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Hot corrosion behavior of ZrO2–MgO coatings in LiCl–Li2O molten salt
Materials Chemistry and Physics 131 (2012) 743–751
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Hot corrosion behavior of ZrO2 –MgO coatings in LiCl–Li2 O molten salt S.H. Cho a,∗ , S.B. Park a , J.H. Lee b,∗ , J.M. Hur a , H.S. Lee a a
Korea Atomic Energy Research Institute, 1045 Daedeokdaero Yuseong-gu, Daejeon 305-353, Republic of Korea Graduate School of Green Energy Technology, Department of Nanomaterials Engineering, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 April 2011 Received in revised form 24 October 2011 Accepted 30 October 2011 Keywords: A. Multilayers B. Plasma deposition C. Corrosion test D. Thermodynamic properties
a b s t r a c t In this study, a magnesia-stabilized zirconia (MgO–ZrO2 ) top coat was applied to the surface of Inconel 713LC with an aluminized NiCoCrAlY bond coat by an optimized plasma spray process. The resulting materials were tested for corrosion behavior at 948 K for 216 h in a LiCl–Li2 O molten salt under an oxidizing atmosphere. The as-coated and tested specimens were analyzed by scanning electron microscopy (SEM)/X-ray energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The bare superalloy reveals an obvious weight loss due to spalling after scale growth and thermal stress. The top coatings exhibited a superior resistance to hot corrosion in the presence of LiCl–Li2 O molten salt when compared to the bare Inconel 713LC and the aluminized bond coatings. These coatings have been found to be beneficial for improving the hot corrosion resistance of the structural materials for high-temperature lithium molten salts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Molten salt technology has been widely used in industrial applications because of its highly desirable physical and chemical characteristics such as high electrical conductivity, high processing rate, and fluid features. However, molten salts can cause corrosion to container materials and various components of electrolysis equipment. Hence, studies on the corrosion of structural materials for handling high-temperature molten salts are common in the literature. Superalloys have been developed for hightemperature applications [1–10]. However, these alloys may not be able to meet both the high-temperature strength requirements and high-temperature corrosion resistance simultaneously. The choice coating materials for thermal barrier type has been found to be the most effective and economical method to enhance corrosion resistance at elevated temperatures without destroying the mechanical properties of a substrate. The top coating materials for the thermal barrier are generally composed of CaO-, CeO2 -, MgO-, or Y2 O3 stabilized ZrO2 , and a bond coat is applied between a substrate and the top coat to enhance adhesion [11–18]. In recent research and development studies, processes such as electrorefining, lithium reduction, and pyrochemical separation have overtaken hydrometallurgical processes in terms of popularity as alternatives for the treatment of spent oxide nuclear fuels. Argonne National Laboratory (ANL) and Korea Atomic Energy
∗ Corresponding authors. Tel.: +82 42 868 2584; fax: +82 42 868 2042. E-mail addresses: [email protected] (S.H. Cho), [email protected] (J.H. Lee). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.10.045
Research Institute (KAERI) have been developing pyrochemical technologies based on a molten salt system [19–22]. In this work, the electrolytic reduction process for a spent oxide nuclear fuel is carried out in a LiCl–Li2 O molten salt at 923 K. The liberation of oxygen at the anode and the resultant high-temperature molten salts create a chemically aggressive environment that is excessively corrosive for ordinary structural materials [19]. Therefore, there is a need for the development of corrosion resistant materials that may be employed is this type of electrolytic reduction technology. However, there are few reports in the literature that investigate the effects of hot lithium molten salts upon the corrosion resistance of structural materials. In this work, a plasma sprayed NiCoCrAlY bond coat and zirconia top coat stabilized with MgO were aluminized using a pack cementation process to improve the corrosion resistance. MgO-stabilized ZrO2 was employed due to the excellent chemical stability that can be achieved using it, especially in LiCl–Li2 O molten salts. [23–26]. Hot corrosion behaviors of the sprayed coatings with an aluminizing process in the presence of lithium molten salt have been investigated under simulated electrolytic reduction conditions. 2. Experimental 2.1. Preparation of specimen The Ni-based superalloy Inconel 713LC (Ni: 74.0, Cr: 11.57, Fe: 0.10, C: 0.05, Si: 0.02, Mo: 4.15, Al: 6.05, Co: 0.08, Ti: 0.76, and Nb: 1.95 wt.%) was used as a substrate material in this study. The specimens of size 70 × 15 × 2 mm3 were degreased and sand-blasted
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S.H. Cho et al. / Materials Chemistry and Physics 131 (2012) 743–751
Table 1 Characteristics of the coating materials. Layer
Product
Composition (wt.%)
Process
Thickness
Top coat Bond coat
METCO. 210NS-1 AMDRY, 365-1
ZrO2 –24MgO Ni–23Co–17Cr–13Al–0.5Y
APS APS
200–250 m 100–150 m
(Al2 O3 , 60 mesh) prior to the aluminizing process and spraying of the coatings by air plasma spraying (APS). 2.2. Coating formulation The specimens were grit blasted with alumina powder to increase adhesion between the bond coat and the substrate. The specimens were first sprayed with a NiCoCrAlY bond coat and then aluminized by a pack cementation process, and an MgO–ZrO2 top coat, using an air plasma spraying system. Table 1 summarizes the properties of the coating materials, and the parameters of the plasma spraying are listed in Table 2. The NiCoCrAlY bond coat provides a rough surface for the mechanical bonding of the ceramic top coat. It protects the underlying alloy substrate against high-temperature oxidation corrosion and minimizes the effect of coefficient of thermal expansion mismatch between the substrate and the ceramic top coat materials. Aluminizing was introduced to increase the Al content of the coating layer [23,26]. For the aluminizing process, specimens with Al(2.0%)–Cr(3.0%)–Al2 O3 (94.9%)–NH4 F(0.1%) mixed powders were placed inside a stainless steel coating box. An aluminum diffusion layer of 30–50 m thickness was formed under a hydrogen atmosphere at 1311–1339 K for 2 h. The air plasma-sprayed MgO–ZrO2 top coat acts as a barrier against the aggressive environment. 2.3. Hot corrosion tests The experimental apparatus is shown in Fig. 1. The LiCl–Li2 O molten salt was introduced into a high-density MgO crucible and then heated at 573 K for 3 h under an argon atmosphere to remove any possible remaining moisture pickup. Once the desired conditions were achieved, the specimens and alumina tube were immersed in the molten salt, and a mixed gas (Ar–10%O2 ) was supplied through an alumina tube. The corrosion tests were carried out at 948 K for 216 h. The Li2 O concentration in LiCl was 3 wt.%. Following the corrosion test, the specimens were withdrawn from the salt and stored under an argon gas atmosphere while the furnace was cooled to room temperature.
