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Some electrochemical aspects of the electroless nickel process with hypophosphite
Electrodeposition and Surface Treatment, 3 (1975) 261 - 273 @ Elsevier Sequoia S.A., Lausanne -Printed in Switzerland
261
SOME ELECTROCHEMICAL ASPECTS OF THE ELECTROLESS NICKEL PROCESS WITH HYPOPHOSPHITE C. H. DE MINJER
Labomtory of Metallurgy, Section of Corrosion and Surface Treatment, University of Technology, Delft (Netherlands) (Received
June 10, 1975)
Summary Several theories are given in the literature to explain the mechanism of the electroless nickel process with hypophosphite as reducing agent. One theory considers electroless deposition as a purely electrochemical process, namely as a combination of several electrochemical reactions occurring at the same electrode. If this is true, a separation of these reactions at different electrodes should be possible. Results of experiments performed to check this assumption are given in this paper. Introduction Brenner and Riddell [ 11 gave the following equation for the electroless deposition of nickel from a solution containing a nickel salt and hypophosphite: Ni2’ + H,PO;
+ H20 + Ni + H2POg + 2H’
(1)
Hydrogen is simultaneously evolved. According to Brenner this occurs by the side reaction H,POr
+ Hz0 + H,PO,
+ H2
(2)
The deposited nickel contains phosphorus. Reactions suggested for the co-deposition of phosphorus are [2] 3H,P&-
-+ H,PO,
+ 2P + Hz0 + 20H-
(3)
or H+H,PO,
-+P+OH-
+H,O
(4)
The H atoms, necessary for reaction (4), are assumed to be formed as intermediate during the reactions given in eqns. (1) or (2). Several theories, reviewed in the literature [3 - 61, are given to explain the mechanism of the process. These theories are based on the assumptions that (a) hydrogen atoms or hydride ions, both formed as intermediates,
262
play an essential role in the reduction of the nickel ions; (b) an intermediate Ni(OH), adsorbed at the surface, is formed; (c) cathodic and anodic reactions occur simultaneously at the electrode surface. An attempt was made to test the validity of this last assumption. In this case it is a prerequisite that anodic and cathodic reactions can be separated using two different electrodes in two different solutions, both the electrodes and the solutions being connected to each other. Neglecting the deposition of phosphorus for the sake of simplicity, the reactions are cathodic Ni2’ + 2e + Ni
(5)
2H’ + 2e + H,
(6)
anodic
H,PO;
+ H20 + H,PO;
+ 2H’ + 2e
(7)
Each reaction has its own characteristic current-potential curve. The electroless deposition at one electrode will occur at a potential where the algebraic sum of the cathodic and anodic currents is zero. Figure l(a) shows this schematically, the current-potential curves being chosen arbitrarily. The potential fM of the electrode is determined by MA=MB+MC For the case in which the anodic and cathodic reactions occur at different electrodes in two solutions, one containing nickel ions without hypophosphite and the other containing hypophosphite without nickel ions and in which both the electrodes and the solutions are connected to each other, the situation schematically given in Fig. l(b) will hold. For ease of study, not only the phosphorus deposition is neglected, but also the hydrogen evolution at the electrode where the oxidation of hypophosphite takes place. The potentials of the electrodes are EM;and EM; with MbA’ = M;B’ + M;C’ The potential difference EM; - fM; is equal to the product of the current and the ohmic resistance of the solution between the electrodes. As this ohmic resistance increases the currents M’,A’, M;B’ and M;C’ will decrease. Therefore, to demonstrate that a separation of the reactions is possible, it is important to have a low ohmic resistance in the solutions between the electrodes. With the normal electroless process all reactions occur at the same electrode surface, which consists of nickel-phosphorus. Figures l(a) and (b) show the same partial current-potential curves. This is only true if the reactions take place at the same electrode material. For the hypophosphite oxidation an electrode with a nickel-phosphorus surface can be chosen. In the compartment with nickel solution without hypophosphite, pure nickel will deposit. The current-potential curves for nickel deposition and hydrogen evolution on pure nickel and on nickel-phosphorus are different,
263
H, PO; 2H’--
Nil*-
Ni
Fig, 1. Electrochemical representation of the electroless nickel process: (a) one electrode; (b) separated anodic and cathodic electrodes.
in
solution
Fig. 2. As Fig. l(b), but both solutions contain hypophosphite, one with nickel, the other nickel-free. following from the fact that pure nickel deposits together with only a small amount of hydrogen, whereas during the deposition of nickel--phosphorus a vivid hydrogen evolution occurs. To avoid complications by changing the current-potential curves in this way, experiments were started using solutions with hypophosphite in both compartments, one with nickel, one without nickel. Figure 2 shows schematically what can be expected when the area of the electrode in the hypophosphite solution is three times that of the electrode in the nickel solution. The co-deposition of phosphorus and the hydrogen evolution in the nickel-free solution are neglected again. Without contact between the two electrodes the potential of the electrode in the nickel solution is EM, just as in Fig. l(a) and nickel-phosphorus deposits. By connecting the electrodes, the potential of the electrode in the nickel solution falls to EM,, that of the electrode in the nickel-free solution l-i%3 t0 EM, and Mac” + MsB” = MsA” + M4D”
264
Since MsB” is larger than MB, this means that the metal deposition rate is increased. The potential difference EM - EM is determined again by the solution resistance between the two elec’tiodes: An increase of the electrode area in the nickel-free solution should increase the effect on the deposition rate of nickel-phosphorus. After a positive effect was found for the case where both solutions contained hypophosphite, some experiments were performed with a solution containing nickel ions without hypophosphite combined with a nickel-free hypophosphite solution. Experimental Electrodes In most experiments nickel sheets with dimensions of 5.5 cm X 1.0 cm X 0.1 cm(area 12 cm2) were used in the nickel ion containing solutions. The sheets were de-greased first with trichloroethylene, then by immersion for one minute in a solution containing about 200 g/l NasPO* * 12 HsO. After rinsing, the sheets were anodically etched in 4N sulfuric acid. After rinsing again the sheets were immersed in deionized water of 95 ‘C, dried in air and weighed. Finally the sheets were activated in 6N HCl for a few seconds, rinsed and immediately used for the experiments. The weight loss due to the HCl activation was negligible. Rinsing was always performed first with running tapwater then with deionized water. To determine the composition of the deposited metal, a nickel-free base metal was used, viz. pure iron sheets with dimensions of 3.0 cm X 3.0 cm X 0.1 cm (area 18 cm2). These dimensions were suitable for X-ray fluorescence analysis. The iron sheets were pretreated in the same way as the nickel sheets, except that activation was performed by immersion in concentrated sulfuric acid for one minute. In the nickel-free solutions an electrode with a large area was required. Therefore a nickel strip with dimensions of 33.5 cm X 3.1 cm X 0.025 cm was formed into a spiral, pretreated like the nickel sheets, and covered with a nickel-phosphorus layer, approximately 20 pm thick. The area was nearly 210 cm2, that is approximately 17 times that of the nickel sheets. Solutions From the many compositions of electroless literature, the following one was chosen: g/l NiC12 6HsO NaH2P02 Hz0 Aminoacetic acid pH (with NaOH) Temperature l
l
30 10 30 4.2 - 4.3 95 “C
nickel solutions
given in the
265
From earlier experiments [7] it was known that this solution is very stable and the pH lies in a range where it has only a small effect on the deposition rate. The aminoacetic acid serves as a buffer and as a complexing agent for the nickel ions. The same composition was used for the nickel-free solution, except that NiCla - 6HsO was replaced by an equivalent amount of NaCI, viz. 14.8 g/l, and the pH was adjusted to 4.2 - 4.3 by adding HCl. Apparatus To check if the deposition rate in an electroless nickel solution increases by connecting the electrode to another electrode in a nickel-free hypophosphite solution, the following two more or less conflicting requirements are important: (a) a low ohmic resistance between the two solutions; (b) no mixing of the solutions. By using two beakers and connecting the solutions by a rather wide, short bridge, the electrical resistance between the two electrodes was found to be approximately 500 R *. The potential of a nickel-phosphorus electrode in the electroless solution was 60 - 80 mV higher than that of a similar electrode in the nickel-free hypophosphite solution. By connecting the two electrodes, a maximum current of 0.16 mA can be expected. This current corresponds to less than 0.2 mg metal per hour. In the electroless solution the average deposition rate is 80 mg nickel-phosphorus per hour on a surface of 12 cm2. An extra amount of 0.2 mg is too small to be considered as an essential difference. Therefore a much lower ohmic resistance between the two solutions was necessary. The apparatus shown in Fig. 3 was chosen. A glsss crucible with a diameter of 90 mm and a height of 40 mm was mounted in a 2 litre beaker. The bottom of this crucible consisted of porous sintered glass. The beaker contained the electroless solution and the crucible contained the nickel-free hypophosphite solution. The nickel sheets were immersed in the beaker, the spiral in the crucible. The electroless solution in the beaker was stirred by bubbling air through it. The electrical resistance between spiral and sheet was as low as 2 - 3 Q . This meant that a maximum current of 25 - 40 mA could be expected, corresponding to an excess of 25 40 mg metal per hour in addition to the 80 mg per hour on an unconnected sheet. To prevent mixing, the solutions were kept at the same level as far as possible. Nevertheless, some mixing did occur by diffusion through the sintered glass bottom. This mixing is not very serious for the nickel salt containing solution, because both the aminoacetic acid and the hypophosphite concentration are the same in both solutions, so that only the nickel concentration will decrease. In this electroless solution the deposition rate is not very sensitive to the nickel concentration and the influence on the *Measured with a Philips conductance of 1000 Hz.
bridge PR 9501/01
Dy 2028 at a frequency
\_
.
e
c
i
I I I I I I
I I
nickel spiral
air
r-7
Fig. 3. Schematic diagram of test apparatus.
preheating spiral ,--‘i
nickelfree solution
nickelfree Solution
mounting bor for crucible and soirol
nickelfree
solution
electroless nit kel solution
267
connected and unconnected sheet will be the same. Diffusion of nickel ions into the nickel-free solution is more objectionable. Though the nickel concentration is kept low by deposition of nickel on the spiral, the potential of the spiral rises and this lowers the effect of connecting the sheet to the spiral. Therefore the solution in the crucible was continuously replenished. A pump of Sage Instruments Inc., Model 220, pumped nickel-free solution from a storage vessel into the crucible. The flow rate could be varied from 100 to 500 ml per hour. The crucible contained approximately 180 ml liquid. Materials used were glass, Teflon and silicon rubber. To prevent cooling of the solution in the crucible the solution coming from the pump was preheated in a glass spiral mounted in the beaker. The level in the crucible was kept constant by a bridge to a funnel with an overflow tube, as can be seen in Fig. 3. Heating of the solution was achieved by means of a conducting tin oxide layer on the outside of the cylindrical part of the beaker. For thermal isolation the beaker was covered with a glass sheet and placed in a wider glass vessel, neither shown in Fig. 3. A contact thermometer and a relay kept the temperature of the liquid constant at 95 f 0.1 “C. Potential measurements The potentials of the nickel sheet and of the spiral were measured. For the spiral potential a saturated calomel electrode was placed in the funnel. For the potential measurement of the sheet a Haber Luggin capillary was placed with its end 0.5 - 1 mm away from the sheet surface. The other end of the capillary was placed in a beaker containing the same electroless solution. A second saturated calomel electrode was immersed in this beaker. The potentials between the electrodes and their respective calomel electrodes were measured with a Fluke differential voltmeter, type 825, combined with a Kipp recorder, type BOO. All potentials are given with respect to the saturated calomel electrode. Analysis The amount of deposited metal was determined by weighing. The nickel to phosphorus ratio was determined either by X-ray fluorescence analysis, X-ray microanalysis or chemical analysis. Iron was used instead of nickel as base metal for these analyses. Only for the X-ray microanalysis, which has a low penetration depth, could nickel also be used. Results Initial experiments First a series of experiments was performed to check the reproducibility of the equipment. Nickel sheets were plated for an hour at six different positions in the electroless nickel solution. Table 1 shows the results. The air inlet was close to the bottom at the right. No significant differences were found; the deposition rate in the upper
