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Effect of environmental condition on the chemical behavior of 690 alloy during fretting wear
Tribology International 146 (2020) 106202
Contents lists available at ScienceDirect
Tribology International journal homepage: http://www.elsevier.com/locate/triboint
Effect of environmental condition on the chemical behavior of 690 alloy during fretting wear Xue Mi a, Hai Xie a, Pan Tang b, Zhen-bing Cai b, Jin-fang Peng b, Min-hao Zhu b, * a b
Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, 610213, China Key Laboratory of Advanced Materials Technology, Ministry of Education, Southwest Jiaotong University, Chengdu, 610031, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical behavior 690 alloy Fretting wear XPS
The fretting capabilities of 690 alloy against 405 stainless steel were evaluated through a series of tube-plate tests in different experimental conditions. The results indicated that as the temperature increased in air, the wear volume and wear depth increased first until reaching a maximum value at 90 � C and then decreased. This was attributed to spinel oxides formed above 200 � C in combination with adhesive wear, which contributed to oxidation resistance and a reduction of interfacial wear, resulting in the decrease of wear volume and wear depth. Metallic nickel, iron and chromium were observed in water, which indicated low oxidation, associated with lower wear damage.
1. Introduction The steam generator is a crucial component in pressurized water reactor (PWR), which is the connection and barrier between primary and secondary class. However, its operation environment is extremely aggressive: high temperature, high pressure and corrosive [1,2]. Addi tional, it is impossible to avoid fretting wear or impact between steam generator tubes and anti-vibration bars [3]. Therefore, extensive effort has been directed towards analyzing and modelling the damage mech anisms of steam generator tubes. Although many studies have been carried out to explore the effect of temperature and environment on the wear behavior of steam generator tubes, there are still some questions on how these factors control the fretting wear process [4–9]. It was generally established that as the temperature increases, the wear volume and wear depth increases first until reaching a critical temperature, and then decreases. This is attributed to the transition of wear mechanisms or the formation of a protective layer [8–15]. Other investigations were centered on studying the composition of the oxide and transfer layers of fretted 690 alloy; e.g., in Ref. [5]. While some work has been devoted to investigating the oxidation and corrosion of steam generator materials at high tempera ture in air or in simulated water [16,17], less effort was directed towards understanding the chemical aspect of the steam generator tubes during fretting wear [5]. The latter has a significant influence on the fretting process, such as friction coefficient, wear volume and the morphology of
wear scars, especially the formation of the wear debris [18]. Depend on their composition, surface oxides or wear debris may play a different role in the fretting procedure, e.g., abrasive particles, load-bearing or lubricant [19]. Therefore, for better understanding of the fretting pro cess, it is absolutely necessary to examine the constituent of the wear particles after fretting tests. In this work, the chemical behavior of 690 alloy was investigated through a series of tube-plate fretting tests in different environmental conditions. After testing, the wear scars were examined through energy dispersive X-ray (EDX), electron probe microanalysis (EPMA) and X-ray Photoelectron Spectroscopy (XPS) to obtain the chemical information. 2. Experimental details The contacting materials studied in this work were 690 alloy and 405 stainless steel. More detail information of the specimens and test rig could be reported elsewhere [9,20]. The schematic diagram of fretting wear tester could be found in Refs. [9,20]. Tube sample (690 alloy) and plate specimen (405 stainless steel) were placed in an orientation, which generated a line contact. The plate sample was stationary and located in a water trough, while the tube specimen was oscillated with a hydraulic system. The fretting direction was parallel to the axis of the 690 alloy tube. To accomplish elevated temperature in water environment, a water supply system was appended to the water tank. During testing, the dynamic friction force and displacement were monitored in real-time. In
* Corresponding author. E-mail addresses: [email protected], [email protected] (M.-h. Zhu). https://doi.org/10.1016/j.triboint.2020.106202 Received 27 October 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 21 January 2020 0301-679X/© 2020 Elsevier Ltd. All rights reserved.
X. Mi et al.
Tribology International 146 (2020) 106202
this paper, two kinds of environmental conditions were selected; air and water solution. The latter was simulated PWR secondary water, which was a mixture of deionized water and ammonia water. Its pH level and conductance were approximately 9.0–9.1 and 3.0 μS/cm at room tem perature (RT). The wear tests were conducted under varied tempera tures ‘T’; RT, 60 � C and 90 � C in water, and RT, 90 � C, 150 � C, 200 � Cand 285 � C in air. Other test parameters; namely, number of cycles ‘N’, frequency ‘f’, displacement amplitude ‘D’ and normal load ‘Fn’ were kept constant at 1 � 105, 5 Hz, 100 μm and 40 N, respectively. After testing, the worn 690 alloy were examined through scanning electron microscopy (SEM, JOEL JSM-6610LV) and 3D optical micro scope (Bruker, Contour GT-I) for obtaining morphology information. Subsequently, the EDX (the appendant of SEM), EPMA (JXA-8230) and XPS (ESCALAB 250Xi) were used to acquire the chemical information of the worn surfaces. Following the wear scar morphology examination and surface chemical analysis, the worn 690 alloy tube were cleaned through APAC method [9,20] to remove wear debris. Consequently, the relatively accurate wear volume can be measured through the 3D optical microscope.
