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Corrosion resistance of 310S and 316L austenitic stainless steel in a quaternary molten salt for concentrating solar power
Available online at www.sciencedirect.com
ScienceDirect
Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000
Energy Procedia 00 (2017) 000–000 ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia
Energy Procedia 142 Energy Procedia 00(2017) (2017)3590–3596 000–000 www.elsevier.com/locate/procedia
9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK
Corrosion resistance of 310S and 316L austenitic stainless steel in a The 15th International Symposium on District Heating and Cooling quaternary molten salt for concentrating solar power Assessing feasibility of using Xiao long Lia ,the Xiaolan Weib，Jianfeng Lua, the Jing heat Dinga,*demand-outdoor ，Weilong Wanga,* temperature function for a long-term district heat demand forecast School of Engineering , Sun Yat-sen University , Guangzhou 510006, PR China
a a
b bSchool
School of Chemistry and Chemical Engineering South China University of Technology, Guangzhou 510640, PR China a
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
This paper evaluates the corrosion of 310S and 316L austenitic stainless steel in a quaternary molten salt KNO33-NaNO22-NaNO33-KCl (wt%, 50.35:38:6.65:5) at 500℃. The corrosion rates were determined using gravimetric analysis, which measures the weight loss over 1848h in order to identify the corrosion products with field-emission Abstract scanning electron microscope (FESEM) and X-ray diffraction (XRD). Theheating resultsnetworks showed are thatcommonly the corrosion rate of these two austenitic hassolutions the highest value at the District addressed in the literature as one ofstainless the moststeels effective for decreasing the initial stage,gas andemissions then decrease a constant value, require which is dueinvestments to the formation ofreturned protective oxide greenhouse from thefollowed building by sector. These systems high which are through thefilm heat on the Due interface preventclimate furtherconditions corrosion.andCompared to 316L, 310S hasheat better corrosion-resistance in the salt, sales. to thetochanged building renovation policies, demand in the future could decrease, since it contains more Cr and Ni element. The corrosion layer thickness of the 310S and 316L is 1.613μm and prolonging the investment return period. 2.903μm based on the cross-section micrographs, which is equal to 0.499μm/year and 1.460μm/year, respectively. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand
©forecast. 2017 The Authors. by Elsevier The districtPublished of Alvalade, locatedLtd. in Lisbon (Portugal), was used as a case study. The district is consisted of 665 © 2017 The Authors. Published by Ltd. committee of the 9th International Conference on Applied Energy. Peer-review under responsibility ofElsevier the scientific buildings that vary in both construction period and typology. Three weather scenarios (low, on medium, and three district Peer-review under responsibility of the scientific committee of the 9th International Conference Appliedhigh) Energy. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Keywords: molten Heat transfer Corrosion, Austenitic Stainless Steel and validated by the authors. comparedQuaternary with results fromsalt, a dynamic heatfluid, demand model, previously developed The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.scenarios, Introduction the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease the rapid numberdevelopment of heating hours 22-139h and during the heating the season (depending on the combination of weather and Withinthe of of economy technology, energy consumption dramatically increases renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the resulting in serious pollution. For decades, new clean energy has been developed and is recognized as a potential coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and substitute of fossil fuel in the future. The concentrated solar power (CSP) with thermal energy storage (TES) makes improve the accuracy of heat estimations. [1]. solar energy utilization moredemand attractive than other renewable energies because of its high feasibility and efficiency [1]
solar energy utilization more attractive than other renewable energies because of its high feasibility and efficiency . [2-7], for its excellent thermal Nitrate molten salt is now being widely used as a heat storage and transfer media[2-7] © 2017 The Authors. Published by Elsevier Ltd. [8] [8] physical properties, such as the commercial Solar Salt (60wt% NaNO33/40wt% KNO33) and Hitec (53wt% KNO33 Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel/fax: +86-020-3933-2319. E-mail address: [email protected]; [email protected]
