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A degradation mode of a carbon steel during thermal cycling under a simulated combustion environment
Materials Science and Engineering A332 (2002) 270– 275 www.elsevier.com/locate/msea
A degradation mode of a carbon steel during thermal cycling under a simulated combustion environment X. Peng *, W.P. Pan, J.T. Riley Combustion Laboratory, Department of Chemistry, Materials Characterization Center, Western Kentucky Uni6ersity, Bowling Green, KY 42101, USA Received 29 May 2001; received in revised form 13 July 2001
Abstract The degradation of A210C carbon steel has been investigated at 560 °C under a simulated coal-firing combustion environment. The simulation was performed by direct introduction into the chamber of flue gas from a freeboard of a coal-fired fluidized-bed combustor (FBC) and meanwhile by covering part of samples using ash deposits collected from the FBC freeboard wall. It shows that oxidation is significantly increased, when the steel is covered with ash deposits. During thermal cycling, the accelerated metal recession in the presence of deposits can be interpreted using a proposed degradation mode, which follows a sequence of scale wrinkling, buckling, and finally spalling. The effect of ash deposits on the steel degradation is also discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon steel; Corrosion; Coal; Combustion; Ash deposits; Fe2O3 scale; Wrinkling; Buckling
1. Introduction In power generation plants, fluidized-bed boilers, such as fluidized-bed combustors (FBC), are in wide commercial use in a range of sizes, while burning different fossil fuels, although mostly coals. High temperature corrosion under the coal combustion environment has been a problem encountered in the materials being employed in the FBC system of the utility industry [1–4]. For a coal-fired FBC system, the degradation of the metal in service under the combustion environment is generally as a result of a synergistic effect of several factors, which can be summarized as follows: (1) attack of the corrosive gases from coal combustion, such as O2, SO2, HCl, H2O, CO, etc.; (2) deposition of fly-ashes (potentially forming aggressive salts at high temperature); and (3) erosion or abrasion induced by impact of the flying particles and the wakes of rising bubbles. The combined multiple factors make it difficult to clarify the degradation process of the metals employed in the FBC. Therefore, laboratory studies
have been more concerned with emphasis on a basic understanding of corrosion mechanisms [5–9]. In this work, we present observations of an A210-C carbon steel, which was covered by ash deposits and exposed a short-term thermal cyclic oxidation exposure under a simulated atmosphere of a freeboard of a coal-fired FBC. Erosion is not concerned in the study, which was intended to understand the degradation of carbon steel in the presence of deposits under exposure of a coal combustion environment.
2. Experimental Samples with dimensions of : 6× 5× 4 mm3 were cut from A210-C carbon steel (chemical composition in weight percent: 0.35C –1.06Mn –0.035P –0.035S – 0.01Si). The samples were finally grounded to 800 grid SiC paper. The corrosion testing was conducted by using the experimental set-up, which is presented schematically in Fig. 1. The corrosive gas atmosphere used
* Corresponding author. Tel.: + 1-270-745-5322; fax: + 1-270-745-5361. E-mail address: [email protected] (X. Peng). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 4 6 - 4
X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275 Table 1 Composition of corrosive flue gases introduced from FBC (in average) O2 (vol.%)
6
H2O (vol.%)
SO2 (vppm) HCl (vppm)
CO (vppm)
0.9
40
ND
135
Fig. 1. Schematic view of the experimental set-up for the corrosion testing by using the flue gas from a freeboard of coal-firing FBC. 1— inlet of gases from a coal-firing FBC, 2— connector, 3 —quartz tube, 4— furnace, 5 — deposit, 6 — crucible boat, 7 —sample, 8 —gas outlet.
simulated that measured at a location 1.9 m above the central plate of the practical coal-firing 0.1MWth WKU –FBC facility [10,11] using an on-line by a ZRF 1312 photo-acoustic multi-gas analyzer and IMR 7000 combustion-gas analyzer. The gas composition is listed in Table 1. Ash deposits collected from the freeboard wall of the FBC were used as the overlying deposit in the test. The chemical compositions by EDX are listed in Table 2; the calcium was mainly from limestone additions made to absorb sulfur oxides and/or reduce chlorine emissions. For corrosion testing, the metal samples were embedded into the deposit powder, which was pre-placed in a crucible boat, and then were pushed into chamber (the hot zone of the silicon tube in a TGA system). Afterwards, the test gas was introduced into the silicon tube, as seen in Fig. 1. The exposure temperature was set at 560 °C. The samples were cyclically oxidized by 3 h exposure at the temperature and subsequently 1 h furnace cooling to room temperature. After certain cycles of cycling oxidation, the samples were weighed. After the testing, the corroded samples were investigated using SEM/EDX. For comparison purposes, the sample was also oxidized by using the same condition of thermal cycling and same combustion gas, but without deposit powder addition. 3. Results Compared to exposure without deposit cover, the low carbon steel exhibited a significantly faster oxidation, and during cooling spallation of thicker and larger pieces of oxides occurred, especially at the edge of both the samples. Fig. 2 is the mass change of the steel under
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Table 2 Elemental composition of deposit powder by EDX (by atomic percentage) Ca
Mg
K
Na
Al
S
Cl
Si
Fe
O
12.1
1.5
0.7
B0.1
3.5
6.8
5.7
5.9
7.7
Bal.
Fig. 2. Mass change of A210-C carbon steel with and without deposit addition after 12 h isothermal and 36h/12 cycles’ cyclic oxidation.
both isothermal and cyclic oxidation conditions. It can been seen that during isothermal condition oxidation is significantly faster with deposit addition than without deposit addition. SEM observation showed that the scale formed was very flat in the absence of ash deposits, but was undulated and separated from the steel in the case of deposit cover, as seen in Fig. 3. Fig. 3 also shows that under both conditions the local separation between the scale and the metal occurred, but more so with deposit addition. Under cyclic oxidation condition, the mass loss occurred after 9h/3 cycles’ thermal cycling. After 36h/12 cycles, the mass loss was 1.7 mg cm − 2 without deposit cover, which is very low compared to 10.3 mg cm − 2 with deposit cover (Fig. 2). The oxides formed are mainly hematite (Fe2O3). Magnetite (Fe3O4) layer and other corrosion phases, such as iron sulfides or iron chlorides were not found obviously (in fact, if the phases formed, they should appear a color contrast different from that of Fe2O3 under SEM), even in the case of deposit cover. EDX analyses of the composition of the scale formed without deposits indicated that the scale contained impurities, which are listed in Table 3. For both conditions, the content of S and Cl in the scale is not much different. It can be presumed that the two elements mainly resulted from the flue gas. Mn and part of Si were from the steel. In the absence of ash deposits, impurities incorporated into the scale, such as Ca, Mg, K, Na, Al, Co, Ni, Si, etc., were detected. Apparently, the cation impurities were from flying ash, which was entrained by the flue gas from the FBC freeboard and entered the furnace. However, in general, the content of these impurities in the scale was greatly increased, when
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the steel was covered with ash deposits. This may be one reason that oxidation becomes faster, when the steel is covered by deposits, as will be discussed later. SEM observation showed after exposure of 36h/12 cycles at 560 °C, local oxide cracking and spalling was observed in the absence of deposit. This implied that the oxide scale formed under the flue gas atmosphere produced by combustion of the coal used was per se fragile and poorly adherent. However, in the case, the
Fig. 4. Secondary electron images of corroded scale formed: (a) general top-view showing surface scale convoluted; (b) fracture section revealing the scale layered and buckled and also the scale spallation appeared between the layers and at the oxide layer-metal interface.
Fig. 3. Backscattered electron images of the cross-sectional samples showing the scale flat without deposit cover (a) but undulated and separated from the metal with deposit cover (b). The bright spots above the scale are copper particles included the polymer, which was employed for preventing the scale exfoliation during sample preparation. Table 3 EDX results of the elemental content within the scale near the interface after 12 h exposure (by atomic percentage)
O S Cl Si Mn Co Ni Al Ca Mg K Na Cr Fe
Without deposit
With deposit
22.1 9 0.5 0.55 9 0.18 0.57 9 0.09 2.33 9 0.56 0.53 9 0.03 0.33 9 0.20 0.10 9 0.05 0.30 9 0.17 B0.10 0.12 9 0.06 B0.10 B0.10 B0.10 Bal.
20.39 0.7 0.639 0.20 0.76 9 0.24 1.369 0.30 0.54 9 0.07 0.73 9 0.25 0.23 9 0.14 1.90 9 0.15 0.49 9 0.09 0.28 9 0.11 0.499 0.18 0.24 9 0.10 0.159 0.03 Bal.
surface scale remained very flat. While with the deposit cover, the scale was wrinkled (convoluted), as seen in Fig. 4a. The white particles (arrowed) are the residual deposits. Most particles could be brushed off the scale surface, indicating loose contact with the oxide. No experimental evidence that would indicate the presence of low-melting salts from reactions between the deposits and the oxide was found during the short-term exposure. As is evident in Fig. 4a, the surface scale was wrinkled and scale cracking (area ‘A’) and buckling (area ‘B’) occurred at certain regions. Fig. 4b is the fracture section of the scale close to the sample edge, revealing the residual scale consisted of two layers, and was convoluted. The residual oxide layers were separated and spallation occurred between the layers or between the oxidemetal interface. Fig. 5 presents SEM observation of the scale cross-section, clearly showing an undulated scalemetal interface. Fig. 5a shows that the undulated scalemetal interface maintained contact, suggesting that the buckling occurred after wrinkling. As will be discussed, wrinkling occurred during oxidation and buckling during cooling. Fig. 5b is another cross-section of the scale formed, showing that the continuous waved oxide layers were split apart into fragments due to occurrence of
X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275
cracks within the layer (normal to the oxide scale-metal interface). The scale-metal interface separation could be frequently observed. Sulfur, sometimes together with other impurities, such as silicon, calcium, potassium (sodium), aluminum, chlorine, etc., could be probed obviously at the interface by the EDX analyses. However, it is not clear in what state these elements appeared as there.
