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Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop
Engineering Failure Analysis 99 (2019) 248–262
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop
T
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A. Ravi Shankara, Ravikumar Solea, K. Thyagarajana, R.P. Georgea, , U. Kamachi Mudalib a b
Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Materials Chemistry & Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure analysis Heat-exchanger failures Intergranular corrosion Galvanic corrosion Hydrogen-assisted cracking
A 400 l capacity nitric acid loop (NAL) test facility simulating inactive plant operating conditions was designed, constructed and evaluation of candidate materials was carried out for nuclear spent fuel reprocessing applications. Austenitic stainless steel coupons were exposed to flowing nitric acid medium at various temperatures in NAL. Assessment of critical components of NAL after 10,000 h operation revealed failure of titanium heater tubes and stainless steel heat exchanger tube sheet welds. Failure analysis and detailed characterisation of corrosion products on the failed tubes were carried out using SEM, EDX and XRD techniques. The brown colour corrosion product on titanium heater tubes comprised iron and small amounts of chromium indicating that failure of titanium tubes was due to deposition of corrosion products of stainless steel, leading to galvanic corrosion. Cracks observed on titanium heater tubes were attributed to hydrogen-induced cracking, resulting in severe degradation. Surface morphology and EDX analysis of the corrosion product on stainless steel heat exchanger tubes revealed dislodged grains from 304LSS and XRD analysis confirmed the deposit as stainless steel. Grain dropping occurred due to intergranular corrosion of 304L SS in nitric acid and deposition of the dislodged grains at the seal weld joints resulted in localised corrosion and failure. Based on the studies carried out, surface heaters instead of immersion heaters and periodic inspection and frequent removal of corrosion products from the nitric acid loop were suggested to mitigate such failures. Based on the results of the investigation, repair and refurbishment of the nitric acid loop was undertaken.
1. Introduction AISI type 304L SS is used as a structural material for various components in the spent nuclear fuel reprocessing plants where nitric acid of different concentrations are employed as the process medium at temperatures ranging from ambient to boiling conditions [1]. Reliability and integrity of structural materials is important for the uninterrupted operation of reprocessing plants. The reprocessing of radioactive spent fuel coupled with corrosive nitric acid environment demands high performance structural materials. To understand the long term corrosion behaviour of conventional structural materials and corrosion assessment of advanced alloys for practical applications, it is essential to investigate their performance in dynamic simulated plant operating conditions. A large scale thermosiphon evaporator mock-up test apparatus made of 304ULC stainless steel was tested by Ueno et al. [2] in boiling nitric acid solution for 36,414 h for life prediction due to corrosion. In our laboratory, a dynamic nitric acid loop (NAL) of 400 l capacity (Fig. 1)
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Corresponding author. E-mail address: [email protected] (R.P. George).
https://doi.org/10.1016/j.engfailanal.2019.02.016 Received 27 March 2017; Received in revised form 26 January 2018; Accepted 14 February 2019 Available online 15 February 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Nitric acid loop of 400 l capacity.
had been designed, fabricated, commissioned and operated for 10,000 h, to evaluate the long term corrosion behaviour of 304L stainless steel (SS), under inactive simulated conditions of reprocessing plants [1,3]. For extending the nitric acid loop operation beyond 10,000 h, assessment of critical components of NAL was carried out. Failure of titanium heater tubes and stainless steel heat exchanger tubes was observed. In the nitric acid loop, nitric acid of 6 M concentration enters at the bottom of the heater through inlet nozzle and exits through the outlet nozzle at 380 K. It is reported that at high concentrations of acid and temperatures, materials such as Cp–Ti [4], Ti–5%Ta [4], Ti–5%Ta–1.8%Nb [1,5,6] and Zircaloy [4,7] exhibit better corrosion resistance. Hence, heater tubes made of Cp-Ti was used in NAL for nitric acid exposure at high temperatures. Titanium is widely used in chemical processing industries for nitric acid applications because of its good corrosion resistance in nitric acid medium [1]. However, some of the titanium heater tubes inside the heater component of the NAL were found to fail with severe degradation after 10,000 h operation. Detailed characterisation of the failed tubes and corrosion products was carried out to understand the mechanism of failure. Stainless steel plate type heat exchanger, shell and tube type heat exchanger, and condenser assembly of NAL were also analysed. Interpretation of the results of this study facilitated to provide recommendations for minimizing corrosion attack in future and refurbishment of NAL. 2. Experimental details 2.1. Nitric acid loop operating conditions The nitric acid loop of 400 l capacity shown in Fig. 1 was used for long term corrosion behaviour of AISI 304L SS used for equipment and piping in Demonstration Fuel Reprocessing Plant (DFRP), Kalpakkam and indigenous nitric acid grade (NAG) 304L SS in different metallurgical conditions. The dynamic NAL was operated at flow velocities up to 1.55 m/s and at temperatures of 313, 333, 353 K, boiling and vapour phase conditions [3]. The individual components of the nitric acid loop, such as heaters, condenser and heat exchangers were dismantled after 10,000 h of operation, for analysis and assessment of degradation. Failure analysis on dismantled titanium heater tubes and stainless steel heat exchangers were carried out. Initially detailed explanation on experiments carried out on failed titanium heater tubes will be discussed in Section 2.2 and for stainless steel heat exchangers detailed explanation is provided in Section 2.3. 2.2. Titanium heater tubes The schematic of the heater with titanium heater tubes is shown in Fig. 2. The failed titanium tubes along with the corrosion 249
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Fig. 2. Schematic of heater with titanium heater tubes.
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Fig. 3. Failed titanium heater tube showing regions of corrosion products, crack and pit.
Fig. 4. SEM micrograph and corresponding EDX spectra of white coloured corrosion product.
products shown in Fig. 3, were collected for failure analysis. Extensive characterisation of the corrosion product and failed tube samples was carried out, to investigate the cause of corrosion. The white coloured corrosion product (shown in Fig. 3a) was also present on the flange portion of the heater. This white coloured product from the flange was scrapped and collected with a spatula for 251
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Table 1 Chemical composition (in wt%) in the regions shown in Fig. 4a. Position
Ti
O
Mg
Region 1 Region 2 Region 3
40.20 35.40 44.19
59.80 63.95 53.92
– 0.66 1.89
Fig. 5. XRD pattern of white coloured corrosion product.
investigation. The brown coloured corrosion product present on the titanium tube (shown in Fig. 3c) was also collected for investigation. During sample collection, every effort was made to minimize contamination. The white coloured corrosion product, brown coloured corrosion product, and titanium tube with cracks (Fig. 3b) and pits (Fig. 3d) were examined by scanning electron microscope (SEM). The unrusted and rusted portions of the titanium tube shown in Fig. 3c were also examined by SEM. Surface morphology and compositional analysis of the corrosion products and titanium tube as discussed above were carried out using SEM (SNE -3000 M model, SEC, Korea) attached with EDX (Bruker). X-ray diffraction (XRD) analysis of the white and brown coloured corrosion products was performed for phase identification using Philips X'pert MPD diffractometer. A 2θ step size of 0.1° was used to determine the peak positions of the phases in the range 10° < 2θ < 80°. XRD patterns obtained were compared with standards in Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction Data (ICDD). Powder Diffraction File2 (PDF-2) was used for the phase analysis and PDF-2 codes of the compounds identified were provided in square brackets in the manuscript.
