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Corrosion properties of Co-based cemented carbides in acidic solutions
International Journal of Refractory Metals & Hard Materials 21 (2003) 135–145 www.elsevier.com/locate/ijrmhm
Corrosion properties of Co-based cemented carbides in acidic solutions Sutha Sutthiruangwong, Gregor Mori
*
Institute of General and Analytical Chemistry, University of Leoben, A-8700 Leoben, Austria Received 28 January 2003; accepted 17 March 2003
Abstract The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H2 SO4 solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H2 SO4 solution. The specimen in H2 SO4 solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Cemented carbides; Chronoamperometry; Corrosion; Magnetic saturation; Pseudopassivity
1. Introduction Cemented carbides or hard metals are heterogeneous materials, which basically suffer galvanic corrosion. Aggressive media preferably attack the binder while the tungsten carbide itself remains immune. This is because of a higher reduction potential of tungsten carbide when compared to the binder. After dissolution of binder, tungsten carbide is no longer present in compact form. There remains a skeleton of tungsten carbide at the surface. This tungsten carbide layer has almost no tensile strength and wear resistance. At room temperature, cemented carbides show an excellent resistance in basic and neutral aqueous solutions. Strong acid solutions such as hydrochloric and sulfuric acid can cause severe corrosion and material degradation [1]. The wear fracturing as well as pullout of WC grains and wear of co*
Corresponding author. Tel.: +43-3842-402-822; fax: +43-384242739-8222. E-mail address: [email protected] (G. Mori). 0263-4368/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0263-4368(03)00027-1
balt by abrasion is possibly assisted by corrosion [2]. The strength of hard metals in wood-cutting environment is adversely affected by corrosive attack [3]. The understanding of corrosion kinetics and thermodynamics is necessary for improvement of cemented carbides in such environments. Corrosion behavior of cemented carbides has been intensively investigated in the recent decade by many researchers. The typical methodologies for corrosion measurement are immersion tests and electrochemical investigations. Most researchers carry out the electrochemical measurement of cemented carbide in deaerated solution. Anodic behavior of cemented carbide is divided into three regions, active, pseudopassive, and transpassive [4,5]. The linear rule of mixtures of Stern is applied to relate the corrosion behavior of cemented carbides to the binder surface area [6]. Corrosion resistance of cemented carbides strongly depends on binder composition. WC dissolution during sintering process leads to alteration of corrosion properties. Refractory metal carbides additions exhibit an improvement in
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corrosion resistance of cemented carbides [7]. Binderless cemented carbide can be obtained by hot isostatic pressing process. It shows a good corrosion resistance but some mechanical properties, especially toughness, are reduced [8]. The present work aims to investigate the corrosion properties of cemented carbide with cobalt based binder. Specimens with various binder characteristics are used in this study.
2. Experimental 2.1. Materials Cemented carbide specimens were prepared by conventional sintering procedure. Properties of materials are listed in Table 1. Magnetic saturation in term of 4pr was measured for each specimen. After polishing with a 1 lm diamond paste, the specimens were cleaned with ethyl alcohol in ultrasonic bath and dried with hot air prior the experiment. 2.2. Electrochemical measurements Conventional electrochemical polarization cell was used for studying electrochemical behavior of cemented carbides at room temperature. Hydrochloric and sulfuric acid solutions with 1 N concentration were used in this study. Argon gas was used as a circulating media in order to maintain a deaerated solution. In case of aerated solution experiment, atmospheric air was directly pumped into the solution. The counter electrode was a platinum plate. Saturated calomel electrode (SCE) was applied as a reference electrode with a potential of )240 mV with respect to standard hydrogen electrode (SHE). Throughout this work all potentials are reported with respect to SCE. The reference electrode was placed outside the cell and connected to the specimen surface
by a Luggin capillary and a salt bridge. A programmable Jaissle potentiostat–galvanostat IMP 88 PC was applied for electrochemical investigation. The potential range of potentiodynamic polarization was )500 to 1000 mV. The solution was bubbled with circulating media for 10 min prior to immersion of the specimens. The corrosion potential was measured immediately after immersion for 30 min before polarization. The scan rate was set at 600 mV/h in positive direction unless mentioned different. Corroded specimens were further investigated by cracking and investigation of fracture surface by scanning electron microscope (SEM). Chronoamperometry was applied to investigate diffusion limitation in pseudopassive region of cemented carbides with 6% and 9% cobalt content and with vanadium carbide addition. Two sets of experiments were carried out at active and pseudopassive potentials. Potentials for chronoamperometry were obtained from potentiodynamic polarization curve. 2.3. Immersion test The specimens were weighed and immersed into 1 N H2 SO4 at room temperature. After 1, 3, 10, and 30 days weight of specimens has been measured for specific mass loss determination. Two sets of immersion tests were carried out in agitated and non-agitated solution.
3. Results 3.1. Electrochemical behavior of cobalt and WC Fig. 1 shows potentiodynamic polarization curves of Co and WC in aerated 1 N H2 SO4 . Corrosion potential
Table 1 Properties of investigated materials Material
Cobalt content (wt.%)
WCCo6min WCCo6med WCCo6max WCCo9SP WCCo9min WCCo9med WCCo9max WCCo15min WCCo15med WCCo15max WCCo6VC0.5 WC Co
6 6 6 9 9 9 9 15 15 15 6 Nil >99.8
Additions (wt.%)
Grain size (lm)
0.5 VC
3 3 3 3 3 3 3 3 3 3 0.7
4pr (lT m3 /kg) 10.2 11.4 12.1 15.1 15.5 17.3 19.0 26.4 28.5 29.3 10.5 201.7
Fig. 1. Potentiodynamic polarization of Co and WC in aerated 1 N H2 SO4 .
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(Ecorr ) and corrosion current density (icorr ) of cobalt are )460 mV and 0.68 mA/cm2 . The cathodic and anodic Tafel constants (bc and ba ) are 225 and 40 mV per decade. WC has a much more noble corrosion potential of 110 mV and a corrosion current density of 6.2 104 mA/cm2 . Tafel constants for cathodic and anodic part are 300 and 102 mV per decade respectively. Pure cobalt does not passivate. Current density increases with increasing anodic overvoltage through the entire range of anodic polarization. WC exhibits a lower corrosion current density than pure cobalt by several orders of magnitude. The current density measured from WC also increases with anodic polarization. Two plateaus are observed at 200 and 600 mV. At approximately 900 mV, current density of WC decreases significantly.
