Nanocry stalline nickel (nc-Ni) coatings were produced by pulse electro deposition using Watts bath with sodium citrate and saccharin added as grain refining agents. The electrochemical nature of nc-Ni coatings, evaluated in 1M H2SO4 solution by electrochemical impedance spectroscopy. The corrosion rate of bulk nickel was lower than that of nc-Ni after stabilisation of free corrosion potential.Defence Science Journal, 2008, 58(4), pp.525-530, DOI:http://dx.doi.org/10.14429/dsj.58.167

Balasubramaniam, R.

Gupta, Naveen

Sachdeva, Deepika

Defence Science Journal

English

Received 14 May 2007, revised 28 September 2007Defence Science Journal, Vol. 58, No. 4, July 2008, pp. 525-530Ó 2008, DESIDOC525Electrochemical Characterisation of Nanocrystalline NickelDeepika Sachdeva, Naveen Gupta, and R. BalasubramaniamIndian Institute of Technology, Kanpur-208 016ABSTRACTNanocrystalline nickel (nc-Ni) coatings were produced by pulse electrodeposition usingWatts bath with sodium citrate and saccharin added as grain refining agents. The electrochemicalnature of nc-Ni coatings, evaluated in 1M H2SO4 solution by electrochemical impedancespectroscopy. The corrosion rate of bulk nickel was lower than that of nc-Ni after stabilisationof free corrosion potential. Keywords:Nickel, electroplating, corrosion, electrochemical impedance spectroscopy, nanocrystallinenickel1 . INTRODUCTIONNanocrystalline materials are being activelystudied because of their unique combination ofproperties and their possible applications in severalareas of modern technology. Electrodeposition isone of the most commercially applicable techniquesfor synthesis of nanocrystalline metals and alloys.Pulse electrodeposition allows high deposition currentdensities in contrast to the direct current depositionroute1. This is possible due to the replenishmentof electrolyte at the electrolyte-substrate interfacereplenishment during the off time periods2, thusallowing current densities even higher than thelimiting current density. This establishes conditionsof large overpotential and low surface diffusionrates, which are typically associated with highernucleation rates and suppressed grain growth rates1.Grain sizes of nano-scale can therefore be obtainedwith increased nucleation rates in pulse electrodeposition.Therefore, pulse electrodeposition route was chosenfor the synthesis of nanocrystalline nickel (nc-Ni)in the present study.Pulse electrodeposited nc-Ni can serve as corrosionand wear resistant coating3. The corrosion propertiesof nc-Ni have also been investigated4,5. Rofagha4,et al. investigated the corrosion behaviour of  nc-Ni (99.99 % purity, 32 nm grain size) and coarsegrained nickel (99.99 %, 100 µm grain size) in 1M H2SO4.They found that the corrosion potential of nc-Niwas more noble compared to that of polycrystallinenickel. Mishra & Balasubramaniam5 have confirmedthese results for nc-Ni. The passive current densities,determined from potentiodynamic polarization curves,of nc-Ni were higher than that of pure bulk Ni4,5.Interestingly, the corrosion rate of nc-Ni decreasedwith decreasing grain size based on polarisationexperiments conducted immediately on immersion5.This was related to a more uniform distribution ofsulphur by pulse electrodeposition method, as hasbeen found for nc-Co. Zeiger7, et al. reportedenhanced corrosion resistance of nc-Fe-8 per centAl in Na2SO4 solution (pH=6), which was attributedto protective oxide film formation due to fast diffusionof Al through the grain boundaries.526DEF SCI J, VOL. 58, NO. 4, JULY 2008The purpose of the present study was to understandthe nature of surface of nc-Ni that forms a passivefilm in 1M H2SO4, using electrochemical impedancespectroscopy (EIS) technique. Rather than lookingat conditions immediately on immersion, the studywas focused on corrosion behaviour after stabilisationof the free corrosion potential. The nc-Ni wasproduced by pulse electrodeposition using Watt’sbaths containing two different grain refining