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
Materials and Corrosion 2012. 63. No. 1 Mechanical degradation failure on leakage Figure 8. SEM morphologies of the different mechanical degradation traces on the three tips: (a) abrasive erosion, (b)flaking away, (c)impact, and (d) cracking nsequence, not only erosion but also impact were both exerted modeled as an ellipse with depth of a mm, i.e., the remaining on this localized area, and a rupture would be generated at last. thickness of the thinned part was (0.7-a)mm. In addition, two Fig. 15b presents the distribution of turbulence dissipation rate situations were considered. In situation 1, the tube was not on the tube wall, which evidently illustrated that relatively clogged by anything, so the pressures on both the inside wall and trong turbulence was also produced onto the tube wall after the the outside wall of the tube were normal, ie, 0.3 and 0. 4 MPa In seashell. This result may explain why the actinomorphous situation 2, the tube was clogged, so the pressure on the inside patterns(Fig. 7) were formed at the rupture tips on the tubes wall of the tube near the thinned part can be regarded as OMPa clogged by a seashell, as well as the diverse mechanical The results were shown in Fig. 18. It is obvious that, in both degradation modes in Fig.8. situations, the maximum stresses increased with the thinning In order for comparison, different inlet velocities of the depths. In fact, this is actually a vicious circle- the greater the natural seawater and different radii of the holes on the seashell stress is the more serious the erosion effect will be. and were both considered in series. Figure 16 displays the effects consequently the larger the thinning depth is; the larger the from these two factors on the maximum stresses onto the tube. thinning depth is, the greater the stress will be in return. Thus, it bviously, they both have a positive correlation, i.e., the erosion is not hard to explain why the thickness of the tube was thinned so effect will be aggravated with increases of the inlet velocity of the quickly. Moreover, it can be also learnt from Fig. 18 that the natural seawater and the diameter of the hole on the seashell. The maximum stresses in situation 2 were all greater than that in former one can be easily controlled in engineering, however situation 1. That is to say, clogging exactly aggravated the erosion the latter one was pretty catastrophic. Once holes were formed on effect on the tube wall a seashell, their radius would be definitely increased by erosion, consequently leading growing stresses on the tube wall. 3.4 Failure analysis Gradually, the thickness of the tube wall at localized area lik n Fig. 15a was thinned, and finally led to a rupture 3.4.1 Clogging and erosion Then, it comes to be interesting how the stresses were Erosion effect was indeed the most serious degradation on the distributed under erosion on the tube wall that was thinned Also titanium tubes in the RCw heat exchangers. Actually, this effect by means of FEM, as shown in Fig. 17, the removed part was was not so severe if the tubes were not clogged, i.e., it was only www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
consequence, not only erosion but also impact were both exerted on this localized area, and a rupture would be generated at last. Fig. 15b presents the distribution of turbulence dissipation rate on the tube wall, which evidently illustrated that relatively strong turbulence was also produced onto the tube wall after the seashell. This result may explain why the actinomorphous patterns (Fig. 7) were formed at the rupture tips on the tubes clogged by a seashell, as well as the diverse mechanical degradation modes in Fig. 8. In order for comparison, different inlet velocities of the natural seawater and different radii of the holes on the seashell were both considered in series. Figure 16 displays the effects from these two factors on the maximum stresses onto the tube. Obviously, they both have a positive correlation, i.e., the erosion effect will be aggravated with increases of the inlet velocity of the natural seawater and the diameter of the hole on the seashell. The former one can be easily controlled in engineering, however the latter one was pretty catastrophic. Once holes were formed on a seashell, their radius would be definitely increased by erosion, consequently leading growing stresses on the tube wall. Gradually, the thickness of the tube wall at localized area like in Fig. 15a was thinned, and finally led to a rupture. Then, it comes to be interesting how the stresses were distributed under erosion on the tube wall that was thinned. Also by means of FEM, as shown in Fig. 17, the removed part was modeled as an ellipse with depth of a mm, i.e., the remaining thickness of the thinned part was (0.7-a) mm. In addition, two situations were considered. In situation 1, the tube was not clogged by