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HCl is smaller than in H2 SO4 . As the potential is increased above 700 mV to transpassive region the current density increases rapidly again. Between 880 and 980 mV the current density starts to decrease dramatically. 3.3. Effect of scan rate
Potentiodynamic polarization curves of cemented carbide with 9% cobalt binder which has a low magnetic saturation of 15.1 lT m3 /kg in aerated and deaerated acidic solutions are shown in Fig. 2. A large difference between aerated and deaerated solution has not been observed. However, there is a difference in cathodic reaction between aerated and deaerated solution. The corrosion potential obtained from aerated solutions is slightly shifted to more noble values. Anodic polarization curves of this material (WCCo9SP) in both aerated and deareated solution are almost identical. The current density decreases by about one order of magnitude when polarizing from active peak potential to pseudopassive region. This is valid for aerated and deaerated solution. HCl shifts the active peak to more noble values and yields to higher active corrosion current densities. Consequently the potential range of pseudopassivity in
Electrochemical behavior of material WCCo9max at different scan rates is shown in Fig. 3. Corrosion potential of all specimens is approximately )425 mV. Anodic polarization curves from different scan rates show almost identical current density responses. A slight decrease in anodic current density is observed with decreasing scan rate. At the last period of polarization, current density decreases sharply above a potential of about 900 mV. The specimen polarized with a 200 mV/h scan rate has been further investigated by SEM. Fig. 4(a) shows a cross section of polarized specimen after cracking. The upper darker layer in Fig. 4(a) is a cobaltdepleted WC skeleton and the lower part is the initial cemented carbide with cobalt binder. Thickness of layer is about 250 lm. Fig. 4(b) is a magnification of the interface between WC skeleton (upper part) and the unaffected hard metal (lower part). The latter contains ductile dimples at the fracture surface. Semiquantitative EDX-analysis confirms optical results. No cobalt peak was obtained in the WC skeleton. A yellow corrosion product at the outer surface of the cemented carbide is observed and shown in Fig. 4(c) and (d). This plate-like product covers the outer surface of cemented carbide above the cobalt-depleted layer. The grain shape of this corrosion product is plate-like in contradiction to polyedric WC grains. Analysis of the outer layer showed high amounts of tungsten and oxygen and no carbon, which indicates a tungsten oxide layer.
Fig. 2. Effect of acid solutions (1 N) and dissolved oxygen on electrochemical behavior of WCCo9SP.
Fig. 3. Effect of scan rate on electrochemical behavior of WCCo9max in aerated 1 N H2 SO4 .
3.2. Effect of acid solutions and dissolved oxygen
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Fig. 4. SEM images of WCCo9max in aerated 1 N H2 SO4 after polarization scan at 200 mV/h: (a) cross sectional view of surface, (b) interface between WC skeleton and hard metal, (c,d) outer oxide layer.
3.4. Effect of magnetic saturation Fig. 5(a)–(c) shows the effect of magnetic saturation on electrochemical behavior of cemented carbides with different binder contents. The electrochemical parameters of investigated material are summarized in Table 2. Free corrosion potential of cemented carbide is about )400 mV, which is between the corrosion potential of cobalt and WC. The minimum current density in the pseudopassive region, ipp , of cemented carbide decreases with decreasing magnetic saturation, 4pr. The amount of decrease of ipp is over one order of magnitude for WCCo6 and still half an order of magnitude for WCCo15. Binder content also influences the electrochemical behavior of cemented carbides. Current density of WCCo6min shows lower ipp than WCCo15min. Apart from the fact that specimens have different magnetic saturations, a possibility for direct comparison of these results will be discussed later. 3.5. Chronoamperometry Chronoamperometric measurements of WCCo9max at two different potentials are presented in Fig. 6. Active and pseudopassive potentials were taken from potentiodynamic polarization curves at values of )350 and +160 mV respectively both resulting in same current response during polarization. After applying the potential step from the corrosion potential to the chosen anodic value onto specimens current densities
first increase sharply and then decrease as a function of time. Current density obtained from the potential in active region ()350 mV) shows lower value than in pseudopassive region (+160 mV). Finally, these two current responses decrease to the same final value of about 4 mA/cm2 . The results of cemented carbide with the same cobalt content but lower magnetic saturation (WCCo9min) are shown in Fig. 7. The current densities immediately increase after the potential step. Then the current density at active potential decreases steadily whereas an oscillating current response has been obtained at pseudopassive potential. The results of chronoamperometric measurements of cemented carbide with 6% cobalt and different magnetic saturation, WCCo6max and WCCo6min, are shown in Figs. 8 and 9 respectively. Analogous to the cemented carbide with 9% cobalt, both chronoamperometric curves at active potential ()360 mV) increases after applying the anodic potential and then decrease steadily with time. At the pseudopassive potential (+200 mV), the sample with a low value of magnetic saturation shows an oscillating current response while the sample with high magnetic saturation yields to a steady decrease. Results for WCCo6VC0.5 cemented carbide which also has a low magnetic saturation value (see Table 1) are shown in Fig. 10. Again two fixed potentials in active and pseudopassive region were used. Current density measured at active potential decreases steadily with time while at pseudopassive potential an oscillating current response has been obtained.
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Fig. 5. Effect of magnetic saturation, 4pr, on electrochemical behavior of cemented carbides with different cobalt binder content in aerated 1 N H2 SO4 . Table 2 Electrochemical parameters of investigated materials in 1 N aerated H2 SO4 Material
Ecorr (mV)
icrit (mA/cm2 )
icorr (mA/cm2 )
ipp (mA/cm2 )
bc (mV/decade)
ba (mV/decade)
WCCo6min WCCo6med WCCo6max WCCo9min WCCo9med WCCo9max WCCo15min WCCo15med WCCo15max WCCo6VC0.5 WC Co
)380 )354 )426 )427 )364 )436 )360 )426 )360 )367 110 )460
6.87 9.72 11.90 11.05 19.05 21.59 24.03 33.87 34.47 0.27 – –
0.070 0.248 0.203 0.170 0.308 0.202 0.385 0.388 0.294 0.019 0.00062 0.680
0.13 0.21 2.56 0.54 4.27 4.94 2.34 8.06 8.16 0.04 – –
155 159 133 110 158 118 163 150 166 79 300 225
29 32 34 41 50 48 53 51 40 72 102 40
3.6. Immersion test Two specimens were weighted during a month immersion test in 1 N sulfuric acid solution. Weight loss data are shown in Fig. 11. Cemented carbide in stirred
acid solution yields higher weight loss when compared to stagnant solution. The rate of weight loss keeps constant throughout the immersion period. The corrosion rates in mm/year are 0.32 and 1.3 for stagnant and stirred solution respectively.