2.3. Stainless steel heat exchangers The stainless steel condenser assembly, plate type heat exchanger and the shell & tube type heat exchanger in NAL were studied. The stainless steel condenser assembly condenses the nitric acid vapour using demineralised (DM) water. It consisted of a shell and 65 tubes seal welded to tube sheets at both ends. The shell and tube type heat exchanger consisting of a shell and 332 tubes, seal welded to tube sheets at both ends was designed to cool the boiling nitric acid from 380 to 313 K. In the condenser and shell and tube type heat exchanger, the acid was on the tube side and DM water was on the shell side with counter current flow pattern. The plate type heat exchanger comprised of a cascaded assembly of 48 corrugated sheets and was designed to cool the DM water (from the shell and tube type heat exchanger) from 318 to 308 K, using process water as the cooling medium. Non destructive tests (NDT) were carried out on the stainless steel condenser assembly, plate type heat exchanger and shell & tube type heat exchanger. Helium leak test (HLT) was conducted on the condenser assembly and shell & tube heat exchanger assembly. Soap bubble test was carried out to locate the leak points in the welded regions of shell and tube type heat exchanger. Each of the corrugated sheets of plate type heat exchanger were checked for pinhole leaks by carrying out, liquid penetrant test (LPT). The corrosion products found on the plate type heat exchanger and the shell & tube type heat exchanger were collected. Surface morphology and compositional analysis of the corrosion products from plate type heat exchanger (pale grey in colour) and from shell and tube type heat exchanger (black in colour) were carried out using SEM. The make and model of SEM used is same as that used for titanium heater tube characterisation as mentioned in Section 2.2. Similarly the parameters employed for obtaining XRD patterns for the corrosion products from plate type heat exchanger (pale grey in colour) and from shell and tube type heat exchanger (black in colour) are same as that used for titanium 252
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Fig. 6. SEM micrograph and corresponding EDX spectra of brown coloured corrosion product. Table 2 Chemical composition (in wt%) in the regions shown in Fig. 6a. Position
Fe
Cr
Ti
O
Region Region Region Region
89.58 79.20 51.03 27.81
10.42 7.79 – –
– – 48.97 72.19
– 13.01 – –
1 2 3 4
heater tube characterisation mentioned in Section 2.2. 3. Results and discussion The study initially focuses on failed titanium heater tubes which is discussed in detail in Section 3.1 as follows and thereafter on stainless steel heat exchangers discussed in Section 3.2. The recommendations to prevent reoccurrence of failure and conclusions were presented in Sections 4 and 5 respectively. 3.1. Titanium heater tubes The titanium heater tube had undergone severe localised corrosion and cracking as revealed in Fig. 3. The white coloured corrosion product collected was analysed and the SEM micrograph of white coloured corrosion product is presented in Fig. 4. The EDX spectra recorded in the regions 1, 2 & 3 of Fig. 4a are presented in Fig. 4b–d respectively. The EDX spectra showed the presence of titanium and oxygen. The chemical compositions from different regions of Fig. 4a are tabulated in Table 1. The results indicated that the white coloured corrosion product predominantly consisted of titanium and oxygen. Some of the regions had trace amounts of magnesium. The hollow heater tube was packed with magnesia and failure of the titanium tube resulted in the dissolution of magnesia in nitric acid. Therefore, it is obvious that the source of magnesium was from the failed tube. The XRD pattern of the white 253
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Fig. 7. XRD pattern of brown coloured corrosion product.
Fig. 8. SEM micrograph and corresponding EDX spectra of unrusted piece.
coloured corrosion product is shown in Fig. 5. The XRD analysis revealed the white coloured corrosion product to be the wellcrystallized phase of rutile TiO2 [21-1276] which is in good agreement with EDX results. Schultz and Covington [8] observed that the anatase TiO2 phase formed during anodizing was readily attacked and dissolved in hot reducing acids while rutile TiO2 formed during thermal oxidation was essentially inert and resistant to corrosion. In the present study, the white corrosion product was identified to be rutile TiO2 and this could be explained as follows. Titanium exhibits high corrosion rate in the vapour and condensate phases of nitric acid, due to the formation of less protective oxide film [4]. Titanium and its alloys corrode significantly in vapour and 254
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Fig. 9. SEM micrograph and corresponding EDX spectra of rusted piece.
Fig. 10. High resolution SEM micrograph of rusted piece of Ti heater tube. Table 3 Chemical composition (in wt%) in the regions shown in Fig. 10a and b. Position
Ti
O
Fe
Cr
Si
Mg
Fig. Fig. Fig. Fig. Fig.
