*3.3. Internal Structure of PEO Coating*

The internal structure of PEO coating was observed by exfoliating the outer layer using a single scratch tester. Figure 5 shows the plan images of the internal structure during different PEO stages. It can be seen in Figure 5 that the cohesion of the PEO coatings was poor and that spalling occurred on the edge of the scratch. The internal structure of the coatings was obvious different from the outer surface. Nodules and pancake-like structures were absent inside the coating. For the coating formed at 5 min (Figure 5a,e), the exposed internal structure was smooth. After 15 min, all the internal structure showed obvious cracks and submicron pores with the different sizes and shapes.

**Figure 5.** (**a**–**d**) SEM images (back-scattered electron mode) and (**e**–**h**) magnified SEM images of the internal structure of PEO coatings formed at (**a**,**e**) 5 min, (**b**,**f**) 15 min, (**c**,**g**) 45 min, and (**d**,**h**) 60 min.
