Kinetics of Ni–Mo electrodeposition from Ni-rich citrate baths E. Beltowska-Lehman ⁎, P. Indyka Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Reymonta 25, 30-059 Cracow, Poland abstract article info Article history: Received 24 November 2009 Received in revised form 4 October 2011 Accepted 14 October 2011 Available online 20 October 2011 Keywords: Induced electrodeposition Ni–Mo coatings Citrate baths The kinetics of Ni–Mo alloy electrodeposition on steel substrates from an aqueous citrate–ammonia complex baths has been investigated by means of steady-state polarisation measurements in a system with a rotating disc electrode (RDE). Partial current densities for discharge of Ni(II) and Mo(VI) ions and hydrogen evolution as a function of molybdate concentration in the bath, cathode potentials and the rate of mass transport were determined. It has been shown that – under all investigated conditions – Ni–Mo alloy deposition is more favourable than pure nickel and the cathodic process is strongly influenced by the Mo(VI) content in the so- lution. The Ni(II) electroreduction rate initially increases, as the cathode potential shifts towards more neg- ative values and the concentration of molybdate grows in the solution. However, for the highest examined MoO 4 2 - content, a considerable decrease in the rate of the process is subsequently observed at certain limit potentials, the values of which depend on molybdate concentration and hydrodynamic conditions. This ef- fect, related to the formation of intermediate molybdenum oxides (characterised by very low overvoltage for hydrogen evolution), becomes less pronounced when the RDE rotation speed is increased. Hydrogen evo- lution is strongly associated with molybdenum deposition. An increase of the molybdate ions concentration in the bath, as well as an increase in the rate of mass transport, leads to an increase in Mo content in deposits and to the reduction of current efficiency. The Ni–Mo coatings electrodeposited from the designed bath (with the current efficiency of about 70%) containing about 30 wt.% Mo, are characterised by a shiny-grey appear- ance and good adhesion to the steel substrate. They are characterised by column growth and amorphous mi- crostructure with randomly distributed nanocrystallites of the MoNi 4 intermetallic phase. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Functional coatings containing molybdenum are characterised by high hardness, high wear resistance, high thermal and corrosion resistance, and good catalytic properties for hydrogen evolution reaction (HER) in al- kaline medium. They also offer an important alternative to hard chromi- um coatings, as chromates are known to be highly toxic and carcinogenic [1]. Ni–Mo coatings can be applied as protective materials e.g. in the automotive and aviation industries, for parts of various ma- chines operating at elevated temperatures and aggressive environments as well as electrodes for HER and fuel cells [2,3]. It is known that pure mo- lybdenum cannot be separately electrodeposited from aqueous solutions, but it can readily be co-deposited with iron-group metals such as nickel to form an alloy. The phenomenon is called “induced co-deposition” [4]. Binary Ni–Mo coatings can be electrodeposited at room temperature from aqueous electrolytes despite a large difference between the melting points of these metals (Ni—1455 °C, Mo—2620 °C) and their limited mutual solubility. The induced co-deposition mechanism is still not clearly understood, even though several hypotheses are presented in the literature [4–12]. The electrodeposition of metal alloys is generally affected by the type of galvanic bath and its operating parameters. Currently, alloys are usually electrodeposited from complex electrolytes. The type and concentration of the organic additives has a considerable impact on the electrocrystallisation process of individual metals, and thus, on the chemical composition, microstructure and functional properties of alloy coatings. Non-toxic citric acid and its salts belong to the most promising complexing compounds used in galvanic baths, which allow a variety of electrodeposited coatings to be produced. Moreover, citrate aque- ous solutions are characterised by buffering, levelling and brightening properties [13], which allow the obtaining of good quality galvanic coatings [14]. These advantages are the reason why mixed sulphate solutions complexed with citrate ions were applied in the present study for Ni–Mo electrodeposition. Molybdenum(VI) and citric acid H 4 Cit (Cit = [C 6 H 4 O 7 ] 4 - ) form relatively stable complex species in aqueous solutions of various pH values, and the equilibria for possible complex formation reactions may be represented by the general equation [15]: pMoO 4 2 - +qHCit 3 - +rH + ↔[(MoO 4 2 - ) p (HCit 3 - ) q (H + ) r ] (2p + 3q - r)- . However, based on results of polarographic investigations of Mo (VI) reduction [16–22] in this presented study, it was determined that the most stable Mo(VI) species in neutral and weakly alkaline so- lutions are an electrochemically inactive MoO 4 2 - molybdate ion and Thin Solid Films 520 (2012) 2046–2051 ⁎ Corresponding author. Tel.: +48 12 295 28 29; fax: +48 12 295 28 04. E-mail address: nmbeltow@imim-pan.krakow.pl (E. Beltowska-Lehman). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.024 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf