Corrosion Mechanism of Alloy 310 Austenitic Steel beneath NaCl

Jun 27, 2013 - Alkali chlorides and sulfates are well-known corrosive species found on superheater and reheater boiler tube deposits. The corrosivity ...
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Corrosion Mechanism of Alloy 310 Austenitic Steel beneath NaCl Deposit under Varying SO2 Concentrations in an Oxy-fuel Combustion Atmosphere Manoj Paneru,* Gosia Stein-Brzozowska, Jörg Maier, and Günter Scheffknecht Institute of Combustion and Power Plant Technology (IFK), University of Stuttgart, Pfaffenwaldring 23, 70569 Stuttgart, Germany ABSTRACT: Alkali chlorides and sulfates are well-known corrosive species found on superheater and reheater boiler tube deposits. The corrosivity of alkali chlorides toward Fe−Cr alloys is connected to the formation of molecular chlorine at the oxide surface. Chlorine can accelerate metal oxidation, establishing a self-sustained cyclic process of metal chloride and metal oxide formation, a corrosion phenomenon referred to as “active oxidation”. In the convective zone of a boiler, chlorine can be released by either sulfation of alkali chlorides by SOx or reaction of alkali salts (NaCl/KCl) present in deposits on heat-exchanger surfaces with metal and/or their oxides or both. To minimize the possible risk of corrosion induced by alkali chlorides, conversion of these chlorides to sulfates is discussed as an attractive solution. The aim of this study is to evaluate the influence of higher SO2 concentrations, during oxy-fuel combustion, on in-deposit sulfation of chloride salts and related corrosion mechanisms. To signify the influence of SO2, one exposure test (OI) was performed without SO2, while the other exposure test (OII) was performed with high SO2 content (1.5%), possibly obtained during oxy-fuel combustion while using a sulfur-rich coal. Synthetic NaCl salt was used as a deposit layer over alloy 310 (Fe−25Cr−21Ni) to evaluate its corrosion behavior at 650 °C in both atmospheres. The exposure atmosphere represents a typical oxy-fuel flue gas composition (test OI, 77.5% CO2 + 4.5% O2 + 18% H2O; test OII, 76% CO2 + 4.5% O2 + 18% H2O + 1.5% SO2). The exposure duration in each test was equal to 350 h. To evaluate the influence of the temperature, FactSage calculations were performed. Electron microprobe analysis (EMPA) was used to observe the extent of corrosion and elemental distribution at sample cross-sections to identify corrosion products. The depth of attack and corrosion progress faced by alloy 310 beneath the NaCl deposit was significantly reduced in the presence of SO2 in the exposed atmosphere. In the absence of SO2, “active oxidation” was the dominant corrosion mechanism, along with strong evidence of internal attack, while in the presence of SO2, none of these corrosion mechanisms was observed. Instead, the local sulfidation attack observed in the presence of SO2 points toward the possibility of type II “hot corrosion”.

1. INTRODUCTION

NaCl + 0.5SO2 + 0.5H 2O + 0.25O2 → 0.5Na 2SO4

Alkali chlorides and sulfates are well-known corrosive deposits found in superheater and reheater boiler tubes.1 Comparatively, alkali chlorides are more corrosive than their sulfates because of their reactivity toward the chromium/chromium oxide and iron/iron oxide layer.2,5−7 NaCl and KCl are common chlorides found in deposits, formed during coal and biomass combustion, respectively.3−7 Numbers of experimental investigations3−7 have been published on corrosion potential of alkali chlorides, with a general agreement that the presence of alkali chloride can cause very high corrosion rates.2 The corrosivity of alkali chlorides toward Fe−Cr alloys is connected to the formation of molecular chlorine at the oxide surface.3 This molecular chlorine accelerates metal oxidation, establishing a selfsustained cyclic process of metal chloride and metal oxide formation, a corrosion phenomenon referred to as “active oxidation”.3−6 In the convective zone of a boiler, chlorine can be released by either sulfation of alkali chlorides by SOx (reaction 1) or reaction of alkali salt present in deposits (on boiler pipe surfaces) with the metal and/or their oxides or both (reaction 2).5 In an oxidizing atmosphere, molecular chlorine is liberated from HCl by reaction 3. © 2013 American Chemical Society

