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Oxidation and dissolution of tungsten carbide powder in water
International Journal of Refractory Metals & Hard Materials 18 (2000) 121±129
Oxidation and dissolution of tungsten carbide powder in water Karin M. Andersson *, Lennart Bergstr om Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden Received 3 January 2000; accepted 1 March 2000
Abstract The oxidation and dissolution of tungsten carbide powder dispersed in water was investigated using X-ray photoelectron spectroscopy (XPS) and leaching studies. We found that the WO3 surface layer on the oxidised tungsten carbide powder dissolves readily at pH > 3 with the tungsten concentration increasing linearly with time. Adding cobalt powder to the tungsten carbide suspension resulted in a signi®cant reduction of the dissolution rate at pH < 10. Electrokinetic studies indicate that the reduced dissolution rate may be related to the formation of surface complexes; experiments showed that Co species in solution adsorb on the oxidised tungsten carbide powder. The experimental data were discussed in relation to theoretical modelling of the WO3 solution chemistry and the Co2 adsorption at oxide/water interfaces. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hard metal; Co; WC; Oxidation; Dissolution
1. Introduction Hard metals are commonly produced following a powder metallurgy route, which involves powder production, mixing, granulation, pressing and sintering [1]. Each of these manufacturing steps must be optimised to yield a dense ®nal product with a homogeneous microstructure. Strength limiting ¯aws can have many origins, but work on ceramics has shown that agglomerates in the powder is a common cause [2]. Hence, large, hard agglomerates must either be broken down or removed from the starting powder to yield a reliable, high-strength material. For ®ne powders, agglomerates are commonly broken down by dispersing and milling the powders in a suitable liquid. Although such a dispersing and milling procedure may be ecient in breaking down the inherent agglomerates, care has to be taken to prohibit the formation of detrimental agglomerates at a later stage, e.g., during spray drying. Recent work on alumina [3], silica [4], and boehmite powders [5] have conclusively linked the formation of hard agglomerates upon drying to the formation of strong interparticle bridges by reprecipitation of dis-
*
Corresponding author. Tel.: +46-8-790-9612; fax: +46-8-20-89-98. E-mail address: [email protected] (K.M. Andersson).
solved material. Hence, the solubility and the dissolution rate of the dispersed powders can have a signi®cant in¯uence on the ®nal properties. For nonoxide powders, such as WC, the response to an aqueous environment will depend on the properties of the oxidised surface layer. Hence, the extent of oxidation and the kinetics of dissolution are important parameters to study. Tungsten has oxides with composition corresponding to a spectrum of oxidation states, between +IV and +VI [6], the latter being the more thermodynamically stable in air at low temperature [7]. Webb et al. [8] investigated the oxidation behaviour of WC at high temperatures in air and they found that the main oxidation product was WO3 . Warren et al. [9] have performed one of the few studies on the oxidation of WC in aqueous media. They also identi®ed WO3 as the oxidation product of hotpressed WC bars in dry and humid atmospheres. Using X-ray photoelectron spectroscopy (XPS), they found that the thickness of the oxide layer of WC, exposed to humid atmospheres at room temperature, increases with increasing humidity. Storing WC in water, however, resulted in a complete removal of the oxide ®lm due to dissolution. WO3 dissolves in water forming tungstate ions by the reaction WO3 H2 O $ WO2ÿ 4 2H
0263-4368/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 0 ) 0 0 0 1 0 - X
1
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It is well known that W(VI) forms a number of dierent polynuclear species in solution [10±14]. Raman studies have shown that at pH > 7.8 the solution is dominated ion; W12 O12ÿ is the main species at by the WO2ÿ 4 42 pH < 7.8 and its protonated form at pH < 5.7 [11]. The situation becomes more complex at lower pH. Aveston 5ÿ et al. [12] showed that W12 O10ÿ 41 and HW6 O21 dominate 5ÿ at pH 5. At pH 4 HW6 O21 protonates to give H3 W6 O3ÿ 21 , which with time forms the stable metatungstate ion [H2 (W3 O10 )4 ]6ÿ [13]. At pH 1 the main tungsten species in solution is the hydrated tungstic acid, WO3 2H2 O [14]. This study deals with the oxidation and dissolution of WC and WC/Co powders in aqueous media. The kinetics of the dissolution process of the tungsten oxide has been investigated and the eect of the Co2 concentration on the WC solubility has been linked to the general adsorption phenomenon of cations at metal oxide/water interfaces. 2. Experimental 2.1. Materials The WC powder had a BET speci®c surface area of 0.49 m2 gÿ1 , a free C-content of 0.01 wt% and a total oxygen content of 0.11 wt%. X-ray diraction analysis (XRD) showed that the WC powder contains small amounts of W2 C. WC/Co (8 vol% Co) powder mixtures were prepared using two dierent procedures. The mixed powder that was used for the pH measurements was prepared by milling WC and Co powders in an ethanol/water medium for 18 h. The WC/Co mixture used for the solubility measurement was prepared inhouse by mixing the WC powder with a Co powder for 10 min, using a FRITSCH Planetary Mono Mill ``pulverisette 6''. The BET speci®c surface area of the Co powder was 1.15 m2 gÿ1 and the total oxygen content 0.78 wt%. XRD showed that the Co powder consists of a mixture of the cubic and the hexagonal phase. Both the Co and WC powder were free from soluble impurities. The NaOH and HCl used for pH adjustments and the added salts, NaCl and CoCl2 6H2 O, were purchased from Merck KgaA, Germany. The deionised water was obtained from a Millipore Milli-Q plus unit (>18.2 M X cmÿ1 resistivity, <9 ppb total organic carbon content). 2.2. Methods The change in pH was measured using an Orion ROSSTM Sure-Flow Combination pH electrode, model 81-72, in 2 vol% suspensions of WC or a WC/Co powder mixture in deionised water. No acid or base was
added to the suspensions, which were kept in closed plastic containers on a shaker (IKA KS 501 digital shaker). The dissolution of WC and WC/Co was investigated at dierent pH values by continuously taking out samples from 2 vol% aqueous suspensions and measuring the concentration of Co and W in solution using inductively coupled plasma (ICP) analysis (Perkin-Elmer Plasma 1000). The suspensions were kept in closed plastic containers and shaken. The pH was kept at set values between pH 3 and 11 and regularly checked and adjusted by addition of acid or base. Samples for analysis were obtained by separating a clear supernatant from the powder using ®ltration. XPS measurements were carried out on WC and WC/ Co powders in order to deduce the degree of oxidation. XPS spectra were recorded using a Kratos AXIS HS XPS, using a MgKa X-ray source operated at 240 W (12 kV/20 mA). The analysis area was 1 mm2 . Powder samples for XPS analysis were dried in vacuum oven at room temperature and 30 Torr for 24 h before being loaded into the XPS apparatus. The dried powders were spread out on the surface of an aluminium sample holder without mounting and degassed in the prevacuum chamber at room temperature. The pressure in the analysis chamber was <1 ´ 10ÿ7 Torr at all times. The sensitivity factors used were 0.25 for C 1s, 0.66 for O 1s, 3.8 for Co 2p and 2.75 for W 4f (supplied by Kratos). The absolute binding energies of the peaks were estimated by using the double C 1s peak at 282.8 and 284.3 eV for the carbide and graphite contributions, respectively, as a reference [15]. The electrokinetic properties were determined using a Zetasizer 2000 (Malvern Instruments, UK). This equipment uses the Doppler shift in the laser light scattering from the particles to obtain an electrophoretic mobility spectrum. The mean electrophoretic mobility, lE , may be used for calculations of the zeta-potential using the Smoluchowski equation [16] lE
ef ; g
2
where e is the dielectric constant of the medium, f the zeta-potential, and g is the viscosity of the medium. Electrokinetic measurements were carried out on dilute WC suspensions (0.01 wt%) at constant ionic strength (5 10ÿ3 M NaCl). The Co2 concentration in the solution was varied from 0 to 10ÿ3 M, by addition of CoCl2 . The pH of the samples was adjusted by adding small amounts of HCl or NaOH solutions and the samples were thoroughly dispersed using an ultrasonic apparatus. The electrophoretic measurements were made just after the preparation (within 10 min) of each sample to minimise the eect of dissolution and oxidation.
