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Flow accelerated corrosion of carbon steel feeder pipes from pressurized heavy water reactors
Journal of Nuclear Materials 429 (2012) 226–232
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at ANM-2011 Conference, Feb 9-11, Mumbai, India
Flow accelerated corrosion of carbon steel feeder pipes from pressurized heavy water reactors J.L. Singh a, Umesh Kumar a,⇑, N. Kumawat a, Sunil Kumar a, Vivekanand Kain b, S. Anantharaman a, A.K. Sinha c a b c
Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Material Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Nuclear Power Corporation of India Limited, Mumbai 400 094, India
a r t i c l e
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Article history: Available online 9 June 2012
a b s t r a c t Detailed investigation of a number of feeder pipes received from Rajasthan Atomic Power Station Unit 2 (RAPS#2) after en-masse feeder pipe replacement after 15.67 Effective Full Power Years (EFPYs) was carried out. Investigations included ultrasonic thickness measurement by ultrasonic testing, optical microscopy, scanning electron microscopy, chemical analysis and X-ray Diffraction (XRD). Results showed that maximum thickness reduction of the feeder had occurred downstream and close to the weld in 32 NB (1.2500 /32.75 mm ID) elbows. Rate of Flow Accelerated Corrosion (FAC) was measured to be higher in the lower diameter feeder pipes due to high flow velocity and turbulence. Weld regions had thinned to a lower extent than the parent material due to higher chromium content in the weld. A weld protrusion has been shown to add to the thinning due to FAC and lead to faster thinning rate at localized regions. Surface morphology of inner surface of feeder had shown different size scallop pattern over the weld and parent material. Inter-granular cracks were also observed along the weld fusion line and in the parent material in 32 NB outlet feeder elbow. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Feeder pipes are integral part of Primary Heat Transport (PHT) system in Pressurized Heavy Water Reactors (PHWRs). The feeder pipes and the headers and pipes connecting to the steam generator are the only components in the PHT of PHWRs that are made from plain carbon steel [1,2]. Flow Accelerated Corrosion (FAC) of feeder piping has been recognized as one of the main concern affecting safety and availability of nuclear power plants. FAC is also a main degradation concern for all the high energy piping/components in the secondary circuit of nuclear reactors. On the secondary circuit components, a few cases of failures due to FAC have been reported worldwide [3–6]. FAC is described as corrosion enhanced by mass transfer, between (a dissolving oxide film on) the base material and a flowing fluid that is unsaturated in the dissolving species. Corrosion proceeds rapidly by electrochemical means and the corrosion rate accelerates as the velocity of the fluid increases. Dissolution of protective magnetite oxide layer into flowing stream of water causes wall thinning (metal loss) of carbon steel [3–11]. If the thinning remains undetected and the remaining thickness is less than ⇑ Corresponding author at: Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. Tel.: +91 22 25596111; fax: +91 22 25505151. E-mail address: [email protected] (U. Kumar). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.05.045
the critical thickness required to withstand the operating pressure, a ductile fracture occurs [3]. Flow rates may change due to change in geometry (elbows/bends, etc.) or due to local disturbance in flow (e.g. flow obstruction by flow measurement devices or a protrusion on the inner surfaces of pipe/welds). Therefore, the local flow rate changes will cause localized changes in thinning rate [12]. The maximum rate of FAC is the key factor in determining the decision to replace the FAC affected pipes. The location of maximum rate of thinning due to FAC is also a key factor so as to allow thinning measurement to be focused on that location. Cracking of some of the outlet feeders has been reported at regions of tensile stresses but the exact mechanism remains unclear with hydrogen from the FAC affected surfaces and materials aging issues expected to be playing a role [13]. Wall thinning of carbon steel feeder pipes due to FAC led to severe thinning in an Indian PHWR (Rajasthan Atomic Power Station unit #2). Based on an exhaustive wall thickness management program, an en-mass feeder pipe replacement was carried out in this reactor after 15.67 EFPY of reactor operation. A detailed investigation has been carried out on seven feeder pipes selected from those removed from the reactor. The objective of this investigation was to examine and understand the nature and extent of thinning caused by FAC in carbon steel feeder pipes of Indian PHWRs. The key findings of this investigation are presented in this paper.
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2. Material and fabrication procedure There are 306 number of feeders connected to each face of the reactor. Each feeder is routed between End Fitting and inlet or outlet header and consists of various straight run and bends that vary from 30 to 90°. Feeder length generally varies from 6 to 20 m [14]. Feeders are designed and fabricated as per the requirement for class 1 components of American Society of Mechanical Engineers (ASMEs) boiler and pressure vessel code Section III [13]. The chemical composition of feeders was in accordance with the particular American Society for Testing and Materials (ASTMs) A-106 Grade B specifications [11,13–15]. Feeder assembly consists of single or double elbow fittings welded at one end to the header and joined on the other end by the gray-loc hub to the End Fitting. Different sizes of feeder pipe were used to balance the fuel channel flow rate in order to maintain consistent outlet temperature. In general, PHT feeder pipes are cold drawn followed by appropriate heat treatment for controlling and achieving the required mechanical properties. All pipes supplied to this specification need to be suitable for cold bending to a minimum mean radius of less than 200 mm or four times the pipe outer diameter. All the butt welding and fittings like elbow and reducer are forged or formed to the finished shape and size by hot working. Fittings manufactured by hot working and/or cold forming processes are appropriately heat treated for controlling and achieving the required mechanical properties. Fittings are finally annealed or normalized.
25 ft/s (7.62 m/s) to a 50 ft/s (15.24 m/s) and temperature at the outlet end was in range of 280–292 °C. A 10 MHz duel element ultrasonic probe of 1 mm tip diameter was used for thickness measurement. A test piece of feeder material of known thickness was taken for finding out the ultrasonic velocity in the material needed for calibration of the instrument. Ultrasonic instrument was set in pulse echo mode. Extrados and intrados of the feeder elbows were marked as 12 and 6’O clock locations respectively, subsequently other clock locations were marked. Dashed lines were drawn axially on the feeder from each clock location at a gap of approximately 5 mm. Ultrasonic thickness measurement were taken at the gap in between the dash line so that paint thickness should not add to actual thickness at the point of measurement. For metallographic examination, outlet feeder elbows were cut axially into two halves along 3 and 9’O clock position. The samples were taken from the location which included the Weld Fusion Zone (WFZ), Heat Affected Zone (HAZ) and the parent material. The ground and polished samples were etched using 2% nital solution for observing the microstructure. The oxide layer on the Inner Diameter (ID) surface of B-10 outlet feeder was analyzed by X-ray Diffraction (XRD) using Mo Ka source. Scanning Electron Microscopy (SEM) examination on the ID surfaces was carried out to observe the scallop or overlapping horse shoe pattern [3,4,8–11,13]. Inductively Coupled Plasma – Atomic Emission Spectrometry (ICP-AES) technique was used for chemical composition analysis of WFZ and the parent material.
3. Experimental procedure 4. Results The feeder pipes received for detailed investigation were highly contaminated and had a contact radiation dose of 2 millirem per hour (mR/h). Decontamination of feeders was carried out with soap solution and soft nylon brush without damaging the inner oxide layer. Details of the feeder dimensions and the reactor operating parameters in these feeders are given in Table 1. These feeders were taken from the outlet end of coolant channel. Diameter and pipe thickness of feeder ranged from 1.2500 to 200 (31.75– 50.8 mm) and 4.85–5.54 mm respectively. Flow rate varied from
4.1. Ultrasonic thickness measurement of feeder Location of the maximum thinning and the percentage reduction in thickness for a number of feeder pipes of different size are given in Table 2. Ultrasonic thickness measurement data of the feeders showed that thickness reduction was prominent only in the 32 NB (31.75 mm) feeder pipe. These feeders were double elbows welded to the straight pipe.
Table 1 The reactor operating parameters for the selected feeders. Sr. no.
Outlet feeders
Nominal bore size (inch/mm)
1 2 3 4 5 6
B-09 B-10 O-03 N-03 Q-13 K-08
32 32 40 40 50 50
NB NB NB NB NB NB
(1.2500 /31.75) (1.2500 /31.75) (1.5000 /38.10) (1.5000 /38.10) (2.0000 /50.80) (2.0000 /50.80)
Initial nominal thickness (mm)
Flow rate (ft/s)/(m/s)
Temperature (°C)
4.85 4.85 5.08 5.08 5.53 5.53
47.0/14.32 49.5/15.08 38.3/11.67 39.6/12.07 25.6/7.80 28.1/8.56
282.0 284.3 284.0 285.6 285.3 292.0
Table 2 Ultrasonic thickness measurement of different size feeder. Feeder no.
ID size and no of elbows
Location and position of minimum thickness in the elbow
Initial nominal thickness (mm)
Measured minimum thickness (mm)
Thickness reduction (%)
B-09
32 NB Double elbow
4.85
1.11
77.1
B-10
32 NB Double elbow
4.85
1.37
71.7
O-03
40 NB Double elbow
5.08
3.62
28.7
N-03
40 NB Double elbow
5.08
3.57
29.7
K-08
50 NB Single elbow
5.54
4.55
17.8
Q-13
50 NB Single elbow
Outlet feeder at12’O clock in second elbow close to weld Outlet feeder at 6’O clock in first elbow close to weld Outlet feeder at 12’O clock in second elbow close to weld Outlet feeder at 12’O clock in second elbow close to weld Outlet feeder at 12’O clock in first elbow close to weld Outlet feeder at 12’O clock in first elbow close to weld
5.54
5.01
9.5
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Fig. 1. Localized thinning of outlet feeders (a) B-09 around the elbow close to the weld at 12’O clock (b) B-10 outlet feeder at 6’O clock intrados of first elbow (c) B-10 outlet feeder at 12’O clock in intrados of second elbow.
Fig. 1 shows the arrangements for thickness measurement and regions of minimum thickness measured on B-09 and B-10 feeder. Localized thinning of B-09 outlet feeder had occurred in the elbow on both sides of the weld at 12’O clock position (Fig. 1a). The thickness zone less than 2.5 mm was encircled on the feeder pipe. Minimum available thickness was 1.11 mm (a reduction of 3.74 mm from initial, nominal thickness) at the intrados of second elbow downstream and close to the second weld. Nevertheless, thickness reduction had occurred everywhere in the feeder elbow. Fig. 2 shows the ID surface of B-09 showing localized thinning adjacent to the weld. A weld protrusion is also seen in the photograph. B-10 feeder had also shown thickness reduction at two different locations and the area with minimum thickness was encircled and shown in Fig. 1b and c. Fig. 1b shows the marked region with the minimum wall thickness of 1.37 mm (a reduction of 3.48 mm from initial, nominal, thickness) at intrados of the first elbow downstream and close to the first weld at 6’O clock position. Fig. 1c shows the marked region with the minimum wall thickness of 2.01 mm at intrados in the second elbow close to the second weld at 12’O clock position.
Higher NB feeder pipe namely O-03, N-03, K-08 and Q-13 had shown thickness reduction in the range of 0.5–1.5 mm. Outlet feeders O-03, N-03 and 40 NB had double elbows whereas K-08 and Q-13 and 50 NB had single elbow welded to the straight segment of the pipe. Coolant flow rate in these feeders was between 25 ft/s (7.62 m/s) and 38 ft/s (11.58 m/s). 4.2. Cross section examination of feeders The appearance of the inside surface of B-10 feeder as observed in the optical microscope is shown in Fig. 3. The photomicrograph in this figure shows the waviness of the inner surface due to FAC by PHT water flowing at a high velocity. It was observed that WFZ had corroded lesser than the parent material. Remaining wall thickness in HAZ, close to the weld was 1.35 mm and increases to a value 1.95 mm in parent material away from the weld. 4.3. Corrosion rate measurement The corrosion rate due to FAC in different sizes of feeder pipes has been listed in Table 3. This corrosion rate has been calculated based on the wall thickness measurement in the laboratory (by ultrasonic testing) at the straight pipe segments, away from the
Fig. 2. Localized thinning of B-09 feeder downstream and adjacent to the weld. A weld protrusion had formed during the reactor operation period.
Fig. 3. Photomicrograph of B-10 outlet feeder section showing waviness of inner surface.
