Electrodeposition of Al-W Alloy Films from Ionic Liquid and Evaluation of Their Corrosion Resistance and Mechanical Properties

Abstract

Introduction The many useful characteristics of Al, particularly its high oxidation and corrosion resistance, make Al coatings effective in extending the lives of metallic materials. However, if Al is exposed to an environment where chloride ions are present (like seawater), the Al passivation layer is destroyed locally, followed by the creation of small holes on the surface. This form of corrosion is called pitting corrosion, and Al–transition metal alloys are known to exhibit a resistance to pitting corrosion. In particular, sputtered Al–W alloy films are reported to show outstanding pitting corrosion resistance [1-3]. Establishing a method for electrodeposition of Al–W films is of importance for industrial applications, because electrodeposition has advantages over other film formation processes including simple equipment, high film formation speed, and applicability in the case of complex surface shapes. Tsuda et al. reported the electrodeposition of Al–W alloys with a high W content from the 1-ethyl-3-methylimidazolium chloride(EMIC)–AlCl3 ionic liquid containing K3W2Cl9 [4]. However, these Al–W alloys had a powdery morphology when the W content was higher than 3 at%, and did not show significant pitting corrosion resistance. In this study, we formed smooth, dense and adherent Al–W alloy coatings by electrodeposition from the EMIC–AlCl3 ionic liquid with the addition of WCl2, and we investigated the pitting corrosion resistance of the resulting coatings. Furthermore, we evaluated the mechanical properties of the Al–W alloy coatings. Experimental The electrolytic bath was prepared by mixing EMIC with anhydrous AlCl3, and tungsten(II) chloride. The tungsten(II) chloride was prepared by reduction of tungsten(VI) chloride by bismuth. The electrodeposition of Al–W films was conducted under galvanostatic conditions using Cu and Al plates as the cathode and anode, respectively. The bath temperature was 80°C. All electrodeposition experiments were conducted in an argon-filled glove box. The electrodeposited films were analyzed by SEM, EDX, and XRD. Partial current density and current efficiency were determined by ICP-AES. The water content of the electrolytic bath was measured by the coulometric Karl Fischer method. Pitting corrosion potentials were determined by linear sweep voltammetry in a deaerated 3.5 wt% NaCl aqueous solution at room temperature. The mechanical properties of Al–W films were analyzed by the nano-indentation method. Results and Discussion The electrodeposited Al–W films were gray and had a rather smooth appearance. Surface and cross-sectional SEM observation showed that these films were composed of dense globular grains, which is characteristic of amorphous deposit. The average grain size was less than 3 μm. At a fixed concentration of tungsten (II) chloride in the bath, the W content of the films was, typically 15–16 at%, independent of the current density. XRD measurements detected no diffraction peaks from these Al–W films, indicating that the films were amorphous. The current efficiency for the Al–W electrodeposition estimated by ICP-AES was less than 87%, which was lower than that for pure Al electrodeposition. Further, the water content was found to be higher in the electrolytic bath with tungsten(II) chloride than in the bath without it. Therefore, the current loss is attributed to the reduction of water. The pitting corrosion potential of the Al–W alloy film was more positive by >800 mV than that of pure Al, indicating that the Al–W films have a high resistance to the pitting corrosion. Nano-indentation showed that the amorphous Al–W alloy films had high hardness and low elastic modulus compared to a pure Al film. As the alloy films were amorphous, the films contained no grain boundaries and dislocations, and this should be responsible for the excellent hardness and elastic modulus. Conclusion Smooth, dense, and adherent Al–16 at% W alloy films were obtained by electrodeposition. The Al–W alloy films exhibited excellent pitting corrosion resistance and mechanical properties compared with pure Al films. These results sufficiently demonstrate the potential applications of the electrodeposited Al–W alloy as a surface protective material.