Materials and Corrosion 2012. 63. No. 1 D:101002maco.201106189 Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part l: electrochemical corrosion Z.-G. Yang,Y Gong and ) .-Z.Yuan Titanium tubes generally exhibit superior resistance against electrochemical corrosions amid seawater for their passive films Tio2. However, hydrogen- assisted corrosion(HAC) is actually the Achilles'heel to titanium materials when he temperature exceeds near 70C. In this event, severe degradations like quick thinning and leakage were frequently detected on a large number of titani tubes exposed to natural seawater environment within heat exchangers in a nuclear power plant, which caused serious safety problems. This paper is the Part I of totally two parts conducted for the whole failure analysis study, mainly focusing on electrochemical aspect of failure causes and their behaviors By means of over ten kinds of characterization methods, the analysis results identified that the hAc induced by the interaction effects between galvanic corrosion and crevice corrosion led to local bulges of the inner walls of some titanium tubes, and then the bulges were quickly thinned and eventually uptured under the eddy erosion from the seawater containing sediment particles. Finally, relevant mechanisms were addressed in detail and prevention methods were proposed as well 1 Introduction tube [5 rcw heat exchangers, within which the desalinated water is conveyed outside the tubes(also called the shell side), and mong all the thirteen nuclear power units presently under the seawater is transported inside the tubes(also called the tube peration in China[1, 2 ] the two 728MWe CANDU 6 units in the side). However, natural seawater usually contains high contents Phase Ill of Qinshan Nuclear Power Plant, which were imported of salts, chloride ions, and even sediment particles, various from Atomic Energy of Canada Limited (AECL), are the first and selective corrosions as well as mechanical degradations are the only two pressurized heavy water reactor(PHWR) units in consequently prone to emerge on these heat exchanger tubes with China, and started commercial operation on 31 December 2002 matrix of titanium, greatly reducing their service lifetimes and 24 July 2003, respectively, with design lifetime of 40 years [ 3) In this event, during about 3 years after actual operatio In a Candu 6 style unit, the recirculating cooling water(2003-2006), failure incidents including clogging, quick thin- (RCW)system consists of two heat exchange loops -the first one ning, and even leakage, etc. frequently occurred on a great cools the power equipments in the nuclear island and the steam number of titanium tubes within the rCW heat exchangers of the equipments in the conventional island by means of desalinated two CAndU 6 nuclear power units in Qinshan Phase Ill, causing water, while the second one cools such warmed desalinated water substantial economic losses as well as potential safety problems by using natural seawater within a specific kind of equipment [6). Materials quality, equipment operation, service environment, called RCW heat exchanger [4]. Hereby, each of the two CANDU routine maintenance, or other factors, which were the main 6 units is equipped in the conventional island with four shell and causes for inducing these premature failures in these cooling equipment, were urgently investigated. Consequently, in order to immediately identify the causes of such failures, investigation Z-G. Yang, Y Gong into four aspects were carried out by referencing our previous Department of Materials Science, Fudan University, Shanghai 200433 successful failure analysis experiences 7-9), including matrix R. Ch materials, environmental media, operation conditions, installa- E-mail:ziyang@fudan.edu.cn tion and maintenance. based on the leaked tubes and the 1-Z Yuan environmental media as seawater, desalinated water. etc.. over 10 Third Qinshan Nuclear Power Co Ltd, Haiyan 314300, Zhejiang kinds of characterization methods were conducted for failure Province(P R China) analysis in series of totally two parts, and totally seven kinds of www.matcorr.com wileyonlinelibrary.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
