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Corrosion behaviour and mechanism of nickel anode in SO42- containing molten Li2CO3-Na2CO3-K2CO3
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Corrosion behaviour and mechanism of nickel anode in SO42- containing molten Li2CO3-Na2CO3-K2CO3 Peilin Wang, Kaifa Du, Yanpeng Dou, Hua Zhu, Dihua Wang* School of Resource and Environmental Science, Hubei International Scientific and Technological Cooperation Base of Sustainable Resources and Energy, Wuhan University, Wuhan 430072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Nickel A. Molten salts C. Anodic films C. Oxidation C. Pitting corrosion
Aiming to develop a corrosion resistant anode for flue-gas capture and utilization, the effect of SO42− on the corrosion behaviour of nickel was systematically investigated under anodic polarization over a temperature range of 650−800 °C in molten Li2CO3-Na2CO3-K2CO3 with 0–2 mol% SO42−. The corrosion of nickel is profoundly accelerated when the concentration of SO42- is over 0.1 mol% at 650 °C. Pitting corrosion and spallation of NiO film caused by the aggressive SO42− were identified. Interestingly, the resistance of nickel anode to pitting corrosion can be effectively enhanced by increasing electrolyte temperature.
1. Introduction Molten salt CO2 capture and electrochemical transformation (MSCCET) process is demonstrated to be a promising approach to efficient utilization of CO2 [1–4], in which CO2 is split into carbon materials or CO on the cathode and O2 on the anode. A stable anode for oxygen evolution is crucial to the success of the process [5–7], but is very challenging due to the very aggressive working environment, i.e, strong anodic polarization in high temperature molten salts [8]. Metallic materials, taking advantages of high electronic conductivity, excellent mechanical robustness and low cost [9–12], are potentially satisfactory anode in molten salts. Among them, nickel-based alloys and/or platinum coating electrode were confirmed to be a stable anode for oxygen evolution in molten carbonates under various experimental conditions [5–7,13,14], which makes the MSCC-ET a competitive technology for the high-flux utilization of CO2 [1,15,16]. Up till now, pure CO2 is the feeding gas in most of researches on the CO2 electrochemical reduction. But the separation and purification of CO2 from industrial flue gas is costly [17]. More recently, the MSCC-ET process was extended to capture and utilize coal-fired flue gas consisting of ∼14 vol% CO2, some SO2 and N2 [18]. In the process, CO2 and SO2 can be captured by molten Li2CO3-Na2CO3-K2CO3-Li2SO4 and electrochemically transformed into sulfur-doped carbon or nanostructured graphite [19]. Nevertheless, there are certain amount of sulfate ions forming by the SO2 capture reaction in molten carbonates [18]. Sulfate, as an oxidizing agent, usually causes hot corrosion of nickel-based alloys at elevated temperatures [20–22]. Although sulfate-
⁎
caused corrosion of nickel and nickel-based alloy have been reported previously [23–26], none of research addresses on the corrosion behaviour of nickel under anodic polarization in molten salt with SO42− so far. Understanding the effect of SO42− on the anodic behaviour of nickel in molten carbonates is urgent for the anode development of the molten salt flue gas capture and electrochemical transformation process. In this work, the anodic behaviour and stability of nickel were investigated by linear sweep voltammetry and potentiostatic electrolysis in molten Li2CO3-Na2CO3-K2CO3 containing various content of SO42−. The anodic corrosion mechanism was explored based on the surface and cross section characterization of the anodic oxide film after potentiostatic electrolysis. Furthermore, the effects of temperature and gaseous atmosphere on the stability of the nickel anode were also evaluated. 2. Materials and methods 2.1. Preparation of molten salt electrolyte Anhydrous Li2CO3, Na2CO3, K2CO3 and Li2SO4 salts with analytical purity were purchased from Sinopharm Chemical Reagent Co., Ltd. An alumina crucible containing 750 g Li2CO3-Na2CO3-K2CO3 (43.5: 31.5: 25.0 in molar ratio) was placed in a sealed vertical tubular furnace. The mixed salt was dried at 300 °C for 48 h to remove moisture and melted at 850 °C under argon atmosphere, and then cooled down to the target temperature. Pre-electrolysis was conducted between a graphite rod anode and a foamed nickel cathode under a constant cell voltage of
Corresponding author. E-mail address: [email protected] (D. Wang).
https://doi.org/10.1016/j.corsci.2020.108450 Received 13 August 2019; Received in revised form 7 December 2019; Accepted 8 January 2020 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Peilin Wang, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2020.108450
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Fig. 1. (a) Anodic polarization curves and (b) Current–time plots of the potentiostatic polarization at 0.5 V of Ni electrodes in molten Li2CO3-Na2CO3-K2CO3 containing various contents of SO42− under an argon atmosphere at 650 °C, scan rate:5 mV/s. Fig. 2. Optical images of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3Na2CO3-K2CO3 without SO42− (a) or containing 0.1 mol% (b), 2.0 mol% (c) SO42− under an argon atmosphere at 650 °C. (d) Photomicrograph of the cross section of the nickel electrodes prepared in molten Li2CO3Na2CO3-K2CO3 containing 2.0 mol% SO42−.
voltammetry (LSV) at 650 °C under argon or synthetic flue gas atmosphere in molten Li2CO3- Na2CO3-K2CO3 with different concentration of SO42−. The synthetic flue gas consists of 14 vol% CO2, 300 ppm SO2 and N2 balance. In the case of anodic polarization measurement under synthetic flue gas atmosphere, the alumina tube that guides the synthetic flue gas into the melt was placed near the anode in order to improve SO2 uptake. Potentiostatic polarization of nickel plates (10 mm × 10 mm × 1 mm) was performed at 0.5 V vs. Ag/Ag2SO4, which is close to the practical operating potential for oxygen evolution at 650 °C. Cyclic polarization measurements were performed under argon atmosphere at different temperatures by sweeping the potential from the open circuit potential (OCP) to 1.2 V at a scan rate of 2 mV/s, and then reversed with the same scan rate. Galvanostatic electrolysis was performed on a computer-controlled DC power source (Shenzhen Neware Electronic Ltd., China) using a Ni sheet (20 mm × 30 mm × 0.1 mm) cathode and a nickel plate anode (10 mm × 10 mm × 1 mm) in molten Li2CO3-Na2CO3-K2CO3 containing 0.1 mol% SO42− at 650 °C.