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Fig. 6. Chronoamperometric measurement of WCCo9max at active potential ()350 mV) and pseudopassive potential (+160 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
Fig. 7. Chronoamperometric measurement of WCCo9min at active potential ()390 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
Fig. 8. Chronoamperometric measurement of WCCo6max at active potential ()360 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
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Fig. 9. Chronoamperometric measurement of WCCo6min at active potential ()360 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
(a)
(b)
Fig. 10. Chronoamperometric measurement of WCCo6VC0.5 with low magnetic saturation at active potential ()330 mV) and pseudopassive potential (+100 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
4. Discussion 4.1. Electrochemical behavior
Fig. 11. Weight loss as a function of time of WCCo9max in 1 N H2 SO4 .
Electrochemical parameters from WCCo9SP are shown in Table 3. Exponential increment of both cathodic and anodic current density in deaerated solutions near corrosion potential indicates an activation controlled process. However the higher cathodic Tafel constants obtained from aerated solutions indicate the combination of activation and diffusion polarization in cathodic region. Oxygen reduction is involved in aerated solutions whereas hydrogen evolution is the dominating cathodic reaction in deaerated solutions. Oxygen containing solution slightly shifts the corrosion potential of cemented carbides to more noble values due to higher reduction potential, Eo , of oxygen (Eqs. (1) and (2)). HCl is more aggressive to cobalt based cemented
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Table 3 Electrochemical parameters of WCCo9SP Media
Ecorr (mV)
icrit (mA/cm2 )
icorr (mA/cm2 )
ipp (mA/cm2 )
bc (mV/decade)
ba (mV/decade)
Aerated H2 SO4 Deaerated H2 SO4 Aerated HCl Deaerated HCl
)317 )384 )301 )340
4.55 4.11 11.53 11.16
0.095 0.075 0.190 0.025
0.16 0.32 0.69 0.46
128 97 153 85
50 50 88 88
carbides than H2 SO4 . Critical current density (icrit ) and minimum current density in pseudopassive region obtained in HCl solution is higher than in H2 SO4 . This suggests that different anions have an influence on corrosion properties of cemented carbides by changing ipp . As presented in Figs. 1–3 at about 700 mV WC oxidation becomes significant and yields to an increase of current density. Eq. (3) shows the involved reaction [9]. Semiquantitative analysis by an EDX system in the SEM confirmed high amounts of oxygen in the surface corrosion product (Fig. 4(d)). Polarization curves in both acid solutions are almost equal above 700 mV. The large applied driving force for oxidation suppresses the influence of anion present in the solution. Beyond 900 mV, current density decreases dramatically. There the obtained current density is even much lower than ipp . This decrease of current density is the result of corrosion product formation on the outer surface of the tungsten carbide skeleton. This product is suspected to be tungsten oxide, WO3 , and it occurs only on the outer surface of the skeleton as can be seen in Fig. 4(d). Thickness of oxide layer is about 10 lm after a further polarization from 900 to 1000 mV within 30 min. The product acts as a barrier and has a very low electrical conductivity when compared to WC. It also retards the possibility of cobalt ions to diffuse out of the tungsten carbide skeleton. 2Hþ þ 2e ! H2 þ
Eo ¼ 240 mVSCE
O2 þ 4H þ 4e ! 2H2 O
ð1Þ
ðpH ¼ 0Þ
o
E ¼ þ989 mVSCE
magnetic saturation, the ratio of magnetic saturation to this maximum value, Rr , is proposed (Eq. (5)). Rr ð%Þ ¼
100 4pr 100 wt:%Co 4pro
ð5Þ
Fig. 12 shows ipp as a function of binder content, including the influence of Rr . WCCo15 with a value of magnetic saturation ratio equal to 100% yields highest ipp whereas WCCo6 with 85% magnetic saturation ratio obtains the lowest. According to this figure, it is clear that a cemented carbide with lowest magnetic saturation ratio exhibits lowest ipp at constant binder content. On the other hand, high tungsten content in binder leads to a high corrosion resistance, which agrees with previous research works [5,10]. Furthermore, ipp decreases with decreasing cobalt binder content at constant magnetic saturation ratios. This means that binder surface area influences the corrosion property of cemented carbide, regardless of which surface area is taken into account in electrochemical measurement. The plot of current density referred to binder surface area, ico , versus potential according to SternÕs law of mixtures in Fig. 13 supports this. 4.3. Pseudopassivity Pseudopassivity in cobalt based cemented carbides has been explained by many researchers. Anodic film such as CoWO4 was described as a cause for current density reduction [5]. In contrast, according to chrono-
ð2Þ þ
WC þ 5H2 O ! WO3 þ CO2 þ 10H þ 10e
ð3Þ
4.2. Influence of binder content Magnetic saturation refers to the binder composition. The amount of tungsten dissolved in the binder during sintering process can be estimated by Eq. (4) where ro is the magnetic saturation of pure cobalt [6] (in G cm3 /g): wt:%Co ð4Þ 100 Magnetic saturation of pure cobalt in term of 4pr is equal to 201.7 lT m3 /kg. According to a specimen with 6% cobalt content, the maximum value of magnetic saturation that can be reached is approximately 12.1 lT m3 /kg or 6% of 201.7 lT m3 /kg. In order to compare the ipp of specimens with different binder content and r ¼ ðro 8 at:%WÞ
Fig. 12. Minimum pseudopassive current density, ipp , as a function of cobalt binder content at different magnetic saturation ratio (Rr ).