+ HCl

(ΔG < 0)

(1)

2NaCl + x M + H 2O + 1.5O2 → Na 2MxO4 + 2HCl (ΔG < 0)

(M = Cr, Fe)

(2)

2HCl + 0.5O2 → Cl 2 + H 2O

(3)

Austenitic steel does not offer the desired corrosion resistance against alkali chlorides.2 Alkali chlorides react with chromium/ chromium oxide, and an unprotective layer of alkali chromates is developed at the alloy surface.5 Destruction of chromia facilitates inward diffusion of corrosive species, such as chlorine and sulfur, present in the flue gas to combine with the alloy elements. Still, austenitic steel is suitable boiler pipe material that can provide adequate mechanical strength (creep strength) up to 750 °C9 (metal temperature). Therefore, austenitic steel is a widely accepted candidate material for superheater tubing to realize the boiler operating at supercritical and ultraSpecial Issue: Impacts of Fuel Quality on Power Production and the Environment Received: March 29, 2013 Revised: June 19, 2013 Published: June 27, 2013 5699

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Table 1. Laboratory Corrosion Testing alloy

salt

test (OI)

test (OII)

alloy 310

NaCl

650 °C, 350 h, 77.5% CO2 + 4.5% O2 + 18% H2O

650 °C, 350 h, 76% CO2 + 4.5% O2 + 18% H2O + 1.5% SO2

3. EXPOSURE TEST Alloy 310, austenitic steel tube (outer diameter of 54 mm and thickness of 8 mm) was radially cut into approximately 20 mm long rings. Each ring was than axially cut into 4−5 arc pieces. Each cut tube arc was rinsed with alcohol at the end to remove the impurities coming from pre-processing. Subsequently, the tube arc was placed inside a ceramic cup and manually covered with a salt layer at both sides, however simultaneously keeping the middle section free. Each sample was continuously exposed in a laboratory furnace under the test conditions as reported (see Table 1). One exposure test, assigned here as OI, was performed in the absence of SO2, while the other, assigned here as OII, was performed in the presence of 1.5% SO2. Both atmospheres represent an oxy-fuel combustion atmosphere. After the exposure, each sample was embedded in the epoxy resin. The embedded samples were than ground and polished. Use of water was avoided during metallographic preparation, owing to water solubility of NaCl. The samples were analyzed using an electron microprobe. The original gray color micrographs obtained from the electron microprobe were converted into a 16 color bit using ImageJ software for further observation and analysis. Some logical operations of ImageJ were also used to process the micrographs to discuss the scale formation.10

supercritical temperatures. To minimize the possible risk of alkali-chloride-induced corrosion, conversion of these chlorides to sulfates is discussed as an attractive solution.5,17 In a real boiler situation, the sulfation of chloride salts occurs either on the surface of ash particles or at the deposit on the boiler tube surface.2 If sulfation occurs on the surface of ash particles suspended in flue gas, then the boiler deposit contains barely any alkali chlorides; however, it might not always be the case. Nevertheless, because the chloride salts have a transition life,2 in-deposit sulfation of chloride salts and its influence on boiler pipe corrosion need to be evaluated. The aim of this study is to evaluate the influence of higher SO2 concentrations typical for oxy-fuel combustion on indeposit sulfation of chloride salts and related corrosion mechanisms. To signify the influence of SO2, one exposure test (OI) was performed without SO2, while the other exposure test (OII) was performed with SO2 (1.5%). Synthetic NaCl salt was used as the deposit layer over alloy 310 (Fe−25Cr−21Ni) to evaluate its corrosion behavior at 650 °C (targeted ultrasupercritical steam temperature). The idea of comparing the null concentration of SO2 to a significantly higher concentration (1.5%) of SO2 was to weigh the corrosion risk posed by chlorine when released via different reaction mechanisms. Comparing the null concentration of SO2 to a higher concentration of SO2 also provides an opportunity to compare the corrosion risk associated during oxy-fuel combustion of high sulfur coal with and without sulfur control.

