Anodic Dissolution of Al–Fe and Al-Fe-Si Alloys in Emimcl–AlCl3 Electrolyte
Authors: Nunomura Junji , Matsushima Hisayoshi , Kyo Yoshihiko , Kojima Yoichi , Ueda Mikito 
Published: December 2024

Al casting alloys, which contain large amounts of Fe, Si, and Cu, are used for automotive engine components. Recycling used Al casting alloys further leads to effective use of resources and reduction of environmental load.

We have reported that various Al alloys can be purified and upgraded by electrorefining using ionic liquids.1,2) In this study, we investigated the anodic dissolution and cathodic deposition behavior of Al–Fe binary alloys and Al–Fe–Si ternary alloys with controlled intermetallic compound.

Pure Fe (99.99%Fe), pure Al (99.999%), Al–1.5%Fe, and Al–1.5%Fe–3.5%Si casting alloys were used as anode samples. Ionic liquid preparation and all electrochemical measurements were performed in an Ar-filled glove box. The ionic liquid electrolyte was prepared by mixing 1-ethyl-3-methylimidazolium chloride (EmImCl) and AlCl3 with a molar ratio of 1 : 2. In the electrochemical measurements, Al–1.5%Fe and Al–1.5%Fe–3.5%Si alloys were used for the working electrode, Pt and Cu plates for the counter electrode, and Al wire for the reference electrode. Anodic polarization measurements and constant potential electrolysis were performed at an experimental temperature of 323 K. Anodic polarization curves were measured from the immersion potential to 1.5 V vs Al/Al(III) at a scanning rate of 1 mV s−
1. Constant potential electrolysis was performed at potentials of 0.4, 0.7, 0.9, and 1.4 V at a charge density of 100 C cm−
2. After constant potential electrolysis, samples were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy dispersive spectroscopy (EDS).

Intermetallic compounds were identified as Al6Fe in the Al–1.5%Fe alloy and β-AlFeSi in the Al–1.5%Fe–3.5%Si alloy by XRD measurements. In the anodic polarization curves of these alloys, oxidation waves were observed at 0.9 V and 1.4 V in the Al–1.5%Fe–3.5%Si alloy, which were higher than the dissolution potential of pure Fe. SEM–EDS analysis of the Al–1.5%Fe–3.5%Si alloy after electrolysis showed a decrease in Al concentration and an increase in Fe and Si concentrations in β-AlFeSi at 0.9 V. At 1.4 V, Al and Fe concentrations decreased and Si concentration increased.

These results indicate that anodic dissolution of Fe in β-AlFeSi in EmImCl–AlCl3 ionic liquid occurs at 1.4 V, which is more noble than the dissolution potential of 0.5 V for pure Fe, and that the addition of Si to the Al–Fe alloy suppressed the anodic dissolution of Fe.

From the above, in the Al–Fe–Si alloy containing β-AlFeSi formed by adding Si in the Al–Fe alloy, the dissolution of Fe into the electrolyte is suppressed, and the purity of Al at the cathode is improved.

Acknowledgement

Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO). We would like to express our gratitude to all parties concerned.

Reference

1) J. Nunomura et al., J. Electrochem. Soc., 169, 082518 (2022).

2) J. Nunomura et al., Electrochim. Acta, 460, 142601 (2023).