Acta Metall Sin  2012, Vol. 48 Issue (11): 1357-1364    DOI: 10.3724/SP.J.1037.2012.00201
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AN ANTIFERROMAGNETIC Fe24Mn4Al5Cr COVAR ALLOY IMPULSE--PASSIVATED BY AN ALTERNATING CURRENT VOLTAGE OVERLAPPING A DIRECT CURRENT VOLTAGE AND ITS CORROSION RESISTANCE
ZHU Xuemei, CAO Xuemei, LIU Ming,  LEI Mingkai,  ZHANG Yansheng
1) School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028
2) Surface Engineering Laboratory, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024
Abstract

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 Ha2SO4 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 μA/cm2, than those of -360 mV and 2.6 μA/cm2 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 $|Z|$, and wider phase degree range,  relative to the constant-passivated films. Correspondently, the impulse-passivated film resistant Rp increased to 54.0 kΩ·cm2 from 14.8 kΩ·cm2 and effective capacitance decreased to 10.2 μF/cm2 from 14.0 μF/cm2, using an equivalent electric circuit of< Rs-(Rp//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.

Fund:

Supported by National Natural Science Foundation of China (No.50725519)