Ind. Eng. Chem. Res. 2009, 48, 1727–1734
1727
Exploring the Water-Soluble Phosphine Ligand as the Environmentally Friendly Stabilizer for Electroless Nickel Plating Ke Wang,† Liang Hong,*,†,‡ and Zhao-Lin Liu‡ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, BLK E5 02-02, 4 Engineering DriVe 4, Singapore 117576, and Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602
Phosphine (R3P) compounds being a typical type of coordination ligand used in homogeneous catalysis are known to possess a tunable affinity with the nickel atom. As such, it deserves to be studied whether a watersoluble phosphine ligand could also be used as a plating stabilizer in the electroless nickel plating (ENP) system in place of hazardous Pb(II) salt and what will be the unique effects generated due to this replacement. Triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (TPPTS) was then chosen as a model phosphine stabilizer to perform the study. TPPTS could indeed work to prevent the ENP process from running out of control. In addition, unlike the existing ENP stabilizers, TPPTS does not reveal a percolation concentration for a sharp ceasing of plating. Also according to the voltammetry investigation, TPPTS can be classified as a cathodic stabilizer. A Ni-P plating layer with high phosphorus content (>15%) has been achieved using10-4 mol/L TPPTS in the plating bath. Furthermore, this work investigated the relation between P weight percent and corrosion resistance of either the as-plated or the annealed Ni-P plating layer by electrochemical impedance spectroscopy (EIS). The annealing was found, according to the depth profile analysis, to drive diffusion of both Ni and P toward the bulk phase, which strongly revamps corrosion resistance of the plating layer. Finally, the stability of the ENP bath was evaluated using the metal-turnover (MTO) test. 1. Introduction Electroless nickel plating (ENP) technology has been widely applied in different industrial sectors to carry out surface modifications for various purposes. ENP is an autocatalytic reaction, meaning that both oxidation of hypophosphite and reduction of Ni2+ ion take place at the freshly generated Ni metal atoms on plating surface. Once a new layer of Ni atoms are deposited, they become the catalyst for the next redox cycle.1-4 Owing to the autocatalytic nature, an ENP bath is essentially a metastable system. As the plating proceeds, tiny particles of nickel or/and nickel phosphite are also generated as byproduct in the plating bath. The Ni atoms at the surface of these particles then act as highly reactive catalytic sites for nickel deposition because they are highly coordination unsaturated. Therefore these particles can trigger out a quick propagation of Ni colloidal particles and hence an overwhelming deposition of nickel black, leading to the failure of the plating bath.5,6 Such a random bath decomposition, consequently, results in a significant increase in operation costs. Technologically, the propagation of metal colloidal particles in an operating ENP bath can be effectively halted by a special kind of chemical known as the stabilizer. Historically, the Pb2+ ion at the parts per million level had been popularly used in commercial ENP baths as the stabilizer until very recently because potentially harmful effects of residual lead in the ENP layer and in the waste solution were realized. The development of lead-free ENP solutions has now become obligatory; for instance, the European Union has imposed the restriction (RoHS Directive) to bar the use of lead in the ENP system. A few nonlead chemicals, such as thiourea7,8 and cupric sulfate,9 have been proposed to be in place of Pb2+. But the presence of sulfur * To whom correspondence should be addressed. E-mail: chehongl@ nus.edu.sg. Fax: +65-6779-1936. Tel: +65-6516-5029. † National University of Singapore. ‡ Institute of Materials Research & Engineering.
(from thiourea) or copper (from cupric sulfate) in the Ni-P plating layer has been found to undermine the corrosion resistance of ENP deposition.10-15 The present work explores the water-soluble phosphine compound triphenylphosphine3,3′,3′′-trisulfonic acid trisodium salt (TPPTS) (Figure 1) as an ENP stabilizer. Phosphine compounds (R3P), in which the P(III) atom carried a lone pair of electrons, have been prevalently used in many homogeneous catalytic reactions as ligand of transition metal complex catalysts. Among them, Ni0-phosphine is an important type of coordination bonding.16-19 Compared to organic S(II) compounds, phosphine compounds possess stronger nucleophilicity to associate with Ni atoms. As a result, the phosphine compounds would preferentially target those Ni atoms that are highly coordination unsaturated on plating surface to repress undesirably high catalytic reactivity. According to the ASTM 733B-04 standard, a Ni-P deposition layer can be classified as low P (1-4 wt %), medium P (5-9 wt %), and high P (10 wt % and above) according to its phosphorus content.20 When the P content exceeds approximately 7-8%, the resulting Ni-P deposition layer has an amorphous matrix, which is desirable to most of industrial applications since its corrosion resistance is largely improved.1,2,21-26 In order to achieve a high-P deposition layer, there is always a tradeoff between deposition rate and phosphorus content. The basic strategy is to suppress the cathodic reaction rate (Ni2+f Ni0) via employing stronger coordination ligands of Ni2+ ion or a higher concentration of lactic acid than that implemented in the mid-P plating bath.27,28 Since the ENP bath
Figure 1. Molecular structure of TPPTS.
