Thin Solid Films 520 (2012) 2046–2051

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Kinetics of Ni–Mo electrodeposition from Ni-rich citrate baths

E. Beltowska-Lehman ⁎, P. IndykaPolish Academy of Sciences, Institute of Metallurgy and Materials Science, Reymonta 25, 30-059 Cracow, Poland

⁎ Corresponding author. Tel.: +48 12 295 28 29; faxE-mail address: nmbeltow@imim-pan.krakow.pl (E.

0040-6090/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.tsf.2011.10.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 November 2009Received in revised form 4 October 2011Accepted 14 October 2011Available online 20 October 2011

Keywords:Induced electrodepositionNi–Mo coatingsCitrate baths

The kinetics of Ni–Mo alloy electrodeposition on steel substrates from an aqueous citrate–ammonia complexbaths has been investigated by means of steady-state polarisation measurements in a system with a rotatingdisc electrode (RDE). Partial current densities for discharge of Ni(II) and Mo(VI) ions and hydrogen evolutionas a function of molybdate concentration in the bath, cathode potentials and the rate of mass transport weredetermined. It has been shown that – under all investigated conditions – Ni–Mo alloy deposition is morefavourable 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 examinedMoO4

2− content, a considerable decrease in the rate of the process is subsequently observed at certain limitpotentials, 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 overvoltagefor 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 concentrationin the bath, as well as an increase in the rate of mass transport, leads to an increase in Mo content in depositsand to the reduction of current efficiency. The Ni–Mo coatings electrodeposited from the designed bath (withthe 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 MoNi4 intermetallic phase.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Functional coatings containingmolybdenumare characterised byhighhardness, 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 andcarcinogenic [1]. Ni–Mo coatings can be applied as protective materialse.g. in the automotive and aviation industries, for parts of various ma-chines operating at elevated temperatures and aggressive environmentsaswell as electrodes for HER and fuel cells [2,3]. It is known that puremo-lybdenumcannot be separately electrodeposited from aqueous solutions,but it can readily be co-deposited with iron-group metals such as nickelto form an alloy. The phenomenon is called “induced co-deposition” [4].Binary Ni–Mo coatings can be electrodeposited at room temperaturefrom aqueous electrolytes despite a large difference between themelting points of these metals (Ni—1455 °C, Mo—2620 °C) and theirlimited mutual solubility. The induced co-deposition mechanism is stillnot clearly understood, even though several hypotheses are presentedin the literature [4–12].

: +48 12 295 28 04.Beltowska-Lehman).

rights reserved.

The electrodeposition of metal alloys is generally affected by thetype of galvanic bath and its operating parameters. Currently, alloysare usually electrodeposited from complex electrolytes. The typeand concentration of the organic additives has a considerable impacton the electrocrystallisation process of individual metals, and thus, onthe chemical composition, microstructure and functional propertiesof alloy coatings.

Non-toxic citric acid and its salts belong to the most promisingcomplexing compounds used in galvanic baths, which allow a varietyof electrodeposited coatings to be produced. Moreover, citrate aque-ous solutions are characterised by buffering, levelling and brighteningproperties [13], which allow the obtaining of good quality galvaniccoatings [14]. These advantages are the reason why mixed sulphatesolutions complexed with citrate ions were applied in the presentstudy for Ni–Mo electrodeposition.

Molybdenum(VI) and citric acid H4Cit (Cit=[C6H4O7]4−) formrelatively stable complex species in aqueous solutions of various pHvalues, and the equilibria for possible complex formation reactionsmay be represented by the general equation [15]: pMoO4

2−+qHCit3−+rH+↔ [(MoO4

2−)p(HCit3−)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 determinedthat the most stable Mo(VI) species in neutral and weakly alkaline so-lutions are an electrochemically inactive MoO4

2− molybdate ion and

2047E. Beltowska-Lehman, P. Indyka / Thin Solid Films 520 (2012) 2046–2051

an electroactive monomer [(MoO4)(HCit)H]4−, the concentration ofwhich decreases with the increase of the pH value [23,24].