prepared for metallographic examination by cold-mounting, grinding, and polishing. The microstructure, morphology and chemical composition of the surface and the cross section of the coatings were characterized by SEM (scanning electron microscopy, JEOL, JSM-6300, JEOL Ltd., Tokyo, Japan) equipped with an EDS (energy dispersive spectrometer, Link ISIS, Oxford Instruments, London, UK). XRD (X-ray Diffraction, Rigaku, DMAX/1200, Rigaku International Corp., Tokyo, Japan) was employed to analyze the phase structural evolution of the coatings and the hot corrosion products. The coating porosity was characterized by means of a surface image analyzer (OLYMPUS BX51, OLYMPUS Corp., Tokyo, Japan). 3. Results and discussion 3.1. Microstructure of the coatings before corrosion Fig. 2 shows the cross sections of the aluminized bond coat and the aluminized bond and top coat systems obtained by plasma spraying. Aluminized NiCoCrAlY and aluminized NiCoCrAlY/MgO–ZrO2 systems were both successfully deposited by plasma spraying, using the parameters outlined in Table 2. As shown in Fig. 2(a), the aluminized bond coat possessed a lamellar structure, which is a characteristic of plasma sprayed coating, including voids and oxide inclusions [23–26]. Fig. 2(b) shows a uniform ceramic coat with voids and micro cracks obtained on an aluminized bond coat surface. The porosity level was determined to range between 3.5 and 5.5% using an image analyzer. Fig. 3 shows the cross-sectional SEM image, elemental distribution, and the XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC. A cross-sectional microstructure of the (NiCoCr)rich region and NiAl–aluminide layer is shown in Fig. 3. It should be noted that the NiAl–aluminide layer contains significant amount of Co, which differs from the initial composition of the top coating layer. The phase of top coating layer was identified as Ni(Co)Al from XRD analysis. Hence, the actual composition of the Ni–Al top coating layer consists of Ni(Co)Al due to Ni and Co forming terminal solid solutions. It is proposed that Co in addition to Ni diffused to the top coating layer during the aluminizing process
2.4. Characterizations Specimens were ultrasonically cleaned in acetone prior and subsequent to the corrosion tests. The initial and final weights of the specimens exposed to the molten salt were recorded to assess the extent of corrosion. Upon completion of the tests, the corroded specimens were cut by a diamond cutter and ultrasonically cleaned in acetone for characterization purposes. Some specimens were Table 2 Parameters of plasma spraying. Parameters
Contents
Apparatus model Plasma gas Gun-to-work distance (mm) Gun traverse rate (mm s−1 ) Gas flow rate (m3 h−1 ) Powder feed rate (g min−1 ) Arc current (A) Arc voltage (V)
METCO-9MC Ar/H2 125 200 Ar: 3.2, H2 : 0.57 40 600 78
Fig. 1. Schematic diagram of the apparatus for the corrosion test.
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Fig. 2. Morphologies in cross section of aluminized NiCoCrAlY (a) and aluminized NiCoCrAlY–MgO–ZrO2 (b) coated Inconel 713LC.
and subsequently formed Ni(Co)Al due to high temperature. The cross-sectional SEM image and the point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC is shown in Fig. 4. In the outer layer (NiAl–aluminide layer), the Al content is ∼36 wt.% and low levels of Cr are also detected. The levels of Co decreases at the surface of the sample and then increases at the interface
between the NiAl–aluminide layer and the bond coating layer. This is convincing evidence of the formation of an Ni(Co)Al phase by diffusion of Co from the bond coat layer. Thus, the Co fraction at the vicinity of interface between the NiAl–aluminide and the bond coat layers is approximately equal to the level at the bond coat layer. Additionally, Co is also detected in levels of ∼3 wt% at the
Fig. 3. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC.
