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August 1990

By: Louis MacDowell & Luz Calle

Abstract

Electrochemical Impedance Spectroscopy (EIS) techniques using AC impedance measurements were chosen to investigate the corrosion resistance of 19 alloys under conditions similar to the Space Transportation System (STS) launch environment. The alloys were Zirconium 702, Hastelloy C-22, Inconel 625, Hastelloy C-276, Hastelloy C-4, Inconel 600, 7Mo +N, Ferralium 255, Inco Alloy G-3, 20Cb-3, SS 904L, Inconel 825, SS 304LN, SS 316L, SS 317L, ES 2205, SS 304L, Hastelloy B-2, and Monel 400. AC impedance data were gathered for each of the following three electrolyte solutions: 3.55% NaCl, 3.55% NaCl-0.1N HCl, and 3.55% NaCl-1.0N HCl. The data were analyzed qualitatively using the Nyquist plot and quantitatively using the Bode plot. Polarization resistance, Rp, values were obtained using the Bode plot. Zirconium 702 was the most corrosion resistant alloy in the three electrolytes. The ordering of the other alloys according to their resistance to corrosion varied as the concentration of hydrochloric acid in the electrolyte increased. The corrosion resistance of Zirconium 702 and Ferralium 255 increased as the concentration of hydrochloric acid in the electrolyte increased. The corrosion resistance of the other 17 alloys decreased as the concentration of the hydrochloric acid in the electrolyte increased.

AC impedance data were gathered for the 18 of the alloys at various immersion times in 3.55% NaCl-0.1N HCl. Alloy SS 304LN was not included in this part of the investigation because of its similarity to SS 304L and due to time constraints.

Polarization resistance, Rp, values were obtained from the Nyquist plots at each immersion time using the EQUIVALENT CIRCUIT software package available with the 388 Electrochemical Impedance software. Hastelloy C-22 showed the highest overall values for Rp while Monel 400 and Inconel 600 had the lowest overall values. There was good general correlation found between the corrosion performance of the alloys from the beach corrosion testing site and the predicted rate of corrosion that was based on the Rp values obtained in the laboratory. These data suggest that Electrochemical Impedance Spectroscopy can be used to predict the corrosion performance of metal alloys.

For additional information, a complete copy of this study is available as NASA Report MTB 610-89A.   Send requests for copies to corrosion@ksc.nasa.gov.

To download a full copy of this report in Adobe's pdf format for local printing, click 610-89a.pdf.  (3.5MB)

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