Cr cermets in H2SO4 solution

Int. Journal of Refractory Metals and Hard Materials 47 (2014) 139–144

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Corrosion behavior of Ti(C,N)-Ni/Cr cermets in H2SO4 solution Shan Chen a,b, Weihao Xiong a,⁎, Zhenhua Yao a, Guopeng Zhang a, Xiao Chen a, Bin Huang a, Qingqing Yang a a b

State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Naval University of Engineering, Wuhan, 430033, China

a r t i c l e

i n f o

Article history: Received 16 March 2014 Accepted 2 July 2014 Available online 17 July 2014 Keywords: Ti(C,N)-based cermets Cr Potentiodynamic polarization Corrosion

a b s t r a c t The effects of partial substitution of Ni with Cr on the microstructure and corrosion resistance of Ti(C,N)-based cermets in H2SO4 solution were investigated in this paper. The results showed that partial substitution of Ni with Cr had a minor effect on the microstructure, whereas the hardness of the Ti(C,N)-based cermets could be improved for dissolution of Cr in Ni binder. The corrosion behavior of Ti(C,N)-based cermets with different Cr content in 0.2 mol/L H2SO4 solution was also studied via potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and corrosion test. The tested Ti(C,N)-Ni/Cr cermets had three typical passive regions. The current of the first passive region was reduced and the passive range was enlarged with the increase of Cr content in binder attributed to the synergistic effect of Ti-based and Cr-based passive layers. But the remaining passive regions were pseudopassive regions. The EIS results also demonstrated that the impendence of the reaction rose with the increase of Cr content in binder. Moreover, the corrosion resistance of cermets in H2SO4 solution was improved remarkably by Cr dissolving in the binder, which gave rise to the enhanced passivation ability. © 2014 Published by Elsevier Ltd.

Introduction Ti(C,N)-based on cermets have been used widely in metal cutting industries for their high performance such as excellent wearresistance, high hot-hardness, perfect chemical stability, very low friction coefficient to metals, superior thermal deformation resistance and high strength [1–4]. They are also used as valves, gears and rings in chemistry industry [5,6]. However, there exists a significant difference in corrosion potentials of the ceramic phase and metal phase when the Ti(C,N)-based cermets served in corrosive circumstance. Modifying compositions and sintering techniques were two popular methods to improve the mechanical properties of Ti(C,N)-based cermets [7–10]. The changing of compositions can also influence the corrosion resistance of the cermets. The corrosion resistance of cermets was affected by the proportion of the binder phase and increased with the decrease of the binder phase Ni. The addition of second carbides plays a role in the electrochemical behavior of Ti(C,N)-based cermets in different solutions [11–14]. B.V. Manoj Kumar et al. had studied the electrochemical behavior of Ti(C,N)-based cermets in 0.2 mol/L sulfuric acid by adding second carbides such as WC, NbC and HfC. Their results showed that all the tested cermets exhibit typical active–passive behavior [11]. Xiong [12–14] reported the effects of Cr3C2, Mo2C and WC on the corrosion resistance of Ti(C,N)-based cermets in HNO3 solution. Their results showed that the corrosion resistance could be improved ⁎ Corresponding author. E-mail address: [email protected] (W. Xiong).

http://dx.doi.org/10.1016/j.ijrmhm.2014.07.010 0263-4368/© 2014 Published by Elsevier Ltd.

remarkably by Cr3C2 or Mo2C addition, but the corrosion rate of cermets increased with WC addition. Some literatures had revealed the beneficial effects of Cr addition on the properties of cermets. For example, the hardness and transverse rupture strength (TRS) could be improved by partial substitution of Ni with Cr, and the high temperature oxidization resistance could be improved when the addition of Cr reached 6 wt.% [15,16]. Our work aims to investigate the effect of Cr addition on the corrosion resistance of Ti(C,N)-based cermets in acid solution, which is a rarely reported topic in this area. In this work, the role of partial substitution of Ni with Cr on the microstructure and corrosion resistance of Ti(C,N)-based cermets in 0.2 mol/L H2SO4 solution was studied. The electrochemical behavior was tested by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and corrosion test. The corrosion mechanism of the Ti(C,N)-based cermets was also addressed. Experimental Materials and characterization The compositions of the cermets with different Cr additions are given in Table 1. Commercially available TiC (2.97 μm), TiN (7.3 μm), WC (4.68 μm), Mo (2.3 μm), Ni (2.25 μm), Cr3C2 (2.8 μm) and C (b30 μm) and Cr (~74 μm) powders were used as starting powders. These powders were first mixed for 48 h at a speed of 250 rpm in a planetary mill. The mass ratio of ball to material was 7:1. The mixed slurries were then dried at 353 K in an infrared stove. After passing

