**1. Introduction**

Fe-based alloys are widely used as structural materials [1] because of their excellent tensile strength and high toughness [2]. However, the corrosion of metal in the working environment will reduce the mechanical properties of metal and shorten its service life. In the corrosion process, the metal is electrochemically oxidized into ions or some compounds [3]. In particular, the presence of grain boundaries can cause intergranular corrosion. The dissolution rate of Fe atoms in the grain boundary region is much greater than the dissolution rate of crystal grains, leading to local corrosion [4]. Intergranular corrosion refers to the boundaries of crystallites of the material being more susceptible to corrosion than their insides, which is mainly caused by the difference in chemical composition between the surface and the interior of the grains and the existence of internal stress.

The surface structure of the Fe-based alloys is complex with defects such as grain boundaries, corners, edges, boundaries, interference layers, etc. [5,6]. Corrosion usually occurs at the defect first, and the corrosion at the defect is more severe than inside the crystal. It can be observed in the experiments that the corrosion is more severe at the grain boundary compared with the inside of the crystal [7]. You et al. found that the corrosion in pure Fe is initiated by pits that quickly become blocked and that it propagates around the pits giving rise to the formation of porous layers [8]. The grain refinement obtained by rolling improved the corrosion resistance of iron in sulfuric acid solution, borate buffer solution, and borate buffer solution with chloride ion [9]. Bennett et al. studied the influence of orientation angle and grain boundary structure on grain boundary corrosion of austenitic and ferritic stainless steels. They pointed out that the grain boundary corrosion mechanism is not only the dissolution of the chromium-depleted alloy but also other mechanisms, but the specific mechanism is not distinct [10]. According to Lapeire's study, grain orientation

**Citation:** Xiao, Z.; Huang, Y.; Liu, Z.; Hu, W.; Wang, Q.; Hu, C. The Role of Grain Boundaries in the Corrosion Process of Fe Surface: Insights from ReaxFF Molecular Dynamic Simulations. *Metals* **2022**, *12*, 876. https://doi.org/10.3390/ met12050876

Academic Editor: Amir Mostafaei

Received: 6 April 2022 Accepted: 17 May 2022 Published: 21 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

<sup>1</sup> College of Materials Science and Engineering, Hunan University, Changsha 410082, China; xiaozigeng@hnu.edu.cn (Z.X.); wyuhu@hnu.edu.cn (W.H.)

can also affect corrosion behavior, and the orientation of adjacent grains plays a dominant role in the dissolution rate [11]. The reduction of the electronic work function at the grain boundary indicates the electrons at the grain boundary are more active, which makes the grain boundary vulnerable to electrochemical attack [12]. Emily et al. believed that the corrosion at the grain boundary is caused by the combined effect of sensitization and crevice corrosion. The precipitation of chromium and the grain boundary has set the local corrosion sensitivity [13].

Up to now, the density functional theory calculations (DFT) and reaction force field (ReaxFF) molecular dynamics (MD) simulations have been usually used to study Fe and Fe-water surface corrosion mechanisms. Hu et al. studied the effects of four typical surface adsorbates [14]. Their works suggested that the strong interaction between oxygen and surface will weaken the bonds' strength between substrate atoms, which will lead to structure deformation and charge redistribution in the substrate. In addition, Bucun et al. studied the effects of oxygen precovering and water adsorption on the surface of Fe substrates [15]. The time and space scales of the above simulations are seriously limited, which can only describe the trend and cannot carry out the complete dynamic evolution of the corrosion process. The ReaxFF-MD provides an alternative method to simulate the interface reaction and can explain the effect of defects (such as grain boundaries) on corrosion from stress changes and energy changes [16,17]. Verners et al. [18] studied nickel-based alloys' stress corrosion cracking behavior. The existence of stress will hinder the twin dislocation failure and activate the new slip to reduce the strength of nickel. DorMohammadi et al. used ReaxFF-MD to study the initial stage of Fe corrosion in pure water under different applied electric fields and temperatures and then studied the passivation mechanism of Fe substrate and the depassivation process of chloride [19–21]. The study of Klu et al. showed that the existence of polycrystalline grain boundaries could significantly increase the diffusion coefficient of carbon, and the release of stress can reduce the diffusion barrier of carbon [22].

In summary, defects in pure iron will preferentially corrode during the corrosion process. Whether the grain boundary as a typical defect will affect the surface corrosion process of pure iron. The computational methods involving simulations of dynamic processes are an important tool for understanding the mechanism of dynamic processes in reactive systems, such as corrosion. The present article, using a reactive force field (ReaxFF) molecular dynamics (MD) simulation, highlights the effect of grain boundaries on the evolution of the corrosion process in a reactive pure Fe-H2O system.