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3. Results and discussion 3.1. Surface state of WC powder XPS measurements show that the as-received WC powder is oxidised on the powder surface. The XPS spectrum in Fig. 1 displays a substantial oxygen peak, in addition to the carbon and tungsten contribution. It is well known that WC is thermodynamically unstable and will oxidise in the presence of water or oxygen at room temperature. Most studies suggest that WO3 is the oxidation product [9]. The W 4f peak in Fig. 2(a) shows contributions from WC at 31.7 and 33.7 eV. In addition there is a contribution from an oxidised surface layer denoted Wox with W 4f peaks at 35.5 and 37.6 eV [15]. Comparing these results with the previous study by Warren et al. [9] suggests that the oxidised surface layer mainly consists of WO3 . We ®nd additional support for a high oxidation state of tungsten, from an analysis of the composition of the surface ®lm. Integrating the O 1s peak and the Wox 4f peaks and estimating the concentration from the respective sensitivity factors results in an atomic ratio of oxygen to Wox exceeding 4. Hence, a lower oxidation state of tungsten than VI is less likely. There are a number of possible oxidation reactions, which are associated with the formation of carbon, methane or carbondioxide. We have estimated the GibbÕs free energies for four dierent reactions using tabulated values of the free energy of formation for the reagents and products of the reactions [7]. It should be noted that there is a potential error in the calculations due to the uncertainty of the value of DG°f of WC [17]. WC s 3H2 O 1 ! WO3 s C s 3H2 g DG° ÿ14:3 kJ molÿ1 ;
3
Fig. 2. XPS spectra of WC powder showing the peaks for: (a) W 4f; (b) O 1s; and (c) C 1s at a high resolution.
WC s 3H2 O 1 ! WO3 s CH4 g H2 g DG° ÿ64:8 kJ molÿ1 Fig. 1. XPS spectrum of the as-received WC powder showing peaks for oxygen, carbon, and tungsten.
WC s 3=2O2 g ! WO3 s C s DG° ÿ725:6 kJ molÿ1
4
5
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WC s 5=2O2 g ! WO3 s CO2 g DG° ÿ1120 kJ molÿ1
6
These estimates show that the energetically perferred reactions involve the formation of CO2 . Indeed, Voorhies has reported CO2 to be one of the products of the electrochemical and chemical oxidation of WC [18]. We also observed gas evolution during the oxidation experiments in aqueous media, which suggests that the formation of free carbon (Eqs. (3) and (5)) is less likely. However, we are unable to tell if oxidation of WC proceeds through reaction (4) or (6) since the composition of the evolved gases is unknown. The C 1s peak in Fig. 2(c) consists of two contributions, the carbide peak at 282.8 eV and the peak corresponding to the free carbon in the sample at 284.3 eV [15]. 3.2. Dissolution of WC in water The dissolution of the oxidised WC powder was studied by measuring the concentration of tungsten in solution. Fig. 3 shows that there are no signi®cant differences in the dissolution behaviour over the studied pH range. All the dissolution curves follow a linear relation over the studied time scale, indicating that the dissolution reaction is a zero-order reaction of a single component, WO3 , decomposing to products. Dissolution rate constants, kp , estimated from the slopes of the curves, varied from 1:3 10ÿ7 mol mÿ2 hÿ1 at pH 3 to 1:9 10ÿ7 mol mÿ2 hÿ1 at pH 11. There is no sign of an asymptotic approach to a plateau value of the W-concentration, which shows that the solution is far from being saturated of W-species in solution under these conditions. Comparing the amount of dissolved W with the oxygen content of the as-received WC powder shows
Fig. 3. Dissolution of WC powder in water at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d).
that the powder must reoxidise and dissolve continuously. The original oxygen content, 0.11 wt%, corresponds to a W-concentration around 5±10 mM in a 2 vol% suspension, which is signi®cantly below the Wconcentration in solution at long leaching times (Fig. 3). There is no substantial dierence in dissolution rate at short and long leaching times, which suggest that it is the dissolution step that is rate limiting. If the reoxidation step would be rate limiting, we should observe a much higher dissolution rate at short leaching times when the original oxide layer is being dissolved. In order to obtain an estimate of the coverage or thickness of the oxide layer as a function of leaching time, we followed the changes in the atomic ratio of tungsten oxide and total tungsten on the surface using XPS. Fig. 4 shows the W 4f peak for the WC powder, after a short (24 h), and long (225 days) immersion time. The W 4f spectrum contains contributions from both WC and WO3 at all immersion times. The strong bulk contribution suggests that the oxide layer is either very thin (less than 5 nm) or inhomogeneous. Gaussian curves were ®tted for the deconvolution of the W 4f peak into WC and WO3 . In the curve ®tting procedure the area ratios for the doublet W 4f7=2 /W 4f5=2 were ®xed to the reported value of 1.33 [19]. The additional peak at about 37 eV, W 5p3=2 , was not included in the quanti®cation. Fig. 5 shows that the WWO3 =Wtot ratio initially increases and then decreases with an asymptotic approach to a constant value at longer immersion times. Hence, the WC powder becomes more oxidised after a short exposure to aqueous media but this oxidised layer decreases at extended immersion times. The initial increase in the degree of oxidation is probably related to hydrolysis. The decrease of the degree of oxidation at longer immersion times shows that the oxide layer is slowly dissolved until steady-state is reached, where the dissolution rate and the reoxidation rate are of similar magnitude. Dissolution of tungsten oxide is also associated with the release of protons (see Eq. (1)). Hence, following the change in pH with time is an indirect way of studying the dissolution process. Fig. 6 shows that the pH of an aqueous suspension of oxidised WC powder decreases with time and approaches a pH value around 2 at long times. We have attempted to determine the stoichiometry of the dominating dissolution reaction at low pH by correlating the observed pH decrease to a corresponding increase in tungsten concentration. Dierent dissolution reactions with various H =W ratios have been assessed and compared to the experimentally determined dissolution rate at pH 3 (Fig. 7). Assuming that only WO2ÿ 4 ions are formed in solution (Eq. (1)), which relates to a ratio of two protons to every tungstate ion, leads to an underestimation of the dissolution rate by
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Fig. 5. The fraction of the W 4f peak relating to the oxidised surface layer (WWO3 =Wtot ) as a function of immersion time.