268 TABLE 1 Sheets at different positions in an electroless nickel solution* Test No.
1 2 3 4 5 6 7 8 9 10 11 12
Potential (mV
Weight increase (mg) Total
Per cm2
-577 -576 -576 -576 -576 -576 -575 -575 -575 -574 -573 -574
99.6
8.30 7.64 7.88 7.78 7.84 7.34 7.35 7.06 6.92 6.79 6.35 6.72
91.7 94.5 93.4 94.1 88.1 88.2 84.7 83.0 81.5 76.2 80.6
Position in solution
upper upper upper upper lower lower lower upper upper upper upper lower
right left middle right right middle left middle right middle left right
*Temperature 95 “C, initial pH 4.42, final pH 4.18, duration 60 min. *Measured with respect to a calomel electrode.
90. NIP m&mt.hr
90.
70.
x
60-
0
x
30.
i 1
2
3
4
5
6
7
8
9
1011
12
test ll*
Fig. 4. The effect of connecting a NiP sheet in an electroless Ni solution to a NiP spiral in a Ni-free solution.
269
left part is nearly 5% lower and in the lower right part 3 - 4% higher than elsewhere. For comparative tests at approximately the same position a more homogeneous stirring was not necessary. The deposition rate decreased with time. This is attributed to a decrease in pH and to depletion of the solution. The total amount of deposited metal was 1.05 g. This corresponds to a reduction of nearly 7.5% in the nickel ion concentration and of approximately 30% in the hypophosphite concentration. The deposition rate of the fresh solution was 8 mg/cm2 h, or approximately 10 pm/h. The potential showed a slight increase with time. Corn bination of electroless solution and nickel-free hypophosphite solution As described in the experimental section, the crucible with sintered glass bottom contained the nickel-free solution, which was continuously replenished. The electroless solution was contained in the beaker outside the crucible. The nickel spiral, covered with nickel-phosphorus, was mounted in the crucible. Two nickel sheets, one connected to the spiral, the other not connected, were suspended in the electroless solution close to each other. The location of the sheets was interchanged. Table 2 gives the results of one of the series of experiments performed. In the first two experiments the sheets were not in the solution at the same time, but one after the other. In experiment 3 neither sheet was connected to the spiral and the results show again that the reproducibility was satisfactory. Between 16 and 22 mg more metal was deposited when the sheet was connected to the spiral (see also Fig. 4). Both on the connected and on the unconnected sheet the amount of metal deposited in an hour decreased with the age of the electroless nickel solution, but the difference remained more or less the same. At low flow rates of the nickel-free solution the difference decreased, probably because the nickel ions diffusing into the crucible were not removed fast enough. This is confirmed by a less negative potential of the spiral at low flow rates. As expected the potential of the sheet connected to the spiral was more negative (8 - 16 mV) than that of the other sheet. At the moment the sheet was connected to the spiral the potential of the spiral mcreased by 15 - 25 mV to a less negative value. The potential of the spiral varied during an experiment. In Table 2 the highest and lowest values are given. Table 3 shows that the amount of additional metal deposited on a connected sheet depends on the area of the spiral in the nickel-free solution. The area of the spiral immersed was only roughly estimated. It is clear that more additional metal was deposited as the area of the spiral immersed in the solution was increased. No essential difference in phosphorus content was found between the layers deposited on the connected and on the unconnected sheets. The phosphorus content varied for all sheets from 10.2 to 10.4 wt.%. For these analyses iron sheets with an area of 18 cm2 were used. On the unconnected iron sheets 1S times more metal was deposited than on the unconnected nickel sheets, corresponding to the ratio of their areas.
210 TABLE 2 Sheets in electroless solution, spiral in nickel-free solution* Test No.
Sheet corm. to spiral or not
Weight increase (mg)
to -643 to -629
no yes
95.4 111.5
16.1
-643 -628 -643
to -649 to -649 to -646
no yes
90.8 110.3
19.5
-627
to -629
-620
to -625
no no no yes no yes no yes no yes no yes no yes no yes
85.3 85.3 81.4 100.6 80.2 96.6 74.7 93.7 72.4 92.8 67.6 89.3 66.4 85.7 60.8 77.3
no yes no yes
52.7 69.8 43.7 60.8
Flow N&free soln. ( cm3/h)
Potential (mV)t
1
216 199
-582 -594
to -583 to -596
-636 -626
2
199 170 201
-582 -592
to -583 to -596
-579 -579 -569 -591 -571 -592 -573 -594 -570 -591 -564 -590 -568 -592 -568 -590 -570 -592 -569 -588
to to to to to to to to to to to to to to to
3
195 125 121 279 239 9
157
10 11
78 354 81
12
Sheet
Spiral
to to to to
-581 -581 -571 -593 -574 -595 -514 -595 -571 -594 -566 -591 -569 -593 -569 -572 -594 -570 -590
-620 -624
to -631
-635
to -638
-630 -620
to -623
-625
to -634
-617
to -624
*Temperature 95 “C, duration 60 min, initial pH 4.3. TMeasured with respect to a saturated calomel electrode.