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Fig. 4. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC.
substrate side despite it not being initially present. Hence, it can be interpreted that the Co contained in the bond coat layer diffused not only to the NiAl–aluminide layer but also to the substrate side where it then forms terminal solid solutions. This phenomenon may be favorable for good bonding between layers due to gradual change of the components. The Al content beneath the NiAl–aluminide layer is about 15 wt.% which is higher than in the original plasma coating of the
bond coat layer. This can be explained by the diffusion of the Al to the bond coat layer from the NiAl–aluminide layer. Hence, Al concentration gradually decreased from 35 wt.% to 6 wt.% at the surface and the substrate, respectively. The 6 wt% at the substrate is an Al alloying composition of 713LC. Fig. 5 shows the cross-sectional SEM image, the elemental distribution and, the XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC. The top coat MgO–ZrO2 layer
Fig. 5. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC.
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Fig. 6. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC.
can be clearly seen at the surface with a well defined interface with the NiAl–aluminide layer. According to XRD results (Fig. 5) MgO-stabilized ZrO2 is the main phase of the outer layer due to the wide range of solid solubility of MgO. The aluminized layer is distinguished from the highest Al content among the layers and good bonding strength is expected between top coat and the NiAl–aluminide layer due to rugged surface condition of the NiAl–aluminide layer. In addition to Al, the NiAl–aluminide layer also contains Co and Cr, in a manner similar to the phenomena observed in the aluminide layer top coat sample shown in Fig. 3. The cross-sectional SEM image and the point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC are shown in Fig. 6. The XRD pattern on the top coat layer showing single phase ZrO2 demonstrates that the MgO content is within the solid solubility limit of ZrO2 as confirmed by EDX results (pointS1 composed of 72 wt% of Zr and 8 wt% of Mg) (Fig. 6). Fig. 6 also shows a cross section of the plasma spray coating layer, which is identified by a triple-phase structure consisting of a (NiCoCr)-rich region, a (ZrO2 –MgO)-rich region, and an aluminide layer. The top coat layer consists of only MgO-stabilized ZrO2 without diffusion of substances from the NiAl–aluminide layer. Hence the interfacial bonding strength between the top coat and the NiAl–aluminide layer derives from the mechanical anchoring effect. Significant levels of Co and Cr are incorporated in the NiAl–aluminide layer due to high solid solubility of these elements as previously discussed.
mainly NiO, Cr2 O3 , NiCr2 O4 , and small amounts of (Al, Nb, Ti)O2 . NiCr2 O4 (spinel-type) is believed to be formed by a solid-state reaction between the corresponding oxides (NiO and Cr2 O3 ) [27]. As the corrosion progressed, (Al, Nb, Ti)O2 was formed; its formation can be attributed to external diffusion of oxygen active elements such as Al, Nb, and Ti [28]. The corresponding cross-sectional SEM image and elemental distribution show that the outer scale is mainly composed of Cr2 O3 , Al2 O3 , TiO2 , and NbO2. An intermediate layer rich in Ni and Al, and an Al2 O3 scale formed close to the alloy. The oxygen active elements such as Al, Nb, and Ti were preferentially oxidized and participated in the corrosion layer [28]. The presence of a depleted zone of Cr beneath the oxide/metal interface is a typical result of hot corrosion [25]. It is anticipated that the Cr depletion and porous oxide layers would accelerate the spallation of the corrosion layer by the internal diffusion of oxygen ions. 3.3.2. Aluminized NiCoCrAlY bond coat Fig. 9 shows the cross-sectional SEM image, the elemental distribution, and the XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h. The morphology and elemental distribution of the coating layers are completely changed after corrosion test. Voids can be occasionally seen at the interface of the two layers. One of the more noticeable changes is that Ni has been mostly eliminated from the aluminide layer and only
3.2. Corrosion kinetics The weight losses of the bare Inconel 713LC and its coated specimens after the corrosion tests in a LiCl–Li2 O molten salt at 948 K are shown in Fig. 7. The weight losses were in the following order: aluminized bond and top coat < aluminized bond coat < bare Inconel 713LC superalloy. After 216 h of hot corrosion, the bare superalloy reveals an obvious weight loss due to the spallation and dissolution of oxide scales and other corroded products [25]. On the other hand, the low weight change of the specimen with top coat by thermal barrier type seems to be imbued with superior protection ability and possesses an improved chemical and thermal stability in a hot lithium molten salt system [17]. 3.3. Corrosion behavior 3.3.1. Bare Inconel 713LC superalloy The cross-sectional SEM image, the elemental distribution, and the XRD pattern of the corrosion products of the bare Inconel 713LC corroded for 216 h are shown in Fig. 8. The corrosion products were
Fig. 7. Weight losses of bare and coated Inconel 713LC corroded at 948 K for 216 h.
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Fig. 8. XRD pattern, cross-sectional SEM image, and the elemental distribution of bare Inconel 713LC corroded at 948 K for 216 h.