Failure analysis of leakage on titanium tubes within heat exchangers in a nuclear power plant. Part I: Electrochemical corrosion Z.-G. Yang*, Y. Gong and J.-Z. Yuan Titanium tubes generally exhibit superior resistance against electrochemical corrosions amid seawater for their passive films TiO2. However, hydrogenassisted corrosion (HAC) is actually the Achilles’ heel to titanium materials when the temperature exceeds near 70 8C. In this event, severe degradations like quick thinning and leakage were frequently detected on a large number of titanium tubes exposed to natural seawater environment within heat exchangers in a nuclear power plant, which caused serious safety problems. This paper is the Part I of totally two parts conducted for the whole failure analysis study, mainly focusing on electrochemical aspect of failure causes and their behaviors. By means of over ten kinds of characterization methods, the analysis results identified that the HAC induced by the interaction effects between galvanic corrosion and crevice corrosion led to local bulges of the inner walls of some titanium tubes, and then the bulges were quickly thinned and eventually ruptured under the eddy erosion from the seawater containing sediment particles. Finally, relevant mechanisms were addressed in detail and prevention methods were proposed as well. 1 Introduction Among all the thirteen nuclear power units presently under operation in China [1, 2], the two 728MWe CANDU 6 units in the Phase III of Qinshan Nuclear Power Plant, which were imported from Atomic Energy of Canada Limited (AECL), are the first and the only two pressurized heavy water reactor (PHWR) units in China, and started commercial operation on 31 December 2002 and 24 July 2003, respectively, with design lifetime of 40 years [3]. In a CANDU 6 style unit, the recirculating cooling water (RCW) system consists of two heat exchange loops – the first one cools the power equipments in the nuclear island and the steam equipments in the conventional island by means of desalinated water, while the second one cools such warmed desalinated water by using natural seawater within a specific kind of equipment called RCW heat exchanger [4]. Hereby, each of the two CANDU 6 units is equipped in the conventional island with four shell and tube [5] RCW heat exchangers, within which the desalinated water is conveyed outside the tubes (also called the shell side), and the seawater is transported inside the tubes (also called the tube side). However, natural seawater usually contains high contents of salts, chloride ions, and even sediment particles, various selective corrosions as well as mechanical degradations are consequently prone to emerge on these heat exchanger tubes with matrix of titanium, greatly reducing their service lifetimes. In this event, during about 3 years after actual operation (2003–2006), failure incidents including clogging, quick thinning, and even leakage, etc. frequently occurred on a great number of titanium tubes within the RCW heat exchangers of the two CANDU 6 nuclear power units in Qinshan Phase III, causing substantial economic losses as well as potential safety problems [6]. Materials quality, equipment operation, service environment, routine maintenance, or other factors, which were the main causes for inducing these premature failures in these cooling equipment, were urgently investigated. Consequently, in order to immediately identify the causes of such failures, investigations into four aspects were carried out by referencing our previous successful failure analysis experiences [7–9], including matrix materials, environmental media, operation conditions, installation, and maintenance. Based on the leaked tubes and the environmental media as seawater, desalinated water, etc., over 10 kinds of characterization methods were conducted for failure analysis in series of totally two parts, and totally seven kinds of Materials and Corrosion 2012, 63, No. 1 DOI: 10.1002/maco.201106189 7 Z.-G. Yang, Y. Gong 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) www.matcorr.com wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
8 Yang, Gong and Yuan Materials and Corrosion 2012. 63. No. 1 different failure mechanisms were simultaneously detected. As a hydraulically expanded (expansion ratio 80%)with carbon steel matter of fact, such a comprehensive failure analysis study from tube sheets cladded by titanium(ASME SA-515 Gr 65 carbon steel engineering practice for titanium tubes that are applied in nuclear [13 78mm-thick, ASME SB-265 Gr 1 Ti[14]3 mm-thick), without power units has been rarely reported. In the Part I of this whole seal welding. Therein, the diameter of supporting holes on all the study, research mainly focused on the electrochemical corrosion plates was 19.25+0.51mm. The schematic diagram of the RCW on the titanium tubes, while in the Part II [10], analysis heat exchanger, as well as the working parameters including flow dominantly concentrated in the mechanical degradation on the directions and input/output temperatures of the desalinated tubes. Concretely in