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Fig. 13. Effect of binder content on electrochemical behavior of cemented carbide with Rr ¼ 85% in aerated 1 N H2 SO4 , current density referred to binder surface area.
amperometric measurement of specimens with high magnetic saturation (low W content in the binder), no passive film has been found in pseudopassive region. After applying an anodic potential step, current densities increase sharply and then decrease with increasing time without any oscillation. Pseudopassive potential yields to a higher current density than the active potential at the beginning, due to the higher anodic overvoltage. These two current densities then converge to the same value of about 4.0 mA/cm2 as the time increases (Fig. 6). If there is a film formation at pseudopassive potential, current density should drop to a lower value because of higher resistance of passive film and if this film is unstable current density should increase again yielding to an oscillating current density response [11]. The current density however decreases smoothly to the steady state value. A different explanation for the pseudopassive behavior is the increasing length of cobalt diffusion path. After dissolution of cobalt metal from anodic polarization, cobalt ions, Co2þ , have to diffuse out through the remaining porous tungsten carbide skeleton shown in Fig. 4. Diffusion in the WC skeleton is slower than free diffusion and much slower than convection. As observed from immersion test in stirred solution, weight loss is much higher when compared to stagnant solution. Porosity, pore size, and tortuosity of the porous layer have an effect on diffusion coefficient. Due to diffusion limitation of mass transport current flow is decreased and causes pseudopassive behavior. No layer (CoWO4 , CoSO4 , a.s.o.) has been found at the interface between WC skeleton and cemented carbide. Fig. 3 shows the effect of scan rate on electrochemical behavior of cemented carbide. Critical current density and ipp decrease with decreasing scan rate. This also suggests that porous WC skeleton inhibits cobalt dissolution. Thickness of porous WC skeleton obtained
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during polarization is much larger at low scan rates. A scan rate of 200 mV/h means that the specimen is 5 times longer polarized when compared to a 1000 mV/h scan rate. Thickness of porous layer depends directly on the integration of current density over time. Therefore the layer thickness for each potential increment obtained from a 200 mV/h experiment increases much faster than that of a 1000 mV/h experiment and yielding to a decline of ipp with decreasing scan rate. In contradiction, WCCo9min that has minimum value of magnetic saturation in the series of 9% cobalt content exhibits a different behavior and indicates precipitation in pseudopassive region. As can be seen in Fig. 7, the oscillating current response is obtained at the pseudopassive potential at +200 mV. The only difference between WCCo9max and WCCo9min is the amount of tungsten dissolved in the binder. For WCCo9min, Rr is equal to 85%, which means that there is a higher amount of tungsten dissolved in the binder when compared to WCCo9max with Rr of about 100%. According to Eq. (4), WCCo9min has dissolved about 2.9 at.% of tungsten in binder whereas WCCo9max confirms only about 0.94 at.% of tungsten. This tungsten in binder plays an important roll in pseudopassive region. At these range of pseudopassive potentials WC is inert. Then the only active site is the binder. Pourbaix diagram of cobalt, Fig. 14, shows the instability of cobalt metal at pseudopassive potential and pH 0. And cobalt is simply dissolved as Co2þ into the solution. Tungsten has a different behavior. At anodic potentials between )390 and )330 mV (equal to )150 and )90 mVSHE ), it is still stable in the solution. At pseudopassive potential of +200 mV (+440 mVSHE ) and pH 0, tungsten is no longer stable and it is oxidized to be oxide compounds as shown in Fig. 15 and Eqs. (6)–(8) [12]. The oxide precipitates have much higher resistivity than the binder as already shown at potential more noble than 900 mV. They drop the current response at the beginning of chronoamperometry at a pseudopassive potential. Anyway these tungsten oxides are not dense and they are not strongly fixed like a true passive film on stainless steel. After their formation, this loose oxide precipitates could be physically removed from the top of binder surface by further slower corrosion of cobalt which leads to an increase of current density after its first step drop. Hence the oscillating current response behavior is obtained. After long times (10 min or more) a decrease of current without any oscillation is obtained. This can be explained by diffusion limitation of mass transport of cobalt through the tortured WC skeleton as already discussed. Oscillating current response was always obtained at dissolution current densities of 2 mA/cm2 or higher. This may be the threshold value that has to be exceeded for inhibition effect of W oxidation on Co dissolution. An analogous chronoamperometric result from magnesium corrosion is reported in a recent publication [13].
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W2 O5 þ H2 O ! 2WO3 þ 2Hþ þ 2e
Eo ¼ 269 mVSCE ð8Þ
Fig. 14. Potential–pH equilibrium diagram for the system cobalt– water, at 25 °C, potential with respect to standard hydrogen electrode [12].
Cemented carbide with 6% cobalt binder content also exhibits the same behavior (Figs. 8 and 9). By using Eq. (4), WCCo6max has tungsten dissolved lower than 0.0035 at.% in binder and WCCo6min binder contains about 3.2 at.% of tungsten. WCCo6max shows no precipitate formation at pseudopassive potential while WCCo6min yields to oxide precipitates formation and leads to an oscillating current response. These can be described in the same manner as in WCCo9. In addition, cemented carbides with a grain size less than 1 lm combined with small amount vanadium carbide and low magnetic saturation (10.5 lT m3 /kg) also exhibits an oscillating current drop in chronoamperometric measurement. Vanadium carbide is a well known additive for grain growth inhibition, which does not dissolve directly into the cobalt binder. It forms a thin layer and deposits around WC grains. This thin layer is not segregated on the surface of WC grains but form a thin crystallized layer on the basal planes of WC grains [14]. Therefore the binder consists mainly of cobalt and dissolved tungsten. According to Eq. (4), the amount of tungsten in binder can be estimated to about 2.7 at.%. This value is in the same order of magnitude with that of WCCo9min and WCCo6min that show the formation of oxide precipitates at pseudopassive potentials. Thus the oxide precipitates formation process that has been explained for WCCo9min can be applied to WCCo6VC0.5 as well. 5. Conclusions The following conclusions can be summarized from the present work:
Fig. 15. Potential–pH equilibrium diagram for the system tungsten– water, at 25 °C, potential with respect to standard hydrogen electrode [12].
W þ 2H2 O ! WO2 þ 4Hþ þ 4e
Eo ¼ 359 mVSCE
1. Dissolved oxygen has small influence on anodic behavior of cobalt based cemented carbides. 2. HCl is more aggressive to cemented carbides than H2 SO4 . 3. Corrosion resistance of cemented carbide increases with decreasing magnetic saturation. 4. Lower cobalt binder content exhibits better corrosion resistance. 5. Pseudopassivity of cobalt based cemented carbides is caused by diffusion limitation of cobalt ion. 6. Tungsten oxide precipitates could be formed in cemented carbides with high tungsten contents in the binder phase yielding to a decreased dissolution rate.
ð6Þ þ
2WO2 þ H2 O ! W2 O5 þ 2H þ 2e
o
E ¼ 271 mVSCE ð7Þ
Future work will be emphasized on the influence of refractory metal carbide additions and the correlation between microstructure and corrosion behavior.
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Acknowledgements The authors wish to thank Austrian Exchange Service € AD) for financial support of SS and Plansee Tizit (O GmbH, Austria, for preparation of cemented carbides.