52.93 17.75 29.54 38.54 29.27
41.57 57.87 46.10 38.87 49.61
5.50 10.43 9.54 12.60 10.80
– 1.40 – – –
– 11.15 14.82 8.72 8.11
– 1.40 – 1.27 2.21
10(a) Region 1 10(a) Region 2 10(a) Region 3 10(b) Region 1 10(b) Region 2
condensate phases of nitric acid, particularly in 20–70 wt% concentration range [9]. It is reported that titanium exhibits excellent corrosion resistance in oxidizing environments [10] and in high oxidizing conditions protective TiO2 passive film is formed, while in less oxidizing conditions, a mixture of Ti2O3 and TiO2 films are formed [11]. Also in recirculating nitric acid process streams, titanium exhibits excellent corrosion resistance while in vapour condensates of nitric acid, less protective titanium oxide films form over titanium where significant corrosion occurs. The severe corrosion of titanium tube in nitric acid in the present study could also be attributed to the nitric acid boiling vapour condensate conditions where non protective titanium oxide films form on the surface. Thus the unprotective and loosely adherent oxide which could form under the prevailing corrosive conditions on the titanium tube could detach and deposit on the flange which might got subsequently oxidised to rutile TiO2. The SEM micrograph and the corresponding EDX patterns of the brown coloured corrosion product collected from the failed titanium tube are shown in Fig. 6a. It is evident from the EDX spectra that the corrosion product consisted of Fe, Cr and O. The chemical compositions obtained from EDX analysis in wt% from different regions of Fig. 6a are tabulated in Table 2. Predominant 255
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Fig. 11. SEM micrograph of cracked titanium heater tube. Table 4 Chemical composition (in wt%) in the regions shown in Fig. 11a. Position
Ti
O
Fe
Cr
Si
Region 1 Region 2 Region 3
30.73 28.45 57.40
48.61 54.45 37.81
11.19 6.25 3.09
4.97 2.83 –
4.50 8.02 1.70
quantity of Fe and small amount of Cr found in the EDX spectra implies that the brown colour corrosion product could be formed due to corrosion of 304L stainless steel samples as well as stainless steel structure of nitric acid loop. This corrosion product eventually deposited on the titanium heater tube. As listed in Table 2, regions 3 and 4 in Fig. 6a exhibited significant amount of both titanium and iron. This indicates that the corrosion product deposited on the titanium tube reacted with titanium and adhered to the surface. The XRD pattern of the brown coloured corrosion product collected from titanium tube is presented in Fig. 7. The XRD peaks matched with hematite α–Fe2O3 [33–0664]. The SEM micrograph of the unrusted piece (shown in Fig. 3c) is presented in Fig. 8a and the corresponding EDX spectra from different regions are presented in Fig. 8b and c. The surface morphology of the unrusted piece indicated insignificant attack and the EDX spectrum of region (1) revealed only titanium (Fig. 8c). However, the EDX spectra of minor particles (in region 2 of Fig. 8a) showed the presence of minor Fe, Si, Mg apart from titanium and oxygen. The chemical composition in wt% obtained from the EDX analysis from region (2) (Fig. 8b) contained Ti – 36.56, O – 53.77, Fe – 2.21, Si – 1.97, Mg – 5.49. The presence of significant amount of oxygen along with titanium and small amount of Fe, Si and Mg indicates that the particles adhering to the surface were essentially oxides. The SEM micrograph of rusted piece presented in Fig. 3c is shown in Fig. 9a and the corresponding EDX spectra recorded for region (1) is presented in Fig. 9b. The surface morphology of the rusted piece revealed isolated regions of rust adhering to the surface of the titanium tube. The rusted region (1) revealed significant amount of oxygen along with titanium and small amount of Fe, Cr, Si and Mg (Ti – 34.82, O – 48.64, Fe – 8.79, Si – 3.67, Mg – 2.66, Cr – 1.42). Detailed investigation of the region (2) adjacent to the rust was also carried out. EDX analysis from this region revealed Ti – 81.8, Fe – 9.27, Cr – 1.97, Mg – 6.97 in wt%. Fig. 10 shows the high 256
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Fig. 12. A failed titanium heater tube showing regions of corrosion products, crack and pit.
Fig. 13. Schematic of shell and tube type heat exchanger used in NAL.
resolution SEM micrograph shown in region 2 of Fig. 9a. The surface morphology revealed isolated particles on the surface (Fig. 10a) and thick deposits as layers and cracks on the rusted surface (Fig. 10b). Chemical composition determined by EDX analysis on the regions shown in Fig. 10a and b are given in Table 3. It is evident that the surface consisted of particles and layers of oxides of titanium and iron. The surface morphology and EDX analysis of the unrusted and rusted surfaces thus confirmed that the corrosion product of stainless steel was depositing on the surface of the titanium heater tube. Characterisation of brown coloured corrosion product on these tubes was identified as α–Fe2O3 (Fig. 7). This shows that brown coloured corrosion products adhering on the titanium tube resulted in galvanic cell formation and led to localised attack. Fig. 11 shows the SEM micrograph of cracked titanium heater tube. The EDX analysis carried out on the regions marked in Fig. 11a yielded the chemical composition in wt% which is tabulated in Table 4. The regions 1, 2 and 3 of Fig. 11a consisted predominantly of oxygen along with titanium and small amount of Fe, Cr and Si. This indicates that the cracked region consisted of significant amount of oxides and corrosion products of SS. Many of the failures of titanium were reported to be due to embedded corroding iron particles present on titanium which acts as catalytic sites for hydrogen ion reduction, resulting in localised corrosion. Galvanic couple formed between Ti and 304L deposits (since iron, chromium are active compared to titanium) also develops sufficient potential for hydrogen formation and adsorption. Hydrogen thus produced on titanium at cathodic sites due to galvanic coupling enters in to titanium through easy diffusion paths where corrosion product deposits are present. Hydriding and embrittlement of titanium tubes occur when the solubility limit of hydrogen exceeds in titanium and the titanium hydrides which 257
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Fig. 14. Schematic of tube sheet of shell and tube type heat exchanger depicting the location of failed weld joints.
subsequently precipitate are brittle and cracks will form resulting in tube rupture. The oriented cracks seen in Fig. 11b and c might be formed due to hydrogen uptake from corrosion reaction at these deposits and subsequently leading to cracking. Covington and Schultz [12] reported that solid solution of iron in titanium has minor effect on pitting corrosion or hydrogen embrittlement, but embedded iron particles on the surface of titanium are very deleterious and serve as sites for pit initiation and hydrogen diffusion. They also predicted that galvanic coupling of titanium and iron leads to hydrogen embrittlement of titanium. Thus, corrosion products from the SS samples and structural material of nitric acid loop deposited on titanium heater tubes is responsible for galvanic couple formation and hydrogen pickup. Owing to the catalytic effect of ferrous ions (producing more hydrogen), the threshold temperature for hydrogen absorption into titanium is reported to decrease from 350 K to 308–313 K [13]. Magnesium present on the surface creates a more negative potential which could also aid in hydrogen pick up. Thus the presence of Mg further accelerates hydrogen pickup and cracking. The crack present at the tube bend (Fig. 12) suggests that the stresses developed at the bends could have contributed to corrosion attack and cracking. Cracking of titanium tubes due to sustained load and cyclic loads was also reported. The present investigation suggests that the corrosion products dissolved in nitric acid streams had deposited and settled on the other components of nitric acid loop, resulting in the formation of crevices and galvanic cells; thus, accelerating the corrosion. Covington [14] reported that immersion of titanium in 35% nitric acid and 5% hydrofluoric acid solution for 2 to 5 min followed by water rinse is an effective method to remove iron contamination on titanium. Apart from this, Schultz and Covington [8] reported that thermal oxidation of titanium offers better resistance towards hydrogen pickup than anodised titanium for heat exchanger tube applications. Thermal oxidation of titanium carried out earlier showed remarkable improvement in corrosion resistance (~80 times) in boiling nitric acid compared to untreated titanium as per ASTM A262 practice-C test [15]. 3.2. Stainless steel heat exchangers Helium leak test (HLT) carried out on the condenser assembly did not show any leaks. Therefore, condenser assembly was recommended for reuse in the NAL, without any modifications. Helium leak test revealed a gross leak in the whole assembly of shell and tube type heat exchanger. Schematic of shell & tube type heat exchanger used in NAL is shown in Fig. 13. The soap bubble test revealed leaks at 16 seal weld joints, out of a total of 664 seal weld joints. This location pertained to the acid outlet side of the heat exchanger. The cross sectional view of the heat exchanger with filled circles indicating the failed weld joints, is shown schematically in Fig. 14. The individual tubes of the shell & tube type heat exchanger did not show any leaks. The corrosion product collected from the shell and tube type heat exchanger was black in colour. The surface morphology of black coloured corrosion product observed in 258
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Fig. 15. High resolution SEM micrograph and corresponding EDX spectra of black coloured corrosion product from shell and tube type heat exchanger.