It is impossible to deposit Ni–Mo alloys containing over 2 wt.% ofMo from the solutions that contain only MoO4

2− and Ni(II) ions with-out adding an appropriate organic compound. The obtained coatingsare of poor quality and contain high amounts of molybdenum oxides[25]. The formation ofmultimolecular heteropolymolybdates (NiMo6O24-

H64−)n, which are difficult to reduce, is the essential reason for this effect

[26]. When e.g. citrate ions are introduced into such solution, largeMoO42

−–Ni(II) clusters get decomposed, which is followed by complexing ofboth metals. However, to break down the clusters a suitable citrate con-centration must be added. An insufficient citrate addition restrains thecomplexing of nickel, since this ligand is preferentially consumed in theprocess of complexing with MoO4

2− and Ni–Mo co-deposition does notoccur [18,27]. At the same time, in aqueous solutions of pH>4 contain-ing only Ni(II) and citrate ions (without molybdate ions), the NiHCit−

complexes are themain nickel electroactive species [24,28]. After intro-ducing an appropriate amount of citrate ligand into solutions containingsalts of both metals (Mo, Ni), Mo(VI) is complexed first, which involvesthe formation of [(MoO4)(HCit)Hr](5− r)− type complexes, only then Ni(II) is complexed, which involves formation of NiHCit− [19]. Under anexcess addition of citrate, the Mo(VI) citrate complex remainsunchanged as [(MoO4)(HCit)Hr](5− r)−, while further coordination ofcitrate to NiHCit− formed inert NiH2Cit24− ions, which gradually be-come the predominant species [19]. Additionally, after introducing am-monia into Ni(II) citrate solutions, for pH>6, citrate and ammoniumcomplexes become the dominant forms, while the share of the latterin the system grows as the pH of the solution increases [29,30].

In the presentwork, the influence of the composition of theNi–Mocit-rate–ammonia electrolyte of two different pH values (8 and 11) on thebehaviour of partial polarisation was investigated under potentiostaticconditions bymeans of a rotating disc electrode system (RDE), which en-sure constant and controlled hydrodynamic conditions. The aim of thepresent study was a better understanding of the induced electrodeposi-tion process and optimisation of the plating bath parameters in order toobtain good quality metallic coatings containing relatively high Mo con-tent with a reasonable current efficiency.

2. Experimental details

TheNi–Mo coatingswere deposited electrochemically from a citrate–ammonia electrolyte, containing 0.2 M NiSO4, 0.3 M Na3C6H5O7, and0.003–0.096 M Na2MoO4. The pH of 8 and 11 was adjusted by addingammonia (0.1 M and 1.5 M respectively). The HySS2009 program forthe investigation of equilibria involving soluble and partially soluble spe-cies (developed by Protonic Software)was used in the calculations of theequilibrium composition of the solutions. The data referring to equilibri-umconstants of Ni(II) andMo(VI) adopted in the calculationswere takenfrom [24]. The electrolysis was carried out in a 0.75 dm3 cell in a systemwith a rotating disc electrode (RDE), supplied by the potentiostat PAR273A. All experiments were carried out at room temperature. The cath-ode was a low-carbon steel disc of 0.028 dm2, rotating between 11 and68 rad s−1, while a platinum sheet (0.05 dm2) was used as anode. Thecathode potentials were referred to the saturated calomel electrode(SCE) via Luggin capillary, located 1 mm from the disc and were cor-rected for ohmic drop by the current interrupt method. The chemicalcomposition and mass of the deposits were determined by the atomicabsorption spectroscopy (AAS) method using spectrophotometer UNI-CAM SP-90. The AAS resultswere complemented by the energy dispersiveX-ray spectroscopy (EDS) technique (EDAX GEMINI 4000 in the FEI E-SEMXL-30). On the basis of the chemical analysis, the quantity of electri-cal charges consumed for nickel andmolybdenum deposition was calcu-lated, assuming the six electron electroreduction process of Mo(VI) toMo(0). Partial current densities determined represent average valuesresulting from all possible changes of current during alloy electrodeposi-tion. Maximal relative error of determining the partial currents was

