*4.1. The Principle of Grain Boundary Corrosion*

Figure 6 shows the variation of shear stress in the Y-direction of the iron substrate; the vertical axis is the difference between the initial stress and the instantaneous stress. During the corrosion process, the stress at the polycrystalline grain boundary was reduced by 2.0 GPa within 200 ps, the stress at the sigma5 twin boundary was reduced by 1.25 GPa, and the sigma3 twin boundary was only reduced by 0.75 GPa, but the stress of the single crystal remained unchanged. The changing trend of stress corresponds to the corrosion phenomenon of the model. Therefore, we believe that the enormous stress relief in the highstress region at the grain boundary will deform the grain boundary structure and cause the atoms at the grain boundary to dissolve, leaving many vacancies on the surface. The existence of vacancies accelerates the diffusion of O ions, thereby promoting the corrosion of grain boundaries.

**Figure 6.** Changes in shear stress of polycrystalline and twin models over simulation time.

The effects of different grain boundaries on the corrosion process were compared by counting the number of dissolved Fe atoms near the grain boundaries and the number of O ions diffused into the Fe substrate. According to Figure 7a, the number of dissolved Fe atoms in polycrystalline and sigma5 twins is the largest due to the early stress release. Within 50 ps, about 250 Fe atoms are dissolved from polycrystalline, and about 100 Fe atoms are dissolved from sigma5 twins. The number of dissolved iron atoms in sigma3 twins and single crystals is roughly the same. Figure 7b shows the number of O ions penetrating into the vicinity of the grain boundaries of the iron substrate. It can be seen that a large number of iron atoms at the grain boundaries of the polycrystalline model are dissolved, and most vacancies are formed on the surface, which causes O ions to penetrate into the substrate quickly. Therefore, the polycrystalline model has the most oxygen atoms penetrating into the substrate. Approximately 100 O ions in sigma5 twins penetrate into the substrate. The number of oxygen atoms in sigma3 twins is 20 more than that in single crystals, but the number of dissolved Fe atoms is the same.

Corrosion at grain boundaries is always accompanied by the massive dissolution of Fe atoms at grain boundaries. Therefore, we simulated the effect of the concentration of vacancies in the Fe substrate on the penetration of O ions in corrosion. Randomly add vacancies in the Fe substrate, and the vacancy concentration is 0%, 5%, and 10%. The result is shown in Figure 8. The concentration of vacancies affects the penetration and depth of O ions penetrate. When the vacancy concentration is 5%, the penetration depth of O ions only increases by 0.25 Å, but when the vacancy concentration rises to 10%, the penetration depth of O ions increases by 1.5 Å. We can conclude that the concentration of vacancies in the Fe substrate reaches a certain level, and O ions can quickly penetrate into the bulk. In intergranular corrosion, the excessive initial stress of the grain boundary causes a large number of Fe atoms at the grain boundary to dissolve to form vacancies, which makes it easier for O ions to penetrate into the substrate to form oxides in accelerated local corrosion.

**Figure 7.** (**a**) The number of Fe atoms dissolved into H2O over time. (**b**) The number of O atoms diffused into the substrate over time.

**Figure 8.** The effect of vacancy concentration on the diffusion of oxygen atoms. The curve represents the average position of oxygen atom diffusion. Simulate by randomly adding vacancies to the Fe substrate proportionally.