4. Discussion According to the results, it is clear that the added ash deposits accelerated metal degradation. It has been proposed that the accelerated corrosion by ash deposits may result from formation of low-melting salts. However, such low-melting salts were not characterized in this work. The thermal cycling and SEM observations suggested that the scale formed with deposit addition is extremely sensitive to spallation, which appeared as a consequence of the contribution of oxide wrinkling (Figs. 4 and 5). Wrinkling is a deformation mode by which elastic stain can be reduced [12]. Wrinkling of thermally-grown oxide has been suggested to be a result
Fig. 5. Backscattered electron images of the scale cross-sections showing the development of the undulant oxide layers and the separation between the layers. (a) Separation between two waved oxide layers. (b) The oxide layers were split into fragments due to formation of cracks normal to the scale-metal interface. The separation between the scale-metal interface is also visible at some region.
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of oxide creep for releasing high compressive stresses, which developed partly due to the scale lateral growth, and as a result growth of new oxides within the scale, due to the inter-diffusion of cations and anions during oxidation [13–15]. In this work, it shows that the oxidation rate of the metal is significantly higher with deposit cover than without deposit cover. Hence, it could be assumed that the growth mechanism of the scale was changed with deposits in respect to oxidation without deposits. The increased lateral or inward growth may occur, as a result of the incorporation of the impurities with low valence, such as Ca, Mg, K, Na, Ni, Co, from the deposits. It is well known that the doping of impurity may change physical defect structure of oxides and thusly have an effect on transport of ion species in them. The incorporation of impurities with low valence probably increased the concentration of oxygen vacancy in the scale. Hence, as a result of an increased attack of oxygen, a fast growing of the scale will occur, with a great contribution of new oxide formed within the scale and/or the metal-scale interface. This leads to the fast build-up of high growth compressive stresses, which may be high enough to wrinkle the oxide and the metal during oxidation. Moreover, compared to oxidation without deposits, the addition of deposits leads to the possibility of a change in content of oxygen, moisture, HCl (Cl2) or SO2 at the metal surface. However, in this study, the deposits were loosely contacted with the metal (suggesting that the change in gas composition may not be evident) and other phases except for iron oxides were not detected in the scale. Taking it into account, this change may be not the main factor responsible for the increased oxidation of the deposit-covered steel during the short-term exposure. The SEM observations proposed that with deposit cover scale buckling occurred during cooling and the buckle usually appeared at the vertex of the undulated interface. During thermal cycling, the development and extension of the buckle will lead to the separation of the scale from the metal and finally scale spallation. Without deposits, the scale separation and spallation also appeared. This implies that the iron oxide scale adhesion to the steel is per se poor. However, compared to the cycling without deposits, the local scale buckling made the scale separation and spallation an easier process in the presence of deposits. In the later case, the degradation of the steel appears to follow a ‘wrinkling– buckling–spalling’ route, as suggested in Fig. 6. The progression of the degradation process during the thermal cycling is presented below. When oxidation starts, rapid growth of the oxide results in the fast accumulation of growing compressive stresses which eventually are sufficient to cause wrinkling of oxide with metal. If the scale is initially adherent to the metal and the compressive stress in the scale
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X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275
Fig. 6. Schematic process of scale damage during the thermal cyclic oxidation.