1.5 V for 2 h to further remove residual water and other impurities from the molten salt. Certain amount of Li2SO4 was weighted accurately and then added into the melt. Seven series of experiments were conducted and the corresponding SO42− addition are 0 mol%, 0.05 mol%, 0.1 mol %, 0.2 mol%, 0.5 mol%, 1.0 mol% and 2.0 mol%, respectively. 2.2. Electrochemical measurements Electrochemical measurements were conducted in a three-electrode system controlled by a CHI1140C electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) in molten Li2CO3Na2CO3-K2CO3 containing various contents of SO42−. Nickel wires (1 mm in diameter, 10 mm deep in electrolyte) and a graphite rod (20 mm in diameter, 30 mm deep in electrolyte) were employed as working electrodes (WE) and counter electrode (CE), respectively. Ag/ Ag2SO4 reference electrode (RE) was prepared by inserting a silver wire into an alumina tube filled with 1.0 g of Li2CO3-Na2CO3-K2CO3 (molar ratio of 43.5 : 31.5 : 25.0) and 0.5 mmol Ag2SO4. The potentials in this work are all referred to the Ag/Ag2SO4 reference electrode. Anodic polarization curves were recorded by linear sweep 2
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Fig. 3. SEM images of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3, without SO42− (a) or containing 0.1 mol% (b) SO42− and 2.0 mol% SO42− (c), (d) under an argon atmosphere at 650 °C, respectively.
film. More serious anodic corrosion happened in the melt with SO42− above 0.2 mol% according to visual observation of the electrode after anodic polarization test. Anodic dissolution of nickel can also be observed from the polarization curves with more frequently metastable pitting signals in the melt with high SO42− concentration. To evaluate the corrosion behaviour of nickel under oxygen evolution, potentiostatic polarizations of nickel plates were conducted at 0.5 V, which is a typical potential for oxygen evolution in the MSCC-ET process. Fig.1b exhibits the current-time curves in molten carbonates with different concentration of SO42−. When the SO42- concentration is below 0.1 mol%, the anodic current sharply declined from more than 1 A to a steady-state of less than 0.4 A within 100 s. The platform current is 0.31 A, 0.35 A and 0.38 A in the melt with 0, 0.05, 0.10 mol% SO42-, respectively, which is mainly due to the oxygen evolution on the protective surface film although the current is slightly increasing with the addition of SO42−. These results indicate a conductive passivation film might form on the surface of the nickel plate within 600 s in the melt when the concentration of SO42− is less than 0.1 mol% under anodic polarization. There always exists an incubation period for pitting corrosion in aqueous solution, but it is unclear in high temperature molten salts. Therefore, potentiostatic electrolysis was prolonged to 11 h in the molten salt with 0.1 mol% SO42−. It was found that the current kept stable all the time, demonstrating a stable film can form in the melt. However, when the concentration of SO42− exceeded 0.2 mol%, the current maintained at a high level and there are lots of spikes on the current curves. The larger current means that a protective film cannot form in the melt. The current spikes should be attributed to the dissolution or spallation of the surface film. Based on the electrochemical measurements, we can find that the addition of SO42− decreases the stability of nickel anode and there exists a critical concentration of SO42−, between 0.1 and 0.2 mol%, at 650 °C. A protective film can maintain on the nickel surface and is not prone to pitting corrosion below the critical concentration, but pitting corrosion and spallation of the film takes place at higher concentration.
2.3. Materials characterization After anodic polarization, the electrodes were washed by deionized water to remove solidified melt, rinsed by anhydrous ethanol, and dried in a vacuum oven at 80 °C for 12 h. To prevent dissolution of the corrosion products and molten salts, some samples were kept in a vacuum oven without washing prior to characterization. The morphology and structure of the samples were characterized by 3-D optical microscope (Keyence VHX-5000), X-ray diffraction spectroscopy (XRD, Shimadzu X-ray 6000 with Cu Kα1 radiation at 40 kV and 250 mA, λ = 0.154 nm) and scanning electron microscopy (SEM, TESCAN MIRA 3 LMH). The anodes were embedded in epoxy resin and ground with SiC papers and then polished with fine diamond paste in cross-section. And then the cross-section of the anodes was detected by SEM and energy dispersive spectrometer (EDS, Oxford X-max 20). 3. Results and discussions 3.1. The effect of SO42− on the anodic behaviour of nickel Fig.1a presents the anodic polarization curves of Ni in molten Li2CO3-Na2CO3-K2CO3 containing various contents of SO42− at 650 °C. Two anodic current peaks (a1 and a2) prior to oxygen evolution are observed in molten carbonate without SO42−, which are related to the oxidation of nickel to Ni(Ⅱ) and Ni(Ⅲ), respectively [14,27]. It displays a typical polarization curve of passivation, and a dark film was observed on the surface of nickel after anodic polarization test. However, the anodic behaviour of nickel was quite different after the addition of SO42−. The passive potential is shifted toward the positive direction and the passive current density of peak a2 is increased, indicating that it is more difficult to form a protective passivation film in the melt with SO42−. With further increasing the concentration of SO42- to 0.2 mol%, the corrosion rate of nickel is profoundly speeded up and the current fluctuation is more remarkable due to the instability of the passive film. As shown in the curves, a current spike represents a metastable pitting event, which involves the spalling and the re-passivation of the surface 3
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Fig. 4. SEM images in the backscattered electrons (BSE) mode and corresponding EDS mapping images of the cross-section of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3 without SO42− (a–c) or with 0.1 mol% (b–d), 2.0 mol% (g–m) SO42- under an argon atmosphere at 650 °C.
