Comparison On The Immersion Corrosion and Electrochemical Performance of WC-Al2O3 Composites and WC-Co Hard Metal in NaCl Solution

doi:10.21203/rs.3.rs-376939/v1

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Comparison On The Immersion Corrosion and Electrochemical Performance of WC-Al2O3 Composites and WC-Co Hard Metal in NaCl Solution

https://doi.org/10.21203/rs.3.rs-376939/v1

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WC-15wt.%Al₂O₃ composites was prepared via hot pressing sintering technology. The corrosion behaviors of WC-Al₂O₃ composites and traditional WC-Co hard alloy in NaCl solution were studied by immersion corrosion and electrochemical technique. The impedance value of WC-Al₂O₃ composite increased more rapidly than WC-Co hard metal during the 24 hours, which indicated that WC-Al₂O₃ composites has more compact passivation film than WC-Co hard metal. The results confirmed that the corrosion performance of WC-Al₂O₃ composites was higher than WC-Co cemented carbide in NaCl solution. The corrosion mechanism of WC-Al₂O₃ composites and WC-Co hard alloy in NaCl solution were also revealed by SEM, EDS, XPS and Raman. The corrosion products of WC-Al₂O₃ composites mainly contain WO₃, while for WC-Co hard alloy are Co (OH)₂, Co₃O₄ and WO₃. The different corrosion mechanism of the two materials is attributed to the Al₂O₃ phase instead of the Co binder, which avoids the galvanic corrosion between WC phase and Co binder.

Ceramics

Ceramic-matrix composites (CMCs)

WC-Al2O3 composite

Corrosion

Sintering

Conventional WC-based hard metals consist of WC phase and metal binder (Fe, Co, Ni, etc.), which could lead to high hardness and fracture toughness of hard alloys [1–6]. Among the conventional WC-based hard metals, because the metal binder is chemically active and susceptible to corrosion, the development of binderless WC-based hard composites has become an inevitable demand. Binderless WC-based hard alloys refer to the hard material without or with a small amount of metal binder (< 0.5%, mass fraction), which is a new hard material in recent years. Binderless WC-based hard alloys can be used as special sealing materials (such as sealing materials for cooling water pumps in nuclear power stations, sealing materials for petroleum and chemical equipment in acidic and alkaline corrosion environments, etc.), high-pressure jet nozzles, fluid mixers, drilling part and valves [7–9].Our group has previously developed the WC-Al₂O₃ composite as a new type of binderless WC-based hard alloys and carried out a series of studies. The research content includes: the ratio of WC and Al₂O₃ composition [10], crystal type and morphology of Al₂O₃[11], sintering process parameters and two-stage hot pressing sintering process [12–13], rare earth materials [14–15], grain growth inhibitor[16–17].Through these studies, WC-Al₂O₃ composite with excellent comprehensive mechanical properties have been prepared, which has attracted worldwide attention.

The corrosion resistance of materials used in corrosive environment is a very important technical index, which is usually studied by electrochemical method. Electrochemical corrosion resistance of WC-based hard metals depends on several factors, which includes the binder composition [18, 19], WC grain size [20–22], binder content [23] and additives [24, 25]. Human et al. [23] confirmed that WC-Co cemented carbide possessed the higher anti-corrosion ability with the decrease of Co content. However, in his other research, it was concluded that the binder phase content had no influence on the corrosion performance of hard alloy [26]. Studies on the corrosion behaviors of binderless hard metals have been rarely reported in domestic and foreign literatures, mainly involving WC-TiC-TaC composite, WC-MgO composite [27, 28]. Ma Y et.al [27] studied the corrosion performance of WC-TiC-TaC composite in NaCl solution, and believed that TiC and TaC could form solid solution with WC, which increased the resistivity and resistance of binderless WC-based hard composite and enhanced the corrosion resistance of the composites. Ouyang Chenxin et.al [28] studied the corrosion performance of WC-MgO composite by immersion corrosion and electrochemical methods in alkaline (pH = 1) and acidic (pH = 13) environments, and believed that the corrosion performance of WC-MgO composites in acidic environment was better than in alkaline environment. Passivation occurred in acidic environment, while no passivation occurred in alkaline environment.

