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Sulfuric acid dew point corrosion in waste heat boiler tube for copper smelting furnace
Engineering Failure Analysis 33 (2013) 29–36
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Sulfuric acid dew point corrosion in waste heat boiler tube for copper smelting furnace R. Ebara a,⇑, F. Tanaka b, M. Kawasaki c a
Department of Mechanical System Engineering, Hiroshima Institute of Technology, 2-1-1, Miyake, Hiroshima 731-5193, Japan Overseas Technical Sales Department, Mitsubishi Materials Corporation, 1-3-2, Ohtemachi, Tokyo 100-8117, Japan c Naoshima Smelter Refinery, Mitsubishi Materials Corporation, Naoshima-Cho, Kagawa 761-03110, Japan b
a r t i c l e
i n f o
Article history: Received 22 January 2011 Received in revised form 31 March 2013 Accepted 3 April 2013 Available online 24 April 2013 Keywords: Waste heat boiler tube Sulfuric acid dew point corrosion Scale General corrosion Corrosion resistant materials
a b s t r a c t This paper presents the sulfuric acid dew point corrosion found on waste heat boiler tube for a copper smelting furnace. Macroscopic and microscopic observation results for the failed boiler tubes are shown. Then the analyzed results by X-ray fluorescence, X-ray diffraction and EPMA on failed boiler tubes are demonstrated. Finally effective alloying elements for waste heat boiler tube material against sulfuric acid dew point corrosion are briefly described on the basis of the exposure test results for three kinds of steels in the radiation section of the waste heat boiler. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Corrosion control in boiler tubes is one of the very important issues for safety and effective operation of boiler. So far various kinds of corrosion behavior in boiler tubes are reported on the basis of results by failure analysis and experiments. In general the major causes of boiler tubes corrosion failure are classified into high temperature corrosion [1–3], molten salt induced corrosion [4–6] and sulfuric acid dew point corrosion [7–9]. The most of the recent boiler tube corrosion have been observed under boiler operation with more aggressive environments than those before. Therefore the failure analysis performs very important role to clarify the cause of boiler tube corrosion in the actual boiler. In this paper it is reported on the sulfuric acid dew point corrosion behavior observed on leaked waste heat boiler tubes at Naoshima Smelter of Mitsubishi Materials Corporation almost four years ago. The desirable materials for waste heat boiler are briefly touched on the basis of exposure test results of a couple of steels in the radiation section of the waste heat boiler. 2. Experimental procedure 2.1. Boiler exhaust gas condition The outline of the waste heat boiler for copper smelting furnace is schematically shown in Fig 1. The temperature and the gas flow rate of boiler exhaust gas was 1513 K and 36,000 Nm3/h, respectively. The exposed gas was mainly composed of 34% SO2, 5%O2, 14%H2O and 39.5%N2. The boiler water temperature was 523 K. ⇑ Corresponding author. E-mail address: [email protected] (R. Ebara). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.04.007
30
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 1. Outline of waste heat boiler for copper smelting furnace.
Table 1 Chemical compositions of STB340 (mass%). Material
C
Si
Mn
P
S
STB340
<0.18
<0.35
0.30–36
0.035 max.
0.035 max.
2.2. Boiler tube material The boiler tube material was STB340. Chemical compositions and mechanical properties of the STB340 are shown in Tables 1 and 2, respectively. 2.3. Experimental procedures on cut out boiler tubes The cut out screen tube surfaces were macroscopically observed by naked eyes. The cross section of the screen tube was observed by optical microscopy. Corrosion products on the boiler tube were analyzed by X-ray fluorescence and X-ray diffraction method. The scale formed on the boiler tube surface was analyzed by EPMA. 2.4. Exposure test In order to find a steel substitute for STB340 steels such as STB410, S-Ten1and SUS310S were exposed at the manhole installed on the water-wall of the screen tube bank in radiation section for about one year. Chemical compositions of these steels are shown in Table 3.