agents,as it is known that different grain refining agentsproduce fine grained deposits in electroplating8-11.The basic aim was to understand how the surfacesof nc-Ni behave when compared with bulk Ni onprolonged exposure to a corrosive environment.2 . EXPERIMENTAL PROCEDURESThe  nc-Ni deposits were obtained on coppersubstrate by pulse electrodeposition. Copper is themost commonly used substrate for studies on nickelplating, primarily due to structural compatibility betweencopper and nickel leading to better adhesion ofelectrodeposits. Copper strips (17 mm x 60 mm x1 mm) were polished using grades 0/0, 2/0, 3/0, 4/0 emery papers and finally degreased with acetonebefore the electrodeposition experiments. The platedarea was of the size 25 mm x 17 mm. Differentanodes of pure nickel, titanium and Pb-Sn alloywere initially tried for the electrodeposition2. ThePb-7 wt per cent Sn alloy was found to be mostsuitable anode, as it was insoluble and could bereused for several experiments. Plating was performedonly on the surface facing the anode. The othersurface was coated with enamel to render it electricallyinsulating. Pulse electrodeposition was performedwith a potentiostat (model 263A, Princeton AppliedResearch (PAR), USA).The parameters used in the pulsing experimentswere ton = 5 ms and toff = 20 ms, as per the procedureadopted earlier by Mishra and Balasubramaniam5.The total time for each experiment was 150 minwith cumulative deposition time of 30 min. Thebath was maintained at 45 °C – 0.01 °C using aconstant temperature bath (Julabo F-32, Germany)along with continuous stirring during electrodepositionto avoid concentration gradients. Each electrodepositionexperiment was performed with a fresh bath. Thebath compositions were: nickel sulphate (NiSO4.6H2O)-240 gm/l, nickel chloride (NiCl2 6H2O)-30 gm/l,and boric acid (H3BO3)-30 gm/l. Two differentgrain refining agents were used separately in thebath: sodium citrate 25 gm/l in one bath and saccharin10 gm/l in the other.The x-ray diffraction (XRD) patterns wereobtained from the electrodeposited surfaces in aRich Seifert 2000D diffractometer using Cu Karadiation. Instrumental broadening was calculatedusing a standard annealed microcrystalline nickelsample of grain size 61 µm. The grain size wasdetermined using Scherer equation and the procedureis described elsewhere5.Electrochemical testing of bulk nickel, copper(substrate) and electrodeposited nickel was performedin order to compare with the electrochemical behaviourof nc-Ni coatings. Electrochemical studies on bulkNi were performed in a round bottom electrochemicalpolarization cell. The bulk Ni sample was mountedin epoxy after providing electrical connection inthe backside. The sample was polished using 1/0, 2/0, 3/0, 4/0 emery papers and cleaned withacetone prior to each test. No mechanical polishingwas performed on the electroplated nc-Ni. Forelectrochemical testing of copper and electroplatednickel, a flat cell was used. The electrolyte usedin all the experiments was freely-aerated 1M H2SO4.Duplicate experiments were conducted and theresults were reproducible.The EIS studies were conducted at free corrosionpotential (FCP) after stabilisation of potential. Thescan was conducted in the frequency range of 100kHz to 10 mHz at a sinusoidal voltage amplitudeof 10 mV (rms). The EIS data were modelled usingZsimpWin software (PAR, USA).3 . RESULTS AND DISCUSSIONSThe results of the pulse electrodepositionexperiments are tabulated in Table 1. The averagegrain size using sodium citrate as grain refiningagent was higher than that obtained using saccharin.The XRD patterns of nc-Ni further revealed someinteresting details (Fig. 1). Texturing was evidentin the electrodeposited nickel using sodium citratein the bath because I(111)/I(200) [i.e., the relativeintensities of (111) and (200) peaks] were found527SACHDEVA, et al.: ELECTROCHEMICAL CHARACTERISATION OF NANOCRYSTALLINE NICKELto be 0.040.05. This ratio is 2.4 for bulk nickelcontaining randomly oriented grains. Similar ratioswere observed in case of nc-Ni obtained with saccharinaddition to the plating bath. These results indicatethat there was preferential growth in case of nc-Niwith sodium citrate as additive and the possible roleof grain refining agent in causing texturing. It isanticipated that adsorption of sodium citrate on {111}planes would retard the growth of {111} planes,while facilitating the growth of {200} planes.The variation of free corrosion potential as afunction of time for bulk and nc-Ni is shown inFig. 