D0:101002mac0201106190 Materials and Corrosion 2012. 63. No. 1 ailure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part ll: Mechanical degradation Y Gong, Z -G Yang and- -Z. Yuan Serious failure incidents like clogging, quick thinning, and leakage frequently occurred on lots of titanium tubes of heat exchangers in a nuclear power plant in China. In the part I of the whole failure analysis study with totally two parts factors mainly involving three kinds of electrochemical corrosions were investigated, including galvanic corrosion, crevice corrosion, and hydroge assisted corrosion. In the current Part ll, through microscopically analyzing the dispersive spectrometry(EDS), another four causes dominantly lying in the g2 ruptures on the leaked tubes by scanning electron microscopy(SEM)and energ aspect of mechanical degradation were determined-clogging, erosion, mechanical damaging, and fretting. Among them, the erosion effect was the primary one, thus the stresses it exerted on the tube wall were also supplementarily evaluated by finite element method( FEM). Based on the analysis results, the different degradation extents and morphologies by erosion on the tubes when they were clogged by different substances such as seashell, rubber debris, and sediments were compared and relevant mechanisms were discussed. Finally, countermeasures were put forward as well. 1 Introduction tubes(the tube side) to cool the desalinated water outside the tubes(the shell side) that has been just utilized in advance to cool Since China now has the largest numbers of nuclear power units the power and the steam equipments in, respectively, the nuclear that are under construction, designed, and planned in the world and the conventional islands, hence immediate determination of [1, 2 ] safety evaluation of the 13 units currently under operation the causes of these premature failures were extremely required to really has an instructive value for those upcoming ones. Indeed, avoid further economic losses and safety problems in the first 3 years after starting commercial operation( from 2003 Consequently, based on our past successful failure analysis to 2006) of the two 728MWe CANDU 6 nuclear power experiences[3-6), systematical study was carried out in series of imported from Atomic Energy of Canada Limited(AECL)) of two parts. The previous Part I[7 primarily focused on scope of Qinshan Phase Ill in Qinshan Nuclear Power Plant, the shell and electrochemical corrosion, such as galvanic corrosion, crevice tube RCW (recirculating cooling water) heat exchangers installed corrosion, and their interaction effect on the initiation of in the conventional island were frequently encountered with hydrogen-assisted corrosion including hydrogen blistering and failure incidents including clogging, quick thinning, and even hydrogen embrittlement. In the current Part II, analysis will leakage on their titanium heat exchange tubes, substantially less mainly orient to the aspect of mechanical degradation, especially than their design lifetime 40 years. Hereby, these RCw heat the typical representative erosion, on basis of microscopically changers are employed to use the natural seawater inside the analyzing the ruptures on the leaked tubes. As a result, different erosion effects on the titanium tubes when being clogged by different substances were comparatively discussed. And finall Y Gong, Z-G. Yang the prevention methods were proposed. Department of Materials Science, Fudan University, Shanghai 200433 Actually, such an engineering practical study of mechanical degradations on titanium tubes that are applied in the E-mail:zgyang@fudan.edu.cn conventional island of a nuclear power unit has been rarely J-Z Yuan reported. Bermudez et al. [8]observed the surface morphologies Third Qinshan Nuclear Power Co Ltd, Haiyan 314300, Zhejiang variation of pure titanium under a simply simulated erosi Province, (P. R. China) corrosion environment; Neville and McDougall [9] detaile o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim wileyonlinelibrary.com www.matcorr.com
Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part II: Mechanical degradation Y. Gong, Z.-G. Yang* and J.-Z. Yuan Serious failure incidents like clogging, quick thinning, and leakage frequently occurred on lots of titanium tubes of heat exchangers in a nuclear power plant in China. In the Part I of the whole failure analysis study with totally two parts, factors mainly involving three kinds of electrochemical corrosions were investigated, including galvanic corrosion, crevice corrosion, and hydrogenassisted corrosion. In the current Part II, through microscopically analyzing the ruptures on the leaked tubes by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS), another four causes dominantly lying in the aspect of mechanical degradation were determined – clogging, erosion, mechanical damaging, and fretting. Among them, the erosion effect was the primary