EIS Study on Dissolution Behavior of Al Alloy Covered with Adhesive
Authors: Kuramoto Yuri , Hoshi Yoshinao 
Published: December 2024

Adhesive bonding technology has an attractive attention in the bonding of the various components of fuel cell vehicle for weight reduction. Because the adhesive degradation may occur due to the occurrence of the corrosion of bonded materials caused by penetration of electrolyte into the adhesive, the development of the corrosion evaluation method for metal materials bonded by adhesive is required. An electrochemical impedance spectroscopy (EIS) is useful method to examine the corrosion of metal materials since the impedance spectrum over a wide frequency range can provide detailed information involving the electrochemical properties [1-3]. This method was applied to the evaluation for the water absorption into the adhesive [4-5]. In the present study, the EIS was used to investigate the dissolution behavior of Al alloys covered with epoxy resin.
Al alloy electrode was fabricated by the following procedure. The Al alloy and copper wire were electrically connected and embedded in UV-cured resin. In this case, an electrodeposition coating was applied to the electrode surface excluding the test surface used for the measurement in order to prevent crevice corrosion. The test surface area was 2.25 cm2. The epoxy resin was selected as the adhesive in the present study. When covering the electrode surface with epoxy resin, the insulating waterproof tape of arbitrary thickness was applied surrounding the electrode surface, and the epoxy resin was poured into it and cured. Electrochemical measurements were carried out by a three-electrode system. The working electrode was the Al alloy electrode or the Al alloy electrode covered with epoxy resin of arbitrary thickness. A platinum wire and a saturated KCl silver/silver chloride electrode were used as the counter electrode and as the reference electrode, respectively. The electrolyte solution was 0.1 M NaCl or 0.1 M NaCl + 0.01 M NaOH. In the polarization curve measurements, the potential scan rate was set at 1 mV / s. The electrochemical impedance was measured with a frequency range of 100 kHz to 10 mHz and a logarithmic sweep of five points per decade. The video recording of electrode surface was conducted during the electrochemical measurements.
The anodic polarization curve and the electrochemical impedance of Al alloys covered with epoxy resin were measured in the test solution. In the anodic polarization curve, an increase of current density was observed at just after starting the measurement. The passive region then appeared owing to the polarization to the noble potential, and the current density was drastically increased at arbitrary potential, implying the occurrence of localized dissolution. When the impedance of the Al alloy covered with epoxy resin was measured during the immersion, the capacitive loops were observed on the Nyquist diagram. The time constants observed in the impedance spectra were different at the early stage and later stage in the immersion. These results implied the occurrence of localized dissolution or the oxide film formation during immersion.

References

[1] Y. Hoshi et al., Mater. Trans., 62, 655 (2021).
[2] Y. Hoshi et al., J. Electrochem. Soc., 166, C3316 (2019).
[3] Y. Hoshi et al., Electrochemistry, 87, 78 (2019).
[4] S. Chauffaille, O. Devos, J. Jumel, M. E. R Shanahan, INT. J. ADHES. ADHES., 30, 602 (2010).
[5] G. D. Davis, R. A. Pethrick and J. Doyle, J. Adhes. Sci. Technol., 23, 507 (2009).

Acknowledgement

This is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).