The mechanism of adhesive wear is also sensitive to the environ mental conditions; medium and temperature. This can be well quanti fied by observing the level of iron content Fe % in alloy 690. As Fig. 1 shows, the Fe % decreased from 19.83% to 9.75% as temperature changing from RT to 90 � C in water, which was induced by the increase of oxygen content. However, in air, as temperature increased from RT to 285 � C, the Fe % showed a considerable increase from 14.39% to 26.55%, which was attributed to transfer of material to the 690 tube sample. 3.2. EPMA analysis To obtain more accurate and detailed results of the chemical composition of worn surfaces, electron probe microanalysis was carried out. As shown in Fig. 2(a), in water at 90 � C, two types of compositions were detected on the worn surface from the backscattered electron imagine. The grey area (position 1) showed high content of nickel and chromium, while high contents of oxygen and iron were detected in the black area (position 2). The latter has remained wear particles, which led to abrasive wear. In air at 285 � C, the wear particles accumulated in the contacting zone and resulted in a rough grey area. On the other side, white speckled with grey in smooth area. The high content of iron (70.12%) and the low content of nickel (0.54%) were counted at a white area (position 3), which was associated with 405 stainless steel transfer layer. The chemical compositions of grey area (position 4 and position 5) were basically same, approximately, O, Cr, Fe and Ni with an atomically ratio of 25:4:10:1. Overall, the main wear particles were iron oxide, which indicated the material transferred from tube to plate.
3. Results 3.1. EDX analysis In water (as shown in Fig. 1), higher nickel content was observed, when compared with that in air, since the presence of water reduced the oxidation action. At RT, one can observe that while the Ni and O con tents were 39.44% and 22.32% (at %), respectively, in water, they changed to 9.51% and 67.98% in air. As the temperature increases in water environment, the oxidation reaction is accelerated, resulting in a higher content of oxygen; changing from 22.32% to 63.77% as the temperature increased from RT and 90 � C. On the other hand, the oxy gen content remained almost the same (~66%) as the temperature increased from RT to 285 � C.
3.3. XPS analysis High-resolution XPS scans of the fretted surfaces were also con ducted to obtain information about the chemical state of the surface oxide. In this paper, the reference peak for charge correction was the C
Fig. 1. The EDX analysis of wear scars. 2
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Fig. 2. Backscattered electron image of the worn surfaces of 690 alloy.
1s peak, whose bonding energy was 284.8 eV. Fig. 3 shows the XPS results in water. As shown in Fig. 3(a), a peak of 574 eV was detected as a result of the existence of metallic chromium in water [21–23]. The bonding energy of Cr 2p3/2 peak was estimated at 576.7 � 0.2 eV, which demonstrated Cr (III) in oxide [22–25]. Furthermore, there was another peak Cr 2p1/2, whose binding energy was about 9.8 eV larger than that of Cr 2p3/2 peak. As shown in Fig. 3(b), the main peak was observed at 711.8 � 0.2 eV from Fe 2p3/2 accom panied with a small peak of metallic iron at 706.6 � 0.2 eV. In the nickel spectrum, the Ni 2p3/2 had an obvious sharp peak at 852.6 � 0.2 eV attributed to the appearance of metallic nickel and a broad feature at 855.9 � 0.2 eV which corresponded to Ni(II) [16,21,23–25]. Addition ally, the satellite peak was obviously separated from the main peak. The binding energy of Ni 2p3/2 peak was roughly 5.6 eV lower than that of satellite peak. In Fig. 3(d), the O 1s peak consisted of two overlapping peaks, one at 530 � 0.2 eV indicating O2 in oxide and one at 531.5 � 0.2 eV due to OH [16,21,24,26]. In general, in water, the spectrum of
all the elements did not change significantly with temperature. Fig. 4 presents the XPS Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in air. There were obvious differences of the position of the peak and the shape of the spectrum between lower temperature (RT and 90 � C) and higher temperature (200 � C and 285 � C). The values of the main peaks of lower temperature were higher than that of higher temperature. The values of the main peaks of Cr 2p and Ni 2p spectrums at higher temperature in air were similar to that in water. On the other hand, the value of Fe 2p3/2 in water was 1 eV higher than that of in higher temperature air. The peak occurring at 710.8 eV was owing to the presence of both Fe (II) and Fe (III) [23–26]. As seen in Fig. 4(b), at higher temperature, a small satellite peak of Fe 2p3/2 for Fe2O3 was observed at roughly 8.4 eV offset the major Fe 2p3/2 peak [26–29]. In Fig. 4(c), the satellite peaks of Ni 2p were observably separated from the main peaks except for 90 � C. At RT, the O 1s spec trum showed three component at 530 � 0.2 eV, 531.5 � 0.2 eV and 535 � 0.2 eV, indicating oxide, hydroxyl ions and satellite, respectively [16,
Fig. 3. XPS high-resolution of Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in water. 3
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Fig. 4. XPS high-resolution of Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in air.