10.1021/ie801456f CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
1728 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Table 1. Composition of the Acidic Hypophosphite Plating Bath Made in-House components
concentration
nickel (II) sulfate hexahydrate sodium hypophosphite monohydrate DL-malic acid sodium acetate lactic acid TPPTS
22.4 g/L 25 g/L 4 g/L 8.5 g/L 21 mL/L variable
stabilizers used in industry are primarily anodic type, they do not promote P% appreciably in the Ni-P plating layer. On the other hand, it may be worth noting that the phosphine ligands have far weaker affinities with the Ni2+ ion than with the Ni0 atom to form a complex. This is an important trait because it allows phosphine to be used as an ENP bath stabilizer, viz., only a substantially small dose of phosphine in the ENP bath is enough to stabilize it. Otherwise the combination of phosphine with Ni2+ ions in the bulk of plating solution will mitigate its role as stabilizer. The other advantage of phosphine stabilizer, besides its low toxicity as compared to Pb2+/Cd2+, lies in its role in inhabiting the cathodic reaction rather than the anodic reaction, which has been identified in this work. As a result, the phosphine stabilizer could also help with raising phosphorus content in the plating layer with the increase in its concentration in the plating solution. This work has also checked the effect of TPPTS used, whose amount falls into the normal stabilizer concentration range (ca. 10-6-10-7 M), on the deposition rate and the properties of the ENP plating layer when the bath service life is extended to 4 MTOs. Moreover, as an appealing finding, the corrosion resistance of the high-P plating layers obtained from the use of TPPTS could be significantly improved via an annealing treatment. This unique outcome is attributed to the migration of both Ni and P atoms toward the bulk of the substrate according to EDX scanning over the cross section layer underneath the plating surface. 2. Experimental Details 2.1. Materials and Reagents. All chemical used were analytical-grade reagents. Deionized (DI) water with a resistivity greater than 15 MΩ · cm, generated by a Millipore Elix10 purification system, was used to prepare solutions and to clean glassware and samples. The brass hull cell test plates (Hiap Guan Electroplating Material Singapore) were used as the plating substrate. The prior plating cleaning included two steps: the brass
Figure 3. Schematic picture of adsorption of TPPTS on the plating surface.
Figure 4. XPS spectra of P2P and the profile from curve fitting.
plates were degreased in an alkaline detergent solution for 5 min with the assistance of ultrasonication, and this was followed by DI water rinsing. The plating was initiated by touching the substrate with a nickel wire. 2.2. Effects of Stabilizer Concentration on Plating Rate. The plating solution was formulated in house, and its main components are listed in Table 1. In this investigation, each data point of a curve was collated from a small plating bath containing 80 mL plating solution, in which the starting pH was adjusted to 5.00 ( 0.03 by the addition of ammonium hydroxide solution (25%) before plating. The temperature of plating solution was maintained at 90 °C by placing the reaction vessel in a water bath (HAAKE DC30/W13). The weight of the Ni-P layer gained (W) on the brass substrate (0.25 × 50 × 37 mm3) after plating for 30 min was determined using an analytical balance (A&D GR-200 Goldbell, Singapore). The deposition rate r was computed by the following formula: W × 104 (1) FAt where r is the deposition rate in micrometers per hour, W the weight of the plating layer in grams, A the plating area in cubic centimeters, t the plating time in hours, and F ) 7.8 g/cm2 is the density of the Ni-P alloy by taking P = 13%. 2.3. Electrochemical Analysis of Oxidation Rate of Hypophosphite. The anodic reaction of hypophosphite in the presence of TPPTS was evaluated using the linear-sweep voltammetry (Autolab, Model JP202). This experiment was carried out in an electrochemical cell equipped with three electrodes (CFC-13 Coating Flat Cell, Scrinberassociate, USA). The working electrode was a plated brass plate (13 cm2), the counter electrode was an expended platinum mesh with a surface area of 25 cm2, and the reference electrode was a standard silver/ silver-chloride electrode (Metrohm 6.0726.100, Switzerland). r)
Figure 2. (a) Influence of concentration of TPPTS on Ni-P deposition rate. (b) Peak values of anodic current density vs the corresponding concentrations of TPPTS.
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1729
Figure 6. Nyquist plots obtained from the as-plated ENP plating layers in 3.5% NaCl solution.