Table 2 Mechanical properties of STB340. Material
Yield strength ry (MPa)
Ultimate tensile strength rB (MPa)
Elongation e (%)
STB340
175
375
35
Table 3 Chemical compositions of exposed steels (mass%). Material
C
Si
Mn
P
S
Cu
Sb
STB410 S-Ten SUS310S
0.32 0.02 0.08
0.35 0.27 1.5
0.55 0.95 2.0
0.035 0.11 0.045
0.035 0.011 0.03
0.29
0.10
Ni
Cr
20
25
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
31
Fig. 2. Investigated screen tubes.
3. Results and discussion 3.1. Examination on failed boiler tubes 3.1.1. Surface and corrosion products Two manholes shown in Fig. 2 were examined. Blue and green colored corrosion products can be identified on the surface of the manhole by naked eyes. Seven screen tubes of (a–g) shown in Fig. 2 was cut out for examination. Yellow green and
Fig. 3. Surface appearance of tube (a).
32
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 4. Surface appearance of tube (b).
Fig. 5. Corrosion products detached from tube (b).
Table 4 Detected elements by X-ray fluorescence analysis. Major elements
Minor elements
Extremely small elements
Cu, Fe, S, Ca
Mo, Si, Br, Pb, K, Cd, Zn, Al Tl, As
Bi, Sn, Mn, Ti, V, P, Cr, Ni
Table 5 Chemical compounds identified by X-ray diffraction. Identified chemical compounds
Presumable chemical compounds
CuSO45H2O CaSO4 CaSO42H2O FeSO45H2O
Fe3O4 CuO MoS2 SiS2 CaS
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
33
Fig. 6. Cross section of tube (a). (a) Cross section of the tube. (b) EPMA analyzed area bracketed in (a).
blue colored corrosion products can be identified on all boiler tube surfaces. Corrosion products and cross section were examined on boiler tubes (a–d). Surface appearance of tubes (a and b) is shown in Figs. 3 and 4, respectively. Corrosion products taken from tube (b) are shown in Fig. 5. Analyzed results by X-ray fluorescence are summarized in Table 4. The major elements detected were Cu, Fe, S and Ca. The minor elements and extremely small elements are supposed to come from raw materials in copper smelting operation and fuel in boiler burning. Table 5 shows results by X-ray diffraction. Chemical compounds such as CuSO4, CaSO4 and FeSO4 were identified. Presumable chemical compounds were Fe3O4, CuO, MoS2, SiS2 and CaS. 3.1.2. Cross sectional observation Minimum thickness of the tube (a) was about 2 mm. Original thickness of this tube was 5 mm. Therefore corrosion rate of this tube can be estimated to be approximately 0.3 mm/year for 10 years service of this boiler. Cross section observed by SEM is shown for tube (a) is shown in Fig. 6. It is obvious that corrosion initiated at the outer surface of the tube. General corrosion was observed and pitting corrosion was not observed on the surface of the tube. The firm scale 750–1500 lm thick was adhered on the corroded area of all observed tubes. The map of elements for the matrix and the scale of specimen (a) is shown in Fig. 7. Distribution of the elements such as C, Fe, O, Cu, S and Ca can be observed on the scale. Other elements were Al, Si, K, Zn, As, Mo and Cd. Quantity of element such as C, O and Fe near the matrix was approximately same as those on the surface of the scale. The S content in the scale was larger than that in the matrix. The Cl content was very small both in the matrix and the scale. These results for tubes (a–d) are summarized in Table 6. 3.2. Cause of the screen tube failure It is clear from the above mentioned examination that corrosion initiated at the outer surface of the boiler tube. Corrosion form is general corrosion without pitting. Relatively thick scale 750–1500 lm thick was adhered on the outer surface of the boiler tube. The scale was not porous and looks to be firm. It can be considered from EPMA analysis on cross section that the contributed elements to corrosion are S, Fe and O. The S and Fe were major elements composed of corrosion products. Therefore it can be concluded that S has prominent effect on general corrosion of the boiler tubes. So far many papers have been reported on corrosion failure of boiler tubes. The primary causes of corrosion failure in boiler tubes are classified into high temperature corrosion, molten salt induced corrosion and sulfuric acid dew point corrosion. High temperature corrosion most frequently occurs on utility power boiler tubes [1]. This corrosion is strongly related to a Na and V in fuel and is quite different from that observed in this examination. Molten salt induced corrosion occurs in waste incineration boiler. In this corrosion Cl in molten salt contribute to corrosion of boiler tube [10]. In this examination Cl content found in the scale was very few. Cracks that features molten salt corrosion were not observed on the scale. Thus corrosion in this examination is quite different from high temperature corrosion and molten salt induced corrosion. On the
34
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 7. EPMA analysis for cross section of tube (a).
