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Brazilian Journal of Chemical Engineering

Print version ISSN 0104-6632On-line version ISSN 1678-4383

Braz. J. Chem. Eng. vol.17 n.4-7 São Paulo Dec. 2000

http://dx.doi.org/10.1590/S0104-66322000000400007 

CONTROL AND OPTIMIZATION OF BATHS FOR ELECTRODEPOSITION OF Co-Mo-B AMORPHOUS ALLOYS

 

S.Prasad, F.A.Marinho and F.S.M.Santana
Department of Chemical Engineering, CCT, UFPB-Campus II, C. P. 10108,
CEP 5819-970, Campina Grande - PB, Brazil

 

(Received: November 25, 1999 ; Accepted: April 6, 2000)

 

 

Abstract. Optimization and control of an electrodeposition process for depositing boron-containing amorphous metallic layer of cobalt-molybdenum alloy onto a cathode from an electrolytic bath having cobalt sulfate, sodium molybdate, boron phosphate, sodium citrate, 1-dodecylsulfate-Na, ammonium sulfate and ammonia or sulfuric acid for pH adjustments has been studied. Detailed studies on bath composition, pH, temperature, mechanical agitation and cathode current density have led to optimum conditions for obtaining satisfactory alloy deposits. These alloys were found to have interesting properties such as high hardness, corrosion resistance, wear resistance and also sufficient ductility. A voltammetric method for automatic monitoring and control of the process has been proposed.
Keywords: Electrodeposition, electrolytic baths, cobalt-molybdenum amorphous alloys.

 

 

INTRODUCTION

The electrodeposition of molybdenum and tungsten has been of considerable interest because of unusual properties of these metals (Brenner, 1963). Of all the metals, tungsten possesses the highest melting point (3410oC), the lowest coefficient of linear thermal expansion (4.3x10-6/oC) and the highest tensile strength (410 kg/mm2). It has very high thermal conductivity (0.487 cal/cm2/cm/oC) and is one of the densest metal (19.3 g/cm3). The metal has unusual mechanical properties. The properties of molybdenum are similar to those of tungsten but not as outstanding. Since molybdenum is cheaper than tungsten it is more economic alternative. For practical applications its most important properties are its high melting point 2610oC and its relatively high mechanical strength at elevated temperatures. Its 0.45% titanium alloy has a tensile strength of about 5000 kg/cm2 at 8700C as compared to about 1000 kg/cm2 for the so-called superstrength alloys now in use. Molybdenum is more ductile than tungsten. It has low coefficient of linear thermal expansion and high Young’s modulus of elasticity. Molybdenum has good resistance to chemical attack; but at elevated temperatures it gets oxidized.

In spite of numerous claims, the electrodeposition of tungsten as well as of molybdenum in pure state from aqueous solutions has been unsuccessful (Devis and Gentry, 1956; Holt, 1956). But their electrolytic induced codeposition occurs with iron-group metals (Brenner, 1963). The most important of the early researches on the deposition of tungsten- and molybdenum-alloys were those of several Russians, particularly the work of Goltz and Kharlamov (1936). They used ammoniacal plating solutions. The deposits obtained were porous and weak. The next development was the use of organic hydroxy acids in these ammoniacal baths to yield more stable baths of much higher concentrations (Vaaler and Holt, 1946; Brenner et al, 1953) . The alloys deposited were crystalline. More recently Watanabe (1985) has reported electrodeposition of several amorphous alloys.

The literature reveals that under proper conditions of bath composition, voltage and current parameters, an amorphous layer may be deposited on a cathode by electrodeposition. But for the most part the electrodeposition of amorphous alloys of these metals has been limited to a few demonstration systems of little direct practical interest, and there are no known instances of electrodeposition of high hardness, wear resistance, moderately ductile amorphous alloys. If a technique could be found to produce such materials it would then be possible, for example, to produce highly wear-resistant barrel liners by replacing conventional low-ductility chromium liner coating with an amorphous layer that would resist spilling of the coating. Many other such applications may be envisioned, including, for example, pump housing, instrument bores, piston rings, cylinder housing and bearings.

Thus there is a need for a process for preparing coatings of high hardness, wear-resistant, corrosion resistant and moderately ductile amorphous alloys by electrodeposition. The work on development, optimization and control of baths for producing such amorphous alloys has therefore been undertaken. The results on electrodeposition of Co-W-B (Prasad, 1994, 1997; Prasad and Marinho, 2000) and Ni-W-B (Prasad, 1993, 1998) have already been reported from this laboratory.

 

EXPERIMENTAL

All reagents used were of analytical grade purity and deionized distilled water was used for preparation of the solutions.

