gineering Failure Analysis 37(2014)29-41 Contents lists available at ScienceDirect ENGINEERING Engineering Failure Analysis ELSEVIER journalhomepagewww.elsevier.com/locate/engfailanal Failure analysis on abnormal wall thinning of heat-transfer Cross Mark titanium tubes of condensers in nuclear power plant Part 1: Corrosion and wear Fei-Jun Chen, Cheng Yao, Zhen-Guo Yang Department of Materials Science, Fudan University, Shanghai 200433, PR China ARTICLE IN FO A BSTRACT Titanium tubes used in condensers in a nuclear power plant in China encountered abnor ACcepable 24 March 20 mal wall thinning, and was thus forced to temporarily stop operation or it could bring 19 November 2013 online 4 December 2013 about catastrophic safety problems. Most of the wall thinning happened at quite regula positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thin- ning failure, we conducted surface deposit analysis, appearance inspection, microstructure hinning nalysis and composition analysis of the diffraction(XRD). ste. reo microscope, scanning electron microscope( SEM)and Energy Dispersive Spect (EDS). The results revealed that the wall thinning was primarily caused by eccent Failure analysis tact wear and three-body contact wear rooted in processing defect of internal boril rosion products deposit and sagging, and foreign particles. Finally, countermeasure proposed for repair and preventio e 2013 Elsevier Ltd. All rights reserved. 1 Introduction Under the background of power crisis, people are relentlessly looking for new and highly effective powers among which nuclear power is the most popular and feasible one. So the safety of nuclear power stations has ever become the priority of ll China has over a dozen of nuclear power stations under operation, one of which located in the southeast has two 700 Mw CANDU units(unit 1 and unit 2) imported from Atomic Energy of Canada Limited(AECL), the only two pressurized heavy water reactor(PHWR)units in the country. Each unit has a vertical structure with four condensers, shown in Fig. 1. The med- ia inside and outside the tubes (also called tube side and shell side)in the condenser are sea water and high purity water steam respectively. Each condenser has two independent shells connected by steam balance channel. within each shell there are two separated horizontal rows of one way heat transfer tube bundles, each of which has independent water inlet and utlet chambers as well as inlet and outlet dynamoelectric isolation valves. So each tube bundle can be isolated for main- tenance and leak emergency treatment. the working parameters of the condensers are listed in Table 1 Each condenser has 9922 heat transfer tubes that fit together as a tower-like structure shown in Fig. 2. The tubes are made of industrial pure titanium in correspondence to Chinese brand TAl, with the length of 17, 370 mm, and specifications of 25. 4 mm x 0.5 mm(outside diameter x wall thickness). Corresponding author. Tel: +86 21 65642523: fax: +86 21 65103056 1350-6307/s- see front matter o 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.11.003
Failure analysis on abnormal wall thinning of heat-transfer titanium tubes of condensers in nuclear power plant Part I: Corrosion and wear 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 Corrosion Wear Failure analysis abstract Titanium tubes used in condensers in a nuclear power plant in China encountered abnormal wall thinning, and was thus forced to temporarily stop operation or it could bring about catastrophic safety problems. Most of the wall thinning happened at quite regular positions on the tubes and these failure tubes were located similarly in the condensers, indicating some common problems. To find out the root cause and mechanism of the thinning failure, we conducted surface deposit analysis, appearance inspection, microstructure analysis and composition analysis of the samples by means of X-ray diffraction (XRD), stereo microscope, scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). The results revealed that the wall thinning was primarily caused by eccentric contact wear and three-body contact wear rooted in processing defect of internal borings, corrosion products deposit and sagging, and foreign particles. Finally, countermeasures were proposed for repair and prevention. 2013 Elsevier Ltd. All rights reserved. 