## 3 Results  
Figure 1 shows the voltage and the current behavior as a function of treatment time.  
Figure 2 illustrates different stages of oxide layer formation that corresponds to different stages on the voltage-time plot presented in Figure 1a. Figure 2a corresponds to Region 1; no discharges were revealed. Figure 2b corresponds to Region 2; microarray discharges start to appear. Figure 2c corresponds to Regions 3-4; the number of microarc discharges increases. Pits visible on the image are an accumulation of small bubbles that appear on the surface during PEO treatment. Black squares in the center of the specimens are the camera focusing locations, e.g., target.  
Figure 1: Electric parameters behavior plots (**a**) voltage as a function of treatment time; (**b**) current as a function of treatment time.  
Figure 2: Different stages of the oxide layer formation: (**a**) anodizing process; (**b**) microarc appearance (presented in a white frame); (**c**) plasma electrolytic oxidation (PEO) treatment and increasing of microarc number discharges (presented in a white frame).  
Surface morphology of the sample subjected to PEO was investigated as well and the obtained images by SEM are presented in Figure 3.  
It is evident from the presented micrographs that the surface morphology of the treated alloy contains round-shaped pores with a diameter in the range of 0.5-2.5 \(\upmu\)m. This is well correlated with treatment obtained in aqueous solution and later compared in discussions of the current work.  
The coating morphology of the PEO-treated specimen was investigated by SEM, and its elemental analysis, performed by EDS, is presented in Figure 4.  
Figure 3: Surface morphology of the Al 1050 alloy after PEO treatment, magnifications: (**a**) 250\(\times\); (**b**) 1000\(\times\); (**c**) 5000\(\times\); (**d**) 10,000\(\times\).  
EDS was performed for qualitative analysis; however, quantification was performed as well and consequently followed by XRD investigation to evaluate coating phase composition. According to EDS investigations, the atomic composition of aluminum and oxygen are 36.7% and 63.3%, respectively. EDS results also indicate that no additional compounds were incorporated into the coating, as it confirmed is usually present after treatment in aqueous solutions originating from the electrolyte.  
The obtained semi-quantitative calculations by XRD show phase composition distribution of the coating. Results in Figure 5 indicate the presence of 55% of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) and 45% of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\).  
Figure 4: Surface area morphology investigated by SEM and elemental composition investigated by energy dispersive X-ray spectroscopy (EDS) after PEO treatment.  
Figure 5: X-ray diffraction pattern of aluminum 1050 alloy surface after PEO treatment.  
Micro-hardness measurements were performed on the oxide coating, and the obtained results are illustrated in Table 2. Different values of the micro-hardness obtained due to the different layers' compositions depend on the depth.  
Figure 6 demonstrates a cross-section micrograph of the obtained coating after PEO treatment and its EDS line scan elemental analysis.  
It is evident from the micrograph that the thickness of the porous outer layer is about 4 \(\upmu\)m and the thickness of the inner dense layer is about 3.5 \(\upmu\)m. According to the linear dependence of the oxygen (O) and aluminum (Al) scans, the oxide layer of the Al\({}_{2}\)O\({}_{3}\) composition is indicated as previously confirmed by XRD analysis.