## 3 Experimental Results
Scanning electron microscopy, shown in Figure 6g, shows the pores in the ceramic layer and the formation of a cracked oxide layer as a result of surface corrosion on the ceramic material. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{4}{c}{**Experimental Parameters Recorded During the Potentiometry Process**} \\ \cline{2-7}  & **OCP** & **E (I = 0)** & **i\({}_{\text{corr}}\)** & **v\({}_{\text{corr}}\)** & **\(\beta_{\text{c}}\)** & **\(\beta_{\text{a}}\)** \\  & **mV** & **(mV)** & **mA/cm\({}^{2}\)** & **mpy** & **(mV/dec)** & **(mV/dec)** \\ \hline Steel substrate & \(-416\) & \(-410.1\) & \(13.01\) & \(152.11\) & \(-135\) & \(318\) \\ \hline Sample 1 & \(-442\) & \(-448.3\) & \(1.51\) & \(17.69\) & \(-18\) & \(346\) \\ \hline Sample 2 & \(-445\) & \(-462.3\) & \(6.11\) & \(71.54\) & - & \(391\) \\ \hline Sample 3 & \(-450\) & \(-415\) & \(0.35\) & \(4.08\) & \(-109\) & \(-84\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Cyclic and linear potentiometry parameters recorded during the electro-corrosion resistance analysis process. Figure 5: Linear potentiometry represented by the Tafel curves in (**a**) and the cyclic one in (**b**). Figure 6: Surface condition of the experimental samples after the electro-corrosion resistance test: (**a–d**) macroscopic appearance by optical microscopy and (**e–h**) microscopic appearance by scanning electron microscopy. The best electro-corrosion behaviour was exhibited by sample 3 as a result of the higher percentage of YSZ in the ceramic layer structure. The surface condition of the materials is shown in Figure 6 at the macro scale (Figure 6a-d) through optical microscopy and at the micro scale (Figure 6e-h) through scanning electron microscopy. A better fit of the data was achieved by replacing the double-layer capacitance by a constant-phase element, i.e., CPE, which expresses the non-ideal behaviour of the electrical double-layer capacitance (capacitance change with frequency). The equivalent circuit consists of a resistor (R\({}_{\text{s}}\)) in series with a parallel combination of a constant-phase element (CPE), a resistor (R\({}_{\text{ct}}\)), and a series combination of resistors and inductances (R\({}_{\text{L}}\), L) as shown in Figure 7. The parameter values that best match  
The Bode spectrum obtained at open circuit potential for the initial uncoated sample in rainwater solution is shown in Figure 8b. Lower impedance values were measured, and the inductive behaviour of the material was observed in the low frequency range. The Bode plot of frequency versus impedance shows that the impedance values decrease slowly, indicating the dissolution of the corrosion products in the solution. The slow but continuous  
Figure 8: Experimental Nyquist and Bode impedance spectra of the experimental samples: (**a**) and (**b**) initial sample, (**c**,**d**) sample 1, (**e**,**f**) sample 2 and (**g**,**h**) sample 3 (samples 1, 2 and 3 are made with ceramic deposits of approximately 60 μm thickness). decrease in impedance indicated an intensification of the corrosion of the initially uncoated steel. Analysis of the Bode spectra in terms of the equivalent circuit (EC) showed the values of the impedance parameters. The EIS spectrum obtained for the initial uncoated material matches very closely with the circuit shown in Figure 7. The experimental values showed good agreement with an error of less than 5%. The results obtained are shown in Table 3. The electrolyte solution continuously passes through the porous Al\({}_{2}\)O\({}_{3}\) ceramic layers with YSZ to react with the steel substrate. Over time, the ceramic layers act as an effective cathode for hydrogen release. Since R\({}_{\text{L}}\) is inversely proportional to the corrosion rate, an increase in R\({}_{\text{L}}\), as shown in Table 3, indicates that the corrosion resistance of the ceramic coatings on the metal substrate has become stronger. Figure 9 shows the main elemental spectra identified on the corroded samples.