Figure 5. Dependence of decomposition volume on the concentration of stabilizer. Table 2. Effect of TPPTS Concentration on the Ni and P Contents in the Ni-P Deposition Layers Pb2+
TPPTS concentration of stabilizers (mol/L) -9
10 10-8 10-7 10-6 10-5 10-4 10-3
Ni wt %
P wt %
Ni wt %
P wt %
87.6 86.3 86.0 85.8 85.5 84.8 84.7
12.4 13.8 14.0 14.2 14.5 15.2 15.3
87.8 87.9 88.0 88.0 88.1
12.2 12.1 12.0 12.0 11.0
The solution charged into the cell contained sodium hypophosphite and the phosphine stabilizer whose concentration was varied, and the pH of solution was maintained at 5.00 ( 0.03. This solution was kept still and its temperature at 70 °C during potential scanning unless otherwise noted. Then the currentpotential curves were obtained from a potential scan with a rate of 10 mV/s. 2.4. Assessment of ENP Bath Stability. The starting conditions of plating bath and the substrate were the same as those described in section 2.2. The pH (5.00 ( 0.05) of the plating bath was maintained by frequently dripping in 25% ammonium hydroxide solution, and the plating solution was agitated by mild air bubbling through the entire process. The stabilizer concentration in this metal-turnover (MTO) testing process was kept around 2 × 10-6 M. The plating solution (1.2 L) and the plating load (defined as the plated area in squared decimeters per volume of plating solution in liters) of 2.5 dm2/L were fixed throughout the MTO test. The plating bath was replenished after about each 30-min interval using a concentrated plating solution. The stabilizer in the plating bath was replenished at the beginning of each MTO, and the brass substrate was changed after completion of each MTO. 2.5. Evaluation of Corrosion Resistance. The corrosion resistance of Ni-P layer was assessed with using electrochemical impedance spectroscopy (EIS). The measurement was conducted in the three-electrode electrochemical system; the same setup as that was used to measure the anodic oxidation rate. All the samples were controlled to have a thickness of about 8 µm. For EIS measurement, an ac voltage in a sine waveform with an amplitude of 10 mV was used as the imposing signal, and the measurement frequency was varied in the range from 100 000 to 0.01 Hz. Before the EIS test, each specimen was immersed in the testing solution for 30 min to reach a stable
Figure 7. Equivalent circuit of the EIS measurement for the as-deposited Ni-P plating layers.
open-circuit potential (OCP). The testing solution was an aqueous NaCl solution (3.5 wt %). 2.6. Heat Treatment of As-Deposited Ni-P Layer. The heat treatment of the as-deposited electroless Ni-P plated brass specimens was carried out in a furnace (Carbolite, CWF1300, England) at a constant heating rate of 10 °C/min from ambient temperature to 500 °C. After calcining at 500 °C for 1 h, the specimens were naturally cooled down to the room temperature within 5 h. The whole annealing process was conducted in the nitrogen atmosphere to prevent oxidation in the plating layer. 2.7. In situ Adsorption of TPPTS on Fresh Nickel Powders. Two aqueous solutions (20 mL of each) containing 0.05 M NiSO4 and 0.05 M NiSO4 + 0.025 M TPPTS were prepared respectively, and an excess of sodium boron hydride (NaBH4, 0.1 g) was added to the solutions under vigorous stirring. The resultant black suspension was centrifuged after the reduction was completed (i.e., the ease of bubbling). The precipitate was washed with DI water and centrifuged, and this cleaning step was repeated five times. The powder obtained was dried at 60 °C and then analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis HiS System) to examine whether there was chemically adsorbed phosphine species on the nickel metal particles. The binding energies of XPS peaks were calibrated using the carbon 1s peak that was corrected to 284.6 eV. The curve fitting was carried out by XPS PEAK (version 4.1). 2.8. Other Instrumental Analyses. The surface morphology of the Ni-P deposits was investigated by field emission scanning electron microscopy (FESEM; JEOL, JSM-6700F, Japan). The energy dispersive X-ray spectrometry (EDX; IncaEnergy 400, Oxford Instrument Ltd.) was employed to determine the chemical composition of the Ni-P deposits. In this measurement, data were collected from the three locations on each sample and the mean value was reported. 2.9. Palladium Titration. The effectiveness of phosphine on holding back nickel deposition on metal colloidal particles was evaluated using palladium titration method as we used before.29 An aqueous PdCl2 solution was prepared by dissolving
1730 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Table 3. Fitting Parameters of Nyquist Plots for the Equivalent Circuit Described in Figure 7 TPPTS conc/M
10-9
10-8
10-7
10-6
10-5
10-4
Rs/Ω Rc/Ω CPE/F n
1.377 3772 1.420 × 10-4 0.9197
1.568 3894 1.313 × 10-4 0.9107
1.574 4341 1.210 × 10-4 0.9179
2.617 4669 1.187 × 10-4 0.9208
1.413 4983 0.8658 × 10-4 0.8871
1.408 5575 0.6638 × 10-4 0.8980
25 mg of PdCl2 in 5 mL concentrated hydrochloric acid (37%). The resulting solution was then diluted by DI water to 500 mL in a volumetric flask. Before titration, the pH of the plating
bath was adjusted to 4.80 ( 0.03 by adding in ammonia solution. The PdCl2 solution was dripped at a rate of ca. 0.05 mL/s into 20 mL ENP solution, keeping the temperature at 60 ( 2 °C. Sufficient magnetic stirring was upheld to disperse PdCl2 added instantaneously. The titration was stopped when the ENP bath suddenly turned black, which was a sign of the large precipitation of metal colloids. The PdCl2 solution consumed represents the volume required (Vd) to cause decomposition of the solution. For each concentration, the titration was repeated 20 times to reduce the accidental error. 3. Results and Discussion
Figure 8. XRD pattern of EN deposit before and after heat treatment.