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J.L. Singh et al. / Journal of Nuclear Materials 429 (2012) 226–232 Table 3 Corrosion rate in straight sections in different feeder pipes. Feeder no. K-08 O-03 N-03 B-09 B-10
Nominal bore size (inch/mm) 50 40 40 32 32
NB NB NB NB NB
00
(2.00 /50.8) (1.5000 /38.1) (1.5000 /38.1) (1.2500 /31.75) (1.2500 /31.75)
Flow rate (ft/s)/(m/s)
Thickness reduction in straight section (mm)
Corrosion rate (mm/year)
28.1/8.56 38.3/11.67 39.6/12.07 47.0/14.32 49.5/15.08
0.57 0.78 0.86 2.06 2.21
0.036 0.051 0.055 0.132 0.141
Fig. 4. Effect of flow velocity in straight segment of feeder pipe on corrosion rate after 15.67 EFPY.
elbows. Wall thickness loss was estimated by subtracting the remaining pipe thickness from the nominal thickness of the respective pipes. These feeders were removed after 15.67 EFPY. Corrosion rate was calculated by dividing the thickness reduction by EFPY. Corrosion rate (thus calculated) vs. flow velocity was plotted and is shown in Fig. 4. Fig. 4 shows a drastic change in corrosion rate beyond a flow velocity of 40 ft/s. This clearly reflects the effect of flow rate (velocity) on the acceleration of corrosion. The flow accelerated corrosion is therefore evident more in smaller diameter pipes which also had a higher flow rate. 4.4. Chemical compositional and XRD analysis of weld and parent metal of feeder The elemental compositional analysis of the weld and parent material by ICP-AES of the B-10 outlet feeder was done. The chemical composition of the WFZ and the parent material are listed in Table 4.This shows that the WFZ had a higher amount of Cr, Mo, and Cu than the parent material. These elements have been shown to provide effective corrosion resistance against FAC [3–5,8,13,16– 19]. Fig. 5 shows the XRD analysis of the oxide layer on the WFZ
Table 4 ICP-AES analysis of B-10 outlet feeder. Sr. no.
1 2 3 4 5 6 7
Element
Cr Cu Mn Mo Ni Si V
Concentration (wt.%) Parent material
Weld fusion zone
0.02 0.04 0.61 0.08 0.07 0.61 0.02
0.29 0.14 0.64 0.43 0.32 0.50 0.02
Fig. 5. XRD pattern of the oxide layer at ID surface (a) on parent material and (b) on weld fusion zone of B-10 feeder. Symbols ‘M’ and ‘Fe’ indicate magnetite and iron peaks respectively. Crystallographic planes are indicated in parenthesis.
and parent material of B-10 feeder. Fig. 5a and b shows the intensity of iron (base material) and magnetite peaks on the parent material and WFZ samples respectively. The intensity of magnetite peaks on the WFZ is quite high compared to the intensity of magnetite peak on the parent material. This shows that WFZ has a thicker magnetite layer compared to that on the parent material. 4.5. SEM examination The B-10 feeder sample was examined in detail under SEM. Fig. 6 shows the of scallop pattern developed on the ID surface of B-10 feeder which is characteristic of single phase FAC [3,4,8– 11,13]. Fig. 6a shows the scallop pattern on the WFZ and Fig. 6b shows the scallop pattern on the parent material. The scallops on
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Fig. 6. SEM photomicrographs showing Scallop at ID of B-10 (a) on the weld fusion zone, (b) on HAZ, close to the weld and (c) dissolution due to eddies formation on the base material.
the weld fusion zone are clearly larger in size and have sharp edges whereas the scallops on parent material downstream and close to the weld are smaller in size and have flattened edges. As the weld region (WFZ) has not undergone much thickness reduction compared to the parent material, it is a reaffirmation of the previously reported observations that a larger scallop size indicates lower FAC rates [3]. Fig. 6c shows the scallops on the base material at a higher magnification showing dissolution of magnetite due to formation of eddies. Fig. 7 shows the SEM photomicrograph of Q-13 outlet feeder. This feeder was of 50 NB size and had a much lower flow rate (7.8 m/s) as compared to B-10 feeder pipe which was 32 NB size having flow rate of 15.08 m/s. Clear and distinct scallop pattern had not developed at the ID surface of Q-13 feeder. This again brings out the fact that at lower velocities, the FAC rate is much
lower and gets reflected in the scallop pattern developed on the inner (affected) surfaces.
Fig. 7. SEM photomicrograph showing scallops in Q-13 (50 NB) feeder. No distinct scallop formed at a lower velocity.
Fig. 8. As-polished B-09 outlet sample showing cracks in the base material and HAZ.
4.6. Inter-granular cracking of feeder B-09 feeder samples comprising of WFZ, HAZ and parent material from the first and second elbows were cut for metallographic examination. An as-polished section of feeder B-09 is shown in Fig. 8. Fig. 8 shows that material was removed (dissolved) from the parent material adjacent to the weld at the ID surface, as indicated by waviness of the ID surface. A possible weld defect at the weld fusion line on the ID surface (indicated by black arrow) is expected to have acted as an additional site for flow disturbance and a site for sustaining more aggressive water chemistry. The HAZ downstream to the weld is the most vulnerable area where factors like turbulence and weld protrusions contributed to maximize the FAC rate leading to excessive localized thinning in B-09 feeder.
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Fig. 8 also shows presence of a fine crack along the weld fusion line (marked by blue arrows) and another fine radial crack (marked by a red arrow) in the parent material at a distance of roughly 5 mm from the weld fusion line. A magnified view of these cracks is shown in Figs. 9 and 10. Fig. 9 shows the crack along weld fusion line. Fig. 10 shows the crack in the parent material started from the ID surface of feeder pipe and was inter-granular in nature. A sample cut from the second elbow of B-09 feeder which included WFZ and adjoining elbow area also showed cracks along the WFZ surrounding the weld (Fig. 11a). A magnified view of the crack in Fig. 11b shows that the crack in HAZ was intergranular in nature.
5. Discussion Fig. 9. Crack along the weld fusion line in HAZ.
Fig. 10. Crack in the base metal of B-09 outlet feeder showing inter-granular propagation from ID surface.
Fig. 11a. As polished specimen of B-09 from the second elbow showing cracks along the weld fusion line.
Fig. 11b. Inter granular crack in the HAZ at higher magnification.
The primary factors causing FAC are the water chemistry of coolant water which is unsaturated in dissolved iron, flow rate, temperature of operation and turbulence. The flow velocity varies a lot from one feeder to another, due to the size of the feeder and the need to maintain required level of cooling in different coolant channels. This is the primary reason for different rates of FAC in different feeders. The second reason is the additional flow disturbance in each feeder due to its unique geometry. Some feeders have one and others two elbows. The bends in each feeder also vary in numbers and degree due to location of each feeder and the geometry necessary to connect to the headers. Based on these two major factors, the FAC effect on feeders is monitored during plant shutdowns on a regular basis by ultrasonic inspection. The plant assesses the remaining thickness in each feeder (at the point of maximum thinning) and decides whether it is safe to operate it further. Unusual high thickness reduction was observed in 32 NB feeder pipe in the elbow region downstream to the weld (Table 2). Excessive grinding of welds and adjacent areas of the feeders could be one of the regions for such dramatic wall thinning in PHWRs or CANDU reactors. Fig. 2 shows the ID surface of elbow of B-09 feeder. A shallow localized depression had developed over an area adjacent to the weld suggests that thickness loss has been associated with localized dissolution and removal of the material from inside surface of the feeder pipe. The present case shows that an additional factor plays an important role in determining the extent and location of maximum FAC rate. Presence of local flow disturbance, e.g. by a weld protrusion, adds to the flow disturbance caused by the geometry and intended global flow rate in each feeder. This local disturbance leads to localized region undergoing more severe thinning. Therefore, local flow disturbances e.g. by weld protrusions, need to be taken into account in determining the location and extent of thinning by FAC. Turbulence resulting from high coolant velocity and flow obstruction like weld protrusion promotes mass transfer between the oxide water interface and the bulk coolant, thereby increasing the rate of dissolution of magnetite film. The temperature of coolant increases from 249 °C at the inlet end to 293 °C at the outlet end of the coolant channel. As coolant water comes out at the outlet end after passing through zirconium alloy pressure tube, it is unsaturated in dissolved iron. Therefore, faster magnetite dissolution occurs in the outlet feeders leading to higher thickness loss [13]. As has been shown in Table 4, the chemical composition of the WFZ and the parent material was slightly different. The chromium and molybdenum levels in the WFZ were higher than those in the base material. This led to differences in FAC rates over these two regions, WFZ and the base material. The WFZ thinned at a lower rate than the base material. Over long operation years, this difference in FAC rates led to formation of weld protrusions and there-
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fore led to even more flow disturbance. Therefore, this type of local disturbance leads to accelerating effect of FAC at the locations just downstream of the weld protrusion as the effect of the weld protrusion goes on increasing with time. Another possibility is that there was a weld protrusion at the location of maximum thinning from the fabrication stage itself. Even if it was present from the fabrication stage, its effect would have increased with years of operation as there was a difference in the FAC rate over the weld regions and the base material. Therefore, the geometry and chemical composition of welds vis-à-vis the base material also needs to be considered in determining the location and extent of FAC. Increased resistance to FAC has also been experimentally shown in high temperature, high pressure water for carbon steel containing slightly higher levels of chromium [8,17]. The FAC rate for a 90° elbow is reported to predict that FAC rate reduce to one-tenth of the FAC rate for carbon steel when Cr content is increased from 0.03% to 0.5%. This is due to the surface oxide changing from Fe3O4 to FeCr2O4 when Cr is added in carbon steel [3,20]. FeCr2O4 oxide is highly stable due to its extremely low solubility in high temperature aqueous solution [16]. Variation in scallops shape and size over weld and parent material was observed in the feeders examined in this study. Small size of scallops on the parent material may be due to formation of vortex or eddies downstream to the weld. Size of scallop can be correlated with FAC rate. Larger the size of scallops as found over the WFZ, smaller the FAC rate. The shape of the scallop is also shown in this study to reflect the flow behavior. The scallops had sharp edges at locations where the dissolution rate was less (WFZ). At the base material (that contained less chromium and molybdenum) the scallops had flattened edges, possibly due to more disturbed flow at such locations at downstream of weld protrusion. The shape of the scallops (Fig. 7c) seemed to reflect the presence of eddies also. Cracking in HAZ and parent material were also observed in the feeder pipes. These cracks were in the elbow bends where high residual tensile stresses are reported to be present [13]. A proper normalizing treatment after fabrication is expected to reduce the levels of stresses in fabricated pipes. However, improper normalizing treatment and/or subsequent welding would have lead to stresses in the weldment. All the cracks were detected in the weldment only. The crack along the weld fusion line shown in Fig. 9 could have been aided by more aggressive water chemistry in the occluded weld defect. Mechanism of such cracking was indicated to be stress corrosion cracking (SCC) due to exposure of feeder surface to hot coolant in elbow region where bending and residual stresses due to welding are present. However, the mechanism of SCC has not been clearly confirmed in feeders. Issues like material aging (at reactor operation temperature for more than 12 EFPY), presence of hydrogen produced by FAC on the material ID surface and possibly corrosion creep have been indicated to play a role in cracking of feeders [13,21]. Research on these issues is underway at this point of time. 6. Conclusions Detailed examination of a number of feeder pipes removed from Rajasthan Atomic Power Station unit #2 after 15.67 EFPY operations has been carried out. The main findings of the examination are given below: FAC has caused thinning of the feeder pipe everywhere but preferentially it has caused localized thinning at the intrados of the elbows downstream to the weld.
Corrosion rate was accelerated due to high flow velocity in smaller diameter feeder pipes. Weld protrusion in the feeder elbows had caused obstruction to the coolant flow and formation of eddies downstream to the weld leading to additional localized thinning close to the weld. Inter-granular cracks were found along weld fusion line and in parent material in the elbow region. The scallop pattern which had developed at the ID surface of feeder pipes indicated degradation due to single phase FAC. Smaller the measured scallop size, higher was the FAC rate and vice versa. Chromium in the weld fusion zone had shown higher resistance against FAC. XRD analysis had shown that a relatively thick oxide layer was present on the weld.