4. RESULTS AND DISCUSSION The results obtained from this experimental analysis are divided according to the exposure atmospheres. On the basis of backscattered electron (BSE) images and elemental maps generated using electron microprobe analysis (EMPA), the possible corrosion mechanism is discussed. 4.1. Test at No SO2 in the Atmosphere (OI-NaCl). The visual observation of the sample surface clearly indicates loose and peeled-off scale after exposure (see Figure 1). Salt seems to be a little sintered after exposure. It was difficult to remove the sample out of the ceramic cup. Apparently, the corrosion product caused the sample to swell and clog in the cup. In the BSE image (see Figure 2), buckling and cracking are clearly visible, along with the porous scale, indicating an accelerated and uncontrolled oxidation. For a closer observation, individual images from three different areas (a, b, and c; see Figure 2) were generated. Area “a” denotes the alloy substrate; area “b” denotes the scale cross-section; and area “c” denotes the outermost cross-section, apparently the deposit corrosion product. The alloy surface clearly indicates the evidence of an internal attack (see a in Figure 6). Large depletion of chromium at the alloy substrate can be observed close to the surface (see Figure 6). The chromium-depleted zone is enriched with iron and nickel (see Figure 6). The high concentration count of iron and nickel in the chromium depletion zone is possibly not due to diffusion of these ions from the alloy substrate to this region but simply due to the loss of chromium from this region. The concentration distribution of chromium in the porous scale seems to increase away from the alloy substrate. Oxygen (more toward the outer scale) and chlorine (more inward) are disturbed throughout the porous scale, while sodium is completely absent (see Figure 6).

2. FACTSAGE EQUILIBRIUM CALCULATION Equilibrium calculations were performed for reactions 4−6 using the FactSage database. Reaction 6 is considered as a possible reaction to incorporate the higher partial pressure of CO2 in the exposure atmosphere. 2NaCl + 0.5Cr2O3 + H 2O + 0.75O2 → Na 2CrO4 + 2HCl (4) 2NaCl + SO2 + H 2O + 0.5O2 → Na 2SO4 + 2HCl

(5)

2NaCl + CO2 + H 2O → Na 2CO3 + 2HCl

(6)

Figure 5 shows the activity versus temperature curve of the equilibrium products for the given inputs. Products with activity equal to 1 are thermodynamically stable products in the given equilibrium condition. For OI (no SO2), in the absence of SO2, Na2CrO4 is a stable solid product until T ≤ 750 °C (see reaction 4). A salt liquid phase (solution of NaCl, NaOH, and Na2CO3) appears to be thermodynamically stable after ca. T ≥ 800 °C; NaCl appears to be significantly dominant. For OII (with SO2), in the presence of SO2, Na2SO4 is a stable solid product until ca. T ≤ 750 °C (see reaction 5). A salt liquid phase (mixture of Na2SO4, NaCl, and NaOH) appears to be thermodynamically stable after ca. T ≥ 650 °C; Na2SO4 appears to be dominant. The mole fraction of NaCl in the salt liquid solution increases with the temperature and exceeds Na2SO4 at ca. T ≥ 950 °C. During both cases, reaction 6 is insignificant because Na2CO3 does not appear as a stable solid/liquid compound. A higher CO2 concentration during the oxy-fuel combustion case may not be important for chloride-related corrosion. 5700

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Figure 3. Sample OII-NaCl (left) before and (right) after 350 h of exposure at 650 °C.

Figure 1. Sample OI-NaCl (left) before and (right) after 350 h of exposure at 650 °C.

Figure 4. BSE image of the OII-NaCl sample after exposure.

Figure 2. BSE image of the OI-NaCl sample after exposure.

The corrosivity of NaCl and the reaction mechanism involved in the high-temperature corrosion of steel caused by chloride salts have been widely studied.2−8,11−17,22 NaCl can react with both chromium or its oxides to release chlorine. This reaction occurs at the alloy deposit interface; therefore, the released chlorine possibly becomes entrapped in the alloy matrix, instead of being released to the exposure atmosphere. The uncontrolled oxidation, formation of porous scale, and depletion of chromium from the alloy substrate can be explained by the reaction mechanism called “active oxidation” triggered by chlorine.3,11 To explain the possible mechanism, the whole reaction sequence of resulting “active oxidation” in the given exposure condition is summarized below based on previous mentioned publications. The exposure atmosphere is oxidizing; therefore, NaCl reacts predominantly with chromium oxide (formed by selective oxidation). The Gibbs free energy of Na2CrO4 formation by reaction 7 is greater than 0, which means that reaction 7 is thermodynamically not favored. However, the reaction with Cr is thermodynamically favored to give the same

product (see reaction 2; M = Cr). Therefore, instead of Cr2O3, Cr might be the actual reacting alloy constituent. Basically, the formation of Na2CrO4 and release of Cl2 is responsible to for “active oxidation”. The simultaneous presence of Na, Cr, and O (see Figure 7) apparently indicates the presence of Na2CrO4. Ideally, consumption of NaCl is not required to continue the corrosion mechanism further because Cl2 is not being consumed during the process of “active oxidation”; Cl2 is constantly recycled within the alloy matrix. 4NaCl + Cr2O3 + 2H 2O + 1.5O2 → 2Na 2CrO4 + HCl (ΔG > 0, 650 °C)

(7)

HCl can directly react with metal to form metal chloride or can release chlorine by reaction 8 and form metal chlorides by reaction 9. 2HCl + 0.5O2 → Cl 2 + H 2O 5701

(8)

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Figure 5. Activity versus temperature for the equilibrium reaction products [right, OI (no SO2); left, OII (with SO2)].

Figure 6. BSE image (top left) and element maps of sample OI-NaCl (substrate cross-section) after 350 h of exposure at 650 °C.

Figure 7. BSE image (left) and element maps of sample OI-NaCl (corrosion product cross-section) after 350 h of exposure at 650 °C.

Table 2. Gibbs Free Energy of Metal Chloride (Reaction 9) and Metal Oxide (Reaction 10) Formations (Calculated from FactSage)

x (Cr, Fe, Ni) + yCl2 → x (Cr, Fe, Ni) chlorides (ΔG < 0, 650 °C)

(9)

x (Cr, Fe) chlorides + yO2 → x (Cr, Fe) oxides + yCl2 (10)

Table 2 shows the Gibbs free energy of various metal chloride formations. The formation of metal chlorides is thermodynamically favorable in the given conditions. A comparison of ΔG (see Table 2) of chromium chloride, iron chloride, and nickel chloride formations shows that the formation of chromium chloride (CrCl3) is highly favored among all metal chloride formations. Therefore, chlorine preferably attacks chromium. Chromium chlorides are highly unstable; therefore, their rapid outward migration results in the loss of chromium from the alloy substrate (see Cr in Figure 6). Most of the chromium chlorides formed eventually migrate toward the scale gas interface and finally oxidize by reaction 10. The outward diffusion of metal chlorides creates microchannels in the scale. The chlorine released during metal chloride oxidation finds its way through these channels again to combine with new metal

metal chloride

ΔG at 650 °C (kJ)

CrCl3 CrCl2 FeCl2

−335 −280 −226

FeCl3

−209

NiCl2

−167

metal oxide Cr2O3 Cr2O3 Fe2O3, Fe3O4, and FeO Fe2O3, Fe3O4, and FeO NiO

ΔG at 650 °C (kJ) −107 −162 −64, −44, and 21 −81, −62, and 44 11

and forms metal chloride, creating a closed loop of continuous metal chloride and metal oxide formations. Table 2 shows ΔG values of metal oxide formation from their respective chlorides. Chromium and iron chlorides finally become oxidized to their respective oxides when their respective ΔG is negative (see the fourth column from the left in Table 2). However, nickel chloride oxidation is not thermodynamically favorable; therefore, it might remain in the 5702

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Figure 8. BSE image (top left) and elemental maps of sample OII-NaCl after 350 h of exposure at 650 °C.