In this work, WC-Al₂O₃ composites was obtained via hot pressing sintering technology. However, the corrosion performance of WC-Al₂O₃ composites in NaCl solution has not been reported. Therefore, the immersion corrosion and electrochemical performance of WC-Al₂O₃ composite in NaCl solution compared to WC-Co hard metal were researched.

2.1 Sample preparation

WC-Al₂O₃ composite used in this work was fabricated by our group. Table 1 presented the compositions of WC-based composites. Raw materials of WC, Al₂O₃(75µm ,150µm, respectively) and WC-6Co hard alloy were used in this work. The preparation process of WC-Al₂O₃ composite have been reported in our previous research [10]. All the samples used for electrochemical corrosion tests were cut into squares from the hot-pressing sintered samples. The exposed area of the electrochemical corrosion samples is 1cm².

Table 1

The compositions of the WC-based composites.
	Composition (wt.%)
	WC	Co	Al₂O₃
WC-6Co	94	6	-
WC-15Al₂O₃	85	-	15

2.2 Immersion corrosion test

According to ASTM standard G31-2004, the specimens were immersed in NaCl solution at 25℃ for obtaining the corrosion rate and macro corrosion morphologies of WC-Co hard alloy and WC-Al₂O₃ composite, respectively. The mass loss after immersion corrosion each stage (7 days, 14days, 21days and 28days, respectively) was weighed for calculating the corrosion rates. Then the optical images of corroded surface after 7 days were observed by digital camera.

2.3 Electrochemical measurements

Electrochemical corrosion tests were conducted in NaCl solution at 25℃ and open to air. The electrolyte solution was prepared from sodium chloride crystals and distilled water. Tafel curves, EIS and OCP were measured by electrochemical workstation (CS310H). The schematic diagram of the electrochemical three-electrode system was shown in Fig. 1. Saturated calomel electrode was used as the reference electrode. WC matrix composite was used as working electrode. Platinum sheet was used as counter electrode. The area of the electrodes is 1cm². The open circuit potential curves were monitored for about 12h in NaCl solution. Subsequently, the EIS test was conducted on the position of OCP value. The test frequency ranges from 100kHZ to 1mHZ. The potentiodynamic polarization was measured in the potential range from − 1000mV vs.SCE to + 1500mV vs.SCE. The scanning rate for all the samples was 2mV/s.

2.4. Microstructural and composition characterization

Prior to microstructural characterization, the specimens were ground with diamond discs from 600 grit to 4000 grit and polished with 1µm diamond pastes. Then the sample surface was degreased in ethanol by using ultrasonic cleaner. The microstructure and chemical composition of the specimens were observed by SEM and EDS. The phase composition of the specimens was characterized by XRD. The composition of the corrosion products of the composites after corrosion test was determined by XPS. The binding energy of C1s standard carbon was used as the reference peak. XPS spectrum is fitted by XPS peak software.

3.1 Microstructure characterization before corrosion tests

The XRD pattern of WC-Co hard metal and WC-Al₂O₃ composite are shown in Fig. 2. The local magnified XRD pattern of Fig. 2(a) was faintly observed in Fig. 2(b). In both materials of WC-Co and WC-Al₂O₃, WC is the main peak. The weak Al₂O₃ and Co peaks also can be observed, respectively. WC-Al₂O₃ composite has no phase transition after hot pressing sintering. It can be seen that WC peak is higher intense than Al₂O₃ and Co peak due to the low amount of Al₂O₃ and Co.

BSE-SEM images of the polished samples are presented in Fig. 3. It could be observed that there is uniform microstructure. As shown in Fig. 3(a), WC-Al₂O₃ composite is almost completely dense, which indicates that densification has occurred in the sample during the sintering process. WA15 composite had the double phase microstructure. Dark regions are dispersed Al₂O₃ particles and the surrounding bright regions are WC particles. The microstructure, mechanical performances and frictional wear properties of WC-Al₂O₃ composite have been reported in our pervious study [29]. As presented in Fig. 3(b), Co binder is visible in the WC-Co hard metal, which uniformly surrounds the WC particles.