one, thus the stresses it exerted on the tube wall were also supplementarily evaluated by finite element method (FEM). Based on the analysis results, the different degradation extents and morphologies by erosion on the tubes when they were clogged by different substances such as seashell, rubber debris, and sediments were compared, and relevant mechanisms were discussed. Finally, countermeasures were put forward as well. 1 Introduction Since China now has the largest numbers of nuclear power units that are under construction, designed, and planned in the world [1, 2], safety evaluation of the 13 units currently under operation really has an instructive value for those upcoming ones. Indeed, in the first 3 years after starting commercial operation (from 2003 to 2006) of the two 728MWe CANDU 6 nuclear power units (imported from Atomic Energy of Canada Limited (AECL)) of Qinshan Phase III in Qinshan Nuclear Power Plant, the shell and tube RCW (recirculating cooling water) heat exchangers installed in the conventional island were frequently encountered with failure incidents including clogging, quick thinning, and even leakage on their titanium heat exchange tubes, substantially less than their design lifetime 40 years. Hereby, these RCW heat exchangers are employed to use the natural seawater inside the tubes (the tube side) to cool the desalinated water outside the tubes (the shell side) that has been just utilized in advance to cool the power and the steam equipments in, respectively, the nuclear and the conventional islands, hence immediate determination of the causes of these premature failures were extremely required to avoid further economic losses and safety problems. Consequently, based on our past successful failure analysis experiences [3–6], systematical study was carried out in series of two parts. The previous Part I [7] primarily focused on scope of electrochemical corrosion, such as galvanic corrosion, crevice corrosion, and their interaction effect on the initiation of hydrogen-assisted corrosion including hydrogen blistering and hydrogen embrittlement. In the current Part II, analysis will mainly orient to the aspect of mechanical degradation, especially the typical representative erosion, on basis of microscopically analyzing the ruptures on the leaked tubes. As a result, different erosion effects on the titanium tubes when being clogged by different substances were comparatively discussed. And finally, the prevention methods were proposed. Actually, such an engineering practical study of mechanical degradations on titanium tubes that are applied in the conventional island of a nuclear power unit has been rarely reported. Bermu´dez et al. [8] observed the surface morphologies variation of pure titanium under a simply simulated erosion– corrosion environment; Neville and McDougall [9] detailedly 18 DOI: 10.1002/maco.201106190 Materials and Corrosion 2012, 63, No. 1 Y. Gong, Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, (P. R. China) E-mail: zgyang@fudan.edu.cn J.-Z. Yuan Third Qinshan Nuclear Power Co. Ltd., Haiyan 314300, Zhejiang Province, (P. R. China) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Mechanical degradation failure on leakage of titani ium tubes analyzed the weight loss, hardness deterioration, etc, on pure a)l titanium and its alloys under erosion. However, all these researches were only conducted in laboratories. As for the actual engineering failure cases, El-Dahshan et al. [10] investigate droplet erosion on the titanium tubes in a condenser of one mSF (multi stage flash) distiller, and Ma et al. [11]carried out a failure analysis study of leakage on titanium tubes within condensers of PTA(P-phthalic acid) production lines, and determined the main cause was fatigue fracture. However, such titanium tubes were seashell not related to nuclear power units. Hence, achievement of current study has critical engineering values in equipment design and mechanical failures prevention of titanium heat exchanger tubes that are used in similar seawater environment for not only nuclear power plants, but also equipment in other industries like thermal power, petrochemical, chemical, metallurgical, and b) 2 Experimental As has been illustrated in the Part I, the titanium tubes with specification ofΦ19×14630×0.71 mm in the rcw heat changers are sustained by 23 carbon steel baffle plates .6 mm-thick) with interval distance of 603 mm, and are mm- thick carbon steel plates cladded with titanium. Diameter of the ustaining holes on all the plates was 19.25+0.51 mm, leaving gaps of less than 0.5 mm between the plate and the tubes due to unsealed welding on the inlet of tube sheet For supplementation, the concrete operation parameters of these tubes are listed Table 1 seen from Fig. 1 that a large number of titanium tubes were clogged by different substances in the inlet of one heat exchanger, for example, by seashells( Fig. 1a), sediments(Fig 1b) and