Acknowledgements The authors would like to thank Mr. D. N. Sah, Raja Ramanna Fellow, Post Irradiation Examination Division, BARC for his guidance and useful suggestions in preparation of the manuscript of the paper. The contribution of R. K. Meena and M. Meena for decontamination, cutting and sample preparation is also acknowledged. References [1] S.S. Bajaj, A.R. Gore, Nucl. Eng. Des. 236 (2006) 701–722. [2] S.A. Bhardwaj, Nucl. Eng. Des. 236 (2006) 861–871. [3] V. Kain, S. Roychowdhury, T. Mathew, A. Bhandakkar, J. Nucl. Mater. 383 (2008) 86–91. [4] V. Kain, S. Roychowdhury, P. Ahmedabai, D.K. Barua, Eng. Fail. Anal. 18 (2011) 2028–2041. [5] F.N. Remy, M. Bouchacourt, Nucl. Eng. Des. 133 (1992) 23–30. [6] Material degradation and related managerial issues at nuclear power plants, in: Proceedings of Technical Meeting Organized by International Atomic Energy Agency, Vienna, Feb 15–18, 2005. [7] M. Naitoh, S. Uchida, S. Koshizuka, H. Ninokata, N. Hiranuma, K. Dosaki, K. Nishida, M. Akiyama, H. Saitoh, J. Nucl. Sci. Technol. 45 (2008) 1116–1128. [8] Multivariable Assessment of Flow Accelerated Corrosion and Steam Generator Fouling: Literature Review, EPRI, Palo Alto, CA, 2003 (1003619). [9] R.B. Dooley, V.K. Chexal, Int. J. Press. Vessel. Pip. 77 (2000) 85–90. [10] J. Robinson, T. Drews, Resolving flow-accelerated corrosion problems in the industrial steam plant, Technical paper: 4689911, Betzdearborn, USA, 1999. [11] S. Nasrazadani, R.K. Nakka, D. Hopkins, J. Stevens, Int. J. Press. Vessel. Pip. 86 (2009) 845–852. [12] K.M. Hwang, T.E. Jin, K.H. Kim, J. Nucl. Sci. Technol. 46 (2009) 469–478. [13] John P. Slade, Tracy S. Gendron, Flow accelerated corrosion and cracking of carbon steel piping in primary water operating experiences at the point lepreau generating station, in: Proceedings of the 12th International Conference on Environmental Degradation of Material in Nuclear Power System-Water Reactors, TMS (The Mineral, Metal and Materials Society), 2005, pp. 773–784. [14] A. Filipovic, E. G. Price, D. Barber, J. Nickerson, Material and fabrication consideration for the CANDU-PHWR heat transport system, in: International Conference on Nuclear Equipment – Welding and Q.A., Sarajivo, Yugoslavia, 1987. [15] A106/A 106M–08, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service, ASTM, USA, PA 19428–2959. [16] J.H. Moon, H.H. Chung, U.C. Kim, J.S.S. Rho, Nucl. Eng. Technol. 37 (2005) 375– 384. [17] D. Cubicciotti, J. Nucl. Mater. 152 (1988) 259–264. [18] I.S. Kim, J. Korean Nucl. Soc. 30 (1998) 148–163. [19] R.L. Tapping, J. Nucl. Mater. 383 (2008) 1–8. [20] T. Shoji, Z. Lu, Y. Takeda, Y. Sato, Towards proactive materials degradation management in NPP – today and future, in: CD Proceedings of the 14th Asia Pacific Corrosion Control Conference (APCCC), Shanghai, China, October 21–24, 2006. [21] C.L. Smith, V.N. Shah, T. Kao, G. Apostolakis, NURGE/CR-5632: Incorporating Aging Effects into Probabilistic Risk Assessment– a Feasibility Study Utilizing Reliability Physics Model, US Nuclear Regulatory Commission, 2001.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Presented at ANM-2011 Conference, Feb 9-11, Mumbai, India
Flow accelerated corrosion of carbon steel feeder pipes from pressurized heavy water reactors J.L. Singh a, Umesh Kumar a,⇑, N. Kumawat a, Sunil Kumar a, Vivekanand Kain b, S. Anantharaman a, A.K. Sinha c a b c
Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Material Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Nuclear Power Corporation of India Limited, Mumbai 400 094, India
a r t i c l e
i n f o
Article history: Available online 9 June 2012
a b s t r a c t Detailed investigation of a number of feeder pipes received from Rajasthan Atomic Power Station Unit 2 (RAPS#2) after en-masse feeder pipe replacement after 15.67 Effective Full Power Years (EFPYs) was carried out. Investigations included ultrasonic thickness measurement by ultrasonic testing, optical microscopy, scanning electron microscopy, chemical analysis and X-ray Diffraction (XRD). Results showed that maximum thickness reduction of the feeder had occurred downstream and close to the weld in 32 NB (1.2500 /32.75 mm ID) elbows. Rate of Flow Accelerated Corrosion (FAC) was measured to be higher in the lower diameter feeder pipes due to high flow velocity and turbulence. Weld regions had thinned to a lower extent than the parent material due to higher chromium content in the weld. A weld protrusion has been shown to add to the thinning due to FAC and lead to faster thinning rate at localized regions. Surface morphology of inner surface of feeder had shown different size scallop pattern over the weld and parent material. Inter-granular cracks were also observed along the weld fusion line and in the parent material in 32 NB outlet feeder elbow. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Feeder pipes are integral part of Primary Heat Transport (PHT) system in Pressurized Heavy Water Reactors (PHWRs). The feeder pipes and the headers and pipes connecting to the steam generator are the only components in the PHT of PHWRs that are made from plain carbon steel [1,2]. Flow Accelerated Corrosion (FAC) of feeder piping has been recognized as one of the main concern affecting safety and availability of nuclear power plants. FAC is also a main degradation concern for all the high energy piping/components in the secondary circuit of nuclear reactors. On the secondary circuit components, a few cases of failures due to FAC have been reported worldwide [3–6]. FAC is described as corrosion enhanced by mass transfer, between (a dissolving oxide film on) the base material and a flowing fluid that is unsaturated in the dissolving species. Corrosion proceeds rapidly by electrochemical means and the corrosion rate accelerates as the velocity of the fluid increases. Dissolution of protective magnetite oxide layer into flowing stream of water causes wall thinning (metal loss) of carbon steel [3–11]. If the thinning remains undetected and the remaining thickness is less than ⇑ Corresponding author at: Post Irradiation Examination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. Tel.: +91 22 25596111; fax: +91 22 25505151. E-mail address: [email protected] (U. Kumar). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.05.045
the critical thickness required to withstand the operating pressure, a ductile fracture occurs [3]. Flow rates may change due to change in geometry (elbows/bends, etc.) or due to local disturbance in flow (e.g. flow obstruction by flow measurement devices or a protrusion on the inner surfaces of pipe/welds). Therefore, the local flow rate changes will cause localized changes in thinning rate [12]. The maximum rate of FAC is the key factor in determining the decision to replace the FAC affected pipes. The location of maximum rate of thinning due to FAC is also a key factor so as to allow thinning measurement to be focused on that location. Cracking of some of the outlet feeders has been reported at regions of tensile stresses but the exact mechanism remains unclear with hydrogen from the FAC affected surfaces and materials aging issues expected to be playing a role [13]. Wall thinning of carbon steel feeder pipes due to FAC led to severe thinning in an Indian PHWR (Rajasthan Atomic Power Station unit #2). Based on an exhaustive wall thickness management program, an en-mass feeder pipe replacement was carried out in this reactor after 15.67 EFPY of reactor operation. A detailed investigation has been carried out on seven feeder pipes selected from those removed from the reactor. The objective of this investigation was to examine and understand the nature and extent of thinning caused by FAC in carbon steel feeder pipes of Indian PHWRs. The key findings of this investigation are presented in this paper.
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2. Material and fabrication procedure There are 306 number of feeders connected to each face of the reactor. Each feeder is routed between End Fitting and inlet or outlet header and consists of various straight run and bends that vary from 30 to 90°. Feeder length generally varies from 6 to 20 m [14]. Feeders are designed and fabricated as per the requirement for class 1 components of American Society of Mechanical Engineers (ASMEs) boiler and pressure vessel code Section III [13]. The chemical composition of feeders was in accordance with the particular American Society for Testing and Materials (ASTMs) A-106 Grade B specifications [11,13–15]. Feeder assembly consists of single or double elbow fittings welded at one end to the header and joined on the other end by the gray-loc hub to the End Fitting. Different sizes of feeder pipe were used to balance the fuel channel flow rate in order to maintain consistent outlet temperature. In general, PHT feeder pipes are cold drawn followed by appropriate heat treatment for controlling and achieving the required mechanical properties. All pipes supplied to this specification need to be suitable for cold bending to a minimum mean radius of less than 200 mm or four times the pipe outer diameter. All the butt welding and fittings like elbow and reducer are forged or formed to the finished shape and size by hot working. Fittings manufactured by hot working and/or cold forming processes are appropriately heat treated for controlling and achieving the required mechanical properties. Fittings are finally annealed or normalized.
25 ft/s (7.62 m/s) to a 50 ft/s (15.24 m/s) and temperature at the outlet end was in range of 280–292 °C. A 10 MHz duel element ultrasonic probe of 1 mm tip diameter was used for thickness measurement. A test piece of feeder material of known thickness was taken for finding out the ultrasonic velocity in the material needed for calibration of the instrument. Ultrasonic instrument was set in pulse echo mode. Extrados and intrados of the feeder elbows were marked as 12 and 6’O clock locations respectively, subsequently other clock locations were marked. Dashed lines were drawn axially on the feeder from each clock location at a gap of approximately 5 mm. Ultrasonic thickness measurement were taken at the gap in between the dash line so that paint thickness should not add to actual thickness at the point of measurement. For metallographic examination, outlet feeder elbows were cut axially into two halves along 3 and 9’O clock position. The samples were taken from the location which included the Weld Fusion Zone (WFZ), Heat Affected Zone (HAZ) and the parent material. The ground and polished samples were etched using 2% nital solution for observing the microstructure. The oxide layer on the Inner Diameter (ID) surface of B-10 outlet feeder was analyzed by X-ray Diffraction (XRD) using Mo Ka source. Scanning Electron Microscopy (SEM) examination on the ID surfaces was carried out to observe the scallop or overlapping horse shoe pattern [3,4,8–11,13]. Inductively Coupled Plasma – Atomic Emission Spectrometry (ICP-AES) technique was used for chemical composition analysis of WFZ and the parent material.
3. Experimental procedure 4. Results The feeder pipes received for detailed investigation were highly contaminated and had a contact radiation dose of 2 millirem per hour (mR/h). Decontamination of feeders was carried out with soap solution and soft nylon brush without damaging the inner oxide layer. Details of the feeder dimensions and the reactor operating parameters in these feeders are given in Table 1. These feeders were taken from the outlet end of coolant channel. Diameter and pipe thickness of feeder ranged from 1.2500 to 200 (31.75– 50.8 mm) and 4.85–5.54 mm respectively. Flow rate varied from
4.1. Ultrasonic thickness measurement of feeder Location of the maximum thinning and the percentage reduction in thickness for a number of feeder pipes of different size are given in Table 2. Ultrasonic thickness measurement data of the feeders showed that thickness reduction was prominent only in the 32 NB (31.75 mm) feeder pipe. These feeders were double elbows welded to the straight pipe.
Table 1 The reactor operating parameters for the selected feeders. Sr. no.
Outlet feeders
Nominal bore size (inch/mm)
1 2 3 4 5 6
B-09 B-10 O-03 N-03 Q-13 K-08
32 32 40 40 50 50
NB NB NB NB NB NB
(1.2500 /31.75) (1.2500 /31.75) (1.5000 /38.10) (1.5000 /38.10) (2.0000 /50.80) (2.0000 /50.80)
Initial nominal thickness (mm)
Flow rate (ft/s)/(m/s)
Temperature (°C)
4.85 4.85 5.08 5.08 5.53 5.53
47.0/14.32 49.5/15.08 38.3/11.67 39.6/12.07 25.6/7.80 28.1/8.56
282.0 284.3 284.0 285.6 285.3 292.0
Table 2 Ultrasonic thickness measurement of different size feeder. Feeder no.
ID size and no of elbows
Location and position of minimum thickness in the elbow
Initial nominal thickness (mm)
Measured minimum thickness (mm)
Thickness reduction (%)
B-09
32 NB Double elbow
4.85
1.11
77.1
B-10
32 NB Double elbow
4.85
1.37
71.7
O-03
40 NB Double elbow
5.08
3.62
28.7
N-03
40 NB Double elbow
5.08
3.57
29.7
K-08
50 NB Single elbow
5.54
4.55
17.8
Q-13
50 NB Single elbow
Outlet feeder at12’O clock in second elbow close to weld Outlet feeder at 6’O clock in first elbow close to weld Outlet feeder at 12’O clock in second elbow close to weld Outlet feeder at 12’O clock in second elbow close to weld Outlet feeder at 12’O clock in first elbow close to weld Outlet feeder at 12’O clock in first elbow close to weld
5.54
5.01
9.5
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Fig. 1. Localized thinning of outlet feeders (a) B-09 around the elbow close to the weld at 12’O clock (b) B-10 outlet feeder at 6’O clock intrados of first elbow (c) B-10 outlet feeder at 12’O clock in intrados of second elbow.
Fig. 1 shows the arrangements for thickness measurement and regions of minimum thickness measured on B-09 and B-10 feeder. Localized thinning of B-09 outlet feeder had occurred in the elbow on both sides of the weld at 12’O clock position (Fig. 1a). The thickness zone less than 2.5 mm was encircled on the feeder pipe. Minimum available thickness was 1.11 mm (a reduction of 3.74 mm from initial, nominal thickness) at the intrados of second elbow downstream and close to the second weld. Nevertheless, thickness reduction had occurred everywhere in the feeder elbow. Fig. 2 shows the ID surface of B-09 showing localized thinning adjacent to the weld. A weld protrusion is also seen in the photograph. B-10 feeder had also shown thickness reduction at two different locations and the area with minimum thickness was encircled and shown in Fig. 1b and c. Fig. 1b shows the marked region with the minimum wall thickness of 1.37 mm (a reduction of 3.48 mm from initial, nominal, thickness) at intrados of the first elbow downstream and close to the first weld at 6’O clock position. Fig. 1c shows the marked region with the minimum wall thickness of 2.01 mm at intrados in the second elbow close to the second weld at 12’O clock position.
