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摘要
An antiferromagnetic Fe24Mn4Al5Cr covar alloy has been impulse-passivated by an alternating current (AC) voltage overlapping a direct current (DC) voltage, in order to improve its corrosion resistance due to the poor corrosion property of austenite matrix with a high Mn content, a low Cr and/or Al content. The impulse-passivated films were obtained in 1.0 mol/L Na2SO4 solution at an AC voltage with modulation amplitude of 180-480 mV during alternating period of 200-400 ms for modification time of 5-15 min, and simultaneously at a DC voltage of 620 mV. The impulse-passivation parameters in 1.0 mol/L Na2SO4 + 0.5 mol/L H2SO4 solution were optimized as amplitude of 380 mV, period of 300 ms, and time of 10 min, by using the potential decline curves. The impulse-passivated films on the covar alloy under the optimum parameters were characterized by Auger electron spectroscopy and X-ray photoelectron spectroscopy (AES/XPS), and evaluated by anodic polarization and electrochemical impedance spectroscopy (EIS), respectively, and compared with those of the constant-passivated films at DC voltage of 620 mV for modification time of 15 min. In the impulse-passivated films on the covar alloy, an enrichment of elements Al and Cr, a lack of Mn in a thick barrier film composed of oxides Al2O3 and Cr2O3. The anodic polarization curves of the impulse-passivated films have a self-passivation with the higher corrosion potential of -100 mV and lower passive current density of 0.7 mu A/cm(2), than those of -360 mV and 2.6 mu A/cm(2) for the constant-passivated films. These electrochemical polarization behaviors were similar to those of the AISI 304 austenitic stainless steel. The EIS of the impulse-passivated films has larger diameter of capacitive arc, higher impedance modulus vertical bar Z vertical bar, and wider phase degree range, relative to the constant-passivated films. Correspondently, the impulse-passivated film resistant R-p increased to 54.0 Omega.cm(2) from 14.8 k Omega.cm(2) and effective capacitance decreased to 10.2 mu F/cm(2) from 14.0 mu F/cm(2), using an equivalent electric circuit of R-s-(R-p//CPE). The high insulation of the impulse-passivated films on the covar alloy led to an improved corrosion resistance of the cover alloy. The impulse-passivated antiferromagnetic Fe24Mn4Al5Cr covar alloy exhibits an application potential in wide industrial field.