2. The free corrosion potential for nc-Ni wasnobler compared to that for bulk Ni. The freecorrosion potentials of nc-Ni produced using twodifferent grain refining agents were similar (Fig.2). The nobler free corrosion potentials for nc-Nihave been explained based on the change in kineticsfor the hydrogen evolution reaction5 on nc-Ni.The EIS was conducted at free corrosion potentialat which the material is in condition of activecorrosion. The data obtained for nc-Ni and bulkNi have been compared in Fig. 3 in the form ofNyquist plots. The shape of the curves obtainedcan be approximated to semicircle, which can bemodelled by considering just one time constant.The EIS data obtained was modelled using Rs(QdlRdl)circuit where, Rs is the solution resistance, Qdl isthe constant phase element for electrical doublelayer and Rdl is the polarization resistance.Qdl is a constant phase element comparable tocapacitors. Double layer capacitance in electrochemicalsystems does not behave like an ideal capacitor,but rather there is some amount of leakage currentthrough it. The impedance of a constant phaseelement is defined as12.Z = [Q (jw)n]-1                           (1)Figure 1. XRD patterns obtained from surface of nc-Ni andbulk Ni using Cu Ka radiation.30 40 50 60 70 80(200)(200)(111)(200)(111)(nc-Ni - Saccharin addition)(nc-Ni - Sodium Citrate addition)( Bulk Ni )2 (degrees)qINTENSITY(ArbitraryUnit)01002003004005006007008001000Parameter Exp 1 Exp 2 Exp 3 Exp 4 Grain refiner  Sodium citrate Sodium citrate Saccharin Saccharin Current density (A/ cm2)  0.30 0.26 0.26 0.26 Current efficiency (%)  67.20 69.15 77.90 70.02 I(1 1 1)/I(2 0 0) 0.04 0.05 1.95 2.50 Calculated grain size (nm) 27 32 18 16 Thickness of nc-Ni deposits (µm)  124 110 123 111 Table 1. Experimental results for pulse electrodeposition.Figure 2. Variation of free corrosion potential as a functionof time for bulk Ni, nc-Ni and copper substrate.0 200 400 600 800 1000-0.25-0.20-0.15-0.10-0.050.000.050.100.150.200.250.300.350.40Bulk Ninc- (Sod. Cit addition)Ninc- (Saccharin addition)NiCopper SubstrateTIME (s)Ecorr(VvsSCE)528DEF SCI J, VOL. 58, NO. 4, JULY 2008Figure 3. Nyquist plots of EIS data obtained at free corrosionpotential for bulk Ni, nc-Ni and copper substrate.0 200 400 600 800 1000 1200 1400 1600 1800 200001002003004005006007008009001000Bulk Ninc- : Sod.Citrate additionNinc- : Saccharin additionNiCopper SubstrateZ (ohms)real(ohms)ZimaginaryFor a constant phase element, the exponent n isless than one which represents its deviation fromideality (n=1).The parameters obtained by modelling of EISdata are tabulated in Table 2. The solution resistancewas almost similar. Qdl for bulk Ni sample as wellas nc-Ni sample was nearly same. Similarly, then values were comparable and close to 1. Thisindicates almost capacitative nature of double layerfor all the three samples. A higher Rdl was obtainedfor bulk Ni compared to nc-Ni. This implies thatbulk Ni was more resistant to anodic dissolutiononce the free corrosion potential had stabilised.This is an interesting result because it indicatesthat once free corrosion conditions have been established,the surface of nc-Ni is more prone to corrosionthan Ni.In order to gain insights on the nature of surfaceof nc-Ni, the surfaces were observed in an atomicforce microscope (AFM) (Fig. 4). The dark regionsin the AFM images indicate the valleys, which probablyindicate the pores. This has been further