OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 after potentiodynamic polarization tests (Fig. 6). The dots in the diagrams indicate the experimental values of the impedance (real and imaginary parts), and the solid lines indicate the approximation by the model – an equivalent electrical circuit (Fig. 6, f). It is important to note that in a corrosive media (3.5 wt. % NaCl) the electrochemical activity of the composites, promoting anodic reactions, is regulated by the ratio CuAl9Mn2 : ER 321. Indeed, the impedance of the initial bronze specimens (Fig. 6, a) before corrosion tests is described by a relatively simple equivalent circuit, including a constant phase element (Q1), solution resistance (Rs) and charge transfer (Rct) and Warburg element (W). The fitted EIS data are given in Table 3. From the Table 3 it follows that the CuAl9Mn2 specimen shows high electrochemical activity because of the low charge transfer resistance (Fig. 3, a; Table 3). The presence of a Warburg element indicates the diffusion of charges (e-, Cu+, Cu2+, Fe2+) through the electrical double layer into the solution. It is interesting to emphasize that after corrosion tests the Rct value decreases by ~4.3 times. This observation can be explained by the fact that anodic processes (oxidation of copper to Cu2+) initiate the dissolution of the natural oxide film and repassivation of the surface, which leads to a change in the kinetics of a charge transfer in the electric double layer (Fig. 6, a; Table 3). For the composite fabricated at the ratio CuAl9Mn2 : ER 321 = 90 : 10 (Fig. 6, b), the equivalent circuit remains the same, and the Rct value decreases by ~3 times (Table 3) due to repassivation of the surface Fig. 6. Nyquist plots of CuAl9Mn2 (a) and composites with a ratio of CuAl9Mn2 : ER 321 = 90 : 10 (b); 25 : 75 (c); 50 : 50 (d); 25 : 75 (e) and equivalent electrical circuits (f)
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