Fig. 6. Change of pH in the solution phase of 2 vol% WC slurry with time. The points represent the experimental values while the solid curve is calculated using the reaction in Eq. (9) and a dissolution rate constant of 1:3 10ÿ7 mol mÿ2 hÿ1 .
Fig. 4. XPS of the W 4f peak of the WC powder after dierent immersion times in water: (a) as received; (b) 24 h; and (c) 225 days.
almost an order of magnitude. Hence, soluble tungsten complexes must form by reactions that produce less than two protons/dissolved tungsten atom. Isopolytungstates will exist at the relatively low pH values of the suspension. Assuming that the formation of H3 W6 O3ÿ 21 according to the reaction
Fig. 7. Amount of dissolved W as a function of time calculated from the pH values in Fig. 6 assuming the formation of WO2ÿ (d), 4 3ÿ H3 W6 O3ÿ 21 N; H3 W6 O21 and WO3 2H2 O , compared with the experimental values at pH 3 (j). The straight lines are only guides for the eye.
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6WO3 3H2 O $ H3 W6 O3ÿ 21 3H
7
dominates the dissolution reaction gives a better correspondence to the experimental data, but the dissolution rate is still underestimated. A further reduction in the H =W ratio can be related to the formation of hydrated tungsten acid according to WO3 2H2 O $ WO3 2H2 O
8
which does not produce protons. Fitting a combination of Eqs. (6) and (7) to the experimental data (see Fig. 7) suggests that the overall dissolution reaction of WC, in slurries with a relatively high solid content, at 2.5 < pH < 3.5 is 15WO3 21H2 O $ H3 W6 O3ÿ 21 9 WO3 2H2 O 3H
9 corresponding to an overall H =W ratio of 1/5. The procedure of estimating the dissolved amount of tungsten from the pH change can of course be inverted. The calculated pH change with time based on Eq. (9) and a dissolution rate constant of 1:3 10ÿ7 is displayed as the solid curve in Fig. 6. 3.3. Oxidation and dissolution of WC/Co mixtures XPS shows that the cobalt in WC/Co (8%) powder mixtures is oxidised. Fig. 8 shows that the peaks of the spectrum of Co 2p ®t the values for cobalt oxide. No peak could be observed at 778.1±778.3 eV, where it
Fig. 8. XPS spectrum of a WC/Co powder mixture showing the peak for Co 2p.
would be expected for pure metallic cobalt [15]. Hence, the oxide layer is suciently thick to shield the bulk contribution. Cobalt metal oxidises in air and water to form CoO. Contrary to the acidic tungsten oxide, CoO is basic and dissolves under the formation of hydroxide ions by the reaction CoO H2 O $ Co2 2OHÿ
10
This fundamental dierence in dissolution behaviour between oxidised WC and Co can be used to analyse the pH changes with time when a hard metal (WC/Co) powder is dispersed in water (Fig. 9). Initially, pH increases with time as a result of the hydrolysis and dissolution of CoO, which appears to be relatively fast. At longer times the pH decreases due to the slower WO3 dissolution. However, the pH changes are relatively small and stay between 8 and 9, which shows that the simultaneous dissolution of the oxidised WC and Co powders buers the suspension. The release of Co in the aqueous medium from a WC/ Co powder mixture is strongly dependent on pH. The results in Fig. 10 show that the dissolution rate is very low at pH > 7 and very high at pH 3. In fact, at pH 3 the dissolution rate constant, kp , is as high as 2:6 10ÿ5 mol mÿ2 hÿ1 , during the ®rst 175 h. This rate constant is two hundred times larger than that of oxidised WC powder. These results corroborate previous work on the corrosion resistance of hard metals, which have shown that erosion of the binder phase (Co) is the main problem [20]. Dissolution of the carbide phase occurs much slower. Comparing the amount of dissolved Co with the total oxygen content of the as-received Co powder, shows that the Co powder must reoxidise and dissolve simultaneously. The original oxygen content of the powder corresponds to a Co concentration in solution of 7.1 mM, a concentration that was reached in the matter of days when the powder was immersed at pH 3 and 5.
Fig. 9. Change of pH in the solution phase of 2 vol% WC/Co (8%) slurry with time. The line is only a guide to the eye.
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CoWO4 may form at the WO3 /water interface in Co2 rich solutions. 3.4. Electrophoretic measurements
Fig. 10. Release of Co in a WC/Co (8%) aqueous slurry with time at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d). The lines are only guides for the eye.
Additional proof is given by the XPS data that the Co powder must be oxidising continuously, where cobalt oxide was detected at low pH also after long leaching times. Fig. 11 shows that the solubility of WC in water decreases dramatically in the presence of Co. At high pH, the dierence in dissolution rate between the WC/Co mixture and the WC powder dispersion (Fig. 3) is marginal, but when pH is reduced below 10 the solubility is drastically reduced. This reduction in solubility of WC corresponds to the onset of Co dissolution, which suggests that Co species are able to passivate the oxidised WC surface. We speculate that Co2 forms some type of surface complex with the tungsten oxide on the WC surface, similar to the complex formation with other metal oxides at low pH [21±23]. Previous studies have shown that passivating CaWO4 complexes can form on the surface of hard metals immersed in Ca2 rich aqueous solutions [24]. Similarly, a surface layer of
Fig. 11. Release of W in a WC/Co (8%) aqueous slurry with time at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d). The lines are only guides for the eye.
The interactions between Co2 ions and the oxidised WC surface were studied in more detail with electrokinetics. Measurement of the electrophoretic mobility and the associated zeta-potential is a very sensitive method to detect ion adsorption at the solid±liquid interface. Fig. 12 shows that the zeta-potential of the oxidised WC powder is essentially constant, and highly negative, over the investigated pH range (3±11). This behaviour is typical for an acidic oxide with a very low isoelectric point (iep), the pH where the zeta-potential is zero. Most oxides have hydrated surfaces dominated by ±OH surface groups. These surface groups are amphoteric and can result in a negative or positive surface charge according to M±OH $ M±Oÿ H
11
M±OH H $ M±OH 2
12
where M is the metal of the oxide. These surface dissociation reaction are pH dependent and the nature of the oxide (acidic or basic) can be related to the dissociation constants. For an acidic oxide, the surface hydroxyl groups dissociate even at low pH, thus resulting in a negative surface charge. Adding CoCl2 to the WC suspension results in an increasing zeta-potential with increasing CoCl2 concentration (Fig. 12). At constant Co-addition, the zetapotential increases with pH and eventually changes sign at a critical pH. This eect can be attributed to the adsorption of Co-species at the oxidised WC surface. Previous studies have shown that the adsorption of
Fig. 12. Zeta-potential of WC powder as a function of pH at dierent CoCl2 concentrations: no CoCl2 (d), 10ÿ5 M CoCl2 (j), 10ÿ4 M CoCl2 (r), 10ÿ3 M CoCl2 (N). All samples contained a 5 mM NaCl background electrolyte.