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Table 1 Nominal composition of the experimental materials (wt.%). Cermet

TiC

TiN

Mo

A B C D

46.1 46. 1 46.1 46.1

10 10 10 10

16 16 16 16

Cermet

Ti

W

Mo

A B C D

38.80 38.98 39.12 38.70

12.82 12.34 12.27 12.72

17.81 18.23 17.96 18.11

WC

Cr3C2

C

Ni

Cr

0.6 0.6 0.6 0.6

0.8 0.8 0.8 0.8

20 18 16 14

/ 2 4 6

Ni

Cr

Co

C

N

16.38 15.15 14.05 11.69

1.17 2.12 3.93 5.95

0.45 0.47 0.41 0.51

10.58 10.70 10.20 10.35

1.99 2.01 2.06 1.97

6.5 6.5 6.5 6.5

electrode and a saturated calomel electrode (SCE) as reference. The working surface of the sample was 0.6 cm2. All the potentials in this paper were reported in the SCE scale. The temperature was maintained at 25 ± 0.1 °C. The electrochemical performance of the cermets was determined from AC measurements, which were carried out using a Parstat SS350 system with commercial software program. After the specimens had reached a stable OCP (Open-Circuit Potential) within 2 h, the potential of the electrode was swept at a rate of 1 mV/s from an initial potential of −200 mV vs. the OCP to a final potential of 1500 mV. The EIS measurements were obtained with an amplitude of 10 mV at frequencies ranging from 100 kHz to 10 mHz. To ensure reproducibility, three measurements were run for each specimen. Corrosion test

the dried powders through a 100 mesh sieve, the powders were uniaxially pressed at 300 MPa for 60 s. The green compacts were sintered at 1718 K in vacuum for 1 h using vacuum sintering furnace (WZDS-20B, Beijing Research Institute of Mechanical and Electrical Technology, China). The microstructures of the cermets were observed by a scanning electron microscope (SEM; QUANTA 200, FEI, Netherland) in back-scattered electron (BSE) mode. Hardness was measured using Rockwell hardness test. The transverse rupture strength (TRS) was measured employing a three-point-bending test (the dimension of specimen 20.0 mm × 6.5 mm × 5.20 mm, span 14.5 mm) at the crosshead speed of 0.5 mm/min. Electrochemical measurements All electrochemical measurements such as potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) were carried out in 0.2 mol/L H2SO4 solution. The tests were carried out in a conventional three-electrode electrochemical glass cell with a platinum counter

Three specimens of each kind of cermets were polished to 0.1 μm with diamond paste and the size of each specimen was measured by a micrometer. The cermets were cleaned by ultrasonic and dried in a drying vessel for 24 h. Then the specimens were weighed using the precision electron balance model BS224S (Beijing Sarturius Co., Ltd., China) with the error of 0.1 mg before and after corrosion test. According to the ASTM G31-72 (Revision 2004), the ratio of solution to the tested surface was 30 ml:1 cm2. The corrosion rate C is as follows: C ¼ △m=At where C is the corrosion rate (g mm−2 h−1), △m is the weight-loss (g), A is the total surface area of sample (mm2), and t is the immersion time (h).The corrosion rate was determined by the average results of three samples. The surface morphology of the cermets after corrosion was observed in the secondary electron (SE) mode to investigate the corrosion mechanism.

Fig. 1. SEM–BSE micrographs of Ti(C, N)-based cermets with different Cr addition: (a) 0 wt.% Cr; (b) 2 wt.% Cr; (c) 4 wt.% Cr; and (d) 6 wt.% Cr.

S. Chen et al. / Int. Journal of Refractory Metals and Hard Materials 47 (2014) 139–144 Table 2 Hardness and TRS of tested cermets 10 μm.