this paper addressed for the Part I, the water and seawater are all listed in Fig. 1(b) hydrogen-assisted corrosion(HAC) including both hydrogen The two target leaked tubes analyzed in the present study blistering(HB)and hydrogen embrittlement(HE), which is were both sampled from the 4#f RCW heat exchanger of unit II generally thought to emerge on titanium only above temperature which suffered the severest degradations on its tubes among all of 70C[11], was actually found occurring around 35-40C when the eight heat exchangers in the two CANDU 6 units-113 tubes it was induced by the interaction effect between galvanic corrosion were found to be heavily thinned, and another 24 tubes were even and crevice corrosion under complicated service conditions. leaked. Furthermore, the ruptures on over 90% of the leaked Relevant mechanisms of these electrochemical corrosions, tubes were located between the first baffle plate and the 78 mm- especially their interaction effect were discussed in detail. Finally, thick tube sheet, and even buried inside this tube sheet, also seen countermeasures and suggestions were also put forward. in Fig. 1(b). Meanwhile, as indicated with the circles in Fig. 2, obvious brown corrosion phenomenon was observed on the 2 Experimenta surface of the tube sheet itself as well Figure 3 presents the external appearances of the two lea 2.1 Visual observation tubes named A and B from this heat exchanger. As shown in Fig 3(a), the rupture on tube a was in shape of 5 x 3 mm ellipse As shown in Fig. 1(a), the RCW heat exchanger is a horizontal and located about 30 mm off the inlet. While for the rupture on der with dimension of about 2400 x 15 000 mm, in which tube B, it seemed relatively round with a diameter of nearly 6 mm, there exist totally 4932 tubes (ASME SB-338 Gr2 titanium[12) and was 70 mm off the inlet, seen in Fig 3(b). Hereby, it must be $ x 14630 x 0.71 mm) in 57 rows and 93 columns. All the noted that both the distances of the two ruptures off the tube tubes are sustained by 23 16 mm-thick carbon steel baffle plates inlets were less than 78 mm, i.e., they were both formed buried ith interval distance of 603 mm. and their two ends were inside the tube sheet. 2.2 Characterization methods hen investigations from three were successively carr out on the two leaked tubes including the evaluation of their matrix materials, the inspection of the two media they contacted e, desalinated water and seawater), and the microscopic analysis of the ruptures on them. As for the first one, oxygen nitrogen hydrogen(ONH)analyzer, carbon sulfur analyzer(CSA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES), were used to inspect their chemical composition optical microscopy (OM) was utilized to observe their metallo- graphic structures; and series of mechanical tests including unit: m desalinated water baffle plies corroSIo seawater305°C tube bundle Figure 1. Illustration of the RCW heat nger:(a)external appearance,( b)scheme and operation Figure 2. Corrosion on the inlet of the tube sheet o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
different failure mechanisms were simultaneously detected. As a matter of fact, such a comprehensive failure analysis study from engineering practice for titanium tubes that are applied in nuclear power units has been rarely reported. In the Part I of this whole study, research mainly focused on the electrochemical corrosion on the titanium tubes, while in the Part II [10], analysis dominantly concentrated in the mechanical degradation on the tubes. Concretely in this paper addressed for the Part I, the hydrogen-assisted corrosion (HAC) including both hydrogen blistering (HB) and hydrogen embrittlement (HE), which is generally thought to emerge on titanium only above temperature of 70 8C [11], was actually found occurring around 35–40 8C when it was induced by the interaction effect between galvanic corrosion and crevice corrosion under complicated service conditions. Relevant mechanisms of these electrochemical corrosions, especially their interaction effect were discussed in detail. Finally, countermeasures and suggestions were also put forward. 