Bath developed for electrodeposition of Ni-Mo-B amorphous alloys (Guimarães et al, 1996) was modified to get a deposit of Co-Mo-B alloy with desirable characteristics. Nickel sulfate was substituted by cobalt sulfate. Boron phosphate was incorporated in the bath to provide a source of boron for creating amorphous nature to the deposit. Boron content was kept as high as possible consistent with solubility. Sodium molybdate and cobalt sulfate were added to the bath as source of molybdenum and cobalt. The concentrations of cobalt and molybdenum were kept as high as possible consistent with remaining soluble at proper mole ratio. Sodium citrate and ammonium sulfate were added as complexing agents for stability of the bath. Their optimum concentrations for the bath were determined experimentally. 1-dodecylsulfate-Na was added as a wetting agent to reduce hydrogen pitting. pH of the bath was adjusted with ammonia or sulfuric acid.

The electrodeposition was realized on a copper sheet cathode of 2.0-2.5 cm2 surface area rotated at a constant rate by EG&G 616 rotor, using a platinum foil of 15.36 cm2 as anode, potentiostat Amel 555B as a source of potential and a thermostat MTA KUTESZ MD2 for controlling the bath temperature. The cathode was polished on Carbimate papers with decreasing grit size and finished on alumina powder of 1 and 0.3 mm. The nature of the deposit was verified by X-ray diffraction and the composition of the deposit was determined by atomic absorption spectroscopy. The deposit was dissolved by digesting in nitric acid for the analysis. Microhardness of the deposits was determined by usual indentation method. Details of the methods and equipment used for the physicochemical determinations can be seen elsewhere (Prasad, 1994).

A large number of electrodeposition experiments was realized for optimization of the bath composition and the operating parameters. The deposition experiments were performed by using the following ranges: cathode current density 20-100 mA/cm2, cathode rotation 5-30 rpm, bath temperature 25-80oC and bath pH 3.0-9.0. After every 20 min of the deposition the cathode was dried and weighed to analyze the results. The result reported is an average of three experiments performed under identical conditions.

Voltammetric experiments were performed to monitor concentration and electrochemical reactivity of metallic species in the bath. The solutions were deaerated by purified argon, and a blanket of argon was maintained over the solution during experiments. An EG&G 273 potentiostat was used as the source of applied potential and as a measuring device.

 

RESULTS AND DISCUSSION

Based on a series of electrodeposition experiments an electrolytic bath composition presented in Table 1 was selected for further studies. The electrodeposition was performed at 45oC on a copper cathode of 2.3 cm2 surface area rotating at a speed of 10 rpm and maintained at a constant current density of 50 mA/cm2. The deposit obtained was sound, smooth and amorphous with an average composition of 51% Co, 47% Mo and 2% B. The maximum cathode current density was around 65%.

 

 

Using the bath presented in Table 1, a series of electrodeposition experiments was performed to study the effect of variation of bath composition and of operating parameters on deposition of the alloy.

 

EFFECT OF THE BATH VARIABLES

Effect of Cobalt and Molybdenum Contents in the Bath

Cobalt sulfate concentration range of 0.0 to 0.4 M was studied. For cobalt sulfate concentration less than 2 x 10-4 M only a thin oxide-like deposit was obtained, which may be a deposit of MoO2 as reported by Chassaing et al (1989) for citrate baths. Further reduction of molybdenum may be stopped due to low hydrogen over-potential of MoO2 layer (Savagado, 1992; Prasad and Guimarães, 1998). On increasing the concentration of cobalt sulfate to the level 0.01 M, a deposit of satisfactory appearance started forming. The cathode current efficiency was very low (<5%). With increase in concentration of cobalt sulfate to 0.40 M the cathode current efficiency increased to about 70% (Fig. 1). The irregular variation in cathode current efficiency (fig. 1) may be ascribed to some change occurring in the alloy composition when cobalt sulfate concentration is raised beyond 0.2 M. The bath with 0.20 M cobalt sulfate gave sound and adherent deposit of acceptable characteristics.

 

 

The sodium molybdate concentration was varied in the range of 0.005-0.30 M for studying its effect on the deposition. It was found that when the concentration of sodium molybdate was less than 0.007 M, the cathode current efficiency increased significantly and the deposit started changing its nature from amorphous to crystalline. On the other hand, an increase in concentration of sodium molybdate beyond 0.02 M deteriorated the quality of the deposit, which may be ascribed to incomplete reduction of molybdenum resulting in dark deposits.

Effect of Complexing Agents.