approximately 7%. The corresponding value for hydrogen evolution wasdetermined from the difference. X-ray diffractometry, using CuKα radia-tion (diffractometer Philips PW 1710), was used to identify the phasecomposition in the deposits. Themicrostructure of the depositswas stud-ied by the following techniques: transmission electron microscopy(TEM) (TECNAI G2 SuperTWIN) and scanning electron microscopy(SEM) (E-SEM FEI XL-30). For the sample SEM and TEM investigation20 kV and 200 kV electron beam energy was applied respectively. Thethin foils for TEM observations were prepared by the Focus Ion Beamtechnique (Quanta 3D FEI), under conditions of 30 kV accelerating voltageand current intensity in the range of 20 nA–0.1 nA.

3. Results and discussion

The cathodic process rate is expressed by the value of current den-sity (i). During the Ni–Mo deposition, the reduction (discharge) of Ni(II), Mo(VI) and H+ ions at the cathode occur at the same time. Theelectrode process may be characterised by polarisation curves i.e.the dependence of current density on electrode potential (i=f(E)).However, the total polarisation curve characterises a complex of phe-nomena occurring at the cathode and does not give information aboutthe process of discharge of individual metal ions. Hence, in presentstudy the method for resolving the total polarisation curve of alloydeposition into partial polarisation curves was used, which allows de-termining the rate of deposition of parent metals and the rate of hy-drogen evolution [4]. For this purpose, the partial currents werecalculated (from Faraday's law: m=k·Q, where m—mass, k—electro-chemical equivalent, Q—charge) based on the results of chemicalanalysis of samples deposited at various potentials (under steady-state potentiostatic conditions) and on quantities of electric chargespassed, assuming that the total current density is equal to the sumof the partial current densities (itot=iNi+iMo+iH). The correspond-ing value for hydrogen evolution (assumption: the only side cathodicreaction) was calculated from the difference between the total chargeQtot and quantities of electrical charges consumed for deposition ofindividual metals (QNi=mNi/kNi, QMo=mMo/kMo). The shape of thestationary cathode potential–current density curves is determinedby the slowest partial process of the electrodeposition reaction(transport of ions to the cathode, charge transfer, crystallisation). Inthe present study, experiments were carried out under laminar flowconditions, using the rotating disc electrode (RDE), which is uniform-ly accessible for mass transport (uniform diffusion layer thickness).Diffusion character of limiting current (ilim) determined experimen-tally can be confirmed by the Levich equation [31] derived for RDE(ilim~ω1/2) in the applied range of angular speed (ω) of disc cathoderotation.

The influence of the concentration of sodium molybdate on thechemical composition of Ni–Mo coatings was examined for a constantcontent of citrate ligand in solutions (0.3 M Na3C6H5O7, 0.2 M NiSO4,1.5 M NH3) of pH 8 and 11, containing MoO4

2− ions at concentrationsvarying within the range between 0.003 M and 0.096 M, under differ-ent hydrodynamic conditions, with the corresponding RDE rotationspeed between 11 and 68 rad s−1. The distributions of Ni(II) species,calculated for initial concentration of the ingredients of the investi-gated bath of pH 8 and 11, are as follows: pH 8—50.4% NiH2Cit24−,20.5% Ni2Cit24−, 22.0% NiHCit−, 7.1% NiOH+; pH 11–0.8% NiH2Cit24−,6.9% Ni2Cit24−, 0.4% NiOH+, 91.9% Ni(NH3)n2+.

Fig. 1 shows the dependence of the partial current density of Ni(II)reduction process on the cathode potential and the concentration of so-dium molybdate in the solution of pH 11, recorded at 11 rad s−1. Thesedependences lead to the conclusion that the Ni(II) electroreduction rateinitially increases (in determined hydrodynamic conditions), as the cath-ode potential shifts towards more negative values and the molybdateconcentration grows in the solution. However, for the highest examinedMoO4

2− content (0.024 M and 0.096 M), a considerable decrease in theprocess rate is subsequently observed at certain limit potentials of less

1.1 1.2 1.3 1.40

1

2

3

4

5i N

i, A d

m-2

-E, V/SCE

0.003 M 0.006 M 0.012 M 0.024 M 0.096 M

Fig. 1. Steady-state partial polarisation curves for Ni(II) ions discharge recorded at11 rad s−1, for increasing molybdate concentrations (0–0.096 M) in the bath of pH 11.