even rubber debris(Fig. Ic)that was the originally liner in the inside wall of the seawater chamber. As a result, these clogged tubes were detected to be thinned. and some of them were even Then, by means of scanning electron microscopy(SEM)and energy dispersive spectrometry(EDS), investigation will dom- inantly focus on the microscopic morphologies and micro-area compositions of the ruptures on the leaked tubes clogged by different substances. Meanwhile, features of the clogging substances as sediment, seashell, and rubber debris were also characterized by optical microscopy (OM), SEM, and EDS. Figure 1 External appearances of clogging in one heat exchanger: (a) Particularly, the stress distribution on the wall of the tube when by seashell, (b)by sediment, (c) by rubber debris it is clogged by a seashell, as well as the erosion effect on the thinned part of the tube wall, were both computationally simulated by commercial finite element method(FEM) software ANsYS. The detailed results are as follows Table 1. Operation parameters of the RCW heat exchanger Media Velocity Outlet o(m/s) T(°C) emp.T(°C) Tube side 2.7 35.1 www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
analyzed the weight loss, hardness deterioration, etc., on pure titanium and its alloys under erosion. However, all these researches were only conducted in laboratories. As for the actual engineering failure cases, El-Dahshan et al. [10] investigated droplet erosion on the titanium tubes in a condenser of one MSF (multi stage flash) distiller, and Ma et al. [11] carried out a failure analysis study of leakage on titanium tubes within condensers of PTA ( p-phthalic acid) production lines, and determined the main cause was fatigue fracture. However, such titanium tubes were not related to nuclear power units. Hence, achievement of current study has critical engineering values in equipment design and mechanical failures prevention of titanium heat exchanger tubes that are used in similar seawater environment for not only nuclear power plants, but also equipment in other industries like thermal power, petrochemical, chemical, metallurgical, and so on. 2 Experimental As has been illustrated in the Part I, the titanium tubes with specification of F19 14 630 0.71 mm in the RCW heat exchangers are sustained by 23 carbon steel baffle plates (16 mm-thick) with interval distance of 603 mm, and are hydraulically expanded at two ends of tube sheet with 78 mmthick carbon steel plates cladded with titanium. Diameter of the sustaining holes on all the plates was 19.25 0.51 mm, leaving gaps of less than 0.5 mm between the plate and the tubes due to unsealed welding on the inlet of tube sheet. For supplementation, the concrete operation parameters of these tubes are listed in Table 1. It can be seen from Fig. 1 that a large number of titanium tubes were clogged by different substances in the inlet of one heat exchanger, for example, by seashells (Fig. 1a), sediments (Fig. 1b), and even rubber debris (Fig. 1c) that was the originally liner in the inside wall of the seawater chamber. As a result, these clogged tubes were detected to be thinned, and some of them were even leaked. Then, by means of scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS), investigation will dominantly focus on the microscopic morphologies and micro-area compositions of the ruptures on the leaked tubes clogged by different substances. Meanwhile, features of the clogging substances as sediment, seashell, and rubber debris were also characterized by optical microscopy (OM), SEM, and EDS. Particularly, the stress distribution on the wall of the tube when it is clogged by a seashell, as well as the erosion effect on the thinned part of the tube wall, were both computationally simulated by commercial finite element method (FEM) software ANSYS. The detailed results are as follows. Materials and Corrosion 2012, 63, No. 1 Mechanical degradation failure on leakage of titanium tubes 19 Table 1. Operation parameters of the RCW heat exchanger Media Flux Q (m3 /s) Velocity v (m/s) Pressure p (MPa) Inlet temp. T (8C) Outlet temp. T (8C) Shell side Desalinated water 2.21 1.0 0.4 41.5 35.0 Tube side Seawater 3.34 2.7 0.3 30.5 35.1 Figure 1. External appearances of clogging in one heat exchanger: (a) by seashell, (b) by sediment, (c) by rubber debris www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
