Electrodeposition of Co-Ni on aluminum plate from chloride bath

Electrodeposition of Co-Ni on aluminum plate from chloride bath

~ Ca"adian Me/allurgical Qua".,I)'. Vol 35. No, 3. pp 24~2S3. 1996 Copyright © 1996Canadian Institute or Mmmg and Metallurgy. Published by Elsevier S...

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Ca"adian Me/allurgical Qua".,I)'. Vol 35. No, 3. pp 24~2S3. 1996 Copyright © 1996Canadian Institute or Mmmg and Metallurgy. Published by Elsevier SCience Ltd Printed In Great Bnlam. All nghts reserved

Pergamon

0008-4433196 SIS00+0 00

PH S0008-4433(96)OOOO2-X

ELECTRODEPOSITION OF Co-Ni ON ALUMINUM PLATE FROM CHLORIDE BATH A. BOUYAGHROUMNI, P. VERSAUn* and O. VITIORI Laboratoire d'Electrochimie Analytique (L1CAS), Universite Claude Bernard Lyon I - CPE, 43. Boulevard du II Novembre 1918, 69622 Villeurbanne, France (Received I August 1993; in reoisedform 31 October 1995)

Abstract-Adherent and compactdeposits of cobalt,nickel and cobalt-nickel alloyswere obtainedfrom a chloride bath at pH 4, on aluminum cathodes, using direct and pulsed current. Pretreatment of the aluminum consisted of a 5 minimmersion in 3.6M H2SO. at 60°C, followed by rinsing withdistilled water, The electrodes wereused immediately after the last rinsing. Pulse time was 5 min at a current density of 20mAcm-2 followed bycurrent interruption for 20 ms.Thethickness of the deposit was3JIm. Theaddition of surfactants to the bath was investigated. The polarization curves with surfactants SDS, SOBS and BRIJ78 leading to the bestdeposits were similar to those recorded in their absence, Whereas the following surfactants promoted theformation ofadherentdeposits: CTAB,FORAFAC1179 and sodium saccharinate reduced theadherence ofthe deposit. Themorphology ofthe depositswasobserved usingscanning electron microscopy and X-raydiffraction wasused to analyze the deposit. At Ni alloycontentlowerthan 18 %, IXCO phasewasidentified. HigherNicontentin the alloyresulted in the presence of IX-CO and P-Ni. Copyright © 1996 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd Resume-Des depots adherents et compacts de cobalt, nickel et d'alliages cobalt-nickel ont ete obtenus sur descathodes en aluminium soiten courant continuo soit en courant pulseen utilisant un bainchlorure de pH = 4. L'aluminium est pretraite par immersion dans une solution d'acide sulfurique 3.6 M a 60 C durant 5 min puis rinceplusieurs fois avecde I'eau distillee. Leselectrodes sont utilisees immediatement apres Iedernier rincage, Lorsque la valeurde la densite de courant est de 20rnAdm- 2l'impulsion de duree 5 ms est suivie d'un temps de repos de 20ms, Le depot possede une epaisseur theorique de 3JIm. Des tensioactifs ont ete ajoutes au bain afin de determiner leur influence. Ainsi, les courbesde polarisation obtenues enpresence des tensioactifs SDS.SOBS et BRIJ78 qui condulsent aux meilleursdepots ressemblent beaucoup a celles obtenues sans tensioactif. Alors que ces tensioactifs permettent d'obtenir des depots adherents la presence de CTAB, FORAFACIl79 ou de saccharinate de sodium reduit l'adherence des depOts. La microscopic electronique a balayage permet de determiner la morphologie des depots et la diffractometrie de rayonsX renseigne sur leurstructure, Lorsque l'al1iage contient moins de 18% de nickel, seule la phase IX-CO est identifiee. Lorsque I'alliage esst plusricheen nickelles deuxphases IX-CO et P-Niapparaissent sur Iespectre, La voltammetrie Ii tension alternative surimposee est similaire a la methode d'impedance faradique. Cette technique fournit des informations precisessur les mecanismes reactionnels aux electrodes, Q