anything, so the pressures on both the inside wall and the outside wall of the tube were normal, i.e., 0.3 and 0.4 MPa. In situation 2, the tube was clogged, so the pressure on the inside wall of the tube near the thinned part can be regarded as 0 MPa. The results were shown in Fig. 18. It is obvious that, in both situations, the maximum stresses increased with the thinning depths. In fact, this is actually a vicious circle – the greater the stress is, the more serious the erosion effect will be, and consequently the larger the thinning depth is; the larger the thinning depth is, the greater the stress will be in return. Thus, it is not hard to explain why the thickness of the tube was thinned so quickly. Moreover, it can be also learnt from Fig. 18 that the maximum stresses in situation 2 were all greater than that in situation 1. That is to say, clogging exactly aggravated the erosion effect on the tube wall. 3.4 Failure analysis 3.4.1 Clogging and erosion Erosion effect was indeed the most serious degradation on the titanium tubes in the RCW heat exchangers. Actually, this effect was not so severe if the tubes were not clogged, i.e., it was only Materials and Corrosion 2012, 63, No. 1 Mechanical degradation failure on leakage of titanium tubes 23 Figure 8. SEM morphologies of the different mechanical degradation traces on the three tips: (a) abrasive erosion, (b) flaking away, (c) impact, and (d) cracking www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
24 1机组3HRCW51 1#机组3#RCW5181 1#机组3#RCW54-77 2006519 Figure 9. External appearances of the ruptures on leaked tubes clogged by rubber debris: (a) ruptures and crack, (b)rolled rubber debris,(c) all, (d) sediment, seashell, and rubber debris. As for the former one once erated on the outside wall of the which was frequently observed in the Part L. As a consequence, erosion effect would be exerted on the bulge by the natural seawater containing sediment particles, quickly thinning its wall thickness, and finally lead to a rupture. Once the rupture w formed, a small amount of seawater would flow through it and simultaneously induce erosion effect on its fringes too, gradually enlarging its size, and would sometimes even create an oriented (see Part I). In terms of th one, the following paragraphs will discuss the different erosion effects on the tubes when being clogged by, respectively, sediment, seashell, and rubber debris. In fact, the erosion effect on the tube when clogged by sediment was not so serious, only two consequences will be Figure 10. External appearance of the rupture evolved from brought about. The first one was still the erosion effect but in mechanical damage on tube outside wall severer extent Our past works[ 12-14] verified this fact. In detail, according to Equation(2), in which Q was the volume flow quantity, u was the fluid velocity, i was the average fluid velocity, and a was the cross-sectional area of the tube, in order to keep aggravated by other defects on the tubes in advance. Summariz. the consistence of the flow, i.e., Q remains constant, i will be g the results in both the Part I and the Part Il, two main kinds of certainly increased if a was reduced due to partly clogging of defects should be ascribed to- the electrochemical corrosions the tube by sediment. Then, it is a common sense that the faster pecially hydrogen blistering, and the clogging primarily by the fluid velocity is, the severer the erosion effect will be on the o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
aggravated by other defects on the tubes in advance. Summarizing the results in both the Part I and the Part II, two main kinds of defects should be ascribed to – the electrochemical corrosions especially hydrogen blistering, and the clogging primarily by sediment, seashell, and rubber debris. As for the former one, once hydrogen blistering was generated on the outside wall of the tube, the tube wall was localizedly bulged inwards the inside wall, which was frequently observed in the Part I. As a consequence, erosion effect would be exerted on the bulge by the natural seawater containing sediment particles, quickly thinning its wall thickness, and finally lead to a rupture. Once the rupture was formed, a small amount of seawater would flow through it and simultaneously induce erosion effect on its fringes too, gradually enlarging its size, and would sometimes even create an oriented eddy erosion effect on its corner (see Part I). In terms of the latter one, the following paragraphs will discuss the different erosion effects on the tubes when being clogged by, respectively, sediment, seashell, and rubber debris. In fact, the erosion effect on the tube when clogged by sediment was not so serious, only two consequences will be brought about. The first one was still the erosion effect but in severer extent. Our past works [12–14] verified this fact. In detail, according to Equation (2), in which Q was the volume