21,24,26,30]. In air, huge changes occurred in the chemical composition as the temperature range from RT to 285 � C. In order to acquire more detail data of Cr 2p2/3, Ni 2p2/3 and Fe 2p2/3 spectra, a software XPSPEAK4.1 was used to decompose peaks. During the analysis, the Gaussian-Lorentzian peak shapes and Shirley back ground were used. Fig. 5 and Fig. 6 show the peak decomposition of Cr 2p2/3 spectra and Ni 2p2/3 spectra at RT and 90 � C in different test environments, respectively. As shown in Fig. 5, the Cr 2p2/3 peak was systematically decomposed into two components in water. Compared with the data in the literatures [21–25]; the peak at 574.2 eV was assigned to metallic Cr, the peak at approximately 576.6 eV to Cr (III). In air, Cr (III) (BE ¼ 576.82 eV at RT and BE ¼ 576.84 eV at 90 � C) and CrO3 (BE ¼ 580.1 eV) were considered to exist on the worn surfaces [23,31]. In literatures [23, 32,33], the peak position of FeCr2O4 and NiCr2O4 were, respectively, 577 � 0.2 eV and 576.2 � 0.2 eV, and Cr2O3 (BE ¼ 576.6 � 0.2 eV)
located between them. Therefore, the presence of Cr (III) peaks in Fig. 5 were characteristic of Cr2O3, FeCr2O4 and NiCr2O4. Besides, the ratio of these three components was different in the different testing environment. In water, the fitting results of Ni 2p2/3 spectra showed four individual peaks at 852.6 eV, 854.6 eV, 856.2 � 0.1 eV and 861.2 � 0.4 eV rep resenting metallic, NiO, Ni(II) and satellite, respectively [23,34] (in Fig. 6). However, there was no metallic Ni peak detected in air. In the published papers, the binding energies of NiFe2O4 and NiCr2O4 were about 856 eV and 855.8 eV, respectively [23]. Thus, the peak of Ni(II) occurring in Fig. 6 might be a consequence of a mixture of NiFe2O4 and NiCr2O4. As shown in Fig. 7(a), at 285 � C, the Cr 2p2/3 spectrum in air showed only one peak at 576.45 eV, which was much closer to the binding en ergy of Cr2O3. As shown in Fig. 7(b), the values obtained from the curve fits for FeO and Fe2O3 spectra were 709.6 eV and 711.6 eV, respectively.
Fig. 5. Results of fitting Cr 2p2/3 spectra at RT and 90 � C in different test environments. 4
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Fig. 6. Results of fitting Ni 2p2/3 spectra at RT and 90 � C in different test environments.
Fig. 7. Results of fitting Cr 2p2/3, Fe 2p2/3 and Ni 2p2/3 spectra at 285 � C.
In addition, one peak located at 710.65 eV due to spinel oxides such as Fe3O4 (BE ¼ 710.8 eV), FeCr2O4 (BE ¼ 710.3 eV) and NiFe2O4 (BE ¼ 710.7 eV) in air [23,26]. Comparing Figs. 6 to 7(c), the temperature elevated in air did not influence on the compounds of nickel. As illus trated in Fig. 7(d), the O 1s spectrum was decomposed into two com ponents, which were oxide and hydroxyl ions. Fig. 8 shows the effect of temperature and environment on the wear volume for 690 alloy tube. It was distinct that the wear volume in air was remarkably larger than that of water. The values of wear volume at RT and 90 � C in air outnumber that of water by approximately 40 and 19, respectively. When the temperature was raised in air, the wear volume of 690 alloy tube increased for RT � T � 90 � C, and then dramatically decreased in the range 90 � C � T � 285 � C. The wear volume of 90 � C
water was very close to that of 200 � C air, which was about double that of 285 � C air. Additionally, the wear volume of 285 � C air was approx imately 2 times of the wear volume of 60 � C water. The figure demon strated the significant effect of the environment on the wear volume. 4. Discussion In air, chromium existed in Cr2O3, FeCr2O4 and CrO3 at lower tem perature (as seen in Fig. 5), but in Cr2O3, FeCr2O4 at higher temperature (as shown in Fig. 7). Slight changes occurred in the existed forms of nickel with changes in temperature and environment. The nickel con sisted of three major forms, NiO, NiFe2O4 and NiCr2O4. With the increasing temperature, more spinel oxides generated. It might be the 5
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smaller at higher temperature, as shown in Figs. 8 and 9. On the other hand, the constituents of the oxide layer almost showed no changes with the temperature increase in water (Figs. 5 and 6). This may be the reason for no transition temperature appeared in water. Unlike in air, the metallic Cr, Fe and Ni could be observed in all tem peratures water. One can, therefore, conclude that in water, the main mechanisms were abrasive wear and delamination [9,20]. Additionally, with increasing temperature, the material damage by delamination was more dominant, leading to the increase of wear volume [9,36]. In comparison with air environment, the main wear mechanism of lower temperature air was same as that of water, however, the wear damage was less. 5. Conclusions Fretting wear performance of 690 alloy tube/405 stainless steel plate has been examined in different environmental conditions. The following conclusions could be obtained through observations and analysis:
Fig. 8. The wear volume of 690 alloy tube in different environmental condi tions after APAC.