2 Experimental 2.1 Visual observation As shown in Fig. 1(a), the RCW heat exchanger is a horizontal cylinder with dimension of about w2400 15 000 mm, in which there exist totally 4932 tubes (ASME SB-338 Gr.2 titanium [12], w19 14 630 0.71 mm) in 57 rows and 93 columns. All the tubes are sustained by 23 16 mm-thick carbon steel baffle plates with interval distance of 603 mm, and their two ends were hydraulically expanded (expansion ratio 80%) with carbon steel tube sheets cladded by titanium (ASME SA-515 Gr.65 carbon steel [13] 78 mm-thick, ASME SB-265 Gr.1 Ti [14] 3 mm-thick), without seal welding. Therein, the diameter of supporting holes on all the plates was 19.25 0.51 mm. The schematic diagram of the RCW heat exchanger, as well as the working parameters including flow directions and input/output temperatures of the desalinated water and seawater are all listed in Fig. 1(b). The two target leaked tubes analyzed in the present study were both sampled from the 4# RCW heat exchanger of unit II, which suffered the severest degradations on its tubes among all the eight heat exchangers in the two CANDU 6 units – 113 tubes were found to be heavily thinned, and another 24 tubes were even leaked. Furthermore, the ruptures on over 90% of the leaked tubes were located between the first baffle plate and the 78 mmthick tube sheet, and even buried inside this tube sheet, also seen in Fig. 1(b). Meanwhile, as indicated with the circles in Fig. 2, obvious brown corrosion phenomenon was observed on the surface of the tube sheet itself as well. Figure 3 presents the external appearances of the two leaked tubes named A and B from this heat exchanger. As shown in Fig. 3(a), the rupture on tube A was in shape of 5 3 mm ellipse and located about 30 mm off the inlet. While for the rupture on tube B, it seemed relatively round with a diameter of nearly 6 mm, and was 70 mm off the inlet, seen in Fig. 3(b). Hereby, it must be noted that both the distances of the two ruptures off the tube inlets were less than 78 mm, i.e., they were both formed buried inside the tube sheet. 2.2 Characterization methods Then investigations from three aspects were successively carried out on the two leaked tubes, including the evaluation of their matrix materials, the inspection of the two media they contacted (i.e., desalinated water and seawater), and the microscopic analysis of the ruptures on them. As for the first one, oxygen nitrogen hydrogen (ONH) analyzer, carbon sulfur analyzer (CSA), and inductively coupled plasma atomic emission spectroscopy (ICP-AES), were used to inspect their chemical compositions; optical microscopy (OM) was utilized to observe their metallographic structures; and series of mechanical tests including 8 Yang, Gong and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 1. Illustration of the RCW heat exchanger: (a) external appearance, (b) scheme and operation parameters Figure 2. Corrosion on the inlet of the tube sheet 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes Results and discussion 3.1 Matrix materials 3.1.1Ch 23456 9 ululuululuuustumllm Chemical compositions of the titanium tubes' matrix materials are listed in Table 1. which are in accordance with the requirements of the ASME SB-338 Gr 2 titanium specification (equals to the TA2 industrial purity titanium in GB/T 3620 1-2007 standard of China (15)), as well as their original values provided by manufacturer RMI. In fact, contents of some of the impurities are even nearly one order of magnitude less than their limitations 3. 1.2 Metallographic structures Figure 4(a)and(b )show the metallographic structures of the titanium tubes in, respectively, transverse and longitudinal directions. It is obvious that the structures in both directions display a similar morphology, i. e, equiaxed polygonal grains -the typical structure of a-titanium, and their average ASTM grain 912314.5.789.1h 3. 1.3 Mechanical test In order to examine whether performances deterioration had occurred on the titanium tubes various mechanical tests were onducted on the two leaked tubes, and the results were also compared with the original values after manufacture. As listed in Table 2, all the mechanical properties of the tubes were eligible according to their standards, and the yield strength and Figure 3. External appearances of the ruptures on the two leaked the tensile strength were even a bit increased than the original tubes: (a)tube a, (b)tube B alues Based on all the analysis results illustrated above, it can be oncluded now that the matrix materials of the titanium tubes tensile test, hardness survey, etc. were also applied to evaluate after service were still qualified, in other words, the failures could their mechanical properties. With respect to the second one, the not be ascribed to the inappropriate selection of tube materials chemical constituents of the desalinated water and the seawater were respectively detected by graphite furnace atomic absorption 3.2 Environmental media pectrometry(GFAAS), ion chromatography(IC), and ICP-AES In terms of the final one, besides further observation of the In this section, inspection will be mainly focused on