Engineering Failure Analysis 37(2014)42-52 Contents lists available at Science Direct VGINEFNING Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/engfailanal Failure analysis on abnormal wall thinning of heat-transfer CrossMark titanium tubes of condensers in nuclear power plant Part ll: erosion and cavitation corrosion Fei-Jun Chen, Cheng Yao, Zhen-Guo Yang epartment of Materials Science, Fudan University, Shanghai 200433, PR China ARTICLE INFO A BSTRACT In Part I of the failure analysis on abnormal wall thinning of heat-transfer titanium tubes eived 24 March 2013 sed in Accepted 19 Nover Available online 4 December 2013 abnormal thinning that commonly happened at the contact part between the tubes and the support plates. This kind of failure was the mainstream failure type in our case and ne main causes were found to be eccentric contact wear and three-body contact m tube Doted in processing defect of internal borings, corrosion products deposit and sagging, cles. however there still some individual failure tubes with different failure sites and modes and were located under the bypass pipes at the shoulder of the tube tower instead of in its lower part, obviously telling another failure story. In Part ll of the Cavitation corrosion failure analysis, material analysis, metallographic examination, mechanical performance tests, macro and microstructure analysis and composition analysis were conducted. The failure causes were found to be erosion and cavitation corrosion and the synergetic effect of them. Finally, corresponder ntermeasures uggested. e 2013 Elsevier Ltd. All rights reserved. 1 Introduction Since nuclear power was utilized, safety concerns have never stopped to bother us. from the disaster of Chernobyl in Russia decades ago, to the nuclear leak in Fukushima in the last two years, historical lessons written in blood have taught us that every detail in a nuclear power station is of critical importance and not a single tiniest potential peril can be ignored. The heat-transfer titanium tubes in condensers of the two 700 MW CANDU units in China- the first and only two pres- surized heavy water reactor(PHWR)units in the country-have encountered abnormal tube wall thinning with a design life of 40 years, the condensers were forced to temporarily stop operation after only 8 years in service because unexpected wall thinning problems were found on the heat-transfer titanium tubes, bringing about heavy economic loss and potential safety threat. Our team was asked to conduct failure analysis of the tubes. The tubes are made of industrial pure titanium in correspondence to Chinese brand TAl, with the length of 17370 mm, and specifications of 25. 4 mm x 0.5 mm(outside diameter x wall thickness). All these specifications have also been mentioned in Part I [ 1] of the failure analysis. Among the dozens of tube samples we got, most of them presented similar failure modes at similar positions. After detailed analysis by various techniques, we primarily ascribed most of the failure cases to eccentric contact wear al three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreig particles, discussed in Part I 1 of the failure analysis. However, we still found another kind of failure case with distinct appearance and failure positions. When we conducted inspection inside the condenser, we discovered the bypass pipes, onding author.Tel:+862165642523;fax:+862165103056 E-mailaddress:zgyang@fudan.edu.cn(Z-G.Yang). 1350-6307/S-see front 2013 Elsevier Ltd. All rights reserved
Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part II: Erosion and cavitation corrosion Fei-Jun Chen, Cheng Yao, Zhen-Guo Yang ⇑ Department of Materials Science, Fudan University, Shanghai 200433, PR China article info Article history: Received 24 March 2013 Accepted 19 November 2013 Available online 4 December 2013 Keywords: Titanium tube Wall thinning Failure analysis Erosion Cavitation corrosion abstract In Part I of the failure analysis on abnormal wall thinning of heat-transfer titanium tubes used in condensers in nuclear power plant, we analyzed the causes and mechanisms of abnormal thinning that commonly happened at the contact part between the tubes and the support plates. This kind of failure was the mainstream failure type in our case and the main causes were found to be eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. However, there were still some individual failure tubes with different failure sites and modes and were located under the bypass pipes at the shoulder of the tube tower instead of in its lower part, obviously telling another failure story. In Part II of the failure analysis, material analysis, metallographic examination, mechanical performance tests, macro- and microstructure analysis and composition analysis were conducted. The failure causes were found to be erosion and cavitation corrosion and the synergetic effect of them. Finally, corresponding countermeasures were suggested. 