Lifespan Estimation of Seal Welded Super Stainless Steels for Water Condenser of Nuclear Power Plants

Transcription

1 Met. Mater. Int., Vol. 20, No. 1 (2014), pp. 69~76 doi: /s Lifespan Estimation of Seal Welded Super Stainless Steels for Water Condenser of Nuclear Power Plants Young Sik Kim 1, *, Sujin Park 1, and Hyun Young Chang 2 1 Andong National University, Materials Research Center for Energy and Clean Technology, School of Materials Science and Engineering, 1375 Gyeongdongro, Andong , Korea 2 KEPCO Engineering & Construction Company, Power Engineering Research Institute, 8 Gumiro, Bundang, Seongnam, Gyeonggi , Korea (received date: 21 May 2013 / accepted date: 5 July 2013) When sea water was used as cooling water for water condenser of nuclear power plants, commercial stainless steels can not be applied because chloride concentration exceeds 20,000 ppm. There are many opinions for the materials selection of tube and tube sheets of a condenser. This work reviewed the application guide line of stainless steels for sea-water facilities and the estimation equations of lifespan were proposed from the analyses of both field data for sea water condenser and experimental results of corrosion. Empirical equations for lifespan estimation were derived from the pit initiation time and re-tubing time of stainless steel tubing in sea water condenser of nuclear power plants. The lifespan of seal-welded super austenitic stainless steel tube/tube sheet was calculated from these equations. Critical pitting temperature of sealwelded PRE 50 grade super stainless steel was evaluated as 60 C. Using the proposed equation in engineering aspect, tube pitting corrosion time of seal-welded tube/tube sheet was calculated as 69.8 years and re-tubing time was estimated as 82.0 years. Key words: metals, welding, corrosion, SEM, sea water 1. INTRODUCTION Copper alloys were used as condenser tube materials in nuclear power plants (NPP) around the world until mid- 1950s. However, corrosion by ammonia from the steam side was raised as a serious issue. From the mid-1950s to the mid-1970s, 304 and 316 stainless steels were used in addition to copper alloys but pitting and crevice corrosion in sea water occurred [1]. From the mid-1970s, super stainless steels were developed, which solved the problems with poor localized corrosion resistance of commercial stainless steels. Titanium alloys have also been developed for the purpose of addressing the disadvantages of copper alloys. Both alloys have been applied to condenser tubes. Austenitic and ferritic super stainless steels were developed, but in the case of ferritic steel, cracking by vibration of the condenser occurs, since its ductile-brittle transition temperature is near the operating temperature of a condenser. Their application has been limited or has ceased. Recently, second generation super austenitic stainless steel has been developed to address the poor pitting corrosion resistance of first generation stainless steels [2]. *Corresponding author: yikim@andong.ac.kr KIM and Springer The tubing materials that have been used for condenser tubes have include 304 and type 316 stainless steels, Admiralty brass, Al-brass, Cupronickel, Titanium, AL-6X, 254 SMO, AL29-4C, AL-6XN (AL-6XN plus), SR-50A, and 654 SMO. The stainless steels used as condenser tube materials have suffered from corrosion depending upon the chloride contents in the cooling water. 304L stainless steel may be used in fresh water conditions with chloride concentration less than 200 ppm, and 316L stainless steel can be applied in water conditions with chloride concentrations less than 1,000 ppm [3]. However, when sea water is used as a cooling medium, commercial stainless steel cannot be used because of high chloride contents over 20,000 ppm. Thus, there are many opinions as to which materials are suitable for sea water condenser tubes. Therefore, this work analyzes the guidelines of material selection for sea water facilities and the field application data of stainless steel. Based on the pit initiation time and re-tubing time of stainless steel tubing in a sea water condenser for nuclear power plants, empirical equations for lifespan estimation were derived. The lifespan of seal-welded super austenitic stainless steel tube/tube sheet was calculated using these equations.

