## 3 Results and Discussion
### Corrosion Susceptibility Assessment Tests  
#### 3.3.1 Electrochemical Impedance Spectroscopy (EIS)  
To evaluate the protective nature of the NiCrAlY layers deposited by the EB-PVD method, the electrochemical impedance spectroscopy (EIS) method was applied, which is a method that does not accelerate reactions at a metal/solution interface [46; 47; 48; 49]. Figure 8a,b shows the Nyquist (a) and Bode diagrams (b) obtained for the 310H stainless steel samples coated with a layer of NiCrAlY and oxidized for different periods of time in water at 550 \({}^{\circ}\)C and 25 MPa. A qualitative characterization of the oxides formed on the sample surfaces after oxidation in an aqueous medium at high temperatures can be made using these diagrams. Following the Nyquist diagrams shown in Figure 8a, it can be observed that for both the uncoated sample and the sample coated with NiCrAlY, a single capacitive semicircle was obtained. In the case of NiCrAlY-coated samples oxidized for different periods of time, two capacitive semicircles were obtained. These can be associated with two created interfaces. The semicircle recorded at medium and low frequencies corresponds to the NiCrAlY-deposited layer and the semicircle recorded at high frequencies correspond to the oxide layer formed on the sample surface following the oxidation tests. Furthermore, these diagrams show an increase in diameter semicircles with the oxidation time increase. This determines a higher polarization resistance value; thus, there is a  
Figure 8: Nyquist diagrams (**a**) and Bode diagrams (**b**) were obtained for uncoated and NiCrAlY-coated 310H stainless steel samples before and after oxidation for different periods in water at 550 \({}^{\circ}\)C and 25 MPa. Figure 7: Schematic diagram of the oxidation process of NiCrAlY coatings. smaller corrosion rate for coated samples that were oxidized for 2160 h, indicating a better corrosion resistance of these samples compared to the other samples. The Bode diagrams (Figure 8b) make the existence of two maximum values of phase angles evident for each analyzed sample, one at medium and low frequencies and another at high frequencies, corresponding to the two semicircles on the Nyquist diagram (Figure 8a). Corrosion processes at the substrate-coating interface are represented by impedances recorded at medium and low frequencies, while corrosion processes at the oxide-solution interface are represented by impedances measured at high frequencies [50; 51; 52; 53; 54]. To obtain the quantitative data, all experimental EIS data were fitted with the ZView 2.90 software. The equivalent electrical circuit (EEC) is represented in Figure 9. The equivalent electrical circuit model is similar to the one used by our group for the study of the uncoated substrate [39], or those that were coated with CrxN and oxidized under similar conditions [22]. The values of the elements of the equivalent circuits determined for all tested samples are presented in Table 5.  
Table 5 shows that in the case of all tested samples, for the capacity of the oxide layer, the capacity of the deposited layer, a capacitive element, could not be used; thus, a distributive element consisting of the constant phase element CPE-T and the CPE-P parameter was used instead. The coating resistance (R\({}_{\text{coat}}\)) shows a progressive increase with the following oxidation times: 11,455 \(\Omega\)\(\times\) cm\({}^{2}\) at 720 h, 2.54 \(\times\) 10\({}^{6}\)\(\Omega\)\(\times\) cm\({}^{2}\) at 1440 h, and 3.1 \(\times\) 10\({}^{6}\)\(\Omega\)\(\times\) cm\({}^{2}\) at 2160 h. This trend indicates a strengthening of the oxide layer's protective properties as the exposure time increases.