Figure 9. Nyquist plots obtained from the heat-treated EN deposits in 3.5% NaCl solution.
3.1. How TPPTS Influences the ENP Deposition Rate. It has been clearly confirmed that an ENP solution without a stabilizer is vulnerable to the formation of tiny in-soluble particles in the plating solution when it operates. Normally a stabilizer-free ENP solution will turn cloudy before long since the generation of colloidal particles in the solution is unavoidable with prolonging of plating.1,2 In this work, we have also proven that the ENP solution composed of ingredients (as shown in Table 1) except TPPTS could remain stable until completion of the first MTO (section 2.4), but the solution becomes less transparent and then turns cloudy since the second MTO cycle. In contrast, a substantially dilute concentration of TPPTS (e.g., 10-6 M) in the ENP solution could retain a clear solution even after completion of four MTO cycles as well as the quality of plating layer. This aspect will be corroborated in the following parts. Unlike the existing ENP stabilizer systems,30 there is not a steep drop of plating rate at the supposedly critical concentration, which is usually observed when the anodic type of stabilizer is used, in the curves of ENP deposition rate vs the concentration of TPPTS (Figure 2a). The deposition rate displays a stagnant response to the increase in the concentration from 10-9 to 10-4 mol/L. Only a slight decrease in deposition rate was observed at the concentration of 10-3 mol/L. It is worthy noting that the deposition rate curve in Figure 2a is below the value (15 µm/h) obtained when no stabilizer is employed in the same plating solution. On the contrary, the sulfur-containing stabilizers promote nickel deposition rate before the critical concentration,7,8 although both R2S and R3P compounds are generally classified as Lewis base. Such a rate-suppression phenomenon implies that the phosphine works in a different way than the sulfurcontaining stabilizers to affect ENP kinetics. Normally, the role of stabilizer in the ENP system has been regarded as inhibition to the anodic process, since the participation of stabilizer molecules in plating surface can tighten up the electron stream when the cathoidc reaction of Ni2+ (eq 2) runs away.31-34 H2PO2- + H2O f H2PO3- + 2H+ + 2eNi2++2e- f Ni0 and H2PO2- + e- + 2H+ f P + 2H2O (2)
Figure 10. Equivalent circuit for EIS measurements on corrosion resistance of the heat-treated Ni-P deposition layers.
From this perspective, we carried out an anodic potentiodynamic scanning measurement, from which the anodic current density of the working electrode (i.e., a Ni-P layer on the brass
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1731 Table 4. Fitting Parameters of Nyquist Plots for the Equivalent Circuit Described in Figure 10 TPPTS Conc./M
10-9
10-8
10-7
10-6
10-5
10-4
Rs/Ω Rc1/Ω CPE1/F n1 Rc2/kΩ CPE2/F n2
1.336 93.8 0.1723 × 10-5 0.7146 110.7 0.2425 × 10-4 0.8753
1.644 68.0 0.3224 × 10-5 0.7503 193.3 0.2716 × 10-4 0.8767
2.527 58.1 0.4895 × 10-5 0.8195 186.1 0.4012 × 10-4 0.8893
0.542 235.9 0.9704 × 10-6 0.7141 139.4 0.5095 × 10-4 0.8958
0.598 205.1 0.8589 × 10-6 0.7405 120.5 0.3881 × 10-4 0.8790
1.575 72.6 0.2852 × 10-5 0.7408 200.1 0.3636 × 10-4 0.8781
substrate) was obtained. In this test, the peak height of current density reflects the oxidation rate of hypophosphite ion on the working electrode (Figure 2b). It is interesting to note that the current density values (∼2.7-2.8 mA/cm2) especially in the low concentration range ( -5, which is in fact the critical concentration of Pb2+ stabilizer, meaning that a concentration above this value will bring about a significant increase in solution stability or inertness to plating. Therefore, a fast increase in Vd is incurred as observed. The Vd ∼ log[TPPTS] curve displays a profile slightly below the Vd ∼ log[Pb2+] curve in the range log[TPPTS] < -4.7. But beyond this margin, Pb2+ manifests a much stronger idling power than TPPTS due to its stronger stabilizing mechanism.30 Also according to the Vd ∼ log C relation, the TPPTS compound presents no substantial difference in the stabilizing capability from Pb2+ ion in the concentration range below 10-6 M, which is the stabilizer concentration level normally adopted in industrial ENP baths. 