Galvanic Deposition of Mo Atop Al 6061 Alloy

Abstract

We report galvanic deposition of Mo films onto 6061 Al alloy from aqueous solutions containing 1 mM HNO 3 and 10 mM MoCl 5 . Deposition for 40 min. yields an 9 μm thick Mo film, which also contains 18 atom% Al. The corrosion resistance of the Mo film is studied by voltammetry and electrochemical impedance spectroscopy in 0.5 M H 2 SO 4 and in 3.5 wt% NaCl electrolyte at pH 2, 7 and 12. These demonstrate that the galvanic Mo film significantly improves the corrosion resistance of the underlying Al 6061 substrate. Galvanic deposition appears to yield elemental, amorphous Mo films.Mo thin films have many technological applications, including back contacts for photovoltaic devices and corrosion-resistant coatings. 1,2 However, Mo thin film deposition currently requires expensive vacuum methods such as magnetron sputtering. 3 Electrochemical methods for thin film growth are often less expensive, easier to scale-up, and more amenable to high-volume manufacturing than vacuum methods. 4 Electrochemical deposition of refractory metals from aqueous electrolytes is difficult due to the relatively cathodic standard reduction potentials, multiple valence states, and complex oxy-anion solution phase chemistry. 5 Mo electrodeposition has been reported mainly from non-aqueous electrolytes such as high temperature molten salts. 6-8 More recently, Mo electrodeposition from aqueous electrolytes has also been reported, 9,10 but either without evidence for complete reduction to Mo(0), 9 or with extremely low current efficiency due to copious hydrogen evolution. 10 Mo alloys with a Mo content less than 50% can also be obtained from aqueous electrolytes. [11][12][13] We report galvanic deposition of compact Mo thin films onto Al 6061 alloy from a simple aqueous electrolyte. Energy dispersive X-ray spectroscopy (EDX) measurements indicate that these films contain primarily Mo and Al. These galvanic Mo films are also characterized by voltammetry and electrochemical impedance spectroscopy (EIS) in 0.5 M H 2 SO 4 and in 3.5 wt% NaCl electrolyte at pH 2, 7 and 12. ExperimentalConcentrated HNO 3 was obtained from J.T. Baker, MoCl 5 was obtained from Alfa Aesar, and 99.999% pure Al rod and 6061 Al foil (76 μm thick) were obtained from ESPI Metals. Al 6061 alloy typically contains 0.8-1.2 wt% Mg, 0.4-0.8 wt% Si, ≤ 0.70 wt% Mg, 0.15-0.40 wt% Cu, 0.04-0.35 wt% Cr, and smaller amounts of Mn, Ti, and Zn. Prior to Mo deposition, 6061 Al alloy was roughened with 150 grit Al 2 O 3 sandpaper and rinsed with water according to ASTM standard B253-11. MoCl 5 is corrosive and water-reactive, so it was stored under vacuum, and all experiments were performed in a chemical fume hood. All experiments were performed at room temperature (20 • C).Voltammetry and open circuit potential measurements were performed with an EG&G PAR model 273A potentiostat/galvanostat with an Al 6061 foil working electrode, Pt counter electrode, and SCE reference electrode. Electrochemical impedance spectroscopy (EIS) measurements were performed with a Solartron 1250 frequency respo...

Applicability of Metals as Liner Materials for Ammonoacidic Crystal Growth

“…In this way, tantalum is regarded as a good candidate for a corrosion‐resistant coating. Molybdenum also exhibits potential as a depositable material with excellent corrosion properties 17, 18.…”

Low temperature electrochemical deposition of highly active elements

“…We recently reported galvanic deposition of compact Ti and Mo [52,53], galvanic deposition of nanoporous Si [54], and galvanic/electroless deposition of compact Si films [55], all deposited on Al alloy substrates. This approach takes advantage of the fact that Al is one of the most active elements, so quite active elements can be noble to Al when present in an electrolyte.…”

“…For Ti deposition, the following electrochemical half-reactions are most likely [52]: Chemical speciation in MoCl5-containing electrolytes is complex, so electrochemical half-reactions during galvanic Mo deposition are uncertain [53]. Since Al oxidation (reactions 6 and 9 above) is integral to galvanic deposition, Ti, Mo and Si deposition are all coupled to Al dissolution and transport through the growing thin film coating.…”

“…Thus Al contamination is intrinsic to these galvanic deposition methods, yielding deposits that contain ~90 atom% Ti, 82 atom% Mo, and 93 atom% Si, as measured by EDX. H2 evolution is observed to a varying extent during Ti, Mo and Si deposition [52][53][54][55], and some hydrogen incorporation into these deposits is likely.…”

“…Much of the evidence cited for galvanic deposition of compact and elemental Ti, Mo and Si from aqueous electrolytes is indirect [52,53,55], based on color retention during prolonged exposure to ambient air, film resistivity measurements by four-point probe, and the ability to perform electrochemical measurements through thick (5-10 µm) coatings. As noted earlier, gradual color change from dark grey to white has long been used as evidence for a porous Si deposit [46].…”