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Co-species on metal oxides is strongly dependent on the surface charge and the speciation of Co(II) in solution [21±23,25±35]. Various models have been developed to describe cation adsorption and the associated changes in electrokinetic behaviour. According to the surface complexation model [21,36± 38], surface complexes form at the interface by reactions such as
CoWO4 have been detected after oxidation of hard metals [39]; hence, there is a de®nite possibility that similar complexes can form during Co2 adsorption on WO3 surface.
M±OH Co2 Clÿ $ M±OCo Clÿ H
The XPS measurements showed that the as-received WC powder is oxidised on the powder surface. Analysis of the binding energies of the W 4f peaks and the atomic ratio between the O 1s and the Wox peaks suggests that the oxidised surface layer mainly consists of WO3 . This WO3 surface layer dissolves readily at all the investigated pH values pH P 3 with the amount of released W increasing linearly with time. Comparing the amount of released W in solution with the oxygen content in the as-received powder indicates that the WC powder must dissolve and reoxidise continuously. As there is no substantial dierence in the dissolution rate at short and long leaching times, we can conclude that it is the dissolution step that is rate limiting. In an unbuered solution, the dissolution of WO3 on the WC surface results in a decreasing pH with time. The H =W ratio in solution matches that of the overall dissolution reaction
13 M± OH2 Co2 $ M± O2 Co 2H
14
Speci®c adsorption of cations causes the negative charge at the oxide/water interface to take positive or less negative values. At high enough pH, Co(OH)2 precipitates at the oxide/water interface [25,26,28±31]. Surface precipitation is usually associated with a charge reversal, from a negative to positive surface charge. This is also observed in Fig. 12 at pH higher than 8 where the OHÿ concentration is suciently high to result in the precipitation of Co(OH)2 on the oxidised WC surface. Surface precipitation is de®ned as precipitation from undersaturated solutions in the presence of a solid and may be related to the interactions between the ions in solution and the charged solid/liquid interface [28±30]. Simple estimates based on the solubility product Co2 OHÿ 2 10ÿ14:2 [7] and the zeta-potentials of the oxidised WC show that surface precipitation should occur at pH 9:5 when Co2 bulk 10ÿ5 M; pH 8:6 when Co2 bulk 10ÿ4 M; pH 8:3 when Co2 bulk 10ÿ3 M: The [Co2 ] and [OHÿ ] at the interface are calculated from the bulk value using the Bolzman distribution equation Co2 surf Co2 bulk eÿ ze/ x= kT ;
15
OHÿ surf OHÿ bulk eÿ ze/ x= kT ;
16
where z is thew valency of the ion, e the charge of electron, / x the potential at the distance x from the interface, k the Boltzman constant and T is the absolute temperature. Approximating surface precipitation with the pH, where charge reversal occurs, we ®nd a relatively good correspondence between the calculations and experiments. The surface precipitation occurs in the predicted pH range and a higher Co-concentration results in surface precipitation at a lower pH. However, it should be noted that a complex oxide may form at longer times; hence, there is a possibility that the surface phase slowly transforms from Co(OH)2 to CoWO4 . Solid solutions of WO3 and Co with the stoichiometry
4. Conclusions and summary
15WO3 21H2 O $ H3 W6 O3ÿ 21 9 WO3 2H2 O 3H
which suggests that polynuclear tungstate species form in solution at low pH. The solubility of WC at pH < 10 decreases dramatically in the presence of Co. This reduction in solubility of WC corresponds to the onset of Co dissolution, which suggests that Co species are able to passivate the oxidised WC surface. Electrokinetic studies show that Co2 forms some type of surface complex with the tungsten oxide on the WC surface. The possibility of surface precipitation of Co(OH)2 and/or CoWO4 was discussed and related to the corrosion inhibition.
Acknowledgements This work has been performed within the Brinell Centre ± Inorganic Interfacial Engineering, supported by the Swedish National Board for Industrial and Technical Development (NUTEK), and the following industrial partners: Erasteel Kloster AB, H oganas AB, Kanthal AB, Sandvik AB, Seco Tools AB, and Uniroc AB. We thank Agneta Bruhn, Seco Tools AB for her help with ICP measurements and Marie Ernstsson, Institute for Surface Chemistry, for her help with XPS measurements and interpretation.
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[22] Katz LE, Hayes KF. Surface complexation modeling I. Strategy for modeling monomer complex formation at moderate surface coverage. J Colloid Interface Sci 1995;170:477±90. [23] Katz LE, Hayes KF. Surface complexation modeling II. Strategy for modeling polymer and precipitation reactions at high surface coverage. J Colloid Interface Sci 1995;170:491±501. [24] Azuma S, Kusaba Y. Eect of pH and calcium (II) ion on the corrosion of tungsten carbide bearing hard alloy in water at elevated temperatures. Corr Eng 1995;44:471±9. [25] Tewari PH, Campbell AB, Lee W. Adsorption of Co2 by oxides from aqueous solution. Can J Chem 1972;50:1642±8. [26] Tewari PH, Lee W. Adsorption of Co (II) at the oxide±water interface. J Colloid Interface Sci 1975;52:77±88. [27] Tamura H, Katayama N, Furuichi R. The Co2 adsorption properties of Al2 O3 , Fe2 O3 , Fe3 O4 ,TiO2 , and MnO2 evaluated by modeling with the Frumkin isotherm. J Colloid Interface Sci 1997;195:192±202. [28] James RO, Healy TW. Adsorption of hydrolyzable metal ions at the oxide±water interface I. Co (II) adsorption on SiO2 and TiO2 as model systems. J Colloid Interface Sci 1972;40:42±52. [29] James RO, Healy TW. Adsorption of hydrolyzable metal ions at the oxide±water interface II. Charge reversal on SiO2 and TiO2 colloids by adsorbed Co (II), La (III) and Th (IV) as model systems. J Colloid Interface Sci 1972;40:53±64. [30] James RO, Healy TW. Adsorption of hydrolyzable metal ions at the oxide±water interface III. A thermodynamic model of adsorption. J Colloid Interface Sci 1972;40:65±81. [31] OÕDay PA, Brown GE, Parks GA. X-ray absorption spectroscopy of cobalt (II) multinuclear surface complexes and surface precipitates on kaolinite. J Colloid Interface Sci 1994;165:269±89. [32] Towels SN, Barger JR, Brown GE, Parks GA. Surface precipitation of Co (II) (aq) on Al2 O3 . J Colloid Interface Sci 1997;187:62±82. [33] Jarezic-Renault N. Ion exchange properties of weakly hydrated, crystalline tin dioxide: Ion-exchange of Cu2 , Zn2 , Co2 , Ni2 , Mn2 . J Inorg Nucl Chem 1978;40:539±44. [34] Duval JE, Kurbatov MH. The adsorption of cobalt and barium ions by hydrous ferric oxide at equilibrium. J Phys Chem 1952;56:982±4. [35] Schenk CV, Dillard JG, Murrey JW. Surface analysis and the adsorption of Co (II) on goethite. J Colloid Interface Sci 1983;95:398±409. [36] Schindler PW, F urst B, Dick R, Wolf PU. Ligand properties of surface silanol groups I. Surface complex formation with Fe3 , Cu2 , Cd2 , and Pb2 . J Colloid Interface Sci 1976;55:469±75. [37] Schindler PW, Stumm W. The surface chemistry of oxide, hydroxide, and oxide minerals. In: Stumm W, editor. Aquatic surface chemistry. New York: Wiley; 1987. p. 83±110. [38] Westall JC. Adsorption mechanisms in aquatic surface chemistry. In: Stumm W, editor. Aquatic surface chemistry. New York: Wiley; 1987. p. 3±32. [39] Voitovich VB, Sverdel VV, Voitovich RF, Golovko EI. Oxidation of WC±Co, WC±Ni and WC±Co±Ni hard metals in the temperature range 500±800°C. Int J Refr Metals Hard Mater 1996;14:289±95.