TABLE 3 Influence of anodic area on extra amount of deposited nickel* Spiral area immersed (%)
Extra weight increase for sheet connected to spiral (mg)
100 75 50 67
29.7 19.2 15.8 18.6 *Temperature 95 “C, duration 60 min, initial pH 4.3.
Per sheet Difference
0.0 19.2 16.4 19.0 20.4 21.7 19.3 16.5 17.1 17.1
271
However, the additional metal deposited by connecting the sheets to the spiral showed no difference. This is an indication that the over-potential of the hypophosphite oxidation in the nickel-free solution is much higher than that of the nickel deposition in the electroless solution. Polarization curves confirmed this. Combination of hypophosphite-free nickel solution and nickel-free hypophosphite solution After the positive results obtained in the foregoing section, a few experiments were performed by combining a nickel-free hypophosphite solution and a hypophosphite-free nickel solution. In this case it is important to keep the nickel solution free from hypophosphite and therefore this solution was held in the crucible and continuously replenished. The beaker contained the nickel-free hypophosphite solution and the nickel spiral. Nickel ions diffusing into this solution raise the potential of the spiral and lessen the effect on the amount of nickel deposited on the sheet connected to the spiral. Another disadvantage was that the spiral had to be mounted before the crucible was placed in the beaker and the solutions were heated. As soon as a temperature of 60 “C was reached the hypophosphite started to decompose and the hypophosphite concentration was already reduced before the experiment could be started at 95 “C. The purpose of the experiment was to see if any effect of connecting a nickel sheet to the spiral could be observed. For an optimum effect the apparatus would have to be adapted. Table 4 shows the results of two series of experiments. For each series a fresh nickel-free solution was used. No nickel deposited on the unconnected sheets. However, on the nickel sheets connected to the spiral 10 - 25 mg per hour were deposited. This amount is of the same order as the additional amount deposited on the connected sheets in the combination of electroless solution and nickel-free hypophosphite solution. The amount decreased with time, owing to depletion of the nickel-free solution. The nickel deposited was dull and looked like electrolytically deposited nickel, e.g. from a Watts bath. This is in contrast to the bright electroless nickel. The potentials given in Table 4 are not very accurate because the point of the Haber Luggin capillary could not be placed close to the sheets in the crucible. However, it is clear that the connected sheets had a more negative potential than the unconnected ones. The potential of the spiral increased 5 - 6 mV to a less negative value at the moment it was connected to the sheet. The phosphorus content of the deposited metal was found to be 0.1 0.2 wt.%. In spite of continuous replenishment of the nickel solution some hypophosphite must have penetrated to the surface of the sheets. The main result of these experiments is that the deposition of nickel and the oxidation of hypophosphite can be separated.
272 TABLE 4 Sheets in hypophosphite-free
solution, spiral in nickel-free solution*
Flow Ni soln. (cm3/h)
Potential (mV)+
-
-378 -622
to -400 to -627
-637
to -639
-418 -614
to -482 to -621
-631
to -634
-423 -610 -434 -618 -651
to to to to to
-626
to -628
-622
to -623
-623 -424 -621
to -628 to -649 to -624
-634
to -637
-630
to -632
-422 -620 -420 -624
to to to to
-625
to -629
-621
-417 -609
to -434 to -618
-616
207 217 254 110 182 367 85
Sheet
Spiral
-442 -618 -459 -623 -569
-463 -622 -433 -626
Sheet corm. to spiral or not
Weight increase (mg)
no yes no yes no yes no yes no
0.7 17.9 -0.3 15.0 -1.9 11.8 -0.4 10.9 -
yes no yes
25.1 -2.6 17.3
to -624
no yes no yes
-0.1 13.7 -0.8 10.4
to -620
no yes
-0.1 12.6
*Temperature 95 ‘C, duration 60 min, initial pH 4.3. TMeasured with respect to a saturated calomel electrode.
Conclusion It is not necessary for the electroless nickel process with hypophosphite as reducing agent that all reactions take place at the same surface. When a nickel-phosphorus electrode in an electroless solution is connected to a nickel-phosphorus electrode in a nickel-free hypophosphite solution, an increased deposition rate is observed. Hypophosphite oxidation and metal deposition occur separately. It is also possible to deposit nickel from a hypophosphite-free nickel solution by connecting the electrode to a nickel-phosphorus electrode in a nickel-free hypophosphite solution. Because the separation of the solutions was not ideal as a consequence of the required low electrical resistance, a small amount of phosphorus, e.g. about 0.1 wt.% , was observed in the deposited nickel. Acknowledgements Thanks are due to Dr. Ir. W. A. Schultze for advice and discussion and to Mr. G. J. Manenschijn and Ir. D. Schalkoord for performing the analyses.
273
References 1 A. Brenner and G. E. Riddell, J. Res. Nat. Bur. Standards, 37 (1946) 1; 39 (1947) 385. 2 K. M. Gorbunova and A. A. Nikiforova, Protection of Metals (transl. from Zashchita Metallov) 5 (1967) 195. 3 T. V. Ivanovskaya and K. M. Gorbunova, Protection of Metals (transl. from Zashchita Metallov) 2 (1966) 477. 4 G. Salvago and P. L. Cavalotti, Plating, 59 (1971) 665. 5 C. Gabrielle and F. Raulin, J. Appl. Electrochem., 1 (1971) 167. 6 G. G. Gawrilow, Chemische (Stromlose) Vernicklung, Eugen G. Leuze Verlag, Saulgaul Wiirtt, 1974, p. 127. 7 C. H. de Minjer and A. Brenner, Plating, 44 (1957) 1297.