3.2. Corrosion mechanism of SO42− to the nickel anode
samples were characterized by optical microscopy, SEM, EDS and XRD. As shown in Fig.2a and Fig.2b, Ni plate was covered by a dense layer of dark green film after polarized in molten carbonates without or with 0.1 mol% SO42−. While for the sample prepared in the melt containing 2.0 mol% SO42−, the surface film is loose and porous (Fig.2c) and parts of film are delaminated from the nickel substrate, moreover, the electrolyte infiltrated into the oxide film (Fig.2d). The spalled film
To investigate the mechanism of the anodic corrosion of nickel in molten carbonates with the addition of SO42−, nickel plates polarized in three typical electrolytes, Li2CO3-Na2CO3-K2CO3 containing 0.1 mol % or 2.0 mol% or without SO42-, were selected for further characterization. The composition and structure of the surface film of the 4
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Furthermore, XRD patterns confirm the oxide films prepared in above two melts mainly consist of NiO (Fig.5), which can behave as a protective layer to the corrosion of nickel substrate [14]. A sandwich structure consisting of oxide film/electrolyte/oxide film on the nickel electrode was observed in molten carbonates containing 2.0 mol% SO42− after anodic polarization (Fig.4g-m). The oxide layer is even completely delaminated from the substrate in some region, with large amount of electrolyte penetration between the film and metal substrate which leads to the internal oxidation of nickel. Na, K and S are distributed in NiO according to the elemental distribution of the crosssection of the sample (Fig.4h-m), demonstrating a highly porous oxide layer which cannot protect the substrate from corrosion. EDS analyses of zone 3 and 4 (Table1) also indicate that Ni dissolved into the electrolyte. Combining the elemental distribution with the XRD results (Fig.5), it is reasonable to speculate that the dissolved corrosion product is NiSO4. The sulfation of NiO has been recognized in the Type-II hot corrosion of nickel [20,23,25,31,32]. Although metal sulfide (i.e. Ni3S2) was also a corrosion product in hot corrosion [23,25,32], it was not detected in the films in this work, which is understandable since nickel electrode is under anodic polarization. These results demonstrate that the corrosion behaviour of nickel under anodic polarization is similar to that of the Type-II hot corrosion but there are some differences of corrosion mechanism. Based on the above measurements, the corrosion mechanism of nickel under anodic polarization in SO42− containing molten carbonates is proposed and illustrated in Fig. 6. NiO is preferentially formed on the surface of nickel and a protective oxide layer can grow in molten carbonates or carbonates with small amount of SO42- due to the low solubility of NiO in the carbonates at 650 °C [33,34]. However, when the concentration of SO42- is over the critical concentration, soluble NiSO4 will generate together with NiO under anodic polarization:
Fig. 5. XRD patterns of nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3 without Li2SO4 (black line) or containing 0.1 mol% (red line), 2.0 mol% (blue line) SO42− under an argon atmosphere at 650 °C (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article). Table 1 The elemental concentration (at%) of selected zones in Fig.4g. Point
Elemental composition(at%)
1 2 3 4 5 6
Ni 10.79 40.79 4.28 0.96 93.79 44.89
O 51.58 34.27 50.72 62.19 0.98 30.64
C 22.44 19.13 24.64 24.42 5.23 20.20
S 0.02 0 0.07 0.35 0 0.01
K 8.99 1.17 11.27 6.52 0 4.26
Na 6.18 4.64 9.02 5.56 0 0
Ni + CO32−−2e-=NiO + CO2(g)
(1)
Ni−2e−=Ni2+(in the presence of SO42-)
(2)
That means, in the presence of plenty amount of SO42−, on the one hand, dense NiO layer is hard to form that leads to a high anodic current at the beginning stage as evidenced in Fig.1b and Fig.4, and on the other hand, SO42− will destroy the NiO film to generate pitting corrosion as evidenced by the spiking current in Fig.1b and morphologies as shown in Fig.3 and Fig.4. The SO42− infiltrates in the film through the corrosion pits, and reacts with the nickel substrate. As it can be seen, the corrosion behaviour is similar to the fluxing process of NiO in hot corrosion process, but the driving force of anodic corrosion is more attributed to the electric field under anodic polarization. With the extension of anodic polarization time, the size of pits becomes larger and larger and the number of voids increases. Therefore, the NiO layer with pits and voids lose its protection to the substrate. With further oxidation and dissolution of nickel substrate, the outside oxide film cracks and is eventually spalled from the substrate due to the interstress in the film and the mechanical action of oxygen bubbles. And then, more electrolyte passes through the cracks and reacts with the substrate, a new layer of NiO film forms on the substrate. The passivation/spallation/repassivation cycles continue as described in the above mechanism until the anode is completely destroyed, leading to the sandwich-like film/ electrolyte/film layers as shown in Fig.4g. Moreover, sulfidation of nickel does not occur since sulfur activity (i.e. pS2) at the nickel/oxide interface maintains a low level under strong anodic polarization comparing with the porous sulfur enriched NiO layer obtained in Type-II hot corrosion process of nickel alloys [23]. In summary, excessive SO42− in molten carbonates reacts with NiO and Ni to form soluble NiSO4 under anodic polarization so that a dense and adhesive NiO film cannot generate, which causes pitting and severe corrosion of the anode.
shall be caused by the mechanical action of oxygen bubbles generated at the electrode surface as well as the surface stress by underneath oxidation. The micro-morphology of the samples was further characterized by SEM. Well defined octahedral particles in a size of 3–5 μm are distributed on the surface of samples obtained in molten carbonates without SO42− (Fig.3a) or with 0.1 mol% SO42− (Fig.3b), but the particle structure tends to irregular and the particle size is disordered with the addition of SO42−, which might result in the film less protective. With increasing the concentration of SO42− to 2.0 mol%, more disordered particles and some visible pits are observed (Fig.3c and Fig.3d). The pits exhibit a bowl-shaped morphology and its size becomes larger with the extension of the anodic polarization time. The diameter of the corrosion pit increases from ∼ 26 μm to ∼39 μm as the polarization time increases from 300 s to 600 s and the bottom of pit is relatively porous. The results confirm that the pitting corrosion takes place when the concentration of SO42− is higher than 0.1 mol%, which is consistently with the electrochemical measurements as shown in Fig.1. In general, the growth of corrosion pit is controlled by the diffusion of corrosion products [28–30]. The composition and structure of the surface film was further characterized by EDS mapping and XRD measurements. The SEM morphology and EDS-mapping of the cross section of the samples are illustrated in Fig.4. The nickel electrode was intimately covered by a dense film in molten carbonates without (Fig.4a) and with small amount of SO42− (0.1 mol%) (Fig.4d). The thickness of the film is ∼12 μm and ∼14 μm, respectively. Ni and O are homogeneously distributed in the oxide layers according to the EDS-mapping (Fig.4a-f).
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Fig. 6. Schematic of the corrosion bahaviour of the Ni electrode after anodic polarization in molten Li2CO3-Na2CO3-K2CO3 containing SO42-.
is profoundly enhanced with increasing the electrolyte temperature. Nickel can work as an effective anode for oxygen evolution at 800 °C in molten carbonates containing 1 mol% SO42−, at 750 °C containing 0.5 mol% SO42−.