1. Introduction Under the background of power crisis, people are relentlessly looking for new and highly effective powers among which nuclear power is the most popular and feasible one. So the safety of nuclear power stations has ever become the priority of all concerns. China has over a dozen of nuclear power stations under operation, one of which located in the southeast has two 700 MW CANDU units (unit 1 and unit 2) imported from Atomic Energy of Canada Limited (AECL), the only two pressurized heavy water reactor (PHWR) units in the country. Each unit has a vertical structure with four condensers, shown in Fig. 1. The media inside and outside the tubes (also called tube side and shell side) in the condenser are sea water and high purity water steam respectively. Each condenser has two independent shells connected by steam balance channel. Within each shell there are two separated horizontal rows of one way heat transfer tube bundles, each of which has independent water inlet and outlet chambers as well as inlet and outlet dynamoelectric isolation valves. So each tube bundle can be isolated for maintenance and leak emergency treatment. The working parameters of the condensers are listed in Table 1. Each condenser has 9922 heat transfer tubes that fit together as a tower-like structure, shown in Fig. 2. The tubes are made of industrial pure titanium in correspondence to Chinese brand TA1, with the length of 17,370 mm, and specifications of 25.4 mm 0.5 mm (outside diameter wall thickness). 1350-6307/$ - see front matter 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.11.003 ⇑ 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) 29–41 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 E: K2B 2A HF1B 1A G界 NERATOR IEH TURBINE SI ⊥:」 GEN ARRANGEMENT(S-1/20c) T/B ocation of Condenser Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Working parameters of the condenser Parameters Media Flow rate Q(m/s) Flow velocity Pressure P(kPa) Inlet temperature Outlet temperature Sea water 130.1 Shell side Steam Main steam flow rate 1033 kg/s(3718 T/h) 4-49 Note: The steam temperature in the condenser in operation was 47C(steam drain temperature of low pressure cylinder). Temperature of condensed water around30° Every tube is sustained by 22 perforated support plates made of carbon steel (IS SS400)with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates(ASTM B265 Gr 1 [ 1+ A515 Gr 65[))whose thickness is 40 mm. It is the titanium steel side that contacts sea water he units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After spection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to var- ious extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perfo- ration would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes Previous work on mechanical performance of thin tubes 3, 4 and failure analysis of tubes used in condensers and plants [5,6 has provided some clues to this problem. Our team [7, 8 completed the failure analysis of leakage on titanium tubes ithin heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochem cal corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences 9-12 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig 3. Corrosion extent varied among dif- ferent support plates and the most serious corrosion happened between the tube sheet and the first support plate( ig. 3(a)). oducts deposit was seen on the surface of many tubes near the support plates(ig. 3(a)and(b))
Every tube is sustained by 22 perforated support plates made of carbon steel (JIS SS400) with thickness of 13 mm and interval distance of 755 mm, and the diameter of the internal borings is 25.6 mm. Two ends of the tubes are welded with titanium clad carbon steel plates (ASTM B265 Gr.1 [1] + A515 Gr.65 [2]) whose thickness is 40 mm. It is the titanium steel side that contacts sea water. The units started commercial operation in August 2002 with actual runtime of about 8 years. During the 5th overhaul in 2010, part of the tubes were found to have suffered from serious wall thinning and thus temporarily stopped operation. After inspection of the failure tubes, it was discovered that the wall thinning mainly happened near the support plates but to various extents and at different positions of the tubes. If the thinned tubes had continued to be used, leaking caused by perforation would have been very likely to happen, which could have brought about catastrophic safety problems. Therefore, we were asked by the plant to conduct failure analysis on the abnormal wall thinning of the titanium