2 70 Young Sik Kim et al. 2. EXPERIMENTAL PROCEDURES 2.1. Experimental alloys The stainless steels evaluated in this study were prepared from high-purity grades of Fe, Cr, Ni, Mo, Cu, Fe-Si, and Fe-Mn. After melting in a high-frequency vacuum induction furnace, only solid sections of the cast ingots were taken to prepare the specimens. The sections were first soaked for 120 min and hot rolled to 4 mm at 1,250. The hot-rolled specimens were annealed at 1,025 for 10 min, water quenched, and acid pickled. A small section was cut after each procedure and used for chemical analysis. Table 1 presents the chemical compositions of the experimental alloys Corrosion Tests Anodic polarization tests were performed in de-aerated 1M NaCl at 50 solution using a potentiostat (Gamry co. DC105). All specimens were ground with #600 grit SiC paper. Epoxy resin was coated on the polished surface, except the area of 1 cm 2. The solutions were de-aerated by purging with pre-purified nitrogen gas. After immersion, the sample was cathodically polarized for 10 min at mv (SCE) and then maintained under open circuit potential for 10 min and subsequently polarized anodically from the corrosion potential at a scan rate of 1 mv/sec. Anodic dissolution tests were performed in de-aerated 1 M NaCl solution at 50 [5] using a potentiostat (Gamry co. DC105). The test conditions were the same as those in the anodic polarization test, except that a potential of +700 mv (SCE) was anodically applied. The critical pitting temperature (CPT) was determined according to ASTM G48 practice A [6] by immersion in 6% FeCl 3. This test was performed with a mock-up specimen for a condenser using sea water as a cooling medium in nuclear power plants. Figure 1 presents the preparation procedure of a mock-up specimen of seal-welded SR-50A tube/tube sheet for the condenser of a nuclear power plant. Figure 1(a) shows the tube expansion, seal welding on the front side, and the cutting of the SR-50A cladding part. Figure 1(b) shows the manual welding on rear side, and Fig. 1(c) shows the dimensions of the mock-up specimen. Fig. 1. Preparation procedure of mock-up specimen for corrosion test of condenser tube. 3. RESULTS AND DISCUSSION 3.1. Materials selection guideline of stainless steels for sea water utilities Pitting corrosion is one of the main degradation mechanisms of sea water facilities. This form of corrosion is a kind of the micro-galvanic corrosion induced by chromium dissolution in the passive film, which leaves behind remained corrodible iron. Proposed mechanisms about the breakdown of the passive film by chloride ion can be summarized as follows; (a) Chloride reacts with metals and the stable compounds more than oxide or hydroxide [7,8], (b) Surface adsorption instead of water molecules affected by the potential difference between the film and the solution [9], (c) chloride increases the vacancy concentration in the passive film and then localized corrosion by the point defect model [10], (d) chloride ion forms unstable metal chloride and then dissolves into the solution [11]. The potential difference between Table 1. Chemical composition of the experimental alloys (w/o) Alloys Fe C Si Mn P S Ni Cr Mo Cu W N PRE 30 SR-50A Bal Mo-6 Bal AL-6X Bal L Bal L Bal *PRE 30 = %Cr + 3.3(Mo + 0.5%W) + 30%N [4]

3 Lifespan Estimation of Seal Welded Super Stainless Steels for Water Condenser of Nuclear Power Plants 71 the passive region and the active region of austenitic stainless steels could be large, depending upon the alloy s composition. Acidic chlorides are representative species that may induce pitting corrosion in stainless steels. Chloride reacts with chromium and forms highly soluble CrCl 3. Adsorbing anions such as chloride ions have an important, and generally adverse, effect in a number of electrochemical reactions, including the adsorption and ordering of adsorbates from a supporting electrolyte, the structure of metal substrates, the kinetics of electrochemical reactions, metal deposition, and corrosion [12]. Thus, chromium dissolves from the passive film and active iron remains. Since chromium dissolves, the chloride, which has an electric driving force, forms spherical pits that penetrate the passive film of the stainless steel. FeCl 3, which is very aggressive to