Ammonium salts are important constituents of the Co-Mo alloy plating baths. They form soluble complex with cobalt ions. But it was found that ammonium ions alone are not adequate because in the presence of molybdate ions, cobalt gradually starts giving a precipitate. The use of certain complexing agents in conjunction with ammonia considerably increases the stability of these baths.

Inorganic complexing agents such as hexametaphosphate and ammoniacal phosphite were found to be unsuccessful in keeping the cobalt in soluble form. Ethylene diammine tetra-acetic acid was unsatisfactory because it complexed with cobalt ion so tightly that the cathode current efficiency was low. The effect of other complexing agents such as hydroxy acetate, mannitol, glycene, glutamic acid, tartarate and citrate were also studied. Tartarate and citrate gave satisfactory results, the latter being the best. 101.0 g/L concentration of sodium citrate gave a stable bath. Concentration higher than this started yielding the deposit with lower content of molybdenum and poor in appearance and soundness; which may be ascribed to the anodic decomposition of the citrate ions (Donten and Osteryoung, 1991).

It was found that in conjunction with the organic hydroxy acids the presence of ammonium salts increased the solubility of cobalt and thus stabilized the bath against gradual precipitation of metal molybdates. They increased the cathode current efficiency of the alloy deposition, and improved the soundness of the deposit. Presence of 17.0 g/L (NH4)2SO4 produced beneficial effects in the bath under study.

In a system as complicated as ammoniacal citrate bath for Co-Mo alloy plating, there is possibility of the formation of many kinds of complexes. Besides this, cobalt may also be present as Co(III) due to anodic oxidation of Co(II). Therefore it is difficult to explain completely the effect of these complexing agents. Some investigations have demonstrated that molybdate may be present in different polymeric states (Prasad and Guimarães, 1998) and may also form citrate complexes (Alcock et al, 1990). Cobalt ions may react with ammonia and citrate ions together to give simple or mixed complexes (Prasad, 1994).

Effect of Ph

The effect of pH on electrodeposition of alloys are specific and usually unpredictable (Brenner, 1963). In some baths, the pH has a large effect and in others a small. The determining factor is the chemical nature of the metallic compounds. The composition and stability of many complexes are a function of pH. As described in the previous paragraph the cobalt-molybdenum bath may contain mixed complexes with ammonia and citrate besides existence of various polymeric species of molybdate. The electrodeposition experiments in a wide range of pH were performed to study the effect on this system. The results of pH range 4.0-9.0 are demonstrated in Fig. 2. The best deposition results were obtained in the pH values ranging from 5.0 to 6.0.

 

 

Effect of Addition Agents

A number of addition agents including triton X-100 and 1-dodecylsulfate-Na as surfactants, 1-4 butyne-diol as leveler, saccharin as stress reducer were incorporated in the bath to help modify surface morphology, reduce pits and surface tension.. An addition of 0.035 g/L 1-dodecylsulfate controlled pitting problem. If the surface of the object to be plated is rough, the use of 0.125 g/L of 1-4 butyne-diol as leveler in combination with the surfactant was found suitable.

 

EFFECT OF OPERATING VARIABLES

The main operating variables are current density, temperature and agitation of the bath. The control of operating variables is much more important in alloy plating than in the deposition of single metal. In the case of deposition of alloys moderate changes in plating conditions may alter the composition and properties of the deposit considerably. In regular type of alloy plating systems it is expected that an increase in current density causes the content of more noble metal in the deposit to decrease; and an increase in temperature or in agitation of the bath causes the content of more noble metal to increase. But the effects of the operating variables on the induced type of codeposition, such as cobalt-molybdenum alloys, are complicated and not consistent enough to be predictable.

Effect of current density

Current density is the most important of the operating variables. The behavior of regular alloy systems could be explained with a fair degree of success by diffusion theory. Unfortunately, no similar, simple explanations are available for explaining all the vagaries of the induced codeposition systems with respect to current density.

The initial experiments of electrodeposition of the Co-Mo alloy were tried in the current density range of 20-100 mA/cm2. It was found that a good quality of amorphous coating could be achieved at a relatively narrow range of cathode current densities. The lower range of preferred current density turned out to be above the hydrogen overvoltage of the cathode and it is postulated that the nascent hydrogen may be involved in the reduction of the molybdate complex (Chassaing et al, 1989). The best operating current density was found to be 50 mA/cm2. Below this current, deposits tend to be crystalline in structure. Higher current density tended toward poor quality deposit and also formation of a black deposit, probably of Co3O4, on the platinum anode. An addition of 1.5 g/L hydrazine sulfate to the bath prevented the formation of the black residue on the anode.

