晶界在浸泡14天之前很明显（图11a-c），在28天时变得模糊。图11c1和d1显示了14d和28d样品的放大区域。可以看出，表面出现了一些微小的孔隙（用箭头标记）。这些孔隙可能源于制备过程中表面的缺陷，或者因为杂质充当阳极，从而导致这些微小区域的电偶腐蚀。总的来说，在整个浸泡过程中没有观察到严重的局部腐蚀坑。

The grain boundaries are obvious before 14 d immersion (Fig. 11a–c) and become obscure at 28 d. Fig. 11c1 and d1 show the enlarged areas of 14 d and 28 d samples. It can be seen that some tiny pores (marked by the arrows) show up on the surfaces. These pores may originate from the defects of the surfaces during the preparation, or because the impurities act as the anode and thus lead to the galvanic corrosion in these micro areas. Overall, no severe localized corrosion pits are observed throughout the immersion.