(1) Compared to air, unoxidized metal elements were observed in water, associated with lower oxidation and wear damage. (2) The chemical state of chromium showed large variations in different testing environments. CrO3 was only formed in air because of higher oxidation. The chemical state of nickel, how ever, almost remained unchanged. (3) The formation of spinel oxides and adhesive layer seemed to protect the surfaces and led to lower volumetric wear damage at
reason for smooth area formatted in the contact zone at higher tem perature as shown in Fig. 1. Spinel oxides, such as Cr2O3, FeCr2O4 and Fe3O4, acted as protective films due to their oxidation resistance [18, 35]. Furthermore, it was evident that transfer layers were observed at higher temperature (as shown in Figs. 1, Figs. 2 and 9), resulting in lack of nickel in partial region [9]. The transfer layers led to a reduction of interfacial contact. Consequently, the wear depth and wear volume were
Fig. 9. The profile micrographs of the worn tubes in elevated temperature air before APAC. The left side is the 3-D profile micrographs; the right side is the cor responding 2-D profile micrographs of the red dash lines in the left side. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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200 � Cand 285 � C. The main constituents of spinel oxides were Cr2O3, FeCr2O4 and Fe3O4. (4) The wear zone was generally characterized by being oxygen-rich and nickel-poor, especially at and above 200 � C. Smooth area, white speckled with grey, could be observed in the fretted area at 285 � C, which indicated the material transferred from tube to plate.
[15] Korashy A, Attia H, Thomson V, et al. Characterization of fretting wear of cobaltbased superalloys at high temperature for aero-engine combustor components. Wear 2015;330:327–37. [16] Zhang Z, Wang J, Han EH, et al. Influence of dissolved oxygen on oxide films of Alloy 690TT with different surface status in simulated primary water. Corrosion Sci 2011;53(11):3623–35. [17] Carrette F, Lafont MC, Chatainier G, et al. Analysis and TEM examination of corrosion scales grown on Alloy 690 exposed to pressurized water at 3RT. Surf Interface Anal 2002;34(1):135–8. [18] Stott FH. The role of oxidation in the wear of alloys. Tribol Int 1998;31(1):61–71. [19] Lee YH, Kim IS. The effect of subsurface deformation on the wear behavior of steam generator tube materials. Wear 2002;253(3):438–47. [20] Mi X, Cai ZB, Xiong XM, et al. Investigation on fretting wear behavior of 690 alloy in water under various temperatures. Tribol Int 2016;100:400–9. [21] Fujimoto S, Kim WS, Sato M, et al. Characterization of oxide films formed on Alloy 600 and Alloy 690 in simulated PWR primary water by using hard X-ray photoelectron spectroscopy. J Solid State Electrochem 2015;19(12):3521–31. [22] Rokosz K, Hryniewicz T, Simon F, et al. XPS analysis of AISI 304L stainless steel surface after electropolishing. Adv Mater Sci 2015;15(1):21–9. [23] Biesinger MC, Payne BP, Grosvenor AP, et al. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Appl Surf Sci 2011;257(7):2717–30. [24] Allen GC, Dyke JM, Harris SJ, et al. The oxidation of Inconel-690 alloy at 600 K in air. Appl Surf Sci 1988;31(2):220–38. [25] Jang HJ, Kwon HS. Effects of film formation conditions on the chemical composition and the semiconducting properties of the passive film on alloy 690. Corr Sci Technol 2006;5(4):141–8. [26] Mills P, Sullivan JL. A study of the core level electrons in iron and its three oxides by means of X-ray photoelectron spectroscopy. J Phys Appl Phys 1983;16(5):723. [27] Yamashita T, Hayes P. Analysis of XPS spectra of Fe2þ and Fe3þ ions in oxide materials. Appl Surf Sci 2008;254(8):2441–9. [28] Grosvenor AP, Kobe BA, Biesinger MC, et al. Investigation of multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds. Surf Interface Anal: Int J Devot Develop Appl Tech Anal Surf Interfaces Thin Films 2004;36(12):1564–74. [29] Nawale AB, Kanhe NS, Patil KR, et al. Magnetic properties of thermal plasma synthesized nanocrystalline nickel ferrite (NiFe2O4). J Alloys Compd 2011;509 (12):4404–13. [30] Schulze PD, Shaffer SL, Hance RL, et al. Adsorption of water on rhenium studied by XPS. J Vac Sci Technol 1983;1(1):97–9. [31] Shuttleworth D. Preparation of metal-polymer dispersions by plasma techniques. An ESCA investigation. J Phys Chem 1980;84(12):1629–34. [32] Allen GC, Harris SJ, Jutson JA, et al. A study of a number of mixed transition metal oxide spinels using X-ray photoelectron spectroscopy. Appl Surf Sci 1989;37(1): 111–34. [33] Moffat TP, Latanision RM, Ruf RR. An X-ray photoelectron spectroscopy study of chromium-metalloid alloys—III. Electrochim Acta 1995;40(11):1723–34. [34] Grosvenor AP, Biesinger MC, Smart RSC, et al. New interpretations of XPS spectra of nickel metal and oxides. Surf Sci 2006;600(9):1771–9. [35] Stott FH, Jordan MP. The effects of load and substrate hardness on the development and maintenance of wear-protective layers during sliding at elevated temperatures. Wear 2001;250(1):391–400. [36] Suh NP. An overview of the delamination theory of wear. Wear 1977;44(1):1–16.