Effect of Temperature

Increase in temperature usually decreases polarization, increases concentration of metal in the cathode diffusion layer and may affect the cathode current efficiency of deposition of metals, particularly those deposited from complex ions.

A series of electrodeposition runs was performed at different temperatures ranging from 25oC to 70oC. The variation of cathode current efficiency with temperature is demonstrated in Fig. 3. The deposit obtained at 45oC was found to have the best physical characteristics such as appearance and hardness.

 

 

Effect of Rotation of Cathode

Rotation of cathode can directly affect the composition of the alloy by reducing the thickness of cathode diffusion layer and also by causing the metal ratio of the diffusion layer to approach more closely to that of the solution in the body of the bath. The concentration of the free complexing agent in the cathode diffusion layer is also reduced by agitation which may have a powerful effect on the potential of one or both of the metals. Also, the possibility exists that the trend of alloy composition resulting from a reduction in the concentration of free complexing agent may be opposite in sense to that resulting from an increase in the metal ion concentration of the cathode diffusion layer. Such possibility may occur in an alloy deposition system where less noble metal alone complexes with the complexing agent. This means that in baths made up with complex ions, the effects of agitation are not as predictable as those in baths made up with simple ions.

The effect of cathode rotation rate observed on efficiency of deposition of the alloy is demonstrated in Fig. 4. It can be seen that the cathode current efficiency approaches to the highest level at 10 rpm. The trend in the efficiency change can not be explained only by simple mass transfer to the cathode, but the explanation should include some complex reaction sequences involving a series of electron transfer steps and modification of electrode surface. The decrease observed in the current efficiency at the rotation higher than 15 rpm may be ascribed to the kinetic correlation of codeposition of cobalt, molybdenum and hydrogen. Molybdenum deposition is activation controlled whereas cobalt and hydrogen discharge is diffusion controlled (Beltowska-Lehman, 1990). It may be presumed that under the prevailing electrodeposition conditions, as the rotation speed is increased from 15 rpm, the concentration of molybdate ions within the cathode layer is also increased, formation of lower-valency molybdenum compounds thus being easier. The latter compounds block the cathode surface and consequently, the percentage of molybdenum in the coating is decreased. A black bloom arising on the cathode surface in such conditions support this mechanism. A similar effect was observed with increased sodium molybdate concentration in the bath. This abnormal effect, i.e. the process rate lowers with higher rotation, may also be correlated to an intermediate active product being formed during the process and taking part in indirect reactions on the electrode. The proposed stepwise reduction of molybdenum is in agreement with the observations of Chassaing et al (1989) in electrodeposition of Ni-Mo alloys.

 

 

Properties of The Electrodeposited Alloy

The most interesting properties of the Co-Mo amorphous alloys are their hardness, corrosion resistance and wear resistance. Bending the deposited foil did not give any crack in the deposit showing its good ductility. Stripping method, which consists of cutting a corner of the test piece and then stripping the deposit from the substrate, showed that the plating adhesion to the copper substrate was satisfactory. Thermal treatment of the plated substrate below the crystallization temperature (<600oC) further enhanced the adhesion. This should be ascribed to the mutual diffusion of the metallic atoms between the substrate and the plate producing a very tenacious bond.

An X-ray diffraction scan of the alloy deposited on copper substrate gave a single broad peak characteristic of an amorphous structure (Fig. 5A). The same sample was next heated. It was found that its crystallization process started at about 620oC. The crystallization process was completed at 850oC (Fig. 5B).

 

 

Vickers micro-hardness of the electroplate has an average range of 800-900. Data as low as 600 has been seen. Hardness as high as 1100 has been noted with some samples. The reason for this wide swing in values is not well understood. Alloy composition, organic complexing and addition agents and hydrogen codeposition may all play an affect.

Corrosion resistance of the Co-Mo deposit has proved to be much superior than hard chrome. No sign of corrosion on the alloy was detected even after seven days of its exposure to H2S and salt environments. The absence of corrosion was verified by using a microscope. The corrosion resistance characteristics were found to be similar to those of the Co-W alloys. Based on the grounds of the Co-W alloys it may be presumed that in neutral and basic media the alloy is protected against corrosion by the formation of a stable oxide layer of Co(III) on its surface while in acidic medium molybdenum protects the alloy by formation of MoO3 or molybdic acid (Prasad, 1994).