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Corrosion properties of Co-based cemented carbides in acidic solutions Sutha Sutthiruangwong, Gregor Mori
*
Institute of General and Analytical Chemistry, University of Leoben, A-8700 Leoben, Austria Received 28 January 2003; accepted 17 March 2003
Abstract The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H2 SO4 solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H2 SO4 solution. The specimen in H2 SO4 solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Cemented carbides; Chronoamperometry; Corrosion; Magnetic saturation; Pseudopassivity
1. Introduction Cemented carbides or hard metals are heterogeneous materials, which basically suffer galvanic corrosion. Aggressive media preferably attack the binder while the tungsten carbide itself remains immune. This is because of a higher reduction potential of tungsten carbide when compared to the binder. After dissolution of binder, tungsten carbide is no longer present in compact form. There remains a skeleton of tungsten carbide at the surface. This tungsten carbide layer has almost no tensile strength and wear resistance. At room temperature, cemented carbides show an excellent resistance in basic and neutral aqueous solutions. Strong acid solutions such as hydrochloric and sulfuric acid can cause severe corrosion and material degradation [1]. The wear fracturing as well as pullout of WC grains and wear of co*
Corresponding author. Tel.: +43-3842-402-822; fax: +43-384242739-8222. E-mail address: [email protected] (G. Mori). 0263-4368/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0263-4368(03)00027-1
balt by abrasion is possibly assisted by corrosion [2]. The strength of hard metals in wood-cutting environment is adversely affected by corrosive attack [3]. The understanding of corrosion kinetics and thermodynamics is necessary for improvement of cemented carbides in such environments. Corrosion behavior of cemented carbides has been intensively investigated in the recent decade by many researchers. The typical methodologies for corrosion measurement are immersion tests and electrochemical investigations. Most researchers carry out the electrochemical measurement of cemented carbide in deaerated solution. Anodic behavior of cemented carbide is divided into three regions, active, pseudopassive, and transpassive [4,5]. The linear rule of mixtures of Stern is applied to relate the corrosion behavior of cemented carbides to the binder surface area [6]. Corrosion resistance of cemented carbides strongly depends on binder composition. WC dissolution during sintering process leads to alteration of corrosion properties. Refractory metal carbides additions exhibit an improvement in
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corrosion resistance of cemented carbides [7]. Binderless cemented carbide can be obtained by hot isostatic pressing process. It shows a good corrosion resistance but some mechanical properties, especially toughness, are reduced [8]. The present work aims to investigate the corrosion properties of cemented carbide with cobalt based binder. Specimens with various binder characteristics are used in this study.
2. Experimental 2.1. Materials Cemented carbide specimens were prepared by conventional sintering procedure. Properties of materials are listed in Table 1. Magnetic saturation in term of 4pr was measured for each specimen. After polishing with a 1 lm diamond paste, the specimens were cleaned with ethyl alcohol in ultrasonic bath and dried with hot air prior the experiment. 2.2. Electrochemical measurements Conventional electrochemical polarization cell was used for studying electrochemical behavior of cemented carbides at room temperature. Hydrochloric and sulfuric acid solutions with 1 N concentration were used in this study. Argon gas was used as a circulating media in order to maintain a deaerated solution. In case of aerated solution experiment, atmospheric air was directly pumped into the solution. The counter electrode was a platinum plate. Saturated calomel electrode (SCE) was applied as a reference electrode with a potential of )240 mV with respect to standard hydrogen electrode (SHE). Throughout this work all potentials are reported with respect to SCE. The reference electrode was placed outside the cell and connected to the specimen surface
by a Luggin capillary and a salt bridge. A programmable Jaissle potentiostat–galvanostat IMP 88 PC was applied for electrochemical investigation. The potential range of potentiodynamic polarization was )500 to 1000 mV. The solution was bubbled with circulating media for 10 min prior to immersion of the specimens. The corrosion potential was measured immediately after immersion for 30 min before polarization. The scan rate was set at 600 mV/h in positive direction unless mentioned different. Corroded specimens were further investigated by cracking and investigation of fracture surface by scanning electron microscope (SEM). Chronoamperometry was applied to investigate diffusion limitation in pseudopassive region of cemented carbides with 6% and 9% cobalt content and with vanadium carbide addition. Two sets of experiments were carried out at active and pseudopassive potentials. Potentials for chronoamperometry were obtained from potentiodynamic polarization curve. 2.3. Immersion test The specimens were weighed and immersed into 1 N H2 SO4 at room temperature. After 1, 3, 10, and 30 days weight of specimens has been measured for specific mass loss determination. Two sets of immersion tests were carried out in agitated and non-agitated solution.
3. Results 3.1. Electrochemical behavior of cobalt and WC Fig. 1 shows potentiodynamic polarization curves of Co and WC in aerated 1 N H2 SO4 . Corrosion potential
Table 1 Properties of investigated materials Material
Cobalt content (wt.%)
WCCo6min WCCo6med WCCo6max WCCo9SP WCCo9min WCCo9med WCCo9max WCCo15min WCCo15med WCCo15max WCCo6VC0.5 WC Co
6 6 6 9 9 9 9 15 15 15 6 Nil >99.8
Additions (wt.%)
Grain size (lm)
0.5 VC
3 3 3 3 3 3 3 3 3 3 0.7
4pr (lT m3 /kg) 10.2 11.4 12.1 15.1 15.5 17.3 19.0 26.4 28.5 29.3 10.5 201.7
Fig. 1. Potentiodynamic polarization of Co and WC in aerated 1 N H2 SO4 .
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(Ecorr ) and corrosion current density (icorr ) of cobalt are )460 mV and 0.68 mA/cm2 . The cathodic and anodic Tafel constants (bc and ba ) are 225 and 40 mV per decade. WC has a much more noble corrosion potential of 110 mV and a corrosion current density of 6.2 104 mA/cm2 . Tafel constants for cathodic and anodic part are 300 and 102 mV per decade respectively. Pure cobalt does not passivate. Current density increases with increasing anodic overvoltage through the entire range of anodic polarization. WC exhibits a lower corrosion current density than pure cobalt by several orders of magnitude. The current density measured from WC also increases with anodic polarization. Two plateaus are observed at 200 and 600 mV. At approximately 900 mV, current density of WC decreases significantly.