Cooling. E-mail address: [email protected]; [email protected]
Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy . 10.1016/j.egypro.2017.12.781
2
Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
3591
/40wt% NaNO2/7wt% NaNO3). However, the thermal stability under the high temperature is not satisfied, especially the corrosion issue brought serious operation safety issue. From this point of view, many scholars start to focus on the thermal stability of molten salt in the operating temperature range. Q. Peng et al. [9] added 5wt% KCl on the basis of Hitec salt, results showed that the best use temperature of the mixed molten salt is improved from 400~500℃ to 550℃, which reduces the loss of NO2- content in the molten salt and delays the deterioration of the molten salt. And also slightly lower the freezing point of molten salt. Eweka and Kerridge[10] studied the reactions between HITEC and copper(II), iron(Ill), cobalt(II) and nickel(II), these transition metals tended to form the most stable oxides, and the nitrite ion (NO2- ) transformed into nitrate iron (NO3-) through nitric oxide and nitrogen dioxide. It is still of importance to continue to investigate the corrosion behavior of various metal matrix in the different molten salts. In this study, We examined the corrosion behavior of 310S and 316L austenitic stainless steel in a new quaternary molten salt, and the mechanism of corrosion was further investigated. 2. Experimental procedure 2.1. Material 2.1.1. Substrate The corrosion tests were performed on commercial austenitic stainless steel 310S and 316L (GoodFellow®) with the composition shown in Table 1. Specimens were manufactured with nominal dimensions of 30×15×2mm and the surface was prepared by sanding with silicon carbide (SiC) abrasive paper. 2.1.2. Preparation and characterization of the salt mixture The alkali metal nitrates that were used as the corrosive agent in this work were KNO 3, NaNO3, NaNO2 and KCl. Some properties of interest and the impurity content of these salts are shown in Table 2. The composition in weight percentage of the multi-component molten salts used was of 50.35wt% KNO3/38wt% NaNO2/6.65wt% NaNO3/5wt% KCl. Table 1 Chemical composition of austenitic stainless steel 310S and 316L (wt%). Alloy O Mg Ti Cr Mn
Fe
Ni
Mo
310S
0.78
0.19
0.00
26.00
1.89
51.78
18.78
0.57
316L
1.13
0.00
0.00
17.22
1.15
68.54
9.63
2.33
Table 2 Properties of KNO3, NaNO3, NaNO2 and KCl. Salt
Melting Point, ℃
Decomposition point， ℃
Impurity content, ppm Cl-
SO42-
KNO3
337
400
145
416
NaNO3
307
380
168
11
NaNO2
271
320
49
35
KCl
771
—
19
Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
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3
2.2 Corrosion study of the substrates 2.2.1. Static corrosion test of the substrates The samples tested using isothermal static immersion were analyzed via gravimetric measurements. Firstly, cleaned specimens were placed in alumina crucibles and completely immersed in the molten mixture with the melt depth of 1.52cm. Then, the crucibles with the solid salt mixture and metal samples were placed in the middle of an electrical tube furnace, and start to be heated to the required temperature. In this experiments, the upper operating rated temperature of 500℃ was selected as the corrosion test temperature. The weight change of metal samples were measured at 168, 336, 504, 672, 840, 1008, 1176, 1344, 1512, 1680, 1848h to obtain the corrosion rate. And for each alloy, three additional identical samples were put in the same crucibles to get the average value for the corrosion tests, and the salt mixture after testing were investigated by SEM and XRD analysis. After removing the samples from the equipment they were cooled slowly in the distilled water at room temperature in order to remove the salt on the surface of metal samples. After that, they were then dried and weighed by five times. The formula (Eq. (1)) was used to calculate the average weight variation over times followed:
m mi m f S0 S0
Where mi is the initial mass of the specimen, mf is the mass of the sample at the selected time and S0 is the initial area of the specimen. The corrosion rate (CR) in micrometers per year can be calculated by the formula (Eq. (2)):
CR
K m S0 T D
Where K is a constant equal to 3.65×106, T is the time of immersion corrosion [days], and D is the alloy density [g/cm3].