Acknowledgement The authors were very appreciated the supported from National Science Foundation of China (51375407, 51575459, U1530136, 51627806). The constructive comments of Professor Helmi Attia, McGill University, are greatly appreciated. References [1] Yun Jae Yong, Park Myung Chul, Su Shin Gyeong, et al. Effects of amplitude and frequency on the wear mode change of Inconel 690 SG tube mated with SUS 409. Wear 2014;313:83–8. [2] Zinkle SJ, Was GS. Materials challenges in nuclear energy. Acta Mater 2013;61: 735–58. [3] Attia MH. Fretting fatigue and wear damage of structural components in nuclear power stations—fitness for service and life management perspective. Tribol Int 2006;39(10):1294–304. [4] Kim DG, Lee YZ. Experimental investigation on sliding and fretting wear of steam generator tube materials. Wear 2001;250(1):673–80. [5] Zhang XY, Ren PD, Peng JF, Zhu MH. Fretting wear behavior of Inconel 690 in hydrazine environments. Trans Nonferrous Metals Soc China 2014;24(2):360–7. [6] Xin L, Yang BB, Wang ZH, et al. Effect of normal force on fretting wear behavior and mechanism of Alloy 690TT in high temperature water. Wear 2016;368:210–8. [7] Wang ZH, Lu YH, Li J, et al. Effect of pH value on the fretting wear behavior of Inconel 690 alloy. Tribol Int 2016;95:162–9. [8] Iwabuchi A. Fretting wear of Inconel 625 at high temperature and in high vacuum. Wear 1985;106(1–3):163–75. [9] Mi X, Wang WX, Xiong XM, et al. Investigation of fretting wear behavior of Inconel 690 alloy in tube/plate contact configuration. Wear 2015;328:582–90. [10] Hong JK, Kim IS. Environment effects on the reciprocating wear of Inconel 690 steam generator tubes. Wear 2003;255(7):1174–82. [11] Waterhouse RB. Fretting at high temperatures. Tribol Int 1981;14(4):203–7. [12] Stott FH. High-temperature sliding wear of metals. Tribol Int 2002;35(8):489–95. [13] Okonkwo PC, Kelly G, Rolfe BF, et al. The effect of temperature on sliding wear of steel-tool steel pairs. Wear 2012;282:22–30. [14] Hager CH, Sanders J, Sharma S, et al. The effect of temperature on gross slip fretting wear of cold-sprayed nickel coatings on Ti6Al4V interfaces. Tribol Int 2009;42(3):491–502.
7
Contents lists available at ScienceDirect
Tribology International journal homepage: http://www.elsevier.com/locate/triboint
Effect of environmental condition on the chemical behavior of 690 alloy during fretting wear Xue Mi a, Hai Xie a, Pan Tang b, Zhen-bing Cai b, Jin-fang Peng b, Min-hao Zhu b, * a b
Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu, 610213, China Key Laboratory of Advanced Materials Technology, Ministry of Education, Southwest Jiaotong University, Chengdu, 610031, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Chemical behavior 690 alloy Fretting wear XPS
The fretting capabilities of 690 alloy against 405 stainless steel were evaluated through a series of tube-plate tests in different experimental conditions. The results indicated that as the temperature increased in air, the wear volume and wear depth increased first until reaching a maximum value at 90 � C and then decreased. This was attributed to spinel oxides formed above 200 � C in combination with adhesive wear, which contributed to oxidation resistance and a reduction of interfacial wear, resulting in the decrease of wear volume and wear depth. Metallic nickel, iron and chromium were observed in water, which indicated low oxidation, associated with lower wear damage.