exceeds the creep/yield strength of the alloy surface, an undulant scale-metal interface will develop, resulting in a tensile stress at the vertex of the interface and compress stress at the valley of the interface during the subsequently cooling (Fig. 6a). Such stresses are produced partly due to the thermal expansion mismatch between the oxide and the metal. As the extent of wrinkling enlarges, the tensile stresses become high enough to trigger the scale buckling (Fig. 6b), probably through the way of bending the oxide from the metal (Fig. 5). The susceptibility of the interface de-bonding in the presents of the ash deposits increases the available energy rate for the buckle propagation [16]. As a buckle propagates, the compressive stresses within the scale are further relaxed. Meanwhile, the buckling-related deformation gives rise to in-plane tensile stresses in the scale. If scale spallation does not occur at once as a consequence of buckle initiation, the stresses will accumulate during the following thermal cycling through further increasing the deflection of the oxide scale [17]. When the stresses exceed, the flexural strength of the oxide cracks form in the scale (Fig. 6c). Cracks may also form in other regions of the curved layer. For instance, if the defect size in a given location is larger, the tensile stresses (or shear stresses) will initiate cracking of the scale. Once cracks open to the gas atmosphere form in the scale, the underlying metal will be exposed to the combustion gases and lead to formation of new oxide on the metal (Fig. 6d). The extension of the buckling-associated fatigue crack growth along the interface produces space for the full development of the secondary oxide layer (Fig. 6e). By this route, separated oxide layers form during the continued cycling, as seen in Fig. 5. Finally, spallation of
layered scale occurs through linking-up of cracks at local regions, as seen in Fig. 6e. Based on the mode and the observations, prevention of the oxide wrinkling under the deposit-free condition is presumed to be a result of a relative slow-growing of the scale, by which the developed stresses could be accommodated by other creep mechanism except wrinkling. In the case, the detachment of the scale from the metal occurs not as readily as it does on the wrinkled interface, due to lack of driving force, such as out-ofplane tensile stresses and shear stresses. As a result, the scale formed was more buckling and spalling-resistant. Hence, the comparison result of this study suggests that during the short-term of thermal cycling, the accelerated degradation of the metal with deposit addition is a result of susceptibility of wrinkling and buckling.
5. Summary Compared to corrosion without ash deposit additions, the degradation of A210-C carbon steel with the deposit cover at 560 °C under the simulated coal-firing combustion environment was faster and the iron oxide scale formed was wrinkled and buckled. The ash deposits have a profound effect on accelerating the degradation of the metal at high temperature by promoting oxide growth and thusly oxide wrinkling, which would favor the scale breakdown, following a route of scale ‘wrinkling– buckling – spalling’ during the thermal cycling. The oxide wrinkling may be a result of an increased oxidation with deposits, which may mainly result from incorporation into the growing iron oxide scale of low valence impurities, such as Ca, Mg, K, Na,
X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275
Co, Ni, from the deposits. This incorporation may change the oxide defect structure and consequently make it faster the lateral and inward growth of the oxide.
Acknowledgements The authors are indebted to Dr K. Liu, graduate students D. Zou, C. Hu, R. Heltsley, M. Zheng, L. Wells and undergraduate student P. Hack for assistance in 10 days’ FBC operation. One of the authors XP is grateful to Dr I.G. Wright at Oak Ridge National Laboratory for helpful comments.
References [1] E.A. Rogers, J.C. Holder, A.J. Minchener, A.J. Page, R.D. Lanauze, G.G. Thurlow, Materials problems in fluidized-bed combustion systems, May 1980, EPRI report No. CS-1449. [2] J. Stringer, I.G. Wright, J. Mater. Energy Sys. 8 (1986) 319.
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[3] A.J. Minchener, A.V. Levy, in: Proc. Conf. on Corrosion –Erosion– Wear of Materials at Elevated Temperature. NACE, Houston, TX, 1987, p. 152. [4] K. Nakagawa, in: Proc. High Temp. Corros. (III), Les Embiez, France, May 1992, J. De Physique. 4 (colloque C9) (1993) 787. [5] W.T. Bakker, R.A. Perkins, Van Liere, J. Mat. Perf. 1 (1985) 9. [6] K. Natesan, Corrosion 41 (1985) 655. [7] H.J. Grabke, Meadowcroft (Eds.), Guideline for Methods of Testing and Research in High-Temperature Corrosion. European Federation of Corrosion Publication Number 14, Institute of Materials, 1995. [8] F.H. Stott, J.F. Norton, Mat. High Temp. 14 (1997) 43. [9] J.R. Nicholls, Mat. High Temp. 14 (1997) 219. [10] K.L. Liu, W.P. Pan, J.T. Riley, Corrosion 56 (2000) 298. [11] X. Peng, K.L. Liu, P. Hack, W.P. Pan, J.T. Riley, D.L. Reuss (Eds.), Technical Meeting of Central States Section on Combustion Fundaments and Applications. Indianapolis, Indiana, USA, April 16 – 18, 2000, p. 80. [12] Z. Suo, Mech. Phys. Solids 43 (1995) 829. [13] F.A. Golightly, F.H. Stott, G.C. Wood, Oxid. Met. 10 (1976) 163. [14] F.N. Rhines, J.S. Wolf, Metall. Trans. 1 (1970) 1701. [15] V.K. Tolpygo, D.R. Clarke, Acta Mater. 46 (1998) 5167. [16] J.S. Wang, A.G. Evans, Acta Mater. 47 (1999) 699. [17] H.E. Evans, Internal Mat. Rev. 40 (1995) 1.