3.3. Will the Increase in TPPTS Concentration Improve Corrosion Resistance of Ni-P Depositon ? Whether or not the phosphine stabilizer could boost the phosphorus content in ENP deposition layer is a meaningful problem to study. This is because the P weight percent strongly affects the corrosion resistance of a Ni-P layer. The conventional method to achieve a high P weight percent deposition layer is through suppression of the reduction rate of Ni2+ by means of using ligands having greater Ks values with Ni2+ ion.2 As far as the present system is concerned, TPPTS raises the P weight percent in a broad concentration range from 10-9-10-4 M through the suppression of the cathodic reaction rate. In light of the effect of stabilizer on P weight percent, the use of Pb2+ results in a lower P weight percent level in general (Table 2) than the use of TPPTS could realize. None of P weight percent values were available in Table 2 when [Pb2+] was higher than 10-5 mol/L because ENP ceased beyond this concentration percolation. Pursuant to the above discussion, Pb2+ is a typical anodic stabilizer so that the P weight percent declines with the increase in [Pb2+], whereas TPPTS is a stabilizer of the cathodic type, which promotes P weight percent with an increase in its concentration. The electrochemical impedance spectroscopy (EIS) was employed to examine the corrosion resistance of ENP deposition layer. The Ni-P deposition layers obtained from the solutions using different concentrations of TPPTS (Figure 6) all display arc-shape Nyquist plots in the scanned frequency range. Nevertheless, these arcs differ in their amplitude as well as the span over the scanned frequency range. The Nyquist plots were fitted by the equivalent circuit as illustrated in Figure 7 to simulate the metal/solution interface where the constant phase element (CPE) resembles a capacitor since the phase angle between the imaginary and the real parts is smaller than 90°.
1732 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009
Figure 11. Depth profile analysis on the cross section for ENP deposits before and after heat treatment. As-deposited EN deposit: a) brass substrate; b) EN deposition layer; Heat-treated EN deposit: a) brass substrate; b) diffusion layer; c) crystallization layer.
Figure 12. Effect of MTO on the deposition rate with using TPPTS as the stabilizer. Table 5. Ni and P Contents in the Ni-P Deposits from Every MTO Round TPPTS: 2 × 10-6 mol/L MTO
Ni wt %
P wt %
1 2 3 4
87.5 87.1 87.3 87.5
12.5 12.9 12.7 12.5
CPE describes inhomogeneous distribution of solid/liquid interface. The fitting results are shown in Table 3, in which the dimensionless parameter n evaluates CPE. With the increase of capacitor component in CPE to its apex, the n value approaches to unity. One may note that accompanying the increase in P weight percent in the deposition layer (Table 3) the Rc values increase but the values of CPE decrease. This trend reflects the improvement on corrosion resistance of the deposition layer. The result is in agreement with the study of other groups.20,35,36 3.4. Study on the Heat Treatment Effect on the Ni-P Deposition Layer. It is well-known that annealing of an ENP deposition layer will cause significant changes in surface properties. In general, heat treatment can drastically boost hardness and wear resistance of the Ni-P layer.37,38 But, controversy arises about the leverage of annealing on the
corrosion resistance of an ENP deposition layer that has been obtained. Singh et al.,39 Ashassi-Sorkhabi et al.,40 and Schenzel et al.41 reported that the corrosion resistance of the annealed EN deposits was increased. Whereas the investigations reporting the negative heat-treatment effect on corrosion resistance are still predominant.42,43 Regarding this, we were inclined to consider that the plating bath conditions, in particular P% and the type of substrate used, compose an impact on the final outcome of annealing. To explore how TPPTS affects this feature, the samples as demonstrated in Figure 6 were subjected to annealing at 500 °C for 1 h. As a result, the amorphous asplated ENP layer was transformed to crystalline NixPy phases (Figure 8), which will become susceptible to corrosion due to the generation of grain-boundary structure. A set of Nyquist plots (Figure 9) of the annealed deposition layers was obtained from the same evaluation system as that of Figure 8. A rather different equivalent circuit (Figure 10) compared with that describing the as-plated samples was obtained from the simulation of the Nyquist plots. It contains the two electric components, CPE1/Rc1 and CPE2/Rc2, which can be interpreted as the two interfacial layers. The parameters of fitting are listed in Table 4. The corrosion resistance of the crystallized outer layer (Rc1) is negligible as compared with that of the diffusion inner layers (Rc2). The corrosion resistance of the EN coatings almost comes from the inner layer. From the Nyquist plots obtained we could see a unique pattern between the P weight percent (the second column in Table 2) and the corrosion resistance (Rc2) except the two samples with both the highest and lowest P weight percent. The pattern shows that a lower P weight percent gives rise to a greater Rc2 after the annealing treatment. Taking for example,the sample prepared from the bath containing 10-5 TPPTS, one can find that the corrosion resistance is enhanced by more than 20 times. To understand this phenomenon, the depth elemental distribution profiles of this sample before and after annealing were analyzed (Figure 11). The annealed surface consists of the three layers except the components of brass, which are an outer crystallized