Figure 9. BSE image (left) and modified elemental maps of sample OII-NaCl after 350 h of exposure at 650 °C.

suffered a lesser extent of corrosion compared to the OI-NaCl sample. The original sharp edge of the NaCl salt crystal appears more granular and sintered after exposure in a SO2-rich atmosphere (see Figure 3). The depth of attack and severity of corrosion (see Figure 4) has significantly reduced in comparison to the OI-NaCl sample (see Figure 2). A BSE image for a closer view (see Figure 8) (cross-section image of the alloy surface area marked “d” in Figure 4) and element maps of the same cross-section (see Figures 8 and 9) show an increasing concentration of chromium and a loss of iron together with nickel at the alloy surface. Enrichment of chromium at the OII-NaCl sample surface is completely different from what was observed with the OI-NaCl sample, where a significant loss of chromium was observed. Iron seems to migrate out from the alloy substrate and is diffused within deposit (see encircled area in Na and Fe in Figure 8). The diffusion of iron from the alloy substrate to the deposit indicates that there has been an interaction between deposit salt and iron. Chlorine is not observed in the entire corrosion product (see Cl in Figure 8). The presence of SO2 in the exposure atmosphere almost completely suppresses chlorine-induced “active oxidation”. The suppression of “active oxidation” in the presence of SO2 in the exposure atmosphere contradicts the previous findings from various authors.4,5,8,15,16 They argued that the presence of SO2 enhances the release of chlorine because of sulfation reactions; therefore, during in-deposit sulfation of alkali chlorides, a high partial pressure of chlorine is generated, which enhances the chlorine-related corrosion, basically “active oxidation”. Most of the investigations that pointed out the synergetic effect of alkali chloride and SO2 were performed either with a lower concentration of SO2 in the exposure atmosphere or at real air-fired boiler operating conditions, where the concentration of SO2 never reached such a high value as used during this investigation. As indicated before, the possible explanation for the contradictory results is apparently connected to the higher

scale as chlorides. Nickel oxides found in the scale are rather formed because of direct oxidation of nickel. The porous scale developed beneath the OI-NaCl sample apparently consists of chromium oxide toward the outer side and iron oxides toward the inner side (see Cr, Fe, and O in Figure 6), with nickel chloride distributed in the scale (see Ni and O in Figure 6). Some nickel oxides can also be found in the scale (see Ni and O in Figure 6) and possibly some sodium chromate at the deposit scale interface. The internal attack (see Figure 6) was consistent with the previous findings.13,14 Grabke et al.14 suggested that the intergranular attack was due to the reaction between the carbides and molecular chlorine. Cr23C6 is the predominant carbide precipitated along the grain boundaries in austenitic steel.13,14 Chlorine can reach the grain boundaries through cracks and fissures and react with chromium carbide in the presence of oxygen, according to reaction 11. Cr23C6 + 23Cl 2 + 3O2 → 23CrCl 2 + 6CO (ΔG < 0, 650 °C)

(11)

Chromium chloride, owing to its high vapor pressure, escapes, creating a string depletion of chromium (see Cr in Figure 6) along the grain boundary. The loss of chromium at the grain boundaries increases the concentration of Fe and Ni (see Fe and Ni in Figure 6). Tsaur et al.11 has pointed out the possibility of a eutectic mixture NaCl−Na2CrO4 melting at 577 °C. It cannot be confirmed whether the formation of the eutectic mixture was a case or not. However, if eutectic melts are formed, then they are accumulated in the scale cracks and further enhance the severity of corrosion. 4.2. Test at SO2-Rich Atmosphere (OII-NaCl). The visual comparison of sample OI-NaCl (see Figure 1) and OII-NaCl (see Figure 3) after exposure clearly indicates the difference in corrosion behavior. The OII-NaCl sample seems to have 5703