Table 6 Detected major elements by EPMA analysis (wt%). Position
C
O
S
Si
Cl
Cu
Ca
Fe
Mo
(a) Matrix Scale (b) Matrix Scale (c) Matrix Scale (d) Matrix Scale
47.7 44.0 21.9 17.5 20.6 22.7 50.7 41.3
10.3 11.8 23.9 20.9 28.2 19.4 21.4 27.5
1.5 2.5 5.1 5.7 8.8 10.5 5.9 8.5
10.1 14.3 0.8 3.7 3.2 6.2 0.6 0.1
0.2 0.2 0.3 0.2 0.5 0.2 0.2 0.5
– 0.8 2.2 2.3 1.1 4.2 1.4 1.8
– – – 0.2 – – – –
28.3 23.2 38.2 42.3 26.1 29.4 9.9 16.4
0.5 0.4 3.6 – 1.4 1.6 1.9 –
Note: the other elements such as Na, Al, P, K, Cr, Mn, Ni, Zn, As, Ag and Cd were excluded in this Table.
other hand the sulfuric acid dew point corrosion is well known low temperature corrosion phenomenon [9]. In this boiler H2SO4 is produced by the reaction with gaseous SO3 in gas and H2O. Then FeSO4 found by X-ray analysis can be produced by the reaction with H2SO4 and boiler tube. That is to say the sulfuric acid dew point corrosion occurs by following reaction.
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
35
Fig. 8. Cross section of exposed materials. (a) STB410 and (b) SUS310S.
SO3 þ H2 O ! H2 SO4 Fe þ H2 SO4 ! FeSO4 þ H2 In fact the manhole in this boiler was opened for about two hours per week in order to remove an accretion on the wall. Moreover, every two months, the manhole was opened for less than 24 h in order to maintain of the copper smelting plant. During these periods the furnace was heated by burner in order to keep melt temperature while the boiler operation was halted. Therefore the boiler tubes experienced cyclic repeat of heating and cooling for about ten years. As a result general corrosion formed on the outer surface of the boiler tubes due to sulfuric acid dew point corrosion. It can be concluded that the reduction of the boiler tube thickness due to sulfuric acid dew point corrosion was the major cause of the water leakage from the boiler tubes. 3.3. Exposure test in the waste heat boiler It was concluded from examination that the major cause of the boiler tube failure was sulfuric acid dew point corrosion. It is not easy to modify the boiler operating condition with copper smelting operation. Consequently it is reasonable to modify the boiler tube material to prevent sulfuric acid dew point corrosion of the boiler tube. After one year exposure test the specimen surface of the exposed steels was colored yellow and blue green as same as that observed on failed boiler tube. The same scales observed on failed boiler tubes were formed on the exposed specimen surfaces. The typical examples of the scales are shown for STB410 and SUS310S in Fig. 8. General corrosion was apparently observed for these steels. The thickness of the scale for STB410, SUS310 and S-Ten1 was 425, 200 and 250 lm respectively. Among these tested steels the thickness of the scale was the smallest for SUS310S. From EPMA analysis for scale (A in Fig. 8) and matrix (B in Fig. 8) Fe and S were mainly detected on the scale. The results by X-ray fluorescence and X-ray diffraction showed that these exposed specimens were suffered from sulfuric acid dew point corrosion during exposure in the boiler. General corrosion that features sulfuric acid dew point corrosion is most frequently observed in the inner wall of the steel stack [11] and the element of an air heater [12,13].