traces of Co and Cr can be detected. XRD analysis reveals that the phase composition of the outer layer consists of LiAlO2 and small amounts of Ni(Co)O. The formation of LiAlO2 can be explained by the oxidation reaction between a LiCl salt containing Li2 O and Al2 O3 . It is believed that the aluminum oxide arises from the following reaction: Ni(Co)Al (4Ni(Co)Al + 3O2 → 2Al2 O3 + 4Ni(Co)) [15,16]. The cross-sectional SEM image and the point quantitative analysis of the aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h is shown in Fig. 10. It should be noted that most of the Ni is eliminated at the aluminide layer with traces of Co found on the surface and discontinuous Ni(Co) concentrations present in the bond coating layer. Cr oxide phase was not detected by XRD analysis, as shown in Fig. 9, however, both EDS mapping and point quantitative analysis results show that a Cr oxide phase formed at the outer surface, as shown in Fig. 10. Hence, it can be suggested that Cr contained in the metallic bond coating layer diffused to Al2 O3 then oxidized as Cr2 O3 , which forms solid solution with Al2 O3 phase [29]. The formation of a dense and stable aluminum oxide phase in the outer layer is attributed to a continuous supply of Al from the aluminide layer. The Al2 O3 layer retards internal
diffusion of oxygen ions. Hence, the weight loss of the specimen with aluminized bond coat is 53% less than that of the bare Inconel 713LC as illustrated in Fig. 7. The formation of voids in the interface may be due to the Kirkendall effect caused by Cr diffusion [30]. 3.3.3. Aluminized NiCoCrAlY bond and MgO–ZrO2 top coat Fig. 11 shows the cross-sectional SEM image, the elemental distribution, and the XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h. The microstructure and the interface among the layers are almost intact after corrosion test. In addition, the elemental distribution of Zr and especially Mg is relatively uniform throughout the top coat layer. XRD result analysis (Fig. 11) reveals that the top coat surface is a Li2 ZrO3 phase, which can be formed by the following chemical reaction: (ZrO2 + Li2 O → Li2 ZrO3 , Gat 948 K = −65.8 kJ). It is interesting to observe that the integrity of the top coat and its interface are maintained after corrosion test. Phase separation of MgO-stabilized ZrO2 to MgO and ZrO2 can be observed. The phase separation might be attributed to the chemical reaction between some of the ZrO2 and Li2 O forming Li2 ZrO3 which releases super saturated MgO from the MgO–ZrO2 solid solution. It
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Fig. 9. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h.
is believed that the interfacial strength is mechanically maintained due to the rugged interface between the top coat and the aluminide layer. The result of thermodynamic equilibrium composition calculations by HSC chemistry is shown in Fig. 12 [31]. The calculated
equilibrium compositions are in good agreement with the experimental data in which ZrO2 and Li2 ZrO3 are main products from the reaction between Li2 O in molten salt and top coat material. Hence, ZrO2 and Li2 O are the only materials involved in the chemical reaction at the surface of the top coat layer, whereas LiCl and
Fig. 10. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h.
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Fig. 11. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h.
Fig. 12. Calculated equilibrium composition of reaction system in molten salt according to temperature (initial composition: ZrO2 : 0.76 mol, MgO: 0.24 mol, Li2 O: 0.03 mol and LiCl: 1 mol).
MgO are inert in the corrosion process from room temperature to 800 ◦ C. The cross-sectional SEM image and the point quantitative analysis of the aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h is shown in Fig. 13. The cross section of plasma spray coating layer is identified as a triple-phase structure consisting of an (NiCoCr)-rich region, a (ZrO2 –MgO)-rich region, and an aluminide layer. The point quantitative analysis of the top coat layer, from S1 to S3, shows a slight increase of Zr and a decrease of O along the depth of the layer cross section leading us to conclude that the compound with a higher oxygen concentration such as Li2 ZrO3 is located at the surface of the top coat layer. The ratio of the components in the aluminide layer is almost identical to that of as coated specimen, except that the oxygen level is slightly increased to ∼3 wt% due to oxidation. The salient fact to note from the point quantitative analysis of bond coat layer of the MgO–ZrO2 coated Inconel 713LC specimen is that the oxygen level is extremely low compared to that of only aluminized NiCoCrAlY coated Inconel 713LC sample. The superior corrosion resistance of the specimen with the top coat may be attributed to the retardation of oxygen diffusion to the bond coat layers, which leads to enhanced chemical and thermal stability and superior protection ability. Finally it can be concluded that the sample bearing both an aluminized bond and top coat possesses better corrosion and
S.H. Cho et al. / Materials Chemistry and Physics 131 (2012) 743–751
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Fig. 13. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h.