the two kinds macroscopic morphologies of the ruptures on the two leaked of liquids, the desalinated water and the seawater. Since the tubes in Fig 3, scanning electron microscopy (SEM) and energy concentrations of the constituents in the desalinated water were dispersive spectrometry(EDS) were adopted to analyze their quite low, GFAAS was particularly adopted, and the results were microscopic morphologies along with micro-area compositions; listed in Table 3. It should be noted that the contents of iron and meanwhile, X-ray photoelectron spectroscopy(XPS), secondary copper elements were relatively high, which means that corrosion ion mass spectrometry (SIMS), and X-ray diffraction(XRD)were had exactly occurred in the RCW system. With respect to the also employed to characterize their near-surface features. seawater, by using ICP-AES and IC, its element constituents were Table 1. Chemical co e titanium tubes(wt%) Element C N Others nal values from RMI <0.3 <0.25 <0.40 GB/T3620.1-2007TA2 <0.30 <0.08 <0.0 <0.10 www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
tensile test, hardness survey, etc. were also applied to evaluate their mechanical properties. With respect to the second one, the chemical constituents of the desalinated water and the seawater were respectively detected by graphite furnace atomic absorption spectrometry (GFAAS), ion chromatography (IC), and ICP-AES. In terms of the final one, besides further observation of the macroscopic morphologies of the ruptures on the two leaked tubes in Fig. 3, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were adopted to analyze their microscopic morphologies along with micro-area compositions; meanwhile, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD) were also employed to characterize their near-surface features. 3 Results and discussion 3.1 Matrix materials 3.1.1 Chemical compositions Chemical compositions of the titanium tubes’ matrix materials are listed in Table 1, which are in accordance with the requirements of the ASME SB-338 Gr.2 titanium specification (equals to the TA2 industrial purity titanium in GB/T 3620.1-2007 standard of China [15]), as well as their original values provided by manufacturer RMI. In fact, contents of some of the impurities are even nearly one order of magnitude less than their limitations. 3.1.2 Metallographic structures Figure 4(a) and (b) show the metallographic structures of the titanium tubes in, respectively, transverse and longitudinal directions. It is obvious that the structures in both directions display a similar morphology, i.e., equiaxed polygonal grains – the typical structure of a-titanium, and their average ASTM grain size is about 8. It should be also pointed out that the sizes of these grains are basically even, without excessively coarsened or fined. 3.1.3 Mechanical test In order to examine whether performances deterioration had occurred on the titanium tubes, various mechanical tests were conducted on the two leaked tubes, and the results were also compared with the original values after manufacture. As listed in Table 2, all the mechanical properties of the tubes were still eligible according to their standards, and the yield strength and the tensile strength were even a bit increased than the original values. Based on all the analysis results illustrated above, it can be concluded now that the matrix materials of the titanium tubes after service were still qualified, in other words, the failures could not be ascribed to the inappropriate selection of tube materials. 3.2 Environmental media In this section, inspection will be mainly focused on the two kinds of liquids, the desalinated water and the seawater. Since the concentrations of the constituents in the desalinated water were quite low, GFAAS was particularly adopted, and the results were listed in Table 3. It should be noted that the contents of iron and copper elements were relatively high, which means that corrosion had exactly occurred in the RCW system. With respect to the seawater, by using ICP-AES and IC, its element constituents were Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 9 Figure 3. External appearances of the ruptures on the two leaked tubes: (a) tube A, (b) tube B Table 1. Chemical compositions of the titanium tubes (wt%) Element Fe C N H O Others Single Total tube in 4# of Unit II 0.069 0.012 0.006 0.0019 0.12 / / original values from RMI 0.07 0.01 0.008 0.0015 0.12 / / ASME SB-338 Gr.2 0.30 0.08 0.03 0.015 0.25 0.10 0.40 GB/T 3620.1-2007 TA2 0.30 0.08 0.03 0.015 0.25 0.10 0.40 www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
