Electrodeposition of Al-W Alloy Films in a 1-Ethyl-3-methyl-imidazolium Chloride-AlCl₃Ionic Liquid Containing W₆Cl₁₂

Abstract

The electrodeposition of Al-W alloy films in a Lewis acidic 1-ethyl-3-methyl-imidazolium chloride (EMIC)-AlCl 3 ionic liquid using W 6 Cl 12 as the W ion source was investigated. W 6 Cl 12 dissolved in the ionic liquid at a higher concentration than other W ion sources used in previous studies. Potentiostatic electrodeposition was performed in a bath containing W 6 Cl 12 at a concentration of 49 mM. Dense Al-W alloy films containing up to 12 at.% W were electrodeposited at potentials more negative than 0 V vs. Al/Al(III). The deposition current density at >0 V was lower than 0.3 mA cm −2 , while that for Al-W alloy films was higher and reached 38 mA cm −2 at −0.5 V. The deposition of W was induced by the deposition of Al. At lower W concentrations, the Al-W alloy films were composed of a super-saturated solid solution, and in the W content range of 9-12 at.% they comprised an amorphous phase. Potentiodynamic polarization and nano-indentation showed that the Al-W alloy films containing 10-12 at.% W exhibited high pitting corrosion resistance, high hardness, and low Young's modulus. Al metal has high oxidation and corrosion resistance, and its chloride-induced pitting corrosion resistance is enhanced by alloying with transition metals. Consequently, Al-transition metal alloys have attracted much research attention as corrosion-protective materials. Among these alloys, Al-W alloys containing approximately 10 at.% are known to exhibit the greatest resistance to pitting corrosion. 1Since the maximum solubility of W in Al phases is only 0.022 at.% at 640• C in the equilibrium state, 2 the formation of Al-W alloy films having high corrosion resistances requires non-equilibrium processes, such as sputtering, 1,3-8 ion implantation, 9 and electrodeposition. 10-12Of these methods, electrodeposition has particular advantages in that a uniform film can be formed relatively quickly and continuously, onto substrates having complex shapes, using simple equipment. Various Al-based alloy films have been electrodeposited in molten salts such as NaCl-AlCl 3 with the addition of ion sources for the alloy component. Recently, electrodeposition using ionic liquids has become more popular owing to their low melting point and low volatility. A representative ionic liquid used for the electrodeposition of Al alloys is Lewis acidic 1-ethyl-3-methylimidazolium chloride (EMIC)-AlCl 3 (where the AlCl 3 /EMIC molar ratio >1 Al-Mn, 18 and Al-Hf 19 are available in the literature. A previous study revealed that electrodeposition in a EMIC-AlCl 3 bath with the addition of W(IV) chloride (WCl 4 ) yielded Al-W alloys, but the W content was lower than 1 at.%.10 Recently, the electrodeposition of Al-W alloy films from an EMIC-AlCl 3 bath with the addition of the W(III) compound K 3 W 2 Cl 9 was reported.11,12 Although the W content of the Al-W alloys reached nearly 96 at.%, the deposits with high W content had powdery morphologies. When the current density was high (>20 mA cm −2 ), dense Al-W alloy films were obtained, but the W content decreased t...

Electrodeposition of Aluminum–Tungsten Alloy Films Using EMIC–AlCl₃–W₆Cl₁₂ Ionic Liquids of Different Compositions

“…56) Because W has a smaller atomic radius than Al, the average interatomic distance in AlW alloy films should decrease with increasing content of W atoms. 13,15,16) Therefore, the higher H and E values of the 17.7 at% W film in comparison to those of the ³12 at% W amorphous films are reasonable.…”

“…The prepared EMICAlCl 3 melt was stored in a 25-mL glass vessel used as an electrolytic cell. W 6 Cl 12 was synthesized by a method described in our previous report, 15,16) and was added to the melt maintained at 80°C throughout the experiment. The bath temperature was controlled by a thermostat (TJA-550, AS ONE) connected to a rubber heater wound around the cell and a thermocouple immersed in the bath.…”

“…Previously, AlW alloy films with <³12 at% W were electrodeposited in bath A at potentials of <¹0.1 V, where the current efficiency was ³90%. 15) Accordingly, about 80% of the current density observed for bath A at <¹0.1 V is attributed to the deposition of Al. Therefore, the lower current density observed for bath B in comparison to bath A reflects the lower deposition rate of Al in bath B.…”

“…In contrast, when the W precursor was replaced with tungsten(II) chloride, W 6 Cl 12 , dense AlW alloy films with up to ³12 at% W were successfully electrodeposited. 15,16) The resultant dense films were confirmed to exhibit a good corrosion resistance.…”

“…16) The W content of the alloy films electrodeposited at a constant current density increased linearly up to ³12 at% with increasing W 6 Cl 12 concentration of the bath in a low W 6 Cl 12 concentration range, but the W content of the films saturated at ³12 at% at a higher W 6 Cl 12 concentration. Potentiostatic electrodeposition at various potentials in an EMIC2AlCl 3 bath saturated with W 6 Cl 12 also failed to yield alloy films with >³12 at% W. 15) One of the parameters that have not yet been explored in this system is the AlCl 3 /EMIC molar ratio of the bath. A decrease in the AlCl 3 /EMIC molar ratio below the previously tested value of 2 is expected to suppress the electrodeposition rate of Al, and therefore, increase the W content of the resulting films.…”