Hardness (HRA) TRS (MPa)

A

B

C

D

91 ± 0.1 1657 ± 35

91.4 ± 0.1 1717 ± 40

92.1 ± 0.1 1747 ± 35

92.7 ± 0.1 1261 ± 35

Results and discussion Microstructures and properties of Ti(C, N)-based cermets The microstructures of the tested Ti(C, N)-based cermets after sintering were characterized by SEM-BSE mode as shown in Fig. 1. The four cermets showed a typical microstructure consisting of hard core, rim phase and binder phase. Ti(C,N) grains were uniformly distributed in Ni-based binder phase. They contained two kinds of core/rim structures: coarse Ti(C,N) grains were generally consisted of black core, white inner rim and grey outer rim; whereas fine Ti(C,N) grains were generally consisted of white core and grey rim. With the increase of Cr content, there were more grains with white core and grey rim can be observed. After sintering at 1718 K for 1 h, we measured the hardness and TRS of four cermets. The data are listed in Table 2. It can be seen from Table 2 that the hardness of the Ti(C,N)-based cermets increased with the content of Cr. The TRS of the cermets firstly increased and reached the peak value when Cr content was 4 wt.%. The as-sintered compositions of Ti(C, N)-based cermets are given in Table 1. The compositions were analysed by chemical analysis method. Trace of Co is obtained from the milling ball. Table 3 shows SEM/EDS analysis of Ni-based binder phase in the tested cermets. In the result, only the metallic constituents of the alloys are given because carbon cannot be accurately quantified by EDS. The content of Cr in Ni-based binder phase increased with the addition of Cr. The effect of partial substitution of Ni with Cr on the mechanical properties of the Ti(C,N)-based cermets with 20 wt.% binder agrees well with the results reported by Yang et al. [15]. Due to the alloying of Cr the binder phase hardened, and with it the hardness of the cermets increased. There was little change about TRS of cermets A, B and C. However, when the Cr content reached 6 wt.%, the TRS of cermet D was much lower than those of the other three cermets. This fact may be attributed to a small amount of undissolved (Mo,Cr)2C as reported by Yang et al. [15]. Cermet D showed a low TRS, so cermets A, B and C were selected for the following corrosion tests.

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passive regions, the first region was attributed to TiOx and the second passive region was attributed to Ni. The results such as critical current density for passivation (icrit), potential for primary passivation (Epp), and the passive current density of the first passive region (ipass1) are shown in Table 4. The cermets exhibit noble ZCP (Zero Current Potential) when compared with pure Ni, which is − 255 mV (vs. SCE) [11]. Partial substitution of Ni with Cr had a minor effect on the ZCP of cermets due to their similar complicated microstructure. However, with the increase of Cr content, the first passive current decreased evidently and the passive range expanded, which indicated that the introduction of Cr additions can enhance the passivation behavior of Ti(C,N)-based cermets. The added Cr was mainly distributed in binder and formed solid solution Ni(Cr). Many researchers have demonstrated that the addition of Cr can reduce the passivation current of Nickel-based alloy [17–19]. Vladimir A. Lavrenko investigated the TiC0.5N0.5 electrochemical behavior in 3.5%NaCl solution, and the results showed that different oxide layers such as TiC0.3N0.3O0.4, TiO and TiO2 formed on the surface. The presence of the first passive region was attributed to the formation of a Ti-based passive layer by following reactions [20]: þ

−

TiCN þ H2 O ¼ TiCNO þ 2H þ 2e

ð1Þ þ

−

TiCN þ 3H2 O ¼ TiO þ CO2 þ 1=2N2 þ 6H þ 6e þ

−

TiCN þ 4H2 O ¼ TiO2 þ CO2 þ 1=2N2 þ 8H þ 8e

ð2Þ ð3Þ

It can be concluded from Pourbaix's potential–pH diagram of different M–H2O system (M = Mo, W and Cr) [21]. The Mo element in the Ti(C,N)-based cermets tested here may be dissolved into the acid solution to form Mo 3 + at the first passivation region. Except for the formation of the Ti-based passive layer, the solution of Cr in Ni binder plays a strong role in maintaining the passivity of Ni at the potential range of 0–0.5 V in oxidizing acid [17]. With the increase of Cr content in binder, a compact passive layer formed on the surface of the cermets, leading to decreased maintaining passivity current density. It means that the formation of first passive region of Ti(C,N)-based cermets with Cr addition was the synergistic effect of Ti-based and Cr-based passive layers. The tested cermets reached a second passive range, which was called a pseudopassive range for its critical density was higher than 0.1 mA cm− 2. The cermets contained more components besides the

Electrochemical measurements Fig. 2 shows the potentiodynamic polarization for Ti(C,N)-based cermets samples in 0.2 mol/L H2SO4 solution. The potentiodynamic polarization curves of the tested cermets consisted of three passive regions which can be distinguished by different passive current densities. The passive region that appeared first on polarization past the critical density was termed as the first passive region; while the passive regions that appeared at higher anodic potentials were called the second and third passive regions, respectively. These results are different from the potentiodynamic curve of Ti(C,N)-based cermets with different secondary carbides in freely aerated 0.2 mol/L H2SO4 solution [11]. In Ref. [11], the potentiodynamic curve of Ti(C,N)–20Ni cermets had two typical

Table 3 Metallic constituents of Ni-based binder phase (at.%).