1. Introduction The steam generator is a crucial component in pressurized water reactor (PWR), which is the connection and barrier between primary and secondary class. However, its operation environment is extremely aggressive: high temperature, high pressure and corrosive [1,2]. Addi tional, it is impossible to avoid fretting wear or impact between steam generator tubes and anti-vibration bars [3]. Therefore, extensive effort has been directed towards analyzing and modelling the damage mech anisms of steam generator tubes. Although many studies have been carried out to explore the effect of temperature and environment on the wear behavior of steam generator tubes, there are still some questions on how these factors control the fretting wear process [4–9]. It was generally established that as the temperature increases, the wear volume and wear depth increases first until reaching a critical temperature, and then decreases. This is attributed to the transition of wear mechanisms or the formation of a protective layer [8–15]. Other investigations were centered on studying the composition of the oxide and transfer layers of fretted 690 alloy; e.g., in Ref. [5]. While some work has been devoted to investigating the oxidation and corrosion of steam generator materials at high tempera ture in air or in simulated water [16,17], less effort was directed towards understanding the chemical aspect of the steam generator tubes during fretting wear [5]. The latter has a significant influence on the fretting process, such as friction coefficient, wear volume and the morphology of
wear scars, especially the formation of the wear debris [18]. Depend on their composition, surface oxides or wear debris may play a different role in the fretting procedure, e.g., abrasive particles, load-bearing or lubricant [19]. Therefore, for better understanding of the fretting pro cess, it is absolutely necessary to examine the constituent of the wear particles after fretting tests. In this work, the chemical behavior of 690 alloy was investigated through a series of tube-plate fretting tests in different environmental conditions. After testing, the wear scars were examined through energy dispersive X-ray (EDX), electron probe microanalysis (EPMA) and X-ray Photoelectron Spectroscopy (XPS) to obtain the chemical information. 2. Experimental details The contacting materials studied in this work were 690 alloy and 405 stainless steel. More detail information of the specimens and test rig could be reported elsewhere [9,20]. The schematic diagram of fretting wear tester could be found in Refs. [9,20]. Tube sample (690 alloy) and plate specimen (405 stainless steel) were placed in an orientation, which generated a line contact. The plate sample was stationary and located in a water trough, while the tube specimen was oscillated with a hydraulic system. The fretting direction was parallel to the axis of the 690 alloy tube. To accomplish elevated temperature in water environment, a water supply system was appended to the water tank. During testing, the dynamic friction force and displacement were monitored in real-time. In
* Corresponding author. E-mail addresses: [email protected], [email protected] (M.-h. Zhu). https://doi.org/10.1016/j.triboint.2020.106202 Received 27 October 2019; Received in revised form 15 January 2020; Accepted 17 January 2020 Available online 21 January 2020 0301-679X/© 2020 Elsevier Ltd. All rights reserved.
X. Mi et al.
Tribology International 146 (2020) 106202
this paper, two kinds of environmental conditions were selected; air and water solution. The latter was simulated PWR secondary water, which was a mixture of deionized water and ammonia water. Its pH level and conductance were approximately 9.0–9.1 and 3.0 μS/cm at room tem perature (RT). The wear tests were conducted under varied tempera tures ‘T’; RT, 60 � C and 90 � C in water, and RT, 90 � C, 150 � C, 200 � Cand 285 � C in air. Other test parameters; namely, number of cycles ‘N’, frequency ‘f’, displacement amplitude ‘D’ and normal load ‘Fn’ were kept constant at 1 � 105, 5 Hz, 100 μm and 40 N, respectively. After testing, the worn 690 alloy were examined through scanning electron microscopy (SEM, JOEL JSM-6610LV) and 3D optical micro scope (Bruker, Contour GT-I) for obtaining morphology information. Subsequently, the EDX (the appendant of SEM), EPMA (JXA-8230) and XPS (ESCALAB 250Xi) were used to acquire the chemical information of the worn surfaces. Following the wear scar morphology examination and surface chemical analysis, the worn 690 alloy tube were cleaned through APAC method [9,20] to remove wear debris. Consequently, the relatively accurate wear volume can be measured through the 3D optical microscope.
The mechanism of adhesive wear is also sensitive to the environ mental conditions; medium and temperature. This can be well quanti fied by observing the level of iron content Fe % in alloy 690. As Fig. 1 shows, the Fe % decreased from 19.83% to 9.75% as temperature changing from RT to 90 � C in water, which was induced by the increase of oxygen content. However, in air, as temperature increased from RT to 285 � C, the Fe % showed a considerable increase from 14.39% to 26.55%, which was attributed to transfer of material to the 690 tube sample. 3.2. EPMA analysis To obtain more accurate and detailed results of the chemical composition of worn surfaces, electron probe microanalysis was carried out. As shown in Fig. 2(a), in water at 90 � C, two types of compositions were detected on the worn surface from the backscattered electron imagine. The grey area (position 1) showed high content of nickel and chromium, while high contents of oxygen and iron were detected in the black area (position 2). The latter has remained wear particles, which led to abrasive wear. In air at 285 � C, the wear particles accumulated in the contacting zone and resulted in a rough grey area. On the other side, white speckled with grey in smooth area. The high content of iron (70.12%) and the low content of nickel (0.54%) were counted at a white area (position 3), which was associated with 405 stainless steel transfer layer. The chemical compositions of grey area (position 4 and position 5) were basically same, approximately, O, Cr, Fe and Ni with an atomically ratio of 25:4:10:1. Overall, the main wear particles were iron oxide, which indicated the material transferred from tube to plate.