stainless steels, remains inside the pits. This is why FeCl 3 is used to evaluate the corrosion resistance of stainless steels in many standards including ASTM. When Cr, Mo, and N are added to stainless steels, the pitting corrosion resistance can be improved. Thus, the relationship between alloying elements and pitting corrosion was derived. The equation below represents the pitting resistance equivalent number - PRE 16 [13]. PRE 16 = %Cr + 3.3%Mo + 16%N (1) It is well-known that the minimum PRE 16 for sea water facilities is 32 [5]. Table 2 shows the representative austenitic stainless steels and their PREs. According to the minimum PRE 16 requirement sea water facilities, alloys with higher PRE than that of 904L can be used. However, it should be noted that this guideline must be applied for nonwelded structures, such as base metals. The key factors that affect pitting corrosion resistance are the chloride contents, ph, and temperature. Generally, when the temperature and chloride content are high and the ph is low, the possibility of pitting corrosion increases. With any chloride content, high temperature and low ph will facilitate pitting corrosion. In contrast, low temperature and high ph can reduce the possibility of pitting corrosion. The presence of Table 2. Commercial austenitic stainless steels and its PREs Alloy PRE 16 Type 304, 304L 18.0 Type 304N, 304LN 19.6 Type 316, 316L 22.6 Type 316N, 316LN 24.2 Type 317, 317L 27.9 Type 317LMN 31.8 Alloy 904L 36.6 AL-6XN 42.7 AL6XN+ 50.0(PRE 30) SR-50A 50.0(PRE 30) *PRE 16 = %Cr + 3.3(Mo + 0.5%W) + 16%N [13] acidic chloride would be the severest condition. Mild environments include chloride-bearing alkaline or high ph solutions. However, once pitting corrosion occurs, pits rapidly grow even in these environments. Increased Mo content of the alloys greatly improves the resistance to pitting corrosion. Thus, high Cr and Mo alloys show the better pitting corrosion resistance. Many works have shown the relations among pitting corrosion, Mo contents, ph, and chloride concentration [5,14]. Alloys bearing more than 6% Mo should be applied for systems in neutral sea water environments [5]. The applicability of alloys depends on the temperature and chloride contents in brackish water and sea water [14]. 904L alloy (PRE 16 ~35) can be used at up to 40 C with 20,000 ppm chloride. In any case, the applicability of alloys for sea water in references [5] and [14] are meaningful for non-welded base metal, but cannot provide any information about lifespan of the alloys Approaches to suggest the lifespan estimation equation Field application data of 1st generation stainless steel can be summarized as follows [15-18]; Calvert Cliffs nuclear power plant in the USA consists of 2 PWR-type reactors. This plant used 70/30 cupronickel but tubes replaced in 1982 because of the many leakages. Unit #1 used titanium tubing and unit #2 used AL-6X (PRE=41) stainless steel tubing, which was not subjected to post-weld heat treatment. At installation, the plant forecast the possible lifespan of the stainless steel tubing as less than 5 years. Tubes less than 5% of the AL-6X showed defects. The corroded parts were mainly the weldments of the tubing. In 1994, about 3.0% of the tubing was plugged after inspection, and among 6 condensers, the #11A condenser was revealed to have had 7.2% of its tubes plugged because of baffle damage. At Bridgeport Harbor NPP, also in the USA, unit #1 had used AL-6X for condenser tubes in 1973, and 25% of tubes had suffered from pitting corrosion after 20 years. Also, 25% of the AL-6X tubes of unit #2 had suffered from pitting corrosion after 13 years. Pitting corrosion occurred at the seam area of tubing in these units, because the AL-6X had not been subjected to post-weld heat treatment. This paper focuses on the N08366 (AL-6X) field data obtained from unit #1 and unit #2 at Bridgeport Harbor. In the view of the environments and materials, the sea water condensers of this plant are similar to those of nuclear power plants in Korea. Important facts provided by the Bridgeport Harbor nuclear power plant are pit initiation after 10 years of operation and re-tubing after 13 to 20 years of operation [15]. Therefore, if AL-6X welded tube without a post-weld heat treatment is used for condenser tubes (with sea water as a cooling medium), it is judged that the pit initiation time will be 10 years and the re-tubing time will be 13 years. This kind of field data and lifespan can provide the very important cri-

4 72 Young Sik Kim et al. Fig. 2. Anodic polarization curves of the experimental alloys in deaerated 1M NaCl at 50 C. teria for estimating the lifespan of various stainless steels. Many reports have revealed corrosion-related data for stainless steels in sea water and chloride solutions, but there is little data for the lifespan in field applications [17~28]. Such field data is meaningful and very important. Figure 2 shows anodic polarization curves of the experimental alloys in de-aerated 50 C 1 M NaCl solution. Test solution was chosen by ASTM G150 [29] which was solution is more aggressive than sea water (about 20,000 ppm chloride) and the test temperature was higher than maximum operation temperature, 40 C of the condenser. As shown in the figure, corrosion potentials are similar to each other. However, by anodic polarization from corrosion potential, the alloys show big different potential behavior depending upon alloy s PRE. 317 L stainless steel having the low PRE was corroded by pitting under 0V (SCE). Pitting of 904L stainless steel has occurred above +200 mv (SCE) and 20Mo-6 stainless steel showed a little higher pitting potential than that of AL-6X. However, PRE 50 grade SR-50A stainless steel did not show any symptom of pitting corrosion (abrupt increased current above 1V (SCE) is due to the evolution of oxygen). As shown in the anodic polarization test, except for the PRE 50 grade alloy, the others showed pitting corrosion in 1M NaCl solution at 50 C. However, it is very difficult to estimate the lifespan when any of the alloys are applied to the operating condenser of nuclear power plants since corrosion damage could be detected after a long period over 20 years. Therefore, an accelerating technique by anodic dissolution was employed. This electrochemical method enables alloys to be evaluated on their applicability in environments with brackish water or sea water [30]. The potential of chlorinated sea water is +600 mv(sce), and that of natural sea water is +300 mv(sce) mv Fig. 3. Anodic dissolution curves at +700 mv (SCE) in deaerated 1 M NaCl at 50 C. (SCE) was applied in this work. A higher potential is applied to accelerate pitting corrosion. Figure 3 shows the anodic dissolution curves at +700 mv (SCE) in de-aerated 1 M NaCl solution at 50 C. As shown in the figure, 317L stainless steel was dissolved greatly by pitting corrosion after the test started. 904L, AL-6X, and 20Mo-6 alloys show increased currents with time, in the order of 904L>AL-6X>20Mo-6. However, in the case of PRE 50 grade SR-50A stainless steel, a stable passive film was formed by the anodic potential and the current reduced with time. The total electric charges by anodic dissolution until 1,800 sec are plotted in Fig. 4. Since these electric charges are related to pitting corrosion, the integrated charges were designated as the pitting corrosion rate. The pitting corrosion rates are reduced in the order of 317L> 904L > AL6X > 20Mo-6 (The value of SR- Fig. 4. Pitting corrosion rate obtained by anodic dissolution for 1,800 sec at +700 mv (SCE) in deaerated 1 M NaCl at 50 C.

5 Lifespan Estimation of Seal Welded Super Stainless Steels for Water Condenser of Nuclear Power Plants 73 Table 3. Pitting corrosion test of the experimental alloys in 6% FeCl 3 Alloys SR-50A 20Mo-6 AL-6X 904L 317L Temperature, C Weight loss (mg) Weight loss (mg/cm 2 ) Pitting sign No Yes No Yes No Yes No Yes Yes CPT, C Fig. 5. Critical pitting temperature of 317L, 904L, AL6X, 20Mo-6 and PRE grade SR-50A by ASTM G48A. 50A does not correspond to the pitting corrosion rate). Table 3 summarizes the results of the pitting corrosion tests of the experimental alloys by ASTM G48 method A. Even though 20Mo-6 and AL-6X have different PREs, the temperature was stepped up by 5 C starting from a temperature close to the corrosion potential. Figure 5 shows the linear relationship between the CPTs and PREs of experimental alloys. A linear relationship can be deduced, as shown in Eq. (2); CPT = 3.2PRE (2) As shown in Table 3, the CPT of AL-6X base metal was 50. Since the CPT difference between base metal and weld metal is C [14], the CPT of welded AL-6X tubing is estimated as C. Therefore, the PRE showing a CPT of C can be calculated according to Eq. (2); When the CPT is evaluated as 20 C and 25 C, PRE 30 can be calculated as 30.7 and Therefore, the calculated PREs of welded AL-6X tubing are estimated as The next step is to find the relationship between the pitting corrosion rate and the PRE of an alloy. Figure 6 shows the relationship of (a) the corroded amount vs. PRE, and (b) the pitting corrosion rates vs. PRE values. (The pitting corrosion rate is obtained by anodic dissolution for 1,800 sec at +700 mv (SCE) in de-aerated 1 M NaCl at 50 C). The relation between them can be written as; Fig. 6. Relationship between (a) corroded amount vs. PRE and (b) pitting corrosion rate vs. PRE values (Pitting corrosion rate obtained by anodic dissolution for 1,800 sec at +700 mv (SCE) in deaerated 1 M NaCl at 50 C). Pitting corrosion rate, A/cm 2 = PRE PRE (3) The pitting corrosion rates of welded AL-6X with estimated PREs of 30.7 and 32.2 can be calculated as A/cm 2 (939.9C/cm 2 ) and A/cm 2 (806.4C/cm 2 ) respectively. The average value is A/cm 2 (873.2C/cm 2 ). Then, the pitting corrosion rate of welded AL-6X tubing can be considered

6 74 Young Sik Kim et al. as A/cm 2. This calculated pitting corrosion rate of welded AL-6X tubing may be correlated with the lifespan of the field data in the Bridgeport Harbor nuclear power plant. Table 4 shows the pitting corrosion rate and the lifespan of the AL-6X used at Bridgeport Harbor. Assuming that the relationship between tube pitting corrosion /re-tubing time and the pitting corrosion rate is inversely proportional, the following equation can be established (it should be noted that this assumption is simply to setup the relationship between them, it has no theoretical basis). Tube pitting corrosion or re-tubing time = a/[pitting corrosion rate] + b (4) where b is the incubation time for tube pitting corrosion, which can be neglected for short periods relative to the whole operating life time of a condenser. Therefore, it was assumed that b is 0. Tube pitting corrosion or re-tubing time = a/[pitting corrosion rate] (Assume b = 0) (5) Thus, the following relations can be derived based on Table 4 and Eqs. (4) and (5). In the case of tube pitting corrosion, 10 years (tube pitting corrosion time) = a/ (pitting corrosion rate), from which can be calculated as a is equal to Therefore, 3.3. Lifespan estimation of seal welded SR-50A tube/tube sheet Figure 7(a) shows the weight loss after repeated pitting tests for SR-50A tube/tube sheet seal-welded mock-up specimens in 6% FeCl 3 at 55 C (dotted line is criterion of pitting generation by ASTM G48 (> 0.1 mg/cm 2 )). From the second test to the fourth test, the weight loss is less than the criterion (0.1 mg/cm 2 ) of ASTM G48 but the weight loss of the first test was 24.3 mg/cm 2. According to only this data, there is a possibility of pitting corrosion in the welded mock-up speci- Tube pitting corrosion time = 4.851/[pitting corrosion rate] (6) In the case of re-tubing, 13 years (re-tubing time) = a/ (pitting corrosion rate), from which is calculated as a is equal to Therefore, Re-tubing time = 6.306/[pitting corrosion rate] (7) From Table 4, Figure 6 and Eq. (3), the following relations can be derived. If we know the PRE or CPT of any stainless steel, the tube pitting corrosion time and re-tubing time can be estimated. However, these empirical equations should be further revised based on application data from many nuclear power plants. They may be applied only for seal-welded super austenitic stainless steel tube/tube sheet for condensers using natural sea water as a cooling water. Tube pitting corrosion time (year) = 0.067e 0.16 PREN (8) Re-tubing time (year) = 0.136e 0.15 PREN (9) Tube pitting corrosion time (year) = 3.689e CPT (10) Re-tubing time (year) = 5.508e CPT (11) Fig. 7. (a) Weight loss by repeated pitting tests for SR-50A tube/tube sheet seal-welded mock-up specimens in 6% FeCl 3 at 55 C (dotted line is pitting sign by ASTM G48 (> 0.1 mg/cm 2 )), (b) Maximum pit depth and average pit depth for the front surface of SR-50A tube/tube sheet seal-welded mock-up specimen in 6% FeCl 3 at 55 C (Pitting criterion by ASTM G48; maximum pit depth > 25 µm). Table 4. Pitting corrosion rate and the lifespan of AL-6X used in Bridgeport Harbor NPP, USA [15] Pitting corrosion rate at +700 mv (SCE) of Field data of AL-6X in Bridgeport Harbor NPP, USA AL-6X as welded tube, A/cm 2 Tube pitting corrosion time, years Re-tubing (25% pitting corroded tubes), years