36
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
One of the authors evaluated corrosion resistance of various kinds of steels under heavy oil fired smoke stack environment and developed corrosion resistant stainless steels such as YUS260 and YUS270 that keep maintenance-free of smokestacks linings [14,15]. Consequently it was clarified that alloying elements such as Cr, Ni, Mo and Cu are effective to prevent general corrosion of steels under sulfuric acid dew point corrosion environment. Therefore SUS310S may substitute STB340 for boiler tube material. 4. Concluding remarks In this paper it is reported on the failure analysis on the waste heat boiler tubes for copper smelting furnace. It can be concluded from the metallurgical investigations that the cause of the boiler tubes failure is the sulfuric acid dew point corrosion due to air leakage and cyclic temperature profile. Development of corrosion resistant materials is desirable to prevent sulfuric acid dew point corrosion in waste heat boiler tubes. Acknowledgements The authors thank Dr. Yoshio Harada, Tocalo Co., Ltd., Prof. Fujimitsu Masuyama, Kyushu Institute of Technology and Dr. Yuuzou Kawahara, Inspection and Research laboratory Co., Ltd. for their useful suggestions and fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Harada Y. Succession of corrosion control techniques for boiler and gas turbine plants. J. Soc. Mat. Sci., Japan 1993;42:905–11 [in Japanese]. Hamada I, Fukuda Y. High temperature corrosion of boiler tubes. J Therm Nucl Power 1999;50:1422–31 [in Japanese]. Kawahara Y. High temperature corrosion in environment and energy conversion equipment. Zairyo-to-Kankyo 2006;55:172–83 [in Japanese]. Spiegel M. Salt melt induced corrosion of metallic materials in waste incineration plants. Mater Corros 1999;50:373–93. Kawahara Y. Recent trends and future subjects on high temperature corrosion prevention technologies in high efficiency waste-to-energy plants. Zairyo-to-Kankyo 2005;54:183–94 [in Japanese]. Ruh A, Spiegel M. Influence of gas phase composition on the kinetics of chloride melt induced corrosion of pure iron (EU-project OPTICORR). Mater Corros 2006;57:237–43. Kowaka M, Moroishi T, Nagano H. Laboratory test methods for sulfur dew point corrosion. J Jpn Inst Met 1970;34:23–32 [in Japanese]. Kowaka M, Nagano H. Mechanism of sulfur dewpoint corrosion. J Jpn Inst Met 1970;34:32–9 [in Japanese]. Harada Y. In: High temperature oxidation and high temperature corrosion in metallic materials. Maruzen; 1982. p. 236–48 [in Japanese]. Kawahara Y. High temperature corrosion mechanisms and alloying elements for materials used in waste incineration environment. Corros Sci 2002;44:223–45. Ebara R. Evaluation and development of corrosion resistant materials for smokestacks. Corros Sci Technol 2007;6:211–8. Masuyama F, Private communication, 2010. Ishihara T, Sulfuric acid dewpoint corrosion in air heater element, The current issue case histories in corrosion failures analysis and corrosion diagnostics, 2008. p. 872–3, [in Japanese] Ebara R, Nakamoto H, Matsumoto T, Sato E, Matsuhashi R, Kozeki T. Development of new corrosion resistant stainless steels. Mitsubishi Tech Rev 1991;28:3–7. Ebara R, Nakamoto H, Matsumoto T, Matsuhashi R, Sato E, Abo H. Corrosion behavior of stainless steels in an alternating dry-wet environment with SO2 gas. Zairyo-to-Kankyo 1991;40:247–53 [in Japanese].