tubes. Previous work on mechanical performance of thin tubes [3,4] and failure analysis of tubes used in condensers and plants [5,6] has provided some clues to this problem. Our team [7,8] completed the failure analysis of leakage on titanium tubes within heat exchangers in a different phase of the same power plant as in our case. The failure was ascribed to electrochemical corrosion and mechanical degradation. However, in our case, the thinning dominantly happened on the outer wall of the tubes at quite regular positions, which implied a different story. To find out the root cause and mechanism of the thinning failure, we conducted a number of experiments for material, microstructure and chemical composition characterization based on previous successful failure analysis experiences [9–12]. 2. Experiments and results 2.1. Visual inspection and sampling To find out the cause of abnormal wall thinning of titanium tubes, we investigated condenser 1B of unit 2 under repair on the spot with the focus on the appearance of the support plates, their connection with the tubes and the surface condition of them both in the water inlet chamber. Some obvious defects found are shown in Fig. 3. Corrosion extent varied among different support plates and the most serious corrosion happened between the tube sheet and the first support plate (Fig. 3(a)). Besides, corrosion products deposit was seen on the surface of many tubes near the support plates (Fig. 3(a) and (b)). Fig. 1. Schematic diagram of the arrangement of four condensers in a unit. Table 1 Working parameters of the condenser. Parameters Media Flow rate Q (m3 /s) Flow velocity V (m/s) Pressure P (kPa) Inlet temperature T (C) Outlet temperature T (C) Tube side Sea water 1,30,100 1.97 – 18.8(note) 27.8 Shell side Steam Main steam flow rate 1033 kg/s (3718 T/h) – 4–4.9 – – Note: The steam temperature in the condenser in operation was 47 C (steam drain temperature of low pressure cylinder). Temperature of condensed water was around 30 C. 30 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 look from the out let: count fron left to right o ber number of tubes in a 406p 16 found in 2010 0动43 failure tubes in Part I Fig. 2. Arrangement of heat transfer tubes in the condenser and the location of the failure tubes a Fig 3. Appearance of support plates in the condenser (a)corrosion products on support plates (b) deposition and sagging of corrosion products
Fig. 2. Arrangement of heat transfer tubes in the condenser and the location of the failure tubes. Fig. 3. Appearance of support plates in the condenser (a) corrosion products on support plates (b) deposition and sagging of corrosion products. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 31
F-f. Chen et aL Engineering Failure Analysis 37(2014)29-41 ig. 4. Black deposit on the outer wall of titanium tubes and its distribution(a) position near the support plate(b) deposit at the bottom. 2Theta [deg J Fig. 5. XRD results of the black deposit on the surface of titanium tubes. 2.2. XRD analysis of the black deposit Black deposit was found at the bottom and the contact part with the support plates of nearly all failure tubes, shown in Fig 4. We scraped some and conducted XRD for composition analysis. The results are shown in Fig. 5. The black deposit inly consists of Fe3 Oa according to the standard powder diffraction file(pDf)card. The Fe3 O4 mainly came from the galvanic corrosion of support plates made of carbon steel under xygen-dehcient con- ditions. But why did it dominantly appear at the contact part between the tubes and the support plates? It can be deduced
2.2. XRD analysis of the black deposit Black deposit was found at the bottom and the contact part with the support plates of nearly all failure tubes, shown in Fig. 4. We scraped some and conducted XRD for composition analysis. The results are shown in Fig. 5. The black deposit mainly consists of Fe3O4 according to the standard powder diffraction file (PDF) card. The Fe3O4 mainly came from the galvanic corrosion of support plates made of carbon steel under oxygen-deficient conditions. But why did it dominantly appear at the contact part between the tubes and the support plates? It can be deduced Fig. 4. Black deposit on the outer wall of titanium tubes and its distribution (a) position near the support plate (b) deposit at the bottom. Fig. 5. XRD results of the black deposit on the surface of titanium tubes. 32 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F-A. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 Fig. 6. Schematic diagram of the cross section of a tube. a Fig. 7. Appearance of defect on the outer wall of titanium tube 1B134027-1 (a)o(b)90(c)180(d)270%. hat this phenomenon was because the support plates were vertical, the corrosion products deposited and sagged into the internal borings by the action of gravity 3. Failure analysis of defective tubes early all the failure tubes are located at the periphery of the lower part of the heat transfer tube tower. So they must from the ones in the current Part L, and they will be separately discussed in Part ll [13] of the failure analyer te different share some common problems. However, there are a few exceptions whose failure mode and location are q To describe defect distribution and location on the tubes, we drew a schematic diagram for illustration As shown in Fig. 6 the top of the tube is defined as o and the bottom is 180. Clockwise rotation goes from 0 to 180 and counterclockwise rotation from 0 to-180. The arrow direction perpendicular into the paper represents the flowing direction of sea water side the tube. The numbering of the tubes is based on their locations in the condenser, which is provided by the plant
that this phenomenon was because the support plates were vertical, the corrosion products deposited and sagged into the internal borings by the action of gravity. 3. Failure analysis of defective tubes Nearly all the failure tubes are located at the periphery of the lower part of the heat transfer tube tower. So they must share some common problems. However, there are a few exceptions whose failure mode and location are quite different from the ones in the current Part I, and they will be separately discussed in Part II [13] of the failure analysis. To describe defect distribution and location on the tubes, we drew a schematic diagram for illustration. As shown in Fig. 6 the top of the tube is defined as 0, and the bottom is 180. Clockwise rotation goes from 0 to 180 and counterclockwise rotation from 0 to 180. The arrow direction perpendicular into the paper represents the flowing direction of sea water inside the tube. The numbering of the tubes is based on their locations in the condenser, which is provided by the plant. Fig. 6. Schematic diagram of the cross section of a tube. Fig. 7. Appearance of defect on the outer wall of titanium tube 1B134027-1 (a) 0 (b) 90 (c) 180 (d) 270. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 33
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 a b Fig 8. Microscopic morphology of wear zone on the titanium tube(a)strip deposit(b)deposit and wear traces. 3.1. Failure analysis of titanium tube 1B134027-1 3.1. 1. Visual inspectio Fig. 7 shows the defect distribution on the outer wall of titanium tube 1B134027-1 from 0 to 270 with intervals of 900 (all the other tubes to be analyzed later in the paper are imaged in order of magnitude of angles). From Fig. 7 we find that the defect is distributed around the tube in the circumferential direction. The wear zone appears smooth and shiny, which is the typical frictional wear morphology. The wear band is about 10 mm wide and the wear extent in the forward direction is greater than that in the backward direction( Fig. 7(b and d). What is more the wear band is right at the contacting area between the tube and the support plate. obviously, it is selective concentrated wear 3.1.2. Microscopic morphology and composition analysis Fig 8 is the microscopic morphology of the wear zone imaged by SEM. There are regularly distributed, parallel fine matches in the longitudinal direction in the wear zone(Fig 8(b)). In the circumferential direction is dark colored deposit (Fig 8(a and b)) formed by the wear products in the internal borings and the sagging of corrosion products on the support plates into them by the action of gravity. EDS was conducted to analyze the element composition of the abrasion Results demonstrate that the mainly consists of iron, oxygen and titanium, shown in Fig 9(b)and Table is primarily the mixtur products generated by the contact wear between the titanium tube and the eel support plate as well as the induced sagging of corrosion products on the support plate into the internal boring