A degradation mode of a carbon steel during thermal cycling under a simulated combustion environment X. Peng *, W.P. Pan, J.T. Riley Combustion Laboratory, Department of Chemistry, Materials Characterization Center, Western Kentucky Uni6ersity, Bowling Green, KY 42101, USA Received 29 May 2001; received in revised form 13 July 2001
Abstract The degradation of A210C carbon steel has been investigated at 560 °C under a simulated coal-firing combustion environment. The simulation was performed by direct introduction into the chamber of flue gas from a freeboard of a coal-fired fluidized-bed combustor (FBC) and meanwhile by covering part of samples using ash deposits collected from the FBC freeboard wall. It shows that oxidation is significantly increased, when the steel is covered with ash deposits. During thermal cycling, the accelerated metal recession in the presence of deposits can be interpreted using a proposed degradation mode, which follows a sequence of scale wrinkling, buckling, and finally spalling. The effect of ash deposits on the steel degradation is also discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon steel; Corrosion; Coal; Combustion; Ash deposits; Fe2O3 scale; Wrinkling; Buckling
1. Introduction In power generation plants, fluidized-bed boilers, such as fluidized-bed combustors (FBC), are in wide commercial use in a range of sizes, while burning different fossil fuels, although mostly coals. High temperature corrosion under the coal combustion environment has been a problem encountered in the materials being employed in the FBC system of the utility industry [1–4]. For a coal-fired FBC system, the degradation of the metal in service under the combustion environment is generally as a result of a synergistic effect of several factors, which can be summarized as follows: (1) attack of the corrosive gases from coal combustion, such as O2, SO2, HCl, H2O, CO, etc.; (2) deposition of fly-ashes (potentially forming aggressive salts at high temperature); and (3) erosion or abrasion induced by impact of the flying particles and the wakes of rising bubbles. The combined multiple factors make it difficult to clarify the degradation process of the metals employed in the FBC. Therefore, laboratory studies
have been more concerned with emphasis on a basic understanding of corrosion mechanisms [5–9]. In this work, we present observations of an A210-C carbon steel, which was covered by ash deposits and exposed a short-term thermal cyclic oxidation exposure under a simulated atmosphere of a freeboard of a coal-fired FBC. Erosion is not concerned in the study, which was intended to understand the degradation of carbon steel in the presence of deposits under exposure of a coal combustion environment.
2. Experimental Samples with dimensions of : 6× 5× 4 mm3 were cut from A210-C carbon steel (chemical composition in weight percent: 0.35C –1.06Mn –0.035P –0.035S – 0.01Si). The samples were finally grounded to 800 grid SiC paper. The corrosion testing was conducted by using the experimental set-up, which is presented schematically in Fig. 1. The corrosive gas atmosphere used
* Corresponding author. Tel.: + 1-270-745-5322; fax: + 1-270-745-5361. E-mail address: [email protected] (X. Peng). 0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 4 6 - 4
X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275 Table 1 Composition of corrosive flue gases introduced from FBC (in average) O2 (vol.%)
6
H2O (vol.%)
SO2 (vppm) HCl (vppm)
CO (vppm)
0.9
40
ND
135
Fig. 1. Schematic view of the experimental set-up for the corrosion testing by using the flue gas from a freeboard of coal-firing FBC. 1— inlet of gases from a coal-firing FBC, 2— connector, 3 —quartz tube, 4— furnace, 5 — deposit, 6 — crucible boat, 7 —sample, 8 —gas outlet.
simulated that measured at a location 1.9 m above the central plate of the practical coal-firing 0.1MWth WKU –FBC facility [10,11] using an on-line by a ZRF 1312 photo-acoustic multi-gas analyzer and IMR 7000 combustion-gas analyzer. The gas composition is listed in Table 1. Ash deposits collected from the freeboard wall of the FBC were used as the overlying deposit in the test. The chemical compositions by EDX are listed in Table 2; the calcium was mainly from limestone additions made to absorb sulfur oxides and/or reduce chlorine emissions. For corrosion testing, the metal samples were embedded into the deposit powder, which was pre-placed in a crucible boat, and then were pushed into chamber (the hot zone of the silicon tube in a TGA system). Afterwards, the test gas was introduced into the silicon tube, as seen in Fig. 1. The exposure temperature was set at 560 °C. The samples were cyclically oxidized by 3 h exposure at the temperature and subsequently 1 h furnace cooling to room temperature. After certain cycles of cycling oxidation, the samples were weighed. After the testing, the corroded samples were investigated using SEM/EDX. For comparison purposes, the sample was also oxidized by using the same condition of thermal cycling and same combustion gas, but without deposit powder addition. 3. Results Compared to exposure without deposit cover, the low carbon steel exhibited a significantly faster oxidation, and during cooling spallation of thicker and larger pieces of oxides occurred, especially at the edge of both the samples. Fig. 2 is the mass change of the steel under
271
Table 2 Elemental composition of deposit powder by EDX (by atomic percentage) Ca
Mg
K
Na
Al
S
Cl
Si
Fe
O
12.1
1.5
0.7
B0.1
3.5
6.8
5.7
5.9
7.7
Bal.