Oxidation and dissolution of tungsten carbide powder in water Karin M. Andersson *, Lennart Bergstr om Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden Received 3 January 2000; accepted 1 March 2000
Abstract The oxidation and dissolution of tungsten carbide powder dispersed in water was investigated using X-ray photoelectron spectroscopy (XPS) and leaching studies. We found that the WO3 surface layer on the oxidised tungsten carbide powder dissolves readily at pH > 3 with the tungsten concentration increasing linearly with time. Adding cobalt powder to the tungsten carbide suspension resulted in a signi®cant reduction of the dissolution rate at pH < 10. Electrokinetic studies indicate that the reduced dissolution rate may be related to the formation of surface complexes; experiments showed that Co species in solution adsorb on the oxidised tungsten carbide powder. The experimental data were discussed in relation to theoretical modelling of the WO3 solution chemistry and the Co2 adsorption at oxide/water interfaces. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hard metal; Co; WC; Oxidation; Dissolution
1. Introduction Hard metals are commonly produced following a powder metallurgy route, which involves powder production, mixing, granulation, pressing and sintering [1]. Each of these manufacturing steps must be optimised to yield a dense ®nal product with a homogeneous microstructure. Strength limiting ¯aws can have many origins, but work on ceramics has shown that agglomerates in the powder is a common cause [2]. Hence, large, hard agglomerates must either be broken down or removed from the starting powder to yield a reliable, high-strength material. For ®ne powders, agglomerates are commonly broken down by dispersing and milling the powders in a suitable liquid. Although such a dispersing and milling procedure may be ecient in breaking down the inherent agglomerates, care has to be taken to prohibit the formation of detrimental agglomerates at a later stage, e.g., during spray drying. Recent work on alumina [3], silica [4], and boehmite powders [5] have conclusively linked the formation of hard agglomerates upon drying to the formation of strong interparticle bridges by reprecipitation of dis-
*
Corresponding author. Tel.: +46-8-790-9612; fax: +46-8-20-89-98. E-mail address: [email protected] (K.M. Andersson).
solved material. Hence, the solubility and the dissolution rate of the dispersed powders can have a signi®cant in¯uence on the ®nal properties. For nonoxide powders, such as WC, the response to an aqueous environment will depend on the properties of the oxidised surface layer. Hence, the extent of oxidation and the kinetics of dissolution are important parameters to study. Tungsten has oxides with composition corresponding to a spectrum of oxidation states, between +IV and +VI [6], the latter being the more thermodynamically stable in air at low temperature [7]. Webb et al. [8] investigated the oxidation behaviour of WC at high temperatures in air and they found that the main oxidation product was WO3 . Warren et al. [9] have performed one of the few studies on the oxidation of WC in aqueous media. They also identi®ed WO3 as the oxidation product of hotpressed WC bars in dry and humid atmospheres. Using X-ray photoelectron spectroscopy (XPS), they found that the thickness of the oxide layer of WC, exposed to humid atmospheres at room temperature, increases with increasing humidity. Storing WC in water, however, resulted in a complete removal of the oxide ®lm due to dissolution. WO3 dissolves in water forming tungstate ions by the reaction WO3 H2 O $ WO2ÿ 4 2H
0263-4368/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 0 ) 0 0 0 1 0 - X
1
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It is well known that W(VI) forms a number of dierent polynuclear species in solution [10±14]. Raman studies have shown that at pH > 7.8 the solution is dominated ion; W12 O12ÿ is the main species at by the WO2ÿ 4 42 pH < 7.8 and its protonated form at pH < 5.7 [11]. The situation becomes more complex at lower pH. Aveston 5ÿ et al. [12] showed that W12 O10ÿ 41 and HW6 O21 dominate 5ÿ at pH 5. At pH 4 HW6 O21 protonates to give H3 W6 O3ÿ 21 , which with time forms the stable metatungstate ion [H2 (W3 O10 )4 ]6ÿ [13]. At pH 1 the main tungsten species in solution is the hydrated tungstic acid, WO3 2H2 O [14]. This study deals with the oxidation and dissolution of WC and WC/Co powders in aqueous media. The kinetics of the dissolution process of the tungsten oxide has been investigated and the eect of the Co2 concentration on the WC solubility has been linked to the general adsorption phenomenon of cations at metal oxide/water interfaces. 2. Experimental 2.1. Materials The WC powder had a BET speci®c surface area of 0.49 m2 gÿ1 , a free C-content of 0.01 wt% and a total oxygen content of 0.11 wt%. X-ray diraction analysis (XRD) showed that the WC powder contains small amounts of W2 C. WC/Co (8 vol% Co) powder mixtures were prepared using two dierent procedures. The mixed powder that was used for the pH measurements was prepared by milling WC and Co powders in an ethanol/water medium for 18 h. The WC/Co mixture used for the solubility measurement was prepared inhouse by mixing the WC powder with a Co powder for 10 min, using a FRITSCH Planetary Mono Mill ``pulverisette 6''. The BET speci®c surface area of the Co powder was 1.15 m2 gÿ1 and the total oxygen content 0.78 wt%. XRD showed that the Co powder consists of a mixture of the cubic and the hexagonal phase. Both the Co and WC powder were free from soluble impurities. The NaOH and HCl used for pH adjustments and the added salts, NaCl and CoCl2 6H2 O, were purchased from Merck KgaA, Germany. The deionised water was obtained from a Millipore Milli-Q plus unit (>18.2 M X cmÿ1 resistivity, <9 ppb total organic carbon content). 2.2. Methods The change in pH was measured using an Orion ROSSTM Sure-Flow Combination pH electrode, model 81-72, in 2 vol% suspensions of WC or a WC/Co powder mixture in deionised water. No acid or base was
