## 3 Results
### Electrochemical Tests
#### 3.2.3 Electrochemical Impedance Spectroscopy (EIS)
The intermediate behavior of the 2% B4C sample implies a balance between resistive and capacitive contributions. The Nyquist diagram reveals, for every sample, three remarkable zones, as seen in Figure 7. High and medium frequencies are described with semicircles, and low frequencies are defined by a straight line, which indicates Warburg element appearance. This indicates that the corrosion mechanism of the samples is changing, from charge transfer to diffusion, with the B4C.3 sample exhibiting the most pronounced Warburg impedance [45; 46]. The overall trend indicates that increasing the HEA content enhances resistance behavior. This can be explained potentially due to agglomeration effects or altered charge  
Figure 6: Bode phase for the studied samples at E\({}_{\text{corr}}\) after 24 h of immersion in 3.5% NaCl solution. transport mechanisms. This must be considered when optimizing these materials for electrochemical applications. A more vertical line in the low-frequency region suggests that the system behaves more like an ideal capacitor. This behavior may be associated with the formation of protective layers or barriers that restrict corrosion, confirming that of all composites, B4C.3 has the highest resistance to corrosion under the studied situation. The experimental EIS data were analyzed using an electrical equivalent-circuit model, and ZSimpWin 3.6 software (Informer Technologies, Los Angeles, CA, USA) was used to accurately represent the impedance response. The equivalent-circuit model that provided the best fit for all studied samples can be seen in Figure 8. A chi-square (\(\chi^{2}\)) parameter close to \(10^{-4}\) suggests a highly accurate fit between experimental and simulated data. Figure 7: Nyquist diagrams for the studied samples at E\({}_{\text{corr}}\) after 24 h of immersion in 3.5% NaCl solution: (**a**) complete plot; (**b**) detail at medium frequencies; (**c**) detail at low frequencies. In contexts where the passivated surface is considered, the letter \"Q\" stands for a Constant Phase Element (CPE). When it comes to the diffusion of ions or electrons in solids or liquids, the \"W\" stands for the Warburg element. This configuration suggests that before reaching the alloy film, the outer layers of the samples offer resistance to dissolution, characterized by a dual-layer passive structure, consisting of both porous and compact regions. After applying [PERSON]'s circuit laws to the circuit and replacing the CPE and Warburg element, we obtained the following equation:  
\[Z_{circuit}=R_{sol}+\frac{A\cdot\left[R_{1}+\frac{R_{2}\cdot B}{R_{2}+B}+ \frac{(R_{3}+C)\cdot D}{(R_{3}+C)+D}\right]}{A+R_{1}+\frac{R_{2}\cdot B}{R_{2}+ B}+\frac{(R_{3}+C)\cdot D}{(R_{3}+C)+D}} \tag{1}\]  
where the letters \"\(A\)\", \"B\", \"C\" and \"D\" stand for  
\[A=\frac{1}{Y_{1}(\textit{jw})^{n_{1}}} \tag{2}\]  
\[B=\frac{1}{Y_{2}(\textit{jw})^{n_{2}}} \tag{3}\]  
\[C=\frac{1}{Y_{W}(\textit{jw})^{0.5}} \tag{4}\]  
Figure 5:0  
The equivalent-circuit parameters for all samples can be seen in Figure 5:1  
This study evaluates the electrochemical behavior of the composites under study, which suggests the appearance of a two-layer passive film. First, a porous outer layer, and secondly, a more compact inner layer, both play a role in the corrosion resistance of the samples. Figure 5:2 The equivalent circuit for the fitting of the EIS data. In the first place, protective and corrosive processes are facilitated by the porous layer, which controls the diffusion of ions and oxygen. Its porosity, thickness and roughness play an important role in chloride ion penetration, which directly affects the compact layer. Finally, the compact layer works as the primary corrosion barrier. It ensures surface passivation by limiting ion and electron transport. Its stability is crucial for long-term protection, as degradation may lead to localized corrosion. Metal ion migration (e.g., Fe\({}^{2+}\) or Cr\({}^{3+}\)) and oxygen diffusion contribute to passivity maintenance by the formation of protective oxides.