1.1 1.2 1.3 1.40.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 M 0.003 M 0.006 M 0.012 M 0.024 M

-E, V/SCE

i Ni,

A d

m-2

Fig. 3. Steady-state partial polarisation curves for Ni(II) ions discharge recorded at11 rad s−1, for increasing molybdate concentrations (0–0.024 M) in the bath of pH 8.

2048 E. Beltowska-Lehman, P. Indyka / Thin Solid Films 520 (2012) 2046–2051

and less negative values, as the Mo(VI) concentration increases in theelectrolyte. The relations determined for partial current densities of Mo(VI) discharge are shown in Fig. 2. The observed effects result mainlyfrom the fact that all Mo(VI) ions could be reduced to metallic molybde-num, when the molybdate concentration in the electrolyte is relativelylow and the deposition rate of the stimulating metal (Ni) is sufficientlyhigh. At a higherMoO4

2− concentration, in turn, a greater amount of mo-lybdate ions reaches the cathode surface and only some of them are re-duced to metallic Mo. The rest is deposited as multivalent oxides thatblock the electrode surface, which results in a decrease of the depositioncurrent densities of nickel andmolybdenum. It should be stressed that atpH 11, nickel is deposited simultaneously with molybdenum even in abath containing 0.096 M of sodium molybdate. At this concentration,the Ni–Mo deposition process was totally inhibited in the solution ofpH 8 (Fig. 3) [32]. An increase in the pH of the bath from 8 to 11 leadsto an increase of the Ni deposition rate. The considerably higher valuesof the partial current density of Ni(II) electro-reduction process for thesame values of the cathode potential were observed. Already at a lowdisc rotation speed (11 rad s−1), the maxima of the i–E curves corre-sponding to Ni(II) reduction process in a bath of pH 11 appears only forthe highest examined concentrations of sodium molybdate (0.024 Mand 0.096 M) (Fig. 1) and this occurs at more negative cathode poten-tials, compared to analogous baths of pH 8 (Fig. 3). The chemical compo-sition of the alloy is determined by the ratio of the rates of the tworeduction reactions. This can explain the decrease of the Mo (orW) con-tent in the Ni–Mo (or Ni–W) alloy accompanied by simultaneous in-crease of the cathodic current efficiency, as observed with an increasein the ammonium ligand concentration [8,30].

1.1 1.2 1.3 1.40.0

0.2

0.4

0.6

0.8

1.0

-E, V/SCE

i Mo, A

dm

-2

0.096 M 0.024 M 0.012 M 0.006 M 0.003 M

Fig. 2. Steady-state partial polarisation curves for Mo(VI) ions discharge recorded at11 rad s−1, for increasing molybdate concentrations (0–0.096 M) in the bath of pH 11.

As could be expected (in a solution of pH 11), after the increase inthe rate of mass transport of Ni(II) ions to the surface of the disc elec-trode obtained by increasing RDE rotation speed (up to 68 rad s−1),practically no influence of the Na2MoO4 concentration on the Ni(II)discharge process is observed (Fig. 4). The effect of the cathode surfaceblocking (by molybdenum compounds of lower valences) disappears.This is due to the fact that under these conditions, the rate of the nickeldeposition process (partly controlled by diffusion) increases.

The partial process of molybdenum deposition from a bath of pH11 is characterised by similar dependences as those observed fornickel. Thus, at low rotation speeds of the disc electrode, the maximaof the partial polarisation curves of the Mo(VI) discharge process areconsiderably lower and flatter than in the case of pH 8 (Figs. 2 and 5).Moreover, they do not appear when the cathode rotation frequency ishigher (Fig. 6).