added to the suspensions, which were kept in closed plastic containers on a shaker (IKA KS 501 digital shaker). The dissolution of WC and WC/Co was investigated at dierent pH values by continuously taking out samples from 2 vol% aqueous suspensions and measuring the concentration of Co and W in solution using inductively coupled plasma (ICP) analysis (Perkin-Elmer Plasma 1000). The suspensions were kept in closed plastic containers and shaken. The pH was kept at set values between pH 3 and 11 and regularly checked and adjusted by addition of acid or base. Samples for analysis were obtained by separating a clear supernatant from the powder using ®ltration. XPS measurements were carried out on WC and WC/ Co powders in order to deduce the degree of oxidation. XPS spectra were recorded using a Kratos AXIS HS XPS, using a MgKa X-ray source operated at 240 W (12 kV/20 mA). The analysis area was 1 mm2 . Powder samples for XPS analysis were dried in vacuum oven at room temperature and 30 Torr for 24 h before being loaded into the XPS apparatus. The dried powders were spread out on the surface of an aluminium sample holder without mounting and degassed in the prevacuum chamber at room temperature. The pressure in the analysis chamber was <1 ´ 10ÿ7 Torr at all times. The sensitivity factors used were 0.25 for C 1s, 0.66 for O 1s, 3.8 for Co 2p and 2.75 for W 4f (supplied by Kratos). The absolute binding energies of the peaks were estimated by using the double C 1s peak at 282.8 and 284.3 eV for the carbide and graphite contributions, respectively, as a reference [15]. The electrokinetic properties were determined using a Zetasizer 2000 (Malvern Instruments, UK). This equipment uses the Doppler shift in the laser light scattering from the particles to obtain an electrophoretic mobility spectrum. The mean electrophoretic mobility, lE , may be used for calculations of the zeta-potential using the Smoluchowski equation [16] lE
ef ; g
2
where e is the dielectric constant of the medium, f the zeta-potential, and g is the viscosity of the medium. Electrokinetic measurements were carried out on dilute WC suspensions (0.01 wt%) at constant ionic strength (5 10ÿ3 M NaCl). The Co2 concentration in the solution was varied from 0 to 10ÿ3 M, by addition of CoCl2 . The pH of the samples was adjusted by adding small amounts of HCl or NaOH solutions and the samples were thoroughly dispersed using an ultrasonic apparatus. The electrophoretic measurements were made just after the preparation (within 10 min) of each sample to minimise the eect of dissolution and oxidation.
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3. Results and discussion 3.1. Surface state of WC powder XPS measurements show that the as-received WC powder is oxidised on the powder surface. The XPS spectrum in Fig. 1 displays a substantial oxygen peak, in addition to the carbon and tungsten contribution. It is well known that WC is thermodynamically unstable and will oxidise in the presence of water or oxygen at room temperature. Most studies suggest that WO3 is the oxidation product [9]. The W 4f peak in Fig. 2(a) shows contributions from WC at 31.7 and 33.7 eV. In addition there is a contribution from an oxidised surface layer denoted Wox with W 4f peaks at 35.5 and 37.6 eV [15]. Comparing these results with the previous study by Warren et al. [9] suggests that the oxidised surface layer mainly consists of WO3 . We ®nd additional support for a high oxidation state of tungsten, from an analysis of the composition of the surface ®lm. Integrating the O 1s peak and the Wox 4f peaks and estimating the concentration from the respective sensitivity factors results in an atomic ratio of oxygen to Wox exceeding 4. Hence, a lower oxidation state of tungsten than VI is less likely. There are a number of possible oxidation reactions, which are associated with the formation of carbon, methane or carbondioxide. We have estimated the GibbÕs free energies for four dierent reactions using tabulated values of the free energy of formation for the reagents and products of the reactions [7]. It should be noted that there is a potential error in the calculations due to the uncertainty of the value of DG°f of WC [17]. WC s 3H2 O 1 ! WO3 s C s 3H2 g DG° ÿ14:3 kJ molÿ1 ;
3
Fig. 2. XPS spectra of WC powder showing the peaks for: (a) W 4f; (b) O 1s; and (c) C 1s at a high resolution.
WC s 3H2 O 1 ! WO3 s CH4 g H2 g DG° ÿ64:8 kJ molÿ1 Fig. 1. XPS spectrum of the as-received WC powder showing peaks for oxygen, carbon, and tungsten.
WC s 3=2O2 g ! WO3 s C s DG° ÿ725:6 kJ molÿ1
4
5
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WC s 5=2O2 g ! WO3 s CO2 g DG° ÿ1120 kJ molÿ1
6
These estimates show that the energetically perferred reactions involve the formation of CO2 . Indeed, Voorhies has reported CO2 to be one of the products of the electrochemical and chemical oxidation of WC [18]. We also observed gas evolution during the oxidation experiments in aqueous media, which suggests that the formation of free carbon (Eqs. (3) and (5)) is less likely. However, we are unable to tell if oxidation of WC proceeds through reaction (4) or (6) since the composition of the evolved gases is unknown. The C 1s peak in Fig. 2(c) consists of two contributions, the carbide peak at 282.8 eV and the peak corresponding to the free carbon in the sample at 284.3 eV [15]. 3.2. Dissolution of WC in water The dissolution of the oxidised WC powder was studied by measuring the concentration of tungsten in solution. Fig. 3 shows that there are no signi®cant differences in the dissolution behaviour over the studied pH range. All the dissolution curves follow a linear relation over the studied time scale, indicating that the dissolution reaction is a zero-order reaction of a single component, WO3 , decomposing to products. Dissolution rate constants, kp , estimated from the slopes of the curves, varied from 1:3 10ÿ7 mol mÿ2 hÿ1 at pH 3 to 1:9 10ÿ7 mol mÿ2 hÿ1 at pH 11. There is no sign of an asymptotic approach to a plateau value of the W-concentration, which shows that the solution is far from being saturated of W-species in solution under these conditions. Comparing the amount of dissolved W with the oxygen content of the as-received WC powder shows
Fig. 3. Dissolution of WC powder in water at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d).