20 Gong, Yang and Yuan Materials and Corrosion 2012. 63. No. 1 6 tumbril Figure 2. Macroscopic morphologies of the seashells clogging in the leaked tubes Figure 4. Microscopic morphologies of the rubber debris clogging in the leaked tubes 3.1.3 Rubber debris Before characterization, let us identify the sources of the rubber debris at first. The rubber liner was initially on the inside wall of the seawater chamber that was installed in front of the inlet of the titanium tubes. However, since such liner was not tightly attached on the inside wall of the chamber due to some uncertain factors in manufacture. corrosion occurred on the surface of the inside wall □s with matrix of carbon steel. With growth of the corrosion products, the liner was detached from the chamber and fractioned into small pieces under the continuous erosion from the seawater. Subsequently, some of the rubber debr flushed into and then clogged the titanium tubes Figure 4 shows that the rubber debris was covered with randomly distributed sediment particles, and its fringe w smooth and thinned. Meanwhile, a linear crack was also present. These evidences obviously verified that the rubber debris had Figure 3. Microscopic morphologies of the sediment clogging in the exactly undergone severe erosion from the seawater containing aked tubes 3.2 Macro- and microscopic observation of ruptures 3 Results and discussion 3.2.1 Tube clogged by sediments 3.1 Environmental media inspection As is displayed in Fig. 5a, there were totally three ruptures on the leaked tube clogged by sediment, and their locations ranged from 3.1.1 Seashell 125 to 145 mm off the inlet. since no other obvious defects existed As is displayed in Fig. 2, the seashell clogging in the leaked tubes on the outside wall of the tube, it is easy to infer that these three like in Fig. la had a size of about 30 X 20mm, close to the ruptures may have been generated by causes inside the tube diameter of the titanium tubes 19 mm. Thus, it's not hard to Indeed, serious plastic deformation and erosion traces were exactly understand why the seashells were easy to be clogged in the present on the tube inside wall near the ruptures, seen in Fig 5b tubes. Meanwhile, perforation with sizes ranging from multiple For further investigation, the rupture named Bin Fig 5b will millimeters to centimeters can be found on the seashells as well, be then observed under SEM. Actually, this rupture whose length which were possibly generated from serious erosion effect in was about 1.5 mm was just next to the crimple, seen in Fig. 6 service After magnifying, a 400 um-long crack can be seen on the tip the bullet- shaped rupture( Fig. 6b), even some small pits with 3.1.2 Sediments diameter of about several microns were around it (marked with As shown in Fig 3, the sediment clogging in the leaked tubes like arrows), which was exactly the evidence of impact effect on the in Fig. 1b had an average particle size of about 50-100 um, and tube wall from the sediments contained in the seawater Impact these particles were agglomerated so compact that the titanium usually leads to severer result on the tube than erosion, for tubes were clogged. example in Fig. 6c, the linear fracture edge manifests that a small o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
3 Results and discussion 3.1 Environmental media inspection 3.1.1 Seashell As is displayed in Fig. 2, the seashell clogging in the leaked tubes like in Fig. 1a had a size of about 30 T 20 mm2 , close to the diameter of the titanium tubes 19 mm. Thus, it’s not hard to understand why the seashells were easy to be clogged in the tubes. Meanwhile, perforation with sizes ranging from multiple millimeters to centimeters can be found on the seashells as well, which were possibly generated from serious erosion effect in service. 3.1.2 Sediments As shown in Fig. 3, the sediment clogging in the leaked tubes like in Fig. 1b had an average particle size of about 50–100 mm, and these particles were agglomerated so compact that the titanium tubes were clogged. 3.1.3 Rubber debris Before characterization, let us identify the sources of the rubber debris at first. The rubber liner was initially on the inside wall of the seawater chamber that was installed in front of the inlet of the titanium tubes. However, since such liner was not tightly attached on the inside wall of the chamber due to some uncertain factors in manufacture, corrosion occurred on the surface of the inside wall with matrix of carbon steel. With growth of the corrosion products, the liner was detached from the chamber and then fractioned into small pieces under the continuous erosion effect from the seawater. Subsequently, some of the rubber debris was flushed into and then clogged the titanium tubes. Figure 4 shows that the rubber debris was covered with randomly distributed sediment particles, and its fringe was smooth and thinned. Meanwhile, a linear crack was also present. These evidences obviously verified that the rubber debris had exactly undergone severe erosion from the seawater containing sediment particles. 