flow quantity, u was the fluid velocity, u was the average fluid velocity, and A was the cross-sectional area of the tube, in order to keep the consistence of the flow, i.e., Q remains constant, u will be certainly increased if A was reduced due to partly clogging of the tube by sediment. Then, it is a common sense that the faster the fluid velocity is, the severer the erosion effect will be on the 24 Gong, Yang and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 9. External appearances of the ruptures on leaked tubes clogged by rubber debris: (a) ruptures and crack, (b) rolled rubber debris, (c) crimples on tube inside wall, (d) long indentations on tube inside wall Figure 10. External appearance of the rupture evolved from a mechanical damage on tube outside wall 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Mechanical degradation failure on leakage 1#机1# RCW NO.47-78 己0064c0 b) 1#机1# RCW NO.47-78 B Figure 12. External appearances of multiple dents along circumference on the inside wall of the tube:(a)location, and (b)inside wall Figure 11. SEM morphologies of the fretting effect on tube surface a)abrasive dust, (b)abrasive pits and cracks When the titanium tube was clogged by seashell, the erosion effect was more serious. At first, the seashell itself would be destroyed under erosion and given rise to one or more inside wall of the tube, as seen in Fig. 5b randomly distributed holes on it(Fig. 2). Then, once the seawater containing sediment particles traversed through such holes, it would be sprayed onto the inside wall of the tube section that was located after the seashell, inducing both erosion and impact effects and itself turned to be turbulent The Fem results testified this procedure. As a result, a U-shaped rupture with multiple However, the sole erosion effect would not directly result in tips was formed(Fig. 7b), even each tip was provided with ruptures. Then, the other consequence came out, i. e, impact. It is actinomorphous patterns Meanwhile, diverse mechanical deg easy to imagine that if the seawater containing sediment particles dation morphologies like impact, cracking, flaking away, etc. flew onto the scaled sediment whose surface was not quite could be also observed(Fig. 8). As a matter of fact, since the uniform, the particles originally in the seawater would bounce average width of the seashells was about 20 mm, absolutely close onto the upside wall of the tube -the exact procedure how impact to the diameter 19 mm of the titanium tubes, hence clogging by was brought about. Comparatively speaking, the erosion effect seashells was the prior trouble for the tubes in the RCW heat actually only uniformly and gradually thinned the tube wall, exchangers, which deserves particular concerns. whereas the impact effect would definitely produce localized pits In terms of clogging by rubber debris, besides the severe een in Fig 6a. Then, under the interaction between both erosion deformation of the tube wall because of the pressing effect and impact, the pits grew and finally evolved into ruptures. Totally from the rubber stripes, the erosion effect was actually more speaking, the comprehensive degradation extent on the titanium complicated. If the rubber stripes were rolled and covered nearly tubes that were clogged by sediment was not as severe as that the entire circumference of the tube inside wall, the erosion mode clogged by seashell and rubber debris. However scaling of was similar to that being clogged by seashell(Fig. 9c); if the sediment was the hardest to be detected and controlled in the tube rubber stripes were fully stretched, the erosion mode was similar side, consequently still deserves attention to that being clogged by sediment( Fig. 9d). However in more www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
inside wall of the tube, as seen in Fig. 5b. Q ¼ Z Z A udA ¼ uA (2) However, the sole erosion effect would not directly result in ruptures. Then, the other consequence came out, i.e., impact. It is easy to imagine that if the seawater containing sediment particles flew onto the scaled sediment whose surface was not quite uniform, the particles originally in the seawater would bounce onto the upside wall of the tube – the exact procedure how impact was brought about. Comparatively speaking, the erosion effect actually only uniformly and gradually thinned the tube wall, whereas the impact effect would definitely produce localized pits, seen in Fig. 6a. Then, under the interaction between both erosion and impact, the pits grew and finally evolved into ruptures. Totally speaking, the comprehensive degradation extent on the titanium tubes that were clogged by sediment was not as severe as that clogged by seashell and rubber debris. However scaling of sediment was the hardest to be detected and controlled in the tube side, consequently still deserves attention. When the titanium tube was clogged by seashell, the erosion effect was more