Electrocrystallization of cobalt from acid chloride solution

Surface Technology, 5 (1977) 169- 204

169

© Elsevier Sequoia S.A., Lausanne- Printed in the Netherlands

ELECTROCRYSTALLIZATION OF COBALT FROM ACID CHLORIDE SOLUTION

JEAN SCOYER and RENE WINAND Department of Metallurgy and Electrochemistry, Free University of Brussels (Belgium)

(Received June 14, 1976)

Summary The characteristics of cobalt deposits have been studied as functions of the electrolysis parameters such as the current density and the pH of the solution. The rest potential, the cathodic overpotential, the current yield, the kinetic parameters and the reaction rate are considered to be properties of an electrochemical nature. Impedance measurements have also been made. The physical properties of the deposits examined were texture, surface appearance, size and shape of grains. The quantitative determination of the proportion of a and ~ phases of cobalt in textured deposits is treated in especial detail. it is concluded that the electrodeposition of cobalt in acid chloride solutions is very different from that in sulphate solutions.

1. Introduction Cobalt is produced industrially by extraction electrolysis from sulphate solutions; numerous studies have been devoted to the behaviour of cobalt in this medium. In electroplating Watts type baths are frequently used. Consequently, various studies have also been devoted to these baths [1]. Deposition of cobalt from acid chloride is rarely used at present, and has therefore had little examination. It seems, however, ripe for development in the future, most particularly because of the progress made in the purification of solutions by solvent extraction. Amongst the few studies published at this time, mention must be made of the technological research by Levin and Borbat [2] who investigated the influence of traces of organic solution originating from the solvent extraction. These impurities do not seem to perturb the electrolysis, the deposited metal reaching a purity of 99.99%. Moreover, the behaviour of inorganic impurities has been studied by Borbat [3] who shows that the presence of zinc ions is notably less harmful than in sulphate solution. Batashev et al. [4] studied the problems involved in the use of platinized titanium insoluble anodes. Keeping in mind the development prospect for

170 electrolysis in acid chloride solutions and taking into c o n s i d e r a t i o n the small a m o u n t of literature available, the s t u d y of the e l e c t r o c r y s t a l l i z a t i o n of c o b a l t in such solutions was u n d e r t a k e n . This investigation is, f u r t h e r m o r e , the logical c o n t i n u a t i o n o f the researches already m a d e in o u r l a b o r a t o r y into the e l e c t r o c r y s t a l l i z a t i o n o f o t h e r metals, b o t h in aqueous solutions and in m o l t e n salts [ 5 - 8 ]. The w o r k p r e s e n t e d is part o f a m o r e c o m p r e h e n s i v e research*; o n l y the results of electrolyses m a d e at r o o m t e m p e r a t u r e will be p r e s e n t e d here.

2. E x p e r i m e n t a l t e c h n i q u e

2.1. Preparation o f solutions and description o f the cell T h e solutions c o n t a i n i n g 50 g 1 1 o f cobalt, were p r e p a r e d by dissolving CoC12- 6 H 2 0 (Merck pro analysis) in tridistilled water. T h e pH was regulated by additions of c o n c e n t r a t e d HC1 (37% Merck pro analysis). B e f o r e the e x p e r i m e n t s the solutions were degassed. T h e e x p e r i m e n t a l apparatus is s h o w n in Fig. 1. T h e electrolyses were carried o u t in a glass cell c o n t a i n i n g a b o u t 500 ml o f solution and t h e r m o s t a t t e d at 25 °C. A stirring r o d with glass blade r o t a t i n g at 250 rev. min 1 ensured good agitation. Nitrogen, b u b b l e d at 0.35 1 min 1, eliminated the dissolved air whilst improving the stirring m e c h a n i s m . T h e r e f e r e n c e e l e c t r o d e , s a t u r a t e d calomel, was placed in an i n t e r m e d i a t e vessel filled with s a t u r a t e d KC1, also t h e r m o started.

T

N2 ~-

il~ii~]~

2

;-

,scm,

Fig. 1. Apparatus: 1, electrolysis cell; 2, hermostatted reference electrode (S.C.E.); 3, pH measuring cell; 4, peristaltic pump; 5, ~H meter; 6, comparator; 7, pH recorder; 8, pH regulating peristaltic micropump.

*In partial fulfilment of the requirements for a Ph.D. degree.

171

2.2. p H regulation During the electrolysis the pH tends to b e c o m e m o r e basic; strict maintenance of the pH is, however, indispensable, as will be shown by the experimental results. The pH was c ont i nuous l y measured in a vessel separated from the cell, in which were inserted a glass electrode and the siphon of a reference electrode (S.C.E.). A peristaltic p u m p caused the solution flow. An auxiliary o u t p u t of the pH meter, Tacussel TS4N, gave a voltage proportional to the pH. This voltage was compared with a p r e d e t e r m i n e d value in a c o m p a r a t o r which was, in fact, a t e m p e r a t u r e regulator supplied by La Pyrom~trie Industrielle. When the required voltage was exceeded the comparator, activated during a chosen period, a peristaltic m i c r o p u m p , Tacussel MPBL, which fed the electrolysis cell a solution having the same Co 2÷ concentration b u t a more acidic pH than the electrolyte, t h e r e b y providing the desired regulation. The facility of having three different settings available (pH o f the corrective solution, duration of injection and rot at i on speed of the m i c r o p u m p ) allowed pH control to +0.05. 2.3. Cathode The cathode assembly is represented in detail in Fig. 2. T he starting cathodes were cobalt disks, purity 99.9%, supplied by M~tallurgie H o b o k e n Overpelt via the Soci~t~ G~n~rale des Minerals. T h e y had a diameter of 20 mm and were 1 mm thick. The electrical c o n t a c t was made by tin soldering a copper wire on the backside o f the plate. The latter was then m o u n t e d with Araldite D. This entire unit was put in a holder which delimited a circular area of 2.5 cm 2 and prevented side effects. The e x t r e m i t y of the siphon was located at a fixed place, 2.5 mm from the cathode. The cathodes were polished under water by means o f em e r y paper, down to the 600 grade and t hen with 1 pm alumina for 30 min (mirror polish). Immediately prior to the e x p e r i m e n t t h e y were cleaned with alcohol, rinsed in distilled water, then etched for a few seconds using the following solution: 60 cm 3 HC1 37% 15 cm 3 HNO3 65% 15 cm 3 CH3COOH glacial 15 cm 3 H 2 0 This solution was also used for the metallographic etching of the deposits. It should be n o ted that the cathodes also u n d e r w e n t an anodic p r e t r e a t m e n t in the electrolysis cell before the cathodic current was switched on. This pret r e a t m e n t is described in § 2.6. 2.4. A n o d e In order to avoid chlorine evolution and to maintain constant Co 2÷ concentration during the e x p e r i m e n t refining electrolysis was preferred. The anode was composed of a m o u n t e d cobalt disk having the same measurements as the cathode.

172

10

,

T

•

._ '! ')

~110 mm 4 Fig. 2. Cathode assembly: 1, cobalt disk; 2, mounting resin; 3, external support in plexiglas; 4, end of the reference capillary, drilled in 3. Fig. 3. Electrical circuit: 1, potentiostat; 2, current adjusting resistances; 3, electrolysis cell; 4, differential voltmeter; 5, standard resistance; 6, ammeter; 7, current integrator; 8, polarity switch; 9, recording electronic millivoltmeter; 10, inversion relay; 11, oscilloscope.

2. 5. Electrical circuit Galvanostatic c o n d i t i o n s c o u l d be achieved b y using the electrical circuit s h o w n in Fig. 3. It was designed in o r d e r t o measure the o h m i c d r o p and the double-layer c a p a c i t a n c e at the beginning o f the electrolysis. F o r this reason, we had to r e n o u n c e the classical use of a p o t e n t i o s t a t . Starting f r o m a stabilized voltage source (20 V p o t e n t i o s t a t , Tacussel P R T 2 0 - 2 X ) the c u r r e n t is regulated b y means o f a set of resistances m o u n t e d in series with the cell. A differential v o l t m e t e r ( H e w l e t t - P a c k a r d 3420 A), placed at the terminals o f a s t a n d a r d resistance, allows the stability o f the c u r r e n t to be c h e c k e d ; slight c u r r e n t f l u c t u a t i o n s m a y be c o m p e n s a t e d b y m o d i f y i n g the voltage furnished b y the p o t e n t i o s t a t . T h e c u r r e n t is also m e a s u r e d b y an a m m e t e r which m u s t be short-cricuited during the switch-on; the a m o u n t of c u r r e n t is d e t e r m i n e d b y means o f a c u r r e n t i n t e g r a t o r (Tacussel IG5). A polarity switch is included in the same circuit loop. T h e c a t h o d i c p o t e n t i a l is m e a s u r e d and r e c o r d e d b y an e l e c t r o n i c m i l l i v o l t m e t e r (Tacussel EPL 1 e q u i p p e d with a T V l l G D plug-in unit). The o h m i c d r o p is d e t e r m i n e d during the c u r r e n t t r a n s i e n t p r o d u c e d b y means of a reversing relay with m e r c u r y w e t t e d c o n t a c t s (Clare H G S / 5 5 1 3 1 TOO). During a l t e r n a t i o n of the reversing device, the o p e n i n g of the first c o n t a c t {B) triggers the oscilloscope ( T e t r o n i x 510 3N), provided with differential amplifiers ( 5 A 2 0 N ) . At this m o m e n t the oscilloscope shows the rest p o t e n t i a l . T h e closing o f the s e c o n d c o n t a c t (A) t h e r e u p o n switches on the c u r r e n t in the electrolysis circuit. This device permits simultaneous m e a s u r e m e n t o f the rest potential, the o h m i c d r o p and the d o u b l e - l a y e r capacitance. T h e quality o f the galvanostatic t r a n s i e n t m a y be c h e c k e d o n the s e c o n d plug-in amplifier of the oscilloscope placed at the terminals of the standard resistance.

173

2. 6. Electrolysis conditions For all the electrolyses, the charge passed was 125 mA h, which leads to a deposit thickness of about 50 pm for a 100% current yield. Before the electrolysis, the cathode, already treated as indicated in § 2.3, was subjected to an anodic etching in the electrolysis cell, the current density being 10 mA cm-2; this was in order to obtain entirely reproducible initial conditions. 2. 7. S u m m a r y o f the standard conditions for all the experiments [Co 2÷] 50 g 1-1 Temperature 25 °C Charge 125 mA h Cathodic area 2.5 cm 2 Rate of agitation 250 rev. min 1 Nitrogen bubbling No inhibitors

3. Experimental m e t h o d

Examination of results We shall first consider the electrochemical investigations and thereafter the techniques for examination of the deposits obtained. 3.1. Electrochemical measurements 3.1.1. Rest potential The rest potential was measured on the oscilloscope, after the anodic attack and immediately before switching on the electrolysis current. 3.1.2. Current yields The current yield was determined by weighing the cathode before and after the electrolysis. In this calculation, allowance was made for the loss of weight due to anodic etching. It should be noted that the determination of the current yields is not very accurate because, in general, the deposit is not detachable from the substratum, so that a small weight can only be evaluated by calculating the difference between two large weights. 3.1.3. Double-layer capacitance The double-layer capacitance was measured during the current transient from the tangent at the origin of the potential versus time curve. 3.1.4. Cathodic potentials The polarization curves were plotted point by point. The potential used was the one which is reached at the steady state during the galvanostatic experiments, the ohmic drop between the working electrode and the Luggin capillary having been deduced.

174 In o u r e x p e r i m e n t s , this o h m i c d r o p was equal to a b o u t 2.5 ~]. T h e potentials, c o r r e c t e d for the o h m i c drop, are p r e s e n t e d semilogarithmically so as to reveal possible Tafel lines.

3.1.5. Determination o f the kinetic parameters It is also possible to plot p o l a r i z a t i o n curves p o i n t b y p o i n t in which o n l y the c u r r e n t actually e f f e c t i n g the c o b a l t d e p o s i t i o n is t a k e n i n t o a c c o u n t (total c u r r e n t X c u r r e n t yield). If the Tafel p l o t o b t a i n e d has the e q u a t i o n U ~ - - E = a + blog Jcr the e x c h a n g e c u r r e n t d e n s i t y Jo is calculated b y the relation log J0 = -- a/b, w h e r e b is the slope of the Tafel line and a its o r d i n a t e for log J = 0. T h e a n o d i c transfer c o e f f i c i e n t a is calculated f r o m the relation b-

RT zF(1 --a)

In this calculation, the n u m b e r o f electrons involved in the e l e c t r o c h e m i c a l r e a c t i o n was c o n s i d e r e d t o be z = 2. (It s h o u l d be n o t e d t h a t some a u t h o r s use z = 1 or even z = 3 [9] .) The validity o f the calculation is e x a m i n e d in the discussion o f the results. T h e calculation o f the e x c h a n g e c u r r e n t d e n s i t y requires k n o w l e d g e o f the equilibrium potential. T h e standard p o t e n t i a l o f the C o / C o 2+ c o u p l e is n o t well established owing t o the high irreversibility o f this c o u p l e and t o the s i m u l t a n e o u s e v o l u t i o n o f h y d r o g e n . T h e value - - 2 7 7 m V / N . H . E . , given by L a t i m e r [ 1 0 ] , was a d o p t e d . O n the o t h e r hand, the activity c o e f f i c i e n t o f CoC12 was calculated f r o m the Tables of Parsons [ 1 1 ] . In the absence o f t h e r m o d y n a m i c data, we neglected in this calculation the influence o f free acid and o f the j u n c t i o n p o t e n t i a l . U n d e r these conditions, an equilibrium p o t e n t i a l of - - 5 3 2 mV/S.C.E, was calculated (see discussion in § 5.5).

3.1.6. Reaction order T h e p o l a r i z a t i o n curves o b t a i n e d at various values o f p H p e r m i t the calculation o f an a p p a r e n t r e a c t i o n order n with regard t o the H ÷ ions according to the f o r m u l a

logJcr ] l o g [ H +]] Ug= constant T h e k n o w l e d g e of n elucidates, in certain cases, the over-all m e c h a n i s m of the reaction.

3.1.7. Impedance measurements To a voltage i m p o s e d b e t w e e n the c a t h o d e and the r e f e r e n c e e l e c t r o d e inserted d i r e c t l y in the solution was s u p e r p o s e d an alternating voltage o f a m p l i t u d e 10 mV. This was e f f e c t e d b y means o f a p o t e n t i o s t a t with rapid response (Tacussel PIT 2 0 - 2 X ) . The f r e q u e n c y r e s p o n s e of the e l e c t r o d e was

175 analysed by means of a transfer function analyser (Solartron 1170 frequency response analyser).

3. 2. Examination of the deposits 3. 2.1. Determination o f the proportion o f allotropic modifications An important characteristic of the cobalt deposits is the proportion of the phases present. It is well known that cobalt has two different allotropic forms: a cobalt, close-packed hexagonal, stable from low temperatures up to 420 °C; and/3 cobalt, face-centered cubic, stable at high temperatures. The phase may be maintained at room temperature because the free energy of the transformation is very weak, about 0.1 kcal/at.g [12]. The determination of the proportion of the phases in an electrolytic deposit is not easy, because the presence of textures in both phases may lead to erroneous interpretations, even from the simple qualitative point of view. Since no satisfactory m e t h o d was found in the literature, we recently proposed a m e t h o d [13] which allowed the determination of the phases in textured deposits. We shall summarize briefly the principle of this method. Figlarz [14] established a formula enabling calculation of the proportion of allotropic modifications present in cobalt powders (with a homogeneous distribution of the crystallographic orientation). The fraction X of ~ phase is given by X 1 --X

•(200) - K - I(10il)

(1)

in which •(200) is the total intensity of the diffraction ray from the (200) plane of/3 Co, 1 ( 1 0 i l ) is that of the ( 1 0 i l ) ray of ~ Co, and K is a constant depending on the wavelength of the radiation used and on the mode of diffraction (transmission or reflexion). This formula is n o t applicable in the case of textured deposits, but we proposed a formula of the same type in which the intensity of the DebyeScherrer rays is replaced by the average intensity obtained during a recording of the pole figure of the same plane. In order to obtain a complete pole figure, it is necessary to use both the reflexion and the transmission method, which necessitates the use of a small specimen and intensity corrections. We would point out that electrochemical deposits present a texture with rotational symmetry, which simplifies the m e t h o d by requiring only one movement of the specimen in the Schulz goniometer. For the present work, we adapted this m e t h o d to thick deposits. Under these conditions, it is impossible to obtain information from the X-ray transmission method. The reflexion method gives intensities which do not require any correction up to a 70 ° angle but, beyond that, the intensity drop is rapid and the data unusable. We have established, however, that the data obtained by reflexion are sufficient for the calculation of the total intensity of the pole figure.

176 This can be d e m o n s t r a t e d on the basis of the existence of equivalent planes. Calculation of the angles made by the planes o f two families in the face-centered cubic system [ 15] allow us to ascertain that there is no family of equivalent planes in which all planes make angles over 70 o with the planes of the ( 2 0 0 } family. The same deduction applies to the close-packed hexagonal system of cobalt with regard to the (1011} family. Accordingly, it is impossible n o t to det ect the existence of a texture, regardless of the plane in which it is presented.

90

70

50

30

10

0

10

:~0

50

70

90

°~

Fig. 4. Schematic pole figure of the (1011) plane of a specimen textured along (1120).

This essential p oi nt being established, we calculated the total intensity of the pole figure, in a four-step operation (as illustrated in Fig. 4). The first step involved subtraction of the background (BG). The latter corresponds to the intensity recorded for angles which do n o t c o m p l y with the Bragg conditions. This intensity was subtracted from the intensity recorded for the characteristic plane. In the second step, we divided the recorded intensity into zones of nont e x t u r e d deposit (NT) and zones corresponding t o one or m o r e textures (T). In the n e x t stage, the r a ndom orientation, giving a constant intensity from 0 to 70 o, was e x t r a p o l a t e d to 90 o. The f o u r th step involved multiplication o f the intensity of a t e x t u r e peak by a correcting factor in order to make allowance for the possible existence of equivalent planes located b e y o n d 70 ° : the intensity of a peak was multiplied by 1.5 if there existed an equivalent plane making a 90 ° angle with the plane of the pole figure, and by n if there were (n -- 1) equivalent planes between 70 o and 90 o. The correcting factors corresponding to the textures e n c o u n t e r e d during our experiments are listed in Tables 1 and 2. The ex tr ap o lat e d intensities corresponding to the non-orientated part and to the one or more t e x t u r e d parts are finally added together to give the total intensity o f the pole figure (total shaded area in figure 4). The pole

177 TABLE 1 C o r r e c t i o n factors for t h e pole figure o f t h e (200) plane Plane s h o w i n g texture

(220) (422)

Angle b e t w e e n t e x t u r e plane and ( 2 0 0 ) p l a n e <70 °

>70 °

45 ° 35 '~ 16' 65 ° 54'

90 ° -

C o r r e c t i o n factor

× 1.5 x1

TABLE 2 C o r r e c t i o n f a c t o r s for t h e pole figure o f t h e ( 1 0 1 1 ) plane Plane s h o w i n g texture

(1010) (1120)

Angle b e t w e e n t e x t u r e plane and (1011 ) plane

C o r r e c t i o n factor

< 70 °

> 70 °

28 ° 05' 63 o 50' 40 ° 11'

-

X1

90 °

X 1.5

figure being symmetrical a bout zero, only the right-hand part is integrated. I n t r o d u c t i o n of these values in formula (1) enabled us to calculate the p r o p o r t i o n o f the phases in a thick and t e x t u r e d electrolytic deposit of cobalt. The pole figure was recorded by means of a Crystalloflex 4 Siemens device, provided with a Schulz t ext ur e goniometer and a scintillation counter. We calculated the value of the coefficient K corresponding to our experimental conditions (reflexion mounting, Ks radiation of a cobalt anticathode, filtered by iron) and obtained the value K = 2.03. 3. 2.2. T e x t u r e The electrolytic deposits generally show textures with rotational symm e t r y ar o u n d an axis perpendicular to the deposit. The study o f these textures and the conditions under which t h e y m ay be p ro d u ced are very i m p o r t a n t because of the a n i s o t r o p y of certain properties, such as the corrosion resistance, or the magnetic properties. We distinguish the textures by noting the plane which is preferably orientated in a direction parallel to the substrate. Some authors characterize the textures by the direction perpendicular to the substrate. These two methods are identical but lead to different symbolism. In the face-centered cubic system, the indices are identical but, since the plane presenting the t e x t u r e does n o t yield a diffraction intensity because of the structure factor, we not e the first equivalent plane:

178 direction [100]

plane (200)

direction [211]

plane (422)

In the close-packed hexagonal system the direction perpendicular to a plane, in general, does not possess the same indices as this plane, viz. direction [210]

plane (1010)

direction [110]

plane (1120)

In order to compare our results, we have quantified the textures, basing our m e t h o d on one proposed by Amblard e t al. [16, 17]. In this method, the texture, which is supposed to follow a Gaussian law on the pole figure, is characterized by two parameters: o, the standard deviation of the Gaussian curve Q, by definition equal to pI/Ial in which I is the total intensity of the D e b y e Scherrer ray of the textured plane and Ial the intensity of the same plane in a random specimen. The coefficient p, half of the multiplicity factor of the plane, is introduced in order to permit a comparison of textures of planes showing different multiplicities, the increase in intensity of a Debye-Scherrer ray at equivalent texture being greater, the weaker the multiplicity. We had to adapt this method in order to make allowance for the presence of the two allotropic modifications of cobalt in our deposits. We therefore define I~l as being the total intensity of the Debye-Scherrer ray of the plane in a random specimen, but constituted of the same proportion of ~ and modifications. This intensity is obtained by multiplying the intensity of a random specimen containing only one allotropic modification by the proportion of this same modification in the actual specimen. On the other hand, when a plane of the cubic lattice diffracts at the same angle as a plane of the hexagonal lattice, it is impossible to separate the intensities of these planes relative to each modification. This is specifically the case for the (220) plane of ~ cobalt which diffracts at the same angle as the (1120) plane of a cobalt. In this case we calculate the average intensity I~(1120)/(220) = I~(112-0) X (% ~) + I~1(220) X (%/3) An identical problem arises in the determination of the multiplicity of planes where we also define an average p as p(1120) X (%a) + p{220) X (%~) i.e.

p(1120)/(220) = 3(%a) + 6(%~) It should be observed that a shift in the proportion of allotropic modification does not affect, in practice, the value of the Q factor in the case of textures along the (119.0)/(220) planes in as much as Ia1(220) is almost equal to twice

179 Ia,(l120). The pole figures recorded for the determination of the proportion of the phases were also used for the study of the textures. To these pole figures were added the pole figures of the plane showing the texture, this being in order to measure or control the standard deviation o. The Debye-Scherrer recordings were realised on a CGR Cristallobloc 30 device equipped with a scintillation counter. The Ks radiation of a cobalt anticathode was selected by means of a bent crystal m o n o c h r o m a t o r (CGR 250).

3. 2. 3. Morphology of deposits Several techniques have been used for the examination of the morphology of the deposits. 3. 2.3.1. Optical examination. This examination permits an assessment of the colour and appearance of the deposit as well as its adherence and the eventual presence of pits. 3. 2.3.2. Roughness. The surface roughness was recorded with a F6rster 5814 Profilograph provided with a sapphire point of 10 pm radius and 60 °

angle. 3.2.3.3. Scanning electron microscopy. The surface of the deposits was examined by scanning electron microscopy (JEOL JSM 50A). 3.2.3.4. Transmission electron microscopy. Replicas of the deposit surfaces were obtained by evaporation of a thick layer of carbon under an angle of 45 ° (direct replica technique). The replicas were detached from the specimen by etching with a ferric chloride solution. Thin foils were obtained by electrolytic etching with a solution of 10% perchloric acid in ethanol-2-butoxy a t - - 1 0 °C. The replicas and the thin foils were examined with a transmission electron microscope (Philips EM 300). 3.2.3.5. Optical microscopy. The metallographic sections reveal the shapes of the grain or clusters of grains and allow the classification of the deposits according to Fischer's theory [18]. The specimens were cut perpendicular to the substrate with a diamond saw (Microslice, Metals Research). The sections were m o u n t e d and polished under water with emery paper for half an hour, down to the 600 grade, and thence with 1 pm alumina. The composition of the etching solution is given in § 2.3. The sections were examined with a Leitz Ortholux microscope. 4. Experimental results We performed several electrolyses at 25 °C, for various values of pH and current density.

180

4.1. Results of the electrochemical measurements 4.1.1. R e s t potent$al The rest p o t e n t i a l for a given pH s h o w s a rather serious dispersion, prob a b l y due to variable q u a n t i t i e s o f dissolved o x y g e n . The average values o b t a i n e d o n the basis o f at least ten e x p e r i m e n t s per pH are listed in Table 3 and p l o t t e d in Fig. 5. I n the same graph we have also p l o t t e d the t h e o r e t i c a l p o t e n t i a l o f a h y d r o g e n e l e c t r o d e ( b r o k e n curve). I t will be n o t e d t h a t the t w o curves a l m o s t run together. F u r t h e r m o r e , we have established t h a t the rest p o t e n t i a l d e p e n d s on the r o t a t i o n a l speed of the stirrer. A n e x p e r i m e n t p e r f o r m e d with a c o b a l t r o t a t i n g disk e l e c t r o d e s h o w e d a linear variation o f the rest p o t e n t i a l with the square r o o t of t h e r o t a t i o n a l speed (Fig. 6).

TABLE 3 Rest potentials pH

4.5

4

3.5

3

2.5

2

1.5

1

EJ=°

--478

--478

--426

--386

--382

--354

--364

--260

(mV/S.C.E.)

-200

E3=O mV/sf.E -~20

-300

"

mYlSc E

-440_ -4bO.

-

400

"\ \ ~

-480. ,~uI/2

- 500

1

2

3

~\ \

"~-

4

pH

5001

10

20

30

40

50

50

(r pm.) I/2

Fig. 5. Rest potential as a function of the pH. Full curve, linear regression from the experimental points; broken curve, theoretical potential of an hydrogen electrode. Fig. 6. Rest potential of a cobalt rotating electrode as a function of the rotational speed (rev. min -1).

4.1.2. Curren t efficiency The c u r r e n t efficiencies are listed in Table 4. T h e variation in c u r r e n t e f f i c i e n c y as a f u n c t i o n o f t h e c u r r e n t d e n s i t y at pH 1 and 1.5 is p l o t t e d in Fig. 7.

4.1.3. Dou ble-layer capacitance The d o u b l e - l a y e r c a p a c i t a n c e had an average value o f 25 p F c m 2. N o s y s t e m a t i c d e p e n d e n c e on c u r r e n t or o n pH was observed. T h e m e a s u r e m e n t dispersion was a b o u t 5 p F cm 2.

181

TABLE 4 Current efficiencies (%) Jc: (mAcm

5

7.5

10

17.5

25

37.5

50

94 70 94 93 87 87 66 22

100 87 94 94 84 94 78 50

100 100 100 94 87 92 82 59

94 98 100 94 89 94 84 65

-

100 95 89 87 75 40

95 90 100 95 84 93 89 71

75

100

125

150

95 96 94 92 86 94 91 72

95

95

200

300

400

600

800

87 78

79

82

2)

pH 4.5 4 3.5 3 2.5 2 1.5 1

%

91

Rc

94

95.3 96 94 96 100 86 93 71

87 74

ix

90

70

50. A

pH 1.5

~7 pH 1

30 2

Log Jc

F i g . 7 . C u r r e n t e f f i c i e n c y as a f u n c t i o n o f t h e c u r r e n t d e n s i t y ( m A

cm-2)

at pH

1 and

1.5.

4.1.4. Cathodic potentials The potentials corrected for the ohmic drop are listed in Table 5 and presented semilogarithmically in Fig. 8. The electrolyses at a current density

TABLE

5

Corrected cathodic potentials (--mV/S.C.E.) Jc: 5 ( m A c m 2)

7.5

10

17.5

25

665 685 755 800 783 775 763 733

815 812 800 785 774 746

678 700 820 822 809 789 785 767

697 703 835 851 822 793 795 786

705 705 850 853 834 811 832 795

707

725

726

745

765

37.5

50

75

100

125

150

200

760 770 880 885 905 875 900 880

780

820 -

830 780 890 900 950 970 920 970

300

400

600

800

pH 4.5 4 3.5 3 2.5 2 1.5 1

832 -

740 735 865 875 872 834 858 823

871 -

SO 4 24

780

800

-

-

954 980

1050 990 1060

1130

182

o f 1 m A c m 2 gave d e p o s i t s w h i c h did n o t cover t h e entire c a t h o d i c area. T h e real current d e n s i t y being u n k n o w n , w e n e g l e c t e d t h e results o f these experiments. Log Jc

2

1

_~-o-

"

Q-O.6

--

& /

"

.-~

~

o

0.7

i - .;i~ :0,8

o ~

Ugc

~-I-"

"

o

I0

V/SCE

Fig. 8. S e m i l o g a r i t h m i c p o l a r i z a t i o n curves p l o t t e d p o i n t b y p o i n t in the g a l v a n o s t a t i c regime. Jc is the current d e n s i t y ( m A c m -2). Curve • ,: + x ~J ,.' • ... pH 4.5 4 3.5 3 2.5 2 1.5 1

Ugc

-650T mY/SCE ~Q jO ,'

/

[ X/ ~

-70C

,'

Log d c r

2

1

@ 06

,/, J ,ll / ,Ij -75C

/

o

/

+

o . / _ B ~ - - o ......

-07

x \ o -B00

~}{

zx ~

/

06

...~Z~>~

\ \

-85G

Ugc I

2

3

4 pH

. "~"

"

©9 V/SeE

Fig. 9. G a l v a n o s t a t i c c a t h o d i c p o t e n t i a l as a f u n c t i o n o f the pH for various current densities ( m A c m 2). Curve • + × ~ pH 5 7.5 10 17.5 Fig. 10. S e m i l o g a r i t h m i c g a l v a n o s t a t i c p o l a r i z a t i o n curves for c o b a l t d e p o s i t i o n p l o t t e d p o i n t by p o i n t , t h e current d e n s i t i e s h a v i n g b e e n c o r r e c t e d for the current e f f i c i e n c y . Jcr is the corrected current d e n s i t y ( m A c m - 2 ) Curve • . + × • ~. pH 4.5 4 3.5 3 2.5 2 1.5 1

183 F o r e a c h p H we m a d e successive e x p e r i m e n t s w i t h increasing c u r r e n t d e n s i t y , l i m i t e d b y the p r o d u c t i o n o f d e p o s i t s o f b a d quality. Figure 9 s h o w s t h e s u d d e n d i s c o n t i n u i t y in t h e i n f l u e n c e o f p H b e t w e e n t h e values 3 a n d 4, a n d Fig. 10 t h e p o l a r i z a t i o n curves o b t a i n e d b y t h e m e t h o d e x p l a i n e d in § 3.1.5.

4.1.5. Kinetic parameters T a b l e 6 gives, f o r various p H , t h e values o f J0, b a n d a. F o r p H 4 - 4.5 t h e c a l c u l a t i o n is b a s e d o n t h e s e c o n d s e g m e n t o f t h e Tafel p l o t , t h e first o n e leading t o negative values for a. In this case, at least a t h r e e - e l e c t r o n mechanism should be considered. TABLE 6 Values o f b, o~ a n d J 0 pH

b (mV)

a

J0 (mA cm -2)

1 1.5 2 2.5 3 3.5 4 4.5

60 44 56 85 75 63 83 92

0.5 0.32 0.46 0.65 0.6 0.52 0.63 0.67

6 x 10 -4 0.14 X 1 0 - 4 2 x 10 -4 50 x 10-4 13 × 1 0 - 4 2.4 x 10 4 0.163 0.28

4.1.6. Reaction order F o r t h e c a l c u l a t i o n o f the r e a c t i o n o r d e r we t o o k t h e values o f t h e c u r r e n t w h i c h w e r e really e f f e c t i v e in c o b a l t d e p o s i t i o n (Jcr), relying on the curves p l o t t e d in Fig. 10. We l i m i t e d ourselves to t h e d o m a i n b e t w e e n p H 1 and 3; a t p H 3.5 t h e t r a n s i t i o n z o n e is r e a c h e d , as s h o w n in Fig. 9. Figure 11 r e p r e s e n t s t h e results t h u s o b t a i n e d for a p o t e n t i a l o f - - 7 8 0 m V / S . C . E . T h e log Jc r t2

10

8

.5

\

4

\

.2

pH I

2

3

Fig. 11. D e t e r m i n a t i o n o f t h e r e a c t i o n o r d e r w i t h respect to t h e H ÷ ions. T h e c a t h o d i c p o t e n t i a l was - - 7 8 0 m V / S . C . E , a n d J c r is t h e c o r r e c t e d c u r r e n t d e n s i t y ( m A c m - 2 ) .

184 slope o f t h e straight line o b t a i n e d b y linear regression gives t h e r e a c t i o n order, b e i n g +0.26 w i t h regard to t h e H ÷ ions.

4.1.7. Impedance measurements T h e m e a s u r e m e n t s w e r e m a d e with a c o b a l t e l e c t r o d e of area 39 m m 2, r o t a t i n g at 5 5 0 0 rev. m i n 1. T h e a u x i l i a r y e l e c t r o d e was a disk o f p u r e c o b a l t T h e m e a s u r e m e n t s w e r e m a d e at pH 2.5, the s o l u t i o n having first b e e n degassed. T h e c a t h o d i c p o t e n t i a l at which t h e m e a s u r e m e n t s w e r e m a d e was - - 8 6 0 m V / S . C . E , and t h e direct c u r r e n t was 3 m A , c o r r e s p o n d i n g t o a curr e n t d e n s i t y of 7.7 m A c m --2.

X (tO

c

10 RL

L

/

I00 Hz

/.

\

~IKHz

6.3Nz

IOKHzt fo

15 "~,25Hz

" 2~

Iq (l-~)

1.5BHz

Fig. 12. Impedance diagram in the complex plane and the corresponding equivalent circuit. Cathodic potential --860 mV/S.C.E., current 3 mA, pH 2.5, [Co 2+ ] 50 g 1-1, rotating electrode 5500 rev. min -1, area 39 mm 2, temperature 25 °C, a.c. voltage 10 mV. The capacitive reactions are shown with positive ordinates. A few characteristic frequencies are listed as parameters on the curve.

T h e f r e q u e n c y r e s p o n s e o f the c a t h o d e is s h o w n in Fig. 12. T h e capacitive a n d i n d u c t i v e loops are r e p r e s e n t e d q u i t e precisely b y semicircles. F r o m t h e s h a p e o f t h e curve an e q u i v a l e n t circuit can be d e d u c e d for the e l e c t r o d e a n d its d i f f e r e n t c o m p o n e n t s d e t e r m i n e d [ 19 ]. This e q u i v a l e n t circuit is s h o w n in Fig. 12. R a , t h e o h m i c resistance, is 6.2 g~. K n o w i n g this value we m a y c o r r e c t t h e c a t h o d i c p o t e n t i a l which, in fact, is equal to - - 8 6 0 + (6.2 X 3) = - - 8 4 1 m V / S . C . E . T h e various c o m p o n e n t s o f the e q u i v a l e n t circuit have t h e f o l l o w i n g values: d o u b l e - l a y e r c a p a c i t a n c e CDc 113 p F t r a n s f e r resistance R t 13.9 ~ R a 28.1 L4.2H T a k i n g into a c c o u n t t h e e l e c t r o d e area, we arrive at t h e f o l l o w i n g values:

185 double-layer R t 5.4 ~ cm R L 11 ~ cm L 1.64 H cm

capacitance 2 2 2

290 pF cm

2

TABLE 7 P e r c e n t a g e o f a p h a s e in t h e d e p o s i t s Jc 5 ( m A / c m -2)

7.5

10

17.5

25

100 79 96 94 72 76 84 74

94 72 97 95 77 78 82 63

37.5

50

75

100

125

150

200

78 72 100 94 93 83 81 59

78

75

NM 75 83 97 94 NM 82 47

300

400

600

800

pH 4.5 4 3.5 3 2.5 2 1.5 1

96 76 82 92 92 89 81 79

98 95 100 85 92 97 95 85

97 98 92 92 88 84

67

72 69 100 97 83 71 82 60

78

-

78 53

73 56

71

85

NM, n o t m e a s u r a b l e : b e n t , cracked ... deposits.

J¢ 100 i~ °/°'~

pH

100 °/o~ 80

100 ~- °/o ~ 1

60 40' O 0 °/o,(

8o

7.5 100 [~ °/o4,

~

1.5

50

~°° I°/°'~ 8o

~

50 100 _o/o 4 @ 80

:oor

2

100 _ e/o,,( 80

2~

60 100 80

50 100 ~o/o,(

10

17.5

25

60 3

100 80

50 100 o/o.(

50

60

80

~5

50 100 . o/o',C

100 % ,( 100

8O

80

4

50 100

50 100

80

80 50

5

log J

4.5

50

200

40

1

2

3

Fig. 13. P e r c e n t a g e of t h e ~ p h a s e as a f u n c t i o n of t h e c u r r e n t d e n s i t y ( m A d i f f e r e n t values o f pH.

4 cm

pH 2) for

Fig. 14. P e r c e n t a g e o f t h e (~ p h a s e as a f u n c t i o n of t h e pH for d i f f e r e n t c u r r e n t densities ( m A c m 2).

186

4.2. Deposit properties 4.2.1. Proportion of the phases in the cathodic deposits T h e p e r c e n t a g e o f t h e a p h a s e in o u r d e p o s i t s is given in T a b l e 7 as a f u n c t i o n o f t h e p H and t h e c a t h o d i c c u r r e n t d e n s i t y . I f the d e p o s i t is b e n t or t o r n , the m e a s u r e m e n t is impossible. Such deposits are d e n o t e d b y NM in o u r Tables. Figures 13 and 14 r e p r e s e n t t h e e v o l u t i o n o f the p e r c e n t a g e o f ~ p h a s e as a f u n c t i o n o f t h e c u r r e n t d e n s i t y a n d o f the p H , respectively. Figures 15 a n d 16 s h o w , as a f u n c t i o n of t h e p H , t h e c u r r e n t d e n s i t y a n d c a t h o d i c p o t e n t i a l leading to d e p o s i t s c o n t a i n i n g the a - p h a s e m i n i m u m (see discussion in § 5.8.2.).

200nnA/cm 2 150 !

~

-

UgmV/ c scE

700 ~

t

1O0

-eoo~_

9O0i ,// ,. / 1000~ a

50

i

~

~

4 pH

,

2

3

i 5 pH

Fig. 15. Current density (mA cm 2) leading to the a-phase minimum. Fig. 16. Cathodic potential leading to the a-phase minimum.

4.2.2. Texture 4.2.2.1. a phase. T a b l e 8 lists, as f u n c t i o n s o f the p H and t h e c u r r e n t d e n s i t y , t h e t e x t u r e s o f t h e a m o d i f i c a t i o n as well as t h e c o r r e s p o n d i n g Q a n d o factors. I t s h o u l d be r e m e m b e r e d t h a t a r a n d o m s p e c i m e n has a Q value e q u a l to p, i.e. Q = 3 for the ( 1 0 1 0 ) p l a n e a n d Q = 3 - 6 for t h e (11~.0)/ ( 2 2 0 ) plane. On t h e o t h e r h a n d , if a t e x t u r e is r a t h e r spread o u t (G > 20 °) the d e p o s i t m a y be c o n s i d e r e d as r a n d o m . Figure 17 s h o w s t h e stability z o n e s o f t h e d i f f e r e n t t e x t u r e s as f u n c t i o n s of t h e c u r r e n t d e n s i t y l o g a r i t h m and t h e pH. T h e z o n e s d e n o t e d b y M are t h o s e in w h i c h the d e p o s i t s s h o w b o t h t e x t u r e s s i m u l t a n e o u s l y ; this has b e e n d e s i g n a t e d b y m i x e d t e x t u r e . T h e p o i n t s give the c o o r d i n a t e s of t h e experim e n t s leading t o a m a x i m u m t e x t u r e , the n u m b e r s a s s o c i a t e d with the p o i n t s being the values o f t h e Q f a c t o r u n d e r t h o s e c o n d i t i o n s . Figure 18 gives t h e stability d o m a i n s of t h e d i f f e r e n t t e x t u r e s as a f u n c t i o n of the c a t h o d e potential a n d t h e pH. T h e s y m b o l s have the s a m e m e a n i n g as in Fig. 17. Figure 19 gives the l o g a r i t h m of Q and t h e s t a n d a r d d e v i a t i o n o for d e p o s i t s ob-

TABLE

8

Texture

of the a phase, with Q and 0 (deg) factors Jc(mA c m - 2 ) : 5

pH

Texture

4.5

(1120) (1010)

4

(1120)

7.5

25

250 3 12 4

898 1

596 2

178 3.5

114 4.5

946 1.5

449 2.4

235 2.6

73 6

5.4 9

5.4 >20

17 9

118 7

6 >20 3.8 >20

6.8 >20

63 7 21

Q o

935 1.4

Q o Q

24.5 3.5 5 >20

5 >20

5 >20

4 >20

6.8 >20 5.6 >20

4.8 >20 4.1 >20

5.1 >20 3.7 >20

5.7 >20 3.7 >20

5.2 >20 3.4

7.3 11 3.9

7.7

18 7

41 5.5

9 9

26 5.5

908 1.4

(1120) (1010)

o

3

(1120) (1010)

Q o Q o

2.5

{1120) (1010)

2

(1130) (1010)

Q o Q o V o Q o

1.5

(1130) (1010)

Q (7 Q o

1

(1130)

(1010)

Q o Q (7

50

75

7.5 >20

24.5 7

12 6

8.6

32 5.5

11.6 8

10.7 7.5

18 11

31 4.5

12 10 5.4 5

6

13.3

19 10

3.3

4.6 32 7.5

150

200

97 5

67 5

NM

300

400

19 14

30 9

34 9

24 9

29 9

27 11

600

800

21 10

15 11

56.6 7

4.7 12 9.6 12

5.6

125

11.6 14

7 8.5

46 5

100

density

17.5

Q G Q (7

37.5

of the pH and the current

10

(lOiO) 3.5

as f u n c t i o n s

3.5

10 5

16 11

16 5.5 6

4.3 9.5

188 pH

r

pH 4. +_ 2_ I .

6+o

7+o

+5o'

8+o

ego

9+0

+Go

Ugc

+obo m'so_mV/scE

Fig. 17. Stability range of the t e x t u r e s of the (~ m o d i f i c a t i o n . M d e n o t e s m i x e d t e x t u r e {1120) + (1010). T h e p o i n t s give t h e c o o r d i n a t e s of t h e e x p e r i m e n t s leading t o a m a x i m u m t e x t u r e ; t h e c o r r e s p o n d i n g Q factor is n o t e d n e x t t o t h e p o i n t . Jc is t h e c a t h o d i c c u r r e n t d e n s i t y ( m A c m - 2). Fig. 18. S t a b i l i t y range of the t e x t u r e s of the (~ m o d i f i c a t i o n as a f u n c t i o n of t h e c a t h o d i c p o t e n t i a l . (See Fig. 17 for the m e a n i n g of the s y m b o l s . )

2 0

-tog Q

IogQ

3

~ _~--~

~ log Jc

i

:Z toq

Oc

Fig. 19. Q and o factors of t h e t e x t u r e of the ~ phase at pH 4.5. Jc is the c u r r e n t d e n s i t y ( m A cm- 2 ) . ( 1 1 2 0 ) t e x t u r e , ,, ( 1 0 1 0 ) t e x t u r e . Fig. 20. Q a n d o factors of the t e x t u r e of the (~ phase at pH 4.

TABLE 9 T e x t u r e o f t h e / ~ p h a s e as a f u n c t i o n o f t h e p H a n d t h e c u r r e n t d e n s i t y Jc(mA cm-2): 5

7.5

37.5

100

125

150

200

(220)

(220) (422)

NM

(220) NP

(220) (422) (220) (220)

(422) NT

NT NT

10

17.5

25

50

NT

NT

(220)

(220)

(220) NP (220) (422)

(220) (220) NT (220) (220) ( 4 2 2 ) (422)

75

300

400

(220) (422) (220) (422)

(220)

600

800

pH

4.5

NT

4

(220) (220) (220) (422) (220) (422)

3.5 3 2.5

(220) (220) (422) (220) (422)

( 4 2 2 ) (422)

(422)

( 4 2 2 ) (422)

(422) (220) (422) (220)

2 1.5 1

(422) NT (220) (422)

(422) (220) (422) NT

(422) (422)

(422) (220) (422)

( 4 2 2 ) (422) (220) (422) (220)

(422)

NT, non-textured deposits; NM, not measurable; NP, phase not present (100% (x).

(220) (422) (220) (422) (220) (422)

(220) (220) (220) (422) NT NM (220) (422) (220) (422)

(220) (422)

( 2 2 0 ) (220) (422) (422)

190

tained at pH 4.5 as a f unct i on of the logarithm of the current density. Figure 20 gives the same results at pH 4.

4.2.2.2. ~ phase. The textures of the ~ phase are listed in Table 9. The values o f the Q and o factors have not been m ent i oned for reasons which will be explained in the discussion o f the results. Figure 21 represents the stability domains of the different textures of the/3 phase as a function of the pH and the current density. The areas marked M are those where the deposits present b o t h textures simultaneously. The NT zone corresponds to n o n -tex tu r ed deposits. The points represent the coordinates of deposits showing the most marked (220) and (422) textures; these deposits have o factors of 6 and 5, respectively.

,l°H 3

I

~

~ tog~c

Fig. 21. Stability range of the textures of the fi modification. M denotes mixed texture (220) + (422), NT non-textured deposits and Jc is the current density (mA cm-2).

4.2.3. Deposit morphology 4. 2.3.1. Appearance. According to the electrolysis conditions, the deposits present different colours varying from pale grey to black. T h e y may be dull or satin-like. The deposits showing a (1120) t e x t u r e are satin-like pale grey. Bright deposits (mirror-like) have also been p r o d u c e d at pH 4 from 25 to 200 mA cm 2 and at pH 3.5 for a current density of 100 mA cm -2. The ( 1 0 i 0 ) t e x t u r e and t he mixed texture, produced at low current densities and a pH between 2 and 3.5, lead to dull deposits with colours varying f r o m dark grey to black. The mixed t ext ur e at pH 4.5 gives pale-grey satinlike deposits. The topographies of cathodes characteristic o f each of these conditions are presented in Figs. 22 - 25 and 27. The reflective power of shiny deposits is illustrated in Fig. 27.

4.2.3.2. Roughness. The roughness of the satin-like specimen is characterized by a very high-frequency relief. The shiny deposits present a roughness which may be i m p o r t a n t but the f r e q u e n c y of which is very weak. The dull deposits have a rather p r o n o u n c e d roughness o f average frequency. The different types of roughness are illustrated in Figs. 22 - 26.

mm

,itmm %

"'~;~i~¸¸~,'~ , ,

I

I

..... I

0

I

2

I

I

I

3

4

5 mrn

Fig. 22. Characteristic a p p e a r a n c e o f satin-like d e p o s i t s o f (1120) texture. Experimental conditions: macrograph pH 4 Jc 5 m A cm 2 roughness DH 3.5 Jc 17.5 m A cm 2 scanning microscopy pH 2.5 Jc 100 m A c m - 2

|

i

i

I

I

J

0

1

2

3

4

5ram

Fig. 23. C h a r a c t e r i s t i c a p p e a r a n c e o f t h e d e p o s i t s w i t h m i x e d t e x t u r e a t p H 4.5. Colour: pale-grey satin-like. E x p e r i m e n t a l c o n d i t i o n s : macrograph Jc 10 m A c m - 2 roughness Jc 10 m A c m 2 scanning microscopy Jc 10 m A c m - 2

b.a

1!tmm

i!i mm

1

t

i

I

I

I

I

I

I

I

I

I

0

~

2

3

4

5rnm

0

I

2

3

4

5ram

)-

Fig. 24. Characteristic a p p e a r a n c e o f d e p o s i t s w i t h ( 1 0 1 0 ) t e x t u r e . Colour: dull dark-grey t o black. E x p e r i m e n t a l conditions: macrograph pH 2 Jc 7.5 m A cm 2 roughness pH 2 Jc 37.5 m A cm 2 scanning microscopy pH 1 Jc 10 m A c m - 2

Fig. 25. C h a r a c t e r i s t i c a p p e a r a n c e o f d e p o s i t s w i t h principal m i x e d t e x t u r e s ( 1 1 2 0 ) - (1010). Colour: dull dark-grey to black. E x p e r i m e n t a l c o n d i t i o n s : macrograph pH 2.5 Jc 10 m A c m 2 roughness pH 2.5 Jc 10 m A c m 2 scanning microscopy pH 3 Jc 10 m A c m 2

193

O

2o'I

.

10

. . . . .

0

i

i

!

2

3

, 4

a rnm 5

Fig. 26. Roughness of a (1120) textured bright deposit. Experimental conditions: pH 3.5, Jc 100 mAcm -2. Fig. 27. Mounting showing the reflective power of shiny deposits. The symbol Co in the upper part of the Figure is reflected in the deposit located in the lower part of the photograph. Experimental conditions: pH 3.5, Jc 100 mA cm -2.

4. 2. 3.3. Scanning electron microscopy. The surface appearance is highly variable according to the preferential orientation of the deposit. For the (1120) texture, the surface exhibits growth pyramids (Fig. 22). No structure could be det ect ed for the shiny deposits, even under the highest magnifications. The deposits with mixed t e x t u r e for pH 4.5 have a rather b r o k e n surface showing crystals delimited by multiple faces (Fig. 23). The surface of deposits showing the (10]:0) t e xt ur e and mixed textures, p r o d u c e d at pH 2 3.5, is f o r m e d by grains of long dihedral shapes, which are sometimes curved (Figs. 24, 25). 4.2.3.4. Electron microscopy. The carbon replicas confi rm ed the observations made with the scanning electron microscope. Figure 28 shows the replicas obtained on specimens presenting the different types of textures. In (a), the surface o f a (1120) t e x t u r e d satin-like deposit has been slightly attacked. The growth pyramids are visible. In (b), the blocks forming the surface of the mixed t e x t u r e d deposits at pH 4.5 appear very clearly. In (c), the dihedral-shaped grains characteristic of the (1010) and mixed principal textures are visible at high magnification. In (d), the replica of a shiny surface shows n o structure.

194

(a)

(c)

~.1 pm,

~pm

,

(b)

2 pm

(d)

= ] pm

,

Fig. 28. Electron microscopy (carbon replicas): characteristic appearance of the surface of deposits with varying textures. (a): (1120) texture satin-like type. (b): mixed texture at pH 4.5. (c): (10i0) and mixed principal textures. (d): (1120) texture bright deposits.

Figure 29 is a transmission m i c r o g r a p h of a thin foil p r e p a r e d f r o m a d e p o s i t o b t a i n e d at pH 1.5 and 100 m A cm 2. T h e grain size is a b o u t 1 pm. The parallel lines in the p h o t o g r a p h are d u e to stacking faults, the f r e q u e n c y o f the latter being very high in c o b a l t e l e c t r o l y t i c deposits.

195

Fig. 29. Transmission electron microscopy: thin foil. Electrolysis conditions: pH 1.5, Jc 100 mA cm-2. Magnification: 70 000 ×.

4. 2. 3.5. Optical microscopy. The optical micrographs reveal deposits with field-textured grains under practically all conditions (FT type of deposits in Fischer's classification). At very high current densities, the grains tend to develop into nodules. It should be emphasized that the grain size revealed by transmission electron microscopy on these foils is such that the observations made by optical microscopy refer to clusters of grains and not to isolated grains. The morphology of deposits of mixed texture at pH 4.5 is different. Indeed, these deposits present grains of great size and the p h e n o m e n o n of epitaxy appears. Figure 30 shows some typical micrographs of these different cases.

5. Interpretation and discussion of results 5.1. Res t potential As the rest potential varies linearly with the pH and with a slope approximately identical to that of a reversible hydrogen electrode (Fig. 5), it may be stated t h a t under our experimental conditions (degassed solutions, stirring) the cobalt electrode measures the'pH of the solution, although in a somewhat inaccurate way. This behaviour was previously observed by Piontelli et al. [20] on cobalt monocrystals in a CoC12 0.5M solution, which also contained 0.5M boric acid. The linear variation of the rest potential versus the square r o o t of the rotational speed (Fig. 6) suggests that the rest potential results from a dynamic pseudo-equilibrium, the mechanism of which is diffusion controlled. The electrodes would thus take a mixed potential between the oxidation potential of the cobalt and the reduction potential of H ÷ ions.

196

20 p m (a)

20 p m

(b)

20 p m (c) Fig. 30. C h a r a c t e r i s t i c o p t i c a l m i c r o g r a p h s . Electrolysis c o n d i t i o n s : (a) pH 4.5 Jc 5 mA cm 2 (b) pH 4 Jc 5 m A cm -2 (c) pH 1.5 Jc 400 m A c m 2

197 5. 2. Current efficiencies The current efficiencies reach 100% at pH 3.5 and are always superior to 85%, except at low current densities and pH 1 and 1.5. For these values of pH, the current yield increases regularly as the current density increases (Fig. 7). Our results are in agreement with those obtained by several workers who studied cobalt electrode deposition from chloride solutions [20 - 22]. 5.3. Double-layer capacitance The difference observed between the values obtained for the doublelayer capacitance during the galvanostatic transient and by the impedance measurement is very significant. However, it should be noted that in the first case the surface has just undergone an anodic etching and is at the rest potential, while in the second case the surface is in the electrolysis regime. The difference observed for the value of the double-layer capacitance could thus be explained either by the presence of an adsorbed species on the electrode in the cathodic regime or by an important modification of the specific surface of the electrode. This fact has to be related to the observations concerning the reaction order. 5. 4. Polarization curves It will be noted that the overpotentials are significant even at low current densities. The discontinuity of the overpotential as a function of the pH has also been noted by other workers, both for chloride [23] and for sulphate electrolytes [24]. It has been attributed by some workers [23] to a change in the nature of the complexes in solution. The polarization curves may be schematized as Tafel plots having one or two segments. The change in their slopes, a phenomenon observed both in the all-sulphate electrolytes and in Watts type electrolyte (mixed sulphatechloride), is generally attributed to a modification of the reaction mechanism as a function of the potential. We shall return to this point in the next section. 5.5. Kinetic parameters The polarization curve~ permit the calculation of J0 and a providing that the Nernst equilibrium potential is precisely known and assuming that the measured overpotential is due only to the transfer. This implies that the crystallization, diffusion and reaction overpotentials are negligible. We believe that these assumptions are unlikely in the case of cobalt electrodeposition for the following reasons: high total overpotential; presence of stable complex in solution; discontinuity in the influence of pH. The principal reason for our objection is that we have ascertained t h a t the different slopes of the Tafel lines correspond, in fact, to deposits having different morphologies and, more particularly, showing different textures (see § 5.9.). We believe, therefore, that the total overpotential includes a not insignificant proportion of crystallization overpotential and also of reaction overpotential, the latter corresponding to the decomposition of the complexes.

198

On the other hand, determining E by calculation is not very satisfactory from a fundamental point of view. Taken strictly, it would be preferable to plot also the anodic polarization curve for the dissolution of cobalt corrected to 100% current yield and to extrapolate the linear parts of bot h curves. Even this m e t h o d , which we are studying at the m o m e n t , will only give correct results if and when the transfer overpotential is predominant. We calculated, however, the values of J0 and ~ in the classical way in order to permit comparison of our results with those given by ot her workers. The values we obtained are of the same order of magnitude as those given by Sheinin et al. [ 2 5 ] . Piontelli et al. [20] measured a Tafel slope of 85 mV and a J 0 value of 0.05 A m -2 for pH 1.4, 2.8 and 4.5 for a 0. 5M CoC12 and 0 . 5 M H3 BO3 solution on a cobalt monocrystal orientated along (1010). These values are in agreement with our results. 5.6. Reaction order It is difficult to draw a definite conclusion from our measurements. It should, however, be noted t hat a reaction order less than uni t y is characteristic for an adsorption p h e n o m e n o n . Several workers have proposed complete mechanisms, including the formation of an intermediate species Co(OH) adsorbed at the electrode [9, 26] for the perchlorate and Watts t y p e of baths. Other workers, by contrast, postulate the adsorption of atomic h y d r o g e n [ 27 ] 5. 7. Impedance measurements The existence of an inductive loop at low frequencies is characteristic for an adsorption p h e n o m e n o n . These measurements confirm the results obtained in the determination of the reaction order. Some authors have also noted the existence of this inductive loop in Watts t y p e baths [3]. Thus, we conclude from our experiments that, for the acid chloride solution, the mechanism of deposition also passes through an adsorption stage. 5.8. Proportion o f the allotropic modifications 5.8.1. Discussion o f the dosage m e t h o d The m e t h o d which was elaborated necessitates a very stable X-ray source and a precise hold on the different settings of the device. Since the measurements extend over several months, it must be ascertained th at there is no drift in the device; the background, especially, must remain constant. A control by means of standards assured identical conditions for all the recordings. The calculation of the total intensity of the pole figure must be achieved with great care in order to remain significant. A certain ambiguity may occur in the recordings of pole figures showing a mixed texture, especially when these textures are very broad. In our case, bot h textures overlap and the interpretation is difficult. The recording of the pole figures of the t e x t u r e d planes allows the estimation of the p r o p o r t i o n of these textures in the pole figure of the (1011) plane.

199

The intensity o f the rays of a D e b y e - S c h e r r e r recording also simplifies the interpretation of the pole figure.

5. 8.2. Interpretation of results The a phase is practically always p r e p o n d e r a n t in our deposits. The m a x i mu m a phase (practically 100%) is obtained at pH 3 - 3.5 over a wide range o f current densities. It should be r e m e m b e r e d that the cathodic overpotential is also maximal for these values of pH (Fig. 9). The result is that the percentage of the a modification is greater, the more negative the cathodic potential. This conclusion is paradoxical if one accepts that the a modification is the stable form at low temperature. Th e percentage of the a modification reaches a relative m a x i m u m for a current density of a b o u t 10 mA cm -2, whatever the pH. The curves, thereafter, pass through a minimum and the corresponding current density is m i n i mu m at pH 3 - 3.5 (see Fig. 15). The potential corresponding to the minimum is constant in the pH range 1.5 - 3.5 (Fig. 16). A t t e n t i o n must be drawn to the fact that for some pH this minimum is very wide, which led us to the i n t r o d u c t i o n of a current or potential interval in Figs. 15 and 16. Figures 13 and 14 also show that the influence o f the pH increases when the cu r r en t density rises. The influence of the current density is minimal at pH 3.5. 5. 8.3. Comparison with the literature As early as 1921, the presence of/3 cobalt in electrolytic deposits was observed for the first time by Hull [ 28 ]. The first study of the influence of the pH on the structure o f electrodeposited cobalt was made by Kersten in 1932 [ 2 9 ] . Since then, the presence of/3 cobalt has been r e p o r t e d by numerous workers studying the m os t varied solutions [1]. As the quantitative d e t e rmin atio n of the phases is very difficult, the greater part of the published results are only qualitative and are generally based on the intensity of the characteristic (200) ray. Even qualitative results could, in this case, be erroneous because the presence and the intensity of the (200) ray is a function, n o t only o f the impor t ance of the face-centered cubic fraction but also, and chiefly, of the t e x t u r e of this modification. If the deposits present a strong t e x t u r e along the (220) plane, the (200) ray may be quasi-non-existent and the phase will n o t be det ect ed because the {220) ray is confused with the (11P~0) ray of the hexagonal modification. Matulis et al. [ 3 0 ] , who studied the influence of additives in sulphate solutions buffered by boric acid, are the only ones, to our knowledge, to have presented quantitative results. In order to eliminate the influence o f the t e x t u r e these authors ground their deposits, then com pared the D e b y e Scherrer recordings with those obtained from mixtures of a and ~ powders. This m e t h o d seems rather dubious to us as the grinding, even when done very carefully, generates cold-working which increases the ~ modification.