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Sulfuric acid dew point corrosion in waste heat boiler tube for copper smelting furnace R. Ebara a,⇑, F. Tanaka b, M. Kawasaki c a
Department of Mechanical System Engineering, Hiroshima Institute of Technology, 2-1-1, Miyake, Hiroshima 731-5193, Japan Overseas Technical Sales Department, Mitsubishi Materials Corporation, 1-3-2, Ohtemachi, Tokyo 100-8117, Japan c Naoshima Smelter Refinery, Mitsubishi Materials Corporation, Naoshima-Cho, Kagawa 761-03110, Japan b
a r t i c l e
i n f o
Article history: Received 22 January 2011 Received in revised form 31 March 2013 Accepted 3 April 2013 Available online 24 April 2013 Keywords: Waste heat boiler tube Sulfuric acid dew point corrosion Scale General corrosion Corrosion resistant materials
a b s t r a c t This paper presents the sulfuric acid dew point corrosion found on waste heat boiler tube for a copper smelting furnace. Macroscopic and microscopic observation results for the failed boiler tubes are shown. Then the analyzed results by X-ray fluorescence, X-ray diffraction and EPMA on failed boiler tubes are demonstrated. Finally effective alloying elements for waste heat boiler tube material against sulfuric acid dew point corrosion are briefly described on the basis of the exposure test results for three kinds of steels in the radiation section of the waste heat boiler. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Corrosion control in boiler tubes is one of the very important issues for safety and effective operation of boiler. So far various kinds of corrosion behavior in boiler tubes are reported on the basis of results by failure analysis and experiments. In general the major causes of boiler tubes corrosion failure are classified into high temperature corrosion [1–3], molten salt induced corrosion [4–6] and sulfuric acid dew point corrosion [7–9]. The most of the recent boiler tube corrosion have been observed under boiler operation with more aggressive environments than those before. Therefore the failure analysis performs very important role to clarify the cause of boiler tube corrosion in the actual boiler. In this paper it is reported on the sulfuric acid dew point corrosion behavior observed on leaked waste heat boiler tubes at Naoshima Smelter of Mitsubishi Materials Corporation almost four years ago. The desirable materials for waste heat boiler are briefly touched on the basis of exposure test results of a couple of steels in the radiation section of the waste heat boiler. 2. Experimental procedure 2.1. Boiler exhaust gas condition The outline of the waste heat boiler for copper smelting furnace is schematically shown in Fig 1. The temperature and the gas flow rate of boiler exhaust gas was 1513 K and 36,000 Nm3/h, respectively. The exposed gas was mainly composed of 34% SO2, 5%O2, 14%H2O and 39.5%N2. The boiler water temperature was 523 K. ⇑ Corresponding author. E-mail address: [email protected] (R. Ebara). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.04.007
30
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 1. Outline of waste heat boiler for copper smelting furnace.
Table 1 Chemical compositions of STB340 (mass%). Material
C
Si
Mn
P
S
STB340
<0.18
<0.35
0.30–36
0.035 max.
0.035 max.
2.2. Boiler tube material The boiler tube material was STB340. Chemical compositions and mechanical properties of the STB340 are shown in Tables 1 and 2, respectively. 2.3. Experimental procedures on cut out boiler tubes The cut out screen tube surfaces were macroscopically observed by naked eyes. The cross section of the screen tube was observed by optical microscopy. Corrosion products on the boiler tube were analyzed by X-ray fluorescence and X-ray diffraction method. The scale formed on the boiler tube surface was analyzed by EPMA. 2.4. Exposure test In order to find a steel substitute for STB340 steels such as STB410, S-Ten1and SUS310S were exposed at the manhole installed on the water-wall of the screen tube bank in radiation section for about one year. Chemical compositions of these steels are shown in Table 3.
Table 2 Mechanical properties of STB340. Material
Yield strength ry (MPa)
Ultimate tensile strength rB (MPa)
Elongation e (%)
STB340
175
375
35
Table 3 Chemical compositions of exposed steels (mass%). Material
C
Si
Mn
P
S
Cu
Sb
STB410 S-Ten SUS310S
0.32 0.02 0.08
0.35 0.27 1.5
0.55 0.95 2.0
0.035 0.11 0.045
0.035 0.011 0.03
0.29
0.10
Ni
Cr
20
25
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
31
Fig. 2. Investigated screen tubes.
3. Results and discussion 3.1. Examination on failed boiler tubes 3.1.1. Surface and corrosion products Two manholes shown in Fig. 2 were examined. Blue and green colored corrosion products can be identified on the surface of the manhole by naked eyes. Seven screen tubes of (a–g) shown in Fig. 2 was cut out for examination. Yellow green and
Fig. 3. Surface appearance of tube (a).
32
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 4. Surface appearance of tube (b).
Fig. 5. Corrosion products detached from tube (b).
