## 3 Experimental Results
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline
**Elements/** & \multicolumn{2}{c}{**Zr \%**} & \multicolumn{2}{c}{**Al \%**} & \multicolumn{2}{c}{**O \%**} & \multicolumn{2}{c}{**Y \%**} & \multicolumn{2}{c}{**Hf \%**} & \multicolumn{2}{c}{**Fe \%**} \\
**Coating** & **wt** & **at** & **wt** & **at** & **wt** & **at** & **wt** & **at** & **wt** & **at** & **wt** & **at** & **wt** & **at** \\ \hline Sample 1 & 5.8 & 1.4 & 45.8 & 38.5 & 47.4 & 62.5 & 0.4 & 0.1 & 0.1 & 0.01 & 0.4 & 0.15 \\ Sample 2 & 12.3 & 2.97 & 40.3 & 32.9 & 46.3 & 64.8 & 0.6 & 0.2 & 0.05 & 0.01 & 0.4 & 0.14 \\ Sample 3 & 17.73 & 4.41 & 34.32 & 28.86 & 46.83 & 66.41 & 0.81 & 0.21 & 0.05 & 0.01 & 0.25 & 0.11 \\ \hline EDS & & & & & & & & & & & & \\ Error\% & & 1.2 & & 1.5 & & 2.15 & & 0.2 & & 0.01 & & 0.01 \\ \hline \end{tabular}  
* St.dev: Zr: \(\pm\)0.8; Al: \(\pm\)1.2; O: \(\pm\)1.5; Y: \(\pm\)0.1; Hf: \(\pm\)0.01; Fe: \(\pm\)0.1.  
\end{table} 
Table 1: Chemical composition of the ceramic coatings. Figure 4: XRD patterns of alumina–YSZ coating (sample 3). The open circuit potential showed a similar average value for all samples coated with ceramic layers, which was slightly higher than the potential of the original uncoated sample. The corrosion current densities (i\({}_{\text{corr}}\)) of all plasma jet sputter-coated systems are quite similar and significantly lower than those of the original material [29]. Corrosion rates are much lower for the coated systems due to the chemical inertness characteristic of ceramic coatings, particularly alumina- and zirconia-based coatings [30]. Sample 2 shows the highest corrosion rate among the ceramic-coated samples, but it is still lower, even by half, compared to the original material, as shown in Table 2. The difference between the coated samples can be attributed to the appearance of a larger number of pores on sample 2, which contributes to the decrease in its corrosion resistance by forming communicating channels through which the electrolyte solution infiltrates until it comes into close contact with the substrate [31]. The much lower values of i\({}_{\text{corr}}\) in comparison to v\({}_{\text{corr}}\) indicate better protection of the samples with ceramic layers against the substrate. A slight enhancement of the anodic reactions (\(\beta\)a \(>\) gc) is observed for the first sample, which is reversed to a lesser extent in sample 3 with ceramic coatings having a higher percentage of YSZ. It is recognised and demonstrated in the literature that ceramic coatings cannot eliminate the infiltration of electrolyte solutions up to the interface of the bonding layer between the metallic substrate and the ceramic layer formed over it. At the same time, and in confirmation, the anodic curves plotted in Figure 5a do not show a passivation plateau, presumably because oxide defects and inclusions in the adhesion layer prevent complete passivation and, as deduced from the very low corrosion potential, allow corrosion of the steel substrate. This suggests that the corrosion resistance of these coatings is not primarily based on passivation, as none of the coatings showed a clear active-passivation transition. Among the ceramic protective layers, Al\({}_{2}\)O\({}_{3}\)-37.5% YSZ appears to provide the best corrosion protection. This is attributed to the superior corrosion resistance of zirconia compared to metallic materials in general and to alumina in most cases. The cyclic potentiometry in Figure 5b shows a generalised surface corrosion nature with a much higher current density for the uncoated sample, which is also confirmed by the intensely corroded surface of the metallic material, as shown in Figure 6a,b. Sample 2 shows a cyclic curve with a different appearance, highlighting the appearance of more intense pitting corrosion zones on the surface in the areas where the interconnected pores help the penetration of the electrolyte solution to the metal substrate, probably due to a higher number of pores in the structure [32].