OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Fe → Fe2++ 2e-, Fe – 2e- + 2H 2O → Fe(OH)2 + 2H + (7) Fe2++2Cl- → FeCl 2 (8) In the cathodic branch of the CV curves, corresponding to the region from +0.05 V to -0.80 V, the peaks of copper reduction are seen (marked by black arrows in Fig. 3), which are shifted relative to each for the specimens having different volume fractions of the steel. In the CuAl9Mn2 specimen, the cathodic reduction of copper (Cu2+ + 2e- → Cu) occurs at the highest potential E = -0.29 V (relative to Ag/AgCl). In turn, in the CuAl9Mn2 + 50 vol. % ER 321 specimen the same reaction takes place at the lowest potential (E = -0.51 V). During reiteration of the CV measurements the same peaks of oxidation and reduction of copper are observed, which indicates the reversible nature of the reaction Cu ⇄ Cu2+ + 2e-. In addition, the following cathodic processes are expected to occur in aqueous solutions, in which the α-Cu phase will act as a reducing agent: O2 + 4е - + 2H 2O → 4OH - (9) O2 + 4H + + 4е- → 4OH- (10) 2H2O + 2е - → Н 2 + 2ОН - (11) 2H+ + 2е- → Н 2 (12) In the specimens containing from 50 to 75 vol. % of the ER 321 steel, the electrochemically active components of the composite are γ-Fe, α-Fe and α-Cu phases. It is reasonable to assume that in the 3.5 wt. % NaCl solution the reactions, associated with the formation of Fe/Cu galvanic couples, initiate. In the presence of strong oxidizing agents (Cl-), both metals (Fe, Cu) in these phases may oxidize. In this case, the electrons released in the reactions (3) and (7) would be captured by copper, which acts as a cathode in the Fe/Cu galvanic couple due to the significant difference in the standard reduction potentials (ECu(2+)/Cu = + 0.34 V, EFe(2+)/Fe = -0.44 V). Based on analysis of the CV curves it follows that the main processes on the surface of the composites (CuAl9Mn2 + ER 321) are anodic oxidation of copper and iron, formation of chlorides (FeCl2, CuCl2), discharge of the oxidizing agent (oxygen) and reduction of copper (reversible reaction). The corrosion rate of the specimens was assessed using a linear potential scan. In contrast to CV curves (Fig. 3), only irreversible anodic processes are observed in the polarization curves (Fig. 4), leading to the appearance of corrosion damage onto the surface of the working electrodes. All specimens are characterized Fig. 4. Voltammograms obtained in 3.5 wt. % NaCl solution for CuAl9Mn2 and composites exhibiting 10 vol. % (a) and 25, 50, 75 vol. % ER 321 (b)
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