3.2 Macro- and microscopic observation of ruptures 3.2.1 Tube clogged by sediments As is displayed in Fig. 5a, there were totally three ruptures on the leaked tube clogged by sediment, and their locations ranged from 125 to 145mm off the inlet. Since no other obvious defects existed on the outside wall of the tube, it is easy to infer that these three ruptures may have been generated by causes inside the tube. Indeed, serious plastic deformation and erosion traces were exactly present on the tube inside wall near the ruptures, seen in Fig. 5b. For further investigation, the rupture named B in Fig. 5b will be then observed under SEM. Actually, this rupture whose length was about 1.5 mm was just next to the crimple, seen in Fig. 6a. After magnifying, a 400mm-long crack can be seen on the tip of the bullet-shaped rupture (Fig. 6b), even some small pits with diameter of about several microns were around it (marked with arrows), which was exactly the evidence of impact effect on the tube wall from the sediments contained in the seawater. Impact usually leads to severer result on the tube than erosion, for example in Fig. 6c, the linear fracture edge manifests that a small 20 Gong, Yang and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 2. Macroscopic morphologies of the seashells clogging in the leaked tubes Figure 3. Microscopic morphologies of the sediment clogging in the leaked tubes Figure 4. Microscopic morphologies of the rubber debris clogging in the leaked tubes 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Mechanical degradation failure on leakage of titanium tubes 21 a 9 123456 mteulmmmuilulllluuluullmlmlil X35 500Nm 24/DEC/0S B c) Figure 5. External appearances of the ruptures on leaked tube clogge by sediment: (a) locations of ruptures, (b)deformed inside wall piece of tube material had already directly dissociated from the tube wall, rather than being gradually flaked away. 3.2.2 Tube clogged by seashell Rupture on this tube clogged by a seashell was located approximately omm off the inlet, as shown in Fig. 7a, ie, it was buried inside the 78mm-thick Ti/carbon steel tube sheet This rupture was generated by the erosion of seawater containing sediments on x156168 24∠DEC/85 logging position of the seashell and its failure morphology looked ke actinomorphous, as seen in Fig. 7b. Also, it should be Figure 6 SEM morphologies of the rupture B on the inside wall particularly pointed out that, on the inside wall of the 9X 5mm (a) total morphology, (b) crack, and (c) dissociation tinomorphous rupture, the erosion traces on the three tips (named A, B, and C)were all in the same actinomorphous shape too Then, the three tips were further detected under SEM. After ruptures and a 250 mm-long crack, seen in Fig. 9a. After cutting summarizing, traces representing four different mechanical off, it was displayed in Fig. 9b that rubber stripes were rolled and degradation mechanisms were observed, including abrasive attached around the entire circumference of the tube inside wall. erosion(Fig. 8a), flaking away(Fig. 8b), impact(Fig. 8c), and As a result, the inside wall was seriously deformed due to the cracking(Fig. 8d), more diverse than the rupture on the leaked pressing effect from the rubber(Fig. 9c), and the crimples were metrically on the two sides of the ruptures. As for the other tube, long indentations instead of crimples caused by pressing 3. 2.3 Tube clogged by rubber debris effect from the rubber stripes were on the inside wall of the tube Two leaked tubes that were both clogged by rubber debris were seen in Fig. 9d, which means the rubber stripes were fully sampled. The first tube was encountered with three round stretched rather than being rolled. www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
piece of tube material had already directly dissociated from the tube wall, rather than being gradually flaked away. 3.2.2 Tube clogged by seashell Rupture on this tube clogged by a seashell was located approximately 40mm off the inlet, as shown in Fig. 7a, i.e., it was buried inside the 78 mm-thick Ti/carbon steel tube sheet. This rupture was generated by the erosion of seawater containing sediments on clogging position of the seashell and its failure morphology looked like actinomorphous, as seen in Fig. 7b. Also, it should be particularly pointed out that, on the inside wall of the 9 T 5 mm2 actinomorphous rupture, the erosion traces on the three tips (named A, B, and C) were all in the same actinomorphous shape too. Then, the three tips were further detected under SEM. After summarizing, traces representing four different mechanical degradation mechanisms were observed, including abrasive erosion (Fig. 8a), flaking away (Fig. 8b), impact (Fig. 8c), and cracking (Fig. 8d), more diverse than the rupture on the leaked tube clogged by sediments. 3.2.3 Tube clogged by rubber debris Two leaked tubes that were both clogged by rubber debris were sampled. The first tube was encountered with three round ruptures and a 250 mm-long crack, seen in Fig. 9a. After cutting off, it was displayed in Fig. 9b that rubber stripes were rolled and attached around the entire circumference of the tube inside wall. As a result, the inside wall was seriously deformed due to the pressing effect from the rubber (Fig. 9c), and the crimples were symmetrically on the two sides of the ruptures. As for the other tube, long indentations instead of crimples caused by pressing effect from the rubber stripes were on the inside wall of the tube, seen in Fig. 9d, which means the rubber stripes were fully stretched rather than being rolled. Materials and Corrosion 2012, 63, No. 1 Mechanical degradation failure on leakage of titanium tubes 21 Figure 5. External appearances of the ruptures on leaked tube clogged by sediment: (a) locations of ruptures, (b) deformed inside wall Figure 6. SEM morphologies of the rupture B on the inside wall: (a) total morphology, (b) crack, and (c) dissociation www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