2013 Elsevier Ltd. All rights reserved. 1. Introduction Since nuclear power was utilized, safety concerns have never stopped to bother us. From the disaster of Chernobyl in Russia decades ago, to the nuclear leak in Fukushima in the last two years, historical lessons written in blood have taught us that every detail in a nuclear power station is of critical importance and not a single tiniest potential peril can be ignored. The heat-transfer titanium tubes in condensers of the two 700 MW CANDU units in China – the first and only two pressurized heavy water reactor (PHWR) units in the country – have encountered abnormal tube wall thinning. With a design life of 40 years, the condensers were forced to temporarily stop operation after only 8 years in service because unexpected wall thinning problems were found on the heat-transfer titanium tubes, bringing about heavy economic loss and potential safety threat. Our team was asked to conduct failure analysis of the tubes. The tubes are made of industrial pure titanium in correspondence to Chinese brand TA1, with the length of 17370 mm, and specifications of 25.4 mm 0.5 mm (outside diameter wall thickness). All these specifications have also been mentioned in Part I [1] of the failure analysis. Among the dozens of tube samples we got, most of them presented similar failure modes at similar positions. After detailed analysis by various techniques, we primarily ascribed most of the failure cases to eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles, discussed in Part I [1] of the failure analysis. However, we still found another kind of failure case with distinct appearance and failure positions. When we conducted inspection inside the condenser, we discovered the bypass pipes, 1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.11.002 ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Engineering Failure Analysis 37 (2014) 42–52 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
F- Chen et aL/ Engineering Failure Analysis 37(2014)42-52 Fig. 1. Bypass pipes. which were designed for shock mitigation of high re steam during start and stop(Fig. 1). As illustrated by the tube rrangement diagram shown in Fig. 2, within t like structure of 9922 heat transfer titanium tubes in each con- denser, samples in Part II are located at the tower under the bypass pipes while the samples we analyzed in Part I 1 were all in the lower part of the tower. So we decided that these special tubes must tell another failure story, which discussed in the current Part ll of the failure analysis. After detailed characterization and analysis, the root cause of the failure was found to be erosion, cavitation corrosion and the synergetic effect of them. Previous work in our lab on the finite element modeling of erosion 5, 6 has been reported but in the current failure case, we are more concerned about the erosion mechanism in real engineering application. actually, avitation corrosion of pure titanium and titanium alloys in electrolyte solutions has been reported 2,3. And cavitation phe- nomenon of commercially pure titanium has been studied in the lab[4. But cavitation corrosion of pure titanium tubes industrially utilized has been rarely touched upon. what's more, erosion and cavitation corrosion interacted and aggravated the wall thinning of titanium tube in our case, which is a relatively novel discovery look fron the outlet: count fron left to right ow number number of tubes in a row failure tubes in PartⅡ 2s is found in 20 Fig. 2. Schematic illustration of the location of failure tubes in Part II in the condenser
which were designed for shock mitigation of high pressure steam during start and stop (Fig. 1). As illustrated by the tube arrangement diagram shown in Fig. 2, within the tower-like structure of 9922 heat transfer titanium tubes in each condenser, samples in Part II are located at the tower shoulder under the bypass pipes while the samples we analyzed in Part I [1] were all in the lower part of the tower. So we decided that these special tubes must tell another failure story, which was discussed in the current Part II of the failure analysis. After detailed characterization and analysis, the root cause of the failure was found to be erosion, cavitation corrosion and the synergetic effect of them. Previous work in our lab on the finite element modeling of erosion [5,6] has been reported but in the current failure case, we are more concerned about the erosion mechanism in real engineering application. Actually, cavitation corrosion of pure titanium and titanium alloys in electrolyte solutions has been reported [2,3]. And cavitation phenomenon of commercially pure titanium has been studied in the lab [4]. But cavitation corrosion of pure titanium tubes industrially utilized has been rarely touched upon. What’s more, erosion and cavitation corrosion interacted and aggravated the wall thinning of titanium tube in our case, which is a relatively novel discovery. Fig. 1. Bypass pipes. Fig. 2. Schematic illustration of the location of failure tubes in Part II in the condenser. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 43