that the powder must reoxidise and dissolve continuously. The original oxygen content, 0.11 wt%, corresponds to a W-concentration around 5±10 mM in a 2 vol% suspension, which is signi®cantly below the Wconcentration in solution at long leaching times (Fig. 3). There is no substantial dierence in dissolution rate at short and long leaching times, which suggest that it is the dissolution step that is rate limiting. If the reoxidation step would be rate limiting, we should observe a much higher dissolution rate at short leaching times when the original oxide layer is being dissolved. In order to obtain an estimate of the coverage or thickness of the oxide layer as a function of leaching time, we followed the changes in the atomic ratio of tungsten oxide and total tungsten on the surface using XPS. Fig. 4 shows the W 4f peak for the WC powder, after a short (24 h), and long (225 days) immersion time. The W 4f spectrum contains contributions from both WC and WO3 at all immersion times. The strong bulk contribution suggests that the oxide layer is either very thin (less than 5 nm) or inhomogeneous. Gaussian curves were ®tted for the deconvolution of the W 4f peak into WC and WO3 . In the curve ®tting procedure the area ratios for the doublet W 4f7=2 /W 4f5=2 were ®xed to the reported value of 1.33 [19]. The additional peak at about 37 eV, W 5p3=2 , was not included in the quanti®cation. Fig. 5 shows that the WWO3 =Wtot ratio initially increases and then decreases with an asymptotic approach to a constant value at longer immersion times. Hence, the WC powder becomes more oxidised after a short exposure to aqueous media but this oxidised layer decreases at extended immersion times. The initial increase in the degree of oxidation is probably related to hydrolysis. The decrease of the degree of oxidation at longer immersion times shows that the oxide layer is slowly dissolved until steady-state is reached, where the dissolution rate and the reoxidation rate are of similar magnitude. Dissolution of tungsten oxide is also associated with the release of protons (see Eq. (1)). Hence, following the change in pH with time is an indirect way of studying the dissolution process. Fig. 6 shows that the pH of an aqueous suspension of oxidised WC powder decreases with time and approaches a pH value around 2 at long times. We have attempted to determine the stoichiometry of the dominating dissolution reaction at low pH by correlating the observed pH decrease to a corresponding increase in tungsten concentration. Dierent dissolution reactions with various H =W ratios have been assessed and compared to the experimentally determined dissolution rate at pH 3 (Fig. 7). Assuming that only WO2ÿ 4 ions are formed in solution (Eq. (1)), which relates to a ratio of two protons to every tungstate ion, leads to an underestimation of the dissolution rate by
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125
Fig. 5. The fraction of the W 4f peak relating to the oxidised surface layer (WWO3 =Wtot ) as a function of immersion time.
Fig. 6. Change of pH in the solution phase of 2 vol% WC slurry with time. The points represent the experimental values while the solid curve is calculated using the reaction in Eq. (9) and a dissolution rate constant of 1:3 10ÿ7 mol mÿ2 hÿ1 .
Fig. 4. XPS of the W 4f peak of the WC powder after dierent immersion times in water: (a) as received; (b) 24 h; and (c) 225 days.
almost an order of magnitude. Hence, soluble tungsten complexes must form by reactions that produce less than two protons/dissolved tungsten atom. Isopolytungstates will exist at the relatively low pH values of the suspension. Assuming that the formation of H3 W6 O3ÿ 21 according to the reaction
Fig. 7. Amount of dissolved W as a function of time calculated from the pH values in Fig. 6 assuming the formation of WO2ÿ (d), 4 3ÿ H3 W6 O3ÿ 21 N; H3 W6 O21 and WO3 2H2 O , compared with the experimental values at pH 3 (j). The straight lines are only guides for the eye.
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6WO3 3H2 O $ H3 W6 O3ÿ 21 3H
7
dominates the dissolution reaction gives a better correspondence to the experimental data, but the dissolution rate is still underestimated. A further reduction in the H =W ratio can be related to the formation of hydrated tungsten acid according to WO3 2H2 O $ WO3 2H2 O
8
which does not produce protons. Fitting a combination of Eqs. (6) and (7) to the experimental data (see Fig. 7) suggests that the overall dissolution reaction of WC, in slurries with a relatively high solid content, at 2.5 < pH < 3.5 is 15WO3 21H2 O $ H3 W6 O3ÿ 21 9 WO3 2H2 O 3H
9 corresponding to an overall H =W ratio of 1/5. The procedure of estimating the dissolved amount of tungsten from the pH change can of course be inverted. The calculated pH change with time based on Eq. (9) and a dissolution rate constant of 1:3 10ÿ7 is displayed as the solid curve in Fig. 6. 3.3. Oxidation and dissolution of WC/Co mixtures XPS shows that the cobalt in WC/Co (8%) powder mixtures is oxidised. Fig. 8 shows that the peaks of the spectrum of Co 2p ®t the values for cobalt oxide. No peak could be observed at 778.1±778.3 eV, where it
Fig. 8. XPS spectrum of a WC/Co powder mixture showing the peak for Co 2p.
would be expected for pure metallic cobalt [15]. Hence, the oxide layer is suciently thick to shield the bulk contribution. Cobalt metal oxidises in air and water to form CoO. Contrary to the acidic tungsten oxide, CoO is basic and dissolves under the formation of hydroxide ions by the reaction CoO H2 O $ Co2 2OHÿ
10
This fundamental dierence in dissolution behaviour between oxidised WC and Co can be used to analyse the pH changes with time when a hard metal (WC/Co) powder is dispersed in water (Fig. 9). Initially, pH increases with time as a result of the hydrolysis and dissolution of CoO, which appears to be relatively fast. At longer times the pH decreases due to the slower WO3 dissolution. However, the pH changes are relatively small and stay between 8 and 9, which shows that the simultaneous dissolution of the oxidised WC and Co powders buers the suspension. The release of Co in the aqueous medium from a WC/ Co powder mixture is strongly dependent on pH. The results in Fig. 10 show that the dissolution rate is very low at pH > 7 and very high at pH 3. In fact, at pH 3 the dissolution rate constant, kp , is as high as 2:6 10ÿ5 mol mÿ2 hÿ1 , during the ®rst 175 h. This rate constant is two hundred times larger than that of oxidised WC powder. These results corroborate previous work on the corrosion resistance of hard metals, which have shown that erosion of the binder phase (Co) is the main problem [20]. Dissolution of the carbide phase occurs much slower. Comparing the amount of dissolved Co with the total oxygen content of the as-received Co powder, shows that the Co powder must reoxidise and dissolve simultaneously. The original oxygen content of the powder corresponds to a Co concentration in solution of 7.1 mM, a concentration that was reached in the matter of days when the powder was immersed at pH 3 and 5.
Fig. 9. Change of pH in the solution phase of 2 vol% WC/Co (8%) slurry with time. The line is only a guide to the eye.
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CoWO4 may form at the WO3 /water interface in Co2 rich solutions. 3.4. Electrophoretic measurements
Fig. 10. Release of Co in a WC/Co (8%) aqueous slurry with time at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d). The lines are only guides for the eye.