3.3. The effect of temperature on the pitting corrosion of nickel anode Temperature is one of the most important factors affecting the initiation and growth of pitting corrosion in aqueous solution [35,36]. In our previous work, it is found that the stability of nickel anode increases with increasing temperature in molten carbonates [14]. It is interesting and important to know how the temperature will affect the pitting corrosion of nickel anode in molten carbonates with certain amount of SO42−. Therefore, cyclic polarization tests were conducted to examine the susceptibility of the nickel anode to pitting corrosion in molten carbonates containing various contents of SO42− at a temperature range 650–800 °C. Fig. 7a shows the cyclic polarization curves of nickel anode in molten carbonates without SO42− at various temperatures. The passivation current decreased with increasing temperature, demonstrating that the stability of the anodic film increased. The back-scanning curves coincided with the sweep curves and no obvious hysteresis loops were observed, indicating that no pitting corrosion occurred in the SO42− free carbonates. Fig.7 b–f present the cyclic polarization curves of nickel in molten carbonates containing different concentration of SO42− (from 0.1 mol% to 2.0 mol%) at different temperature. It can be found that the critical SO42− concentration which induces to pitting corrosion increases with increasing temperature, i.e., 0.1 mol % at 650 °C, 0.2 mol% at 700 °C, 0.5 mol% at 750 °C, 1.0 mol% at 800 °C respectively (Fig.7 b–d). Below the critical concentration, passivation film can form on the nickel surface at the temperature. Pitting corrosion and spallation of the film will take place as evidenced by the increasing current and current oscillation on the curves. The formation of a protective film is prone to form on the nickel electrode at higher temperature, which is most likely to be ascribed to the increasing stability of NiO layer at the higher temperature [14]. Furthermore, the re-passivation potential (Erp) follows the order of 650 °C＜700 °C＜750 °C, which indicates that the resistance to pitting corrosion of nickel anode
3.4. The effect of the atmosphere on the pitting corrosion The application of the MSCC-ET process is preferentially operated under the flue gas atmosphere with ∼14 % CO2 and some SO2 to reduce the cost for CO2 separation and purification. To explore the effect of the composition of atmosphere on the corrosion behaviour of nickel anode, the measurements of anodic polarization curve and galvanostatic electrolysis were conducted under Ar and synthetic flue gas atmosphere at 650 °C (Fig.8). Although the nickel electrode can maintain a typical passivation curve in the molten salt with 0.05 mol% SO42− under the synthetic flue gas (Fig.8a), the passivation peak is much larger than that under argon atmosphere, and it becomes unstable in the melt with 0.1 mol% SO42− (Fig.8b). Fig.8c shows the cell voltage-time plot during the electrolysis under argon atmosphere. After an electrochemical oxidation of nickel at the initial stage to grow a protective oxide film, the cell voltage increased to 2.3 V by shifting the anodic reaction to the oxygen evolution. While under synthetic flue gas, the cell voltage is below 2.0 V and a regular voltage oscillation curve is observed (Fig.8d). The low cell voltage demonstrates that the anodic reaction is mainly due to the dissolution/oxidation of nickel rather than oxygen evolution, while the voltage oscillation is caused by the periodical forming and stripping of the oxide film due to its poor protection and adhesion. The passivation/spallation/re-passivation takes place under the experimental condition. The results demonstrate that the stability of nickel is affected by temperature, concentration of SO42− and the composition of atmosphere. According to the Lux-Flood model [34,37], the basicity of molten carbonates is controlled by the PCO2 in the gas phase above the melt as 6
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Fig. 7. Cyclic polarization curves of Ni electrode in molten Li2CO3-Na2CO3-K2CO3 without SO42− (a) or with 0.1 mol% (b), 0.2 mol% (c), 0.5 mol% (d), 1.0 mol% (e), 2.0 mol% (f) SO42− under an argon atmosphere at various temperature, scan rate: 2 mV/s.
concentration but pitting corrosion and anodic dissolution of nickel take place at higher concentration. The pitting corrosion resistance and stability of nickel anode is greatly enhanced by increasing temperature, but the corrosion rate is accelerated with increasing SO42− concentration. The formation of soluble NiSO4 causes pitting corrosion and prevents the surface from forming a dense and adhesive NiO layer, and eventually caused severe corrosion of the anode. Nevertheless, the nickel anode performs poor stability under synthetic flue gas atmosphere with a much higher anodic current and a fluctuating anodic potential. Understanding the failure mechanism of nickel anode and the critical SO42- concentration will be beneficial for the development of nickel-based inert anodes and optimization of electrolysis conditions in molten salt flue gas capture and electrochemical transformation process. The pre-formation of oxide layer which can block the sulfate ions might be an effective way to improve the durability of nickel-based inert anode and will be investigated in future.
the following reaction: Li2CO3⇌CO2+Li2O
(3)
The basicity of the molten salt is reduced in CO2 / flue gas atmosphere comparing with in Ar, which weaken the stability of the NiO film. Furthermore, the acidic dissolution of NiO can be accelerated by the presence of SO3 according to the following reactions [25,26,38]: 2SO2+O2 = 2SO3
(4)
NiO + SO3=NiSO4
(5)
4. Conclusions Nickel can be passivated in SO42− containing molten carbonates when the concentration of SO42− is lower than a critical concentration. The critical SO42−concentration which induces pitting of nickel anode in molten carbonates is 0.1 mol% at 650 °C, 0.2 mol% at 700 °C, 0.5 mol % at 750 °C and 1.0 mol% at 800 °C, respectively. In an initial period of anodic polarization, a dense and protective NiO film forms on the nickel surface and is not prone to pitting corrosion below the critical
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. 7
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Fig. 8. Anodic polarization curves of the Ni electrodes in molten Li2CO3-Na2CO3-K2CO3 containing 0.05 mol% (a) or 0.1 mol% (b) SO42− under various atmospheres at 650 °C, scan rate: 5 mV/s. Evolution of the cell voltage during constant current electrolysis for 40 min at Ianode = 200 mA/cm2 in molten Li2CO3-Na2CO3-K2CO3 containing 0.1 mol% SO42at 650 °C under argon atmosphere (c) or synthetic flue gas atmosphere (d).