3.2 Corrosion behavior after immersion corrosion test

Figure 4(a, b) illustrates the optical morphologies of WC-Co hard metal and WC-Al₂O₃ composite after immersion tests for 7 days in NaCl solution, respectively. As presented in Fig. 4(a), obvious corrosion products appeared on the surface of WC-Co hard alloy. However, as presented in Fig. 4(b), no corrosion layer was visible on the corroded surface of WC-Al₂O₃ composite. Figure 4(c) shows the mass loss curve of the samples after immersion corrosion test for 7, 14,21 and 28 days, respectively. It could be found that the dissolution rate increased obviously with immersion time in the sample of WC-Co hard metal. However, there were no significant differences in WC-Al₂O₃ composite at any time point. Hence, it may be speculated that the corrosion performance of WC-Al₂O₃ composite in NaCl solution is higher than WC-Co hard metal. The previous literatures [23] have reported the corrosion mechanism of WC-Co hard alloy in NaCl solution, while for WC-Al₂O₃ composite was not yet clear.

3.3 Electrochemical response

3.3.1 OCP monitoring

Figure 5 shows the OCP curves of all specimens measured in NaCl solution for 12h. The open circuit potential becomes stable with immersion time, which implies that the corroded surface of WC-based composites is stable gradually and not easily affected to pitting in electrolyte solution [30]. As shown in Fig. 5, WC-Al₂O₃ composites possesses more positive E_OCP value than WC-Co hard alloy. Katiyar et al. [30] reported that the higher positive E_OCP value implies the lower tendency to corrode from corrosion thermodynamics. The E_OCP value of WC-Al₂O₃ composite increases slightly with immersion time. This indicates that the passive film becomes dense at the final stage of immersion, which could hinder further corrosion of WC-Al₂O₃ composite [31]. However, the E_OCP value of WC-Co hard alloy decreases slightly with immersion time, which indicates the active dissolution of Co binder. Because the corrosion potential of WC phase is more positive than the Co binder, the Co ions are preferred to enter the electrolyte ^[38,39]. Hence, it could be speculated that the corrosion stability of WC-Al₂O₃ composites was higher than WC-Co hard metal by replacing the Co binder with Al₂O₃ particle.

3.3.2 Tafel curves

Figure 6 presents the tafel curves of WC-Al₂O₃ composites and WC-Co hard alloy in NaCl solution. The polarization curves of WC-based composites indicate the typical anodic behavior. The corrosion current density(i_corr) increases exponentially with the corrosion potential (E_corr). Subsequently, it tends to the steady state [31]. The cathodic polarization branches display the oxygen reaction in NaCl solution. This phenomenon is controlled by activation in WC-based composites. All the composites exhibit the passivation behavior, which was marked by the decrease or almost constant in the corrosion current density at higher potentials. In the anodic polarization curve, WC-Al₂O₃ composite shows the passivation behavior and the passivation current density(i_pass)is less than 10µA/cm²[23]. However, WC-Co cemented carbide exhibits the pseudo-passivation behavior, because the passivation current density (i_pass) is much higher than 10µA/ cm². The pseudo-passivation behaviors occurred when the pseudo-passivation current density of the composite at higher potential was several orders of magnitude higher than the passivation current density. The pseudo-passivation observed here for WC-Co carbide alloy is consistent with other reported study [32]. On one hand, Co binder has a low corrosion potential compared with WC phase [32], so it can be said that Co binder has the priority of dissolution, and WC phase is protected by the cathode. On the other hand, WC phase could be oxidized at anodic potential. Therefore, the oxidation of WC phase and active dissolution of Co binder could take place in NaCl solution.