10 Yang, Gong and Yuan Materials and Corrosion 2012. 63. No. 1 a)ent revealed in Table 4, which conformed to the normal compositions of natural seawater- a high content of chloride ions 3. 3 Rupture analysis of tube a 3.3.1 SEM and EDs After magnifying the rupture in Fig. 3(a), it can be obviously learnt from Fig. 5(a) that the fringes of this rupture were bent inwards to the inside wall, which may be attributed to a relatively large force exerting on the outside wall of the tube. Meanwhile, rown colored rust covered around this rupture, which should be the corrosion products of iron oxides originated from the Ti/ carbon steel tube sheet. The inside wall around the rupture was pretty smooth, and the extent of rusting was not as severe as that on the outside wall too [Fig. 5(b)]. Under SEM, microscopic morphology of the area marked with rectangular in Fig. 5(b) was presented in Fig. 6(a). Besides the smooth fringe(site 001), some corrosion substances(site 002) were also scaling on the inside wall of the rupture. By means of EDS, the former one was exactly the titanium matrix material of the tube[Fig. 6(c)), while the latter one actually also contained iron and oxygen elements other than titanium [Fig. 6(d). It can be inferred that such corrosion substances were introduced from the outside wall of the tube after the rupture was formed. Furthermore, it must be particularly noted that the corner of the inside wall exhibited an unusual morphology seeming like oriented eddy erosion, seen in Fig. 6(b). Just due to this erosion effect, almost no corrosion substances were scaling around the corner, completely contrast to the site 002 in Fig. 6(a). Also, Figure 4. Metallographic structures of the titanium tubes: (a) transverse,(b)longitudinal another significant evidence also revealed that densely distributed pits existed near the corner with eddy erosion trace. This fact means that such kind of erosion was even accompanied with impact because of the sediment particles contained in the natural Table 2. Mechanical properties of the titanium tubes(wt%) Yield strength Uo?(MPa) Tensile strength o,(MPa) Elongation 8s (% Hardness HvI 35.0 riginal values from RMI 296-3 435-494 ASME SB-338 Gr 2 275-450 ≥20 GB/T3624-199516 370-530 165-225 Table 3. GFAAS results of the desalinated water (ppb, equals to ug/L) Elemen Fe Cu T desalinated water Table 4. Element constituents of the seawater(ppm, equals to mg/L) Cu Seawater 5.19×103 1.16×102 0.13 <0.002 <0002 0.015 o 2012 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.com
revealed in Table 4, which conformed to the normal compositions of natural seawater – a high content of chloride ions. 3.3 Rupture analysis of tube A 3.3.1 SEM and EDS After magnifying the rupture in Fig. 3(a), it can be obviously learnt from Fig. 5(a) that the fringes of this rupture were bent inwards to the inside wall, which may be attributed to a relatively large force exerting on the outside wall of the tube. Meanwhile, brown colored rust covered around this rupture, which should be the corrosion products of iron oxides originated from the Ti/ carbon steel tube sheet. The inside wall around the rupture was pretty smooth, and the extent of rusting was not as severe as that on the outside wall too [Fig. 5(b)]. Under SEM, microscopic morphology of the area marked with rectangular in Fig. 5(b) was presented in Fig. 6(a). Besides the smooth fringe (site 001), some corrosion substances (site 002) were also scaling on the inside wall of the rupture. By means of EDS, the former one was exactly the titanium matrix material of the tube [Fig. 6(c)], while the latter one actually also contained iron and oxygen elements other than titanium [Fig. 6(d)]. It can be inferred that such corrosion substances were introduced from the outside wall of the tube after the rupture was formed. Furthermore, it must be particularly noted that the corner of the inside wall exhibited an unusual morphology seeming like oriented eddy erosion, seen in Fig. 6(b). Just due to this erosion effect, almost no corrosion substances were scaling around the corner, completely contrast to the site 002 in Fig. 6(a). Also, another significant evidence also revealed that densely distributed pits existed near the corner with eddy erosion trace. This fact means that such kind of erosion was even accompanied with impact because of the sediment particles contained in the natural 10 Yang, Gong and Yuan Materials and Corrosion 2012, 63, No. 1 Figure 4. Metallographic structures of the titanium tubes: (a) transverse, (b) longitudinal Table 2. Mechanical properties of the titanium tubes (wt%) Yield strength s0.2 (MPa) Tensile strength sb (MPa) Elongation d5 (%) Hardness HV1 Sample 1 435 535 35.0 174 Sample 2 427 545 31.5 166 Average 431 540 33.3 170 original values from RMI 296–351 435–494 32–39 / ASME SB-338 Gr.2 275–450 345 20 / GB/T 3624-1995[16] 250 370–530 20 165–225 Table 3. GFAAS results of the desalinated water (ppb, equals to mg/L) Element Fe Cu Ti Mn Ni Cr desalinated water 137 39.2 <20 2.5 <5 0.7 Table 4. Element constituents of the seawater (ppm, equals to mg/L) Element Cl Mg Al Cu Fe Ti Mn Seawater 5.19 103 1.16 102 0.13 <0.002 0.088 <0.002 0.015 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.com