22 Gong, Yang and Yuan 2,63,No. Multiple dents occurred on the circumferential line of the tube and they were pretty unusual. The dents were uniformly distributed along the circumferential line on the inside wall of the tube and their surrounding surfaces were very smooth, see 2 3 6789 the marked line in Fig. 12. These regular dents were actually acked to the inlet tube sheet for joining. In this failure analys kinds of peculiar flaws were found to appear on totally three tubes. With respect to the ruptures, some dents were perforated nd leaked As shown in Fig. 13a, there were two overlapping dents in one rupture, one was shallow, and the other was deeper. That meant the track head exerted on the inside wall of the tube happened to be slightly slided. When the dent was shallow, the erosion traces by seawater containing sediments were actually in orderly directions(Fig. 13b), however as for the deep erosion traces were oriented to one direction(marked with arrows in Fig. 13c) and their depths were even larger due to erosion of sediments. In general, with respect to the shallow one, those d erosion traces were actually embedded in the TiOz passive film which exhibited really good hardness, thus the erosion extent by seawater containing sediment particles was not so severe and the traces were in disorder. However, once the TiOz passive film with thickness of several dozens of nanometers was removed under B sustaining erosion of seawater, the fresh pure titanium was exposed to erosion, which was much more ductile than TiOz,so the erosion traces were oriented and deeper. Under sustained erosion from seawater containing sediments, some dents on the Figure 7. External appearances of the rupture on leaked tube clogged tube would gradually become deeper and larger with time, and by a seashell:(a)location, and( b)inside wall 3.3 Finite element method As learnt from the engineers in Qinshan Nuclear Power Plant, seashell was the main kind of substance clogging in the titanium 3.2.4 Mechanical damaging on tube outside wall tubes than sediment and rubber debris thus the erosion effect on Rupture location on this tube was 1715 mm off the inlet, near the the tube when clogged by a seashell was particularly qualitatively third baffle plate (interval distance 603 mm), seen in Fig. 10. evaluated by FEM. Figure 14a was the 1/2 FEM model established Actually, a mechanical scratch whose direction was roughly for simulation, in which the seashell was simplified as a 1 mm marked along the red line was across this rupture. It is not hard to thick round plate containing a p=8 mm hole in the center, and infer that this scratch was definitely brought about by some hard was located 45 to the axial direction of the tube. The fluid machine during the installation process of the tubes. Then, how simulating the natural seawater in the tube was displayed in this mechanical damage eventually evolved into a rupture was Fig. 14b, whose inlet velocity was 2.7 m/s, and the velocity wondered. Considering the configuration of the RCW heat through the hole on the seashell can be calculated by Equation(1) exchangers, gaps less than 0.5 mm were always left between the in which, Vo, Ao, respectively, denotes the inlet velocity and the ustaining plates and the tubes. In the Part I, the detailed cross-sectional area of the tube, and As was the area of the hole mechanisms of galvanic corrosion and crevice corrosion on titanium tubes aroused by such gaps was discussed. Actually, besides these electrochemical corrosions. another mechanical Vs= Vo degradation would be also brought about by the gaps in service, i.e., fretting, which would induce periodical contacting between the tubes and the plates. As a consequence, not only abrasive It is clearly shown in Fig. 15a that the existence of the seashell dusts of metal particles were produced on the tube surface, seen dramatically reduced the pressure on the tube inside wall that was in Fig. 1la; but also abrasive pits and cracks were engendered on located after the seashell, however the localized area facing toward the tube surface, seen in Fig. 11b. Since this mechanical scratch the hole was exceptional. This was actually the exact area was near the third baffle plate, it was gradually aggravated under undergoing severest erosion effect by the natural seawater, let such continuous fretting effect and finally evolved into a rupture. alone the fact that rigid sediment particles were also present. as a o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