Ni-P layer with a thickness of ca. 2 µm and two inner Ni-P diffusion layers that can be found at the depths of ca. 6 and 9 µm, respectively. It can be found that these two diffusion layers behave like a narrow wedge, being plugged into the brass matrix and having a thickness slightly less than 1 µm. It is estimated that both Ni and P atoms retain an amorphous assembly in these two diffusion layers since the sample reveals an obviously improved corrosion resistance. According to the fact that crystallization in the Ni-P plating layer causes loss of corrosion resistance possessed originally by its amorphous counterpart, the outer crystallized layer is not considered to help promote corrosion resistance. Hence, the two diffusion Ni-P layers constitute two anticorrosion screens. Two factors are deemed to be critical, the presence of P atoms in the diffusion layer and the state that they are sandwiched in the brass matrix. The squeezing environment might help to restrict crystallization to take place in these two Ni-P diffusion layers during annealing. For a deposition layer with a lower P weight percent, the diffusion of Ni-P atoms might be more easily to take place, and hence, diffusion layers are more easily formed. In light of the two boundary cases of P weight percent, e.g., 12.4% and 15.3%, their corrosion resistance is mainly affected by the P weight percent in the diffusion layers.
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1733
3.5. Performance of TPPTS in a Continuous ENP Process. The ENP bath stabilization capability of TPPTS was further examined in a four-MTO (metal-turnover) continuous plating process (as stated in section 2.4). The plating solution still remained homogeneous and clear after completion of 4-MTO despite a very slight falling of plating rate (Figure 12). This decrease in ENP rate with the prolongation of reaction time is due to the accumulation of stabilizer, byproduct, buffers etc. in the solution. The P% in the plating layer could be roughly maintained stable throughout the four MTOs (Table 5). On the other hand, it can be observed by FESEM that the surface morphologies generated are basically smooth, expect there are some codeposited particles in the surface of sample prepared from the fourth MTO. 4. Conclusions This work investigated a lead-free electroless nickel plating (ENP) bath in which water-soluble phosphine ligand, triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (TPPTS), was employed as the bath stabilizer. The variation of ENP deposition rate with the stabilizer concentration was investigated. TPPTS does not reveal a concentration percolation, over which the plating rate undergoes a rapid declination with the increase in concentration. Besides this, TPPTS also reveals a weak rate-suppression effect due to the very low concentration (i.e., 10-7 mol/L). Furthermore, this work has confirmed, by the linear-sweep voltammetry test, that TPPTS does not deactivate the oxidation of hypophosphite on the plating surface known as the anodic process. In addition, the stabilization capability of TPPTS to the increasing of metal colloidal particles in the plating solution was examined by means of PdCl2-HCl solution titration, and it was found to be very similar to that of Pb2+ stabilizer in the concentration range below the critical concentration of Pb2+. On the basis of the traits of TPPTS in the ENP system and the fact that it does not form a complex with the Ni2+ ion, this phosphine ligand is regarded as an ENP stabilizer of cathodic type. As far as the TPPTS stabilizing mechanism is concerned, it works through adsorption on coordination unsaturated Ni atoms on the plating surface. As a result of being a cathodic type stabilizer, TPPTS also brings about a rise of P% in the Ni-P deposition layer and therefore an improvement on corrosion resistance of the Ni-P deposition layer. The corrosion resistance was assessed using the impedance frequency approach. A high-P plating layer (P% > 14) can be achieved when the content of TPPTS is still very low (10-7 mol/L) and P% ) 15 is realized at the TPPTS concentration level of 10-4 mol/L. It is noteworthy that the corrosion resistance of the plating layers was revamped by about 20 times on average when these plated brass plates were subjected to annealing due to the formation of double diffusion Ni-P layers inside the brass substrate. In the last part of this work, the stabilization capability of TPPTS was tested in a four-MTO process. The result showed that the properties, for example, surface morphology, deposit composition, and plating rate, do not exhibit a perceivable change with the extension of operation time. TPPTS is therefore regarded as a stabilizer that has great potential to replace lead in the future industrial applications due to several advantages that this compound could offer. Beside the application in the ENP system, it is anticipated that TPPTS can be also used in electroless cobalt plating systems since they are similar.