Table 2 presents the corrosion parameters from the Tafel curves. The significant corrosion parameters were obtained according to ASTM Standard G5-94. The i_corr, E_corr and the Tafel slopes were determined by Tafel extrapolation. The E_corr value reflected the thermodynamic characteristic of the composites [33]. The previous research reported that the higher corrosion potential implies better chemical stability and lower corrosion tendency, and the lower i_corr reflects lower corrosion rate [34]. It could be observed that the E_corr of WC- Al₂O₃ composite was higher than that of WC-Co hard alloy, implying that WC-Co hard alloy has lower corrosion tendency. Because the i_corr values can imply the dynamics characteristic of corrosion process, the i_corr value reflects the corrosion rate more accurately than E_corr value. WC-Al₂O₃ composite shows the lower i_corr value (18.25 µA/cm²), while WC-Co hard metal shows the higher value (32.56 µA/cm²). The variation of i_corr value could be explained from the following reasons. The main reason was the formation of dense passivation film on the surface of WC-Al₂O₃ composite, which could decrease the corrosion rate of the composites. In addition, the i_pass of WC-Al₂O₃ composite (∼8.55µA/cm²) is lower than WC-Co hard alloy(∼2020.4µA/cm²). In short, it can be concluded that WC-Al₂O₃ composites had more positive E_corr value, lower i_corr and i_pass values, indicating that WC-Al₂O₃ composites possessed better corrosion performance in NaCl solution from corrosion thermodynamics and corrosion kinetics.

Table 2

The corrosion parameters of the composites in NaCl solution.
Samples	E_corr (V_SCE, mV)		βa(mV/°)	βc(mV/°)	i_corr (µA cm^− 2)	i_pass (µA cm^− 2)
Samples	OCP	Polarisation curves	βa(mV/°)	βc(mV/°)	i_corr (µA cm^− 2)	i_pass (µA cm^− 2)
WC-6Co	-376.2	-359.6	134.4	298.8	18.25	2020.4
WC-15Al₂O₃	-238.8	-312.1	326.6	278.3	32.56	8.55

3.3.3EIS analysis

Figure 7 presented the EIS results of WC- based composites in NaCl solution. The Nyquist spectrum of all the composites in NaCl solution both have similar capacitive loops (Fig. 7(a)and(d)), which indicate that both of two composites have similar corrosion processes. The capacitive arc radius of the Nyquist plots at low frequencies can reflect the corrosion performance of the materials. Hence, it could be inferred that WC-Al₂O₃ composites possesses higher corrosion performance than WC-Co carbide alloy owning to the larger capacitive loop of WC-Al₂O₃ composite. The results are consistent with the polarization results. As presented in Fig. 7(b) and (e), the impedance value of WC-Al₂O₃ composite increases more rapidly than WC-Co hard metal during the 24 hours, which indicates that WC-Al₂O₃ composite possesses more dense passivation film than WC-Co hard metal. The passivation film on the surface of WC-Al₂O₃ composite could inhibit further corrosion of the material. According to the Bode results (Fig. 7(c)and(f)), the impedance of WC matrix composite was fitted by two time constants, one of which was related to the corrosion process at low frequencies, and the other was related to the passivation layer at high frequencies [35]. It could be observed that the phase angle curves of WC-Al₂O₃ composite is closer to the low-frequency region than WC-Co hard alloy. It has also been reported that a wider phase angle curve means an improved corrosion resistance of the materials in previous studies [21].

Figure 7(g) presented the equivalent circuit of WC-based composites. In the circuit, R_ct is the resistance of charge transfer; R_s and R_f represent the resistances of corrosion solution and corrosion products, respectively; Q_f and Q_ct are the capacitances of corrosion products and double-charge layer, respectively. The corrosion parameters of WC-based composites during corrosion process were presented in Table 3. The variations in Q_f and R_f occur on the composites and reflect the growth of passivation film on the surface of the composites. Compared with WC-Co hard alloy, the values of decreased Y₀ (Q_f) of WC-Al₂O₃ composite in NaCl solution showed more dense passivation film on the surface of the composites [31]. The value of Q_ct in NaCl solution was higher than the typical of a double layer (1 ~ 100µF/cm²). This phenomenon could be explained by an increase in the number of corrosion products (oxides), which will expand the electrochemical active region, if semiconducting, the double layer could be formed. The most significant corrosion parameter for the composite is the charge transfer resistance (R_ct), which is related to Faraday process and it can reflect the corrosion rate of the materials. The higher the R_ct, the higher resistance to corrosion. The R_ct value of WC-Co hard metal in NaCl solution was 5071 Ω·cm2, which was lower than WC-Al₂O₃ composites. In addition, the polarization resistance (R_p) is shown in Fig. 7(h). The polarization resistance (R_p) is equal to the sum of the solution resistance (R_f)and charge transfer resistance (R_ct) [29, 40]. The R_p value of WC-Al₂O₃ composite is about one order of magnitude higher than WC-Co hard alloy. As the increase of corrosion time, the R_p value of WC-Al₂O₃ composites increased from 9336 to 24180 ohm. cm².However, for WC-Co cemented carbide, R_p value keeps a stable value near 5000 ohm.cm².Therefore, it could be concluded that compared with WC-Co hard alloy, WC-Al₂O₃ composites has higher corrosion resistance. The result is consistent with the polarization curve.

