## 3 Results
### Electrochemical Tests
#### 3.2.3 Electrochemical Impedance Spectroscopy (EIS)  
EIS is an advanced method, being highly effective and accurate for analyzing interfacial properties associated with surface processes in metallic alloys. Unlike other electrochemical techniques, EIS offers significant advantages, such as operating under steady-state conditions, efficiently measuring small signals and covering a broad frequency range from 100 mHz up to 100 kHz. Electrochemical impedance spectroscopy results of the different composites are presented as Bode-phase impedance plots, recorded at a consistent potential in a 3.5% NaCl solution. Due to the high number of samples under study, the Bode impedance plot is presented in Figure 5 and the Bode-phase plot in Figure 6; this way, the conducted test can be interpretated easier. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & **Monolithic** & **0.5\% HEA** & **1\% HEA** & **2\% HEA** & **3\% HEA** \\  & **B4C** & **B4C** & **B4C** & **B4C** & **B4C** \\ \hline E\({}_{\text{corr}}\) (mV vs. Ref) & \(-600.665\) & \(-749.920\) & \(-478.421\) & \(-357.533\) & \(-230.693\) \\ I\({}_{\text{corr}}\) (uA/cm\({}^{2}\)) & 0.296 & 0.834 & 0.161 & 0.111 & 0.041 \\ \(\beta_{\text{c}}\) (mV) & 49.7 & 84.2 & 55.9 & 77.0 & 91.1 \\ \(\beta_{\text{a}}\) (mV) & 65.6 & 108.4 & 66.3 & 60.3 & 125.2 \\ Equivalent weight (g/eq) & 3.946 & 3.963 & 3.980 & 4.014 & 4.049 \\ Measured density (g/cm\({}^{3}\)) & 2.43 \(\pm\) 0.02 & 2.49 \(\pm\) 0.01 & 2.54 \(\pm\) 0.01 & 2.61 \(\pm\) 0.02 & 2.66 \(\pm\) 0.02 \\ Surface (cm\({}^{2}\)) & 0.996 & 1.035 & 0.843 & 0.935 & 0.898 \\ Corrosion rate (mpy) & 62.158 \(\times\) 10\({}^{-3}\) & 165.183 \(\times\) 10\({}^{-3}\) & 38.544 \(\times\) 10\({}^{-3}\) & 23.516 \(\times\) 10\({}^{-3}\) & 8.951 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Corrosion parameters for all samples tested. Figure 5: Bode impedance for the studied samples at Ecorr after 24 h of immersion in 3.5% NaCl solution. In Figure 5, it is observed that the highest measure of impedance is attributed to B4C.3, which registered a value of 60.047 k\(\Omega\). This indicates that the 3% HEA composite has higher corrosion resistance as result of its capacity to oppose electric current. This is followed by B4C.2, presenting a similar result as obtained for the corrosion potential, as when the HEA volume percentage increases in the sample composition, the higher its corrosion resistance behavior. B4C.05 and B4C.1 presented similar results, indicating a very similar behavior. The data obtained from the EIS measurements of B4C composites demonstrated a very high resistance to corrosion in artificial seawater, especially B4C.3. On the other hand, Bode-phase analysis of B4C-reinforced samples reveals significant differences in their electrochemical behaviors across frequency ranges (see Figure 7). At low frequencies, the B4C.3 sample shows the most negative phase angle (\(-75^{\circ}\)), which indicates a strong capacitive behavior and a strong passive layer. The B4C.05 sample shows a lower phase shift (\(--30^{\circ}\)), which implies a higher charge transfer rate and lower capacitive response. These results suggest that lower HEA concentrations lead to a thinner or less effective passive layer. At medium frequencies, all the samples transition towards a mix between capacitive and resistive response. The 3% HEA B4C sample maintains the highest phase shift. This indicates superior electrochemical stability, which matches the corrosion potential and corrosion rate test results. At high frequencies, it can be seen how all composites converge towards lower phase angles (\(--10^{\circ}\) to \(-30^{\circ}\)), dominated by electrolyte resistance. The B4C.3 sample consistently demonstrates the best capacitive properties, implying improved corrosion resistance. On the other hand, the 0.5% and 1% B4C samples show the weakest capacitive response. This suggests that lower HEA concentrations result in a less effective passive layer.