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None. Acknowledgement This work was supported by the National Natural Science Foundation of China (51874211). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.corsci.2020.108450. References [1] H. Yin, X. Mao, D. Tang, W. Xiao, L. Xing, H. Zhu, D. Wang, D.R. Sadoway, Capture and electrochemical conversion of CO2 to value-added carbon and oxygen by molten salt electrolysis, Energy Environ. Sci. 6 (2013) 1538. [2] S. Licht, Efficient solar-driven synthesis, carbon capture, and desalinization, STEP: solar thermal electrochemical production of fuels, metals, bleach, Adv. Mater. 23 (2011) 5592–5612. [3] H.V. Ijije, C. Sun, G.Z. Chen, Indirect electrochemical reduction of carbon dioxide to carbon nanopowders in molten alkali carbonates: process variables and product properties, Carbon 73 (2014) 163–174. [4] L. Hu, Y. Song, J. Ge, J. Zhu, S. Jiao, Capture and electrochemical conversion of CO2 to ultrathin graphite sheets in CaCl2-based melts, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 21211–21218. [5] K. Du, R. Yu, M. Gao, Z. Chen, X. Mao, H. Zhu, D. Wang, Durability of platinum coating anode in molten carbonate electrolysis cell, Corros. Sci. 153 (2019) 12–18. [6] X. Cheng, H. Yin, D. Wang, Rearrangement of oxide scale on Ni-11Fe-10Cu alloy under anodic polarization in molten Na2CO3-K2CO3, Corros. Sci. 141 (2018) 168–174. [7] K. Zheng, K. Du, X. Cheng, R. Jiang, B. Deng, H. Zhu, D. Wang, Nickel-iron-Copper alloy as inert anode for ternary molten carbonate electrolysis at 650°C, J. Electrochem. Soc. 165 (2018) E572–E577. [8] M. Sarvghad, T. Chenu, G. Will, Comparative interaction of cold-worked versus annealed inconel 601 with molten carbonate salt at 450°C, Corros. Sci. 116 (2017) 88–97. [9] V. Chapman, B.J. Welch, M. Skyllas-Kazacos, Anodic behaviour of oxidised Ni–Fe alloys in cryolite–alumina melts, Electrochim. Acta 56 (2011) 1227–1238. [10] D. Tang, K. Zheng, H. Yin, X. Mao, D.R. Sadoway, D. Wang, Electrochemical growth of a corrosion-resistant multi-layer scale to enable an oxygen-evolution inert anode
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characterisation methods for investigation of corrosion of bipolar plate materials in molten carbonate fuel cell - Part I. Electrochemical study, Corros. Sci. 41 (1999) 1497–1513. J. Jun, G.S. Frankel, N. Sridhar, Effect of chloride concentration and temperature on growth of 1D Pit, J. Solid State Electrochem. 19 (2015) 3439–3447. J. Jun, G.S. Frankel, N. Sridhar, Further modeling of chloride concentration and temperature effects on 1D pit growth, J. Electrochem. Soc. 163 (2016) C823–C829. P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils, I. Introduction and pit growth kinetics, Corros. Sci. 44 (2002) 927–941. Y. Liu, Y. Wu, J. Wang, Y. Ning, Oxidation behavior and microstructure degeneration of cast Ni-based superalloy M951 at 900 °C, Appl. Surf. Sci. 479 (2019) 709–719. P.S. Sidky, M.G. Hocking, The hot corrosion of nickel-based ternary alloys and superalloys for gas-turbine applications.1.cOrrosion in SO2/O2 atmospheres,
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Corrosion behaviour and mechanism of nickel anode in SO42- containing molten Li2CO3-Na2CO3-K2CO3 Peilin Wang, Kaifa Du, Yanpeng Dou, Hua Zhu, Dihua Wang* School of Resource and Environmental Science, Hubei International Scientific and Technological Cooperation Base of Sustainable Resources and Energy, Wuhan University, Wuhan 430072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. Nickel A. Molten salts C. Anodic films C. Oxidation C. Pitting corrosion
Aiming to develop a corrosion resistant anode for flue-gas capture and utilization, the effect of SO42− on the corrosion behaviour of nickel was systematically investigated under anodic polarization over a temperature range of 650−800 °C in molten Li2CO3-Na2CO3-K2CO3 with 0–2 mol% SO42−. The corrosion of nickel is profoundly accelerated when the concentration of SO42- is over 0.1 mol% at 650 °C. Pitting corrosion and spallation of NiO film caused by the aggressive SO42− were identified. Interestingly, the resistance of nickel anode to pitting corrosion can be effectively enhanced by increasing electrolyte temperature.
1. Introduction Molten salt CO2 capture and electrochemical transformation (MSCCET) process is demonstrated to be a promising approach to efficient utilization of CO2 [1–4], in which CO2 is split into carbon materials or CO on the cathode and O2 on the anode. A stable anode for oxygen evolution is crucial to the success of the process [5–7], but is very challenging due to the very aggressive working environment, i.e, strong anodic polarization in high temperature molten salts [8]. Metallic materials, taking advantages of high electronic conductivity, excellent mechanical robustness and low cost [9–12], are potentially satisfactory anode in molten salts. Among them, nickel-based alloys and/or platinum coating electrode were confirmed to be a stable anode for oxygen evolution in molten carbonates under various experimental conditions [5–7,13,14], which makes the MSCC-ET a competitive technology for the high-flux utilization of CO2 [1,15,16]. Up till now, pure CO2 is the feeding gas in most of researches on the CO2 electrochemical reduction. But the separation and purification of CO2 from industrial flue gas is costly [17]. More recently, the MSCC-ET process was extended to capture and utilize coal-fired flue gas consisting of ∼14 vol% CO2, some SO2 and N2 [18]. In the process, CO2 and SO2 can be captured by molten Li2CO3-Na2CO3-K2CO3-Li2SO4 and electrochemically transformed into sulfur-doped carbon or nanostructured graphite [19]. Nevertheless, there are certain amount of sulfate ions forming by the SO2 capture reaction in molten carbonates [18]. Sulfate, as an oxidizing agent, usually causes hot corrosion of nickel-based alloys at elevated temperatures [20–22]. Although sulfate-
⁎
caused corrosion of nickel and nickel-based alloy have been reported previously [23–26], none of research addresses on the corrosion behaviour of nickel under anodic polarization in molten salt with SO42− so far. Understanding the effect of SO42− on the anodic behaviour of nickel in molten carbonates is urgent for the anode development of the molten salt flue gas capture and electrochemical transformation process. In this work, the anodic behaviour and stability of nickel were investigated by linear sweep voltammetry and potentiostatic electrolysis in molten Li2CO3-Na2CO3-K2CO3 containing various content of SO42−. The anodic corrosion mechanism was explored based on the surface and cross section characterization of the anodic oxide film after potentiostatic electrolysis. Furthermore, the effects of temperature and gaseous atmosphere on the stability of the nickel anode were also evaluated. 2. Materials and methods 2.1. Preparation of molten salt electrolyte Anhydrous Li2CO3, Na2CO3, K2CO3 and Li2SO4 salts with analytical purity were purchased from Sinopharm Chemical Reagent Co., Ltd. An alumina crucible containing 750 g Li2CO3-Na2CO3-K2CO3 (43.5: 31.5: 25.0 in molar ratio) was placed in a sealed vertical tubular furnace. The mixed salt was dried at 300 °C for 48 h to remove moisture and melted at 850 °C under argon atmosphere, and then cooled down to the target temperature. Pre-electrolysis was conducted between a graphite rod anode and a foamed nickel cathode under a constant cell voltage of
Corresponding author. E-mail address: [email protected] (D. Wang).
https://doi.org/10.1016/j.corsci.2020.108450 Received 13 August 2019; Received in revised form 7 December 2019; Accepted 8 January 2020 0010-938X/ © 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Peilin Wang, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2020.108450
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Fig. 1. (a) Anodic polarization curves and (b) Current–time plots of the potentiostatic polarization at 0.5 V of Ni electrodes in molten Li2CO3-Na2CO3-K2CO3 containing various contents of SO42− under an argon atmosphere at 650 °C, scan rate:5 mV/s. Fig. 2. Optical images of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3Na2CO3-K2CO3 without SO42− (a) or containing 0.1 mol% (b), 2.0 mol% (c) SO42− under an argon atmosphere at 650 °C. (d) Photomicrograph of the cross section of the nickel electrodes prepared in molten Li2CO3Na2CO3-K2CO3 containing 2.0 mol% SO42−.