Table 3

EIS fitting parameters of WC-Al₂O₃ and WC-Co samples.
Samples	Time/h	Rs(Ω cm²)	Q_f		R_f(Ω cm²)	Q_ct		Rct(Ω cm²)	Rp(Ω cm²)
Samples	Time/h	Rs(Ω cm²)	Yo(µF/cm2)	n	R_f(Ω cm²)	Yo(µF/cm2)	n	Rct(Ω cm²)	Rp(Ω cm²)
WC-15Al₂O₃	1	7.38	133.4	0.86	7116	1299	0.91	2220	9336
	3	7.09	133.1	0.86	10270	1239	0.99	3257	13527
	6	6.95	129.8	0.87	10460	1174	1	3901	14361
	12	6.77	127.9	0.90	16250	799	1	7050	23300
	24	6.81	118.6	0.90	16851	608	1	7329	24180
WC-6Co	1	5.29	97.2	0.93	3389	6447	1	617	4006
	3	5.37	127.9	0.91	3817	9211	1	697	4514
	6	5.45	140.4	0.88	3878	8177	1	937	4815
	12	5.30	141.6	0.88	4059	66.99	1	945	5004
	24	5.30	168	0.88	4139	65.82	1	932	5071

3.4 Characterization of the corroded surface

3.4.1 Corrosion Morphologies

Figure 8 presents the typical SEM images of WC-Co hard metal and WC-Al₂O₃ composite after corrosion tests, respectively. Figure 9 and Fig. 10 show the EDS analysis results of corrosion products for WC-Co hard metal and WC-Al₂O₃ composite, respectively. Compared to Fig. 3, the remarkable distinctions can be found between the polished specimens and the corroded specimens. As presented in Fig. 8(a), a large number of loose corrosion products and many pores can be seen on the surface of WC-Co hard metal, which means that the electrolyte solution could penetrate the loose corrosion products and accelerate further corrosion of the material. According to the EDS analysis results of WC-Co hard metal, as shown in Fig. 10, W, C, Co, O exist on the corroded surface, which is attributed to the formation of oxide compounds. It also can be inferred that cobalt is far more corrosive than tungsten in NaCl solution. Ali Fazili et al [36] has reported that Co binder was preferentially dissolved, subsequently a part of WC particles was corroded in NaCl solution.

Turning to WC-Al₂O₃ composite, as presented in Fig. 8(a) and (b), different corrosion morphologies could be observed on the surface of WC-Co hard metal and WC-Al₂O₃ composites. As presented in Fig. 8(b), it could be observed that dense passive film was formed on the surface of WC-Al₂O₃ composite. According to the EDS results of WC-Al₂O₃ composite, it could be found that the main elements are W and O, which could correspond in EDS mapping. The O content of the original composite is only 15 wt.%, but the O content increases after corrosion tests. Hence, it could be speculated that the corrosion products of WC-Al₂O₃ composites contains WO₃.

3.4.2 Raman and XPS results

Figure 11 presents the Raman spectra of WC-Co hard metal and WC-Al₂O₃ composites after corrosion tests. As illustrated in Fig. 11(a), it could be observed that the positions of the main peak on the surface of WC-Co hard metal are 705cm^− 1(WO₃), 460cm^− 1(Co (OH)₂), 525cm^− 1(Co₃O₄) [15, 38], indicating that the corrosion process of WC-Co hard alloy is attributed to the preferential dissolution of Co binder and the oxidation of WC phase. However, as presented in Fig. 11(b), it could be observed that the positions of the main peak on the surface of WC-Al₂O₃ composites are 952cm^− 1(WO₃) and 705cm^− 1(WO₃) [30, 37, 38, 39], indicating that the corrosion product of WC-Al₂O₃ composites contains mainly tungsten oxide. In addition, the Raman results in this work are consistent with the previous literature [20, 37].