For the Mo deposition process at the potentials more negativethan −1.3 V/SCE (pH 11), a linear dependence of the partial currentas a function of the square root of the electrode rotation speed is ob-served (Fig. 7). The fact that the straight lines determined for all mo-lybdate concentrations in the solution pass through the origin of thecoordinate system confirms that the cathode surface is not blocked.For pH 8, they fail to pass through the point in the case of higherMoO4

2− concentrations (Fig. 7).The observed differences in the rates of deposition of nickel and

molybdenum from baths with varying content of sodium molybdateand pH values (8 and 11) are associated mainly with the change ofNi(II) complex types and of their concentrations. In the bath of pH11 electroactive Ni(II) ammonium complexes with a positive charge

1.1 1.2 1.3 1.40

2

4

6

8

10

0.024 M 0.012 M 0.006 M 0.003 M 0.096 M

-E, V/SCE

i Ni, A

dm

-2

Fig. 4. Steady-state partial polarisation curves for Ni(II) ions discharge recorded at68 rad s−1, for increasing molybdate concentrations (0–0.096 M) in the bath of pH 11.

1.1 1.2 1.3 1.40.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.024 M 0.012 M 0.006 M 0.003 M

i Mo, A

dm

-2

-E, V/SCE

Fig. 5. Steady-state partial polarisation curves for Mo(VI) ions discharge recorded at11 rad s−1, for increasing molybdate concentrations (0–0.024 M) in the bath of pH 8.

0 1 2 3 4 5 6 7 8 90.0

0.3

0.6

0.9

1.2 pH 11

0.096 M 0.024 M 0.012 M 0.006 M 0.003 M

pH 8 0.024 M 0.012 M 0.006 M 0.003 M

i Mo, A

dm

-2

rad1/2s-1/2

Fig. 7. Effect of electrode rotation speed on the partial current density for Mo(VI) dis-charge in Ni–Mo codeposition at −1.3 V/SCE from solutions containing different mo-lybdate concentrations of pH 8 and 11.

2049E. Beltowska-Lehman, P. Indyka / Thin Solid Films 520 (2012) 2046–2051

are the dominant forms. They undergo electroreduction on the cath-ode more easily than Ni(II) citrate complexes with a negative charge(e.g. [Ni(HCit)2]4−, [Ni2Cit2]4−and [NiHCit]−) that are dominant inthe solution of pH 8 [32]. Therefore, the process of nickel depositionon the steel electrode from electrolyte solutions of pH 8 occurs atmore negative potentials than in the case of solutions of pH 11.

For separate nickel deposition from analogous solutions with pH 11,the partial current density related to hydrogen evolution is relativelylow and does not depend on the rotation speed of the disc electrode.On the other hand, when the bath contains Na2MoO4, the characteristicmaximumappears in the polarisation curve corresponding to the processof hydrogen evolution within the range of potentials that precede thealloy deposition potential. The maximum increases as the concentrationof molybdate ions grows in the bath and the thickness of the diffusionlayer is reduced. Thus, the maximum is shifted towards the more nega-tive values of the cathodepotential (Fig. 8). The observed effect can be re-lated to the progressive blocking of the cathode surface by lower valencymolybdenum compounds or to the increased thickness of such deposit. Itconfirms the hypothesis that the induced co-deposition is a multi-stageprocess and its first stage consists in the incomplete reduction of molyb-date ions. It should be stressed that unlike MoO3 oxide characterised byhigh resistance (band gap=2.9 eV), lower valency Mo oxides, whichare semiconductors with n-type electrical conductivity, have electronicproperties close to metals (energy gap≈0). Their conductivity wasfound to increase as the Mo oxidation state is reduced [21,33,34].