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 able 1 Chemical composition of TAl in-service titanium tube and the standard values(wt). Fe N H Other elements Total of the rest elements Measured values International values 03 ≤0.1 Note: International values are from GB/T 3620. 1-2007titanium and titanium alloy brand and their chemical composition"[8 Fig 3. Metallographic structure of one in-service titanium tube 200x kN Force-displacement curve 12 4. Force-d Table 2 Mechanical properties of in-service titanium tubes. Tensile strength(ab/MPa) Elongation percentage ( Average 327 379 Note: Sample 1 and sample 2 were taken from tubes 2A032012-l and 2A032012-5, 6 in condenser 2A of unit 1
Table 1 Chemical composition of TA1 in-service titanium tube and the standard values (wt%). Chemical element Fe C N O H Other elements Total of the rest elements Measured values 0.058 0.012 0.004 0.1 0.0026 – – SB338 values 0.30 0.08 0.03 0.25 0.15 60.1 60.4 International values 0.20 0.08 0.03 0.18 0.015 60.1 60.4 Note: International values are from GB/T 3620.1-2007 ‘‘titanium and titanium alloy brand and their chemical composition’’ [8]. Fig. 3. Metallographic structure of one in-service titanium tube 200. Fig. 4. Force–displacement curve of tensile test sample. Table 2 Mechanical properties of in-service titanium tubes. Item Yielding strength (r0.2/MPa) Tensile strength (rb/MPa) Elongation percentage (d5/%) Sample 1 365 440 37.0 Sample 2 290 450 38.8 Average 327 445 37.9 Note: Sample 1 and sample 2 were taken from tubes 2A032012-1 and 2A032012-5, 6 in condenser 2A of unit 1. 44 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 3 Comparison of mechanical properties of in-service tube and the standard values. Yielding strength(oo.2/MPa) Tensile strength(ob/MPa) Elongation percentage(6s/6) 445 B338 value International value 370-530 Note: International values are from Chinese standard GB/T 3624-1995 110]. Fig. 5. Macro-and microscopic morphologies of tensile fracture of titanium tube(a)macroscopic appearance of tensile fracture(b)magnified fracture edge. 2. Experiments and results 1. Material analysis of titanium tubes 1. 1 Chemical composition To better analyze this individual case, we started from its material analysis and tried to decide whether the failure came from material pre We analyzed the material of one failure tube of condenser 1B in unit 1 as a sample ICP-AES, infrared carbon sulfur analyzer and nitrogen hydrogen oxygen gas chromatograph were applied to measure the content of Fe, C, and n, H, o respec- tively. The results are listed in Table 1. It presents that the content of impurity element in the in-service titanium tube is strictly controlled, far below the upper limit of requirements of ASME SB338 titanium specification [7(equals to the TAl industrial pure titanium in GB/T 3620 1-2007 standard of China 8), indicating that the material of the in-service titanium ubes is qualified
2. Experiments and results 2.1. Material analysis of titanium tubes 2.1.1. Chemical compositions To better analyze this individual case, we started from its material analysis and tried to decide whether the failure came from material problems. We analyzed the material of one failure tube of condenser 1B in unit 1 as a sample. ICP-AES, infrared carbon sulfur analyzer and nitrogen hydrogen oxygen gas chromatograph were applied to measure the content of Fe, C, and N, H, O respectively. The results are listed in Table 1. It presents that the content of impurity element in the in-service titanium tube is strictly controlled, far below the upper limit of requirements of ASME SB338 titanium specification [7] (equals to the TA1 industrial pure titanium in GB/T 3620.1-2007 standard of China[8]), indicating that the material of the in-service titanium tubes is qualified. Table 3 Comparison of mechanical properties of in-service tube and the standard values. Item Yielding strength (r0.2/MPa) Tensile strength (rb/MPa) Elongation percentage (d5/%) Ti tube sample 327 445 37.9 SB338 value 275–450 P345 P20 International value P250 370–530 P20 Note: International values are from Chinese standard GB/T 3624-1995 [10]. Fig. 5. Macro- and microscopic morphologies of tensile fracture of titanium tube (a) macroscopic appearance of tensile fracture (b) magnified fracture edge. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 45