the SEM micrograph (Fig. 15a) revealed more faceted grains, indicating that the particles were intergranular grains dislodged from 304L SS. The selectively dissolved regions of inclusions/carbide precipitates within the grains were also observed as shown in Fig. 15a. The EDX spectra for the regions 1 and 2 are shown in Fig. 15b and c respectively. The chemical compositions (in wt%) from region 1 is Fe – 67.26, Cr – 24.50, Ni – 6.77, Al – 0.97, Si – 0.50 while from region 2 is Fe – 67.86, Cr – 22.02, Ni – 8.89, Al – 0.73, Si – 0.50. These results clearly indicated that the corrosion product was predominantly 304L SS grains dislodged from 304L SS due to severe intergranular corrosion. For the plate type heat exchanger liquid penetrant test revealed that none of the corrugated sheets had any pin holes or leaks. The corrosion product collected from plate type heat exchanger was pale grey in colour. The SEM micrograph of this corrosion product is shown in Fig. 16a. The corrosion product observed shows some of the particles exhibiting faceted morphology. The EDX spectra of the regions 1 and 2 are shown in Fig. 16b and c respectively. The chemical composition (in wt%) in the regions 1 and 2 were Fe – 67.61, Cr – 28.11, Al – 2.60, Si – 1.69 and Fe – 59.67, Cr – 35.10, Al – 1.84, Si – 3.39 respectively. This indicates that the corrosion product was predominantly stainless steel particles. The high resolution SEM micrograph of the corrosion product collected from the plate type heat exchanger is shown in Fig. 17. The EDX spectra for the regions 1 and 2 (Fig. 17b and c respectively) showed the chemical composition in wt% for the regions 1 and 2 to be Fe – 73.45, Cr – 21.81, Si – 3.17, Al – 1.57 and Fe – 69.79, Cr – 20.84, Ni – 9.37 respectively. The SEM micrograph revealed that the particles were intergranular grains dislodged from 304L SS. Some of the inclusions/carbide precipitates present in the grains also have dissolved selectively, as indicated by the arrow shown in Fig. 17a. In order to corroborate that the particles are dislodged intergranular grains of 304L SS, XRD analysis of both pale grey coloured and black coloured corrosion products was carried out. The XRD patterns of the pale grey coloured and black coloured corrosion products shown in Fig. 18a and b respectively revealed that the major reflections corresponded to the austenite phase (Fe0.7Cr0.19Ni0.11) [33–0397]. This confirms that the corrosion product particles were indeed 304L SS. Corrosion of stainless steel in nitric acid is influenced by the autocatalytic reduction of nitric acid and the presence of redox species in the acid [16]. Since the condenser assembly is exposed to acid vapour, accumulation of oxidizing ions due to corrosion would be less in the condenser assembly compared to that in the shell and tube type heat exchanger. The regions where dislodged grains deposited due to intergranular corrosion results in the confinement of nitric acid under the deposits. This causes accumulation of cations of Fe and Cr due to corrosion and further increases their concentration in these confined regions. The reduction products of nitric acid as well as Fe3+ and Cr3+ ions produced during corrosion catalyze the nitric acid reduction reaction [16]. This 259
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Fig. 16. SEM micrograph and corresponding EDX spectra of pale grey coloured corrosion product.
autocatalytic nitric acid reduction mechanism shifts the corrosion potential of stainless steel towards transpassive region leading to accelerated intergranular corrosion [16]. Thus, the nitric acid confined at the dislodged grain deposits of seal joints could have accelerated localised corrosion, resulting in failure. Based on the failure analysis carried out, it is recommended that the plate type heat exchanger may be re-assembled and used in the loop after cleaning the corrosion products from the corrugated plates. The leakage locations at the seal weld joints of the shell and tube type heat exchanger can be repaired and reused in the loop. 3.3. Refurbishment of NAL Shell and tube type heat exchanger was cleaned thoroughly and all the failed seal weld joints were repaired. After weld repair, leak test was conducted to ensure no leak in the heat exchanger. The whole component is then cleaned, pickled, passivated and reinstalled in the loop. Immersion type heater unit was re-engineered to avoid failure of immersion type titanium heater tubes. Surface tape heaters were fixed by wounding around the heater vessel and insulted with mineral wool and placed aluminium cladding over the insulation. As the existing titanium top flange had holes for inserting immersion heaters, the top flange was suitably modified to close the vessel top. New pumps were installed in the loop to allow uninterrupted operation of the loop. The piping spool and supports were suitably modified to fit with inlet and outlet ports of the pumps. Replacement of dump valve, repair and reassembling of chiller unit allowed operation of the loop to its earlier state. Successful refurbishment reduced the investment cost and extended the operational life of nitric acid loop and also availability of the loop for carrying out R&D activities. 4. Recommendations to prevent reoccurrence of failure Based on results of this study, the following recommendations have been made: I. Frequent removal of corrosion products from nitric acid process streams is essential to avoid formation of galvanic cells and crevices. If feasible, frequent removal of corrosion products from the system using filters, scrubbers, etc. is suggested. II. Surface modification of titanium tubes by pickling, passivation and thermal oxidation can be considered to minimize corrosion attack and hydrogen induced cracking. III. Use of corrosion resistant materials such as Zircaloy and Ti–Ta–Nb alloy, as Cp–Ti undergoes accelerated corrosion in the vapour and condensate phases of nitric acid. 260
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Fig. 17. High resolution SEM micrograph and corresponding EDX spectra of corrosion product collected from plate type heat exchanger.
IV. Instead of immersion heater, surface heater can be used so that heater is avoided from acid contact. V. Periodic inspection of the components and contamination of nitric acid is needed to avoid failure. VI. Checking of the acidity level of DM water, after every campaign, and if required dismantling of the heat exchanger assembly are recommended.
5. Conclusions The failed Ti heater tubes and corrosion products collected from the tube were carefully examined to understand the corrosion mechanisms that had led to the failure of the component. The composition, morphology and phase analysis of the corrosion products and failed tube suggested that the failure was due to the deposition and adherence of iron over titanium, leading to galvanic corrosion. This accelerated corrosion could have resulted in higher hydrogen uptake causing the tubes to crack. The leaks in the shell and tube type heat exchanger, mainly in the seal weld regions of the nitric acid outlet, confirmed that the failure occurred due to either prolonged exposure of boiling nitric acid under deposits or poor welding or poor weld joint geometry at these regions. Based on the results of the investigation, mechanism of corrosion and causes for the failure of components were proposed. Based on the failure analysis and non-destructive testing carried out, refurbishment of the loop was carried out and recommendations to prevent reoccurrence of such failures were made.
Acknowledgements The authors are grateful to Dr.G. Panneerselvam of Chemistry Group for recording XRD patterns.
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Fig. 18. XRD patterns of (a) pale grey colour and (b) black colour corrosion products.