The obtained results and data from other works [7,9,11,35–37]constitute a basis for proposing further stages of the process of in-duced electrodeposition of Ni–Mo alloys from citrate baths. As for

1.1 1.2 1.3 1.40.0

0.5

1.0

1.5

2.0 0.096 M 0.024 M 0.012 M 0.006 M 0.003 M

i Mo, A

dm

-2

-E, V/SCE

Fig. 6. Steady-state partial polarisation curves for Mo(VI) ions discharge recorded at68 rad s−1, for increasing molybdate concentrations (0–0.096 M) in the bath of pH 11.

separate molybdate solutions, in a citrate bath for Ni–Mo deposition,at low polarisation, the partial current of the stimulating metal (Ni) isvery low and the electroactive Mo(VI) ions undergo only partial re-duction:

MoO4ð Þ HCitð ÞH½ �4� þ H2Oþ 2e→MoO2 þ HCit3� þ 3OH�:

Due to this reaction, part of the cathode surface gets covered withmolybdenum oxides/hydroxides (MoO2 in the greatest amounts [37])characterised by strong electrocatalytic properties in the process of hy-drogen evolution, that is the major reaction at potentials less negativethan about −1.2 V/SCE. Fig. 9 presents the SEM image of depositobtained in the cathode potential range corresponding to themaximumof hydrogen evolution. The chemical composition evaluated by the EDSmicroanalysis was as follows: 13.6±0.5 wt.% Mo (4.3 at.%), 36.3±0.7 wt.% O (69.5 at.%), and 50.1±1.0 wt.% Ni (26.2 at.%). The depositsobtained in such conditions revealed the substantial oxygen amount. Itis obvious that such a high content of oxygen is due to formation of Niand/or Mo oxides during the deposition process [38].

The rise in molybdate concentrations in the bath and an increase inmass transport rate lead to progressive blocking of the cathode surfaceby the insoluble semiconducting (n-type) mixed-valence molybdenumoxide film of increasing thickness. With increasing polarisation the par-tial current for hydrogen evolution rapidly decreases whereas the Ni(II)discharge rate increases. It is probable that (under these conditions)with the participation of Ni(II) ions, an intermediate (mixed oxide) prod-uct e.g. MoO2Ni4 (according to [7]) (MoO2+4Ni2++8e=MoO2Ni4) isformed, which subsequently undergoes reduction to the alloy. Assuming

1.1 1.2 1.3 1.4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

i H2, A

dm

-2

-E, V/SCE

0.096 M (68 rads-1)

0.096 M (11 rads-1)

0.024 M (11 rads-1)

0.012M (11 rads-1) 0 M (11 rads-1)

Fig. 8. Partial polarisation curves for hydrogen evolution during Ni–Mo deposition fromsolution of pH 11 containing different molybdate concentration recorded at 11 and68 rad s−1.

Fig. 9. SEM image of the deposit (scraped from the substrate) obtained at−1.15 V/SCEby electric charge of 100 C (0.2 M NiSO4, 0.3 M Na3C6H5O7, 0.012 M Na2MoO4, pH 8,68 rad s−1).

1 μμm

10 μm

200nm

Fig. 10. SEM morphology with SEM and TEM cross-section images of typical as-deposited Ni–Mo coating (30 wt.% Mo) deposited at −1.35 V/SCE (0.2 M NiSO4,0.3 M Na3C6H5O7, 0.096 M Na2MoO4, pH 11, 68 rad s−1).

35 40 45 50 55 60 65 70 75 80

1000

1500

2000

2500

3000

3500

4000

400

MoN

i 4+

200

α-F

e

312

MoN

i 4

002

MoN

i 4

310

MoN

i 4

220

MoN

i 4 + 1

10 α

-Fe

121

MoN

i 4

Inte

nsi

ty [

cou

nts

]

2 Theta [deg]

Fig. 11. XRD (CuKα) and TEM diffraction patterns of Ni–Mo coating (30 wt.% Mo), de-posited at −1.35 V/SCE (0.2 M NiSO4, 0.3 M Na3C6H5O7, 0.096 M Na2MoO4, pH 11,68 rad s−1).

