*4.2. Characterization of Corrosion at the Grain Boundary*

The dynamic evolution of the chemical composition of the Fe/water interface system is shown in Figure 9a. In the early 5 ps of corrosion, due to the adsorption and dissociation of many water molecules, a large number of byproducts such as H3O+, H2, OH−, and H atoms are generated, which means that the early corrosion rate is high. After 10ps, the system enters the slow-corrosion state. After the dissociation of water molecules, O ions diffuse into the Fe substrate, and H ions enter the solution to form H3O+. Therefore, as the corrosion progresses, the amount of H3O+ gradually increases. Figure 9b shows the changes in the number of water molecules over the simulation time. To avoid interference, we only count the changes of water molecules above the grain boundaries. The changing trend of water molecules in all models is the same. The significant reduction of water molecules in the first 50 ps means a faster corrosion rate. After the initial passivation, the number of water molecules slowly decreases. As the angle of the grain boundary increases, the number of water molecules decreases. It also shows that the existence of grain boundaries will affect the adsorption and dissociation of water molecules and the subsequent penetration of oxygen atoms.

The atomic charge distribution at the end of the pure Fe-H2O system simulation is depicted in Figure 10. The charge of the Fe atoms at the bottom of the metal substrate has fluctuated around 0 e. The charges of O and H atoms in the solution fluctuate slightly around −0.775 e and 0.320 e, respectively. These atomic charges are consistent with the previous simulation articles [19]. Charge exchange mainly occurs in the electric double layer, where the Fe atom loses electrons, and the charge of the surface Fe atoms rises to nearly 0.7 e. In addition, the charge of oxygen atoms diffused into the Fe substrate decreases to 0.3 e. When hydrogen atoms enter the Fe substrate as interstitial atoms or combine with Fe atoms in the solution, the hydrogen atoms possess a specific negative charge of about 0.2 e. In contrast, the hydrogen atoms in the OH− ions still carry a positive charge. The related research reveals that the H atoms spread into the Fe substrate possess a negative charge [32,33]. Therefore, during the corrosion process, there is a process of redistribution of electric charge on the pure Fe-H2O system.

**Figure 9.** (**a**) The change of ions numbers of polycrystalline during the simulation time and (**b**) the evolution of water molecules in the simulation time.

**Figure 10.** Charge distributions of the polycrystal pure Fe-H2O system at 500 ps.

The radial distribution functions (RDFs) can study the different Fe oxide phases in simulation can be studied by the radial distribution functions (RDFs). Generally, the partial radial distribution function (RDF), g (r), defined as the probability of an atom at a distance from the origin, which is used to characterize the bonding and the structure of the formed oxide film [34,35]. Figure 11 shows the RDFs for the O-H bond and the Fe-O bond at the end of the simulation. The RDFs were calculated every 0.4 ps and took an average of 450 to 500 ps. It has two prominent peaks for the Fe-O bond length. The first peaks at 1.78 Å correspond to the interaction of Fe ions and OH- ions, but this peak spans from 1.5 Å to 2.5 Å covering the peaks of FeO, Fe2O3, and Fe3O4. These results show that the oxide film formed on the Fe surface mainly consists of Fe hydroxides and oxides in the corrosion process. The transformation trend is the transformation of Fe hydroxide to Fe oxide.

**Figure 11.** Partial radial distribution functions for the oxide film of polycrystal model at 500 ps.