Higher NB feeder pipe namely O-03, N-03, K-08 and Q-13 had shown thickness reduction in the range of 0.5–1.5 mm. Outlet feeders O-03, N-03 and 40 NB had double elbows whereas K-08 and Q-13 and 50 NB had single elbow welded to the straight segment of the pipe. Coolant flow rate in these feeders was between 25 ft/s (7.62 m/s) and 38 ft/s (11.58 m/s). 4.2. Cross section examination of feeders The appearance of the inside surface of B-10 feeder as observed in the optical microscope is shown in Fig. 3. The photomicrograph in this figure shows the waviness of the inner surface due to FAC by PHT water flowing at a high velocity. It was observed that WFZ had corroded lesser than the parent material. Remaining wall thickness in HAZ, close to the weld was 1.35 mm and increases to a value 1.95 mm in parent material away from the weld. 4.3. Corrosion rate measurement The corrosion rate due to FAC in different sizes of feeder pipes has been listed in Table 3. This corrosion rate has been calculated based on the wall thickness measurement in the laboratory (by ultrasonic testing) at the straight pipe segments, away from the
Fig. 2. Localized thinning of B-09 feeder downstream and adjacent to the weld. A weld protrusion had formed during the reactor operation period.
Fig. 3. Photomicrograph of B-10 outlet feeder section showing waviness of inner surface.
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J.L. Singh et al. / Journal of Nuclear Materials 429 (2012) 226–232 Table 3 Corrosion rate in straight sections in different feeder pipes. Feeder no. K-08 O-03 N-03 B-09 B-10
Nominal bore size (inch/mm) 50 40 40 32 32
NB NB NB NB NB
00
(2.00 /50.8) (1.5000 /38.1) (1.5000 /38.1) (1.2500 /31.75) (1.2500 /31.75)
Flow rate (ft/s)/(m/s)
Thickness reduction in straight section (mm)
Corrosion rate (mm/year)
28.1/8.56 38.3/11.67 39.6/12.07 47.0/14.32 49.5/15.08
0.57 0.78 0.86 2.06 2.21
0.036 0.051 0.055 0.132 0.141
Fig. 4. Effect of flow velocity in straight segment of feeder pipe on corrosion rate after 15.67 EFPY.
elbows. Wall thickness loss was estimated by subtracting the remaining pipe thickness from the nominal thickness of the respective pipes. These feeders were removed after 15.67 EFPY. Corrosion rate was calculated by dividing the thickness reduction by EFPY. Corrosion rate (thus calculated) vs. flow velocity was plotted and is shown in Fig. 4. Fig. 4 shows a drastic change in corrosion rate beyond a flow velocity of 40 ft/s. This clearly reflects the effect of flow rate (velocity) on the acceleration of corrosion. The flow accelerated corrosion is therefore evident more in smaller diameter pipes which also had a higher flow rate. 4.4. Chemical compositional and XRD analysis of weld and parent metal of feeder The elemental compositional analysis of the weld and parent material by ICP-AES of the B-10 outlet feeder was done. The chemical composition of the WFZ and the parent material are listed in Table 4.This shows that the WFZ had a higher amount of Cr, Mo, and Cu than the parent material. These elements have been shown to provide effective corrosion resistance against FAC [3–5,8,13,16– 19]. Fig. 5 shows the XRD analysis of the oxide layer on the WFZ
Table 4 ICP-AES analysis of B-10 outlet feeder. Sr. no.
1 2 3 4 5 6 7
Element
Cr Cu Mn Mo Ni Si V
Concentration (wt.%) Parent material
Weld fusion zone
0.02 0.04 0.61 0.08 0.07 0.61 0.02
0.29 0.14 0.64 0.43 0.32 0.50 0.02
Fig. 5. XRD pattern of the oxide layer at ID surface (a) on parent material and (b) on weld fusion zone of B-10 feeder. Symbols ‘M’ and ‘Fe’ indicate magnetite and iron peaks respectively. Crystallographic planes are indicated in parenthesis.
and parent material of B-10 feeder. Fig. 5a and b shows the intensity of iron (base material) and magnetite peaks on the parent material and WFZ samples respectively. The intensity of magnetite peaks on the WFZ is quite high compared to the intensity of magnetite peak on the parent material. This shows that WFZ has a thicker magnetite layer compared to that on the parent material. 4.5. SEM examination The B-10 feeder sample was examined in detail under SEM. Fig. 6 shows the of scallop pattern developed on the ID surface of B-10 feeder which is characteristic of single phase FAC [3,4,8– 11,13]. Fig. 6a shows the scallop pattern on the WFZ and Fig. 6b shows the scallop pattern on the parent material. The scallops on
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Fig. 6. SEM photomicrographs showing Scallop at ID of B-10 (a) on the weld fusion zone, (b) on HAZ, close to the weld and (c) dissolution due to eddies formation on the base material.
the weld fusion zone are clearly larger in size and have sharp edges whereas the scallops on parent material downstream and close to the weld are smaller in size and have flattened edges. As the weld region (WFZ) has not undergone much thickness reduction compared to the parent material, it is a reaffirmation of the previously reported observations that a larger scallop size indicates lower FAC rates [3]. Fig. 6c shows the scallops on the base material at a higher magnification showing dissolution of magnetite due to formation of eddies. Fig. 7 shows the SEM photomicrograph of Q-13 outlet feeder. This feeder was of 50 NB size and had a much lower flow rate (7.8 m/s) as compared to B-10 feeder pipe which was 32 NB size having flow rate of 15.08 m/s. Clear and distinct scallop pattern had not developed at the ID surface of Q-13 feeder. This again brings out the fact that at lower velocities, the FAC rate is much
lower and gets reflected in the scallop pattern developed on the inner (affected) surfaces.
Fig. 7. SEM photomicrograph showing scallops in Q-13 (50 NB) feeder. No distinct scallop formed at a lower velocity.
Fig. 8. As-polished B-09 outlet sample showing cracks in the base material and HAZ.
4.6. Inter-granular cracking of feeder B-09 feeder samples comprising of WFZ, HAZ and parent material from the first and second elbows were cut for metallographic examination. An as-polished section of feeder B-09 is shown in Fig. 8. Fig. 8 shows that material was removed (dissolved) from the parent material adjacent to the weld at the ID surface, as indicated by waviness of the ID surface. A possible weld defect at the weld fusion line on the ID surface (indicated by black arrow) is expected to have acted as an additional site for flow disturbance and a site for sustaining more aggressive water chemistry. The HAZ downstream to the weld is the most vulnerable area where factors like turbulence and weld protrusions contributed to maximize the FAC rate leading to excessive localized thinning in B-09 feeder.
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Fig. 8 also shows presence of a fine crack along the weld fusion line (marked by blue arrows) and another fine radial crack (marked by a red arrow) in the parent material at a distance of roughly 5 mm from the weld fusion line. A magnified view of these cracks is shown in Figs. 9 and 10. Fig. 9 shows the crack along weld fusion line. Fig. 10 shows the crack in the parent material started from the ID surface of feeder pipe and was inter-granular in nature. A sample cut from the second elbow of B-09 feeder which included WFZ and adjoining elbow area also showed cracks along the WFZ surrounding the weld (Fig. 11a). A magnified view of the crack in Fig. 11b shows that the crack in HAZ was intergranular in nature.