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Failure analysis of titanium heater tubes and stainless steel heat exchanger weld joints in nitric acid loop
T
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A. Ravi Shankara, Ravikumar Solea, K. Thyagarajana, R.P. Georgea, , U. Kamachi Mudalib a b
Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India Materials Chemistry & Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Failure analysis Heat-exchanger failures Intergranular corrosion Galvanic corrosion Hydrogen-assisted cracking
A 400 l capacity nitric acid loop (NAL) test facility simulating inactive plant operating conditions was designed, constructed and evaluation of candidate materials was carried out for nuclear spent fuel reprocessing applications. Austenitic stainless steel coupons were exposed to flowing nitric acid medium at various temperatures in NAL. Assessment of critical components of NAL after 10,000 h operation revealed failure of titanium heater tubes and stainless steel heat exchanger tube sheet welds. Failure analysis and detailed characterisation of corrosion products on the failed tubes were carried out using SEM, EDX and XRD techniques. The brown colour corrosion product on titanium heater tubes comprised iron and small amounts of chromium indicating that failure of titanium tubes was due to deposition of corrosion products of stainless steel, leading to galvanic corrosion. Cracks observed on titanium heater tubes were attributed to hydrogen-induced cracking, resulting in severe degradation. Surface morphology and EDX analysis of the corrosion product on stainless steel heat exchanger tubes revealed dislodged grains from 304LSS and XRD analysis confirmed the deposit as stainless steel. Grain dropping occurred due to intergranular corrosion of 304L SS in nitric acid and deposition of the dislodged grains at the seal weld joints resulted in localised corrosion and failure. Based on the studies carried out, surface heaters instead of immersion heaters and periodic inspection and frequent removal of corrosion products from the nitric acid loop were suggested to mitigate such failures. Based on the results of the investigation, repair and refurbishment of the nitric acid loop was undertaken.
1. Introduction AISI type 304L SS is used as a structural material for various components in the spent nuclear fuel reprocessing plants where nitric acid of different concentrations are employed as the process medium at temperatures ranging from ambient to boiling conditions [1]. Reliability and integrity of structural materials is important for the uninterrupted operation of reprocessing plants. The reprocessing of radioactive spent fuel coupled with corrosive nitric acid environment demands high performance structural materials. To understand the long term corrosion behaviour of conventional structural materials and corrosion assessment of advanced alloys for practical applications, it is essential to investigate their performance in dynamic simulated plant operating conditions. A large scale thermosiphon evaporator mock-up test apparatus made of 304ULC stainless steel was tested by Ueno et al. [2] in boiling nitric acid solution for 36,414 h for life prediction due to corrosion. In our laboratory, a dynamic nitric acid loop (NAL) of 400 l capacity (Fig. 1)
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Corresponding author. E-mail address: [email protected] (R.P. George).
https://doi.org/10.1016/j.engfailanal.2019.02.016 Received 27 March 2017; Received in revised form 26 January 2018; Accepted 14 February 2019 Available online 15 February 2019 1350-6307/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Nitric acid loop of 400 l capacity.
had been designed, fabricated, commissioned and operated for 10,000 h, to evaluate the long term corrosion behaviour of 304L stainless steel (SS), under inactive simulated conditions of reprocessing plants [1,3]. For extending the nitric acid loop operation beyond 10,000 h, assessment of critical components of NAL was carried out. Failure of titanium heater tubes and stainless steel heat exchanger tubes was observed. In the nitric acid loop, nitric acid of 6 M concentration enters at the bottom of the heater through inlet nozzle and exits through the outlet nozzle at 380 K. It is reported that at high concentrations of acid and temperatures, materials such as Cp–Ti [4], Ti–5%Ta [4], Ti–5%Ta–1.8%Nb [1,5,6] and Zircaloy [4,7] exhibit better corrosion resistance. Hence, heater tubes made of Cp-Ti was used in NAL for nitric acid exposure at high temperatures. Titanium is widely used in chemical processing industries for nitric acid applications because of its good corrosion resistance in nitric acid medium [1]. However, some of the titanium heater tubes inside the heater component of the NAL were found to fail with severe degradation after 10,000 h operation. Detailed characterisation of the failed tubes and corrosion products was carried out to understand the mechanism of failure. Stainless steel plate type heat exchanger, shell and tube type heat exchanger, and condenser assembly of NAL were also analysed. Interpretation of the results of this study facilitated to provide recommendations for minimizing corrosion attack in future and refurbishment of NAL. 2. Experimental details 2.1. Nitric acid loop operating conditions The nitric acid loop of 400 l capacity shown in Fig. 1 was used for long term corrosion behaviour of AISI 304L SS used for equipment and piping in Demonstration Fuel Reprocessing Plant (DFRP), Kalpakkam and indigenous nitric acid grade (NAG) 304L SS in different metallurgical conditions. The dynamic NAL was operated at flow velocities up to 1.55 m/s and at temperatures of 313, 333, 353 K, boiling and vapour phase conditions [3]. The individual components of the nitric acid loop, such as heaters, condenser and heat exchangers were dismantled after 10,000 h of operation, for analysis and assessment of degradation. Failure analysis on dismantled titanium heater tubes and stainless steel heat exchangers were carried out. Initially detailed explanation on experiments carried out on failed titanium heater tubes will be discussed in Section 2.2 and for stainless steel heat exchangers detailed explanation is provided in Section 2.3. 2.2. Titanium heater tubes The schematic of the heater with titanium heater tubes is shown in Fig. 2. The failed titanium tubes along with the corrosion 249
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Fig. 2. Schematic of heater with titanium heater tubes.
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Fig. 3. Failed titanium heater tube showing regions of corrosion products, crack and pit.
Fig. 4. SEM micrograph and corresponding EDX spectra of white coloured corrosion product.
products shown in Fig. 3, were collected for failure analysis. Extensive characterisation of the corrosion product and failed tube samples was carried out, to investigate the cause of corrosion. The white coloured corrosion product (shown in Fig. 3a) was also present on the flange portion of the heater. This white coloured product from the flange was scrapped and collected with a spatula for 251
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Table 1 Chemical composition (in wt%) in the regions shown in Fig. 4a. Position
Ti
O
Mg
Region 1 Region 2 Region 3
40.20 35.40 44.19
59.80 63.95 53.92
– 0.66 1.89
Fig. 5. XRD pattern of white coloured corrosion product.
investigation. The brown coloured corrosion product present on the titanium tube (shown in Fig. 3c) was also collected for investigation. During sample collection, every effort was made to minimize contamination. The white coloured corrosion product, brown coloured corrosion product, and titanium tube with cracks (Fig. 3b) and pits (Fig. 3d) were examined by scanning electron microscope (SEM). The unrusted and rusted portions of the titanium tube shown in Fig. 3c were also examined by SEM. Surface morphology and compositional analysis of the corrosion products and titanium tube as discussed above were carried out using SEM (SNE -3000 M model, SEC, Korea) attached with EDX (Bruker). X-ray diffraction (XRD) analysis of the white and brown coloured corrosion products was performed for phase identification using Philips X'pert MPD diffractometer. A 2θ step size of 0.1° was used to determine the peak positions of the phases in the range 10° < 2θ < 80°. XRD patterns obtained were compared with standards in Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction Data (ICDD). Powder Diffraction File2 (PDF-2) was used for the phase analysis and PDF-2 codes of the compounds identified were provided in square brackets in the manuscript.