voltammetry (LSV) at 650 °C under argon or synthetic flue gas atmosphere in molten Li2CO3- Na2CO3-K2CO3 with different concentration of SO42−. The synthetic flue gas consists of 14 vol% CO2, 300 ppm SO2 and N2 balance. In the case of anodic polarization measurement under synthetic flue gas atmosphere, the alumina tube that guides the synthetic flue gas into the melt was placed near the anode in order to improve SO2 uptake. Potentiostatic polarization of nickel plates (10 mm × 10 mm × 1 mm) was performed at 0.5 V vs. Ag/Ag2SO4, which is close to the practical operating potential for oxygen evolution at 650 °C. Cyclic polarization measurements were performed under argon atmosphere at different temperatures by sweeping the potential from the open circuit potential (OCP) to 1.2 V at a scan rate of 2 mV/s, and then reversed with the same scan rate. Galvanostatic electrolysis was performed on a computer-controlled DC power source (Shenzhen Neware Electronic Ltd., China) using a Ni sheet (20 mm × 30 mm × 0.1 mm) cathode and a nickel plate anode (10 mm × 10 mm × 1 mm) in molten Li2CO3-Na2CO3-K2CO3 containing 0.1 mol% SO42− at 650 °C.
1.5 V for 2 h to further remove residual water and other impurities from the molten salt. Certain amount of Li2SO4 was weighted accurately and then added into the melt. Seven series of experiments were conducted and the corresponding SO42− addition are 0 mol%, 0.05 mol%, 0.1 mol %, 0.2 mol%, 0.5 mol%, 1.0 mol% and 2.0 mol%, respectively. 2.2. Electrochemical measurements Electrochemical measurements were conducted in a three-electrode system controlled by a CHI1140C electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) in molten Li2CO3Na2CO3-K2CO3 containing various contents of SO42−. Nickel wires (1 mm in diameter, 10 mm deep in electrolyte) and a graphite rod (20 mm in diameter, 30 mm deep in electrolyte) were employed as working electrodes (WE) and counter electrode (CE), respectively. Ag/ Ag2SO4 reference electrode (RE) was prepared by inserting a silver wire into an alumina tube filled with 1.0 g of Li2CO3-Na2CO3-K2CO3 (molar ratio of 43.5 : 31.5 : 25.0) and 0.5 mmol Ag2SO4. The potentials in this work are all referred to the Ag/Ag2SO4 reference electrode. Anodic polarization curves were recorded by linear sweep 2
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Fig. 3. SEM images of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3, without SO42− (a) or containing 0.1 mol% (b) SO42− and 2.0 mol% SO42− (c), (d) under an argon atmosphere at 650 °C, respectively.
film. More serious anodic corrosion happened in the melt with SO42− above 0.2 mol% according to visual observation of the electrode after anodic polarization test. Anodic dissolution of nickel can also be observed from the polarization curves with more frequently metastable pitting signals in the melt with high SO42− concentration. To evaluate the corrosion behaviour of nickel under oxygen evolution, potentiostatic polarizations of nickel plates were conducted at 0.5 V, which is a typical potential for oxygen evolution in the MSCC-ET process. Fig.1b exhibits the current-time curves in molten carbonates with different concentration of SO42−. When the SO42- concentration is below 0.1 mol%, the anodic current sharply declined from more than 1 A to a steady-state of less than 0.4 A within 100 s. The platform current is 0.31 A, 0.35 A and 0.38 A in the melt with 0, 0.05, 0.10 mol% SO42-, respectively, which is mainly due to the oxygen evolution on the protective surface film although the current is slightly increasing with the addition of SO42−. These results indicate a conductive passivation film might form on the surface of the nickel plate within 600 s in the melt when the concentration of SO42− is less than 0.1 mol% under anodic polarization. There always exists an incubation period for pitting corrosion in aqueous solution, but it is unclear in high temperature molten salts. Therefore, potentiostatic electrolysis was prolonged to 11 h in the molten salt with 0.1 mol% SO42−. It was found that the current kept stable all the time, demonstrating a stable film can form in the melt. However, when the concentration of SO42− exceeded 0.2 mol%, the current maintained at a high level and there are lots of spikes on the current curves. The larger current means that a protective film cannot form in the melt. The current spikes should be attributed to the dissolution or spallation of the surface film. Based on the electrochemical measurements, we can find that the addition of SO42− decreases the stability of nickel anode and there exists a critical concentration of SO42−, between 0.1 and 0.2 mol%, at 650 °C. A protective film can maintain on the nickel surface and is not prone to pitting corrosion below the critical concentration, but pitting corrosion and spallation of the film takes place at higher concentration.
2.3. Materials characterization After anodic polarization, the electrodes were washed by deionized water to remove solidified melt, rinsed by anhydrous ethanol, and dried in a vacuum oven at 80 °C for 12 h. To prevent dissolution of the corrosion products and molten salts, some samples were kept in a vacuum oven without washing prior to characterization. The morphology and structure of the samples were characterized by 3-D optical microscope (Keyence VHX-5000), X-ray diffraction spectroscopy (XRD, Shimadzu X-ray 6000 with Cu Kα1 radiation at 40 kV and 250 mA, λ = 0.154 nm) and scanning electron microscopy (SEM, TESCAN MIRA 3 LMH). The anodes were embedded in epoxy resin and ground with SiC papers and then polished with fine diamond paste in cross-section. And then the cross-section of the anodes was detected by SEM and energy dispersive spectrometer (EDS, Oxford X-max 20). 3. Results and discussions 3.1. The effect of SO42− on the anodic behaviour of nickel Fig.1a presents the anodic polarization curves of Ni in molten Li2CO3-Na2CO3-K2CO3 containing various contents of SO42− at 650 °C. Two anodic current peaks (a1 and a2) prior to oxygen evolution are observed in molten carbonate without SO42−, which are related to the oxidation of nickel to Ni(Ⅱ) and Ni(Ⅲ), respectively [14,27]. It displays a typical polarization curve of passivation, and a dark film was observed on the surface of nickel after anodic polarization test. However, the anodic behaviour of nickel was quite different after the addition of SO42−. The passive potential is shifted toward the positive direction and the passive current density of peak a2 is increased, indicating that it is more difficult to form a protective passivation film in the melt with SO42−. With further increasing the concentration of SO42- to 0.2 mol%, the corrosion rate of nickel is profoundly speeded up and the current fluctuation is more remarkable due to the instability of the passive film. As shown in the curves, a current spike represents a metastable pitting event, which involves the spalling and the re-passivation of the surface 3
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Fig. 4. SEM images in the backscattered electrons (BSE) mode and corresponding EDS mapping images of the cross-section of the nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3 without SO42− (a–c) or with 0.1 mol% (b–d), 2.0 mol% (g–m) SO42- under an argon atmosphere at 650 °C.