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 a Fig. 6. Appearance of tubes 052003-17, 18(a) top of the tube (b)bottom of the tube. 2.1.2. Metallographic structure Fig. 3 is the metallographic structure of one in-service titanium tube. It is the typical metallographic structure of pure a-Ti -equiaxed polygonal grains that are evenly distributed with uniform size and clear grain boundary with no visible 2.1.3. Mechanical tests of titanium tubes The outer wall and inner wall of the in-service titanium tubes were exposed to high purity water steam and sea water espectively. Tests should be done to judge whether their mechanical performance had deteriorated after long time service The sample preparation and tensile testing of titanium tubes were done according to the standard of ASME SA370 [9) Fig 4 is the force-displacement curve. Table 2 shows the measured value of mechanical properties of in-service titanium tubes in unit 1 Table 3 further displays the comparison between the measured values and the standard values of AsME SB338 2]and Chinese GB T 3624-95 10 It can be learned from Fig 4 that the force-displacement curve of the titanium tube basically conforms to the tensile behavior of metal. Meanwhile, the comparison of several mechanical property indexes in Table 3 presents that the yielding strength, tensile strength and elongation percentage do not show any obvious deterioration and that all the performances have met the ASME SB338 standard [7] and Chinese GB/T 3624 standard [10. Therefore, the performance of the tube mate- rial is qualified and has not deteriorated Fig 5 is a group of pictures of microscopic and macroscopic fracture morphologies of an in-service titanium tube after tensile test. The fracture takes on a look of indention( Fig. 5(a))and the fracture surface is not smooth with tangles. also duc tile shear zones and dimples( fig. 5())are seen from the microscopic morphology of fracture edge, which is characteristic of ductile fracture. So the sample must have undergone sufficient plastic deformation before fracture At high magnification, there are hardly any inclusions in the dimples, indicating that the microscopic structure of the material is fine( Fig. 5(b)). Therefore, we can safely conclude that the mechanical performance of the in-service tube is good without indications of Till now, we have ruled out the possibility that the failure was caused by material problems
2.1.2. Metallographic structure Fig. 3 is the metallographic structure of one in-service titanium tube. It is the typical metallographic structure of pure a-Ti -equiaxed polygonal grains that are evenly distributed, with uniform size and clear grain boundary with no visible inclusions. 2.1.3. Mechanical tests of titanium tubes The outer wall and inner wall of the in-service titanium tubes were exposed to high purity water steam and sea water respectively. Tests should be done to judge whether their mechanical performance had deteriorated after long time service. The sample preparation and tensile testing of titanium tubes were done according to the standard of ASME SA370 [9]. Fig. 4 is the force–displacement curve. Table 2 shows the measured value of mechanical properties of in-service titanium tubes in unit 1. Table 3 further displays the comparison between the measured values and the standard values of ASME SB338 [2] and Chinese GB/T 3624-95 [10]. It can be learned from Fig. 4 that the force–displacement curve of the titanium tube basically conforms to the tensile behavior of metal. Meanwhile, the comparison of several mechanical property indexes in Table 3 presents that the yielding strength, tensile strength and elongation percentage do not show any obvious deterioration and that all the performances have met the ASME SB338 standard [7] and Chinese GB/T 3624 standard [10]. Therefore, the performance of the tube material is qualified and has not deteriorated. Fig. 5 is a group of pictures of microscopic and macroscopic fracture morphologies of an in-service titanium tube after tensile test. The fracture takes on a look of indention (Fig. 5(a)) and the fracture surface is not smooth with tangles. Also ductile shear zones and dimples (Fig. 5(b)) are seen from the microscopic morphology of fracture edge, which is characteristic of ductile fracture. So the sample must have undergone sufficient plastic deformation before fracture. At high magnification, there are hardly any inclusions in the dimples, indicating that the microscopic structure of the material is fine (Fig. 5(b)). Therefore, we can safely conclude that the mechanical performance of the in-service tube is good without indications of deterioration. Till now, we have ruled out the possibility that the failure was caused by material problems. Fig. 6. Appearance of tubes 052003-17, 18 (a) top of the tube (b) bottom of the tube. 46 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52
F- Chen et aL/ Engineering Failure Analysis 37(2014)42-52 a Fig. 7. Appearance of tubes 052003-17, 18 imaged by stereo microscope(a)wavy defect zone(b)pits(c) boundary of defect zone and non-defect zone. 2.2. Failure analysis of titanium tubes 052003-17, 18 2.2.1. Visual inspection Titanium tube 052003-17, 18 belong to unit 2 and the failure position is on the upper part of the tube wall instead of near the contact part with the support plates commonly seen in Part I 1. Fig 6 depicts the appearance of titanium tube 052003-17, 18. From the picture we can see a lot of pitting defects distributed in a strip in the longitudinal direction on the top of the tube at 45(Fig. 6(a)). The pitting defects only appear on the top of the tube and the bottom is flawless( Fig. 6(b)). The boundary of the defect strip is very clear. Since the media on the shell side is pure water steam, no foreign ions are present. So the pits must be created by cavitation corrosion instead of pitting corrosion. 2.2.2. Microscopic morphology observation by stereo microsc Fig 7 is the surface morphology of titanium tube 052003-17, 18 imaged by stereo microscope. In the defect zone, wave structure and pits are seen(Fig. 7(a and b)) And the boundary between the defected part and the normal part where no defect is found is very clear(Fig. 7(c)). The wave structure matches the typical pattern of erosion
2.2. Failure analysis of titanium tubes 052003-17, 18 2.2.1. Visual inspection Titanium tube 052003-17, 18 belong to unit 2 and the failure position is on the upper part of the tube wall instead of near the contact part with the support plates commonly seen in Part I [1]. Fig. 6 depicts the appearance of titanium tube 052003-17, 18. From the picture we can see a lot of pitting defects distributed in a strip in the longitudinal direction on the top of the tube at 45 (Fig. 6(a)). The pitting defects only appear on the top of the tube and the bottom is flawless (Fig. 6(b)). The boundary of the defect strip is very clear. Since the media on the shell side is pure water steam, no foreign ions are present. So the pits must be created by cavitation corrosion instead of pitting corrosion. 2.2.2. Microscopic morphology observation by stereo microscope Fig. 7 is the surface morphology of titanium tube 052003-17, 18 imaged by stereo microscope. In the defect zone, wave structure and pits are seen (Fig. 7(a and b)). And the boundary between the defected part and the normal part where no defect is found is very clear (Fig. 7(c)). The wave structure matches the typical pattern of erosion. Fig. 7. Appearance of tubes 052003-17, 18 imaged by stereo microscope (a) wavy defect zone (b) pits (c) boundary of defect zone and non-defect zone. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 47
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 ax312431p Heit288528 Im width1:219205mwdh2:215869m Lax 420.8/lum soMt3747/mdh1:219242mdh:215.96 Fig 8. Structure of the pits on tubes 052003-17, 18 and depth measurement(a)appearance of pit a(b)appearance of pit B(c)3-D contour map of pit A(d) 3-D contour map of pit B(e)depth of pit A(f) depth of pit B. shown in Fig 8. The depth of pit A is 0.288 mm, so the thinning rate of the tube wall is 41.14%. And the depth of pit e s To evaluate the extent of erosion and cavitation corrosion, we measured the depth of two pits by stereo microsco 0.377 mm which means that thickness of the tube wall has declined by 53.85% From these figures we can conclude that cav- itation corrosion is rather threatening. The depth of a lot of pits exceeds 40% of the thickness of the tube wall and the depth of some even reaches 60%. It is highly possible that perforation will occur if the tubes continue to be used, which will endan- ger the safety of both machines and personnel, so the tubes have to be replaced as soon as possible
To evaluate the extent of erosion and cavitation corrosion, we measured the depth of two pits by stereo microscope, shown in Fig. 8. The depth of pit A is 0.288 mm, so the thinning rate of the tube wall is 41.14%. And the depth of pit B is 0.377 mm which means that thickness of the tube wall has declined by 53.85%. From these figures we can conclude that cavitation corrosion is rather threatening. The depth of a lot of pits exceeds 40% of the thickness of the tube wall and the depth of some even reaches 60%. It is highly possible that perforation will occur if the tubes continue to be used, which will endanger the safety of both machines and personnel, so the tubes have to be replaced as soon as possible. Fig. 8. Structure of the pits on tubes 052003-17, 18 and depth measurement (a) appearance of pit A (b) appearance of pit B (c) 3-D contour map of pit A (d) 3-D contour map of pit B (e) depth of pit A (f) depth of pit B. 48 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52
F- Chen et aL/ Engineering Failure Analysis 37(2014)42-52 Fig 9. Microscopic morphology of defects on tubes 052003-17, 18(a)wavy erosion zone(b)a large pit with thin, raised edge. 2.2.3. Microscopic morphology and composition analysis Fig. 9 is the microscopic morphology of the titanium tube imaged by SEM. From this figure it can be learned that the wavy races within the defect zone are somewhat directional (Fig. g(a)), and the pits are quite deep with raised thin edges (Fig 9(b)). To measure the element composition within the pits, EDS was conducted with the results shown in Fig. 10 and Table 4. The main constituent within the erosion and cavitation corrosion zone is titanium. The rest are iron and oxygen which come from the corrosion products of carbon steel support plates. Therefore, no foreign ions or substance are found and the defect must be created by the high pressure steam released from the bypass pipe. cording to the arrangement of tubes in Fig. 2, tube 052003-17, 18 are located at the shoulder of the tower structure. The fact that the defect zone is situated at the top of the tube at 45 and has clear boundary indicates that it is formed by impact of high pressure steam at a certain angle. 3. Discussion of failure mechanisms Erosion is caused by the lashing of solid or fluid in the form of loose particles against the surface of a material at a certain ngel and velocity. In this case, when liquid drops in the high pressure steam lashed against the titanium tubes right under the bypass pipes at a certain velocity, energy exchange happened. The energy of these liquid drops which they got from the team medium carrying them transfered to the outer wall of the tubes. when the energy was high enough or the time was long enough for plastic deformation of the tube wall to occur, indentation appeared. The material originally in the ndentation was pushed around the indentation to form a raised edge where thin peaks could be observed, as presented
2.2.3. Microscopic morphology and composition analysis Fig. 9 is the microscopic morphology of the titanium tube imaged by SEM. From this figure it can be learned that the wavy traces within the defect zone are somewhat directional (Fig. 9(a)), and the pits are quite deep with raised thin edges (Fig. 9(b)). To measure the element composition within the pits, EDS was conducted with the results shown in Fig. 10 and Table 4. The main constituent within the erosion and cavitation corrosion zone is titanium. The rest are iron and oxygen which come from the corrosion products of carbon steel support plates. Therefore, no foreign ions or substance are found and the defect must be created by the high pressure steam released from the bypass pipe. According to the arrangement of tubes in Fig. 2, tube 052003-17, 18 are located at the shoulder of the tower structure. The fact that the defect zone is situated at the top of the tube at 45 and has clear boundary indicates that it is formed by impact of high pressure steam at a certain angle. 3. Discussion of failure mechanisms 3.1. Erosion Erosion is caused by the lashing of solid or fluid in the form of loose particles against the surface of a material at a certain angel and velocity. In this case, when liquid drops in the high pressure steam lashed against the titanium tubes right under the bypass pipes at a certain velocity, energy exchange happened. The energy of these liquid drops which they got from the steam medium carrying them transfered to the outer wall of the tubes. When the energy was high enough or the time was long enough for plastic deformation of the tube wall to occur, indentation appeared. The material originally in the indentation was pushed around the indentation to form a raised edge where thin peaks could be observed, as presented Fig. 9. Microscopic morphology of defects on tubes 052003-17, 18 (a) wavy erosion zone (b) a large pit with thin, raised edge. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 49
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 态的 01Ju201117:30:54 b 000200400600 10.001200140016 Fig. 10. Microscopic morphology and EDS analysis of the pits(a)microscopic morphology of pits( b)EdS analysis of the pits Table 4 Surface composition in the defect zone of tubes 052003-17, 18. 16.26 7.52 in Fig 9(a and b). That was how the wavy erosion zone formed and the explanation for its morphology. what's more, the high pressure steam lashed against the tube wall at a fixed angle, which sheded light on the directional erosion traces There were two unique factors favoring erosion in our case. Firstly, the flow velocity in the heat exchange tubes was normally no more than 2 m/s, which was mentioned in Part I [1]. but the flow velocity in the bypass valve was nearly 20 m/s, making the erosion force much greater. Secondly, most erosion cases reported happened in the pipelines wher the flow rushed along them. However, erosion in our case happened on the tubes under the bypass pipes, which meant that the steam released lashed straight against the tube wall and the impact force was thus much greater. As a result, the tube wall was more vulnerable to damage
