OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Ta b l e 3 Parameters of equivalent electrical circuits for CuAl9Mn2 and CuAl9Mn2/ER 321 composites before and after potentiodynamic polarization Before corrosion Element of the electrical circuit CuAl9Mn2 CuAl9Mn2 : ER 321 90 : 10 75 : 25 50 : 50 25 : 75 Solution resistance, Ohm‧cm 2 5 5 70 4 67 Charge transfer resistance, Ohm‧cm 2 1,385 1,158 7,269 21,630 177,000 Warburg element, kOhm/с–0.5 0.21 0.26 – 15.75 – After corrosion Solution resistance, Ohm‧cm 2 5 4.9 69 43 71 Charge transfer resistance, Ohm‧cm 2 324 396 13,210 25,510 154,200 Warburg element, kOhm/с–0.5 1.05 0.64 – 16.50 – layer. With an increase in the share of steel in the composite up to 25 vol. % the Nyquist diagram is a semicircle without a linear part in the low frequency region, therefore the equivalent circuit does not include the Warburg element responsible for the charge diffusion. The Rct value increases from 7,269 to 13,210 Ohm‧cm2 (Fig. 6, c, Table 3) after a potentiodynamic polarization test, which indicates a higher passivation ability of this composite in comparison with the sample having 10 vol. % of the ER 321 steel. A composite with the ratio CuAl9Mn2 : ER 321 = 50 : 50 has a quite high charge transfer resistance before and after corrosion tests (Table 3). The presence of a semicircle in the Nyquist diagram (Fig. 6, d) and a linear dependence, when both real and imaginary parts > 10,000 Ohm‧cm2, indicate oxidation of the material and the release (diffusion) of the charged particles into solution. In the impedance spectra of the composite with the ratio CuAl9Mn2 : ER 321 = 25 : 75, only a part of the semicircle is present (Fig. 6, e), which indicates a significant charge transfer resistance Rct (~177,000 Ohm‧cm2). At the same time, an electrical discharge of the double layer does not occur and diffusion of charges into the solution is not occurred (Fig. 6, c, Table 3). As can be seen from Table 3, the charge transfer resistance of this specimen decreases slightly after corrosion. In this case, the anodic oxidation of iron and copper will be hindered if a repassivation of the surface layer takes place after appearance of the corrosion damage. We conclude that the least electrochemically active specimen is the composite with the ratio CuAl9Mn2 : ER 321 = 25 : 75, since it is characterized by the maximum value of Rct, which, in turn, is inversely proportional to the density of corrosion current. Summarizing the data of electrochemical experiments (EIS, CV, linear polarization), we conclude that the composite with 75 vol. % of the steel could be considered to be the most corrosion resistant one. The lowest corrosion resistance was found for the CuAl9Mn2 and the composite with 10 vol. % of the ER 321 steel. Based on electrochemical reactions (1)–(12) and literature data on the corrosion properties of Fe-Cu alloys [21] and bronze [22, 23], we can expect that the possible corrosion products of the studied CuAl9Mn2/ER 321 composites are insoluble or poorly soluble compounds – copper and iron oxides. In the X-ray diffraction patterns of the CuAl9Mn2 (Fig. 7, a) and the composite CuAl9Mn2 – 10 vol. % ER 321 (Fig. 7, b), subjected to corrosion tests in the cyclic potential sweep mode, the main diffraction lines from the matrix phases are recorded (α-Cu, β′, α-(Fe,Cr), Cu3Al), found earlier in [13]. In this case, additional reflections belonging to the Cu2O and FeCl2 phases are detected (Fig. 7, a, b). These phases (Cu2O, FeCl2) can be formed as a result of microgalvanic corrosion occurring primarily at the grain boundaries between α-(Fe,Cr) ferrite particles and the α-Cu matrix in chloride-containing electrolytes. Analysis of the corrosion damage (Fig. 7, c, d) on the composite’s surface has shown that the mechanisms of localized corrosion are realized in the CuAl9Mn2 alloy, leading to the appearance of shallow pits
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