A B C D

Ni

Ti

W

Mo

Cr

75.29 62.92 69.44 67.12

16.82 18.74 11.97 10.5

1.54 3.43 1.90 1.52

4.62 9.67 8.13 8.89

/ 3.43 7.31 10.06

Fig. 2. Dynamic-potential curves of Ti(C,N)-based cermets with different Cr addition in 0.2 mol/L H2SO4 solution (A: 0 wt.% Cr; B: 2 wt.% Cr; and C: 4 wt.% Cr).

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Table 4 Passivation parameters of tested cermets from potentiodynamic polarization curves. Cermet

ZCP (mV) vs. SCE

Icrit (μA/cm2)

Epp (mV vs. SCE

Ipass1 (μA/cm2)

Passive range1 (mV)

A B C

−107 −97 −105

135 28 21.3

91 104 9

61 36.6 32.4

387 440 580

Ti(C,N) cores and the Ni-based binder, the microstructures of the tested cermets were complicated and the electrochemical behavior of them was complex. It can be seen from Fig. 2 that the current density of the second passive region also decreased with the addition of Cr content. Cermet A was attributed to the effect of Ni and Mo, whereas other cermets were attributed to the effect of Ni, Mo and Cr. The anodic current increased with positive scanning of potential. There was no difference in current density among the tested cermets when the scanning potential reached 1.5 V, because the passive film broke down as indicated by an increase in current density (Fig. 2). Fig. 3 presents the impedance spectra in the form of Nyquist plots obtained from cermets specimens with different Cr contents in a 0.2 mol/L H2SO4 solution for 24 h, 120 h and 240 h, respectively. The Nyquist plot of cermet A was different from the figures of other cermets for its semi-circle shape. The increase in the diameter of the arc indicated an improvement of the film and charge transfer resistance by adding Cr element. This implied that the addition of Cr promoted the formation of the passive film because more Cr dissolved into the binder. The electrochemical response to the impedance tests for the materials under consideration was simulated using equivalent circuits. Fig. 4 shows the equivalent circuits used to simulate the EIS data, where Rs is the solution resistance, capacitance (Cdl) and the charge transfer resistance (Rct). In our experiments, the interface between solution and sample does not work like a perfect capacitor. Hence, modeling the interface with a constant phase element (CPE) is appropriate [22,23]. The fitted EIS data obtained by ZSimpWin program are summarized in Table 5. For all the tested cermets, the charge transfer resistance decreased with immersion time. However, the charge transfer resistances of cermets with Cr were higher than that of the cermets without Cr. High Rct value indicated good corrosion resistance; therefore, the corrosion rates were ranked in the following order: A N B N C cermets. This showed that partial substitution of Ni with Cr improved the corrosion resistance of Ti(C,N)-based cermets. Compared with the hard phase, the binder dissolved in H2SO4 solution preferentially, which influenced the totality of the passive film and the Rct decreased with duration of immersion time. Corrosion rate test Fig. 5 shows the corrosion rates of cermets with different Cr content as a function of corrosion time in 0.2 mol/L H2SO4 solution. It is found that the trend of corrosion rates was basically consistent with the increase of the immersion time: the corrosion rate was gradually reduced with immersion time. In Fig. 5, it is clear that the corrosion rates of the cermets decreased evidently with the increase of Cr content. It is noticeable that the corrosion rate of cermet A decreased sharply compared to cermet C at the early stage, which indicated that the corrosion rate of cermet A was much higher than that of cermet B and C. The corrosion rate of cermet B also decreased remarkably in the first experiment period and attained a stable corrosion rate with the immersion time. At the beginning of immersion, the binder corroded preferentially and dissolved in the solution, which led to enhanced corrosion process of cermets. However, it was more difficult for the corrosive solution to corrode the deeper interior of specimens with the increasing corrosion Fig. 3. Nyquist plots of cermets in 0.2 mol/L H2SO4 solutions for different immersion time: (a) 24 h, (b) 120 h, and (c) 240 h.

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Fig. 4. Equivalent circuit for fitting the EIS data.