INTRODUCTION

the deposit becomes progressively less shiny and microstructural failures have been noticed (9). By superimposing an The alloys of metals of the iron group often have a high hard- alternating current to the direct current, it is usually observed ness, high internal tension an.d particula~ magnetic properties. that the cathodic overpotential is shifted and the Co:Ni ratio Although Co-Ni films are WIdely used In protective and dec- modified [10, II). It has been claimed [II] that the a.c. current orative plating applications, they are also used as permanent may increase the coercive strength, H" of a magnetic alloy and magnetic memories with a high commutation speed [I, 2]. decrease its remanent induction Hr. The structure of the alloy The addition of organic compounds to the electroplating depends on the Co:Ni ratio [12-15] and for high Co contents bath may modify the electrodeposit morphology, thus changing the properties can be changed drastically [16-18]. the grain size, the roughness or the brightnes~ .of the deposit. In the present work, the codeposition of nickel and cobalt on These additives may limit or stop the dendritic growth, and aluminum plates was studied in order to obtain magnetic thin change the Co.Ni ratio [3-5). The addition of gelatin can stop layers. The influence of surfactants was investigated, and a the codeposition completely (6). The nature of the amon and strong influence on the morphology of the deposits was the current density can change the granularity and the chemical outlined. The use of pulsed currents was compared to direct compositions [2, 4, 7, 8].When the current density is increased, current, for several Co:Ni ratios and textural or structural properties were observed both by X-ray analysis and scanning • To whomcorrespondence should be addressed. electronic microscopy, SEM. 245

A. BOUYAGHROUMNI et af.: ELECTRODEPOSITION OF Co-Ni

246

Table I. Surfactantsemployed in electrodeposition studies Surfactant

Formula

Sodiump-octylbenzene sulfonate(SOBS) CH, (CH,), C. H. SO, Na Sodium dodecylsulfonate (SDS) CH, (CH,)" OSO, Na O-benzoic sulfimide sodiumsalt hydrate(SAC) C, II. a, SNNa.xH,O Polyoxyethylene (20) stearyl ether (BRlJ 78) C" H17 (OCH, CH,)20 OH Cetyltrimethylamine bromide (CTAB) CH, (CH,)" N(CH,) Br Polyftuoroalkylammonium iodide(FORAFAC 1179) C, r, (C,F.), C, H. so,N (RR'R")l

EXPERIMENTAL A Pilovitnum signal generator and a PJT 24-1 potentiostat (TacusseI, France) were used. Voltamrnograms were recorded on a TA4, XVt recorder (Erdam, France). A GSTP4 signal generator and an UAP3 sinewave generator containing a phase sensitive detector were connected to a PRT 30-0 I potentiostat (Tacussel, France). Faradaic and capacitive components to the total current are recorded on a L6512, XVt recorder (Linseis, Germany). The working electrode was a 4 em long, I em wide and 0.1 em thick aluminum plate. The aluminum had a purity of 99.8% (commercial purity). Spectrographic assays showed the presence of the following impurities: 0.1% Mg, 0.05% Fe, 0.03% Cu and 0.02% Si. Typically I em was immersed to obtain the voltammograms, whereas the electrode was immersed 2 em for the electrodeposition experiments. The aluminum surface was first treated with trichloroethylene followed by rinsing with acetone. Then it was immersed for 5min in 3.6M H,SO., at 60"C, and rinsed in distilled water at 60°C for 30 s [19]. Rinsing was then carried out at room temperature with distilled water. The electrode was used immediately after this procedure. A saturated calomel electrode was used as reference, and immersed in a separate compartment filled with 0.5 M NaC!. The auxiliary electrode was an 8 mm diameter carbon rod. The counterelcctrode was separated from the bulk by an agar-agar salt bridge. This limited the pH variations and formation of catholyte hypochlorite ions, resulting from the chlorine evolution. The scan rate was 0.5 mV S-I. This low value has been chosen in order to keep the aluminum electrode in quasi stationary conditions, since the solution was vigorously stirred. Thus, no anomalous concentration gradient might arise. In these conditions reproducibility was reached. Increasing the scan rate up to 10mV S-I did not change the curves but induced a noticeable capacitive current. X-ray diffraction studies were made with a D 500 diffractometer (Siemens, Germany). The SEM photomicrographs were obtained using a microscope 35 CF model (JEOL, U.S.A.). The bath had two constant components: 0.5 M NaCI and 0.4 M II ,B0 1. In selected experiments 10-' M concentration of surfactant was added. To this bath were added nickel and cobalt salts NiCl l.6H lO and CoCI,.6H lO with different Co:Ni ratios, pH was fixed at 4 to.1 and the temperature was set to 25 to.I°e. A medium pH value tends to enhance the rate of nucleation, resulting in a higher cathode current efficiency [9]. The surfactants are listed in Table I. The deposits, plated with and without surfactants, were obtained at a cathodic d.c. current ofO.76Adm- l for 20min.