Literature Cited (1) Riedel, W. Electroless Nickel Plating; ASM International, Finishing Publications: Stevenage, Hertfordshire, U.K., 1991. (2) Mallory, G. O., Haydu, J. B., Eds. Electroless Plating-Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (3) Schlesinger, M., Paunovic, M., Eds. Modern Electroplating, 4th ed.; John & Sons, Inc.: New York, 2000. (4) Winowlin Jappes, J. T.; Ramamoorthy, B.; Kesavan Nair, P. A study on the influence of process parameters on efficiency and crystallinity of electroless Ni-P deposits. J. Mater. Process. Technol. 2005, 169, 308–313. (5) Yin, X.; Hong, L.; Chen, B.-H.; Ko, T.-M. Modeling the stability of electroless plating bath s diffusion of nickel colloidal particles from the plating frontier. J. Colloid Interface Sci. 2003, 262, 89–96. (6) Cheong, W. J.; Luan, B. L.; Shoesmith, D. W. The effects of stabilizers on the bath stability of electroless Ni deposition and the deposit. Appl. Surf. Sci. 2004, 229, 282–300. (7) Han, K. P.; Jing, L. F. Stabilization Effict of Electroless Nickel Plating by Thiourea. Met. Finish. 1997, 5, 73–75. (8) Lin, K. L.; Hwang, J.-W. Effect of thiourea and lead acetate on the deposition of electroless nickel. Mater. Chem. Phys. 2002, 76, 204–211. (9) Chen, C.-H.; Chen, B.-H.; Hong, L. Role of Cu2+ as an Additive in an Electroless Nickel-Phosphorus Plating System: A Stabilizer or a Codeposit. Chem. Mater. 2006, 18, 2959–2968. (10) Petukhov, I. V.; Shcherban, M. G. Effect of lead chloride on chemical deposition of Ni-P coatings. Prot. Met. 1999, 35, 556–570. (11) Das, L.; Chin, D. T. Effect of bath stabilizers on electroless nickel deposition on ferrous substrates. Plat. Surf. Finish. 1996, 83, 55–61. (12) Cheong, W. J.; Luan, B. L.; Shoesmith, D. W. The effects of stabilizers on the bath stability of electroless Ni deposition and the deposit. Appl. Surf. Sci. 2004, 229, 282–300. (13) Armyanov, S.; Georgieva, J.; Tachev, D.; Valova, E.; Nyagolova, N.; Mehta, S.; Leibman, D.; Ruffini, A. Electroless deposition of Ni-Cu-P alloys in acidic solutions. Electrochem. Solid-State Lett. 1999, 2, 323–325. (14) Tarozaite, R.; Selskis, A. Electroless nickel plating with Cu2+ and dicarboxylic acids additives. Trans. Inst. Met. Finish. 2006, 84, 105–112. (15) Touir, R.; Larhzil, H.; Ebntouhami, M.; Cherkaoui, M.; Chassaing, E. Electroless deposition of copper in acidic solutions using hypophosphite reducing agent. J. Appl. Electrochem. 2006, 36, 69–75. (16) Tamao, K. Discovery of the cross-coupling reaction between grignard reagents and C(sp2) halides catalyzed by nickel-phosphine complexes. J. Organomet. Chem. 2002, 653, 23–26. (17) Norris, C. M.; Schreiner, S. Chemistry in environmentally benign media: Synthesis and characterization of water-soluble nickel-phosphine complexes. Abstr. Pap. Am. Chem. Soc. 1999, 217, U1012U1012 227-INOR Part 1. (18) Miedaner, A.; Noll, B. C.; DuBois, D. L. Synthesis and characterization of palladium and nickel complexes with positively charged triphosphine ligands and their use as electrochemical CO2-reduction catalysts. Organometallics 1997, 16, 5779–5791. (19) Baird, I. R.; Smith, M. B.; James, B. R. Nickel(II) and nickel(0) complexes containing 2-pyridylphosphine ligands, including water-soluble species. Inorg. Chim. Acta 1995, 235, 291–297. (20) Cui, G. F.; Li, N.; Li, D. Y.; Zheng, J.; Wu, Q. L. The physical and electrochemical properties of electroless deposited nickel-phosphorus black coatings. Surf. Coat. Technol. 2006, 200, 6808–6814. (21) Martyak, N. M. Characterization of Thin Electroless Nickel Coatings. Chem. Mater. 1994, 6, 1667–1674. (22) Mai, Q. X.; Daniels, R. D.; Harpalani, H. B. Structural Changes Induced by Heating in Electroless Nickel-Phosphorus Alloys. Thin Solid Films 1988, 166, 235–247. (23) Lambert, M. R.; Duquette, D. J. A Study of Electroless Nickel Coatings Containing Low Phosphorus. Thin Solid Film 1989, 177, 207– 223. (24) Kumar, P. S.; Nair, P. K. Studies on Crystallization of Electroless Ni-P Deposits. J. Mater. Proc. Tech. 1996, 56, 511–520. (25) Baskaran, I.; Sankara Narayanan, T. S. N.; Stephen, A. Effect of accelerators and stabilizers on the formation and characteristics of electroless Ni-P deposits. Mater. Chem. Phys. 2006, 99, 117–126. (26) Krolikowski, A.; Karbownicka, B.; Jaklewicz, O. Anodic dissolution of amorphous Ni-P alloys. Electrochim. Acta 2006, 51, 6120–6127. (27) Mallory, G. O., Jr. Method for depositing electroless nickel phosphorus alloys. US Patent 6,020,021, 1998. (28) Cheng, Y. H.; Zou, Y.; Cheng, L.; Liu, W. Effect of complexing agents on properties of electroless Ni-P deposits. Mater. Sci. Technol. 2008, 24, 457–460.
