Surface and CoatingsTechnology,31 (1987)343 - 349 343

ELECTRODEPOSITIONOF NICKEL FROM N,N-DIMETHYL-FORMAMIDE

H. K. SRIVASTAVA andP. K. TIKOO

Department of Chemistry,Banaras Hindu University, Varanasi221005(India)

(ReceivedJune23, 1986)

Summary

The electrodepositionof nickel from nickel chloride dissolvedin pureN,N-dimethylformamide(DMF) was carried out. The presenceof a littlehydrochloric acid (specific gravity, 1.18) was essentialfor electrolysistooccur. Cathodecurrent efficienciesas high as 95% - 100%wererecorded.A bright depositwith a cathodecurrentefficiency of 97.2%was obtainedunder operatingconditionsof pH 4.7, temperature40 °Candcurrentdensity0.5 A dm2. An NiCl

2 6H20-DMF electrolyteresultedin minimum evolu-tion of hydrogengasduringelectrodeposition.X-ray analysisconfirmedthedepositionof nickel in its purestform with anf.c.c. lattice.A smallconcen-tration of Co

2’ played a vital role in enhancingthe cathodecurrent effi-ciency as well as significantly improving the hardness.Scanningelectronmicrographsrevealedafine-grainedstructurevoid of crackswhenthe coatingwas deposited in the presenceof an optimum concentrationof cobaltchloride.

1. Introduction

Electrolytesdissolvedin waterandusedfor platingallow hydrogenionsto dischargewith the platingcations.Excesshydrogenthat is evolveddisturbsthe plating rate,decreasesthe cathodecurrent efficiency andunfavourablyaffectsthe microstructureof the depositand otherpropertiessuch as hard-ness, brittleness,ductility and tensile strength.Preferentialselectionof anon-aqueoussolventenhancesthe probability of ion solvation—ioncomplexformation owing to differentdielectricalproperties,the differentnatureofhydrogenbonding,alteredion size andresonance.

Tákei extensivelystudied[1] the electrodepositionof nickelandcopperfrom organicsolvents,particularlyfrom atrifluoroacetate—amidebath.Elec-trodepositionof copper,cadmium,zinc, cobaltandnickel from perchloratesolutions in dimethylformamide(DMF), CH

3OH and CH3CN have beenstudied[2]. The electrodepositionof manganeseandaluminiumfrom forma-mide anddimethylaniinerespectivelyhasbeencarriedout by Bar.teckiand

0257-8972/87/$3.50 © ElsevierSequoia/Printedin The Netherlands

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Inger-Stocka[3]. Dimethyl suiphoxide(DMSO) hasbeenusedas apotentsolventfor electrodepositionof nickelby Biallozoretal. [4]. Electrodeposi-tion of manganesefrom MnCI2—DMSO andMnC12-DMF bathshasalso beenstudied [5]. Comparativelyimproveddepositsof nickel from ethyleneglycolhave been reportedby Dill [6]. Beryllium, lithium tungstenand molyb-denum,which cannotbe depositedin aqueoussolutions,havebeensuccess-fully plated in organicsolvents [7]. Likewise, aluminium,which cannotbeplatedfrom aqueousbaths,hasbeensuccessfullyplatedfrom ethylbenzene[8, 9] andtetrahydrofuran[10].

In the presentpaperwe describeour studyof the physicalcharacteris-tics of nickel electrodepositedfrom its chloride salt dissolvedin pureDMF.

2. Experimentaldetails

Copper strips of area 4 cm2 (2 cm X 2 cm) and thickness1 mm were

used as the cathode,the basemetalon which nickel waselectrodeposited.Thesestripswere first ground to obtain asmoothsurface.A mirror-imagesurfacewithout stains or pits wasobtainedafter discpolishing. Degreasingwas carriedout by cleaningin hot soapsolution (Idipol), electrolyticclean-ing andsolventcleaning.The oxide layerwasremovedby pickling with 10%sulphuric acid. Following each operation, the copperplate was cleanedusing cold and hot double-distilled water. High purity nickel sheetswereused as anodes.The DMF was purified usingamethoddescribedby Searset al. [11]. A copper-stripcathodewas placedbetweentwo parallelanodesandan interelectrodedistanceof 2 cm wasmaintainedthroughouttheexper-iment. A coppercoulometerwasusedto measurethecurrent.The operatingconditionsaregiven in Table 1.