m·S0-1 (mg·cm-2)
A 310S B 316L
B
(b)
9
0.16
A 0.08
Corrosion rate (m/y)
(a)
0.24
A 310S B 316L
6
3
B A
0.00
0
7
14
21
28
35
42
49
Time (days)
56
63
70
77
0
0
7
14
21
28
35
42
49
56
63
70
77
Time (days)
Fig. 1. Gravimetric corrosion curve (a) and Corrosion rate curve (b) of the 310S and 316L specimens at 500℃ by static isothermal immersion tests. 2.2.2. Characterization of the samples The previously tested specimens were characterized by field-emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD). FESEM-EDX was carried out by a FEI® Quanta 400F field-emission Scanning Electron Microscope. It was used to study the morphology and chemical composition of the surface and cross-section of the corroded samples after the isothermal immersion test. For cross-sectional of the corroded samples was mounted in a phenolic resin
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Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
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(SND®), grounding them with 240, 320, 400, 600, 1000, and 2400-grit silicon carbide papers. Distilled water was sprayed on the papers during the grounding process.
Fig. 2. Surface micrograph of the stainless steels (x2500) (a) 310S and (b) 316L after corrosion (1008h). XRD was carried out for the phase analysis of the corrosion products and the scales that on the surface of the specimens. XRD was performed using a PANalytical® (Empyrean model). The XRD patterns obtained were compared to the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS) to identify the phase. 3. Results and discussion 3.1. Static corrosion test The corrosion behaviour of the 310S and 316L austenitic stainless steel specimens in the quaternary molten salts was studied by static isothermal tests carried out at 500℃ for 1848h. The weight of the specimens subjected to static isothermal immersion tests was measured in the interval of 168h. The corrosion rate curves of the alloys was plotted in Fig.1. The results showed that the corrosion rate of these two austenitic stainless steels showed the highest value at the initial stage, and then gradually decrease to the constant value. By comparison, the corrosion rate of 310S is smaller than that of 316L.
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Fig. 3. Cross-sectional SEM image and the elemental distribution of 310S: (a) SEM image in the quaternary molten salts at 500℃ for 1008h, showing the elemental distribution of (b)O, (c)Fe, (d)Cr, (e)Ni and (f) EDX analysis .
Fig. 4. Cross-sectional SEM image and the elemental distribution of 316L: (a) SEM image in the quaternary molten salts at 500℃ for 1008h, showing the elemental distribution of (b)O, (c)Fe, (d)Cr, (e)Ni and (f) EDX analysis. 3.2 Characterization of the corroded samples The studies of the microstructure of the samples were performed through FESEM with the aim of determining the corrosion products formed on the substrate. Fig. 2 (a) and (b) show the surface appearance of the samples after 1008h of exposure to the quaternary molten salt in the static isothermal immersion tests. The surface appearance of the two samples is quite different. Both samples formed a dense corrosion layer composed of Fe 2O3, NiFe2O4 and NiCr2O4, the corrosive layer of 316L is looser than 310S, which were also detected in the cross-section of the samples shown in Fig. 3 and Fig. 4. The EDX analysis and element mappings of the cross-sections were carried out (see Fig. 3 and Fig. 4). It is clear that the thickness of the oxide layer on the sample is as follows: 310S <316L, as was observed in the cross-section micrographs (see Fig. 3 and Fig. 4). XRD measurements after the static isothermal immersion tests are presented in Fig. 5. XRD measurements confirmed the observed species in FESEM-EDX, i.e. It shows the predominant presence of Fe2O3 and as corrosion by-products. After 1848h of the testing, 316L samples exposed to the prepared molten salt had corrosion rate up to 1.14μm/year, which is greater than that of 310S. Similarly, according to the superficies micrographs, 316L is much
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Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
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more corrosive than 310S, since the surface of the 316L samples appeared quite more detached (see Fig.2 (a) and (b). This is due to that 310S has a higher nickel-chromium content than 316L.