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HCl is smaller than in H2 SO4 . As the potential is increased above 700 mV to transpassive region the current density increases rapidly again. Between 880 and 980 mV the current density starts to decrease dramatically. 3.3. Effect of scan rate
Potentiodynamic polarization curves of cemented carbide with 9% cobalt binder which has a low magnetic saturation of 15.1 lT m3 /kg in aerated and deaerated acidic solutions are shown in Fig. 2. A large difference between aerated and deaerated solution has not been observed. However, there is a difference in cathodic reaction between aerated and deaerated solution. The corrosion potential obtained from aerated solutions is slightly shifted to more noble values. Anodic polarization curves of this material (WCCo9SP) in both aerated and deareated solution are almost identical. The current density decreases by about one order of magnitude when polarizing from active peak potential to pseudopassive region. This is valid for aerated and deaerated solution. HCl shifts the active peak to more noble values and yields to higher active corrosion current densities. Consequently the potential range of pseudopassivity in
Electrochemical behavior of material WCCo9max at different scan rates is shown in Fig. 3. Corrosion potential of all specimens is approximately )425 mV. Anodic polarization curves from different scan rates show almost identical current density responses. A slight decrease in anodic current density is observed with decreasing scan rate. At the last period of polarization, current density decreases sharply above a potential of about 900 mV. The specimen polarized with a 200 mV/h scan rate has been further investigated by SEM. Fig. 4(a) shows a cross section of polarized specimen after cracking. The upper darker layer in Fig. 4(a) is a cobaltdepleted WC skeleton and the lower part is the initial cemented carbide with cobalt binder. Thickness of layer is about 250 lm. Fig. 4(b) is a magnification of the interface between WC skeleton (upper part) and the unaffected hard metal (lower part). The latter contains ductile dimples at the fracture surface. Semiquantitative EDX-analysis confirms optical results. No cobalt peak was obtained in the WC skeleton. A yellow corrosion product at the outer surface of the cemented carbide is observed and shown in Fig. 4(c) and (d). This plate-like product covers the outer surface of cemented carbide above the cobalt-depleted layer. The grain shape of this corrosion product is plate-like in contradiction to polyedric WC grains. Analysis of the outer layer showed high amounts of tungsten and oxygen and no carbon, which indicates a tungsten oxide layer.
Fig. 2. Effect of acid solutions (1 N) and dissolved oxygen on electrochemical behavior of WCCo9SP.
Fig. 3. Effect of scan rate on electrochemical behavior of WCCo9max in aerated 1 N H2 SO4 .
3.2. Effect of acid solutions and dissolved oxygen
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Fig. 4. SEM images of WCCo9max in aerated 1 N H2 SO4 after polarization scan at 200 mV/h: (a) cross sectional view of surface, (b) interface between WC skeleton and hard metal, (c,d) outer oxide layer.
3.4. Effect of magnetic saturation Fig. 5(a)–(c) shows the effect of magnetic saturation on electrochemical behavior of cemented carbides with different binder contents. The electrochemical parameters of investigated material are summarized in Table 2. Free corrosion potential of cemented carbide is about )400 mV, which is between the corrosion potential of cobalt and WC. The minimum current density in the pseudopassive region, ipp , of cemented carbide decreases with decreasing magnetic saturation, 4pr. The amount of decrease of ipp is over one order of magnitude for WCCo6 and still half an order of magnitude for WCCo15. Binder content also influences the electrochemical behavior of cemented carbides. Current density of WCCo6min shows lower ipp than WCCo15min. Apart from the fact that specimens have different magnetic saturations, a possibility for direct comparison of these results will be discussed later. 3.5. Chronoamperometry Chronoamperometric measurements of WCCo9max at two different potentials are presented in Fig. 6. Active and pseudopassive potentials were taken from potentiodynamic polarization curves at values of )350 and +160 mV respectively both resulting in same current response during polarization. After applying the potential step from the corrosion potential to the chosen anodic value onto specimens current densities
first increase sharply and then decrease as a function of time. Current density obtained from the potential in active region ()350 mV) shows lower value than in pseudopassive region (+160 mV). Finally, these two current responses decrease to the same final value of about 4 mA/cm2 . The results of cemented carbide with the same cobalt content but lower magnetic saturation (WCCo9min) are shown in Fig. 7. The current densities immediately increase after the potential step. Then the current density at active potential decreases steadily whereas an oscillating current response has been obtained at pseudopassive potential. The results of chronoamperometric measurements of cemented carbide with 6% cobalt and different magnetic saturation, WCCo6max and WCCo6min, are shown in Figs. 8 and 9 respectively. Analogous to the cemented carbide with 9% cobalt, both chronoamperometric curves at active potential ()360 mV) increases after applying the anodic potential and then decrease steadily with time. At the pseudopassive potential (+200 mV), the sample with a low value of magnetic saturation shows an oscillating current response while the sample with high magnetic saturation yields to a steady decrease. Results for WCCo6VC0.5 cemented carbide which also has a low magnetic saturation value (see Table 1) are shown in Fig. 10. Again two fixed potentials in active and pseudopassive region were used. Current density measured at active potential decreases steadily with time while at pseudopassive potential an oscillating current response has been obtained.
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Fig. 5. Effect of magnetic saturation, 4pr, on electrochemical behavior of cemented carbides with different cobalt binder content in aerated 1 N H2 SO4 . Table 2 Electrochemical parameters of investigated materials in 1 N aerated H2 SO4 Material
Ecorr (mV)
icrit (mA/cm2 )
icorr (mA/cm2 )
ipp (mA/cm2 )
bc (mV/decade)
ba (mV/decade)
WCCo6min WCCo6med WCCo6max WCCo9min WCCo9med WCCo9max WCCo15min WCCo15med WCCo15max WCCo6VC0.5 WC Co
)380 )354 )426 )427 )364 )436 )360 )426 )360 )367 110 )460
6.87 9.72 11.90 11.05 19.05 21.59 24.03 33.87 34.47 0.27 – –
0.070 0.248 0.203 0.170 0.308 0.202 0.385 0.388 0.294 0.019 0.00062 0.680
0.13 0.21 2.56 0.54 4.27 4.94 2.34 8.06 8.16 0.04 – –
155 159 133 110 158 118 163 150 166 79 300 225
29 32 34 41 50 48 53 51 40 72 102 40
3.6. Immersion test Two specimens were weighted during a month immersion test in 1 N sulfuric acid solution. Weight loss data are shown in Fig. 11. Cemented carbide in stirred
acid solution yields higher weight loss when compared to stagnant solution. The rate of weight loss keeps constant throughout the immersion period. The corrosion rates in mm/year are 0.32 and 1.3 for stagnant and stirred solution respectively.
