Petch relationship. However, the correlation between the grain size and its corrosion behavior is still an open field for investigation. Onyeji et al. [8] studied the corrosion behavior of two X65 steels with the same chemical composition but different grain sizes in aerated and deaerated brine solutions. The authors reported that the steel with coarser grains showed a higher corrosion resistance in both solutions. Li et al. [12] observed that the corrosion resistance of nanocrystallized lowcarbon steels in 0.05 M H2SO4 + 0.05 M Na2SO4 aqueous solution increased with decreasing the grain size. Palumbo et al. [13] observed that an increase in grain refinement leads to an increase in the volume fraction of intercrystalline areas such as grain boundaries and triple junctions. Many authors argued that the grain boundaries and triple junctions have higher energies compared to the bulk and, as such, are more chemically active with respect to the adjacent matrix [7,8,11,12,14–17]. Therefore, the grain refinement enhances the reactivity of the surface, which may cause a preferential dissolution of the grains [7,8,11,12,14–16,18]. However, it is worth mentioning that there is not an unanimous consensus regarding the effect of the grain size on the corrosion resistance of ferrous alloys. Some studies showed that the environment plays a crucial role. Wang et al. [7] reported that the grain refinement decreased the corrosion resistance of the low alloy steel in a 3.5 wt.% NaCl solution, but the same steel showed an improvement in corrosion resistance in a 0.1 M NaHCO3 solution. Similar behavior was observed by Zeiger et al. [16]. The authors found that the grain refinement led to a decrease in the corrosion resistance of the steel in a Na2SO4 solution with pH = 1, but the corrosion resistance increased in a Na2SO4 solution with pH = 6. The little consensus reported in the literature is related to the difficulty of isolating the effect of the grain size from other microstructural changes introduced during the grain refinement processes such as, for example, rolling or plastic deformation. Consequently, a case-by-case study is needed to understand the corrosion effect of the grain size of a given metal in a given environment.

The objective of this work is to study the corrosion behavior of two electrical steel sheets with similar chemical composition, but different grain sizes in pure water saturated with CO2. To this end, the study was carried out using weight loss and electrochemical measurements. The scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDS) and x-ray photoelectron spectroscopy (XPS) analyses were employed to characterize the corrosion product layer and to support the gravimetric and electrochemical results. Moreover, to highlight the micro-galvanic activities occurring at the grain boundaries and triple junctions on the metal surface, Volta potential measurements were performed.

### **2. Experimental Procedures**