6000
(a) 310S
Fe2O3
3000
2000
0 hour
NiCr2O4
4000
Fe2O3
Fe3O4
Intensity (counts)
Intensity (counts)
6000
Cr0.19Fe0.7Ni0.11
(b) 316L
Cr0.19Fe0.7Ni0.11
0 hour
1008 hours
1008 hours 0
20
40
60
2º
80
100
0
20
40
60
80
100
2º
Fig. 5. XRD analysis of the 310S (a) and 316L (b) specimens before and after 1008h of static isothermal immersion test. Different degrees of corrosion are also observed in the cross-sectional micrographs (see Fig. 3 and Fig. 4). The oxide layer of 2.903μm was observed on the 316L sample, while the 310S sample had a thinner layer of 1.613μm, which is due to less Cr and Ni content in 316L. As a result of the X-ray diffraction analysis (see Figure 5), the main corrosion products of 310S are Fe2O3 and Fe3O4, while the 316L corrosion products are Fe2O3 and NiCr2O4. For XRD results, The Gibbs free energy calculation results for the corrosion reaction are as follows: (4) (6) (7) (8) (9) (10) (11) ), the Gibbs free energy of the above reaction is all According to the Gibbs free energy criterion ( less than 0, indicating that the above reaction can be carried out spontaneously under the experimental conditions. And the spontaneous degree of reaction(3)(4)(7)(8)(10)(11) is greater, which is the decisive factor to control the corrosion rate. 4. Conclusions The isothermal immersion corrosion test of austenitic stainless steels has been conducted in contact with a new quaternary molten salt. The stainless steel of 310S and 316L were completely immersed in the quaternary molten salt under static isothermal conditions at 500℃ for 1848h. The results showed that 310S has smaller corrosion rate, which means it had the better corrosion resistance than 316L. The morphology of stainless steel were also obtained by FESEM, and it was found that a oxide layer has been formed on the interface between the substrate and salts. According to the elements analysis and possible corrosion reaction, it was confirmed that the component of this cover is mainly Fe2O3, NiFe2O4 and NiCr2O4. The corrosion rate reaches maximum value in a short time and then gradually decrease to a constant value. After calculation, the annul corrosion rate of 310S and 316L samples is 0.825μm/year and 1.14μm/year, respectively.
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Acknowledgments This work was supported by the funding of Nature Science Foundation of China (U1507113), Nature Science Foundation of China (51436009), Science and Technology Planning Project of Guang Dong Province (2015A010106006), Nature Science Foundation of Guangdong (2016A030313362). References [1]. Liu M, Tay N H S, Bell S, et al. Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies [J]. Renewable & Sustainable Energy Reviews, 2016, 53:1411-1432. [2]. García-Martín G, Lasanta M I, Encinas-Sánchez V, et al. Evaluation of corrosion resistance of A516 Steel in a molten nitrate salt mixture using a pilot plant facility for application in CSP plants [J]. Solar Energy Materials & Solar Cells, 2017, 161:226-231. [3]. Fernández A G, Pérez F J. Improvement of the corrosion properties in ternary molten nitrate salts for direct energy storage in CSP plants [J]. Solar Energy, 2016, 134:468-478. [4]. Dorcheh A S, Galetz M C. Slurry aluminizing: A solution for molten nitrate salt corrosion in concentrated solar power plants [J]. Solar Energy Materials & Solar Cells, 2016, 146:8-15. [5]. Dorcheh A S, Durham R N, Galetz M C. Corrosion behavior of stainless and low-chromium steels and IN625 in molten nitrate salts at 600 °C [J]. Solar Energy Materials & Solar Cells, 2016, 144:109-116. [6]. Fernández A G, Cortes M, Fuentealba E, et al. Corrosion properties of a ternary nitrate/nitrite molten salt in concentrated solar technology [J]. Renewable Energy, 2015, 80:177-183. [7]. Kruizenga A M, Mcconohy G. Molten Nitrate Salts at 600 and 680(deg)C: Thermophysical property changes and corrosion of High Temperature Nickel Alloys.[J]. Solar Energy, 2013, 103(103):242-252. [8]. Zhao C Y, Wu Z G. Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems [J]. Solar Energy Materials & Solar Cells, 2011, 95(12):3341–3346. [9]. Peng Q, Wei X, Ding J, et al. RESEARCH ON THE PREPARATION AND PROPERTIES OF MULTI-COMPONENT MOLTEN SALTS [J]. Acta Energiae Solaris Sinica, 2009, 30(12):1621-1626. [10] Eweka, Kerridge. Molten sodium nitrite-sodium nitrate-potassium nitrate eutectic: The reactions and spectra of iron(Ill), cobalt(II),nickel(II) and copper(II) compounds. Thermochimica Acta 1996, 290: 133-8.