Figure 12 XPS spectra of corroded surface of WC-Co hard metal: (a) wide scan survey spectra, (b) Co2p, (c)W4f, (d) O1s, (e) C1s.

3.5 Corrosion mechanism

Figure 14 displays the corrosion current transients of WC-Co hard alloy and WC-Al₂O₃ composites in NaCl solution. The applied potentials were − 300 and 750 mV vs SCE in the passivation region for WC-Co and WC-Al₂O₃ composite, respectively. With the increase of corrosion time, the corrosion current density decreased to a stable value, and the potentiostatic polarization behavior of WC-Al₂O₃ and WC-Co composite was analyzed. Lekatou et al. [40] believed that the current density increased and stabilized to a stable value with immersion time, indicating that the passivation behavior of the material has occurred. As shown in Fig. 14(a) and (b), WC-Co hard metal shows the greatest steady current density value with the increasing immersion time, implying that the corrosion behavior of WC-Co hard metal is general corrosion corresponding with the dissolution of Co binder. However, as shown in Fig. 14(c) and (d), the current density of WC-Al₂O₃ composites indicated an obvious fluctuation, which implied that the passivation film of WC-Al₂O₃ composites was broken in NaCl solution. Hence, it could be inferred that the corrosion behavior of WC-Co hard metal in NaCl solution is general corrosion by the dissolution of Co binder, but for WC-Al₂O₃ composites is general corrosion by the oxidation of WC phase and the dissolution of Al₂O₃ phase.

Figure 15 shows the schematic illustrations of the corrosion mechanism for WC-Co hard alloy and WC-Al₂O₃ composite, respectively. As presented in Fig. 15(a1), after removing the loose corrosion products, WC skeleton is left, which imply that Co binder has been dissolved. Combined with EDS, XPS and Raman analysis, it can be further confirmed that the corrosion behavior of WC-Co cemented carbide in NaCl solution exhibits general corrosion characteristic. Katiyar et al [38] have reported that Co binder plays cathodic protection role for WC phase, which inhibit the corrosion of WC phase. However, when Co binder was totally dissolved, subsequently WC phase was also oxidized to WO₃. Therefore, the corrosion mechanism of WC-Co cemented carbide in NaCl solution is the electrochemical dissolution of Co binder and the chemical oxidation of WC phase.

As written in Reaction (1), Co binder would react with water to form CoO [17].

Co + H₂O→CoO + 2H⁺+2e⁻ (1)

This is oxidized to Co₃O₄, which is the final product. Therefore, CoO is not detected by Raman and XPS. This process can be expressed as the following reactions:

3CoO + H₂O→Co₃O₄(corrosion products) + 2H⁺+2e⁻ (2)

In addition, the most significant corrosion process for WC-Co hard metal are, respectively, cobalt dissolution of the anode and oxygenation reaction of the cathode. The reactions are as follows [30]:

Co → Co²⁺ + 2e⁻ (3)

2H₂O + O₂ + 4e⁻ →4OH⁻ (4)

Then, the OH⁻ ions react with Co²⁺ ions to form cobalt hydroxide:

Co²⁺ + 2OH⁻→Co (OH)₂(corrosion products) (5)

After Co binder is dissolved, WC phase exposed to NaCl solution begins to corrode. The reaction is as follows:

WC + 5H₂O→WO₃(corrosion products) + CO₂ + 10H⁺+10e⁻ (6)

As presented in Fig. 15(b1), after removing the loose corrosion products, the dense corrosion layer is still on the surface of WC-Al₂O₃ composites. In addition, the corroded surface of WC-Al₂O₃ composite showed some cracks, which may be caused by stress due to the polarization process and new phase formation [41]. Combined with EDS, XPS and Raman analysis, it can be inferred that the corrosion behavior of WC-Al₂O₃ composites exhibit the general corrosion characteristic. The corrosion of WC phase can cause the spalling of Al₂O₃ phase. The previous works [28, 42] confirmed that ceramic phase (such as Al₂O₃ and MgO) is a good insulator, and WC phase has excellent conductivity, so WC phase cannot form a galvanic couple with Al₂O₃ phase. The literatures [43] reported that chemical corrosion occurred on the surface of Al₂O₃ phase. Hence, the corrosion process of WC-Al₂O₃ composites is the electrochemical oxidation of WC phase and the chemical dissolution of Al₂O₃ phase in NaCl solution. As expressed in Reaction (8), leading to the high hydration of surface film.