Nature Anionic Anionic Anionic Neutral Cationic Cationic

NiCb and CoCI 2 were fixed at 0.1 M. When using pulsed currents, no surfactant was added and the following parameters were chosen: Tp = 5ms; T, = 20ms; J p = z Adm " and J, = OAdm- l . A systematic study of the Ni:Co ratio in the deposit was undertaken with several baths in which the INjl+I:ICol+1 ratios were fixed successively at: 1:9; 1:4; 3:7; 2:3; I: I; 3:2; 7:3; 4: I and 9: I. These ratios are expressed in mol l" of each metal, but the sum of the concentrations of Nj2+ and COH is kept constant and equal to 0.2 mol l", All of the deposits obtained with direct or pulsed currents were 3 tim thick, a value calculated from the charge flowing through the electrode, and by weighing the cathode. (The current efficiencywas found to range between 96 and 100%.) A qualitative adherence test was used. It consists of stroking the electrode each 3 mm with a steel point, doing parallel strokes crossing at 90°, according to the norm ASTM B 571 [20]. The composition of the alloys was determined by differential pulse polarography, with 10 M KSCN as a supporting electrolyte. First, the alloy was dissolved in 2.5 M HNO, [21] and the solution, after dilution, was adjusted to pH 6. The peak potentials are -0.78 and -1.14 V/SCE for Ni and Co, respectively, and separation is complete [22].

RESULTS AND DISCUSSION

Direct current First, the cathodic polarization curves for the aluminum electrode were recorded, when immersed in electrolytes containing Ni'", Co H or the mixture Ni'+ + Col+. In Fig. I the curves for nickel electrodeposition in the presence of several surfactants are compared. With SDS, SOBS and BRIJ 78 surfactants the cathodic curves are nearly the same. On the other hand, with CTAB and F 1179, the increase of the current is about 200 mV more cathodic. For SAC, the current is similar up to -1.1 V/SCE and then increases markedly (curve 4, Fig. I). These first investigations show that, at least, two classes may be distinguished for the surfactants under study. A similar behavior is observed for cobalt electrodeposition (Fig. 2). The two sulfonated surfactants, SDS and SOBS give curves close to those recorded without additive. For SOS it is observed that the current is slightly larger than with SOBS. The additive BRIJ 78 seems to shift the curve by about IOOmV in the cathodic direction, as CT AB and F 1179 do with a 200mV shift. But here, SAC behaves quite similarly to the sulfonated surfactants. This is a difference from nickel electrodepositio n. After examination of NiH and COH separately, it was

247

A. BOUYAGHROUMNI et al.: ELECIRODEPOSITION OF Co-Ni POTENTIAL Vvs

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SOBS enhance the electrodeposition of these cations since the curves I and 2 (Fig. 3) are slightly shifted towards less negative potentials than the blank (the mixture ofNi'+ and Co2+ without surfactant). However. BRIJ 78 and SAC act as in the case of cobalt alone--curves 3 and 4. Figs 3 and 2, respectively, and CTAB and F 1179 do not improve in any way the electrodeposition at low current densities--curves 5 and 6, Fig. 3. It is to be noticed that, when the current density is higher than about 15rnA em", then all the curves tend to be similar. The major influence of the surfactants on these deposits is observed before the complete coverage of the aluminum electrode is obtained . For a 0.1 M Co! "-0.1 M Ni'+ bath. without surfactant, or with the surfactants SDS and SOBS, the electrodeposits are regular and adherent to the aluminum plate. Despite a cathodic curve close to that of SDS and SOBS, in presence of SAC. the deposit is brighter, but non-adherent. With CTAB and F 1179, the deposits are gray or black. made up of finelydivided metallic powder and poorly adherent. Table 2 reports the open circuit potential of aluminum in various electrolytes. The aluminum electrode has been prepared as described in the experimental section and no deposit preexisted on the electrode surface. Only from the values obtained with the bath containing Ni2• -{;Oh it is possible to distinguish two kinds of surfactants. The first class comprises of SDS, SOBSand BRIJ 78: the open circuit potentials are close to those recorded without any additive. The second class is comprised of SAC. CTAB and F 1179, which lead to more cathodic values.

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recorded in a bath without surfactant or were even slightly lower in the presence of Ni(II). To conclude. these experiments confirmed the quality of the obtained deposits. The cationic additives were strongly adsorbed at negative potentials and this adsorption blocked the nucleation centers. When an overpotential existed at the beginning of the deposition. the slope dJldE of the curve was

more important and the current densities were larger. The obtained deposits were generally non-coherent, non-adhesive and had a burned appearance. If the anionic additives altered only slightly the aluminum bath interface, they played a regulatory role in the formation of deposits. these last ones appearing adherent and compact. In the presence of SAC, the deposits had a correct aspect but did not present any adhesion. This lack

A. BOUYAGHROUMNI et al.: ELECfROOEPOSITION OF Co-Ni

251

Table 5. Influence of pulse amplitude on cathodic current efficiencies (same conditions as forTable2)

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I~ ~urreas Fi ' Ig 6 Nature of surfactants: (a) 1: without, 2: crAB, 3: dltlOnS ., . . . . . BRIJ 78,4: FORAFAC 1179; (b) I. without. 2. SOBS, 3. SOS, 4. SAC.

of adhesion was probably associated to a poor wettability of the cathode surface.

pulsed current Nowadays pulsed techniques are used more frequently for tal electrodeposition [25, 26], mainly because they lead to me . than .IS the case Wit. h diirect b . ht and more compact deposits ;:rent. Pu lse electrolysisis a powerful mea~s of perturbing the d orption--desorption phenomena occunng at metal-elec:r:lyte interface. After severalpreliminary investigations, a 5ms

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pulse duration and a 20ms current interruption time was found to give good deposits. In the same way, experiments ranging from I to 4 A dm - 2 showed that a 2 A drn" current density was very efficient(current efficiency 98%)(Table 5).