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Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000
Energy Procedia 00 (2017) 000–000 ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia
Energy Procedia 142 Energy Procedia 00(2017) (2017)3590–3596 000–000 www.elsevier.com/locate/procedia
9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK
Corrosion resistance of 310S and 316L austenitic stainless steel in a The 15th International Symposium on District Heating and Cooling quaternary molten salt for concentrating solar power Assessing feasibility of using Xiao long Lia ,the Xiaolan Weib，Jianfeng Lua, the Jing heat Dinga,*demand-outdoor ，Weilong Wanga,* temperature function for a long-term district heat demand forecast School of Engineering , Sun Yat-sen University , Guangzhou 510006, PR China
a a
b bSchool
School of Chemistry and Chemical Engineering South China University of Technology, Guangzhou 510640, PR China a
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
Abstract
This paper evaluates the corrosion of 310S and 316L austenitic stainless steel in a quaternary molten salt KNO33-NaNO22-NaNO33-KCl (wt%, 50.35:38:6.65:5) at 500℃. The corrosion rates were determined using gravimetric analysis, which measures the weight loss over 1848h in order to identify the corrosion products with field-emission Abstract scanning electron microscope (FESEM) and X-ray diffraction (XRD). Theheating resultsnetworks showed are thatcommonly the corrosion rate of these two austenitic hassolutions the highest value at the District addressed in the literature as one ofstainless the moststeels effective for decreasing the initial stage,gas andemissions then decrease a constant value, require which is dueinvestments to the formation ofreturned protective oxide greenhouse from thefollowed building by sector. These systems high which are through thefilm heat on the Due interface preventclimate furtherconditions corrosion.andCompared to 316L, 310S hasheat better corrosion-resistance in the salt, sales. to thetochanged building renovation policies, demand in the future could decrease, since it contains more Cr and Ni element. The corrosion layer thickness of the 310S and 316L is 1.613μm and prolonging the investment return period. 2.903μm based on the cross-section micrographs, which is equal to 0.499μm/year and 1.460μm/year, respectively. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand
©forecast. 2017 The Authors. by Elsevier The districtPublished of Alvalade, locatedLtd. in Lisbon (Portugal), was used as a case study. The district is consisted of 665 © 2017 The Authors. Published by Ltd. committee of the 9th International Conference on Applied Energy. Peer-review under responsibility ofElsevier the scientific buildings that vary in both construction period and typology. Three weather scenarios (low, on medium, and three district Peer-review under responsibility of the scientific committee of the 9th International Conference Appliedhigh) Energy. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Keywords: molten Heat transfer Corrosion, Austenitic Stainless Steel and validated by the authors. comparedQuaternary with results fromsalt, a dynamic heatfluid, demand model, previously developed The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation 1.scenarios, Introduction the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease the rapid numberdevelopment of heating hours 22-139h and during the heating the season (depending on the combination of weather and Withinthe of of economy technology, energy consumption dramatically increases renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the resulting in serious pollution. For decades, new clean energy has been developed and is recognized as a potential coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and substitute of fossil fuel in the future. The concentrated solar power (CSP) with thermal energy storage (TES) makes improve the accuracy of heat estimations. [1]. solar energy utilization moredemand attractive than other renewable energies because of its high feasibility and efficiency [1]
solar energy utilization more attractive than other renewable energies because of its high feasibility and efficiency . [2-7], for its excellent thermal Nitrate molten salt is now being widely used as a heat storage and transfer media[2-7] © 2017 The Authors. Published by Elsevier Ltd. [8] [8] physical properties, such as the commercial Solar Salt (60wt% NaNO33/40wt% KNO33) and Hitec (53wt% KNO33 Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel/fax: +86-020-3933-2319. E-mail address: [email protected]; [email protected]
