## 4 Discussion  
When a positive voltage is applied to a sample immersed in the electrolyte, redistribution of the charge begins. Negatively charged ions of electrolyte migrate to the metal's surface by the influence of the external electric field, resulting in the formation of the adsorbed ions on the surface of the anode and leading to the distribution of the voltage drop across the electrolytic cell. As shown in Figure 2a,b, a typical anodizing process proceeds up to 30 V and causes the formation of a thin oxide layer that prevents further chemical reaction. At the same time, the negatively charged ions accumulate on the anode surface, forming a so-called quasi-cathode [19; 20]. These ions simultaneously redistribute the voltage between electrodes in the electrolytic bath, leading to an increase in the electric field intensity between the anode and local quasi-cathode until a sufficient value for the ion diffusion process is reached through the barrier layer. This oxide layer affects the current magnitude, which decreases due to the added resistance of the formed layer. While the coating thickness increases, the electrical resistance grows alongside the current reduction simultaneously, leading to the redistribution of potential between the electrodes. The increase in the voltage between the anode and quasi-cathode provides further appearance of the electrochemical process leads to the anodic growth of porous oxide layer by the mean of microarc discharge \begin{table}
\begin{tabular}{c c c} \hline \hline
**Base Aluminum [HV\({}_{10}\)]** & **Inner Layer [HV\({}_{10}\)]** & **Outer Layer [HV\({}_{10}\)]** \\ \hline
42 \(\pm\) 2.2 & 756 \(\pm\) 3.1 & 1051 \(\pm\) 2.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Micro-hardness test results of different alloy coating depth related to Figure 6: Base metal: point 4; inner layer: point 3; outer layer: point 2. Figure 6: EDS line scan of the aluminum 1050 alloy cross section after PEO treatment and its microphotograph: 1: resin; 2: outer layer; 3: inner layer; 4: aluminum base. channels [21] as shown in Figure 2b,c. Finally, a high voltage leads to the heating and electrical breakdown of the barrier layer, resulting in a microarc discharge. Evaluation of morphological changes showed that formed alloy subjected to PEO treatment has a round-shaped porosity with a diameter in the range of 0.5-2.5 \(\upmu\)m. The morphology and the size distribution of pores are well correlated with the work by [PERSON] et al. in aqueous solution electrolyte [22]. Micrographs shown in Figure 6 illustrate the porous structure of the outer layer, while the inner layer has no porosity at all. SEM images in Figures 4 and 6 show the morphology of the obtained coating and are well correlated with the morphology usually obtained in the PEO process performed in aqueous solution electrolyte [23]. No cracks were found in the formed coating. We suggest that the low cooling rate is responsible for this. However, the coating produced in aqueous solution is usually formed from the oxide layer with cracks, which is attributed to the high cooling rate of the coating formation [24]. In addition, [PERSON] et al. described a temperature profile of the process in their work and found that the plasma temperature in discharges is around 10.000 K [25]. EDS analysis, line scan, and XRD investigations indicated that the coating is free of any contamination, which is, in contrast to the coating obtained in aqueous solution, where contamination originates from the electrolyte [26]. Typically, electrolytes for aluminum alloys in PEO treatment are made of alkali metal hydroxides and sodium silicate. Therefore, a formed crystalline aluminum oxide coating usually contains impurities of silicon in a form of oxide. XRD results demonstrate the presence of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) and \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) phases in the obtained coating.