Stability and Performance of the Deposition Bath

100 mL of the bath presented in Table I was electrolyzed at 45oC for 8 h. A copper foil of 2.50 cm2 area was used as cathode and a platinum foil of 15.36 cm2 worked as anode. It was found that during the 8 h of electrolysis at 50 mA/cm2 cathode current density the deposition efficiency dropped from about 65% to 29%. The fall in current efficiency during the course of electrolysis should be caused mainly by decrease in Co(II) and molybdate concentrations. Anodic degradation of ammonia (Gerisher and Mauerer, 1970; Bard, 1978) and citrate ions (Minevski and Adzic, 1988) may also have contributed in this fall.

It is usually assumed that, during electrolysis with positively polarized noble metal anode, the anodic reaction is simply the oxidation of water to oxygen. However, under conditions of low efficiency and large cell potentials, other oxidation processes are possible which may consume or transform components of the bath. The simplest problem is that of oxidation of metal ions, e.g. Co(II) to Co(III) (El-Halim and Khalil, 1987). The organic compounds such as complexants, surfactants, brighteners and wetting agents can be oxidized in aqueous solution at potentials less positive than the potential of oxygen evolution. For example, carboxylic acid group can be oxidized to carbon dioxide with simultaneous production of radicals (Minevski and Adzic, 1988). Alcohols and aldehydes can be oxidized to various products depending on substrate and oxidation conditions (Capon and Parson, 1973; Takamura and Sakamoto, 1980). Ammonia can be oxidized at potentials lower than about 1.2 V (Gerisher and Mauerer, 1970; Bard, 1978). These oxidation reactions may be responsible for low stability and poor performance of the baths. Moreover, cobalt compounds are known to catalyze anodic processes of organic compounds (Wendt and Schneider, 1986), and to increase the overpotential for oxygen evolution (Armstrong et al, 1988). Thus the presence of a cobalt salt in an electroplating bath in which an inert Pt-anode is used should result in a faster decomposition of citrate and ammonium ions.

Electrodeposition in The Cell with Anode in a Separate Compartment

Some electrodeposition experiments were realized on a cell with anode in a separate compartment to see if the deposition performance improves. The alloy plating experiments as described in the above paragraph were repeated by putting Pt-anode in a compartment containing 1.0 M Na2SO4 at pH 5.5 separated from the cathode by a glass frit. During the 8 h of platting the cathode current efficiency decreased from 65% to 43% in contrast to 29% for unseparated cell. This shows that separating the anodic and cathodic processes during the electroplating of the alloy is an effective method for improving the stability and efficiency of the bath.

Monitoring and Controlling the Alloy Deposition Baths

The above description shows that anodic decomposition of the bath ingredients is the main cause of its low stability. Keeping the anode in a separate compartment shows great improvement in the stability. But during the electrolysis, as time passes, the concentration of the metallic components falls due to their removal in the form of the deposit. There should be some device to monitor the concentration of these components so that they may be replenished at right moment to maintain the quality of the deposit and efficiency of the process. As the alloy deposited has a definite composition, monitoring the concentration of any one ingredient may reveal the concentration of the remaining components.

There are several methods to analyze the bath solutions. But the preferred method should be simple, rapid, capable of being readily automated, and should be applicable to the diversity of the bath problems. A squarewave voltammetric method (Osteryoung and O’Dea, 1986) has been developed which fulfills all these requirements. By performing a series of squarewave voltammetric experiments the experimental parameters were determined. It was found that the bath solution diluted ten times with ammonia buffer of pH 9.5 gives a very well shaped net-current peak at –1.38 V corresponding to Co(II) reduction (Fig. 6). The peak height is linearly related with the Co(II) concentration (Fig. 7). The results obtained show that the method is very satisfactory for monitoring the Co(II) concentration even in very high speed plating baths as this technique provides results in a time scale of seconds. The process can easily be automated by coupling it with a computer. The computer may control automatically: the polarographic analyzer for realizing squarewave voltammetry; rinsing, sampling and cell drain functions; and calculation of results and then replenishment of the ingredients (Selk, 1982).

 

 

 

CONCLUSION

The results of this study suggest that the optimum composition of the bath for deposition of amorphous Co-Mo alloy is as presented in Table I. The most suitable values of operating parameters were: bath temperature 45oC, cathode current density 50 mA/cm2 and cathode rotation speed 10 rpm. The average composition of the alloy deposited was 51% Co, 47% Mo and 2% B. The alloy deposited was sound, thick, hard and corrosion resistant. The maximum cathode current efficiency was found to be 65%. Anodic decomposition of the bath ingredients, such as of ammonium and citrate ions, was responsible for degradation and aging of the bath. The stability of the bath was improved by using anode in a separate compartment.

 

ACKNOWLEDGEMENT

The authors are grateful to the CNPq, Brasília, for financial assistance and to Janet G. Osteryoung for helpful discussions.

 

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