oxidation resistance than material having only an aluminized bond coat [15]. 4. Conclusions (1) The weight losses of the bare Inconel 713LC and its coated specimens after the corrosion tests in a LiCl–Li2 O molten salt at 948 K are in the order of aluminized bond and top coat < aluminized bond coat < bare superalloy (Inconel 713LC). (2) The phase composition of the aluminized bond coat surface corroded at 948 K for 216 h consists of LiAlO2 and Ni(Co)O, and that of the aluminized bond and top coat surface is Li2 ZrO3 , which was formed from MgO-stabilized ZrO2 by the chemical reaction with Li2 O in the molten salt. Compared with the bare superalloy (Inconel 713LC), the aluminized NiCoCrAlY coat shows a much better hot corrosion resistance in the presence of LiCl–Li2 O molten salt as a result of the formation of a continuous and protective Al2 O3 scale. (3) The superior corrosion resistance of the specimen with the top coat can be attributed to not only the chemical and thermal stability and the superior protection ability but also the Al2 O3 layer formed by oxidation of Ni(Co)Al in the aluminized bond coat, because they retard the internal diffusion of oxygen ions and also external diffusion of the alloying elements from the metallic bond coat. Acknowledgment This work was funded by the National Mid- and Long-term Atomic Energy Research & Development Program supported by the Ministry of Education, Science and Technology of Korea. References [1] R.A. Rapp, Corros. Sci. 44 (2002) 209.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Hot corrosion behavior of ZrO2 –MgO coatings in LiCl–Li2 O molten salt S.H. Cho a,∗ , S.B. Park a , J.H. Lee b,∗ , J.M. Hur a , H.S. Lee a a
Korea Atomic Energy Research Institute, 1045 Daedeokdaero Yuseong-gu, Daejeon 305-353, Republic of Korea Graduate School of Green Energy Technology, Department of Nanomaterials Engineering, Chungnam National University, 79 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 9 April 2011 Received in revised form 24 October 2011 Accepted 30 October 2011 Keywords: A. Multilayers B. Plasma deposition C. Corrosion test D. Thermodynamic properties
a b s t r a c t In this study, a magnesia-stabilized zirconia (MgO–ZrO2 ) top coat was applied to the surface of Inconel 713LC with an aluminized NiCoCrAlY bond coat by an optimized plasma spray process. The resulting materials were tested for corrosion behavior at 948 K for 216 h in a LiCl–Li2 O molten salt under an oxidizing atmosphere. The as-coated and tested specimens were analyzed by scanning electron microscopy (SEM)/X-ray energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The bare superalloy reveals an obvious weight loss due to spalling after scale growth and thermal stress. The top coatings exhibited a superior resistance to hot corrosion in the presence of LiCl–Li2 O molten salt when compared to the bare Inconel 713LC and the aluminized bond coatings. These coatings have been found to be beneficial for improving the hot corrosion resistance of the structural materials for high-temperature lithium molten salts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Molten salt technology has been widely used in industrial applications because of its highly desirable physical and chemical characteristics such as high electrical conductivity, high processing rate, and fluid features. However, molten salts can cause corrosion to container materials and various components of electrolysis equipment. Hence, studies on the corrosion of structural materials for handling high-temperature molten salts are common in the literature. Superalloys have been developed for hightemperature applications [1–10]. However, these alloys may not be able to meet both the high-temperature strength requirements and high-temperature corrosion resistance simultaneously. The choice coating materials for thermal barrier type has been found to be the most effective and economical method to enhance corrosion resistance at elevated temperatures without destroying the mechanical properties of a substrate. The top coating materials for the thermal barrier are generally composed of CaO-, CeO2 -, MgO-, or Y2 O3 stabilized ZrO2 , and a bond coat is applied between a substrate and the top coat to enhance adhesion [11–18]. In recent research and development studies, processes such as electrorefining, lithium reduction, and pyrochemical separation have overtaken hydrometallurgical processes in terms of popularity as alternatives for the treatment of spent oxide nuclear fuels. Argonne National Laboratory (ANL) and Korea Atomic Energy
∗ Corresponding authors. Tel.: +82 42 868 2584; fax: +82 42 868 2042. E-mail addresses: [email protected] (S.H. Cho), [email protected] (J.H. Lee). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.10.045
Research Institute (KAERI) have been developing pyrochemical technologies based on a molten salt system [19–22]. In this work, the electrolytic reduction process for a spent oxide nuclear fuel is carried out in a LiCl–Li2 O molten salt at 923 K. The liberation of oxygen at the anode and the resultant high-temperature molten salts create a chemically aggressive environment that is excessively corrosive for ordinary structural materials [19]. Therefore, there is a need for the development of corrosion resistant materials that may be employed is this type of electrolytic reduction technology. However, there are few reports in the literature that investigate the effects of hot lithium molten salts upon the corrosion resistance of structural materials. In this work, a plasma sprayed NiCoCrAlY bond coat and zirconia top coat stabilized with MgO were aluminized using a pack cementation process to improve the corrosion resistance. MgO-stabilized ZrO2 was employed due to the excellent chemical stability that can be achieved using it, especially in LiCl–Li2 O molten salts. [23–26]. Hot corrosion behaviors of the sprayed coatings with an aluminizing process in the presence of lithium molten salt have been investigated under simulated electrolytic reduction conditions. 2. Experimental 2.1. Preparation of specimen The Ni-based superalloy Inconel 713LC (Ni: 74.0, Cr: 11.57, Fe: 0.10, C: 0.05, Si: 0.02, Mo: 4.15, Al: 6.05, Co: 0.08, Ti: 0.76, and Nb: 1.95 wt.%) was used as a substrate material in this study. The specimens of size 70 × 15 × 2 mm3 were degreased and sand-blasted
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Table 1 Characteristics of the coating materials. Layer
Product
Composition (wt.%)
Process
Thickness
Top coat Bond coat
METCO. 210NS-1 AMDRY, 365-1
ZrO2 –24MgO Ni–23Co–17Cr–13Al–0.5Y
APS APS
200–250 m 100–150 m
(Al2 O3 , 60 mesh) prior to the aluminizing process and spraying of the coatings by air plasma spraying (APS). 2.2. Coating formulation The specimens were grit blasted with alumina powder to increase adhesion between the bond coat and the substrate. The specimens were first sprayed with a NiCoCrAlY bond coat and then aluminized by a pack cementation process, and an MgO–ZrO2 top coat, using an air plasma spraying system. Table 1 summarizes the properties of the coating materials, and the parameters of the plasma spraying are listed in Table 2. The NiCoCrAlY bond coat provides a rough surface for the mechanical bonding of the ceramic top coat. It protects the underlying alloy substrate against high-temperature oxidation corrosion and minimizes the effect of coefficient of thermal expansion mismatch between the substrate and the ceramic top coat materials. Aluminizing was introduced to increase the Al content of the coating layer [23,26]. For the aluminizing process, specimens with Al(2.0%)–Cr(3.0%)–Al2 O3 (94.9%)–NH4 F(0.1%) mixed powders were placed inside a stainless steel coating box. An aluminum diffusion layer of 30–50 m thickness was formed under a hydrogen atmosphere at 1311–1339 K for 2 h. The air plasma-sprayed MgO–ZrO2 top coat acts as a barrier against the aggressive environment. 2.3. Hot corrosion tests The experimental apparatus is shown in Fig. 1. The LiCl–Li2 O molten salt was introduced into a high-density MgO crucible and then heated at 573 K for 3 h under an argon atmosphere to remove any possible remaining moisture pickup. Once the desired conditions were achieved, the specimens and alumina tube were immersed in the molten salt, and a mixed gas (Ar–10%O2 ) was supplied through an alumina tube. The corrosion tests were carried out at 948 K for 216 h. The Li2 O concentration in LiCl was 3 wt.%. Following the corrosion test, the specimens were withdrawn from the salt and stored under an argon gas atmosphere while the furnace was cooled to room temperature.
