Electrodeposition of Al-W-Mn Alloy from Lewis Acidic AlCl3−1-Ethyl-3-Methylimidazolium Chloride Ionic Liquid

It is well-known that Al-W alloy prepared by non-equilibrium alloying methods, e.g., RF sputtering deposition, show the most superior chloride-induced pitting corrosion potential among Al-transition metal binary alloys. If the W content exceeds 10 atomic percent (at %), the pitting potential is capable of extending the passive region more than 2600 mV against pure Al.¹ Recently we have established the electrodeposition process of the binary Al-W alloy from the Lewis acidic 66.7-33.3 percent mole fraction (mol %) AlCl₃–[C₂mim]Cl ionic liquid.² Depending on the electrodeposition conditions, it was possible to prepare Al-W alloys with W content approaching 96 at %. However, Al-W alloys with a high chloride pitting potential comparable to those produced by other non-equilibrium methods were not obtained because of their powdery morphology and fragile nature. Current challenge faced by Al-W alloy electrodeposition is to find out a new approach to form a smooth and dense layer onto a metal substrate possibly leading to higher chloride-induced pitting potential. In this study, we examined ternary Al-W-Mn alloy electrodeposition from the Lewis acidic 66.7-33.3 mol % AlCl₃–[C₂mim]Cl with both K₃[W₂Cl₉] and MnCl₂ because the addition of Mn to binary Al-transition metal alloys often gives a favorable chloride pitting potential.³

The procedures used for the preparation and purification of the AlCl₃−[C₂mim]Cl IL were identical to those described in previous articles.^2-4 All the electrochemical experiments were conducted using a three-electrode cell. A Teflon^® sheathed platinum rotating disk electrode was employed for the working electrode.  A coil of 1.00 mm diameter aluminum wire was used for the counter electrode. The reference electrode was an aluminum wire immersed in a neat 66.7-33.3 mol % AlCl₃–[C₂mim]Cl, but was separated from the bulk IL by porosity E glass frits.  All experiments were carried out in an Ar gas-filled glove box with O₂ and H₂O < 1 ppm.  Alloy samples were deposited from the 66.7-33.3 m/o AlCl₃−[C₂mim]Cl with K₃[W₂Cl₉] and MnCl₂ onto Teflon^®-sheathed 2.0-mm-diameter copper cylinder electrodes.

Cyclic voltammograms were recorded at a Pt stationary disk electrode in the 66.7 mol % AlCl₃–[C₂mim]Cl before and after addition of K₃[W₂Cl₉] and/or MnCl₂ (Figure 1). In the neat melt, Al deposition/stripping occurred at 0 V vs. Al(III)/Al by the electrochemical reactions of the [Al₂Cl₇]^-. After addition of K₃[W₂Cl₉] or MnCl₂ to the neat IL, the Al deposition/stripping behavior altered as reported in previous articles.^2,3 Interestingly the electrochemical behavior varied significantly after addition of both K₃[W₂Cl₉] and MnCl₂. Three unknown oxidation peaks related to Al alloy stripping appeared. The main oxidation peak observed at ca. 0.73 V was more positive than those in the K₃[W₂Cl₉] or the MnCl₂ solutions.  Based on this, the Al-W-Mn alloy was prepared on Cu substrates rotated at a fixed rate of 1000 rpm by a dc galvanostatic method at 353 K. As is the case for binary Al-W alloy electrodeposition in the 66.7 mol % AlCl₃–[C₂mim]Cl,² the W content in the ternary Al-W-Mn alloys decreased with increasing the applied current density independently of the K₃[W₂Cl₉] and MnCl₂ concentration in the IL. The Mn content greatly varied with the solution composition and the applied current density. X-ray diffraction patterns of the samples are shown in Figure 2. As we expected, XRD pattern for Al_92.1W_0.2Mn_7.7 suggested an amorphous phase formation, which is usually signaled by the disappearance of the fcc Al reflections and the development of a broad reflection centered at about 2θ = 41°, due to the high Mn content. Surprisingly the pattern for Al_88.6W_3.6Mn_7.8 did not show the broad reflection while the Mn content in these two samples is about the same. It is difficult to explain this unexpected result; however, there is no doubt that W atoms in the alloys prevent an amorphous phase formation in Al-W-Mn alloy system. This result is very similar to the fact found during the electrodeposition of Al-Mo-Ti alloy.⁴ The chloride-induced pitting potential of the Al-W-Mn alloys produced in this study exceeded that for Al-W and Al-Mn binary alloy electrodeposits. Thus, the Mn addition to Al-W alloy electrodeposits is a highly effective approach in the improvement of surface morphology and chloride-induced pitting potential.

References

1. B. A. Shaw, T. L. Fritz, G. D. Davis, and W. C. Moshier, J. Electrochem. Soc., 137, 1317 (1990).

2. T. Tsuda, Y. Ikeda, T. Arimura, A. Imanishi, S. Kuwabata, C. L. Hussey, and G. R. Stafford, ECS Trans., 50(11), 239 (2012).

3. T. Tsuda, C. L. Hussey, and G. R. Stafford, J. Electrochem. Soc., 152, C620 (2005).

4. T. Tsuda, S. Arimoto, S. Kuwabata, and C. L. Hussey, J. Electrochem. Soc., 155, D256 (2008).