5. Discussion Fig. 9. Crack along the weld fusion line in HAZ.
Fig. 10. Crack in the base metal of B-09 outlet feeder showing inter-granular propagation from ID surface.
Fig. 11a. As polished specimen of B-09 from the second elbow showing cracks along the weld fusion line.
Fig. 11b. Inter granular crack in the HAZ at higher magnification.
The primary factors causing FAC are the water chemistry of coolant water which is unsaturated in dissolved iron, flow rate, temperature of operation and turbulence. The flow velocity varies a lot from one feeder to another, due to the size of the feeder and the need to maintain required level of cooling in different coolant channels. This is the primary reason for different rates of FAC in different feeders. The second reason is the additional flow disturbance in each feeder due to its unique geometry. Some feeders have one and others two elbows. The bends in each feeder also vary in numbers and degree due to location of each feeder and the geometry necessary to connect to the headers. Based on these two major factors, the FAC effect on feeders is monitored during plant shutdowns on a regular basis by ultrasonic inspection. The plant assesses the remaining thickness in each feeder (at the point of maximum thinning) and decides whether it is safe to operate it further. Unusual high thickness reduction was observed in 32 NB feeder pipe in the elbow region downstream to the weld (Table 2). Excessive grinding of welds and adjacent areas of the feeders could be one of the regions for such dramatic wall thinning in PHWRs or CANDU reactors. Fig. 2 shows the ID surface of elbow of B-09 feeder. A shallow localized depression had developed over an area adjacent to the weld suggests that thickness loss has been associated with localized dissolution and removal of the material from inside surface of the feeder pipe. The present case shows that an additional factor plays an important role in determining the extent and location of maximum FAC rate. Presence of local flow disturbance, e.g. by a weld protrusion, adds to the flow disturbance caused by the geometry and intended global flow rate in each feeder. This local disturbance leads to localized region undergoing more severe thinning. Therefore, local flow disturbances e.g. by weld protrusions, need to be taken into account in determining the location and extent of thinning by FAC. Turbulence resulting from high coolant velocity and flow obstruction like weld protrusion promotes mass transfer between the oxide water interface and the bulk coolant, thereby increasing the rate of dissolution of magnetite film. The temperature of coolant increases from 249 °C at the inlet end to 293 °C at the outlet end of the coolant channel. As coolant water comes out at the outlet end after passing through zirconium alloy pressure tube, it is unsaturated in dissolved iron. Therefore, faster magnetite dissolution occurs in the outlet feeders leading to higher thickness loss [13]. As has been shown in Table 4, the chemical composition of the WFZ and the parent material was slightly different. The chromium and molybdenum levels in the WFZ were higher than those in the base material. This led to differences in FAC rates over these two regions, WFZ and the base material. The WFZ thinned at a lower rate than the base material. Over long operation years, this difference in FAC rates led to formation of weld protrusions and there-
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fore led to even more flow disturbance. Therefore, this type of local disturbance leads to accelerating effect of FAC at the locations just downstream of the weld protrusion as the effect of the weld protrusion goes on increasing with time. Another possibility is that there was a weld protrusion at the location of maximum thinning from the fabrication stage itself. Even if it was present from the fabrication stage, its effect would have increased with years of operation as there was a difference in the FAC rate over the weld regions and the base material. Therefore, the geometry and chemical composition of welds vis-à-vis the base material also needs to be considered in determining the location and extent of FAC. Increased resistance to FAC has also been experimentally shown in high temperature, high pressure water for carbon steel containing slightly higher levels of chromium [8,17]. The FAC rate for a 90° elbow is reported to predict that FAC rate reduce to one-tenth of the FAC rate for carbon steel when Cr content is increased from 0.03% to 0.5%. This is due to the surface oxide changing from Fe3O4 to FeCr2O4 when Cr is added in carbon steel [3,20]. FeCr2O4 oxide is highly stable due to its extremely low solubility in high temperature aqueous solution [16]. Variation in scallops shape and size over weld and parent material was observed in the feeders examined in this study. Small size of scallops on the parent material may be due to formation of vortex or eddies downstream to the weld. Size of scallop can be correlated with FAC rate. Larger the size of scallops as found over the WFZ, smaller the FAC rate. The shape of the scallop is also shown in this study to reflect the flow behavior. The scallops had sharp edges at locations where the dissolution rate was less (WFZ). At the base material (that contained less chromium and molybdenum) the scallops had flattened edges, possibly due to more disturbed flow at such locations at downstream of weld protrusion. The shape of the scallops (Fig. 7c) seemed to reflect the presence of eddies also. Cracking in HAZ and parent material were also observed in the feeder pipes. These cracks were in the elbow bends where high residual tensile stresses are reported to be present [13]. A proper normalizing treatment after fabrication is expected to reduce the levels of stresses in fabricated pipes. However, improper normalizing treatment and/or subsequent welding would have lead to stresses in the weldment. All the cracks were detected in the weldment only. The crack along the weld fusion line shown in Fig. 9 could have been aided by more aggressive water chemistry in the occluded weld defect. Mechanism of such cracking was indicated to be stress corrosion cracking (SCC) due to exposure of feeder surface to hot coolant in elbow region where bending and residual stresses due to welding are present. However, the mechanism of SCC has not been clearly confirmed in feeders. Issues like material aging (at reactor operation temperature for more than 12 EFPY), presence of hydrogen produced by FAC on the material ID surface and possibly corrosion creep have been indicated to play a role in cracking of feeders [13,21]. Research on these issues is underway at this point of time. 6. Conclusions Detailed examination of a number of feeder pipes removed from Rajasthan Atomic Power Station unit #2 after 15.67 EFPY operations has been carried out. The main findings of the examination are given below: FAC has caused thinning of the feeder pipe everywhere but preferentially it has caused localized thinning at the intrados of the elbows downstream to the weld.
Corrosion rate was accelerated due to high flow velocity in smaller diameter feeder pipes. Weld protrusion in the feeder elbows had caused obstruction to the coolant flow and formation of eddies downstream to the weld leading to additional localized thinning close to the weld. Inter-granular cracks were found along weld fusion line and in parent material in the elbow region. The scallop pattern which had developed at the ID surface of feeder pipes indicated degradation due to single phase FAC. Smaller the measured scallop size, higher was the FAC rate and vice versa. Chromium in the weld fusion zone had shown higher resistance against FAC. XRD analysis had shown that a relatively thick oxide layer was present on the weld.
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