Materials and Corrosion 2012. 63. No. 1 Electrochemical corrosion failure of leakage on titanium tubes 711.80eV, i.e., the FeO(OH)(Fig 8): ere was only one titanium spectrum here, and its binding energy was 458.53eV, i.e., the TiOz. Now it can be confirmed that some kind of corrosion which would bring about an irregular chemical alence on titanium only occurred on the outside wall of the tube and was probably related to the unknown force exerting on the tub 3.3.3SMS Considering the well-known fact that titanium exhibits strong nteraction with hydrogen [18]. thus the titanium with irregular chemical covalence on the outside wall near the rupture was possibly related to the hydrogen element, which can only be sensitively detected by surface analysis technique SIMS [19).As a displayed in Fig 9(a), hydrogen element in form of H2 gas was indeed present, while it did not exist on the inside wall in contrast, seen in Fig. 9(b). This result verified the conclusion we put forward above that corrosion only took place on the outside wall of the tube, and it was very probably the HAC. However, it still needs further method to determine the actual chemical compositions of the titanium hydrides 3.4 XRD It is clearly shown in Fig. 10 that a kind of crystalline with irregular stoichiometric number as TiH1924 was present other than pure titanium near the rupture. Then, the actual chemical omposition of the titanium hydride can be finally ascertained. In fact, the peak ofTiH1924 can be regarded as TiH2(20), the product of hydrogen absorption reaction. 3. 4 Rupture analysis of tube B 3.4.1 Macroscopic morphologies of the rupture on tube A: (a) As shown in Fig. 11(a), a crack with length of 3 mm was engendered from the rupture along the axial direction of the tube and exhibited brittle fracture morphology, while the inside wall seawater,whose mechanism will be detailedly discussed in Part II near the crack was even seriously deformed, seen in Fig. 11(b) of this study Similar to that on tube A, the rupture fringes here were also bent inwards to the inside wall but with severer extent. So it can be 3.3.2XPS udged that the two ruptures on both tubes a and b were aroused For the purpose of characterizing the surface features of both the by the same cause, but the degradation extent on tube b was outside wall and the inside wall near the rupture, XPS was utilized severer than that on tube A. to determine the chemical valences of relevant elements. As Consequently, similar microscopic analysis and near-surface shown in Fig. 7(a), totally four kinds of main elements were characterization were not needed to conduct again on tube B present on the outside wall, among them the two metal elements Instead, cross-sections of the rupture were observed. Figure 12( should be paid special attention to since they were from the and(b)illustrated that the surface of the outside wall was matrix materials of the tube and the plate. The electron binding deteriorated so severe that both delamination and descaling energy of iron was 711.80eV, which corresponded to the occurred on it, which were the evidence of crevice corrosion. On ompound FeO(OH)(17]. However for titanium, it had two the contrary, the inside wall of the tube was really smooth, seen in close spectra, that is to say two species of chemical valences Fig. 12(a). However, the fringe of the rupture was seriously existed. After locally magnifying(Fig. 7(b)), the binding energy of thinned due to the erosion effect from the seawater containing the left one was 458.48ev, while that of the right one was sediment particles, see Fig. 12(c) 455.74eV. Referring to the handbook [17 the former one represented TiO2, however the latter one could not correspond to 4 Failure analysis any compound. In other words, an irregular chemical valence was introduced on the titanium due to corrosion, and needed Based on the analysis results presented above, it can be concluded subsequent analysis from the electrochemical point of view that serious corrosion had is In order for comparison, the inside wall near the rupture was occurred on the titanium tubes of the 4# RCW heat exchanger in detected by XPS. The binding energy of iron was still Unit Il. Furthermore, since almost all the rupture locations on the www.matcorr.com o 2012 WILEY-VCH Verlag GmbH& Co KGaA, Weinheim