7 Lifespan Estimation of Seal Welded Super Stainless Steels for Water Condenser of Nuclear Power Plants 75 men. However, as shown in Figure 1, the mock-up specimen for the condenser was made in several steps.; The tube and tube sheet of the front side to be evaluated was autogenously welded by non-filler GTAW method after tube expansion, in which a carbon steel tube sheet was clad in PRE 50 grade SR-50A (CPT of non-welded SR-50A is 85 C, and as discussed the others, high corrosion resistance of super stainless steels is due to the high PREN and the synergistic effect of Mo and N in the alloy; When Mo and N coexist in stainless steel, corrosion resistance in chloride solutions is drastically increased. This synergistic effect of N and Mo is mainly due to the formation of nitro-oxyanions and molybdate ion in the passive film [31-33]. After welding, only the seal-welded PRE 50 grade SR-50A section was cut, after which its rear side was manually welded. The weight loss by pitting corrosion on the back side of specimen could not be used as an indication of the pitting corrosion on the seal weld in the field case. Figure 7(b) shows the maximum pit depth and average pit depth for the front surface of the SR-50A tube/tube sheet seal-welded mock-up specimen in 6% FeCl 3 at 55 C. The pitting criterion by ASTM G48 is a maximum pit depth of > 25 µm. As shown in this figure, the pit depths after 4 repeated tests were less than 25 µm and thus it can be judged that no-pitting corrosion occurred. Figure 8(a) shows the weight loss after repeated pitting tests for SR-50A tube/tube sheet seal-welded mockup specimens in 6% FeCl 3 at 60 (The dotted line is the criterion of pitting generation by ASTM G48 (>0.1 mg/cm 2 )). The weight loss after 4 repeated tests exceeded the criterion of ASTM G 48. Therefore, the pitting presence should be examined by the pit depth criterion. Figure 8(b) shows the maximum pit depth and average pit depth for the front surface of SR- 50A tube/tube sheet seal-welded mock-up specimen in 6% FeCl 3 at 60 C. The pit depths after 4 repeated tests exceeded the criterion of ASTM G48. It can be defined that 60 C is the CPT of SR-50A tube/tube sheet seal-welded mock-up specimen. Finally, using this CPT, and Eqs. (10) and (11), the tube pitting corrosion time and re-tubing time of seal-welded SR- 50A tube/tube sheet were calculated as 69.8 years and 82.0 years, respectively. 4. CONCLUSIONS This work analyzed the materials selection guidelines for sea water facilities and the field application data of stainless steels. Empirical equations for lifespan estimation were derived from the pit initiation time and re-tubing time of stainless steel tubing in sea water condenser of nuclear power plants. The lifespan of seal-welded super austenitic stainless steel tube/tube sheet was calculated from these equations. The pit initiation time and re-tubing time in a condenser using sea water as the cooling medium were estimated as 69.8 years and 82.0 years respectively. REFERENCES Fig. 8. (a) Weight loss by repeated pitting tests for SR-50A tube/tube sheet seal-welded mock-up specimens in 6% FeCl 3 at 60 C (dotted line is pitting sign by ASTM G48 (> 0.1 mg/cm 2 )), (b) Maximum pit depth and average pit depth for the front surface of SR-50A tube/tube sheet seal-welded mock-up specimen in 6% FeCl 3 at 60 C (Pitting criterion by ASTM G48; maximum pit depth > 25 µm). 1. J. G. Kim, A Study of the Safety Evaluation Techniques on the Secondary-Side Components at Nuclear Power Plants, pp , KINS/AR-045/90, Korea Institute of Nuclear Safety (1991). 2. I. S. Hwang, Industry and Energy, Report on the Corrosion Failure of Water Condenser of Uljin, pp.15-28, NPP #5, Ministry of Commerce, Korea (2006). 3. British Stainless Steel Association, Stainless Steel Advisory Service, Sheet No (2001). 4. M. B. Rockel, W. Herda, and U. Brill, Stainless Steels 91,

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