261
SOME ELECTROCHEMICAL ASPECTS OF THE ELECTROLESS NICKEL PROCESS WITH HYPOPHOSPHITE C. H. DE MINJER
Labomtory of Metallurgy, Section of Corrosion and Surface Treatment, University of Technology, Delft (Netherlands) (Received
June 10, 1975)
Summary Several theories are given in the literature to explain the mechanism of the electroless nickel process with hypophosphite as reducing agent. One theory considers electroless deposition as a purely electrochemical process, namely as a combination of several electrochemical reactions occurring at the same electrode. If this is true, a separation of these reactions at different electrodes should be possible. Results of experiments performed to check this assumption are given in this paper. Introduction Brenner and Riddell [ 11 gave the following equation for the electroless deposition of nickel from a solution containing a nickel salt and hypophosphite: Ni2’ + H,PO;
+ H20 + Ni + H2POg + 2H’
(1)
Hydrogen is simultaneously evolved. According to Brenner this occurs by the side reaction H,POr
+ Hz0 + H,PO,
+ H2
(2)
The deposited nickel contains phosphorus. Reactions suggested for the co-deposition of phosphorus are [2] 3H,P&-
-+ H,PO,
+ 2P + Hz0 + 20H-
(3)
or H+H,PO,
-+P+OH-
+H,O
(4)
The H atoms, necessary for reaction (4), are assumed to be formed as intermediate during the reactions given in eqns. (1) or (2). Several theories, reviewed in the literature [3 - 61, are given to explain the mechanism of the process. These theories are based on the assumptions that (a) hydrogen atoms or hydride ions, both formed as intermediates,
262
play an essential role in the reduction of the nickel ions; (b) an intermediate Ni(OH), adsorbed at the surface, is formed; (c) cathodic and anodic reactions occur simultaneously at the electrode surface. An attempt was made to test the validity of this last assumption. In this case it is a prerequisite that anodic and cathodic reactions can be separated using two different electrodes in two different solutions, both the electrodes and the solutions being connected to each other. Neglecting the deposition of phosphorus for the sake of simplicity, the reactions are cathodic Ni2’ + 2e + Ni
(5)
2H’ + 2e + H,
(6)
anodic
H,PO;
+ H20 + H,PO;
+ 2H’ + 2e
(7)
Each reaction has its own characteristic current-potential curve. The electroless deposition at one electrode will occur at a potential where the algebraic sum of the cathodic and anodic currents is zero. Figure l(a) shows this schematically, the current-potential curves being chosen arbitrarily. The potential fM of the electrode is determined by MA=MB+MC For the case in which the anodic and cathodic reactions occur at different electrodes in two solutions, one containing nickel ions without hypophosphite and the other containing hypophosphite without nickel ions and in which both the electrodes and the solutions are connected to each other, the situation schematically given in Fig. l(b) will hold. For ease of study, not only the phosphorus deposition is neglected, but also the hydrogen evolution at the electrode where the oxidation of hypophosphite takes place. The potentials of the electrodes are EM;and EM; with MbA’ = M;B’ + M;C’ The potential difference EM; - fM; is equal to the product of the current and the ohmic resistance of the solution between the electrodes. As this ohmic resistance increases the currents M’,A’, M;B’ and M;C’ will decrease. Therefore, to demonstrate that a separation of the reactions is possible, it is important to have a low ohmic resistance in the solutions between the electrodes. With the normal electroless process all reactions occur at the same electrode surface, which consists of nickel-phosphorus. Figures l(a) and (b) show the same partial current-potential curves. This is only true if the reactions take place at the same electrode material. For the hypophosphite oxidation an electrode with a nickel-phosphorus surface can be chosen. In the compartment with nickel solution without hypophosphite, pure nickel will deposit. The current-potential curves for nickel deposition and hydrogen evolution on pure nickel and on nickel-phosphorus are different,
263
H, PO; 2H’--
Nil*-
Ni
Fig, 1. Electrochemical representation of the electroless nickel process: (a) one electrode; (b) separated anodic and cathodic electrodes.