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Fig. 6. Chronoamperometric measurement of WCCo9max at active potential ()350 mV) and pseudopassive potential (+160 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
Fig. 7. Chronoamperometric measurement of WCCo9min at active potential ()390 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
Fig. 8. Chronoamperometric measurement of WCCo6max at active potential ()360 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
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Fig. 9. Chronoamperometric measurement of WCCo6min at active potential ()360 mV) and pseudopassive potential (+200 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
(a)
(b)
Fig. 10. Chronoamperometric measurement of WCCo6VC0.5 with low magnetic saturation at active potential ()330 mV) and pseudopassive potential (+100 mV) in aerated 1 N H2 SO4 : (a) potentiodynamic scan, (b) chronoamperometry.
4. Discussion 4.1. Electrochemical behavior
Fig. 11. Weight loss as a function of time of WCCo9max in 1 N H2 SO4 .
Electrochemical parameters from WCCo9SP are shown in Table 3. Exponential increment of both cathodic and anodic current density in deaerated solutions near corrosion potential indicates an activation controlled process. However the higher cathodic Tafel constants obtained from aerated solutions indicate the combination of activation and diffusion polarization in cathodic region. Oxygen reduction is involved in aerated solutions whereas hydrogen evolution is the dominating cathodic reaction in deaerated solutions. Oxygen containing solution slightly shifts the corrosion potential of cemented carbides to more noble values due to higher reduction potential, Eo , of oxygen (Eqs. (1) and (2)). HCl is more aggressive to cobalt based cemented
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Table 3 Electrochemical parameters of WCCo9SP Media
Ecorr (mV)
icrit (mA/cm2 )
icorr (mA/cm2 )
ipp (mA/cm2 )
bc (mV/decade)
ba (mV/decade)
Aerated H2 SO4 Deaerated H2 SO4 Aerated HCl Deaerated HCl
)317 )384 )301 )340
4.55 4.11 11.53 11.16
0.095 0.075 0.190 0.025
0.16 0.32 0.69 0.46
128 97 153 85
50 50 88 88
carbides than H2 SO4 . Critical current density (icrit ) and minimum current density in pseudopassive region obtained in HCl solution is higher than in H2 SO4 . This suggests that different anions have an influence on corrosion properties of cemented carbides by changing ipp . As presented in Figs. 1–3 at about 700 mV WC oxidation becomes significant and yields to an increase of current density. Eq. (3) shows the involved reaction [9]. Semiquantitative analysis by an EDX system in the SEM confirmed high amounts of oxygen in the surface corrosion product (Fig. 4(d)). Polarization curves in both acid solutions are almost equal above 700 mV. The large applied driving force for oxidation suppresses the influence of anion present in the solution. Beyond 900 mV, current density decreases dramatically. There the obtained current density is even much lower than ipp . This decrease of current density is the result of corrosion product formation on the outer surface of the tungsten carbide skeleton. This product is suspected to be tungsten oxide, WO3 , and it occurs only on the outer surface of the skeleton as can be seen in Fig. 4(d). Thickness of oxide layer is about 10 lm after a further polarization from 900 to 1000 mV within 30 min. The product acts as a barrier and has a very low electrical conductivity when compared to WC. It also retards the possibility of cobalt ions to diffuse out of the tungsten carbide skeleton. 2Hþ þ 2e ! H2 þ
Eo ¼ 240 mVSCE
O2 þ 4H þ 4e ! 2H2 O
ð1Þ
ðpH ¼ 0Þ
o
E ¼ þ989 mVSCE
magnetic saturation, the ratio of magnetic saturation to this maximum value, Rr , is proposed (Eq. (5)). Rr ð%Þ ¼
100 4pr 100 wt:%Co 4pro
ð5Þ
Fig. 12 shows ipp as a function of binder content, including the influence of Rr . WCCo15 with a value of magnetic saturation ratio equal to 100% yields highest ipp whereas WCCo6 with 85% magnetic saturation ratio obtains the lowest. According to this figure, it is clear that a cemented carbide with lowest magnetic saturation ratio exhibits lowest ipp at constant binder content. On the other hand, high tungsten content in binder leads to a high corrosion resistance, which agrees with previous research works [5,10]. Furthermore, ipp decreases with decreasing cobalt binder content at constant magnetic saturation ratios. This means that binder surface area influences the corrosion property of cemented carbide, regardless of which surface area is taken into account in electrochemical measurement. The plot of current density referred to binder surface area, ico , versus potential according to SternÕs law of mixtures in Fig. 13 supports this. 4.3. Pseudopassivity Pseudopassivity in cobalt based cemented carbides has been explained by many researchers. Anodic film such as CoWO4 was described as a cause for current density reduction [5]. In contrast, according to chrono-
ð2Þ þ
WC þ 5H2 O ! WO3 þ CO2 þ 10H þ 10e
ð3Þ
4.2. Influence of binder content Magnetic saturation refers to the binder composition. The amount of tungsten dissolved in the binder during sintering process can be estimated by Eq. (4) where ro is the magnetic saturation of pure cobalt [6] (in G cm3 /g): wt:%Co ð4Þ 100 Magnetic saturation of pure cobalt in term of 4pr is equal to 201.7 lT m3 /kg. According to a specimen with 6% cobalt content, the maximum value of magnetic saturation that can be reached is approximately 12.1 lT m3 /kg or 6% of 201.7 lT m3 /kg. In order to compare the ipp of specimens with different binder content and r ¼ ðro 8 at:%WÞ
Fig. 12. Minimum pseudopassive current density, ipp , as a function of cobalt binder content at different magnetic saturation ratio (Rr ).