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The reaction sequence that describes the “hot corrosion” mechanism can explain the formation of alkali−metal sulfate complexes. Na2SO4 can react with iron oxide to form alkali metal trisulfate according to reaction 12.

concentration of SO2 (1.5%, ∼15 000 ppm) implied during this investigation. The partial pressure of SO2 in the exposure atmosphere might be high enough to start the sulfation reaction right away at the deposit gas interface (i.e., away from the alloy surface). The chlorine released therefore does not reach the surface to react with alloy constituents and, instead, escapes to the atmosphere. Apparently, it can be said that the presence of a significantly higher SO2 concentration in the boiler atmosphere can be helpful in minimizing the risk of chlorine-related corrosion. However, it should be noted that the laboratory test results cannot be generalized for a real case, where other trace flue gases and deposit mineral species might also have other influences. A further investigation is necessary to identify appropriate S/Cl (or adequate SO2 partial pressure) for chloride sulfation right away at the deposit gas interface. In the presence of SO2, the sulfation reaction is highly favored (see reaction 12). Boonsongsup et al.18 studied the overall kinetics of NaCl(s) sulfation in a temperature range of 400−600 °C and found that the reaction kinetics depends upon adsorption of SO2 and O2 on the surface of NaCl to form SO3. In the experimental condition investigated during this study, conversion of SO2 to SO3 might be favored because the exposure temperature (650 °C) is within the temperature range of maximum SO2 to SO3 conversion and Fe2O3 can act as a catalyst to promote this reaction. The corrosion mechanism of the OII-NaCl sample is therefore dominated by the interaction between Na2SO4 (i.e., sulfated NaCl) and the alloy constituents. A cross-section observation shows a three-layered scale comprising a nickel sulfide sandwiched between an outer iron oxide and an inner chromium oxide layer (see Figure 4). The chromium oxide layer is thicker and seems to be growing locally inward from the original tube surface. The only sulfur source in the given atmosphere is SO2 in the bulk gas. The direct sulfidation of the alloy constituents induced by SO2 in the bulk gas is almost impossible in austenitic steel because chromium in the form of chromia is highly resistant against sulfur attack. Therefore, the sulfidation attack is apparently induced by Na2SO4 (i.e., sulfated NaCl). The outward migration of iron from the alloy substrate and its distribution within the deposit and the presence of oxygen and sulfur in the same region (see Na, Fe, S, and O in Figure 8) indicate the presence of some alkali iron sulfate complexes. The elemental maps have been evaluated for the presence of alkali chromates as well, but the presence was not noticeable because it can be seen that chromium does not seem to be distributed within the deposit (see Figure 8). Eliaz et al.19 has discussed the mechanism of “hot corrosion” and distinguished it into two types: type I “hot corrosion” mainly occurring within the temperature range of 850−950 °C and type II “hot corrosion” mainly occurring within the temperature range of 650−850 °C. Type I “hot corrosion” cannot be an issue for this experimental study. Type II “hot corrosion” is characterized by sulfidation and a depletion region beneath the porous and non-protective scale and the localized form of attack,19 both resembling the results obtained during this experimental study. For type II “hot corrosion”, the formation of eutectics has been suggested by Eliaz et al.19 FactSage equilibrium calculations suggest the presence of salt liquid in the exposure temperature range (see Figure 5). Birks21 also suggested the presence of NaCl−Na2SO4 eutectics in a similar study.