200 In the absence of inhibitors, these ~/uthors observed a p r o p o r t i o n of modification between 50 and 100%, depending on the pH, the m a x i m u m being reached at pH 3. It appears, therefore, that the quantitative m et hods which avoid the influence of the texture, either by grinding the deposits ( m e t h o d of Matulis et al. ) or by integration of the intensity of the pole figure ( m e t h o d which we have proposed), give comparable results: a m axi m um of modification at pH 3 - 3.5. These results disagree with qualitative data in the literature which show an increase in the/3 modification when the pH decreases [31, 3 2 ] . 5. 9. T e x t u r e 5. 9.1. Discussion o f the m e t h o d The validity of the m e t h o d and the hypothesis on which it is based have been discussed by Amblard et al. [ 17] . So m e additional difficulties arise in the application of this m e t h o d to cobalt deposits. It is impossible to separate the influence of the t e x t u r e {1120) of the modification from the (220) texture of the ~ modification. For this reason, the Q factor of the (220) t e xt ur e could n o t be calculated in Table 9. A n o t h e r problem arises with the (422) texture. It is impossible to measure the diffraction intensity of this plane because the cobalt Ka radiation has t o o high a wavelength and the Bragg conditions cannot be met. Therefore, we c a n n o t calculate Q(422). ]'he use of a m o l y b d e n u m Ka radiation permits the detection of the (422) plane but the latter diffracts at the same angle as the (3030) plane, third order of the {1010) plane. We must also draw attention to the fact that a text ur e along (422) may reinforce the (311) ray owing to the small angle made by these planes in the face-centered cubic system (10 °). A rigorous interpretation of the t e x t u r e is t herefore only possible on the basis of co mp let e pole figures such as those we obtained with the Schulz goniometer. An examination of the D e b y e - S c h e r r e r spectrum, or even of the pole figures recorded in a simplified device (in which the specimen rotates around an axis perpendicular to the plane defined by the incident and reflected beams) risks a false conclusion (texture 311) due to a p h e n o m e n o n called " p s e u d o t e x t u r e " [ 17 ]. The determination of the o factor also requires care. This factor is generally determined on the basis of the pole figure recorded in following the t e x t u r e plane. This m e t h o d is n o t usable for the (220) t e x t u r e because, under these conditions, the o factor of the (1120) t e x t u r e of the a phase would be measured, this t e x t u r e always being stronger than the one along (220). The same would apply for the (422} texture, this plane diffracting at the same angle as the third order of the (1010) plane of the a modification. We have measured, therefore, the e factor of these textures on the basis of the pole figure of the (200) plane. This measurement is possible only in a few favorable cases, as textures which are too broad overlap. 5. 9.2. I n t e r p r e t a t i o n and discussion o f restdls 5. 9.2.1. a phase. The (0001) basal plane of the a phase of our deposits is always perpendicular to the substrate. Two different textures have been

201 observed: the (1010) plane parallel to the substrate and the (1120) plane parallel to the surface. These textures exist separately or simultaneously, according to the electrolysis conditions. Their stability zones have been plotted in Figs. 17 and 18. The passage from the (1010) texture to a (1190) texture occurs without transition, except at low current densities and pH 2 - 3, where there is a transition zone in which both textures coexist. A mixed zone was also observed at pH 4.5 and low current densities. Once more we must distinguish between the results obtained at pH 4 and 4.5 and those at more acidic pH; for these high values of pH we obtain very strong textures along the (llP~0) plane and the Q factor reaches practically 1000. Such deposits give Debye-Scherrer recordings containing one ray instead of the thirteen rays easily detectable in the spectrum of a random specimen. These strong textures are produced over a rather wide range of pH and current densities. It is therefore possible to obtain them without having to maintain drastic electrolysis conditions. Figures 19 and 20 indicate that it is also possible to choose electrolysis conditions leading to predetermined Q (or o) factors. For the more acidic solutions, the textures are much less marked. In the transition zone of pH 3 - 3.5, the textures are very weak and very wide and are practically non-existent at low current densities. The (10i0) texture is also much less marked than the (1120) texture; indeed, the m a x i m u m of the (1120) texture has a Q factor which is 20 times higher than that of the maximum of the (1010) texture, the multiplicity of the planes being identical. The texture modifications are accompanied by changes in the appearance of the deposits and in certain cases by variations of the Tafel slope. This is particularly noticeable at pH 2 and 1.5. 5. 9.2.2. ~ phase. The ~ modification appears with two different textures: when either the (220) plane or the (422) plane is parallel to the surface. These textures exist alone or together (mixed texture M). There also exist zones in which the deposit is not textured (NT). The passage from the (422) to the (220) texture is progressive, either going through a mixed texture zone or through a non-textured zone. The ~-phase textures are notably weaker and much broader than those of the a phase. The (422) texture corresponds with the (10]0) texture of the a phase, but extends slightly more towards the high current densities. The (220) zone is present only at high pH and is thus less extended than the (1120) texture. 5. 9.3. Comparison with the literature Most of the published results concern sulphate or Watts type solutions. However, few results on the acid chloride medium have been published [20, 22, 33, 34]. These electrolytes generally contain boric acid or inhibitors. It is impossible, therefore, to compare the textures obtained under these speci-

202 tic conditions with our results as it is well known that the boric acid acts n o t only as a buffer but also as an inhibitor. It has indeed been established that the addition of boric acid increases the overpotentials and modifies the orientation of the deposit [ 23] .

5.10. Morphology of deposits 5.10.1. Cathode appearance The colour, appearance, roughness and shape of the surface crystals are modified when the texture changes. From this point of view, distinction must be made between the two mixed t e x t u r e zones, as their respective deposits have totally different appearances. The bright deposits are obtained in the transition zone where the potential gradient as a function of pH is maximal (see Fig. 9). T hey present a medium (1120) texture. Maurin [35] studied the m o r p h o l o g y of mixedt e x t u re deposits obtained in Watts t y p e baths and showed that one grain is constituted of two parts joined in such a way as to maintain the parallelism of the close-packed planes. One of the parts presents the (1120) texture, the other one the (1010) texture. T he blackish colour of these deposits is explained by the multiple reflection of light on the facets o f the dihedrals [35]. On the other hand, the satin-like appearance of the deposits of the (1120) texture comes from the very high frequency of roughness which they show. Consequently, it is possible to choose electrolysis conditions leading t o a chosen surface appearance.

5.10.2. Optical microscopy The variations in grain shape at pH 4.5 are in agreement with Fischer's t h e o r y for low inhibition. At other values of pH, the overpotentials are not abl y more i m p o r t a n t and the deposit always presents grain shapes of the FT type. The m o r p h o l o g y of the grains confirms the differences existing between the two mixed textures.

6. Comparison with the sulphate baths Numerous results concerning the sulphate baths have been published. However, in order to allow a simpler comparison with our results, we have made a series of electrolyses in sulphate solution, all other conditions remaining unchanged. These experiments were c o n d u c t e d at pH 4. The cathode potentials, listed at the end of Table 5, are more negative than those measured in chloride solutions, which confirms the observations made by other authors [ 2 5 ] . T he current yields are above 95%. On the other hand, the deposits obtained in sulphate solutions are of bad quality; t h e y show rents, pits and cracks and it is impossible, under these conditions, to submit them to X-ray analysis. The deposits produced in chloride baths are far better.

203 7. Conclusions T h e b e h a v i o u r of c o b a l t in c h l o r i d e s o l u t i o n is v e r y d i f f e r e n t f r o m t h a t in s u l p h a t e s o l u t i o n . T h e o v e r p o t e n t i a l s are l o w e r a n d t h e d e p o s i t s of a b e t t e r q u a l i t y . Bright d e p o s i t s have even b e e n p r o d u c e d , a n d w i t h o u t an additive. O u r m e t h o d for t h e q u a n t i t a t i v e d e t e r m i n a t i o n o f the relative p r o p o r tion o f a a n d ~ m o d i f i c a t i o n s e n a b l e d us to ascertain t h a t t h e d e p o s i t s are m a i n l y c o n s t i t u t e d o f t h e h e x a g o n a l v a r i e t y , even at t h e m o s t acidic p H . T h e m a x i m u m p r o p o r t i o n of t h e a m o d i f i c a t i o n (100%) is o b t a i n e d trader c o n d i t i o n s leading to t h e highest c a t h o d i c p o t e n t i a l s . T h e t w o a l l o t r o p i c m o d i f i c a t i o n s c o n s t i t u t i n g the d e p o s i t s h o w preferential o r i e n t a t i o n s . T h e h e x a g o n a l phase is t e x t u r e d a l o n g the ( 1 1 2 0 ) a n d (1010) planes, the c u b i c p h a s e along (220) and (422). All the p r o p e r t i e s , e l e c t r o c h e m i c a l as well as s t r u c t u r a l , p r e s e n t a d i s c o n t i n u i t y in t h e pH r a n g e b e t w e e n 3 and 4. At high p H , the o v e r p o t e n t i a l s are low, t h e (11P~0) t e x t u r e s are v e r y s t r o n g and t h e d e p o s i t s are satin-like. A t p H 1, it is possible t o m a i n t a i n g o o d quality deposits, i.e. a d h e r e n t , n o t b e n t , n o t c r a c k e d a n d f o r m e d w i t h a g o o d c u r r e n t yield u p t o v e r y high c u r r e n t densities (800 m A c m 2), even w i t h o u t drastic stirring. T h e electrolysis o f c o b a l t in chloride s o l u t i o n s , t h e r e f o r e , seems v e r y p r o m i s i n g and its s t u d y merits f u r t h e r d e v e l o p m e n t , especially a t higher temperatures.

Acknowledgements We wish t o t h a n k very sincerely P r o f e s s o r J a c q u e s Charlier of t h e P h y sical M e t a l l u r g y D e p a r t m e n t o f t h e Universit~ Libre de Bruxelles and Professor J e a n V e r e e c k e n o f the Vrije Universiteit Brussel f o r m o s t p r o f i t a b l e discussions. F u r t h e r m o r e , we t h a n k P r o f e s s o r V e r e e c k e n f o r k i n d l y p u t t i n g at o u r disposal the s c a n n i n g e l e c t r o n m i c r o s c o p e in his d e p a r t m e n t .

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