serious. At first, the seashell itself would be destroyed under erosion and given rise to one or more randomly distributed holes on it (Fig. 2). Then, once the seawater containing sediment particles traversed through such holes, it would be sprayed onto the inside wall of the tube section that was located after the seashell, inducing both erosion and impact effects, and itself turned to be turbulent. The FEM results testified this procedure. As a result, a U-shaped rupture with multiple tips was formed (Fig. 7b), even each tip was provided with actinomorphous patterns. Meanwhile, diverse mechanical degradation morphologies like impact, cracking, flaking away, etc., could be also observed (Fig. 8). As a matter of fact, since the average width of the seashells was about 20 mm, absolutely close to the diameter 19 mm of the titanium tubes, hence clogging by seashells was the prior trouble for the tubes in the RCW heat exchangers, which deserves particular concerns. In terms of clogging by rubber debris, besides the severe deformation of the tube wall because of the pressing effect from the rubber stripes, the erosion effect was actually more complicated. If the rubber stripes were rolled and covered nearly the entire circumference of the tube inside wall, the erosion mode was similar to that being clogged by seashell (Fig. 9c); if the rubber stripes were fully stretched, the erosion mode was similar to that being clogged by sediment (Fig. 9d). However in more Materials and Corrosion 2012, 63, No. 1 Mechanical degradation failure on leakage of titanium tubes 25 Figure 11. SEM morphologies of the fretting effect on tube surface: (a) abrasive dust, (b) abrasive pits and cracks Figure 12. External appearances of multiple dents along circumference on the inside wall of the tube: (a) location, and (b) inside wall www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
26 Gong, Yang and Yuan Materials and Corrosion 2012. 63. No. 1 shallow 1 5kU 3/APR/86 Figure 14. FEM model of the tube clogged by a seashell: (a) total geometry and(b)fluid in tube 15kV X585日从n 38/APR/86 b)r82 15kU X1, 000 10Mm 30/APR/86 703.89 363,23 Figure 13. SEM logies of imperforated dents on the inside wall: 6022.57 (a)morphology of a dent, (b)erosion traces with disorderly directions, 10681.91 and (c)oriented erosion traces equent situations, the rubber debris within be was in amorphous shapes, thus the two erosion may be Figure 15. FEM results:(a)pressure and(b)turbulence dissipation rate simultaneously present. So it can be identified that the degradation extent on the titanium tubes by clogging of rubber debris was the severest. Fortunately, it was the easiest to be ruptures including both perforated and imperforated dents on the detected and prevented. inside walls of the titanium tubes were all brought about in the installation process when they were tracked by mechanical hands 3.4.2 Mechanical damage and fretting So we can define it the artificial mechanical damage. Once these In fact, considering the actual engineering conditions of the damages in shape of dents were formed, two kinds of results failure cases in Section 3. 2.5. it is not hard to determine that these would be brought about. on one hand. the natural seawater o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
frequent situations, the rubber debris within the tube was in amorphous shapes, thus the two erosion modes may be simultaneously present. So it can be identified that the degradation extent on the titanium tubes by clogging of rubber debris was the severest. Fortunately, it was the easiest to be detected and prevented. 3.4.2 Mechanical damage and fretting In fact, considering the actual engineering conditions of the failure cases in Section 3.2.5, it is not hard to determine that these ruptures including both perforated and imperforated dents on the inside walls of the titanium tubes were all brought about in the installation process when they were tracked by mechanical hands. So we can define it the artificial mechanical damage. Once these damages in shape of dents were formed, two kinds of results would be brought about. On one hand, the natural seawater 26 Gong, Yang and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 13. SEM morphologies of imperforated dents on the inside wall: (a) morphology of a dent, (b) erosion traces with disorderly directions, and (c) oriented erosion traces Figure 14. FEM model of the tube clogged by a seashell: (a) total geometry and (b) fluid in tube Figure 15. FEM results: (a) pressure and (b) turbulence dissipation rate 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Mechanical degradation failure on leakage a)23 case 1 1.7 2.32.52.72.93.13.3 /m·s b)35 k25 2 Figure 18. Maximum stresses on the thinned part of the tube with different depth 15 1.52.02.53.03.54.0 rd / mm Similarly, the scratch in Section 3. 2. 