prepared for metallographic examination by cold-mounting, grinding, and polishing. The microstructure, morphology and chemical composition of the surface and the cross section of the coatings were characterized by SEM (scanning electron microscopy, JEOL, JSM-6300, JEOL Ltd., Tokyo, Japan) equipped with an EDS (energy dispersive spectrometer, Link ISIS, Oxford Instruments, London, UK). XRD (X-ray Diffraction, Rigaku, DMAX/1200, Rigaku International Corp., Tokyo, Japan) was employed to analyze the phase structural evolution of the coatings and the hot corrosion products. The coating porosity was characterized by means of a surface image analyzer (OLYMPUS BX51, OLYMPUS Corp., Tokyo, Japan). 3. Results and discussion 3.1. Microstructure of the coatings before corrosion Fig. 2 shows the cross sections of the aluminized bond coat and the aluminized bond and top coat systems obtained by plasma spraying. Aluminized NiCoCrAlY and aluminized NiCoCrAlY/MgO–ZrO2 systems were both successfully deposited by plasma spraying, using the parameters outlined in Table 2. As shown in Fig. 2(a), the aluminized bond coat possessed a lamellar structure, which is a characteristic of plasma sprayed coating, including voids and oxide inclusions [23–26]. Fig. 2(b) shows a uniform ceramic coat with voids and micro cracks obtained on an aluminized bond coat surface. The porosity level was determined to range between 3.5 and 5.5% using an image analyzer. Fig. 3 shows the cross-sectional SEM image, elemental distribution, and the XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC. A cross-sectional microstructure of the (NiCoCr)rich region and NiAl–aluminide layer is shown in Fig. 3. It should be noted that the NiAl–aluminide layer contains significant amount of Co, which differs from the initial composition of the top coating layer. The phase of top coating layer was identified as Ni(Co)Al from XRD analysis. Hence, the actual composition of the Ni–Al top coating layer consists of Ni(Co)Al due to Ni and Co forming terminal solid solutions. It is proposed that Co in addition to Ni diffused to the top coating layer during the aluminizing process
2.4. Characterizations Specimens were ultrasonically cleaned in acetone prior and subsequent to the corrosion tests. The initial and final weights of the specimens exposed to the molten salt were recorded to assess the extent of corrosion. Upon completion of the tests, the corroded specimens were cut by a diamond cutter and ultrasonically cleaned in acetone for characterization purposes. Some specimens were Table 2 Parameters of plasma spraying. Parameters
Contents
Apparatus model Plasma gas Gun-to-work distance (mm) Gun traverse rate (mm s−1 ) Gas flow rate (m3 h−1 ) Powder feed rate (g min−1 ) Arc current (A) Arc voltage (V)
METCO-9MC Ar/H2 125 200 Ar: 3.2, H2 : 0.57 40 600 78
Fig. 1. Schematic diagram of the apparatus for the corrosion test.
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Fig. 2. Morphologies in cross section of aluminized NiCoCrAlY (a) and aluminized NiCoCrAlY–MgO–ZrO2 (b) coated Inconel 713LC.
and subsequently formed Ni(Co)Al due to high temperature. The cross-sectional SEM image and the point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC is shown in Fig. 4. In the outer layer (NiAl–aluminide layer), the Al content is ∼36 wt.% and low levels of Cr are also detected. The levels of Co decreases at the surface of the sample and then increases at the interface
between the NiAl–aluminide layer and the bond coating layer. This is convincing evidence of the formation of an Ni(Co)Al phase by diffusion of Co from the bond coat layer. Thus, the Co fraction at the vicinity of interface between the NiAl–aluminide and the bond coat layers is approximately equal to the level at the bond coat layer. Additionally, Co is also detected in levels of ∼3 wt% at the
Fig. 3. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC.
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Fig. 4. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC.
substrate side despite it not being initially present. Hence, it can be interpreted that the Co contained in the bond coat layer diffused not only to the NiAl–aluminide layer but also to the substrate side where it then forms terminal solid solutions. This phenomenon may be favorable for good bonding between layers due to gradual change of the components. The Al content beneath the NiAl–aluminide layer is about 15 wt.% which is higher than in the original plasma coating of the
bond coat layer. This can be explained by the diffusion of the Al to the bond coat layer from the NiAl–aluminide layer. Hence, Al concentration gradually decreased from 35 wt.% to 6 wt.% at the surface and the substrate, respectively. The 6 wt% at the substrate is an Al alloying composition of 713LC. Fig. 5 shows the cross-sectional SEM image, the elemental distribution and, the XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC. The top coat MgO–ZrO2 layer
Fig. 5. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC.
