*3.1. Fracture Cross-Sectional Morphology and Chemical Composition of the PEO Coating*

Free-standing coatings were obtained by electrochemically dissolving the aluminum substrate in the NaCl solution. Figure 1 shows the SEM images of the fracture cross-section of the free-standing PEO coatings and the corresponding EDS results at different treatment times. Figure 1a–d shows that all aluminum/coating interfaces had a wavy-jagged appearance, which may be a result of discontinuous oxidation of the aluminum substrate. A dense barrier layer with a relatively constant thickness of ~1 μm existed near the aluminum/coating interface.

In the initial stage (Figure 1a), the coating was compact with a thickness of ~1.2 μm. At 15 min (Figure 1b), the fracture cross-section displayed a clear longitudinal profile of the strip pores, which was thought to be the residue of the discharge channels in the PEO process. As shown in Figure 1c,d, the PEO coating with a three-layer structure was clearly revealed at 45–60 min, and large cavities were present in the coating. Discontinuous nodules were distributed over the outer surface. The internal structure was filled with a large number of micropores, and some cracks appeared to traverse the entire outer-layer thickness.

EDS analysis was conducted to achieve a better understanding of the cross-sectional structure. As shown in Figure 1e,f, the EDS point analysis (Point 4) revealed that the nodules were rich in Si. A small amount of Si was also detected at the edge of the cavity (Point 2). Very little P was detected at Point 2 (Figure 1d), since the oxides containing P were hard to deposit in the coatings [26]. It is noted that the PEO coatings contain a certain amount of Si and P in addition to aluminum, and these elements may also combine with oxygen, causing a higher O/Al ratio than expected. Electrolyte evaporation, condensation, decomposition, and deposition were caused by the heat of the plasma discharges, which resulted in the incorporation of the electrolyte composition into the PEO coating.

Figure 1g gives the typical EDS mappings corresponding to the cross section of the PEO coating (Figure 1d). It can be seen that Si was higher in nodules at the surface, suggesting that the silicate in the electrolyte was prone to be deposited to form Si-rich nodules at the surface of PEO coating. However, the P mostly distributed around the cavity, showing that the electrolyte had been penetrated into the cavity during the PEO process. The higher level of P in the inner coating might be related to the short-circuit transport of electrolyte components through the outer coating, which would be left inside the coating [27].

**Figure 1.** (**a**–**d**) SEM images of fracture cross section of PEO coatings formed at (**a**) 5 min, (**b**) 15 min, (**c**) 45 min, and (**d**) 60 min. (**e**,**f**) The corresponding EDS results to the coating of (**c**) and (**d**). (**g**) Typical EDS mappings of PEO coatings marked in Figure 1d.