in
solution
Fig. 2. As Fig. l(b), but both solutions contain hypophosphite, one with nickel, the other nickel-free. following from the fact that pure nickel deposits together with only a small amount of hydrogen, whereas during the deposition of nickel--phosphorus a vivid hydrogen evolution occurs. To avoid complications by changing the current-potential curves in this way, experiments were started using solutions with hypophosphite in both compartments, one with nickel, one without nickel. Figure 2 shows schematically what can be expected when the area of the electrode in the hypophosphite solution is three times that of the electrode in the nickel solution. The co-deposition of phosphorus and the hydrogen evolution in the nickel-free solution are neglected again. Without contact between the two electrodes the potential of the electrode in the nickel solution is EM, just as in Fig. l(a) and nickel-phosphorus deposits. By connecting the electrodes, the potential of the electrode in the nickel solution falls to EM,, that of the electrode in the nickel-free solution l-i%3 t0 EM, and Mac” + MsB” = MsA” + M4D”
264
Since MsB” is larger than MB, this means that the metal deposition rate is increased. The potential difference EM - EM is determined again by the solution resistance between the two elec’tiodes: An increase of the electrode area in the nickel-free solution should increase the effect on the deposition rate of nickel-phosphorus. After a positive effect was found for the case where both solutions contained hypophosphite, some experiments were performed with a solution containing nickel ions without hypophosphite combined with a nickel-free hypophosphite solution. Experimental Electrodes In most experiments nickel sheets with dimensions of 5.5 cm X 1.0 cm X 0.1 cm(area 12 cm2) were used in the nickel ion containing solutions. The sheets were de-greased first with trichloroethylene, then by immersion for one minute in a solution containing about 200 g/l NasPO* * 12 HsO. After rinsing, the sheets were anodically etched in 4N sulfuric acid. After rinsing again the sheets were immersed in deionized water of 95 ‘C, dried in air and weighed. Finally the sheets were activated in 6N HCl for a few seconds, rinsed and immediately used for the experiments. The weight loss due to the HCl activation was negligible. Rinsing was always performed first with running tapwater then with deionized water. To determine the composition of the deposited metal, a nickel-free base metal was used, viz. pure iron sheets with dimensions of 3.0 cm X 3.0 cm X 0.1 cm (area 18 cm2). These dimensions were suitable for X-ray fluorescence analysis. The iron sheets were pretreated in the same way as the nickel sheets, except that activation was performed by immersion in concentrated sulfuric acid for one minute. In the nickel-free solutions an electrode with a large area was required. Therefore a nickel strip with dimensions of 33.5 cm X 3.1 cm X 0.025 cm was formed into a spiral, pretreated like the nickel sheets, and covered with a nickel-phosphorus layer, approximately 20 pm thick. The area was nearly 210 cm2, that is approximately 17 times that of the nickel sheets. Solutions From the many compositions of electroless literature, the following one was chosen: g/l NiC12 6HsO NaH2P02 Hz0 Aminoacetic acid pH (with NaOH) Temperature l
l
30 10 30 4.2 - 4.3 95 “C
nickel solutions
given in the
265
From earlier experiments [7] it was known that this solution is very stable and the pH lies in a range where it has only a small effect on the deposition rate. The aminoacetic acid serves as a buffer and as a complexing agent for the nickel ions. The same composition was used for the nickel-free solution, except that NiCla - 6HsO was replaced by an equivalent amount of NaCI, viz. 14.8 g/l, and the pH was adjusted to 4.2 - 4.3 by adding HCl. Apparatus To check if the deposition rate in an electroless nickel solution increases by connecting the electrode to another electrode in a nickel-free hypophosphite solution, the following two more or less conflicting requirements are important: (a) a low ohmic resistance between the two solutions; (b) no mixing of the solutions. By using two beakers and connecting the solutions by a rather wide, short bridge, the electrical resistance between the two electrodes was found to be approximately 500 R *. The potential of a nickel-phosphorus electrode in the electroless solution was 60 - 80 mV higher than that of a similar electrode in the nickel-free hypophosphite solution. By connecting the two electrodes, a maximum current of 0.16 mA can be expected. This current corresponds to less than 0.2 mg metal per hour. In the electroless solution the average deposition rate is 80 mg nickel-phosphorus per hour on a surface of 12 cm2. An extra amount of 0.2 mg is too small to be considered as an essential difference. Therefore a much lower ohmic resistance between the two solutions was necessary. The apparatus shown in Fig. 3 was chosen. A glsss crucible with a diameter of 90 mm and a height of 40 mm was mounted in a 2 litre beaker. The bottom of this crucible consisted of porous sintered glass. The beaker contained the electroless solution and the crucible contained the nickel-free hypophosphite solution. The nickel sheets were immersed in the beaker, the spiral in the crucible. The electroless solution in the beaker was stirred by bubbling air through it. The electrical resistance between spiral and sheet was as low as 2 - 3 Q . This meant that a maximum current of 25 - 40 mA could be expected, corresponding to an excess of 25 40 mg metal per hour in addition to the 80 mg per hour on an unconnected sheet. To prevent mixing, the solutions were kept at the same level as far as possible. Nevertheless, some mixing did occur by diffusion through the sintered glass bottom. This mixing is not very serious for the nickel salt containing solution, because both the aminoacetic acid and the hypophosphite concentration are the same in both solutions, so that only the nickel concentration will decrease. In this electroless solution the deposition rate is not very sensitive to the nickel concentration and the influence on the *Measured with a Philips conductance of 1000 Hz.
bridge PR 9501/01
Dy 2028 at a frequency
\_
.
e
c
i
I I I I I I
I I
nickel spiral
air
r-7
Fig. 3. Schematic diagram of test apparatus.
preheating spiral ,--‘i
nickelfree solution
nickelfree Solution
mounting bor for crucible and soirol
nickelfree
solution
electroless nit kel solution
267
connected and unconnected sheet will be the same. Diffusion of nickel ions into the nickel-free solution is more objectionable. Though the nickel concentration is kept low by deposition of nickel on the spiral, the potential of the spiral rises and this lowers the effect of connecting the sheet to the spiral. Therefore the solution in the crucible was continuously replenished. A pump of Sage Instruments Inc., Model 220, pumped nickel-free solution from a storage vessel into the crucible. The flow rate could be varied from 100 to 500 ml per hour. The crucible contained approximately 180 ml liquid. Materials used were glass, Teflon and silicon rubber. To prevent cooling of the solution in the crucible the solution coming from the pump was preheated in a glass spiral mounted in the beaker. The level in the crucible was kept constant by a bridge to a funnel with an overflow tube, as can be seen in Fig. 3. Heating of the solution was achieved by means of a conducting tin oxide layer on the outside of the cylindrical part of the beaker. For thermal isolation the beaker was covered with a glass sheet and placed in a wider glass vessel, neither shown in Fig. 3. A contact thermometer and a relay kept the temperature of the liquid constant at 95 f 0.1 “C. Potential measurements The potentials of the nickel sheet and of the spiral were measured. For the spiral potential a saturated calomel electrode was placed in the funnel. For the potential measurement of the sheet a Haber Luggin capillary was placed with its end 0.5 - 1 mm away from the sheet surface. The other end of the capillary was placed in a beaker containing the same electroless solution. A second saturated calomel electrode was immersed in this beaker. The potentials between the electrodes and their respective calomel electrodes were measured with a Fluke differential voltmeter, type 825, combined with a Kipp recorder, type BOO. All potentials are given with respect to the saturated calomel electrode. Analysis The amount of deposited metal was determined by weighing. The nickel to phosphorus ratio was determined either by X-ray fluorescence analysis, X-ray microanalysis or chemical analysis. Iron was used instead of nickel as base metal for these analyses. Only for the X-ray microanalysis, which has a low penetration depth, could nickel also be used. Results Initial experiments First a series of experiments was performed to check the reproducibility of the equipment. Nickel sheets were plated for an hour at six different positions in the electroless nickel solution. Table 1 shows the results. The air inlet was close to the bottom at the right. No significant differences were found; the deposition rate in the upper