in Fig. 9(a and b). That was how the wavy erosion zone formed and the explanation for its morphology. What’s more, the high pressure steam lashed against the tube wall at a fixed angle, which sheded light on the directional erosion traces. There were two unique factors favoring erosion in our case. Firstly, the flow velocity in the heat exchange tubes was normally no more than 2 m/s, which was mentioned in Part I [1], but the flow velocity in the bypass valve was nearly 20 m/s, making the erosion force much greater. Secondly, most erosion cases reported happened in the pipelines where the flow rushed along them. However, erosion in our case happened on the tubes under the bypass pipes, which meant that the steam released lashed straight against the tube wall and the impact force was thus much greater. As a result, the tube wall was more vulnerable to damage. Fig. 10. Microscopic morphology and EDS analysis of the pits (a) microscopic morphology of pits (b) EDS analysis of the pits. Table 4 Surface composition in the defect zone of tubes 052003-17, 18. Element O Ti Fe Wt% 16.26 77.52 6.22 50 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)42-52 cavities solid wall 777777 Fig. 11. Illustration of the microjet mechanism of cavitation corrosion. 3.2. Cavitation corrosion In the flowing fluid, when the pressure in a local region suddenly drops below the steam pressure corresponding to the local temperature for some certain reason, some of the fluid vaporizes. Then the gas dissolved in the fluid escapes and forms cavities. This process is called cavitation. On the one hand, the boiling point of water declines with the fall of pressure. On the other hand, the pressure of the fluid decreases with the increase of its flowing velocity, which can be explained by the b oulli equation in hydromechanics where P, P, D, C stand for the pressure, density, flowing velocity of the fluid and constant respectively. The moment the high pressure steam rushed out of the bypass pipes, its pressure dropped dramatically, leading to the increase of its velocity and piling point also declined. in this tation corrosion to the tubes. No consensus on the mechanism of cavitation corrosion has been reached yet. Rayleigh 11 proposed the theory of cavitation systematically in 1917 and nowadays more researchers tend to explain the mechanism as the combination of microjet mechanism and impact wave mechanism. According to the former one [12-14. when the cavities burst under the action of pressure gradient or near the boundary they deform into oblate spheres or shoe-shaped spheres and then divide and finally burst. Right before burst, a microjet forms and passes through the divided cavity and lashes the tube wall, causing cavitation corrosion, see Fig. 11. According to the latter one 15-17. cavities will burst when they reach high pressure regions in the fluid, which turns their potential energy into kinetic energy that flows within a small region. In this way, impact waves form in the fluid. When they contact the tubes, they will cause stress pulses and pulsing regional plastic deformation and even work hardening to the tube wall Plastic deformation and pits will form on the tube wall by repeating fluid impact waves. By the interaction of these two mechanisms, cavitation corrosion of the tube wall oc- curs due to the release of high pressure steam. There's also one favorable condition for cavitation corrosion in our case. That is the oxygen-deficient environment and the fast flow. It is commonly known that there exists a compact oxide layer on the surface of titanium which makes it corrosion- resistant. In oxygen-sufficient environment, once the passivation layer is damaged, it is re-formed immediately, preventing from further corrosion or degradation. But in the oxygen deficient condenser and with fast steam flow continual away the passivation layer damaged by the mechanical action of erosion in our case, it is not easy to form again. So all will undergo continuously cavitation corrosion 3.3. Synergetic effect of erosion and cavitation corrosion In in Part ll, erosion and cavitaion corrosion occured at the same time and both of them caused mechanical deg- radation. Apart from the factors we have mentioned that benefited erosion and cavitaion corrosion, the interaction between them was also one ruling cause. On the one hand cavitation corrosion left pits on the tube wall, where the passivation oxide layer had already been removed and the surface structure ruined. Then the pits served as vulnerable points of erosion, the result of which was the mechanical degradation of tube wall. In this way, erosion damage was made easier. On the other hand, erosion destroyed the surface structure of tube wall and created wavy pattern, making the tube wall structure loose and irregular. So the mechanical performance of the tube wall deteriorated and it was more vulnerable to deformation and consequently more vulnerable to cavitation corrosion By the interaction between these two processes, the pits and wavy zone were enlarged and deepened. As a result, serious wall thinning was observed 4. Conclusions 1. The titanium tubes in the condenser are TAl level industrial a-phase pure titanium tubes. their chemical composition, metallographic structure, microscopic morphology and mechanical properties all meet national and international standards. So the material is qualified