3Na 2SO4 + 3SO2 + Fe2O3 → 2Na 2Fe(SO4 )3

(12)

Alkali metal trisulfate reacts with iron from the substrate according to reaction 13. 2Na 2Fe(SO4 )3 + 19Fe → 6Fe3O4 + 3FeS + Na 2S

(13)

Na2S is oxidized to Na2SO4 by reaction 14, and FeS is oxidized to Fe3O4 by reaction 15. Na 2S + 2O2 → Na 2SO4

(14)

3FeS + 5O2 → Fe3O4 + 3SO2

(15)

Nickel because of its higher affinity toward sulfur reacts with SO2 (during FeS oxidation) released to the alloy matrix and develops nickel sulfide beneath iron oxide. This reaction sequence establishes the outward migration of iron from the alloy substrate. The vacant space left in the substrate by outward migration of iron is filled by chromium oxide formed because of the inward diffusion of oxygen through porous iron oxide. Nickel sulfide is thus sandwiched between iron and chromium oxide.

5. CONCLUSION To evaluate the influence of the temperature, equilibrium calculations were performed using FactSage. In the absence of SO2, Na2Cr2O4 appears as a stable liquid apparently because the available FactSage database does not take into account a eutectic mixture of Na2CrO4 and NaCl with a melting point at 557 °C. In the presence of SO2, FactSage confirms the formation of a salt liquid in the exposure temperature possibly because of the formation of a eutectic mixture of NaCl and Na2SO4. Only the role of salt deposit and influence of SO2 on corrosion are reported here. It should also be noted that the exposure atmosphere has a high amount of H2O vapor. The effect of water vapor on vaporization of the Cr2O3 scale has been well-discussed by Asteman et al.20 A possible influence of a higher fraction of CO2 in NaCl deposit transformation was not incorporated in this study. Moreover, the thermodynamic study did not suggest a significant interaction between NaCl and CO2. The following conclusions can be made on the basis of the results from performed laboratory exposures under reasonable oxy-fuel combustion conditions. (1) Influence on salt: In the absence of SO2, the interaction of NaCl with the alloy constituents (Cr or Cr2O3) to form Na2CrO4 was dominant, while in the presence of SO2, NaCl predominantly reacts with exposure gas atmosphere to form Na2SO4. (2) Influence on the corrosion mechanism of alloy 310: The corrosion mechanism was overruled by “active oxidation”, along with a strong evidence of an internal attack in the absence of SO2 in the exposure atmosphere. In the presence of SO2, neither the “active oxidation” nor the evidence of internal attack was significant; instead, a strongly observed local sulfidation attack points toward the possibility of type II “hot corrosion”. When the reaction between chromium oxide and NaCl is dominant in releasing chlorine from the salt, chlorine poses a higher corrosion risk because of its possibility to react with alloy 5704

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(17) Aho, M.; Ferrer, E. Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass. Fuel 2005, 84 (2−3), 201−212. (18) Boonsongsup, L.; Lisa, K.; Frederick, W. J., Jr. Kinetics of the sulfation of NaCl at combustion conditions. Ind. Eng. Chem. Res. 1997, 36 (10), 4212−4216. (19) Eliaz, N.; Shemesh, G.; Latanision, R. M. Hot corrosion in gas turbine components. Eng. Failure Anal. 2002, 9 (1), 31−43. (20) Asteman, H.; Svensson, J.-E.; Johansson, L.-G. Oxidation of 310 steel in H2O/O2 mixtures at 600 °C: The effect of water-vaporenhanced chromium evaporation. Corros. Sci. 2002, 44 (11), 2635− 2649. (21) Birks, N. Investigation into the Role of Sodium Chloride Deposited on Oxide and Metal Substrates in the Initiation of Hot Corrosion; Department of Metallurgical and Materials Engineering, University of Pittsburgh: Pittsburgh, PA, 1983; NASA Grant NAG 3-44 Final Report. (22) Bryers, R. W. Fireside slagging, fouling, and high-temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels. Prog. Energy Combust. Sci. 1996, 22 (1), 29−120.

constituents, including carbides precipitated along grain boundaries of an austenitic alloy. These results should not be generalized for all concentrations of SO2; however, as observed when partial pressure of SOx in the exposed atmosphere is high enough to start alkali chloride sulfation right away at the deposit surface, risk of “active oxidation” is significantly reduced.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49-0-71168563491. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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