confirmedby the porosity measurement using the electrochemicaldata. The total coating porosity can be evaluatedby the following13 equation:F =  /10Eps corr apcRR- D b·                     (2)where, F is the total coating porosity, Rps the polarisationresistance of the susbtrate obtained from EIS data(Rdl of copper from Table 2), Rpc the polarizationresistance of the coated system (Table 2), DEcorris the difference of the corrosion potential betweenthe coating and the substrate which is approximatelyof the order of 0.25 V (Fig. 2). The anodic Tafelslope (ßa) of the substrate was determined by anodicpolarization of copper substrate in 1M H2SO4 solutionand was found to be 40 mV/decade, which is typicalof activation controlled anodic polarisation.According to this, the nc-Ni coating on coppersubstrate possessed 0.00025 per cent porosity withsodium citrate as additive agent and 0.0014 percent porosity with saccharin as additive. This iscomparable to the porosity values reported in theliterature using electrochemical data13. Thus, theporous nature of the nc-Ni deposits could be relatedto the lower corrosion resistance of nc-Ni afterstabilisation of free corrosion potential.4 . SUMMARYThe nc-Ni deposits were obtained by pulseelectrodeposition using Watts’s bath using sodiumcitrate and saccharin as grain refining agents. Thegrain size was higher for the nc-Ni deposited usingsodium citrate but, on the contrary, porosity was Bulk Ni nc-Ni  (Sodium citrate) nc-Ni  (Saccharin) Copper  (Substrate) Rs (ohm.cm2) 0.868 0.444 1.963 2.593 Qdl (? S.sn.cm2)  28.90 57.59 52.88 69.22 ndl 0.89 0.82 0.87 0.80 Rdl (ohm.cm2) 1587 361 376 1411  Table 2. Parameters obtained by modelling EIS data obtained at free corrosion potential for bulk Ni and nc-Ni.529SACHDEVA, et al.: ELECTROCHEMICAL CHARACTERISATION OF NANOCRYSTALLINE NICKELfound to be approximately an order lower than thatwith saccharin addition. Texturing was observedin case of nc-Ni obtained with sodium citrate asadditive. The nature of surfaces of nc-Ni wascompared with that of bulk Ni by electrochemicalimpedance spectroscopy studies in 1M H2SO4 afterstabilisation at free corrosion potential. Polarisationresistance for nc-Ni was lower compared to thatof bulk Ni. The porous nature of nc-Ni depositshas been related their lower resistance.REFERENCES1. Zimmerman, A.F.; Clark, D.G.; Aust, K.T. &Erb, U. Pulse electrodeposition of Ni-SiCnanocomposite. Materials Letters, 2002, 52,85-90. Figure 4. (a) and (c) 2-D AFM images of surface of nc-Ni revealing its porous nature for sodium citrate and saccharin additions,respectively, and (b) and (d) 3-D AFM images of surface of nc-Ni revealing its porous nature for sodium citrate andsaccharin additions, respectively.(a) (c)(b) (d)530DEF SCI J, VOL. 58, NO. 4, JULY 20082. Mishra, R. Studies on nanocrystalline nickelsynthesised by electrodeposition. Departmentof Materials and Metallurgical Engineering, IndianInstitute of  Technology, Kanpur, India. MTechThesis.3. 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Youssef, Kh.M.S.; Koch, C.C.& Fedkiw, P.S.Improved corrosion behaviour of nanocrystallinezinc produced by pulse-current electrodeposition.Corrosion Science, 2004, 46, 51-64.10. Rofagha.; Splinter, S.J.; Erb, U.& McIntyre,N.S.  XPS characterization of the passive filmsformed on nanocrystalline nickel in sulphuricacid. Nanostructural Materials, 1994, 4, 69-78.11. Afshar, A.; Dolati, A.G. & Ghorbani, M.Electrochemical characterization of the Ni-Fealloy electrodeposition from chloride-citrate-glycolicacid solutions. Mat. Chem. Phys., 2003, 77(2),352-58.12. Electrochemical Impedance Spectroscopy: Aprimer.http://www.gamry.com/App_Notes/EIS_Primer/EIS_Primer.htm13. Ahn, S.H.; Lee, J.H.; Kim, H.G. & Kim, J.G.A study on the quantitative determination ofthrough-coating porosity in PVD-grown coatings.Appl. Surf. Sci., 2004, 233, 105-14.

Electrochemical Characterisation of Nanocrystalline Nickel

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