seawater, whose mechanism will be detailedly discussed in Part II of this study. 3.3.2 XPS For the purpose of characterizing the surface features of both the outside wall and the inside wall near the rupture, XPS was utilized to determine the chemical valences of relevant elements. As shown in Fig. 7(a), totally four kinds of main elements were present on the outside wall, among them the two metal elements should be paid special attention to since they were from the matrix materials of the tube and the plate. The electron binding energy of iron was 711.80 eV, which corresponded to the compound FeO(OH) [17]. However for titanium, it had two close spectra, that is to say two species of chemical valences existed. After locally magnifying [Fig. 7(b)], the binding energy of the left one was 458.48 eV, while that of the right one was 455.74 eV. Referring to the handbook [17], the former one represented TiO2, however the latter one could not correspond to any compound. In other words, an irregular chemical valence was introduced on the titanium due to corrosion, and needed subsequent analysis. In order for comparison, the inside wall near the rupture was also detected by XPS. The binding energy of iron was still 711.80 eV, i.e., the FeO(OH) (Fig. 8); as for the titanium, there was only one titanium spectrum here, and its binding energy was 458.53 eV, i.e., the TiO2. Now it can be confirmed that some kind of corrosion which would bring about an irregular chemical valence on titanium only occurred on the outside wall of the tube, and was probably related to the unknown force exerting on the tube. 3.3.3 SIMS Considering the well-known fact that titanium exhibits strong interaction with hydrogen [18], thus the titanium with irregular chemical covalence on the outside wall near the rupture was possibly related to the hydrogen element, which can only be sensitively detected by surface analysis technique SIMS [19]. As displayed in Fig. 9(a), hydrogen element in form of H2 gas was indeed present, while it did not exist on the inside wall in contrast, seen in Fig. 9(b). This result verified the conclusion we put forward above that corrosion only took place on the outside wall of the tube, and it was very probably the HAC. However, it still needs further method to determine the actual chemical compositions of the titanium hydrides. 3.3.4 XRD It is clearly shown in Fig. 10 that a kind of crystalline with irregular stoichiometric number as TiH1.924 was present other than pure titanium near the rupture. Then, the actual chemical composition of the titanium hydride can be finally ascertained. In fact, the peak of TiH1.924 can be regarded as TiH2 [20], the product of hydrogen absorption reaction. 3.4 Rupture analysis of tube B 3.4.1 Macroscopic morphologies As shown in Fig. 11(a), a crack with length of 3 mm was engendered from the rupture along the axial direction of the tube and exhibited brittle fracture morphology, while the inside wall near the crack was even seriously deformed, seen in Fig. 11(b). Similar to that on tube A, the rupture fringes here were also bent inwards to the inside wall but with severer extent. So it can be judged that the two ruptures on both tubes A and B were aroused by the same cause, but the degradation extent on tube B was severer than that on tube A. Consequently, similar microscopic analysis and near-surface characterization were not needed to conduct again on tube B. Instead, cross-sections of the rupture were observed. Figure 12(a) and (b) illustrated that the surface of the outside wall was deteriorated so severe that both delamination and descaling occurred on it, which were the evidence of crevice corrosion. On the contrary, the inside wall of the tube was really smooth, seen in Fig. 12(a). However, the fringe of the rupture was seriously thinned due to the erosion effect from the seawater containing sediment particles, see Fig. 12(c). 4 Failure analysis Based on the analysis results presented above, it can be concluded from the electrochemical point of view that serious corrosion had occurred on the titanium tubes of the 4# RCW heat exchanger in Unit II. Furthermore, since almost all the rupture locations on the Materials and Corrosion 2012, 63, No. 1 Electrochemical corrosion failure of leakage on titanium tubes 11 Figure 5. Macroscopic morphologies of the rupture on tube A: (a) outside wall, (b) inside wall www.matcorr.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim