## 3 Results and Discussion  
### Oxidation Kinetics  
To carry out the gravimetric analysis, every 720 h of oxidation in water at a temperature of 550 \({}^{\circ}\)C and a pressure of 25 Mpa, three stainless steel samples coated with NiCrAlY were taken out of the autoclave, after which they were washed, dried, and weighed with an accuracy of 1 \(\times\) 10\({}^{-5}\) g, using the KERN analytical balance. Based on the mass variation, oxidation kinetics were also determined for all coated samples after testing them in water at high temperatures, as can be seen in Figure 2. The oxidation kinetics are represented by the mass variation as a function of the oxidation time. Gravimetric analysis reveals that all tested samples gained weight over time due to oxidation. The mass values determined by weighing in the case of the tested samples fell between 5.36065 mg \(\times\) dm\({}^{-2}\) and 7.14577 mg \(\times\) dm\({}^{-2}\). The oxidation kinetics (Figure 2) were also determined for all tested samples using the obtained mass gains values. The oxidation process is described as follows by the kinetic Equation (4):  
\[\frac{\Delta m}{s}=kt^{n} \tag{4}\]  
where  
\(\Delta\)m is the mass gain, [mg];  
s is the sample surface, [dm\({}^{2}\)];  
k is the rate constant of the oxidation reaction;  
t is the exposure time, [h];In our previous studies [39], the substrate was tested under the same conditions and for the same periods of time. The oxidation process was characterized by parabolic kinetics because n = 0.564. This fact indicates that oxidation is controlled by the diffusion of metal ions and oxygen through a generally protective oxide. Average values of oxide thicknesses (um) and oxidation rates (um \(\times\) year\({}^{-1}\)) calculated from mass measurements for 310H stainless steel samples coated with NiCrAlY and oxidized for 720 h, 1440 h, 2160 h, and 2880 h in water at 550 \({}^{\circ}\)C and 25 Mpa are graphically shown in Figure 3.  
Figure 3 shows that as the oxidation period increased, the oxide thicknesses increased, and the corrosion rates decreased. The lowest value of the corrosion rate of 0.27 um \(\times\) year\({}^{-1}\), obtained for a coated sample and oxidized for the longest period of time, shows a better corrosion resistance. It should be mentioned that compared to the data obtained by electron microscopy, where layer thicknesses are determined only on the areas visualized under the microscope (the samples may have a thicker or thinner layer in other areas), global layer thicknesses were calculated by gravimetric analysis. \begin{table}
\begin{tabular}{c c c c} \hline
**Kinetics Equation** & **k** & **n** & **R2** \\ \hline y = 1.3029 \(\times\) t\({}^{0.2154}\) & 1.3029 & 0.2154 & 0.9991 \\ \hline \end{tabular}
\end{table} 
Table 4: Kinetic parameters determined for NiCrAlY-coated 310H stainless steel samples after oxidation in water at supercritical temperature. Figure 2: Oxidation kinetics of NiCrAlY-coated 310H steel samples at high temperature. Morphological and Structural Characterization of the Oxides Formed Following Oxidation 3.2.1. Grazing Incidence X-Ray Diffraction (GIXRD)  
Figure 4 shows the X-ray diffraction spectra obtained for the 310H substrates coated with a layer of NiCrAIY before and after exposure for 720 h, 1440 h, and 2160 h in water at 550 \({}^{\circ}\)C and 25 Mpa. The spectra were obtained at the incidence angle of 5\({}^{0}\). The diffractograms presented in Figure 4 show the changes induced in the composition of the NiCrAIY layer following the sample oxidation in the environment simulating the conditions in an SCW reactor. Thus, in the figure above, on the surface of the coated and unoxidized samples, the NiCrAIY phase was identified as a deposited layer, while on the surface of all oxidized samples, chromium oxide (Cr\({}_{2}\)O\({}_{3}\)) and the Corundum phase (Al\({}_{2}\)O\({}_{3}\)) were identified. The diffraction spectrum of Cr\({}_{2}\)O\({}_{3}\) (PDF card no.