3.2. Corrosion mechanism of SO42− to the nickel anode
samples were characterized by optical microscopy, SEM, EDS and XRD. As shown in Fig.2a and Fig.2b, Ni plate was covered by a dense layer of dark green film after polarized in molten carbonates without or with 0.1 mol% SO42−. While for the sample prepared in the melt containing 2.0 mol% SO42−, the surface film is loose and porous (Fig.2c) and parts of film are delaminated from the nickel substrate, moreover, the electrolyte infiltrated into the oxide film (Fig.2d). The spalled film
To investigate the mechanism of the anodic corrosion of nickel in molten carbonates with the addition of SO42−, nickel plates polarized in three typical electrolytes, Li2CO3-Na2CO3-K2CO3 containing 0.1 mol % or 2.0 mol% or without SO42-, were selected for further characterization. The composition and structure of the surface film of the 4
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Furthermore, XRD patterns confirm the oxide films prepared in above two melts mainly consist of NiO (Fig.5), which can behave as a protective layer to the corrosion of nickel substrate [14]. A sandwich structure consisting of oxide film/electrolyte/oxide film on the nickel electrode was observed in molten carbonates containing 2.0 mol% SO42− after anodic polarization (Fig.4g-m). The oxide layer is even completely delaminated from the substrate in some region, with large amount of electrolyte penetration between the film and metal substrate which leads to the internal oxidation of nickel. Na, K and S are distributed in NiO according to the elemental distribution of the crosssection of the sample (Fig.4h-m), demonstrating a highly porous oxide layer which cannot protect the substrate from corrosion. EDS analyses of zone 3 and 4 (Table1) also indicate that Ni dissolved into the electrolyte. Combining the elemental distribution with the XRD results (Fig.5), it is reasonable to speculate that the dissolved corrosion product is NiSO4. The sulfation of NiO has been recognized in the Type-II hot corrosion of nickel [20,23,25,31,32]. Although metal sulfide (i.e. Ni3S2) was also a corrosion product in hot corrosion [23,25,32], it was not detected in the films in this work, which is understandable since nickel electrode is under anodic polarization. These results demonstrate that the corrosion behaviour of nickel under anodic polarization is similar to that of the Type-II hot corrosion but there are some differences of corrosion mechanism. Based on the above measurements, the corrosion mechanism of nickel under anodic polarization in SO42− containing molten carbonates is proposed and illustrated in Fig. 6. NiO is preferentially formed on the surface of nickel and a protective oxide layer can grow in molten carbonates or carbonates with small amount of SO42- due to the low solubility of NiO in the carbonates at 650 °C [33,34]. However, when the concentration of SO42- is over the critical concentration, soluble NiSO4 will generate together with NiO under anodic polarization:
Fig. 5. XRD patterns of nickel electrodes polarized at 0.5 V for 600 s in molten Li2CO3-Na2CO3-K2CO3 without Li2SO4 (black line) or containing 0.1 mol% (red line), 2.0 mol% (blue line) SO42− under an argon atmosphere at 650 °C (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article). Table 1 The elemental concentration (at%) of selected zones in Fig.4g. Point
Elemental composition(at%)
1 2 3 4 5 6
Ni 10.79 40.79 4.28 0.96 93.79 44.89
O 51.58 34.27 50.72 62.19 0.98 30.64
C 22.44 19.13 24.64 24.42 5.23 20.20
S 0.02 0 0.07 0.35 0 0.01
K 8.99 1.17 11.27 6.52 0 4.26
Na 6.18 4.64 9.02 5.56 0 0
Ni + CO32−−2e-=NiO + CO2(g)
(1)
Ni−2e−=Ni2+(in the presence of SO42-)
(2)
That means, in the presence of plenty amount of SO42−, on the one hand, dense NiO layer is hard to form that leads to a high anodic current at the beginning stage as evidenced in Fig.1b and Fig.4, and on the other hand, SO42− will destroy the NiO film to generate pitting corrosion as evidenced by the spiking current in Fig.1b and morphologies as shown in Fig.3 and Fig.4. The SO42− infiltrates in the film through the corrosion pits, and reacts with the nickel substrate. As it can be seen, the corrosion behaviour is similar to the fluxing process of NiO in hot corrosion process, but the driving force of anodic corrosion is more attributed to the electric field under anodic polarization. With the extension of anodic polarization time, the size of pits becomes larger and larger and the number of voids increases. Therefore, the NiO layer with pits and voids lose its protection to the substrate. With further oxidation and dissolution of nickel substrate, the outside oxide film cracks and is eventually spalled from the substrate due to the interstress in the film and the mechanical action of oxygen bubbles. And then, more electrolyte passes through the cracks and reacts with the substrate, a new layer of NiO film forms on the substrate. The passivation/spallation/repassivation cycles continue as described in the above mechanism until the anode is completely destroyed, leading to the sandwich-like film/ electrolyte/film layers as shown in Fig.4g. Moreover, sulfidation of nickel does not occur since sulfur activity (i.e. pS2) at the nickel/oxide interface maintains a low level under strong anodic polarization comparing with the porous sulfur enriched NiO layer obtained in Type-II hot corrosion process of nickel alloys [23]. In summary, excessive SO42− in molten carbonates reacts with NiO and Ni to form soluble NiSO4 under anodic polarization so that a dense and adhesive NiO film cannot generate, which causes pitting and severe corrosion of the anode.
shall be caused by the mechanical action of oxygen bubbles generated at the electrode surface as well as the surface stress by underneath oxidation. The micro-morphology of the samples was further characterized by SEM. Well defined octahedral particles in a size of 3–5 μm are distributed on the surface of samples obtained in molten carbonates without SO42− (Fig.3a) or with 0.1 mol% SO42− (Fig.3b), but the particle structure tends to irregular and the particle size is disordered with the addition of SO42−, which might result in the film less protective. With increasing the concentration of SO42− to 2.0 mol%, more disordered particles and some visible pits are observed (Fig.3c and Fig.3d). The pits exhibit a bowl-shaped morphology and its size becomes larger with the extension of the anodic polarization time. The diameter of the corrosion pit increases from ∼ 26 μm to ∼39 μm as the polarization time increases from 300 s to 600 s and the bottom of pit is relatively porous. The results confirm that the pitting corrosion takes place when the concentration of SO42− is higher than 0.1 mol%, which is consistently with the electrochemical measurements as shown in Fig.1. In general, the growth of corrosion pit is controlled by the diffusion of corrosion products [28–30]. The composition and structure of the surface film was further characterized by EDS mapping and XRD measurements. The SEM morphology and EDS-mapping of the cross section of the samples are illustrated in Fig.4. The nickel electrode was intimately covered by a dense film in molten carbonates without (Fig.4a) and with small amount of SO42− (0.1 mol%) (Fig.4d). The thickness of the film is ∼12 μm and ∼14 μm, respectively. Ni and O are homogeneously distributed in the oxide layers according to the EDS-mapping (Fig.4a-f).