3. Results 3.1. EDX analysis In water (as shown in Fig. 1), higher nickel content was observed, when compared with that in air, since the presence of water reduced the oxidation action. At RT, one can observe that while the Ni and O con tents were 39.44% and 22.32% (at %), respectively, in water, they changed to 9.51% and 67.98% in air. As the temperature increases in water environment, the oxidation reaction is accelerated, resulting in a higher content of oxygen; changing from 22.32% to 63.77% as the temperature increased from RT and 90 � C. On the other hand, the oxy gen content remained almost the same (~66%) as the temperature increased from RT to 285 � C.
3.3. XPS analysis High-resolution XPS scans of the fretted surfaces were also con ducted to obtain information about the chemical state of the surface oxide. In this paper, the reference peak for charge correction was the C
Fig. 1. The EDX analysis of wear scars. 2
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Fig. 2. Backscattered electron image of the worn surfaces of 690 alloy.
1s peak, whose bonding energy was 284.8 eV. Fig. 3 shows the XPS results in water. As shown in Fig. 3(a), a peak of 574 eV was detected as a result of the existence of metallic chromium in water [21–23]. The bonding energy of Cr 2p3/2 peak was estimated at 576.7 � 0.2 eV, which demonstrated Cr (III) in oxide [22–25]. Furthermore, there was another peak Cr 2p1/2, whose binding energy was about 9.8 eV larger than that of Cr 2p3/2 peak. As shown in Fig. 3(b), the main peak was observed at 711.8 � 0.2 eV from Fe 2p3/2 accom panied with a small peak of metallic iron at 706.6 � 0.2 eV. In the nickel spectrum, the Ni 2p3/2 had an obvious sharp peak at 852.6 � 0.2 eV attributed to the appearance of metallic nickel and a broad feature at 855.9 � 0.2 eV which corresponded to Ni(II) [16,21,23–25]. Addition ally, the satellite peak was obviously separated from the main peak. The binding energy of Ni 2p3/2 peak was roughly 5.6 eV lower than that of satellite peak. In Fig. 3(d), the O 1s peak consisted of two overlapping peaks, one at 530 � 0.2 eV indicating O2 in oxide and one at 531.5 � 0.2 eV due to OH [16,21,24,26]. In general, in water, the spectrum of
all the elements did not change significantly with temperature. Fig. 4 presents the XPS Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in air. There were obvious differences of the position of the peak and the shape of the spectrum between lower temperature (RT and 90 � C) and higher temperature (200 � C and 285 � C). The values of the main peaks of lower temperature were higher than that of higher temperature. The values of the main peaks of Cr 2p and Ni 2p spectrums at higher temperature in air were similar to that in water. On the other hand, the value of Fe 2p3/2 in water was 1 eV higher than that of in higher temperature air. The peak occurring at 710.8 eV was owing to the presence of both Fe (II) and Fe (III) [23–26]. As seen in Fig. 4(b), at higher temperature, a small satellite peak of Fe 2p3/2 for Fe2O3 was observed at roughly 8.4 eV offset the major Fe 2p3/2 peak [26–29]. In Fig. 4(c), the satellite peaks of Ni 2p were observably separated from the main peaks except for 90 � C. At RT, the O 1s spec trum showed three component at 530 � 0.2 eV, 531.5 � 0.2 eV and 535 � 0.2 eV, indicating oxide, hydroxyl ions and satellite, respectively [16,
Fig. 3. XPS high-resolution of Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in water. 3
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Fig. 4. XPS high-resolution of Cr 2p, Fe 2p, Ni 2p and O 1s spectra at different temperatures in air.
21,24,26,30]. In air, huge changes occurred in the chemical composition as the temperature range from RT to 285 � C. In order to acquire more detail data of Cr 2p2/3, Ni 2p2/3 and Fe 2p2/3 spectra, a software XPSPEAK4.1 was used to decompose peaks. During the analysis, the Gaussian-Lorentzian peak shapes and Shirley back ground were used. Fig. 5 and Fig. 6 show the peak decomposition of Cr 2p2/3 spectra and Ni 2p2/3 spectra at RT and 90 � C in different test environments, respectively. As shown in Fig. 5, the Cr 2p2/3 peak was systematically decomposed into two components in water. Compared with the data in the literatures [21–25]; the peak at 574.2 eV was assigned to metallic Cr, the peak at approximately 576.6 eV to Cr (III). In air, Cr (III) (BE ¼ 576.82 eV at RT and BE ¼ 576.84 eV at 90 � C) and CrO3 (BE ¼ 580.1 eV) were considered to exist on the worn surfaces [23,31]. In literatures [23, 32,33], the peak position of FeCr2O4 and NiCr2O4 were, respectively, 577 � 0.2 eV and 576.2 � 0.2 eV, and Cr2O3 (BE ¼ 576.6 � 0.2 eV)
located between them. Therefore, the presence of Cr (III) peaks in Fig. 5 were characteristic of Cr2O3, FeCr2O4 and NiCr2O4. Besides, the ratio of these three components was different in the different testing environment. In water, the fitting results of Ni 2p2/3 spectra showed four individual peaks at 852.6 eV, 854.6 eV, 856.2 � 0.1 eV and 861.2 � 0.4 eV rep resenting metallic, NiO, Ni(II) and satellite, respectively [23,34] (in Fig. 6). However, there was no metallic Ni peak detected in air. In the published papers, the binding energies of NiFe2O4 and NiCr2O4 were about 856 eV and 855.8 eV, respectively [23]. Thus, the peak of Ni(II) occurring in Fig. 6 might be a consequence of a mixture of NiFe2O4 and NiCr2O4. As shown in Fig. 7(a), at 285 � C, the Cr 2p2/3 spectrum in air showed only one peak at 576.45 eV, which was much closer to the binding en ergy of Cr2O3. As shown in Fig. 7(b), the values obtained from the curve fits for FeO and Fe2O3 spectra were 709.6 eV and 711.6 eV, respectively.
