(Keynote) EIS Study of Galvanic Corrosion of Al Alloy Coupled with High Strength Steel in Simulated Atmospheres
Authors: Nishikata Atsushi , Tran Nam Van , Ooi Azusa , Tada Eiji 
Published: December 2020

Introduction

Weight reduction of automobiles is required to improve fuel efficiency. Therefore, the uses of lightweight materials such as Al alloys and CFRP is expected to increase. When an automobile is made of multi-materials, there is a concern that galvanic corrosion and crevice corrosion may occur at the joint parts.
We successfully applied EIS to study corrosion of metals covered with thin solution films [1–4]. A two-electrode cell, which was composed of two identical metal plates embedded in epoxy resin with a small gap distance, was used to measure the EIS. Under very thin solution films, uneven current distribution must be considered in the EIS measurements because the current lines may concentrate on the nearest edge of the two electrodes when AC voltage is applied. The EIS characteristics of a metal/solution film interface was successfully described by an equivalent circuit of transmission line (TML) [2,3]. We reported that accurate charge transfer resistance can be obtained by curve fitting the measured EIS with a TML circuit, even if the current lines are not evenly distributed on the electrode surface [1].
In this study, EIS was applied to the study of galvanic corrosion of Al alloy coupled with high strength steel under thin NaCl solution films with different thicknesses. The monitoring of the galvanic corrosion of All alloy was also carried out under a wet-dry cyclic condition by EIS.

Experimental

Al alloy (6016–T4, 10 mm×10 mm) and high-strength steel (980 MPa, 10 mm×10 mm) were used as specimens. They were embedded in the epoxy resin at a gap distance of 0.1 mm. A solution film of 2 M NaCl was placed on the cell surface. Three different films thickness, 0.025, 0.050 and 0.1 mm were used. The EIS was measured under the condition that the potential difference of Al alloy and high strength steel was set to zero by potentiostat. Amplitude of AC voltage applied between the two electrodes was 10 mV and the frequency range was 10 kHz to 10 mHz. Using the same cell, the galvanic corrosion under wet-dry cyclic conditions was also monitored by continuously measuring two impedance at high and low frequency. To investigate the effect of crevice, the cells with and without a crevice were employed.

Result and Discussion

All Nyquist plots of the galvanic couple measured under thin solution films showed two capacitive semi-circles. One semicircle with a smaller time constant can be expressed by a parallel combination of double layer capacitance Cdl and charge transfer resistance Rct for the All alloy. Another semicircle with a larger time constant can be given by Cdl / Rct for the high strength steel. The corrosion rate (CR) of the Al alloy estimated from the Rct increases with decreasing the solution film thickness. This tendency is consistent with that of the galvanic current between the two electrodes.
The galvanic corrosion behavior under the wet-dry cycles in the presence of chlorides was monitored. In above experiment, the optimum frequency for monitoring Rct of the Al alloy was determined to be 0.3 Hz. Using this frequency, the corrosion rate of the Al alloy coupled with the high strength steel was monitored under the wet-dry cycles. The corrosion rate of Al alloy changed in response to the wet-dry cycles. The galvanic corrosion behavior will be discussed in the conference on the basis of the EIS monitoring data.

Acknowledgements

This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References

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