3.1. Failure analysis of titanium tube 1B134027-1 3.1.1. Visual inspection Fig. 7 shows the defect distribution on the outer wall of titanium tube 1B134027-1 from 0 to 270 with intervals of 90 (all the other tubes to be analyzed later in the paper are imaged in order of magnitude of angles). From Fig. 7 we find that the defect is distributed around the tube in the circumferential direction. The wear zone appears smooth and shiny, which is the typical frictional wear morphology. The wear band is about 10 mm wide and the wear extent in the forward direction is greater than that in the backward direction (Fig. 7(b and d)). What is more, the wear band is right at the contacting area between the tube and the support plate. Obviously, it is selective concentrated wear. 3.1.2. Microscopic morphology and composition analysis Fig. 8 is the microscopic morphology of the wear zone imaged by SEM. There are regularly distributed, parallel fine scratches in the longitudinal direction in the wear zone (Fig. 8(b)). In the circumferential direction is dark colored deposit (Fig. 8(a and b)) formed by the wear products in the internal borings and the sagging of corrosion products on the support plates into them by the action of gravity. EDS was conducted to analyze the element composition of the abrasion deposit. Results demonstrate that the deposit mainly consists of iron, oxygen and titanium, shown in Fig. 9(b) and Table 2. This deposit is primarily the mixture of wear products generated by the contact wear between the titanium tube and the carbon steel support plate as well as the gravity induced sagging of corrosion products on the support plate into the internal boring. Fig. 8. Microscopic morphology of wear zone on the titanium tube (a) strip deposit (b) deposit and wear traces. 34 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F- Chen et aL/ Engineering Failure Analysis 37(2014)29-41 a 20um e:edax32genesis'genmaps, spe 01-Jul-2011 18: 28: 5 eLa 1.002003.00400 7008009.0 merry.ke Fig 9. Composition analysis of the wear zone on tube1B134027-1(a) strp deposit(b)EDS analysis of the strip deposit. EDS results of the strip deposit. WE6 29.34 3.2. Failure analysis of titanium tube 1B160012-14 3. 2. 1. Visual inspection Fig. 10 is the exterior appearance of titanium tube 1B160012-14. The wear zone is mainly distributed from 0 to 1700 (Fig. 10(a and b)), but there are no obvious wear traces in the backward direction (Fig. 10(c and d)). the wear zone in the forward direction looks shiny and smooth and is not abraded seriously. But a visible S-shaped furrow crack is found in the wear zone(Fig. 10(e)), which is about 7 mm long parallel to the longitudinal direction and 10 mm long in the circum- ferential direction 3. 2. Microscopic To further observe the morphology characteristics of the furrow crack, 3-D stereo microscope was applied to image the surface wear zone of the tube, shown in Fig. 11. From Fig. 11(a)we can learn that the furrow crack is obviously deeper than
3.2. Failure analysis of titanium tube 1B160012-14 3.2.1. Visual inspection Fig. 10 is the exterior appearance of titanium tube 1B160012-14. The wear zone is mainly distributed from 0 to 170 (Fig. 10(a and b)), but there are no obvious wear traces in the backward direction (Fig. 10(c and d)). The wear zone in the forward direction looks shiny and smooth and is not abraded seriously. But a visible S-shaped furrow crack is found in the wear zone (Fig. 10(e)), which is about 7 mm long parallel to the longitudinal direction and 10 mm long in the circumferential direction. 3.2.2. Microscopic observation by 3-D stereo microscope To further observe the morphology characteristics of the furrow crack, 3-D stereo microscope was applied to image the surface wear zone of the tube, shown in Fig. 11. From Fig. 11(a) we can learn that the furrow crack is obviously deeper than Fig. 9. Composition analysis of the wear zone on tube1B134027-1 (a) strip deposit (b) EDS analysis of the strip deposit. Table 2 EDS results of the strip deposit. Element O Ti Fe Wt% 29.34 3.56 67.10 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 35