Fig. 5. Results of fitting Cr 2p2/3 spectra at RT and 90 � C in different test environments. 4
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Fig. 6. Results of fitting Ni 2p2/3 spectra at RT and 90 � C in different test environments.
Fig. 7. Results of fitting Cr 2p2/3, Fe 2p2/3 and Ni 2p2/3 spectra at 285 � C.
In addition, one peak located at 710.65 eV due to spinel oxides such as Fe3O4 (BE ¼ 710.8 eV), FeCr2O4 (BE ¼ 710.3 eV) and NiFe2O4 (BE ¼ 710.7 eV) in air [23,26]. Comparing Figs. 6 to 7(c), the temperature elevated in air did not influence on the compounds of nickel. As illus trated in Fig. 7(d), the O 1s spectrum was decomposed into two com ponents, which were oxide and hydroxyl ions. Fig. 8 shows the effect of temperature and environment on the wear volume for 690 alloy tube. It was distinct that the wear volume in air was remarkably larger than that of water. The values of wear volume at RT and 90 � C in air outnumber that of water by approximately 40 and 19, respectively. When the temperature was raised in air, the wear volume of 690 alloy tube increased for RT � T � 90 � C, and then dramatically decreased in the range 90 � C � T � 285 � C. The wear volume of 90 � C
water was very close to that of 200 � C air, which was about double that of 285 � C air. Additionally, the wear volume of 285 � C air was approx imately 2 times of the wear volume of 60 � C water. The figure demon strated the significant effect of the environment on the wear volume. 4. Discussion In air, chromium existed in Cr2O3, FeCr2O4 and CrO3 at lower tem perature (as seen in Fig. 5), but in Cr2O3, FeCr2O4 at higher temperature (as shown in Fig. 7). Slight changes occurred in the existed forms of nickel with changes in temperature and environment. The nickel con sisted of three major forms, NiO, NiFe2O4 and NiCr2O4. With the increasing temperature, more spinel oxides generated. It might be the 5
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smaller at higher temperature, as shown in Figs. 8 and 9. On the other hand, the constituents of the oxide layer almost showed no changes with the temperature increase in water (Figs. 5 and 6). This may be the reason for no transition temperature appeared in water. Unlike in air, the metallic Cr, Fe and Ni could be observed in all tem peratures water. One can, therefore, conclude that in water, the main mechanisms were abrasive wear and delamination [9,20]. Additionally, with increasing temperature, the material damage by delamination was more dominant, leading to the increase of wear volume [9,36]. In comparison with air environment, the main wear mechanism of lower temperature air was same as that of water, however, the wear damage was less. 5. Conclusions Fretting wear performance of 690 alloy tube/405 stainless steel plate has been examined in different environmental conditions. The following conclusions could be obtained through observations and analysis:
Fig. 8. The wear volume of 690 alloy tube in different environmental condi tions after APAC.
(1) Compared to air, unoxidized metal elements were observed in water, associated with lower oxidation and wear damage. (2) The chemical state of chromium showed large variations in different testing environments. CrO3 was only formed in air because of higher oxidation. The chemical state of nickel, how ever, almost remained unchanged. (3) The formation of spinel oxides and adhesive layer seemed to protect the surfaces and led to lower volumetric wear damage at
reason for smooth area formatted in the contact zone at higher tem perature as shown in Fig. 1. Spinel oxides, such as Cr2O3, FeCr2O4 and Fe3O4, acted as protective films due to their oxidation resistance [18, 35]. Furthermore, it was evident that transfer layers were observed at higher temperature (as shown in Figs. 1, Figs. 2 and 9), resulting in lack of nickel in partial region [9]. The transfer layers led to a reduction of interfacial contact. Consequently, the wear depth and wear volume were
Fig. 9. The profile micrographs of the worn tubes in elevated temperature air before APAC. The left side is the 3-D profile micrographs; the right side is the cor responding 2-D profile micrographs of the red dash lines in the left side. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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200 � Cand 285 � C. The main constituents of spinel oxides were Cr2O3, FeCr2O4 and Fe3O4. (4) The wear zone was generally characterized by being oxygen-rich and nickel-poor, especially at and above 200 � C. Smooth area, white speckled with grey, could be observed in the fretted area at 285 � C, which indicated the material transferred from tube to plate.
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