*The Corrosion Process at the Grain Boundary*

In order to study the corrosion process of a perfect crystal, we simulate the pure Fe-H2O interfacial corrosion of Fe (110) single crystal for that the (110) surface is the most stable close-packed one. The corrosion processes of the single crystal are shown in Figure 2. It can be found that water molecules are adsorbed on the surface of the Fe substrate at 0.14 ps (Figure 2a); then the hydrogen-oxygen bond of the water molecule begins to stretch (Figure 2b); and finally, the water molecules decomposed completely into OH− and free H+ ions (Figure 2c). The chemical reaction during the corrosion process is shown below:

**Figure 2.** (**a**) The adsorption of water molecules on the Fe substrate; (**b**) the elongation of the O-H bond in the adsorbed water molecules; and (**c**) the breaking of the O-H bond and the formation of OH−.

Step 1: The water molecules adsorbed on the surface of the Fe substate dissociate to form OH<sup>−</sup> and H+:

$$\text{H}\_2\text{O} \rightarrow \text{OH}^- + \text{H}^+$$

OH− is adsorbed on the surface of the substrate to generate Fe(OH):

$$\text{Fe} + \text{OH}^- \rightarrow \text{FeOH} + \text{e}^-$$

The charge of the Fe atoms on the surface rises to 0.4 e, and the charge of the O atoms drops to −0.69 e.

Step 2: The Fe(OH) on the surface dissolves into the water:

$$\text{Fe} + \text{FeOH} + \text{OH}^- \rightarrow \text{FeOH} + \text{FeOH}^+ + 2\text{e}^-$$

The charge of the Fe atoms on the surface rises to 0.57 e Step 3: Fe is oxidized to form Fe2+:

$$\mathrm{FeOH}^+ + \mathrm{H}^+ \rightarrow \mathrm{Fe}^{2+} + \mathrm{H}\_2\mathrm{O}$$

Step 4: The adsorbed OH<sup>−</sup> also dissociate into O and H ions, forming additional H3O+ and iron oxides.

During the process of the adsorption of H2O on the surface of Fe substrate to form Fe(OH)2, the surface Fe atoms are oxidized, and the average charge increases to 0.7 e. The O atoms adsorbed on the surface are reduced and the average charge drops to −0.775 e, H ions also penetrate into the Fe substrate with an average charge of −0.25 e. Finally, the adsorbed OH<sup>−</sup> also dissociate into O and H ions, forming additional H3O+ and iron oxides.

In Fe single crystal, O ions penetration uniformly into the Fe substrate to form oxides (Figure 3a). The Fe (110) surface with a sigma3 twins is also studied, as shown in Figure 3b. It can be found that the corrosion process of the sample with sigma3 twins is similar to that of single crystals. After water molecules decompose on the Fe surface, Fe atoms will dissolve into the water solution, O ions will penetrate into the Fe substrate to form oxides, and H ions will diffuse into the Fe substrate. The existence of the twin grain boundary does not affect the corrosion rate of the Fe substrate. This situation is because the sigma3 grain boundaries are relatively stable and not easy to corrode.

**Figure 3.** The snapshots at different corrosion stages for the different pure Fe-H2O models in 300 K: (**a**) Fe single crystal (110); (**b**) Fe sigma3 (110) [111] twins; (**c**) Fe sigma5 (02-1) [012] twins; (**d**) polycrystals. The yellow, red, and white spheres represent Fe, oxygen, and hydrogen atoms.

However, the corrosion of sigma5 twins is different from that of sigma3 twins. The most stable (210) surface is used to test the effect of different grain boundaries on corrosion. As shown in Figure 3c, the penetration of O ions at the grain boundary is faster, and the corrosion depth at the grain boundary is more than four atomic layers. This phenomenon is related to the structure and stability of the grain boundary. As the angle of the grain boundary increases, the corrosion at the grain boundary will be easier.

To verify the above speculation, we simulate the corrosion of the polycrystalline grain boundary. According to Figure 3d, polycrystalline consists of three grains, and irregular grain boundaries are obtained by rotating the grain in the middle. The corrosion phenomenon of polycrystal is similar to sigma5 twins, but the phenomenon of grain boundary corrosion is more prominent. Many Fe atoms on the grain boundary are dissolved and O ions penetrate rapidly into the grain boundaries to form oxides. The number of dissolved Fe atoms in polycrystals is far greater than that of sigma5 twins, and the corrosion rate of grain boundaries is speedy. This may be related to the reduction of Fe work function (ionization energy) during H2O adsorption obtained by the multielectron intercalation theory [17] and may also be related to stress.

It is found that O ions penetrate faster at the polycrystalline and sigma5 twin grain boundaries, and local corrosion occurs at the grain boundaries, as shown in Figure 4. In the polycrystalline model, O ions first penetrate into the grain boundaries and then along the grain boundaries to the surroundings to form oxides. In the twinned system, a small number of O ions are distributed on the sigma5 twin grain boundary. However, it is impossible to judge whether it penetrates along the grain boundaries, and the O ions in the sigma3 twins exhibit uniform penetration. During the corrosion process, the stress changes at the grain boundaries, as shown in Figure 5. The stress at the polycrystalline grain boundary decreases in a wide range during the corrosion process, and the stress at the sigma5 twin is only reduced at the grain boundary, but the stress at the sigma3 twin remains unchanged. The variation range of stress is consistent with the penetration range of O ions.

**Figure 4.** Distribution of oxygen atoms in Fe substrate during corrosion. (**a**) Fe single crystal (110); (**b**) Fe sigma3 (110) [111] twins; (**c**) Fe sigma5 (02-1) [012] twins; (**d**) polycrystals.

**Figure 5.** The stress change of the Fe substrate during the corrosion process. (**a**,**b**) are the polycrystalline; (**c**,**d**) are the sigma5 twins; (**e**,**f**) are the sigma3 twins.

#### **4. Discussion**