3.2.4 Mechanical damaging on tube outside wall Rupture location on this tube was 1715 mm off the inlet, near the third baffle plate (interval distance 603 mm), seen in Fig. 10. Actually, a mechanical scratch whose direction was roughly marked along the red line was across this rupture. It is not hard to infer that this scratch was definitely brought about by some hard machine during the installation process of the tubes. Then, how this mechanical damage eventually evolved into a rupture was wondered. Considering the configuration of the RCW heat exchangers, gaps less than 0.5 mm were always left between the sustaining plates and the tubes. In the Part I, the detailed mechanisms of galvanic corrosion and crevice corrosion on titanium tubes aroused by such gaps was discussed. Actually, besides these electrochemical corrosions, another mechanical degradation would be also brought about by the gaps in service, i.e., fretting, which would induce periodical contacting between the tubes and the plates. As a consequence, not only abrasive dusts of metal particles were produced on the tube surface, seen in Fig. 11a; but also abrasive pits and cracks were engendered on the tube surface, seen in Fig. 11b. Since this mechanical scratch was near the third baffle plate, it was gradually aggravated under such continuous fretting effect and finally evolved into a rupture. 3.2.5 Mechanical damaging on inside wall of the tube Multiple dents occurred on the circumferential line of the tube and they were pretty unusual. The dents were uniformly distributed along the circumferential line on the inside wall of the tube and their surrounding surfaces were very smooth, see the marked line in Fig. 12. These regular dents were actually introduced when long titanium tubes were tracked to the inlet of the tube sheet for joining. In this failure analysis event, such kinds of peculiar flaws were found to appear on totally three tubes. With respect to the ruptures, some dents were perforated and leaked, and some were not. As shown in Fig. 13a, there were two overlapping dents in one rupture, one was shallow, and the other was deeper. That meant the track head exerted on the inside wall of the tube happened to be slightly slided. When the dent was shallow, the erosion traces by seawater containing sediments were actually in disorderly directions (Fig. 13b), however as for the deeper one, the erosion traces were oriented to one direction (marked with arrows in Fig. 13c) and their depths were even larger due to erosion of sediments. In general, with respect to the shallow one, those erosion traces were actually embedded in the TiO2 passive film, which exhibited really good hardness, thus the erosion extent by seawater containing sediment particles was not so severe and the traces were in disorder. However, once the TiO2 passive film with thickness of several dozens of nanometers was removed under sustaining erosion of seawater, the fresh pure titanium was exposed to erosion, which was much more ductile than TiO2, so the erosion traces were oriented and deeper. Under sustained erosion from seawater containing sediments, some dents on the tube would gradually become deeper and larger with time, and finally were perforated. 3.3 Finite element method As learnt from the engineers in Qinshan Nuclear Power Plant, seashell was the main kind of substance clogging in the titanium tubes than sediment and rubber debris, thus the erosion effect on the tube when clogged by a seashell was particularly qualitatively evaluated by FEM. Figure 14a was the 1/2 FEM model established for simulation, in which the seashell was simplified as a 1 mmthick round plate containing a F ¼ 8 mm hole in the center, and was located 458 to the axial direction of the tube. The fluid simulating the natural seawater in the tube was displayed in Fig. 14b, whose inlet velocity was 2.7 m/s, and the velocity through the hole on the seashell can be calculated by Equation (1), in which, V0, A0, respectively, denotes the inlet velocity and the cross-sectional area of the tube, and As was the area of the hole. Vs ¼ V0 A0 As (1) It is clearly shown in Fig. 15a that the existence of the seashell dramatically reduced the pressure on the tube inside wall that was located after the seashell, however the localized area facing toward the hole was exceptional. This was actually the exact area undergoing severest erosion effect by the natural seawater, let alone the fact that rigid sediment particles were also present. As a 22 Gong, Yang and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 7. External appearances of the rupture on leaked tube clogged by a seashell: (a) location, and (b) inside wall 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com