Cooling. E-mail address: [email protected]; [email protected]
Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy . 10.1016/j.egypro.2017.12.781
2
Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
3591
/40wt% NaNO2/7wt% NaNO3). However, the thermal stability under the high temperature is not satisfied, especially the corrosion issue brought serious operation safety issue. From this point of view, many scholars start to focus on the thermal stability of molten salt in the operating temperature range. Q. Peng et al. [9] added 5wt% KCl on the basis of Hitec salt, results showed that the best use temperature of the mixed molten salt is improved from 400~500℃ to 550℃, which reduces the loss of NO2- content in the molten salt and delays the deterioration of the molten salt. And also slightly lower the freezing point of molten salt. Eweka and Kerridge[10] studied the reactions between HITEC and copper(II), iron(Ill), cobalt(II) and nickel(II), these transition metals tended to form the most stable oxides, and the nitrite ion (NO2- ) transformed into nitrate iron (NO3-) through nitric oxide and nitrogen dioxide. It is still of importance to continue to investigate the corrosion behavior of various metal matrix in the different molten salts. In this study, We examined the corrosion behavior of 310S and 316L austenitic stainless steel in a new quaternary molten salt, and the mechanism of corrosion was further investigated. 2. Experimental procedure 2.1. Material 2.1.1. Substrate The corrosion tests were performed on commercial austenitic stainless steel 310S and 316L (GoodFellow®) with the composition shown in Table 1. Specimens were manufactured with nominal dimensions of 30×15×2mm and the surface was prepared by sanding with silicon carbide (SiC) abrasive paper. 2.1.2. Preparation and characterization of the salt mixture The alkali metal nitrates that were used as the corrosive agent in this work were KNO 3, NaNO3, NaNO2 and KCl. Some properties of interest and the impurity content of these salts are shown in Table 2. The composition in weight percentage of the multi-component molten salts used was of 50.35wt% KNO3/38wt% NaNO2/6.65wt% NaNO3/5wt% KCl. Table 1 Chemical composition of austenitic stainless steel 310S and 316L (wt%). Alloy O Mg Ti Cr Mn
Fe
Ni
Mo
310S
0.78
0.19
0.00
26.00
1.89
51.78
18.78
0.57
316L
1.13
0.00
0.00
17.22
1.15
68.54
9.63
2.33
Table 2 Properties of KNO3, NaNO3, NaNO2 and KCl. Salt
Melting Point, ℃
Decomposition point， ℃
Impurity content, ppm Cl-
SO42-
KNO3
337
400
145
416
NaNO3
307
380
168
11
NaNO2
271
320
49
35
KCl
771
—
19
Xiao long Li et al. / Energy Procedia 142 (2017) 3590–3596 Author name / Energy Procedia 00 (2017) 000–000
3592
3
2.2 Corrosion study of the substrates 2.2.1. Static corrosion test of the substrates The samples tested using isothermal static immersion were analyzed via gravimetric measurements. Firstly, cleaned specimens were placed in alumina crucibles and completely immersed in the molten mixture with the melt depth of 1.52cm. Then, the crucibles with the solid salt mixture and metal samples were placed in the middle of an electrical tube furnace, and start to be heated to the required temperature. In this experiments, the upper operating rated temperature of 500℃ was selected as the corrosion test temperature. The weight change of metal samples were measured at 168, 336, 504, 672, 840, 1008, 1176, 1344, 1512, 1680, 1848h to obtain the corrosion rate. And for each alloy, three additional identical samples were put in the same crucibles to get the average value for the corrosion tests, and the salt mixture after testing were investigated by SEM and XRD analysis. After removing the samples from the equipment they were cooled slowly in the distilled water at room temperature in order to remove the salt on the surface of metal samples. After that, they were then dried and weighed by five times. The formula (Eq. (1)) was used to calculate the average weight variation over times followed:
m mi m f S0 S0
Where mi is the initial mass of the specimen, mf is the mass of the sample at the selected time and S0 is the initial area of the specimen. The corrosion rate (CR) in micrometers per year can be calculated by the formula (Eq. (2)):
CR
K m S0 T D
Where K is a constant equal to 3.65×106, T is the time of immersion corrosion [days], and D is the alloy density [g/cm3].