F-I. Chen et aL/ Engineering Failure Analysis 37 (2014)29-41 d Fig 10. Appearance of tube 1B160012-14(a)o(b)90(c)180(d)270(e)S-shaped furrow crack magnified from(b) the surrounding wear scratches. So it should have been created by adhesive wear by the irregular motion of a hard adhering to the surface of the titanium tube in micro-vibration. The width of the crack is about 255-285 um(Fig. 11 define it more accurately it is three-body wear between the hard particle, the outer wall of the titanium tube internal boring, and it is also a kind of concentrated wear. 3.2.3. Micro-morphology and component analysis Fig 12 presents the surface micro-morphology of the titanium tube. Scratches in the wear zone are somewhat directional ( Fig. 12(b)), and there is layered deposit on the tube wall( Fig. 12(b)). EDS results of the deposit is shown in Fig. 13 and Table 3. The deposit mainly consists of iron oxides. 4. Discussion of failure mechanisms corosion The black deposit on the surface of titanium tubes has proved to be Fe3 O4. which comes from the galvanic corrosion of arbon steel support plates under oxygen-deficient condition. It is commonly hat there is a compact oxide film on the surface of pure titanium, which raises its electrode potential from-1.63 to 0.0V[15]. As a result, carbon steel whose electrode potential is-0.6V[15, will be the anode during galvanic on. The half-cell and total reactions are
the surrounding wear scratches. So it should have been created by adhesive wear by the irregular motion of a hard particle adhering to the surface of the titanium tube in micro-vibration. The width of the crack is about 255–285 lm (Fig. 11(b)). To define it more accurately, it is three-body wear between the hard particle, the outer wall of the titanium tube, and the internal boring, and it is also a kind of concentrated wear. 3.2.3. Micro-morphology and component analysis Fig. 12 presents the surface micro-morphology of the titanium tube. Scratches in the wear zone are somewhat directional (Fig. 12(b)), and there is layered deposit on the tube wall (Fig. 12(b)). EDS results of the deposit is shown in Fig. 13 and Table 3. The deposit mainly consists of iron oxides. 4. Discussion of failure mechanisms 4.1. Galvanic corrosion The black deposit on the surface of titanium tubes has proved to be Fe3O4, which comes from the galvanic corrosion of carbon steel support plates under oxygen-deficient condition. It is commonly known that there is a compact oxide film on the surface of pure titanium, which raises its electrode potential from 1.63 V [14] to 0.0 V [15]. As a result, carbon steel, whose electrode potential is 0.6 V [15], will be the anode during galvanic corrosion. The half-cell and total reactions are as follows: Fig. 10. Appearance of tube 1B160012-14 (a) 0 (b) 90 (c) 180 (d) 270 (e) S-shaped furrow crack magnified from (b). 36 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41
F- Chen et aL/ Engineering Failure Analysis 37(2014)29-41 al Fig. 11. Microscopic morphology of the furrow crack imaged by stereo microscope(a) furrow crack in the wear zone(b)measurement of furrow width. Anode. 2Fe-4e= 2Fe Cathode: 02+2H20+4e= 40H- Total reaction: 2Fe+O2 +2H20= 2Fe(OH)2 Then the ultra-active Fe(oHh formed soon turns into magnetite by the reaction 6Fe(OH)2+O2= 2Fe3O4+6H2O ut for those tubes of the outside bundles near the water inlet and outlet, enough oxygen brought in ric oxide by the reaction: 4Fe(OH)2+O2=2Fe203+4H2O 4Fe3 O4+02=6Fe20 That is why tubes of the outside bundles are covered by red corrosion products instead of black Fe3 O4 on the tubes of the
Anode : 2Fe 4e ¼ 2Fe2þ Cathode : O2 þ 2H2O þ 4e ¼ 4OH Total reaction : 2Fe þ O2 þ 2H2O ¼ 2FeðOHÞ2 Then the ultra-active Fe(OH)2 formed soon turns into magnetite by the reaction: 6FeðOHÞ2 þ O2 ¼ 2Fe3O4 þ 6H2O But for those tubes of the outside bundles near the water inlet and outlet, enough oxygen brought in during inspection can turn the iron oxides into the red ferric oxide by the reaction: 4FeðOHÞ2 þ O2 ¼ 2Fe2O3 þ 4H2O and 4Fe3O4 þ O2 ¼ 6Fe2O3 That is why tubes of the outside bundles are covered by red corrosion products instead of black Fe3O4 on the tubes of the inside bundles. Fig. 11. Microscopic morphology of the furrow crack imaged by stereo microscope (a) furrow crack in the wear zone (b) measurement of furrow width. F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41 37