The microhardnessof nickelwasmeasuredon a TukonWilsonhardnesstesterusinga136°diamondpyramid indenterwith an appliedloadof 25 gf.The durationof electrolysiswas 180 mm for thosecoatingsusedfor hard-nessmeasurements,and the valuesfor hardnessare the averageof severalvaluestakenat differentlocations.

TABLE 1

Bath compositionandconditionsof electrolysisin pureDMF

Nickel chloride(M) 0.2(NiC1

2-6H20)pH (variedby addingHC1) 1.7 - 4.7Currentdensity(A dm

2) 0.15 - 0.5Temperature(°C) 20 - 70Cobaltchloride(M) 0.02 - 0.10

CoC12 6H20

Durationof electrolysis(mm) 60Agitation nil

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3. Resultsanddiscussion

NiC12 ~6H2Odissolvedin pure DMF doesnot leadto electrodepositionunless some HC1 is added. The absenceof electrodepositionis probablybecausethe secondarynitrogen atoms of DMF coordinate through thecentral nickel ion, therebyforming aNiC12 XDMF complex.Complexesoftransition metals with amides,coordinatedthrough nitrogen, have beenreported [12]. When HC1 is added,the DMF of the complexis released.Aone-to-onecomplex of DMF:HC1 was reportedby Richards [13]. In thisprocesssomenickel ions are freedfor ionization.

The pH is usually varied by the additionof HC1 in the caseof nickelchloride,andby H2SO4 in the caseof nickel sulphate.WhenDMF is usedasa solvent,nickel chloride is the bestelectrolyteas H2SO4 reactswith DMFwhereasHC1doesnot [14].

3.1. Effect ofpHAt a current density(CD) of 0.15 A dm

2 andatemperatureof 40 C,the cathodecurrent efficiency(CCE) decreasedsharplywith increasingpH.However, the CCE increasedwhen the pH was increasedfrom 3.8to 4.7, asindicatedin Table 2.

TABLE 2

Characteristicsof nickelelectrodepositedfrom NiC12 6H20—DMF bathat currentdensity

0.15 A dm2 andtemperature40 °C

pH Cathode Natureof depositcurrentefficiency(%)

1.7 99.7 Blackishat edges,greyishnearedgesandsemi-brightat centre

2.7 87.1 Grey3.3 36.6 Grey3.8 30.0 Dull4.7 39.0 Light greyish

3.2. Effect of current densityThe effect of current densityon the natureof the electrodepositand

on the cathodecurrent efficiency was determinedand the resultsare pre-sentedin Table 3. An optimumcurrent densityof 0.5 A dm2 producedabright depositwith a high cathodecurrent efficiency of 97.2%. However,low hardnessvalueswereobtainedin all cases,asindicatedin Table3.

3.3. Effect of temperatureThe influenceof temperatureon cathodecurrentefficiency, natureof

deposit, average theoretical thickness and diamond pyramid hardnessis

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TABLE 3

Characteristicsof nickel electrodepositedfrom NiCl2~6H20—DMF bath at pH 4.7 andtemperature40 °C

Current Cathode Nature of Hardness Averagedensity current deposit (VPN)~ theoretical(A dm~

2) efficiency thickness(%) (.im)

0.15 39.0 Light greyish 155 2.50.30 67.2 Hazybright 182 3.70.40 86.4 Semi-bright 164 4.40.50 97.2 Bright 157 5.8

aVickerspyramidnumber.

TABLE 4

Characteristicsof nickel electrodepositedfrom NiCI26H2O—DMF bath at pH 4.7 and

currentdensity 0.5 A dm2

Temper- Cathode Nature of Hardness Averageature current deposit (VPN) theoretical(°C) efficiency thickness

(%) (bLm)

20 75.0 Mirror imagea 147 4.830 88.2 Too brightb 152 5.340 97.2 Bright 157 5.850 80.2 Hazy bright 255 6.260 96.6 Grey 164 4.470 88.6 Blackish 157 5.8

apeelingeffect.bMild peelingat edges.

shown in Table 4. Cathodecurrent efficienciesvaried between75.0% and97.2% with no clear correlation with temperature.Hardnessvalueswerefairly constantfor coatingsproducedat all temperatures,except for thatdepositedat 50 °Cwhich wassignificantly harder.

3.4. Effect of Co2~ionsTable 5 shows the results of adding different concentrationsof Co2~

ions to the solution.Thehardnessrosesteeplyon increasingthe cobaltchlo-ride concentrationup to 0.06 M, at which point it hadamaximumvalueof676 VPN. The high valueof hardnessmay be attributedto the presenceofcobalt ions, which facilitate the dissociationof the NiCl

2 XDMF complex.The rate constantfor solventexchangeof the Co

2~ion [15] is much higherthanthat for theNi2~ion [16] in DMF.