3.2. Cavitation corrosion In the flowing fluid, when the pressure in a local region suddenly drops below the steam pressure corresponding to the local temperature for some certain reason, some of the fluid vaporizes. Then the gas dissolved in the fluid escapes and forms cavities. This process is called cavitation. On the one hand, the boiling point of water declines with the fall of pressure. On the other hand, the pressure of the fluid decreases with the increase of its flowing velocity, which can be explained by the Bernoulli equation in hydromechanics: P þ qm2=2 ¼ C; where P, q, v, C stand for the pressure, density, flowing velocity of the fluid and constant respectively. The moment the high pressure steam rushed out of the bypass pipes, its pressure dropped dramatically, leading to the increase of its velocity and the boiling point of water also declined. In this way, cavitation happened and the high velocity steam caused serious cavitation corrosion to the tubes. No consensus on the mechanism of cavitation corrosion has been reached yet. Rayleigh [11] proposed the theory of cavitation systematically in 1917 and nowadays more researchers tend to explain the mechanism as the combination of microjet mechanism and impact wave mechanism. According to the former one [12–14], when the cavities burst under the action of pressure gradient or near the boundary, they deform into oblate spheres or shoe-shaped spheres and then divide and finally burst. Right before burst, a microjet forms and passes through the divided cavity and lashes the tube wall, causing cavitation corrosion, see Fig. 11. According to the latter one [15–17], cavities will burst when they reach high pressure regions in the fluid, which turns their potential energy into kinetic energy that flows within a small region. In this way, impact waves form in the fluid. When they contact the tubes, they will cause stress pulses and pulsing regional plastic deformation and even work hardening to the tube wall. Plastic deformation and pits will form on the tube wall by repeating fluid impact waves. By the interaction of these two mechanisms, cavitation corrosion of the tube wall occurs due to the release of high pressure steam. There’s also one favorable condition for cavitation corrosion in our case. That is the oxygen-deficient environment and the fast flow. It is commonly known that there exists a compact oxide layer on the surface of titanium which makes it corrosionresistant. In oxygen-sufficient environment, once the passivation layer is damaged, it is re-formed immediately, preventing titanium from further corrosion or degradation. But in the oxygen deficient condenser and with fast steam flow continually carrying away the passivation layer damaged by the mechanical action of erosion in our case, it is not easy to form again. So the tube wall will undergo continuously cavitation corrosion. 3.3. Synergetic effect of erosion and cavitation corrosion In our case in Part II, erosion and cavitaion corrosion occured at the same time and both of them caused mechanical degradation. Apart from the factors we have mentioned that benefited erosion and cavitaion corrosion, the interaction between them was also one ruling cause. On the one hand, cavitation corrosion left pits on the tube wall, where the passivation oxide layer had already been removed and the surface structure ruined. Then the pits served as vulnerable points of erosion, the result of which was the mechanical degradation of tube wall. In this way, erosion damage was made easier. On the other hand, erosion destroyed the surface structure of tube wall and created wavy pattern, making the tube wall structure loose and irregular. So the mechanical performance of the tube wall deteriorated and it was more vulnerable to deformation and consequently more vulnerable to cavitation corrosion. By the interaction between these two processes, the pits and wavy zone were enlarged and deepened. As a result, serious wall thinning was observed. 4. Conclusions 1. The titanium tubes in the condenser are TA1 level industrial a-phase pure titanium tubes. Their chemical composition, metallographic structure, microscopic morphology and mechanical properties all meet national and international standards. So the material is qualified. Fig. 11. Illustration of the microjet mechanism of cavitation corrosion. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 42–52 51