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Fig. 6. Schematic of the corrosion bahaviour of the Ni electrode after anodic polarization in molten Li2CO3-Na2CO3-K2CO3 containing SO42-.
is profoundly enhanced with increasing the electrolyte temperature. Nickel can work as an effective anode for oxygen evolution at 800 °C in molten carbonates containing 1 mol% SO42−, at 750 °C containing 0.5 mol% SO42−.
3.3. The effect of temperature on the pitting corrosion of nickel anode Temperature is one of the most important factors affecting the initiation and growth of pitting corrosion in aqueous solution [35,36]. In our previous work, it is found that the stability of nickel anode increases with increasing temperature in molten carbonates [14]. It is interesting and important to know how the temperature will affect the pitting corrosion of nickel anode in molten carbonates with certain amount of SO42−. Therefore, cyclic polarization tests were conducted to examine the susceptibility of the nickel anode to pitting corrosion in molten carbonates containing various contents of SO42− at a temperature range 650–800 °C. Fig. 7a shows the cyclic polarization curves of nickel anode in molten carbonates without SO42− at various temperatures. The passivation current decreased with increasing temperature, demonstrating that the stability of the anodic film increased. The back-scanning curves coincided with the sweep curves and no obvious hysteresis loops were observed, indicating that no pitting corrosion occurred in the SO42− free carbonates. Fig.7 b–f present the cyclic polarization curves of nickel in molten carbonates containing different concentration of SO42− (from 0.1 mol% to 2.0 mol%) at different temperature. It can be found that the critical SO42− concentration which induces to pitting corrosion increases with increasing temperature, i.e., 0.1 mol % at 650 °C, 0.2 mol% at 700 °C, 0.5 mol% at 750 °C, 1.0 mol% at 800 °C respectively (Fig.7 b–d). Below the critical concentration, passivation film can form on the nickel surface at the temperature. Pitting corrosion and spallation of the film will take place as evidenced by the increasing current and current oscillation on the curves. The formation of a protective film is prone to form on the nickel electrode at higher temperature, which is most likely to be ascribed to the increasing stability of NiO layer at the higher temperature [14]. Furthermore, the re-passivation potential (Erp) follows the order of 650 °C＜700 °C＜750 °C, which indicates that the resistance to pitting corrosion of nickel anode
3.4. The effect of the atmosphere on the pitting corrosion The application of the MSCC-ET process is preferentially operated under the flue gas atmosphere with ∼14 % CO2 and some SO2 to reduce the cost for CO2 separation and purification. To explore the effect of the composition of atmosphere on the corrosion behaviour of nickel anode, the measurements of anodic polarization curve and galvanostatic electrolysis were conducted under Ar and synthetic flue gas atmosphere at 650 °C (Fig.8). Although the nickel electrode can maintain a typical passivation curve in the molten salt with 0.05 mol% SO42− under the synthetic flue gas (Fig.8a), the passivation peak is much larger than that under argon atmosphere, and it becomes unstable in the melt with 0.1 mol% SO42− (Fig.8b). Fig.8c shows the cell voltage-time plot during the electrolysis under argon atmosphere. After an electrochemical oxidation of nickel at the initial stage to grow a protective oxide film, the cell voltage increased to 2.3 V by shifting the anodic reaction to the oxygen evolution. While under synthetic flue gas, the cell voltage is below 2.0 V and a regular voltage oscillation curve is observed (Fig.8d). The low cell voltage demonstrates that the anodic reaction is mainly due to the dissolution/oxidation of nickel rather than oxygen evolution, while the voltage oscillation is caused by the periodical forming and stripping of the oxide film due to its poor protection and adhesion. The passivation/spallation/re-passivation takes place under the experimental condition. The results demonstrate that the stability of nickel is affected by temperature, concentration of SO42− and the composition of atmosphere. According to the Lux-Flood model [34,37], the basicity of molten carbonates is controlled by the PCO2 in the gas phase above the melt as 6
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Fig. 7. Cyclic polarization curves of Ni electrode in molten Li2CO3-Na2CO3-K2CO3 without SO42− (a) or with 0.1 mol% (b), 0.2 mol% (c), 0.5 mol% (d), 1.0 mol% (e), 2.0 mol% (f) SO42− under an argon atmosphere at various temperature, scan rate: 2 mV/s.
concentration but pitting corrosion and anodic dissolution of nickel take place at higher concentration. The pitting corrosion resistance and stability of nickel anode is greatly enhanced by increasing temperature, but the corrosion rate is accelerated with increasing SO42− concentration. The formation of soluble NiSO4 causes pitting corrosion and prevents the surface from forming a dense and adhesive NiO layer, and eventually caused severe corrosion of the anode. Nevertheless, the nickel anode performs poor stability under synthetic flue gas atmosphere with a much higher anodic current and a fluctuating anodic potential. Understanding the failure mechanism of nickel anode and the critical SO42- concentration will be beneficial for the development of nickel-based inert anodes and optimization of electrolysis conditions in molten salt flue gas capture and electrochemical transformation process. The pre-formation of oxide layer which can block the sulfate ions might be an effective way to improve the durability of nickel-based inert anode and will be investigated in future.
the following reaction: Li2CO3⇌CO2+Li2O
(3)
The basicity of the molten salt is reduced in CO2 / flue gas atmosphere comparing with in Ar, which weaken the stability of the NiO film. Furthermore, the acidic dissolution of NiO can be accelerated by the presence of SO3 according to the following reactions [25,26,38]: 2SO2+O2 = 2SO3
(4)
NiO + SO3=NiSO4
(5)
4. Conclusions Nickel can be passivated in SO42− containing molten carbonates when the concentration of SO42− is lower than a critical concentration. The critical SO42−concentration which induces pitting of nickel anode in molten carbonates is 0.1 mol% at 650 °C, 0.2 mol% at 700 °C, 0.5 mol % at 750 °C and 1.0 mol% at 800 °C, respectively. In an initial period of anodic polarization, a dense and protective NiO film forms on the nickel surface and is not prone to pitting corrosion below the critical
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. 7
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Fig. 8. Anodic polarization curves of the Ni electrodes in molten Li2CO3-Na2CO3-K2CO3 containing 0.05 mol% (a) or 0.1 mol% (b) SO42− under various atmospheres at 650 °C, scan rate: 5 mV/s. Evolution of the cell voltage during constant current electrolysis for 40 min at Ianode = 200 mA/cm2 in molten Li2CO3-Na2CO3-K2CO3 containing 0.1 mol% SO42at 650 °C under argon atmosphere (c) or synthetic flue gas atmosphere (d).
Declaration of Competing Interest
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