4 was also engendered Figure 16. Influencing factors on the maximum stresses on the tube in the installation process, i. e, was also an artificial mechanical wall: (a)inlet velocities and(b)radius of hole on seashell damage. However, since the damaged location was so near to the baffle plate, another critical cause as fretting was induced As was mentioned above, the 0.5 mm gap between the baffle plates and the tubes in the rCw heat exchangers left an action space for plates and tubes to periodically mutually contact, i.e., the fretting effect. As a result, the damaged part near the fretting gradually evolved into a rupture due to fatigue acture 4 Conclusions and recommendations 1. Sediments. seashells. and rubber debris, which were taken from the inlet of seawater chamber. were the three main foreign substances clogging in the failed tubes of the rcw heat exchangers and resulted in different erosion effects and traces. Clogging by sediments was the hardest to be detected, clogging by seashell was the most frequent, and clogging by rubber debris was the severest to 2. Mechanical damage, which was induced during installation of some tubes, was another kind of critical factor for the failures of the tubes 3. Fretting between the baffle plates and titanium tubes was also one important mechanical degradation factor on the Figure 17. FEM model of the thinned part of the tube tubes, particularly whe npanied with mechanical damages. 4. To sum up, totally four kinds of mechanical degradations that ontaining sediment particles would exert erosion and / or impact should be accounted for premature failures of the titanium effect on the dents, thinning the tube walls of the imperforated tubes were found in this Part IL, i.e., clogging, erosion, ones and enlarging the sizes of the perforated ones. On the other mechanical damage, and fretting. Subjected to complex hand, if such erosion effect was not so severe, microbes originally interaction of multiple degradation modes, the localized growing in the seawater would be attached on the fringes of the defects were initiated on the tubes, and eventually were perforated dents by flushing with the seawater, and would ther ruptured for the sustained erosion of seawater containing gradually grow and eventually the tube sediment particles www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
containing sediment particles would exert erosion and/or impact effect on the dents, thinning the tube walls of the imperforated ones and enlarging the sizes of the perforated ones. On the other hand, if such erosion effect was not so severe, microbes originally growing in the seawater would be attached on the fringes of the perforated dents by flushing with the seawater, and would then gradually grow and eventually clog the tube. Similarly, the scratch in Section 3.2.4 was also engendered in the installation process, i.e., was also an artificial mechanical damage. However, since the damaged location was so near to the baffle plate, another critical cause as fretting was induced. As was mentioned above, the 0.5 mm gap between the baffle plates and the tubes in the RCW heat exchangers left an action space for plates and tubes to periodically mutually contact, i.e., the fretting effect. As a result, the damaged part near the fretting gradually evolved into a rupture due to fatigue fracture. 4 Conclusions and recommendations 1. Sediments, seashells, and rubber debris, which were taken from the inlet of seawater chamber, were the three main foreign substances clogging in the failed tubes of the RCW heat exchangers and resulted in different erosion effects and traces. Clogging by sediments was the hardest to be detected, clogging by seashell was the most frequent, and clogging by rubber debris was the severest to the tubes. 2. Mechanical damage, which was induced during installation of the heat exchangers, on both the outside and the inside walls of some tubes, was another kind of critical factor for the failures of the tubes. 3. Fretting between the baffle plates and titanium tubes was also one important mechanical degradation factor on the tubes, particularly when accompanied with mechanical damages. 4. To sum up, totally four kinds of mechanical degradations that should be accounted for premature failures of the titanium tubes were found in this Part II, i.e., clogging, erosion, mechanical damage, and fretting. Subjected to complex interaction of multiple degradation modes, the localized defects were initiated on the tubes, and eventually were ruptured for the sustained erosion of seawater containing sediment particles. Materials and Corrosion 2012, 63, No. 1 Mechanical degradation failure on leakage of titanium tubes 27 Figure 16. Influencing factors on the maximum stresses on the tube wall: (a) inlet velocities and (b) radius of hole on seashell Figure 17. FEM model of the thinned part of the tube Figure 18. Maximum stresses on the thinned part of the tube with different depth www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