Additional proof is given by the XPS data that the Co powder must be oxidising continuously, where cobalt oxide was detected at low pH also after long leaching times. Fig. 11 shows that the solubility of WC in water decreases dramatically in the presence of Co. At high pH, the dierence in dissolution rate between the WC/Co mixture and the WC powder dispersion (Fig. 3) is marginal, but when pH is reduced below 10 the solubility is drastically reduced. This reduction in solubility of WC corresponds to the onset of Co dissolution, which suggests that Co species are able to passivate the oxidised WC surface. We speculate that Co2 forms some type of surface complex with the tungsten oxide on the WC surface, similar to the complex formation with other metal oxides at low pH [21±23]. Previous studies have shown that passivating CaWO4 complexes can form on the surface of hard metals immersed in Ca2 rich aqueous solutions [24]. Similarly, a surface layer of
Fig. 11. Release of W in a WC/Co (8%) aqueous slurry with time at dierent pH values: pH 3 (), pH 5 ( ), pH 7 (.), pH 8 (N), pH 9 (r), pH 10 (j), pH 11(d). The lines are only guides for the eye.
The interactions between Co2 ions and the oxidised WC surface were studied in more detail with electrokinetics. Measurement of the electrophoretic mobility and the associated zeta-potential is a very sensitive method to detect ion adsorption at the solid±liquid interface. Fig. 12 shows that the zeta-potential of the oxidised WC powder is essentially constant, and highly negative, over the investigated pH range (3±11). This behaviour is typical for an acidic oxide with a very low isoelectric point (iep), the pH where the zeta-potential is zero. Most oxides have hydrated surfaces dominated by ±OH surface groups. These surface groups are amphoteric and can result in a negative or positive surface charge according to M±OH $ M±Oÿ H
11
M±OH H $ M±OH 2
12
where M is the metal of the oxide. These surface dissociation reaction are pH dependent and the nature of the oxide (acidic or basic) can be related to the dissociation constants. For an acidic oxide, the surface hydroxyl groups dissociate even at low pH, thus resulting in a negative surface charge. Adding CoCl2 to the WC suspension results in an increasing zeta-potential with increasing CoCl2 concentration (Fig. 12). At constant Co-addition, the zetapotential increases with pH and eventually changes sign at a critical pH. This eect can be attributed to the adsorption of Co-species at the oxidised WC surface. Previous studies have shown that the adsorption of
Fig. 12. Zeta-potential of WC powder as a function of pH at dierent CoCl2 concentrations: no CoCl2 (d), 10ÿ5 M CoCl2 (j), 10ÿ4 M CoCl2 (r), 10ÿ3 M CoCl2 (N). All samples contained a 5 mM NaCl background electrolyte.
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Co-species on metal oxides is strongly dependent on the surface charge and the speciation of Co(II) in solution [21±23,25±35]. Various models have been developed to describe cation adsorption and the associated changes in electrokinetic behaviour. According to the surface complexation model [21,36± 38], surface complexes form at the interface by reactions such as
CoWO4 have been detected after oxidation of hard metals [39]; hence, there is a de®nite possibility that similar complexes can form during Co2 adsorption on WO3 surface.
M±OH Co2 Clÿ $ M±OCo Clÿ H
The XPS measurements showed that the as-received WC powder is oxidised on the powder surface. Analysis of the binding energies of the W 4f peaks and the atomic ratio between the O 1s and the Wox peaks suggests that the oxidised surface layer mainly consists of WO3 . This WO3 surface layer dissolves readily at all the investigated pH values pH P 3 with the amount of released W increasing linearly with time. Comparing the amount of released W in solution with the oxygen content in the as-received powder indicates that the WC powder must dissolve and reoxidise continuously. As there is no substantial dierence in the dissolution rate at short and long leaching times, we can conclude that it is the dissolution step that is rate limiting. In an unbuered solution, the dissolution of WO3 on the WC surface results in a decreasing pH with time. The H =W ratio in solution matches that of the overall dissolution reaction
13 M± OH2 Co2 $ M± O2 Co 2H
14
Speci®c adsorption of cations causes the negative charge at the oxide/water interface to take positive or less negative values. At high enough pH, Co(OH)2 precipitates at the oxide/water interface [25,26,28±31]. Surface precipitation is usually associated with a charge reversal, from a negative to positive surface charge. This is also observed in Fig. 12 at pH higher than 8 where the OHÿ concentration is suciently high to result in the precipitation of Co(OH)2 on the oxidised WC surface. Surface precipitation is de®ned as precipitation from undersaturated solutions in the presence of a solid and may be related to the interactions between the ions in solution and the charged solid/liquid interface [28±30]. Simple estimates based on the solubility product Co2 OHÿ 2 10ÿ14:2 [7] and the zeta-potentials of the oxidised WC show that surface precipitation should occur at pH 9:5 when Co2 bulk 10ÿ5 M; pH 8:6 when Co2 bulk 10ÿ4 M; pH 8:3 when Co2 bulk 10ÿ3 M: The [Co2 ] and [OHÿ ] at the interface are calculated from the bulk value using the Bolzman distribution equation Co2 surf Co2 bulk eÿ ze/ x= kT ;
15
OHÿ surf OHÿ bulk eÿ ze/ x= kT ;
16
where z is thew valency of the ion, e the charge of electron, / x the potential at the distance x from the interface, k the Boltzman constant and T is the absolute temperature. Approximating surface precipitation with the pH, where charge reversal occurs, we ®nd a relatively good correspondence between the calculations and experiments. The surface precipitation occurs in the predicted pH range and a higher Co-concentration results in surface precipitation at a lower pH. However, it should be noted that a complex oxide may form at longer times; hence, there is a possibility that the surface phase slowly transforms from Co(OH)2 to CoWO4 . Solid solutions of WO3 and Co with the stoichiometry
4. Conclusions and summary
15WO3 21H2 O $ H3 W6 O3ÿ 21 9 WO3 2H2 O 3H
which suggests that polynuclear tungstate species form in solution at low pH. The solubility of WC at pH < 10 decreases dramatically in the presence of Co. This reduction in solubility of WC corresponds to the onset of Co dissolution, which suggests that Co species are able to passivate the oxidised WC surface. Electrokinetic studies show that Co2 forms some type of surface complex with the tungsten oxide on the WC surface. The possibility of surface precipitation of Co(OH)2 and/or CoWO4 was discussed and related to the corrosion inhibition.
Acknowledgements This work has been performed within the Brinell Centre ± Inorganic Interfacial Engineering, supported by the Swedish National Board for Industrial and Technical Development (NUTEK), and the following industrial partners: Erasteel Kloster AB, H oganas AB, Kanthal AB, Sandvik AB, Seco Tools AB, and Uniroc AB. We thank Agneta Bruhn, Seco Tools AB for her help with ICP measurements and Marie Ernstsson, Institute for Surface Chemistry, for her help with XPS measurements and interpretation.
K.M. Andersson, L. Bergstr om / International Journal of Refractory Metals & Hard Materials 18 (2000) 121±129
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