F-f. Chen et aL/ Engineering Failure Analysis 37(2014)29-41 a -3 mm- b Fig. 12. Microscopic morphology of the wear zone in tube 1B160012-14 (a) irregularly curved crack(b)deposit morphology 4. 2. Eccentric contact wear and three-body contact wear Let us analyze the appearance of the wear zone of the first failure case. Firstly, the wear zone was a band around the tube cause of gravity induced contact between the tube and the internal boring. But as the wear zone was also distributed else- where, we might think that it was caused by the abrasive wear of the corrosion products that had sagged into the internal orings because of gravity. Then the second point must be noted. The wear extent was different in the forward direction and the backward direction of the tubes. If the wear was only caused by the corrosion products, the wear extent should uniform in these two directions. Therefore, there must be processing defects on the wall of the internal borings. Only the unevenly distributed negative deviation of the diameter of the internal borings could cause this special selective concen- rated wear phenomenon and the localized offset of the tube resulting from the grindings depositing in the internal borings aggravated the wear extent. The thinning process was like this: under the undulation of steam and water during operation, irregular relative vibration of the tubes and the internal borings caused the contact wear between them and with the pro- cessing defects distributed unevenly in the internal borings, eccentric wear occurred in different positions around the tubes and to various extents. Meanwhile, the corrosion products on the vertical support plates kept sagging into the internal bor- ings because of gravity. The position change of the corrosion products and grindings led to the offset of the tube. The result ras the three body contact wear between the tube, the grindings and corrosion products, and the internal boring In con- clusion, the localized wall thinning of titanium tube 1B134027-1 was rooted in eccentric contact wear ascribed to the poor processing of the internal borings and the corrosion products and grindings. Then it comes to the second failure case. The main wear morphology of titanium tube 1B160012-14 was similar with tube 1B134027-1, i.e. wear at different positions around the tube and to various extents. However, there was an irregular S- shaped furrow crack with a circumferential length of 10 mm
4.2. Eccentric contact wear and three-body contact wear Let us analyze the appearance of the wear zone of the first failure case. Firstly, the wear zone was a band around the tube instead of just at the bottom. It could be easily understood that wear was supposed to happen at the bottom of the tube because of gravity induced contact between the tube and the internal boring. But as the wear zone was also distributed elsewhere, we might think that it was caused by the abrasive wear of the corrosion products that had sagged into the internal borings because of gravity. Then the second point must be noted. The wear extent was different in the forward direction and the backward direction of the tubes. If the wear was only caused by the corrosion products, the wear extent should be uniform in these two directions. Therefore, there must be processing defects on the wall of the internal borings. Only the unevenly distributed negative deviation of the diameter of the internal borings could cause this special selective concentrated wear phenomenon. And the localized offset of the tube resulting from the grindings depositing in the internal borings aggravated the wear extent. The thinning process was like this: under the undulation of steam and water during operation, irregular relative vibration of the tubes and the internal borings caused the contact wear between them and with the processing defects distributed unevenly in the internal borings, eccentric wear occurred in different positions around the tubes and to various extents. Meanwhile, the corrosion products on the vertical support plates kept sagging into the internal borings because of gravity. The position change of the corrosion products and grindings led to the offset of the tube. The result was the three body contact wear between the tube, the grindings and corrosion products, and the internal boring. In conclusion, the localized wall thinning of titanium tube 1B134027-1 was rooted in eccentric contact wear ascribed to the poor processing of the internal borings and the corrosion products and grindings. Then it comes to the second failure case. The main wear morphology of titanium tube 1B160012-14 was similar with tube 1B134027-1, i.e. wear at different positions around the tube and to various extents. However, there was an irregular Sshaped furrow crack with a circumferential length of 10 mm. Fig. 12. Microscopic morphology of the wear zone in tube 1B160012-14 (a) irregularly curved crack (b) deposit morphology. 38 F.-J. Chen et al. / Engineering Failure Analysis 37 (2014) 29–41