Al₂O₃ + 3 H₂O→2Al (OH)₃ (7)

The adsorption of Cl⁻ onto the WC-Al₂O₃ composite easily transforms Al (OH)₃ to soluble AlCl₃. Diffusion of AlCl₃ from the surface of samples to electrolyte leads to the corrosion of WC-Al₂O₃ composite [43].

Al (OH)₃ + 2Cl⁻→AlCl₃ + 2OH (8)

The most important anodic and cathodic processes of WC-Al₂O₃ composites are tungsten carbide oxidation of the anode and oxygenation reaction of the cathode in NaCl solution, respectively. The specific electrochemical reaction is as follows:

WC + 5H₂O→WO₃ (corrosion products) + CO₂ + 10H⁺+10e⁻ (9)

2H₂O + O₂ + 4e⁻→4OH⁻ (10)

In summary, combined with the results of polarization curve, it could be rigorously inferred that the corrosion current density increased during anodic polarization due to the oxidation of WC phase. WC-Al₂O₃ composite could form the dense passivation film after corrosion test in NaCl solution. Hence, it could be confirmed that the corrosion performance of WC-Al₂O₃ composite is higher than WC-Co cemented carbide.

In this work, the corrosion behavior of WC-Al₂O₃ composite and WC-Co hard metal in NaCl solution were investigated by immersion corrosion test and electrochemical test. Some important conclusions were as follows.:

(1) WC-Al₂O₃ composites has higher corrosion performance than WC-Co hard alloy in NaCl solution, due to the lower i_corr of 18.25µA/cm² and more positive E_corr of -312.1mV, compared to the i_corr of 32.56µA/cm² and E_corr of -359.6mV for WC-Co hard metal. In addition, the corrosion current density is different from the passivation current density, indicating the passivation trend. The pseudo-passivation of WC-Co hard metal was observed in NaCl solution (i_pass > > 10µA/cm²). However, WC-Al₂O₃ composite exhibits the passivation behavior (i_pass<10µA/cm²).

(2) The EIS results also proved higher corrosion performance of WC-Al₂O₃ composite, due to the higher value of polarization resistance (R_p). The impedance value of WC-Al₂O₃ composite increased more rapidly than WC-Co hard metal, indicating that dense passivation film was formed on the surface of WC-Al₂O₃ composites, and its stability and compactness enhanced with the increase of immersion time.

(3) The corrosion behavior of conventional WC-Co hard metal is general corrosion with preferential dissolution of Co binder and subsequent oxidation of WC phase. The corrosion products of WC-Co cemented carbides contain mainly Co (OH)₂, Co₃O₄ and a small amount of WO₃.In addition, the corrosion mechanism of WC-Al₂O₃ composite is general corrosion, in which WC phase is oxidized and a small amount of Al₂O₃ phase is dissolved. The corrosion products of WC-Al₂O₃ composites contain mainly WO₃.

Acknowledgements

The work was supported by the Fundamental Research Funds for the Central Universities under grant CUSF-DH-D-2020050.

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Comparison On The Immersion Corrosion and Electrochemical Performance of WC-Al2O3 Composites and WC-Co Hard Metal in NaCl Solution

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Abstract

Figures

1. Introduction

2. Experimental Procedure

2.1 Sample preparation

2.2 Immersion corrosion test

2.3 Electrochemical measurements

2.4. Microstructural and composition characterization

3. Results And Discussion

3.1 Microstructure characterization before corrosion tests

3.2 Corrosion behavior after immersion corrosion test

3.3 Electrochemical response

3.3.1 OCP monitoring

3.3.2 Tafel curves

3.4 Characterization of the corroded surface

3.4.1 Corrosion Morphologies

3.4.2 Raman and XPS results

3.5 Corrosion mechanism

4. Conclusion

Declarations

Acknowledgements

References

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