268 TABLE 1 Sheets at different positions in an electroless nickel solution* Test No.
1 2 3 4 5 6 7 8 9 10 11 12
Potential (mV
Weight increase (mg) Total
Per cm2
-577 -576 -576 -576 -576 -576 -575 -575 -575 -574 -573 -574
99.6
8.30 7.64 7.88 7.78 7.84 7.34 7.35 7.06 6.92 6.79 6.35 6.72
91.7 94.5 93.4 94.1 88.1 88.2 84.7 83.0 81.5 76.2 80.6
Position in solution
upper upper upper upper lower lower lower upper upper upper upper lower
right left middle right right middle left middle right middle left right
*Temperature 95 “C, initial pH 4.42, final pH 4.18, duration 60 min. *Measured with respect to a calomel electrode.
90. NIP m&mt.hr
90.
70.
x
60-
0
x
30.
i 1
2
3
4
5
6
7
8
9
1011
12
test ll*
Fig. 4. The effect of connecting a NiP sheet in an electroless Ni solution to a NiP spiral in a Ni-free solution.
269
left part is nearly 5% lower and in the lower right part 3 - 4% higher than elsewhere. For comparative tests at approximately the same position a more homogeneous stirring was not necessary. The deposition rate decreased with time. This is attributed to a decrease in pH and to depletion of the solution. The total amount of deposited metal was 1.05 g. This corresponds to a reduction of nearly 7.5% in the nickel ion concentration and of approximately 30% in the hypophosphite concentration. The deposition rate of the fresh solution was 8 mg/cm2 h, or approximately 10 pm/h. The potential showed a slight increase with time. Corn bination of electroless solution and nickel-free hypophosphite solution As described in the experimental section, the crucible with sintered glass bottom contained the nickel-free solution, which was continuously replenished. The electroless solution was contained in the beaker outside the crucible. The nickel spiral, covered with nickel-phosphorus, was mounted in the crucible. Two nickel sheets, one connected to the spiral, the other not connected, were suspended in the electroless solution close to each other. The location of the sheets was interchanged. Table 2 gives the results of one of the series of experiments performed. In the first two experiments the sheets were not in the solution at the same time, but one after the other. In experiment 3 neither sheet was connected to the spiral and the results show again that the reproducibility was satisfactory. Between 16 and 22 mg more metal was deposited when the sheet was connected to the spiral (see also Fig. 4). Both on the connected and on the unconnected sheet the amount of metal deposited in an hour decreased with the age of the electroless nickel solution, but the difference remained more or less the same. At low flow rates of the nickel-free solution the difference decreased, probably because the nickel ions diffusing into the crucible were not removed fast enough. This is confirmed by a less negative potential of the spiral at low flow rates. As expected the potential of the sheet connected to the spiral was more negative (8 - 16 mV) than that of the other sheet. At the moment the sheet was connected to the spiral the potential of the spiral mcreased by 15 - 25 mV to a less negative value. The potential of the spiral varied during an experiment. In Table 2 the highest and lowest values are given. Table 3 shows that the amount of additional metal deposited on a connected sheet depends on the area of the spiral in the nickel-free solution. The area of the spiral immersed was only roughly estimated. It is clear that more additional metal was deposited as the area of the spiral immersed in the solution was increased. No essential difference in phosphorus content was found between the layers deposited on the connected and on the unconnected sheets. The phosphorus content varied for all sheets from 10.2 to 10.4 wt.%. For these analyses iron sheets with an area of 18 cm2 were used. On the unconnected iron sheets 1S times more metal was deposited than on the unconnected nickel sheets, corresponding to the ratio of their areas.
210 TABLE 2 Sheets in electroless solution, spiral in nickel-free solution* Test No.
Sheet corm. to spiral or not
Weight increase (mg)
to -643 to -629
no yes
95.4 111.5
16.1
-643 -628 -643
to -649 to -649 to -646
no yes
90.8 110.3
19.5
-627
to -629
-620
to -625
no no no yes no yes no yes no yes no yes no yes no yes
85.3 85.3 81.4 100.6 80.2 96.6 74.7 93.7 72.4 92.8 67.6 89.3 66.4 85.7 60.8 77.3
no yes no yes
52.7 69.8 43.7 60.8
Flow N&free soln. ( cm3/h)
Potential (mV)t
1
216 199
-582 -594
to -583 to -596
-636 -626
2
199 170 201
-582 -592
to -583 to -596
-579 -579 -569 -591 -571 -592 -573 -594 -570 -591 -564 -590 -568 -592 -568 -590 -570 -592 -569 -588
to to to to to to to to to to to to to to to
3
195 125 121 279 239 9
157
10 11
78 354 81
12
Sheet
Spiral
to to to to
-581 -581 -571 -593 -574 -595 -514 -595 -571 -594 -566 -591 -569 -593 -569 -572 -594 -570 -590
-620 -624
to -631
-635
to -638
-630 -620
to -623
-625
to -634
-617
to -624
*Temperature 95 “C, duration 60 min, initial pH 4.3. TMeasured with respect to a saturated calomel electrode.