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Fig. 6. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC.
can be clearly seen at the surface with a well defined interface with the NiAl–aluminide layer. According to XRD results (Fig. 5) MgO-stabilized ZrO2 is the main phase of the outer layer due to the wide range of solid solubility of MgO. The aluminized layer is distinguished from the highest Al content among the layers and good bonding strength is expected between top coat and the NiAl–aluminide layer due to rugged surface condition of the NiAl–aluminide layer. In addition to Al, the NiAl–aluminide layer also contains Co and Cr, in a manner similar to the phenomena observed in the aluminide layer top coat sample shown in Fig. 3. The cross-sectional SEM image and the point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC are shown in Fig. 6. The XRD pattern on the top coat layer showing single phase ZrO2 demonstrates that the MgO content is within the solid solubility limit of ZrO2 as confirmed by EDX results (pointS1 composed of 72 wt% of Zr and 8 wt% of Mg) (Fig. 6). Fig. 6 also shows a cross section of the plasma spray coating layer, which is identified by a triple-phase structure consisting of a (NiCoCr)-rich region, a (ZrO2 –MgO)-rich region, and an aluminide layer. The top coat layer consists of only MgO-stabilized ZrO2 without diffusion of substances from the NiAl–aluminide layer. Hence the interfacial bonding strength between the top coat and the NiAl–aluminide layer derives from the mechanical anchoring effect. Significant levels of Co and Cr are incorporated in the NiAl–aluminide layer due to high solid solubility of these elements as previously discussed.
mainly NiO, Cr2 O3 , NiCr2 O4 , and small amounts of (Al, Nb, Ti)O2 . NiCr2 O4 (spinel-type) is believed to be formed by a solid-state reaction between the corresponding oxides (NiO and Cr2 O3 ) [27]. As the corrosion progressed, (Al, Nb, Ti)O2 was formed; its formation can be attributed to external diffusion of oxygen active elements such as Al, Nb, and Ti [28]. The corresponding cross-sectional SEM image and elemental distribution show that the outer scale is mainly composed of Cr2 O3 , Al2 O3 , TiO2 , and NbO2. An intermediate layer rich in Ni and Al, and an Al2 O3 scale formed close to the alloy. The oxygen active elements such as Al, Nb, and Ti were preferentially oxidized and participated in the corrosion layer [28]. The presence of a depleted zone of Cr beneath the oxide/metal interface is a typical result of hot corrosion [25]. It is anticipated that the Cr depletion and porous oxide layers would accelerate the spallation of the corrosion layer by the internal diffusion of oxygen ions. 3.3.2. Aluminized NiCoCrAlY bond coat Fig. 9 shows the cross-sectional SEM image, the elemental distribution, and the XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h. The morphology and elemental distribution of the coating layers are completely changed after corrosion test. Voids can be occasionally seen at the interface of the two layers. One of the more noticeable changes is that Ni has been mostly eliminated from the aluminide layer and only
3.2. Corrosion kinetics The weight losses of the bare Inconel 713LC and its coated specimens after the corrosion tests in a LiCl–Li2 O molten salt at 948 K are shown in Fig. 7. The weight losses were in the following order: aluminized bond and top coat < aluminized bond coat < bare Inconel 713LC superalloy. After 216 h of hot corrosion, the bare superalloy reveals an obvious weight loss due to the spallation and dissolution of oxide scales and other corroded products [25]. On the other hand, the low weight change of the specimen with top coat by thermal barrier type seems to be imbued with superior protection ability and possesses an improved chemical and thermal stability in a hot lithium molten salt system [17]. 3.3. Corrosion behavior 3.3.1. Bare Inconel 713LC superalloy The cross-sectional SEM image, the elemental distribution, and the XRD pattern of the corrosion products of the bare Inconel 713LC corroded for 216 h are shown in Fig. 8. The corrosion products were
Fig. 7. Weight losses of bare and coated Inconel 713LC corroded at 948 K for 216 h.
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Fig. 8. XRD pattern, cross-sectional SEM image, and the elemental distribution of bare Inconel 713LC corroded at 948 K for 216 h.