m·S0-1 (mg·cm-2)
A 310S B 316L
B
(b)
9
0.16
A 0.08
Corrosion rate (m/y)
(a)
0.24
A 310S B 316L
6
3
B A
0.00
0
7
14
21
28
35
42
49
Time (days)
56
63
70
77
0
0
7
14
21
28
35
42
49
56
63
70
77
Time (days)
Fig. 1. Gravimetric corrosion curve (a) and Corrosion rate curve (b) of the 310S and 316L specimens at 500℃ by static isothermal immersion tests. 2.2.2. Characterization of the samples The previously tested specimens were characterized by field-emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray analysis (EDX) and X-Ray Diffraction (XRD). FESEM-EDX was carried out by a FEI® Quanta 400F field-emission Scanning Electron Microscope. It was used to study the morphology and chemical composition of the surface and cross-section of the corroded samples after the isothermal immersion test. For cross-sectional of the corroded samples was mounted in a phenolic resin
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(SND®), grounding them with 240, 320, 400, 600, 1000, and 2400-grit silicon carbide papers. Distilled water was sprayed on the papers during the grounding process.
Fig. 2. Surface micrograph of the stainless steels (x2500) (a) 310S and (b) 316L after corrosion (1008h). XRD was carried out for the phase analysis of the corrosion products and the scales that on the surface of the specimens. XRD was performed using a PANalytical® (Empyrean model). The XRD patterns obtained were compared to the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS) to identify the phase. 3. Results and discussion 3.1. Static corrosion test The corrosion behaviour of the 310S and 316L austenitic stainless steel specimens in the quaternary molten salts was studied by static isothermal tests carried out at 500℃ for 1848h. The weight of the specimens subjected to static isothermal immersion tests was measured in the interval of 168h. The corrosion rate curves of the alloys was plotted in Fig.1. The results showed that the corrosion rate of these two austenitic stainless steels showed the highest value at the initial stage, and then gradually decrease to the constant value. By comparison, the corrosion rate of 310S is smaller than that of 316L.
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Fig. 3. Cross-sectional SEM image and the elemental distribution of 310S: (a) SEM image in the quaternary molten salts at 500℃ for 1008h, showing the elemental distribution of (b)O, (c)Fe, (d)Cr, (e)Ni and (f) EDX analysis .
Fig. 4. Cross-sectional SEM image and the elemental distribution of 316L: (a) SEM image in the quaternary molten salts at 500℃ for 1008h, showing the elemental distribution of (b)O, (c)Fe, (d)Cr, (e)Ni and (f) EDX analysis. 3.2 Characterization of the corroded samples The studies of the microstructure of the samples were performed through FESEM with the aim of determining the corrosion products formed on the substrate. Fig. 2 (a) and (b) show the surface appearance of the samples after 1008h of exposure to the quaternary molten salt in the static isothermal immersion tests. The surface appearance of the two samples is quite different. Both samples formed a dense corrosion layer composed of Fe 2O3, NiFe2O4 and NiCr2O4, the corrosive layer of 316L is looser than 310S, which were also detected in the cross-section of the samples shown in Fig. 3 and Fig. 4. The EDX analysis and element mappings of the cross-sections were carried out (see Fig. 3 and Fig. 4). It is clear that the thickness of the oxide layer on the sample is as follows: 310S <316L, as was observed in the cross-section micrographs (see Fig. 3 and Fig. 4). XRD measurements after the static isothermal immersion tests are presented in Fig. 5. XRD measurements confirmed the observed species in FESEM-EDX, i.e. It shows the predominant presence of Fe2O3 and as corrosion by-products. After 1848h of the testing, 316L samples exposed to the prepared molten salt had corrosion rate up to 1.14μm/year, which is greater than that of 310S. Similarly, according to the superficies micrographs, 316L is much
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more corrosive than 310S, since the surface of the 316L samples appeared quite more detached (see Fig.2 (a) and (b). This is due to that 310S has a higher nickel-chromium content than 316L.
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Fig. 5. XRD analysis of the 310S (a) and 316L (b) specimens before and after 1008h of static isothermal immersion test. Different degrees of corrosion are also observed in the cross-sectional micrographs (see Fig. 3 and Fig. 4). The oxide layer of 2.903μm was observed on the 316L sample, while the 310S sample had a thinner layer of 1.613μm, which is due to less Cr and Ni content in 316L. As a result of the X-ray diffraction analysis (see Figure 5), the main corrosion products of 310S are Fe2O3 and Fe3O4, while the 316L corrosion products are Fe2O3 and NiCr2O4. For XRD results, The Gibbs free energy calculation results for the corrosion reaction are as follows: (4) (6) (7) (8) (9) (10) (11) ), the Gibbs free energy of the above reaction is all According to the Gibbs free energy criterion ( less than 0, indicating that the above reaction can be carried out spontaneously under the experimental conditions. And the spontaneous degree of reaction(3)(4)(7)(8)(10)(11) is greater, which is the decisive factor to control the corrosion rate. 4. Conclusions The isothermal immersion corrosion test of austenitic stainless steels has been conducted in contact with a new quaternary molten salt. The stainless steel of 310S and 316L were completely immersed in the quaternary molten salt under static isothermal conditions at 500℃ for 1848h. The results showed that 310S has smaller corrosion rate, which means it had the better corrosion resistance than 316L. The morphology of stainless steel were also obtained by FESEM, and it was found that a oxide layer has been formed on the interface between the substrate and salts. According to the elements analysis and possible corrosion reaction, it was confirmed that the component of this cover is mainly Fe2O3, NiFe2O4 and NiCr2O4. The corrosion rate reaches maximum value in a short time and then gradually decrease to a constant value. After calculation, the annul corrosion rate of 310S and 316L samples is 0.825μm/year and 1.14μm/year, respectively.
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Acknowledgments This work was supported by the funding of Nature Science Foundation of China (U1507113), Nature Science Foundation of China (51436009), Science and Technology Planning Project of Guang Dong Province (2015A010106006), Nature Science Foundation of Guangdong (2016A030313362). References [1]. Liu M, Tay N H S, Bell S, et al. Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies [J]. Renewable & Sustainable Energy Reviews, 2016, 53:1411-1432. [2]. García-Martín G, Lasanta M I, Encinas-Sánchez V, et al. Evaluation of corrosion resistance of A516 Steel in a molten nitrate salt mixture using a pilot plant facility for application in CSP plants [J]. Solar Energy Materials & Solar Cells, 2017, 161:226-231. [3]. Fernández A G, Pérez F J. Improvement of the corrosion properties in ternary molten nitrate salts for direct energy storage in CSP plants [J]. Solar Energy, 2016, 134:468-478. [4]. Dorcheh A S, Galetz M C. Slurry aluminizing: A solution for molten nitrate salt corrosion in concentrated solar power plants [J]. Solar Energy Materials & Solar Cells, 2016, 146:8-15. [5]. Dorcheh A S, Durham R N, Galetz M C. Corrosion behavior of stainless and low-chromium steels and IN625 in molten nitrate salts at 600 °C [J]. Solar Energy Materials & Solar Cells, 2016, 144:109-116. [6]. Fernández A G, Cortes M, Fuentealba E, et al. Corrosion properties of a ternary nitrate/nitrite molten salt in concentrated solar technology [J]. Renewable Energy, 2015, 80:177-183. [7]. Kruizenga A M, Mcconohy G. Molten Nitrate Salts at 600 and 680(deg)C: Thermophysical property changes and corrosion of High Temperature Nickel Alloys.[J]. Solar Energy, 2013, 103(103):242-252. [8]. Zhao C Y, Wu Z G. Thermal property characterization of a low melting-temperature ternary nitrate salt mixture for thermal energy storage systems [J]. Solar Energy Materials & Solar Cells, 2011, 95(12):3341–3346. [9]. Peng Q, Wei X, Ding J, et al. RESEARCH ON THE PREPARATION AND PROPERTIES OF MULTI-COMPONENT MOLTEN SALTS [J]. Acta Energiae Solaris Sinica, 2009, 30(12):1621-1626. [10] Eweka, Kerridge. Molten sodium nitrite-sodium nitrate-potassium nitrate eutectic: The reactions and spectra of iron(Ill), cobalt(II),nickel(II) and copper(II) compounds. Thermochimica Acta 1996, 290: 133-8.