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Fig. 13. Effect of binder content on electrochemical behavior of cemented carbide with Rr ¼ 85% in aerated 1 N H2 SO4 , current density referred to binder surface area.
amperometric measurement of specimens with high magnetic saturation (low W content in the binder), no passive film has been found in pseudopassive region. After applying an anodic potential step, current densities increase sharply and then decrease with increasing time without any oscillation. Pseudopassive potential yields to a higher current density than the active potential at the beginning, due to the higher anodic overvoltage. These two current densities then converge to the same value of about 4.0 mA/cm2 as the time increases (Fig. 6). If there is a film formation at pseudopassive potential, current density should drop to a lower value because of higher resistance of passive film and if this film is unstable current density should increase again yielding to an oscillating current density response [11]. The current density however decreases smoothly to the steady state value. A different explanation for the pseudopassive behavior is the increasing length of cobalt diffusion path. After dissolution of cobalt metal from anodic polarization, cobalt ions, Co2þ , have to diffuse out through the remaining porous tungsten carbide skeleton shown in Fig. 4. Diffusion in the WC skeleton is slower than free diffusion and much slower than convection. As observed from immersion test in stirred solution, weight loss is much higher when compared to stagnant solution. Porosity, pore size, and tortuosity of the porous layer have an effect on diffusion coefficient. Due to diffusion limitation of mass transport current flow is decreased and causes pseudopassive behavior. No layer (CoWO4 , CoSO4 , a.s.o.) has been found at the interface between WC skeleton and cemented carbide. Fig. 3 shows the effect of scan rate on electrochemical behavior of cemented carbide. Critical current density and ipp decrease with decreasing scan rate. This also suggests that porous WC skeleton inhibits cobalt dissolution. Thickness of porous WC skeleton obtained
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during polarization is much larger at low scan rates. A scan rate of 200 mV/h means that the specimen is 5 times longer polarized when compared to a 1000 mV/h scan rate. Thickness of porous layer depends directly on the integration of current density over time. Therefore the layer thickness for each potential increment obtained from a 200 mV/h experiment increases much faster than that of a 1000 mV/h experiment and yielding to a decline of ipp with decreasing scan rate. In contradiction, WCCo9min that has minimum value of magnetic saturation in the series of 9% cobalt content exhibits a different behavior and indicates precipitation in pseudopassive region. As can be seen in Fig. 7, the oscillating current response is obtained at the pseudopassive potential at +200 mV. The only difference between WCCo9max and WCCo9min is the amount of tungsten dissolved in the binder. For WCCo9min, Rr is equal to 85%, which means that there is a higher amount of tungsten dissolved in the binder when compared to WCCo9max with Rr of about 100%. According to Eq. (4), WCCo9min has dissolved about 2.9 at.% of tungsten in binder whereas WCCo9max confirms only about 0.94 at.% of tungsten. This tungsten in binder plays an important roll in pseudopassive region. At these range of pseudopassive potentials WC is inert. Then the only active site is the binder. Pourbaix diagram of cobalt, Fig. 14, shows the instability of cobalt metal at pseudopassive potential and pH 0. And cobalt is simply dissolved as Co2þ into the solution. Tungsten has a different behavior. At anodic potentials between )390 and )330 mV (equal to )150 and )90 mVSHE ), it is still stable in the solution. At pseudopassive potential of +200 mV (+440 mVSHE ) and pH 0, tungsten is no longer stable and it is oxidized to be oxide compounds as shown in Fig. 15 and Eqs. (6)–(8) [12]. The oxide precipitates have much higher resistivity than the binder as already shown at potential more noble than 900 mV. They drop the current response at the beginning of chronoamperometry at a pseudopassive potential. Anyway these tungsten oxides are not dense and they are not strongly fixed like a true passive film on stainless steel. After their formation, this loose oxide precipitates could be physically removed from the top of binder surface by further slower corrosion of cobalt which leads to an increase of current density after its first step drop. Hence the oscillating current response behavior is obtained. After long times (10 min or more) a decrease of current without any oscillation is obtained. This can be explained by diffusion limitation of mass transport of cobalt through the tortured WC skeleton as already discussed. Oscillating current response was always obtained at dissolution current densities of 2 mA/cm2 or higher. This may be the threshold value that has to be exceeded for inhibition effect of W oxidation on Co dissolution. An analogous chronoamperometric result from magnesium corrosion is reported in a recent publication [13].
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W2 O5 þ H2 O ! 2WO3 þ 2Hþ þ 2e
Eo ¼ 269 mVSCE ð8Þ
Fig. 14. Potential–pH equilibrium diagram for the system cobalt– water, at 25 °C, potential with respect to standard hydrogen electrode [12].
Cemented carbide with 6% cobalt binder content also exhibits the same behavior (Figs. 8 and 9). By using Eq. (4), WCCo6max has tungsten dissolved lower than 0.0035 at.% in binder and WCCo6min binder contains about 3.2 at.% of tungsten. WCCo6max shows no precipitate formation at pseudopassive potential while WCCo6min yields to oxide precipitates formation and leads to an oscillating current response. These can be described in the same manner as in WCCo9. In addition, cemented carbides with a grain size less than 1 lm combined with small amount vanadium carbide and low magnetic saturation (10.5 lT m3 /kg) also exhibits an oscillating current drop in chronoamperometric measurement. Vanadium carbide is a well known additive for grain growth inhibition, which does not dissolve directly into the cobalt binder. It forms a thin layer and deposits around WC grains. This thin layer is not segregated on the surface of WC grains but form a thin crystallized layer on the basal planes of WC grains [14]. Therefore the binder consists mainly of cobalt and dissolved tungsten. According to Eq. (4), the amount of tungsten in binder can be estimated to about 2.7 at.%. This value is in the same order of magnitude with that of WCCo9min and WCCo6min that show the formation of oxide precipitates at pseudopassive potentials. Thus the oxide precipitates formation process that has been explained for WCCo9min can be applied to WCCo6VC0.5 as well. 5. Conclusions The following conclusions can be summarized from the present work:
Fig. 15. Potential–pH equilibrium diagram for the system tungsten– water, at 25 °C, potential with respect to standard hydrogen electrode [12].
W þ 2H2 O ! WO2 þ 4Hþ þ 4e
Eo ¼ 359 mVSCE
1. Dissolved oxygen has small influence on anodic behavior of cobalt based cemented carbides. 2. HCl is more aggressive to cemented carbides than H2 SO4 . 3. Corrosion resistance of cemented carbide increases with decreasing magnetic saturation. 4. Lower cobalt binder content exhibits better corrosion resistance. 5. Pseudopassivity of cobalt based cemented carbides is caused by diffusion limitation of cobalt ion. 6. Tungsten oxide precipitates could be formed in cemented carbides with high tungsten contents in the binder phase yielding to a decreased dissolution rate.
ð6Þ þ
2WO2 þ H2 O ! W2 O5 þ 2H þ 2e
o
E ¼ 271 mVSCE ð7Þ
Future work will be emphasized on the influence of refractory metal carbide additions and the correlation between microstructure and corrosion behavior.
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Acknowledgements The authors wish to thank Austrian Exchange Service € AD) for financial support of SS and Plansee Tizit (O GmbH, Austria, for preparation of cemented carbides.
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