TABLE 3 Influence of anodic area on extra amount of deposited nickel* Spiral area immersed (%)
Extra weight increase for sheet connected to spiral (mg)
100 75 50 67
29.7 19.2 15.8 18.6 *Temperature 95 “C, duration 60 min, initial pH 4.3.
Per sheet Difference
0.0 19.2 16.4 19.0 20.4 21.7 19.3 16.5 17.1 17.1
271
However, the additional metal deposited by connecting the sheets to the spiral showed no difference. This is an indication that the over-potential of the hypophosphite oxidation in the nickel-free solution is much higher than that of the nickel deposition in the electroless solution. Polarization curves confirmed this. Combination of hypophosphite-free nickel solution and nickel-free hypophosphite solution After the positive results obtained in the foregoing section, a few experiments were performed by combining a nickel-free hypophosphite solution and a hypophosphite-free nickel solution. In this case it is important to keep the nickel solution free from hypophosphite and therefore this solution was held in the crucible and continuously replenished. The beaker contained the nickel-free hypophosphite solution and the nickel spiral. Nickel ions diffusing into this solution raise the potential of the spiral and lessen the effect on the amount of nickel deposited on the sheet connected to the spiral. Another disadvantage was that the spiral had to be mounted before the crucible was placed in the beaker and the solutions were heated. As soon as a temperature of 60 “C was reached the hypophosphite started to decompose and the hypophosphite concentration was already reduced before the experiment could be started at 95 “C. The purpose of the experiment was to see if any effect of connecting a nickel sheet to the spiral could be observed. For an optimum effect the apparatus would have to be adapted. Table 4 shows the results of two series of experiments. For each series a fresh nickel-free solution was used. No nickel deposited on the unconnected sheets. However, on the nickel sheets connected to the spiral 10 - 25 mg per hour were deposited. This amount is of the same order as the additional amount deposited on the connected sheets in the combination of electroless solution and nickel-free hypophosphite solution. The amount decreased with time, owing to depletion of the nickel-free solution. The nickel deposited was dull and looked like electrolytically deposited nickel, e.g. from a Watts bath. This is in contrast to the bright electroless nickel. The potentials given in Table 4 are not very accurate because the point of the Haber Luggin capillary could not be placed close to the sheets in the crucible. However, it is clear that the connected sheets had a more negative potential than the unconnected ones. The potential of the spiral increased 5 - 6 mV to a less negative value at the moment it was connected to the sheet. The phosphorus content of the deposited metal was found to be 0.1 0.2 wt.%. In spite of continuous replenishment of the nickel solution some hypophosphite must have penetrated to the surface of the sheets. The main result of these experiments is that the deposition of nickel and the oxidation of hypophosphite can be separated.
272 TABLE 4 Sheets in hypophosphite-free
solution, spiral in nickel-free solution*
Flow Ni soln. (cm3/h)
Potential (mV)+
-
-378 -622
to -400 to -627
-637
to -639
-418 -614
to -482 to -621
-631
to -634
-423 -610 -434 -618 -651
to to to to to
-626
to -628
-622
to -623
-623 -424 -621
to -628 to -649 to -624
-634
to -637
-630
to -632
-422 -620 -420 -624
to to to to
-625
to -629
-621
-417 -609
to -434 to -618
-616
207 217 254 110 182 367 85
Sheet
Spiral
-442 -618 -459 -623 -569
-463 -622 -433 -626
Sheet corm. to spiral or not
Weight increase (mg)
no yes no yes no yes no yes no
0.7 17.9 -0.3 15.0 -1.9 11.8 -0.4 10.9 -
yes no yes
25.1 -2.6 17.3
to -624
no yes no yes
-0.1 13.7 -0.8 10.4
to -620
no yes
-0.1 12.6
*Temperature 95 ‘C, duration 60 min, initial pH 4.3. TMeasured with respect to a saturated calomel electrode.
Conclusion It is not necessary for the electroless nickel process with hypophosphite as reducing agent that all reactions take place at the same surface. When a nickel-phosphorus electrode in an electroless solution is connected to a nickel-phosphorus electrode in a nickel-free hypophosphite solution, an increased deposition rate is observed. Hypophosphite oxidation and metal deposition occur separately. It is also possible to deposit nickel from a hypophosphite-free nickel solution by connecting the electrode to a nickel-phosphorus electrode in a nickel-free hypophosphite solution. Because the separation of the solutions was not ideal as a consequence of the required low electrical resistance, a small amount of phosphorus, e.g. about 0.1 wt.% , was observed in the deposited nickel. Acknowledgements Thanks are due to Dr. Ir. W. A. Schultze for advice and discussion and to Mr. G. J. Manenschijn and Ir. D. Schalkoord for performing the analyses.
273
References 1 A. Brenner and G. E. Riddell, J. Res. Nat. Bur. Standards, 37 (1946) 1; 39 (1947) 385. 2 K. M. Gorbunova and A. A. Nikiforova, Protection of Metals (transl. from Zashchita Metallov) 5 (1967) 195. 3 T. V. Ivanovskaya and K. M. Gorbunova, Protection of Metals (transl. from Zashchita Metallov) 2 (1966) 477. 4 G. Salvago and P. L. Cavalotti, Plating, 59 (1971) 665. 5 C. Gabrielle and F. Raulin, J. Appl. Electrochem., 1 (1971) 167. 6 G. G. Gawrilow, Chemische (Stromlose) Vernicklung, Eugen G. Leuze Verlag, Saulgaul Wiirtt, 1974, p. 127. 7 C. H. de Minjer and A. Brenner, Plating, 44 (1957) 1297.