Influence of metal ion concentration. For nickel concentrations ranging from 0.02 to 0.18 moll- J, the current efficiency increased from 82 to 97% (Table 6). The deposits were bright and adherent. SEM investigations showed a typical " cauliflower" crystallization [Fig. 9(a)]. For cobalt ion concentrations ranging from 0.02 to 0.18 moll-I, the current efficiency increased from 85 to 94% (Table 6). The deposits were gray, regular, unburnished and adherent. SEM observations showed that for concentrations lower than 0.06 moll:", the deposit was made of small particles with an average diameter of I JIm. However, for concentrations above 0.06 mol l", cobalt crystallized as fine needles [Fig. 9(b)]. For a cobalt-nickel mixture. the [Nil +]:[C0 2 +] ratio plays an important role. At a total metal concentrat ion of 0.2 moll -I, it was observed that for ratios lower than I, large needles are initiated by the presence of cobalt [Fig. 9(c)]. When the ratio was higher than I . nickel influenced the deposit morphology, and the SEM photograph was identical to that shown in Fig. 9(a). In Table 7 the current efficiencies for the Ni-Co deposit are given. Influence of the [Co l +]:[Ni2 +] ratio on the nickel percentage in thedeposit. As shown in Fig. 10the nickel percentage decreased from 47 to 2% while the Co:Ni rat io was changing from 1:9 to 9: I. The decrease was very abrupt and it appeared that cobalt was more easily deposited than nickel. For a Co:Ni rat io of I :I in the electrolytic bath and in the absence of any surfactant the alloy contained 14% ofNi when using pulsed current and only Table 7. Infuence of the Ni:Co ratioon thecathodic current efficiencies when using pulsed current (Joul.. = 2Adm- 1, [Ni1+ + [CoH ) ~ 0.2M) Ni:Coratio

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252

A. BOUYAG HROUM NI et al.: ELECTRODEPOSITION OF Co-Ni

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Table 8. Nature of the phases and nickel percentage in the case of NiCo deposits obtained with several (Col+]:INi"J ratios (1."0< =

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mixtu re of a -Co and P-Ni phases is obt ained with progre ssively more P-N i phase. As the coba lt co ntent decre ased . the f.c.c. crysta l gra d ua lly appea red and the h.c.p. structure decreased. These result s are presented in Table 8. By hysteresis loop recording. it has been shown that the introd uction of a surfacta nt like BRIJ 78 or SDS in the electrolytic bath leads to the production of allo ys having magnetic properties different from those of allo ys obt ained without surfactant [27J. T his can probably be expla ined by a decrease in the corresponding alloy granularity and in its nickel percentage. In the case of alloys obt ained in the presence of surfactant in the electro lytic bath. the values of the satura tion magn etizat ion B;... and of the remanence Dr are higher (only slightly for Dr) than those observed for samples preparated without any surfactant. Howe ver, in all cases the coercive force He stays the same order of magnitud e.

CONCLUSIONS I I 'X. using d irect current. The cathodic current efficiency was 9!i'X. in the fi rst case and only 92% in the second case. The for mer value, d ose to I O()% . indica ted that the hydrogen evolution was Inw and th us favored compact and bright deposits. X-ray analys is of the deposit showed the progressive cha nge in structure. For high Co:Ni rat ios in the bath it was o bserved that the 7 - (' (1 pha se was present up to a J: I value. Thu s a

Electrodeposition of cobalt, nickel and cob alt nickel alloys is possible with either direct or pulsed current from a bath contain ing 0.5 M NaCi and 0.4 M H1BO). The limited sur face treatment of aluminum by 3 M H 2SO. at 60"C appears to be suitable to obtain high current efficiency and good depo sit adherence. It is clearly demon strated from alternating current data that so me surfactan ts stro ngly adsorbed on aluminum have a large

A . BOUY AGHROUMNI et al.: ELECfRODEPOSITION OF Co-Ni

negative effect on the deposit aspect and adherence. Pulsed currents are helpful in impro..ing the quality of the deposits (smooth surface and good adherence to substrate).

REFERENCES I. S. N, Srimati, S. M . Mayanna and B. S. Sheshadri, Surf. Technol. 16, 277 (1982).

.

2. J. Przyluski, J. Bielinski and M . Kucharski, Surf. Technol. 9,179 (1979) .

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