Fig. 2. Mass change of A210-C carbon steel with and without deposit addition after 12 h isothermal and 36h/12 cycles’ cyclic oxidation.
both isothermal and cyclic oxidation conditions. It can been seen that during isothermal condition oxidation is significantly faster with deposit addition than without deposit addition. SEM observation showed that the scale formed was very flat in the absence of ash deposits, but was undulated and separated from the steel in the case of deposit cover, as seen in Fig. 3. Fig. 3 also shows that under both conditions the local separation between the scale and the metal occurred, but more so with deposit addition. Under cyclic oxidation condition, the mass loss occurred after 9h/3 cycles’ thermal cycling. After 36h/12 cycles, the mass loss was 1.7 mg cm − 2 without deposit cover, which is very low compared to 10.3 mg cm − 2 with deposit cover (Fig. 2). The oxides formed are mainly hematite (Fe2O3). Magnetite (Fe3O4) layer and other corrosion phases, such as iron sulfides or iron chlorides were not found obviously (in fact, if the phases formed, they should appear a color contrast different from that of Fe2O3 under SEM), even in the case of deposit cover. EDX analyses of the composition of the scale formed without deposits indicated that the scale contained impurities, which are listed in Table 3. For both conditions, the content of S and Cl in the scale is not much different. It can be presumed that the two elements mainly resulted from the flue gas. Mn and part of Si were from the steel. In the absence of ash deposits, impurities incorporated into the scale, such as Ca, Mg, K, Na, Al, Co, Ni, Si, etc., were detected. Apparently, the cation impurities were from flying ash, which was entrained by the flue gas from the FBC freeboard and entered the furnace. However, in general, the content of these impurities in the scale was greatly increased, when
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the steel was covered with ash deposits. This may be one reason that oxidation becomes faster, when the steel is covered by deposits, as will be discussed later. SEM observation showed after exposure of 36h/12 cycles at 560 °C, local oxide cracking and spalling was observed in the absence of deposit. This implied that the oxide scale formed under the flue gas atmosphere produced by combustion of the coal used was per se fragile and poorly adherent. However, in the case, the
Fig. 4. Secondary electron images of corroded scale formed: (a) general top-view showing surface scale convoluted; (b) fracture section revealing the scale layered and buckled and also the scale spallation appeared between the layers and at the oxide layer-metal interface.
Fig. 3. Backscattered electron images of the cross-sectional samples showing the scale flat without deposit cover (a) but undulated and separated from the metal with deposit cover (b). The bright spots above the scale are copper particles included the polymer, which was employed for preventing the scale exfoliation during sample preparation. Table 3 EDX results of the elemental content within the scale near the interface after 12 h exposure (by atomic percentage)
O S Cl Si Mn Co Ni Al Ca Mg K Na Cr Fe
Without deposit
With deposit
22.1 9 0.5 0.55 9 0.18 0.57 9 0.09 2.33 9 0.56 0.53 9 0.03 0.33 9 0.20 0.10 9 0.05 0.30 9 0.17 B0.10 0.12 9 0.06 B0.10 B0.10 B0.10 Bal.
20.39 0.7 0.639 0.20 0.76 9 0.24 1.369 0.30 0.54 9 0.07 0.73 9 0.25 0.23 9 0.14 1.90 9 0.15 0.49 9 0.09 0.28 9 0.11 0.499 0.18 0.24 9 0.10 0.159 0.03 Bal.
surface scale remained very flat. While with the deposit cover, the scale was wrinkled (convoluted), as seen in Fig. 4a. The white particles (arrowed) are the residual deposits. Most particles could be brushed off the scale surface, indicating loose contact with the oxide. No experimental evidence that would indicate the presence of low-melting salts from reactions between the deposits and the oxide was found during the short-term exposure. As is evident in Fig. 4a, the surface scale was wrinkled and scale cracking (area ‘A’) and buckling (area ‘B’) occurred at certain regions. Fig. 4b is the fracture section of the scale close to the sample edge, revealing the residual scale consisted of two layers, and was convoluted. The residual oxide layers were separated and spallation occurred between the layers or between the oxidemetal interface. Fig. 5 presents SEM observation of the scale cross-section, clearly showing an undulated scalemetal interface. Fig. 5a shows that the undulated scalemetal interface maintained contact, suggesting that the buckling occurred after wrinkling. As will be discussed, wrinkling occurred during oxidation and buckling during cooling. Fig. 5b is another cross-section of the scale formed, showing that the continuous waved oxide layers were split apart into fragments due to occurrence of
X. Peng et al. / Materials Science and Engineering A332 (2002) 270–275
cracks within the layer (normal to the oxide scale-metal interface). The scale-metal interface separation could be frequently observed. Sulfur, sometimes together with other impurities, such as silicon, calcium, potassium (sodium), aluminum, chlorine, etc., could be probed obviously at the interface by the EDX analyses. However, it is not clear in what state these elements appeared as there.
