OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 by the presence of a passive state region, limited by a narrower potential window: from -0.3 V to -0.1 V for the initial bronze and CuAl9Mn2 + 10 vol. % ER 321 (Fig. 4, a); from -0.4 V to -0.2 V for composites containing 25, 50 and 75 vol. % ER 321 (Fig. 4, b). A sharp increase in anodic currents is accompanied by an intense dissolution of the outer surface layer at E = -0.1 V. It is important to note that composites with a larger volume fraction of steel (≥ 25 %) demonstrate lower values of the anodic currents (Fig. 4, b), which indicates a formation of a protective oxide film with apparent dielectric properties onto the surface of the composites. Fig. 5 shows the voltammetric dependences, plotted in logarithmic coordinates, of the same specimens in 3.5 wt. % NaCl solution. Extrapolation of these curves by the Tafel function, which takes into account the slopes of the cathodic (bc) and anodic (ba) branches, allows to calculate the corrosion current densities and polarization resistance (Rp) using the SternGeary equation [20]. The corrosion parameters are given in Table 2. Two important conclusions could be made from the analysis of the Tafel curves. With an increase in steel content in the composites, the corrosion potential (Ecor, Table 2) shifts to the region of the positive potentials. This is, most likely, due to the fact that the surface of the samples is passivated not by copper oxides, but by nickel and chromium oxides. This suggestion is supported by the lower standard reduction potential of Ni and Cr (ECr(3+)/Cr = -0.744 V, ENi(2+)/Ni = -0.257 V) than that of copper (ECu(2+)/Cu = +0.34 V). The densities of corrosion currents naturally decrease from 11.010 to 0.512 μA/cm2 for the composites obtained at the ratios CuAl9Mn2 : ER 321 = 90 : 10; 25 : 75; 50 : 50 and 25 : 75, and the polarization resistance, on the contrary, increases by almost an order of magnitude (Table 2). It can be concluded that the formation of the austenitic γ-Fe phase in the surface layer of the samples with a volume fraction of steel ≥ 50 % more effectively prevents the development of corrosion processes than alloying the α-Cu phase with nickel and chromium. Thus, using the EBAM method, the composites characterized by a lower (~9.5 times) corrosion rate in the marine environment could be fabricated. EIS study of the electrochemical behavior of the specimens In order to reveal the electrochemical properties of the CuAl9Mn2/ER 321 composites, the impedance spectra were obtained relative to the open-circuit potential and plotted in Nyquist coordinates before and Fig. 5. Potentiodynamic polarization curves obtained in the same electrolyte (3.5 wt. % NaCl solution) for the CuAl9Mn2 and composites Ta b l e 2 Corrosion parameters determined from polarization curves by Tafel extrapolation of the CuAl9Mn2 and composites Sample Corrosion parameters Ecor, V Icor, μA/cm2 β а, V βc, V Rp, Ohm‧cm 2 CuAl9Mn2 -0.207 11.390 0.066 0.287 2,048 CuAl9Mn2 – 10% ER 321 -0.218 11.010 0.071 0.209 2,091 CuAl9Mn2 – 25% ER 321 -0.194 4.945 0.125 0.098 4,810 CuAl9Mn2 – 50% ER 321 -0.147 1.043 0.125 0.098 16,300 CuAl9Mn2 – 75% ER 321 -0.149 0.512 0.039 0.061 20,100
RkJQdWJsaXNoZXIy MTk0ODM1