2050 E. Beltowska-Lehman, P. Indyka / Thin Solid Films 520 (2012) 2046–2051

such a compound is formed, it could be expected that the following over-all reaction occurs during the process:

MoO4ð Þ HCitð ÞH½ �4� þ 4Ni2þ þ 3H2Oþ 14e¼ Moþ 4Niþ 7OH� þHCit3�:

The reduction process of the intermediate product could be alsostimulated by nickel deposition on the cathode surface and the relat-ed changes in the Fermi level in the substrate. At more negative po-tentials nickel deposition becomes the predominant reaction andhydrogen evolution increases again as a side reaction: 2H2O+2e=H2↑+2OH−. On an unblocked cathode surface, nickel is alsodeposited in an independent reaction, for pH 11 mainly from ammo-nium complexes of the [Ni(NH3)n]2+ type:

Ni NH3ð Þ3� �2þ þ 2e ¼ Niþ 3NH3↑

and for pH 8 mainly from the electroactive citrate complex:

NiHCit� þ 2e ¼ NiþHCit3�:

The Ni–Mo coatings (containing max. about 30 wt.% Mo) electrode-posited in such conditions are characterised by a shiny-grey appearanceand good adhesion to the steel substrate. The Ni–Mo coatings are alsocharacterised by columnar growth, regular, globular morphology andamorphous microstructure with randomly distributed nanocrystallitesof the intermetallic compound MoNi4, about 5–30 nm in size (Figs. 10and 11). Similar microstructures have also been observed in Ni–Mo al-loys electrodeposited from weakly acid baths of pH 4 [39].

4. Conclusions

It has been shown that the deposition of nickel from citrate–am-monia solutions containing molybdate ions (with the concentrationunder a determined limit value) occurs with a higher rate, comparedto its separate deposition, which means that the formation of Ni–Moalloys is energy favourable. The increase in the rate of all partial ca-thodic processes (Ni and Mo deposition, hydrogen evolution), as therotation speed of the disc electrode increases, demonstrates signifi-cant influence of the mass transport to the cathode on the inducedprocess of Ni–Mo alloy deposition.

The multi-step mechanism of induced co-deposition is confirmedby the observed maximum of hydrogen evolution at low polarisationand by the effect of the decrease of nickel deposition rate as a certainlimit threshold of molybdate concentration in the bath is exceeded.The first stage of the mechanism consists of an incomplete reductionof the electroactive citrate complex of Mo(VI) to Mo oxides of lowervalency, characterised by a low overvoltage for hydrogen evolution.Due to the increasing resistance of the incompletely reduced Mofilm (I stage) only to its limited thickness, the intermediate – e.g.mixed oxide involving both metals: Ni and Mo – could be formedon the cathode (II stage), which is later, with increasing polarisation,reduced to Ni–Mo alloy (III stage). The deposition process is totallyinhibited after exceeding such a limited thickness and hydrogen evo-lution is the only cathodic reaction.

Furthermore, it has been shown that as the pH of the bath for theNi–Mo electrodeposition is increased to exceed 8 (which is realisedby adding ammonia), the rate of Ni deposition clearly increases,which is related mainly to a change in the type of Ni(II) complexesand their concentration. In a bath of pH 11, positive Ni(II) ammoniacomplexes are much more preferred to electroreduction on the cath-ode than negatively-charged citrate complexes of Ni(II) that are dom-inant in solutions of lower pH. Thus, the ratio of the rates of theelectroreduction of Ni(II) and Mo(VI) ions (this ratio determines thealloy composition) is much higher at pH 11 and alloys obtained insuch conditions contain less molybdenum (up to about 30 wt.%),but are deposited with a higher current efficiency.

2051E. Beltowska-Lehman, P. Indyka / Thin Solid Films 520 (2012) 2046–2051

The designed stable citrate–ammonia bath containing consider-able excess of Ni(II) relative to Mo(VI) content allows electrodeposit-ing high quality protective Ni–Mo coatings (metallic, homogenous,smooth, compact, adhering well to steel substrates), characterisedby amorphous microstructure with randomly distributed nanocrys-tallites of the intermetallic compound MoNi4.

Acknowledgements

The results presented in this paper have been obtained within theproject “KomCerMet” (contract no. POIG.01.03.01-14-013/08-00 withthe Polish Ministry of Science and Higher Education) in the frame-work of the Operational Programme Innovative Economy 2007–2013.

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