4. Discussion According to the results, it is clear that the added ash deposits accelerated metal degradation. It has been proposed that the accelerated corrosion by ash deposits may result from formation of low-melting salts. However, such low-melting salts were not characterized in this work. The thermal cycling and SEM observations suggested that the scale formed with deposit addition is extremely sensitive to spallation, which appeared as a consequence of the contribution of oxide wrinkling (Figs. 4 and 5). Wrinkling is a deformation mode by which elastic stain can be reduced [12]. Wrinkling of thermally-grown oxide has been suggested to be a result
Fig. 5. Backscattered electron images of the scale cross-sections showing the development of the undulant oxide layers and the separation between the layers. (a) Separation between two waved oxide layers. (b) The oxide layers were split into fragments due to formation of cracks normal to the scale-metal interface. The separation between the scale-metal interface is also visible at some region.
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of oxide creep for releasing high compressive stresses, which developed partly due to the scale lateral growth, and as a result growth of new oxides within the scale, due to the inter-diffusion of cations and anions during oxidation [13–15]. In this work, it shows that the oxidation rate of the metal is significantly higher with deposit cover than without deposit cover. Hence, it could be assumed that the growth mechanism of the scale was changed with deposits in respect to oxidation without deposits. The increased lateral or inward growth may occur, as a result of the incorporation of the impurities with low valence, such as Ca, Mg, K, Na, Ni, Co, from the deposits. It is well known that the doping of impurity may change physical defect structure of oxides and thusly have an effect on transport of ion species in them. The incorporation of impurities with low valence probably increased the concentration of oxygen vacancy in the scale. Hence, as a result of an increased attack of oxygen, a fast growing of the scale will occur, with a great contribution of new oxide formed within the scale and/or the metal-scale interface. This leads to the fast build-up of high growth compressive stresses, which may be high enough to wrinkle the oxide and the metal during oxidation. Moreover, compared to oxidation without deposits, the addition of deposits leads to the possibility of a change in content of oxygen, moisture, HCl (Cl2) or SO2 at the metal surface. However, in this study, the deposits were loosely contacted with the metal (suggesting that the change in gas composition may not be evident) and other phases except for iron oxides were not detected in the scale. Taking it into account, this change may be not the main factor responsible for the increased oxidation of the deposit-covered steel during the short-term exposure. The SEM observations proposed that with deposit cover scale buckling occurred during cooling and the buckle usually appeared at the vertex of the undulated interface. During thermal cycling, the development and extension of the buckle will lead to the separation of the scale from the metal and finally scale spallation. Without deposits, the scale separation and spallation also appeared. This implies that the iron oxide scale adhesion to the steel is per se poor. However, compared to the cycling without deposits, the local scale buckling made the scale separation and spallation an easier process in the presence of deposits. In the later case, the degradation of the steel appears to follow a ‘wrinkling– buckling–spalling’ route, as suggested in Fig. 6. The progression of the degradation process during the thermal cycling is presented below. When oxidation starts, rapid growth of the oxide results in the fast accumulation of growing compressive stresses which eventually are sufficient to cause wrinkling of oxide with metal. If the scale is initially adherent to the metal and the compressive stress in the scale
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Fig. 6. Schematic process of scale damage during the thermal cyclic oxidation.