traces of Co and Cr can be detected. XRD analysis reveals that the phase composition of the outer layer consists of LiAlO2 and small amounts of Ni(Co)O. The formation of LiAlO2 can be explained by the oxidation reaction between a LiCl salt containing Li2 O and Al2 O3 . It is believed that the aluminum oxide arises from the following reaction: Ni(Co)Al (4Ni(Co)Al + 3O2 → 2Al2 O3 + 4Ni(Co)) [15,16]. The cross-sectional SEM image and the point quantitative analysis of the aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h is shown in Fig. 10. It should be noted that most of the Ni is eliminated at the aluminide layer with traces of Co found on the surface and discontinuous Ni(Co) concentrations present in the bond coating layer. Cr oxide phase was not detected by XRD analysis, as shown in Fig. 9, however, both EDS mapping and point quantitative analysis results show that a Cr oxide phase formed at the outer surface, as shown in Fig. 10. Hence, it can be suggested that Cr contained in the metallic bond coating layer diffused to Al2 O3 then oxidized as Cr2 O3 , which forms solid solution with Al2 O3 phase [29]. The formation of a dense and stable aluminum oxide phase in the outer layer is attributed to a continuous supply of Al from the aluminide layer. The Al2 O3 layer retards internal
diffusion of oxygen ions. Hence, the weight loss of the specimen with aluminized bond coat is 53% less than that of the bare Inconel 713LC as illustrated in Fig. 7. The formation of voids in the interface may be due to the Kirkendall effect caused by Cr diffusion [30]. 3.3.3. Aluminized NiCoCrAlY bond and MgO–ZrO2 top coat Fig. 11 shows the cross-sectional SEM image, the elemental distribution, and the XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h. The microstructure and the interface among the layers are almost intact after corrosion test. In addition, the elemental distribution of Zr and especially Mg is relatively uniform throughout the top coat layer. XRD result analysis (Fig. 11) reveals that the top coat surface is a Li2 ZrO3 phase, which can be formed by the following chemical reaction: (ZrO2 + Li2 O → Li2 ZrO3 , Gat 948 K = −65.8 kJ). It is interesting to observe that the integrity of the top coat and its interface are maintained after corrosion test. Phase separation of MgO-stabilized ZrO2 to MgO and ZrO2 can be observed. The phase separation might be attributed to the chemical reaction between some of the ZrO2 and Li2 O forming Li2 ZrO3 which releases super saturated MgO from the MgO–ZrO2 solid solution. It
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Fig. 9. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h.
is believed that the interfacial strength is mechanically maintained due to the rugged interface between the top coat and the aluminide layer. The result of thermodynamic equilibrium composition calculations by HSC chemistry is shown in Fig. 12 [31]. The calculated
equilibrium compositions are in good agreement with the experimental data in which ZrO2 and Li2 ZrO3 are main products from the reaction between Li2 O in molten salt and top coat material. Hence, ZrO2 and Li2 O are the only materials involved in the chemical reaction at the surface of the top coat layer, whereas LiCl and
Fig. 10. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY coated Inconel 713LC corroded at 948 K for 216 h.
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Fig. 11. Cross-sectional SEM image, elemental distribution, and XRD pattern of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h.
Fig. 12. Calculated equilibrium composition of reaction system in molten salt according to temperature (initial composition: ZrO2 : 0.76 mol, MgO: 0.24 mol, Li2 O: 0.03 mol and LiCl: 1 mol).
MgO are inert in the corrosion process from room temperature to 800 ◦ C. The cross-sectional SEM image and the point quantitative analysis of the aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h is shown in Fig. 13. The cross section of plasma spray coating layer is identified as a triple-phase structure consisting of an (NiCoCr)-rich region, a (ZrO2 –MgO)-rich region, and an aluminide layer. The point quantitative analysis of the top coat layer, from S1 to S3, shows a slight increase of Zr and a decrease of O along the depth of the layer cross section leading us to conclude that the compound with a higher oxygen concentration such as Li2 ZrO3 is located at the surface of the top coat layer. The ratio of the components in the aluminide layer is almost identical to that of as coated specimen, except that the oxygen level is slightly increased to ∼3 wt% due to oxidation. The salient fact to note from the point quantitative analysis of bond coat layer of the MgO–ZrO2 coated Inconel 713LC specimen is that the oxygen level is extremely low compared to that of only aluminized NiCoCrAlY coated Inconel 713LC sample. The superior corrosion resistance of the specimen with the top coat may be attributed to the retardation of oxygen diffusion to the bond coat layers, which leads to enhanced chemical and thermal stability and superior protection ability. Finally it can be concluded that the sample bearing both an aluminized bond and top coat possesses better corrosion and
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Fig. 13. Cross-sectional SEM image and point quantitative analysis of aluminized NiCoCrAlY and MgO–ZrO2 coated Inconel 713LC corroded at 948 K for 216 h.
oxidation resistance than material having only an aluminized bond coat [15]. 4. Conclusions (1) The weight losses of the bare Inconel 713LC and its coated specimens after the corrosion tests in a LiCl–Li2 O molten salt at 948 K are in the order of aluminized bond and top coat < aluminized bond coat < bare superalloy (Inconel 713LC). (2) The phase composition of the aluminized bond coat surface corroded at 948 K for 216 h consists of LiAlO2 and Ni(Co)O, and that of the aluminized bond and top coat surface is Li2 ZrO3 , which was formed from MgO-stabilized ZrO2 by the chemical reaction with Li2 O in the molten salt. Compared with the bare superalloy (Inconel 713LC), the aluminized NiCoCrAlY coat shows a much better hot corrosion resistance in the presence of LiCl–Li2 O molten salt as a result of the formation of a continuous and protective Al2 O3 scale. (3) The superior corrosion resistance of the specimen with the top coat can be attributed to not only the chemical and thermal stability and the superior protection ability but also the Al2 O3 layer formed by oxidation of Ni(Co)Al in the aluminized bond coat, because they retard the internal diffusion of oxygen ions and also external diffusion of the alloying elements from the metallic bond coat. Acknowledgment This work was funded by the National Mid- and Long-term Atomic Energy Research & Development Program supported by the Ministry of Education, Science and Technology of Korea. References [1] R.A. Rapp, Corros. Sci. 44 (2002) 209.
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