Application of in-situ EIS to Galvanic Corrosion of Aluminum Alloy Coupled with High-Tensile Steel

Abstract

Toward multi-materials for the weight reduction of automobile bodies, the light materials have been used in the automotive industry. However, it may pose serious issues for degradation due to the galvanic corrosion. In this study, an in-situ electrochemical impedance spectroscopy (in-situ EIS) 1-4 was applied to the evaluation of galvanic corrosion of aluminum alloy coupled with high-tensile steel. This method allows for the simultaneous measurement of impedance spectra of aluminum alloy and high-tensile steel.An electrochemical measurement was performed by a three-electrode system. The working electrode (WE) was an aluminum alloy plate, the counter electrode (CE) was a high-tensile steel plate. The reference electrode (RE) was a saturated KCl-Ag/AgCl (SSE), which was set near the WE and CE. To simulate the galvanic corrosion environment of automotive materials, the aluminum alloy plate and high-tensile steel plate are set face-to-face. The distance of both electrodes was about 100 μm. The 3D impedance measurement was carried out for 2 weeks. In this measurement, the potential difference between the WE and the CE was set to 0 V to simulate galvanic corrosion 5. The frequency range was 10 kHz to 0.1 Hz at 5 frequencies per decade and AC potential amplitude was 10 mV. The electrochemical cell was placed in the temperature and humidity chamber for the wet and dry cyclic corrosion test.The in-situ EIS was successfully carried out during the wet and dry cyclic corrosion test in the temperature and humidity chamber. The three-electrode system enables the simultaneous measurement of impedance of aluminum alloy plate - high-tensile steel plate (WE-CE), aluminum alloy plate (WE), and high-tensile steel (CE). In the wet condition, the capacitive loop and part of large loop in the high and low frequency ranges were observed in the instantaneous impedance of the aluminum alloy plate-high tensile strength plate (WE-CE) determined from 3D impedance 1-4 at arbitrary time. In this case, the instantaneous impedance of the aluminum alloy plate (WE) and high-tensile steel plate (CE) described the capacitive loop and part of large loop on the Nyquist diagram respectively. It was confirmed that the capacitive loop and part of large loop in the high and low frequency ranges observed in the instantaneous impedance of the aluminum alloy - high tensile strength (WE-CE) were related to the aluminum alloy plate (WE) and high tensile strength plate (CE), respectively. The galvanic corrosion behavior during the wet and dry cyclic corrosion test was investigated by evaluating the charge transfer resistance estimated from the instantaneous impedance of the aluminum alloy plate (WE) and high-tensile steel plate (CE) to discuss the time variation of the charge transfer resistance.