Althoughthe tracesof cobaltions presenthadno significantinfluenceon conductivity or viscosity, therewasamarkedinfluenceon anodepoten-

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TABLE 5

Characteristicsof nickel electrodepositedfrom NiCl2 6H20—DMF, HC1, CoCI2 . 6H20bathat currentdensity 0.5 A dm

2 andtemperature60 °C

Cobalt Nature of Cathode Hardness Averagechloride deposit current (VNP) theoreticalconcentration efficiency thickness(M) (%) (pm)

0.02 Greyish 93.0 504 7.00.04 Cornersbright and 95.8 611 6.8

centralportion greyish0.06 Bright regionextends 95.1 676 6.90.08 Greyishregionextends 98.5 372 7.60.10 Brightnessdiminishes 98.4 232 6.9

tial [17]. It was concludedthat the presenceof a smallamountof cobaltchloride did not severelyaffect the compositionof the electrodepositbe-cause the polarographichalf-wave potential for reduction of Co2~ion ismuch lower thanthatof Ni2~ion in DMF [18].

3.5. X-ray analysisX-ray examinationof electrodepositedmaterial from the nickel chlo-

ride—DMF bathat CD 0.5 A dm2, pH 4.7 and temperature40 °Cgaveonestrongly diffused line at adistanceof 228.20and two weaklydiffused linesat distancesof 243.10 and 291.90 from the main zero line position of139.45. For Cu Ka radiation (A = 1.5405) and for first-order reflectionmaxima(n = 1), the Braggequationd = nA/(2 sin 0) gavetheratio of spacingas

d111:d2~:d220= 2.0396:1.7625:1.2479

which aggreeswell with the literaturevaluefor nickel, which is aface-centredcubic crystal. The latticeparameterwas calculatedto be 3.5299,which is incloseagreementwith the literaturevalueof 3.5230.

The analysis led to the conclusionthat the electrodepositedmaterialwas not a clathrateor acomplexcompoundof nickel but waspure nickelwith an f.c.c. lattice.

3.6. Microstructureof electrodepositsScanningelectronmicrographyof the electrodepositsat pH 4,7 andCD

0.5 A dm2 revealeda fine-grainedstructureat and above40 °C.However,

microcracksobservedat 50 °Cmaybe causedby the high internal stressesdevelopedas a resultof denseandcomparativelyharderdeposits.At 60 °Candabove,lamellar,crack-freestructureswereobtained(Fig. 1). The micro-cracks observedat certain typical portions of edgesmay arise becauseahigher current density always exists at edges (Fig. 2). Scanningelectronmicrographsof nickel electrodepositedin the presenceof cobalt chloriderevealedafine-grainedstructure(Fig. 3),evenat the edges.

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Fig. 1. Scanningelectronmicrographof nickel electrodepositedat currentdensity0.5 Adm2, pH 4.7 andtemperature60 °C(centralportion).

Fig. 2. Scanningelectronmicrographof nickel electrodepositedat currentdensity 0.5 Adm~2,pH 4.7 andtemperature60 °C(at edge).

~ L0um~

Fig. 3. Scanningelectronmicrographof nickel electrodepositedin presenceof 0.06 Mcobalt chloride at current density 0.5 A dm

2 andtemperature60 °C(neartheedge),shownat two differentmagnifications.

4. Conclusions

(i) Bright depositsof nickel can be obtainedusingthe following solu-tion andparameters

NiC126H2O(0.2M)—DMF, HC1, at pH 4.7,40 °Cand0.5 A dm

2

(ii) Harddepositsof nickel of about676 VPN canbe obtainedusingthefollowing solutionandparameters

NiCl2~6H2O(0.2M)—DMF, HC1, CoCI26H2O(0.06M), at 60 °Cand 0.5 A

dm2

Acknowledgments

We aremost thankfulto Prof.S. M. Verma,Headof theDepartmentofChemistry, for providing the necessaryfacilities. We are also thankful to

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Dr. G. V. S. Shastryand Mr. S. N. Lal for their help in measuringhardnessand preparingscanningelectronmicrographs.One of us (H.K.S.) gratefullyacknowledgesa teachingresearchfellowshipawardsponsoredby theUniver-sity GrantsCommission,India.

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