Table 5 EIS data of the specimens at various immersion intervals. Specimen

A

B

C

Immersion time (h)

Rs (Ω cm2)

24 120 240 24 120 240 24 120 240

3.903 3.539 3.53 2.167 2.307 2.883 3.244 2.823 2.78

Rct (Ω cm2)

CPE1 C (mF/cm2)

n (01)

0.427 0.156 0.278 0.134 0.152 0.157 0.311 0.781 0.845

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

3163 1484 1437 18,356 6778 4434 23,832 12,098 10,537

time. Furthermore, the corrosion products covered around the surface of samples and reduced the direct contact area between cermets and aggressive medium with immersion time. Hence, the corrosion products had some degree of protective effect and consequently the corrosion rate reduced. It corresponded with Xiong's research [13]. The corrosion micrographs of the tested cermets immersed in 0.2 mol/L H2SO4 solution for 25 days were investigated by SEM-SE as shown in Fig. 6. The corrosion became more slight with the increase of Cr content, which coincided with the corrosion rate. When the surface of Ti(C, N)-based cermets exposed to 0.2 mol/L H2SO4 solution, the binder dissolved preferentially because of the significant difference in corrosion potentials between the ceramic phase and metal phase. From Fig. 6(a), it can be observed that the binder without Cr corroded seriously in Ti(C, N)-based cermets, the binder had dissolved in the solution and left pores, so that the ceramics phase particles could be observed apparently at some part of specimen. The remained ceramics phase particles had the core and rim structure. In Fig. 6(b), there also exists part of binder on the surface. In Fig. 6(c), the remaining binder phase exists widely at the attacked surface. It can be concluded that the corrosion of binder became slight with the increase of Cr content in the binder. From the above discussion, the metal binder phase was the most sensitive component in the cermets without Cr addition, when the

Fig. 6. SEM–SE micrographs of the corrosion surface of Ti(C, N)-based cermets immersed in 0.2 mol/L H2SO4 solution for 25 days: (a) 0 wt.% Cr; (b) 2 wt.% Cr; and (c) 4 wt.% Cr.

Fig. 5. Corrosion rates of cermets versus time in 0.2 mol/L H2SO4 solution.

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material was immersed in H2SO4 solution. As reported in Ref. [14], when the scanning potential reached 0.3 V with respect to saturated calomel electrode, the degradation of Ti(C, N)-based material has been explained by the metal binder phase selective dissolution and the oxidation of compositions based on reactions (1)–(3). However, when the cermets were immersed in 0.2 mol/L H2SO4 solution, the corrosion potential was about − 0.1 V. According to the Pourbaix diagram of the Ni–H2 O system at 25 °C, the Ni binder can reacts with H2SO4. The reaction is as following [21]: þ

Ni þ 2H ¼ Ni

2þ

þ 2e

ð4Þ

The degradation of binder phase was the main corrosion behavior of the cermets. After the binder phase dissolution, the unsupported ceramic particles fell off from the attacked surface, resulting in the formation of the deep and big corrosion pits. The Cr atoms distributed mainly in the binder phase when partial Ni was substituted with Cr. The addition of Cr improved the passive ability of Ti(C,N)-based cermets in oxidizing acid, namely, the binder phase without Cr showed lower corrosion resistance. Conclusions In this paper, the effects of partial substitution of Ni with Cr on the microstructure and corrosion resistance of Ti(C,N)-based cermets in 0.2 mol/L H2SO4 solution were investigated. The conclusions of this study were as follows: (1) Partial substitution of Ni with Cr had no obvious effect on the microstructure of Ti(C,N)-Ni based cermets which had a typical core/rim structure. Nevertheless, the hardness of the cermets was improved by Cr dissolving in the Ni binder. (2) The tested cermets represented three typical passive regions in 0.2 mol/L H2SO4 solution. The current of the first passive region decreased and the passive region enlarged with the increase of Cr addition. The rest passive regions were pseudopassive ranges. The EIS tests showed that the impendence of the cermets in 0.2 mol/L H2SO4 solution increased with Cr content and decreased with accumulation of immersion time. (3) Ni binder phase was the most vulnerable composition in the H2SO4 environment compared with (Ti,M) (C,N) ceramic phase in the Ti(C,N)-based cermets system. The addition of Cr can improve the corrosion resistance of Ti(C,N)-based cermets in 0.2 mol/L H2SO4 solution, because Cr can dissolve in Ni binder and improve the passive ability. The corrosion rate was the smallest when the Cr content attained 4%. Acknowledgments This research was supported by the National S&T Major Project of China under Project No. 2009ZX0401-022. The authors also thank the

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