Table 4 Detected elements by X-ray fluorescence analysis. Major elements
Minor elements
Extremely small elements
Cu, Fe, S, Ca
Mo, Si, Br, Pb, K, Cd, Zn, Al Tl, As
Bi, Sn, Mn, Ti, V, P, Cr, Ni
Table 5 Chemical compounds identified by X-ray diffraction. Identified chemical compounds
Presumable chemical compounds
CuSO45H2O CaSO4 CaSO42H2O FeSO45H2O
Fe3O4 CuO MoS2 SiS2 CaS
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
33
Fig. 6. Cross section of tube (a). (a) Cross section of the tube. (b) EPMA analyzed area bracketed in (a).
blue colored corrosion products can be identified on all boiler tube surfaces. Corrosion products and cross section were examined on boiler tubes (a–d). Surface appearance of tubes (a and b) is shown in Figs. 3 and 4, respectively. Corrosion products taken from tube (b) are shown in Fig. 5. Analyzed results by X-ray fluorescence are summarized in Table 4. The major elements detected were Cu, Fe, S and Ca. The minor elements and extremely small elements are supposed to come from raw materials in copper smelting operation and fuel in boiler burning. Table 5 shows results by X-ray diffraction. Chemical compounds such as CuSO4, CaSO4 and FeSO4 were identified. Presumable chemical compounds were Fe3O4, CuO, MoS2, SiS2 and CaS. 3.1.2. Cross sectional observation Minimum thickness of the tube (a) was about 2 mm. Original thickness of this tube was 5 mm. Therefore corrosion rate of this tube can be estimated to be approximately 0.3 mm/year for 10 years service of this boiler. Cross section observed by SEM is shown for tube (a) is shown in Fig. 6. It is obvious that corrosion initiated at the outer surface of the tube. General corrosion was observed and pitting corrosion was not observed on the surface of the tube. The firm scale 750–1500 lm thick was adhered on the corroded area of all observed tubes. The map of elements for the matrix and the scale of specimen (a) is shown in Fig. 7. Distribution of the elements such as C, Fe, O, Cu, S and Ca can be observed on the scale. Other elements were Al, Si, K, Zn, As, Mo and Cd. Quantity of element such as C, O and Fe near the matrix was approximately same as those on the surface of the scale. The S content in the scale was larger than that in the matrix. The Cl content was very small both in the matrix and the scale. These results for tubes (a–d) are summarized in Table 6. 3.2. Cause of the screen tube failure It is clear from the above mentioned examination that corrosion initiated at the outer surface of the boiler tube. Corrosion form is general corrosion without pitting. Relatively thick scale 750–1500 lm thick was adhered on the outer surface of the boiler tube. The scale was not porous and looks to be firm. It can be considered from EPMA analysis on cross section that the contributed elements to corrosion are S, Fe and O. The S and Fe were major elements composed of corrosion products. Therefore it can be concluded that S has prominent effect on general corrosion of the boiler tubes. So far many papers have been reported on corrosion failure of boiler tubes. The primary causes of corrosion failure in boiler tubes are classified into high temperature corrosion, molten salt induced corrosion and sulfuric acid dew point corrosion. High temperature corrosion most frequently occurs on utility power boiler tubes [1]. This corrosion is strongly related to a Na and V in fuel and is quite different from that observed in this examination. Molten salt induced corrosion occurs in waste incineration boiler. In this corrosion Cl in molten salt contribute to corrosion of boiler tube [10]. In this examination Cl content found in the scale was very few. Cracks that features molten salt corrosion were not observed on the scale. Thus corrosion in this examination is quite different from high temperature corrosion and molten salt induced corrosion. On the
34
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
Fig. 7. EPMA analysis for cross section of tube (a).
Table 6 Detected major elements by EPMA analysis (wt%). Position
C
O
S
Si
Cl
Cu
Ca
Fe
Mo
(a) Matrix Scale (b) Matrix Scale (c) Matrix Scale (d) Matrix Scale
47.7 44.0 21.9 17.5 20.6 22.7 50.7 41.3
10.3 11.8 23.9 20.9 28.2 19.4 21.4 27.5
1.5 2.5 5.1 5.7 8.8 10.5 5.9 8.5
10.1 14.3 0.8 3.7 3.2 6.2 0.6 0.1
0.2 0.2 0.3 0.2 0.5 0.2 0.2 0.5
– 0.8 2.2 2.3 1.1 4.2 1.4 1.8
– – – 0.2 – – – –
28.3 23.2 38.2 42.3 26.1 29.4 9.9 16.4
0.5 0.4 3.6 – 1.4 1.6 1.9 –
Note: the other elements such as Na, Al, P, K, Cr, Mn, Ni, Zn, As, Ag and Cd were excluded in this Table.
other hand the sulfuric acid dew point corrosion is well known low temperature corrosion phenomenon [9]. In this boiler H2SO4 is produced by the reaction with gaseous SO3 in gas and H2O. Then FeSO4 found by X-ray analysis can be produced by the reaction with H2SO4 and boiler tube. That is to say the sulfuric acid dew point corrosion occurs by following reaction.
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
35
Fig. 8. Cross section of exposed materials. (a) STB410 and (b) SUS310S.
SO3 þ H2 O ! H2 SO4 Fe þ H2 SO4 ! FeSO4 þ H2 In fact the manhole in this boiler was opened for about two hours per week in order to remove an accretion on the wall. Moreover, every two months, the manhole was opened for less than 24 h in order to maintain of the copper smelting plant. During these periods the furnace was heated by burner in order to keep melt temperature while the boiler operation was halted. Therefore the boiler tubes experienced cyclic repeat of heating and cooling for about ten years. As a result general corrosion formed on the outer surface of the boiler tubes due to sulfuric acid dew point corrosion. It can be concluded that the reduction of the boiler tube thickness due to sulfuric acid dew point corrosion was the major cause of the water leakage from the boiler tubes. 3.3. Exposure test in the waste heat boiler It was concluded from examination that the major cause of the boiler tube failure was sulfuric acid dew point corrosion. It is not easy to modify the boiler operating condition with copper smelting operation. Consequently it is reasonable to modify the boiler tube material to prevent sulfuric acid dew point corrosion of the boiler tube. After one year exposure test the specimen surface of the exposed steels was colored yellow and blue green as same as that observed on failed boiler tube. The same scales observed on failed boiler tubes were formed on the exposed specimen surfaces. The typical examples of the scales are shown for STB410 and SUS310S in Fig. 8. General corrosion was apparently observed for these steels. The thickness of the scale for STB410, SUS310 and S-Ten1 was 425, 200 and 250 lm respectively. Among these tested steels the thickness of the scale was the smallest for SUS310S. From EPMA analysis for scale (A in Fig. 8) and matrix (B in Fig. 8) Fe and S were mainly detected on the scale. The results by X-ray fluorescence and X-ray diffraction showed that these exposed specimens were suffered from sulfuric acid dew point corrosion during exposure in the boiler. General corrosion that features sulfuric acid dew point corrosion is most frequently observed in the inner wall of the steel stack [11] and the element of an air heater [12,13].
36
R. Ebara et al. / Engineering Failure Analysis 33 (2013) 29–36
One of the authors evaluated corrosion resistance of various kinds of steels under heavy oil fired smoke stack environment and developed corrosion resistant stainless steels such as YUS260 and YUS270 that keep maintenance-free of smokestacks linings [14,15]. Consequently it was clarified that alloying elements such as Cr, Ni, Mo and Cu are effective to prevent general corrosion of steels under sulfuric acid dew point corrosion environment. Therefore SUS310S may substitute STB340 for boiler tube material. 4. Concluding remarks In this paper it is reported on the failure analysis on the waste heat boiler tubes for copper smelting furnace. It can be concluded from the metallurgical investigations that the cause of the boiler tubes failure is the sulfuric acid dew point corrosion due to air leakage and cyclic temperature profile. Development of corrosion resistant materials is desirable to prevent sulfuric acid dew point corrosion in waste heat boiler tubes. Acknowledgements The authors thank Dr. Yoshio Harada, Tocalo Co., Ltd., Prof. Fujimitsu Masuyama, Kyushu Institute of Technology and Dr. Yuuzou Kawahara, Inspection and Research laboratory Co., Ltd. for their useful suggestions and fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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