2.3. Stainless steel heat exchangers The stainless steel condenser assembly, plate type heat exchanger and the shell & tube type heat exchanger in NAL were studied. The stainless steel condenser assembly condenses the nitric acid vapour using demineralised (DM) water. It consisted of a shell and 65 tubes seal welded to tube sheets at both ends. The shell and tube type heat exchanger consisting of a shell and 332 tubes, seal welded to tube sheets at both ends was designed to cool the boiling nitric acid from 380 to 313 K. In the condenser and shell and tube type heat exchanger, the acid was on the tube side and DM water was on the shell side with counter current flow pattern. The plate type heat exchanger comprised of a cascaded assembly of 48 corrugated sheets and was designed to cool the DM water (from the shell and tube type heat exchanger) from 318 to 308 K, using process water as the cooling medium. Non destructive tests (NDT) were carried out on the stainless steel condenser assembly, plate type heat exchanger and shell & tube type heat exchanger. Helium leak test (HLT) was conducted on the condenser assembly and shell & tube heat exchanger assembly. Soap bubble test was carried out to locate the leak points in the welded regions of shell and tube type heat exchanger. Each of the corrugated sheets of plate type heat exchanger were checked for pinhole leaks by carrying out, liquid penetrant test (LPT). The corrosion products found on the plate type heat exchanger and the shell & tube type heat exchanger were collected. Surface morphology and compositional analysis of the corrosion products from plate type heat exchanger (pale grey in colour) and from shell and tube type heat exchanger (black in colour) were carried out using SEM. The make and model of SEM used is same as that used for titanium heater tube characterisation as mentioned in Section 2.2. Similarly the parameters employed for obtaining XRD patterns for the corrosion products from plate type heat exchanger (pale grey in colour) and from shell and tube type heat exchanger (black in colour) are same as that used for titanium 252
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Fig. 6. SEM micrograph and corresponding EDX spectra of brown coloured corrosion product. Table 2 Chemical composition (in wt%) in the regions shown in Fig. 6a. Position
Fe
Cr
Ti
O
Region Region Region Region
89.58 79.20 51.03 27.81
10.42 7.79 – –
– – 48.97 72.19
– 13.01 – –
1 2 3 4
heater tube characterisation mentioned in Section 2.2. 3. Results and discussion The study initially focuses on failed titanium heater tubes which is discussed in detail in Section 3.1 as follows and thereafter on stainless steel heat exchangers discussed in Section 3.2. The recommendations to prevent reoccurrence of failure and conclusions were presented in Sections 4 and 5 respectively. 3.1. Titanium heater tubes The titanium heater tube had undergone severe localised corrosion and cracking as revealed in Fig. 3. The white coloured corrosion product collected was analysed and the SEM micrograph of white coloured corrosion product is presented in Fig. 4. The EDX spectra recorded in the regions 1, 2 & 3 of Fig. 4a are presented in Fig. 4b–d respectively. The EDX spectra showed the presence of titanium and oxygen. The chemical compositions from different regions of Fig. 4a are tabulated in Table 1. The results indicated that the white coloured corrosion product predominantly consisted of titanium and oxygen. Some of the regions had trace amounts of magnesium. The hollow heater tube was packed with magnesia and failure of the titanium tube resulted in the dissolution of magnesia in nitric acid. Therefore, it is obvious that the source of magnesium was from the failed tube. The XRD pattern of the white 253
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Fig. 7. XRD pattern of brown coloured corrosion product.
Fig. 8. SEM micrograph and corresponding EDX spectra of unrusted piece.
coloured corrosion product is shown in Fig. 5. The XRD analysis revealed the white coloured corrosion product to be the wellcrystallized phase of rutile TiO2 [21-1276] which is in good agreement with EDX results. Schultz and Covington [8] observed that the anatase TiO2 phase formed during anodizing was readily attacked and dissolved in hot reducing acids while rutile TiO2 formed during thermal oxidation was essentially inert and resistant to corrosion. In the present study, the white corrosion product was identified to be rutile TiO2 and this could be explained as follows. Titanium exhibits high corrosion rate in the vapour and condensate phases of nitric acid, due to the formation of less protective oxide film [4]. Titanium and its alloys corrode significantly in vapour and 254
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Fig. 9. SEM micrograph and corresponding EDX spectra of rusted piece.
Fig. 10. High resolution SEM micrograph of rusted piece of Ti heater tube. Table 3 Chemical composition (in wt%) in the regions shown in Fig. 10a and b. Position
Ti
O
Fe
Cr
Si
Mg
Fig. Fig. Fig. Fig. Fig.
52.93 17.75 29.54 38.54 29.27
41.57 57.87 46.10 38.87 49.61
5.50 10.43 9.54 12.60 10.80
– 1.40 – – –
– 11.15 14.82 8.72 8.11
– 1.40 – 1.27 2.21
10(a) Region 1 10(a) Region 2 10(a) Region 3 10(b) Region 1 10(b) Region 2
condensate phases of nitric acid, particularly in 20–70 wt% concentration range [9]. It is reported that titanium exhibits excellent corrosion resistance in oxidizing environments [10] and in high oxidizing conditions protective TiO2 passive film is formed, while in less oxidizing conditions, a mixture of Ti2O3 and TiO2 films are formed [11]. Also in recirculating nitric acid process streams, titanium exhibits excellent corrosion resistance while in vapour condensates of nitric acid, less protective titanium oxide films form over titanium where significant corrosion occurs. The severe corrosion of titanium tube in nitric acid in the present study could also be attributed to the nitric acid boiling vapour condensate conditions where non protective titanium oxide films form on the surface. Thus the unprotective and loosely adherent oxide which could form under the prevailing corrosive conditions on the titanium tube could detach and deposit on the flange which might got subsequently oxidised to rutile TiO2. The SEM micrograph and the corresponding EDX patterns of the brown coloured corrosion product collected from the failed titanium tube are shown in Fig. 6a. It is evident from the EDX spectra that the corrosion product consisted of Fe, Cr and O. The chemical compositions obtained from EDX analysis in wt% from different regions of Fig. 6a are tabulated in Table 2. Predominant 255
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Fig. 11. SEM micrograph of cracked titanium heater tube. Table 4 Chemical composition (in wt%) in the regions shown in Fig. 11a. Position
Ti
O
Fe
Cr
Si
Region 1 Region 2 Region 3
30.73 28.45 57.40
48.61 54.45 37.81
11.19 6.25 3.09
4.97 2.83 –
4.50 8.02 1.70
quantity of Fe and small amount of Cr found in the EDX spectra implies that the brown colour corrosion product could be formed due to corrosion of 304L stainless steel samples as well as stainless steel structure of nitric acid loop. This corrosion product eventually deposited on the titanium heater tube. As listed in Table 2, regions 3 and 4 in Fig. 6a exhibited significant amount of both titanium and iron. This indicates that the corrosion product deposited on the titanium tube reacted with titanium and adhered to the surface. The XRD pattern of the brown coloured corrosion product collected from titanium tube is presented in Fig. 7. The XRD peaks matched with hematite α–Fe2O3 [33–0664]. The SEM micrograph of the unrusted piece (shown in Fig. 3c) is presented in Fig. 8a and the corresponding EDX spectra from different regions are presented in Fig. 8b and c. The surface morphology of the unrusted piece indicated insignificant attack and the EDX spectrum of region (1) revealed only titanium (Fig. 8c). However, the EDX spectra of minor particles (in region 2 of Fig. 8a) showed the presence of minor Fe, Si, Mg apart from titanium and oxygen. The chemical composition in wt% obtained from the EDX analysis from region (2) (Fig. 8b) contained Ti – 36.56, O – 53.77, Fe – 2.21, Si – 1.97, Mg – 5.49. The presence of significant amount of oxygen along with titanium and small amount of Fe, Si and Mg indicates that the particles adhering to the surface were essentially oxides. The SEM micrograph of rusted piece presented in Fig. 3c is shown in Fig. 9a and the corresponding EDX spectra recorded for region (1) is presented in Fig. 9b. The surface morphology of the rusted piece revealed isolated regions of rust adhering to the surface of the titanium tube. The rusted region (1) revealed significant amount of oxygen along with titanium and small amount of Fe, Cr, Si and Mg (Ti – 34.82, O – 48.64, Fe – 8.79, Si – 3.67, Mg – 2.66, Cr – 1.42). Detailed investigation of the region (2) adjacent to the rust was also carried out. EDX analysis from this region revealed Ti – 81.8, Fe – 9.27, Cr – 1.97, Mg – 6.97 in wt%. Fig. 10 shows the high 256
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Fig. 12. A failed titanium heater tube showing regions of corrosion products, crack and pit.
Fig. 13. Schematic of shell and tube type heat exchanger used in NAL.
resolution SEM micrograph shown in region 2 of Fig. 9a. The surface morphology revealed isolated particles on the surface (Fig. 10a) and thick deposits as layers and cracks on the rusted surface (Fig. 10b). Chemical composition determined by EDX analysis on the regions shown in Fig. 10a and b are given in Table 3. It is evident that the surface consisted of particles and layers of oxides of titanium and iron. The surface morphology and EDX analysis of the unrusted and rusted surfaces thus confirmed that the corrosion product of stainless steel was depositing on the surface of the titanium heater tube. Characterisation of brown coloured corrosion product on these tubes was identified as α–Fe2O3 (Fig. 7). This shows that brown coloured corrosion products adhering on the titanium tube resulted in galvanic cell formation and led to localised attack. Fig. 11 shows the SEM micrograph of cracked titanium heater tube. The EDX analysis carried out on the regions marked in Fig. 11a yielded the chemical composition in wt% which is tabulated in Table 4. The regions 1, 2 and 3 of Fig. 11a consisted predominantly of oxygen along with titanium and small amount of Fe, Cr and Si. This indicates that the cracked region consisted of significant amount of oxides and corrosion products of SS. Many of the failures of titanium were reported to be due to embedded corroding iron particles present on titanium which acts as catalytic sites for hydrogen ion reduction, resulting in localised corrosion. Galvanic couple formed between Ti and 304L deposits (since iron, chromium are active compared to titanium) also develops sufficient potential for hydrogen formation and adsorption. Hydrogen thus produced on titanium at cathodic sites due to galvanic coupling enters in to titanium through easy diffusion paths where corrosion product deposits are present. Hydriding and embrittlement of titanium tubes occur when the solubility limit of hydrogen exceeds in titanium and the titanium hydrides which 257
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Fig. 14. Schematic of tube sheet of shell and tube type heat exchanger depicting the location of failed weld joints.
subsequently precipitate are brittle and cracks will form resulting in tube rupture. The oriented cracks seen in Fig. 11b and c might be formed due to hydrogen uptake from corrosion reaction at these deposits and subsequently leading to cracking. Covington and Schultz [12] reported that solid solution of iron in titanium has minor effect on pitting corrosion or hydrogen embrittlement, but embedded iron particles on the surface of titanium are very deleterious and serve as sites for pit initiation and hydrogen diffusion. They also predicted that galvanic coupling of titanium and iron leads to hydrogen embrittlement of titanium. Thus, corrosion products from the SS samples and structural material of nitric acid loop deposited on titanium heater tubes is responsible for galvanic couple formation and hydrogen pickup. Owing to the catalytic effect of ferrous ions (producing more hydrogen), the threshold temperature for hydrogen absorption into titanium is reported to decrease from 350 K to 308–313 K [13]. Magnesium present on the surface creates a more negative potential which could also aid in hydrogen pick up. Thus the presence of Mg further accelerates hydrogen pickup and cracking. The crack present at the tube bend (Fig. 12) suggests that the stresses developed at the bends could have contributed to corrosion attack and cracking. Cracking of titanium tubes due to sustained load and cyclic loads was also reported. The present investigation suggests that the corrosion products dissolved in nitric acid streams had deposited and settled on the other components of nitric acid loop, resulting in the formation of crevices and galvanic cells; thus, accelerating the corrosion. Covington [14] reported that immersion of titanium in 35% nitric acid and 5% hydrofluoric acid solution for 2 to 5 min followed by water rinse is an effective method to remove iron contamination on titanium. Apart from this, Schultz and Covington [8] reported that thermal oxidation of titanium offers better resistance towards hydrogen pickup than anodised titanium for heat exchanger tube applications. Thermal oxidation of titanium carried out earlier showed remarkable improvement in corrosion resistance (~80 times) in boiling nitric acid compared to untreated titanium as per ASTM A262 practice-C test [15]. 3.2. Stainless steel heat exchangers Helium leak test (HLT) carried out on the condenser assembly did not show any leaks. Therefore, condenser assembly was recommended for reuse in the NAL, without any modifications. Helium leak test revealed a gross leak in the whole assembly of shell and tube type heat exchanger. Schematic of shell & tube type heat exchanger used in NAL is shown in Fig. 13. The soap bubble test revealed leaks at 16 seal weld joints, out of a total of 664 seal weld joints. This location pertained to the acid outlet side of the heat exchanger. The cross sectional view of the heat exchanger with filled circles indicating the failed weld joints, is shown schematically in Fig. 14. The individual tubes of the shell & tube type heat exchanger did not show any leaks. The corrosion product collected from the shell and tube type heat exchanger was black in colour. The surface morphology of black coloured corrosion product observed in 258
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Fig. 15. High resolution SEM micrograph and corresponding EDX spectra of black coloured corrosion product from shell and tube type heat exchanger.
the SEM micrograph (Fig. 15a) revealed more faceted grains, indicating that the particles were intergranular grains dislodged from 304L SS. The selectively dissolved regions of inclusions/carbide precipitates within the grains were also observed as shown in Fig. 15a. The EDX spectra for the regions 1 and 2 are shown in Fig. 15b and c respectively. The chemical compositions (in wt%) from region 1 is Fe – 67.26, Cr – 24.50, Ni – 6.77, Al – 0.97, Si – 0.50 while from region 2 is Fe – 67.86, Cr – 22.02, Ni – 8.89, Al – 0.73, Si – 0.50. These results clearly indicated that the corrosion product was predominantly 304L SS grains dislodged from 304L SS due to severe intergranular corrosion. For the plate type heat exchanger liquid penetrant test revealed that none of the corrugated sheets had any pin holes or leaks. The corrosion product collected from plate type heat exchanger was pale grey in colour. The SEM micrograph of this corrosion product is shown in Fig. 16a. The corrosion product observed shows some of the particles exhibiting faceted morphology. The EDX spectra of the regions 1 and 2 are shown in Fig. 16b and c respectively. The chemical composition (in wt%) in the regions 1 and 2 were Fe – 67.61, Cr – 28.11, Al – 2.60, Si – 1.69 and Fe – 59.67, Cr – 35.10, Al – 1.84, Si – 3.39 respectively. This indicates that the corrosion product was predominantly stainless steel particles. The high resolution SEM micrograph of the corrosion product collected from the plate type heat exchanger is shown in Fig. 17. The EDX spectra for the regions 1 and 2 (Fig. 17b and c respectively) showed the chemical composition in wt% for the regions 1 and 2 to be Fe – 73.45, Cr – 21.81, Si – 3.17, Al – 1.57 and Fe – 69.79, Cr – 20.84, Ni – 9.37 respectively. The SEM micrograph revealed that the particles were intergranular grains dislodged from 304L SS. Some of the inclusions/carbide precipitates present in the grains also have dissolved selectively, as indicated by the arrow shown in Fig. 17a. In order to corroborate that the particles are dislodged intergranular grains of 304L SS, XRD analysis of both pale grey coloured and black coloured corrosion products was carried out. The XRD patterns of the pale grey coloured and black coloured corrosion products shown in Fig. 18a and b respectively revealed that the major reflections corresponded to the austenite phase (Fe0.7Cr0.19Ni0.11) [33–0397]. This confirms that the corrosion product particles were indeed 304L SS. Corrosion of stainless steel in nitric acid is influenced by the autocatalytic reduction of nitric acid and the presence of redox species in the acid [16]. Since the condenser assembly is exposed to acid vapour, accumulation of oxidizing ions due to corrosion would be less in the condenser assembly compared to that in the shell and tube type heat exchanger. The regions where dislodged grains deposited due to intergranular corrosion results in the confinement of nitric acid under the deposits. This causes accumulation of cations of Fe and Cr due to corrosion and further increases their concentration in these confined regions. The reduction products of nitric acid as well as Fe3+ and Cr3+ ions produced during corrosion catalyze the nitric acid reduction reaction [16]. This 259
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Fig. 16. SEM micrograph and corresponding EDX spectra of pale grey coloured corrosion product.
autocatalytic nitric acid reduction mechanism shifts the corrosion potential of stainless steel towards transpassive region leading to accelerated intergranular corrosion [16]. Thus, the nitric acid confined at the dislodged grain deposits of seal joints could have accelerated localised corrosion, resulting in failure. Based on the failure analysis carried out, it is recommended that the plate type heat exchanger may be re-assembled and used in the loop after cleaning the corrosion products from the corrugated plates. The leakage locations at the seal weld joints of the shell and tube type heat exchanger can be repaired and reused in the loop. 3.3. Refurbishment of NAL Shell and tube type heat exchanger was cleaned thoroughly and all the failed seal weld joints were repaired. After weld repair, leak test was conducted to ensure no leak in the heat exchanger. The whole component is then cleaned, pickled, passivated and reinstalled in the loop. Immersion type heater unit was re-engineered to avoid failure of immersion type titanium heater tubes. Surface tape heaters were fixed by wounding around the heater vessel and insulted with mineral wool and placed aluminium cladding over the insulation. As the existing titanium top flange had holes for inserting immersion heaters, the top flange was suitably modified to close the vessel top. New pumps were installed in the loop to allow uninterrupted operation of the loop. The piping spool and supports were suitably modified to fit with inlet and outlet ports of the pumps. Replacement of dump valve, repair and reassembling of chiller unit allowed operation of the loop to its earlier state. Successful refurbishment reduced the investment cost and extended the operational life of nitric acid loop and also availability of the loop for carrying out R&D activities. 4. Recommendations to prevent reoccurrence of failure Based on results of this study, the following recommendations have been made: I. Frequent removal of corrosion products from nitric acid process streams is essential to avoid formation of galvanic cells and crevices. If feasible, frequent removal of corrosion products from the system using filters, scrubbers, etc. is suggested. II. Surface modification of titanium tubes by pickling, passivation and thermal oxidation can be considered to minimize corrosion attack and hydrogen induced cracking. III. Use of corrosion resistant materials such as Zircaloy and Ti–Ta–Nb alloy, as Cp–Ti undergoes accelerated corrosion in the vapour and condensate phases of nitric acid. 260
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Fig. 17. High resolution SEM micrograph and corresponding EDX spectra of corrosion product collected from plate type heat exchanger.
IV. Instead of immersion heater, surface heater can be used so that heater is avoided from acid contact. V. Periodic inspection of the components and contamination of nitric acid is needed to avoid failure. VI. Checking of the acidity level of DM water, after every campaign, and if required dismantling of the heat exchanger assembly are recommended.
5. Conclusions The failed Ti heater tubes and corrosion products collected from the tube were carefully examined to understand the corrosion mechanisms that had led to the failure of the component. The composition, morphology and phase analysis of the corrosion products and failed tube suggested that the failure was due to the deposition and adherence of iron over titanium, leading to galvanic corrosion. This accelerated corrosion could have resulted in higher hydrogen uptake causing the tubes to crack. The leaks in the shell and tube type heat exchanger, mainly in the seal weld regions of the nitric acid outlet, confirmed that the failure occurred due to either prolonged exposure of boiling nitric acid under deposits or poor welding or poor weld joint geometry at these regions. Based on the results of the investigation, mechanism of corrosion and causes for the failure of components were proposed. Based on the failure analysis and non-destructive testing carried out, refurbishment of the loop was carried out and recommendations to prevent reoccurrence of such failures were made.
Acknowledgements The authors are grateful to Dr.G. Panneerselvam of Chemistry Group for recording XRD patterns.
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Fig. 18. XRD patterns of (a) pale grey colour and (b) black colour corrosion products.
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