exceeds the creep/yield strength of the alloy surface, an undulant scale-metal interface will develop, resulting in a tensile stress at the vertex of the interface and compress stress at the valley of the interface during the subsequently cooling (Fig. 6a). Such stresses are produced partly due to the thermal expansion mismatch between the oxide and the metal. As the extent of wrinkling enlarges, the tensile stresses become high enough to trigger the scale buckling (Fig. 6b), probably through the way of bending the oxide from the metal (Fig. 5). The susceptibility of the interface de-bonding in the presents of the ash deposits increases the available energy rate for the buckle propagation [16]. As a buckle propagates, the compressive stresses within the scale are further relaxed. Meanwhile, the buckling-related deformation gives rise to in-plane tensile stresses in the scale. If scale spallation does not occur at once as a consequence of buckle initiation, the stresses will accumulate during the following thermal cycling through further increasing the deflection of the oxide scale [17]. When the stresses exceed, the flexural strength of the oxide cracks form in the scale (Fig. 6c). Cracks may also form in other regions of the curved layer. For instance, if the defect size in a given location is larger, the tensile stresses (or shear stresses) will initiate cracking of the scale. Once cracks open to the gas atmosphere form in the scale, the underlying metal will be exposed to the combustion gases and lead to formation of new oxide on the metal (Fig. 6d). The extension of the buckling-associated fatigue crack growth along the interface produces space for the full development of the secondary oxide layer (Fig. 6e). By this route, separated oxide layers form during the continued cycling, as seen in Fig. 5. Finally, spallation of
layered scale occurs through linking-up of cracks at local regions, as seen in Fig. 6e. Based on the mode and the observations, prevention of the oxide wrinkling under the deposit-free condition is presumed to be a result of a relative slow-growing of the scale, by which the developed stresses could be accommodated by other creep mechanism except wrinkling. In the case, the detachment of the scale from the metal occurs not as readily as it does on the wrinkled interface, due to lack of driving force, such as out-ofplane tensile stresses and shear stresses. As a result, the scale formed was more buckling and spalling-resistant. Hence, the comparison result of this study suggests that during the short-term of thermal cycling, the accelerated degradation of the metal with deposit addition is a result of susceptibility of wrinkling and buckling.
5. Summary Compared to corrosion without ash deposit additions, the degradation of A210-C carbon steel with the deposit cover at 560 °C under the simulated coal-firing combustion environment was faster and the iron oxide scale formed was wrinkled and buckled. The ash deposits have a profound effect on accelerating the degradation of the metal at high temperature by promoting oxide growth and thusly oxide wrinkling, which would favor the scale breakdown, following a route of scale ‘wrinkling– buckling – spalling’ during the thermal cycling. The oxide wrinkling may be a result of an increased oxidation with deposits, which may mainly result from incorporation into the growing iron oxide scale of low valence impurities, such as Ca, Mg, K, Na,
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Co, Ni, from the deposits. This incorporation may change the oxide defect structure and consequently make it faster the lateral and inward growth of the oxide.
Acknowledgements The authors are indebted to Dr K. Liu, graduate students D. Zou, C. Hu, R. Heltsley, M. Zheng, L. Wells and undergraduate student P. Hack for assistance in 10 days’ FBC operation. One of the authors XP is grateful to Dr I.G. Wright at Oak Ridge National Laboratory for helpful comments.
References [1] E.A. Rogers, J.C. Holder, A.J. Minchener, A.J. Page, R.D. Lanauze, G.G. Thurlow, Materials problems in fluidized-bed combustion systems, May 1980, EPRI report No. CS-1449. [2] J. Stringer, I.G. Wright, J. Mater. Energy Sys. 8 (1986) 319.
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[3] A.J. Minchener, A.V. Levy, in: Proc. Conf. on Corrosion –Erosion– Wear of Materials at Elevated Temperature. NACE, Houston, TX, 1987, p. 152. [4] K. Nakagawa, in: Proc. High Temp. Corros. (III), Les Embiez, France, May 1992, J. De Physique. 4 (colloque C9) (1993) 787. [5] W.T. Bakker, R.A. Perkins, Van Liere, J. Mat. Perf. 1 (1985) 9. [6] K. Natesan, Corrosion 41 (1985) 655. [7] H.J. Grabke, Meadowcroft (Eds.), Guideline for Methods of Testing and Research in High-Temperature Corrosion. European Federation of Corrosion Publication Number 14, Institute of Materials, 1995. [8] F.H. Stott, J.F. Norton, Mat. High Temp. 14 (1997) 43. [9] J.R. Nicholls, Mat. High Temp. 14 (1997) 219. [10] K.L. Liu, W.P. Pan, J.T. Riley, Corrosion 56 (2000) 298. [11] X. Peng, K.L. Liu, P. Hack, W.P. Pan, J.T. Riley, D.L. Reuss (Eds.), Technical Meeting of Central States Section on Combustion Fundaments and Applications. Indianapolis, Indiana, USA, April 16 – 18, 2000, p. 80. [12] Z. Suo, Mech. Phys. Solids 43 (1995) 829. [13] F.A. Golightly, F.H. Stott, G.C. Wood, Oxid. Met. 10 (1976) 163. [14] F.N. Rhines, J.S. Wolf, Metall. Trans. 1 (1970) 1701. [15] V.K. Tolpygo, D.R. Clarke, Acta Mater. 46 (1998) 5167. [16] J.S. Wang, A.G. Evans, Acta Mater. 47 (1999) 699. [17] H.E. Evans, Internal Mat. Rev. 40 (1995) 1.