1734 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 (29) Yin, X.; Hong, L.; Chen, B. H.; Ko, T. M. Modeling the stability of electroless plating bathsdiffusion of nickel colloidal particles from the plating frontier. J. Colloid Interface Sci. 2003, 262, 89–96. (30) Yin, X.; Hong, L.; Chen, B.-H. Role of a Pb+ Stabilizer in the Electroless Nickel Plating System: A Theoretical Exploration. J. Phys. Chem. B 2004, 108, 10919–10929. (31) Ohno, I.; Wakabayashi, O.; Haruyama, S. Anodic Oxidation of Reductants in Electroless Plating. J. Electrochem. Soc. 1985, 132, 2323– 2330. (32) Feldstein, N.; Lancsek, T. S. A New Technique for Investigating the Electrochemical Behavior of Electroless Plating Baths and the Mechanism of Electroless Nickel Plating. J. Electrochem. Soc. 1971, 118, 869– 874. (33) Han, K. P.; Fang, J. L. Effect of Cysteine on the Kinetics of Electroless Nickel Deposition. J. Appl. Electrochem. 1996, 26, 1273–1277. (34) Sotskaya, N. V.; Ryabinina, E. I.; Karvchenko, T. A.; Shikhaliev, Kh. S. The Role of Organic Additives in the Electroless Nickel Plating Bath. Zashchita MetalloV 2003, 39, 276–280. (35) Rajam, K. S.; Rajagopal, I.; Rajagopalan, S. R. Phosphorus content and heat treatment effects on the corrosion resistance of electroless nickel. Plat. Surf. Finish. 1990, 77, 63–66. (36) Lo, P.-H.; Tsai, W.-T.; Lee, J.-T.; Hung, M.-P. Role of phosphorus in the electrochemical behavior of electroless Ni-P alloys in 3.5 wt.% NaCl solutions. Surf. Coat. Technol. 1994, 67, 27–34.
(37) Keong, K. G.; Sha, W.; Malinov, S. Hardness evolution of electroless nickel-phosphorus deposits with thermal processing. Surf. Coat. Technol. 2003, 168, 263–274. (38) Wing, L. M. The use of electroless nickel on automotive components. Trans. Inst. Met. Finish. 1997, 75, B11–B14. (39) Singh, D.; Balasubramaniam, R.; Dube, R. K. Effect of coating time and coating time on corrosion behavior of electrolesss nickelphosphorus coated powder-metallurgy iron specimens. Corros. 1995, 51, 581–585. (40) Ashassi-Sorkhabi, H.; Rafizadeh, S. H. Effect of coating time and heat treatment on structures and corrosion characteristics of electroless Ni-P alloy deposits. Surf. Coat. Technol. 2004, 176, 318–326. (41) Schenzel, H. G.; Kreye, H. Improved corrosion resistance of electroless nickel phosphorus coatings. Plat. Surf. Finish. 1990, 77, 50–54. (42) Singh, D. D. N.; Ghosh, R. Electroless nickel-phosphorus coatings to proctect steel reinforcement bars from chloride induced corrosion. Surf. Coat. Technol. 2006, 201, 90–101. (43) Duncan, R. N. Performance of electroless nickel coated steel in oil-field environments. Mater. Perform. 1983, 22, 28–34.
ReceiVed for reView September 27, 2008 ReVised manuscript receiVed December 10, 2008 Accepted December 13, 2008 IE801456F
