Galvanic Deposition of Ti atop Al 6061 Alloy

Abstract

For the first time, galvanic deposition of Ti is reported from aqueous solutions containing 17 mM HF and 10 mM K 2 TiF 6 at pH 2.73 onto Al 6061 alloy. X-ray diffraction yields peaks consistent with a polycrystalline Ti deposit, and electrical resistivity measurements are also consistent with metallic Ti, not TiO 2 . Elemental analysis by energy dispersive X-ray (EDX) spectroscopy demonstrates the as-deposited film contains ∼90 atom% Ti. The galvanic Ti deposit improves the corrosion resistance of the underlying Al substrate, as illustrated by voltammetry and electrochemical impedance spectroscopy studies in several electrolytes. Deposition for 30 h. yields a 14 μm thick, silver-gray Ti film.Ti has numerous applications in the microelectronics, aerospace, and biomedical industries due to its excellent mechanical properties, corrosion resistance and biocompatibility. 1,2 Ti is a refractory metal with a high strength to weight ratio and low modulus of elasticity, and is therefore used in dental implants and prostheses. Ti provides a favorable foreign body response after implantation, which leads to mineralization and titanium osseointegration. 3,4 Thin films of Ti are also employed for their corrosion resistance, 5,6 and as diffusion barrier layers within electronic devices. 7 Ti thin films are typically deposited by evaporation and sputtering methods using expensive vacuum systems. 8-10 Electrochemical deposition is generally simpler and less expensive, but Ti electrodeposition from aqueous electrolytes is difficult due to the highly cathodic potential required. For this reason, Ti electrodeposition is typically performed from high temperature molten salts. 1 We report the room temperature galvanic deposition of Ti thin films onto Al 6061 alloy. X-ray diffraction and electrical resistivity measurement of the galvanic Ti film are consistent with a polycrystalline Ti deposit rather than TiO 2 . Energy dispersive X-ray spectroscopy (EDX) indicates that these films contain 89-91 atom% Ti. The corrosion resistance of Ti atop Al 6061 alloy is studied by voltammetry and electrochemical impedance spectroscopy (EIS). ExperimentalReagent grade chemicals were used for all experiments. Semiconductor grade HF was obtained from J.T Baker, while K 2 TiF 6 was obtained from Sigma Aldrich. 99.999% pure Al and Al 6061 alloy rods 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. The Al rods were cut into stubs of approximately 1.5 cm. The Al electrodes were polished sequentially with 600, 1500, 2400, 4000 grit Al 2 O 3 sand papers, and finally with 50 nm Al 2 O 3 , then rinsed with acetone and twice distilled water.For studies of corrosion resistance in four model electrolytes, electrochemistry was performed in a three-electrode cell with the Ti/Al film working electrode, Pt counter electrode and SCE reference electrode. Voltammetry was performed from −1070 mV to -650 mV vs. SCE with a scan rate of 0...

Low temperature electrochemical deposition of highly active elements

“…Nevertheless, it is difficult to completely exclude the possible formation of sub-oxides, or other species not completely reduced to zero valence. However, in the case of Ti deposition, polycrystalline Ti deposits of grain size ~20 µm are indicated by the x-ray diffraction results shown in Figure 5 [52]. In addition, F incorporation and/or Ti film porosity is observed when the pH is varied away from the optimal range of 2.6-3.0, providing further evidence for deposition of compact, elemental Ti is optimized in that pH range.…”

“…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.…”

“…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.…”

“…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].…”