Electrodeposition of cobalt and cobalt-aluminum alloys from a room temperature chloroaluminate molten salt

• Published in: Journal of the Electrochemical Society 1996-11, Vol.143 (11), p.3448-3455
• Main Authors: MITCHELL, J. A, PITNER, W. R, HUSSEY, C. L, STAFFORD, G. R
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemistry; Electrochemistry; Exact sciences and technology; General and physical chemistry; High temperature electrochemistry (including molten salts); Metallic coatings; Metals. Metallurgy; Production techniques; Surface treatment
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.1837235

The electrodeposition of magnetic cobalt-aluminum alloys was investigated in the Lewis acidic Al chloride-1-methyl-3-ethylimidazolium chloride [60.0-40.0 mole % (m /o)] molten salt containing electrogenerated Co(II) at 25 deg C. rotating disk electrode voltammetry indiated that it is possible to produce alloy deposits containing up to 62 atomic (a/o) Al at potentials positive of that for the bulk deposition of Al. The onset of the underpotential-driven Al codeposition process occurred at approx0.40 V vs. the Al/Al(III) couple in a 5.00 mmol liter exp -1 Co(II) solution but decreased as the Co(II) concentration increased. The Co-Al alloy composition displayed an inverse dependence on the Co(II) concentration but tended to become independent of concentration as the potential was decreased to 0 V. A rotating ring-disk electrode voltammetry technique was developed to analyze the composition and structure of the Co-Al alloy deposits. This technique takes advantage of the fact that the mass-transport-limited reduction of cobalt(II) occurs at potentials considerably more positive than that at which Al codeposition occurs. Scanning electron microscopy and energy dispersive x-ray analysis of bulk electrodeposits revealed that deposit morphology depends strongly upon Al content/deposition potential; deposits produced at 0.40 V from 50.0 mmol liter exp -1 Co(II) solutions consisted of 10-20 mu m diam multifaceted nodules of pure hcp Co, whereas those obtained at 0.20 V were dense and fine grained, containing approx4 a/o Al. Deposits produced at 0 V had the visual appearance of a loosely adherent black powder. X-ray diffraction measurements revealed a lattice expansion and a decrease in grain size as the hcp Co was alloyed with increasing amounts of Al.