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Laboratory Studies of Potassium-Halide-Induced High-Temperature Corrosion of Superheater Steels. Part 2: Exposures in Wet Air Hao Wu,* Dorota Bankiewicz, Patrik Yrjas, and Mikko Hupa Laboratory of Inorganic Chemistry, Process Chemistry Centre, Åbo Akademi University, Piispankatu 8, FIN-20500 Turku, Finland ABSTRACT: In Part 1 of this work, the high-temperature corrosion behavior of three different superheater materials (10CrMo9-10, AISI347, and Sanicro28) under KBr and KF salts in dry air was studied. The results were compared to an earlier published paper in which the same steels were exposed to KCl. In this work, a continuation of the studies with the same steels covered with KCl, KBr, and KF and exposed to wet air (30 vol % H2O) at 450 and 550 °C was carried out. The corrosion behavior of the test steels in both dry and wet air was compared. It was found that the differences in the structure and thickness of the oxide layers formed depend not only on the gas atmosphere and temperature but also on the composition of the steel and the type of potassium halide. Generally, AISI 347 was found to corrode the least, although Sanicro 28 in some cases seemed to perform better.

1. INTRODUCTION Disastrous corrosion of waterwalls and superheaters is a common problem in waste-fired and cofired boilers.1−6 Thus, there is a longstanding interest in control of the quality of solid waste fuels due to their highly changeable compositions and to the abundance of elements forming corrosive deposits. Regarding high-temperature corrosion, the most corrosive species found in waste combustors’ deposits are mainly Cl and S which, together with Zn, Pb, and alkali metals such as K and Na, may form low-melting-point, highly aggressive deposits.7,8 In addition to Cl, there have been indications that other halogens such as Br might be similarly aggressive and induce corrosion.3,9,10 Vainikka et al.9 reported that during combustion, Br forms mainly water-soluble salts which physically and chemically resemble the corresponding Cl salts. The available data on the corrosive behavior of Br in waste combustors are limited due to the scarcer presence of this halogen in waste-derived fuels when compared to Cl. However, Vainikka and Hupa11 reported that Br can be present in solid waste fuels even as a major element with amounts exceeding 1 wt %. The main sources of Br in waste fuels are brominated flame retardants (BFRs), which are often used in electric and electronic equipment, textiles, upholstery, and insulation foams.9,12 It was reported that during combustion, Br decomposes in a manner similar to Cl present in polyvinyl chloride (PVC).13 The other halogen considered to have properties similar to the above-mentioned Cl and Br is F. Fluorine can be present in waste fuels in amounts similar to bromine. According to the ECN Phyllis 2 database,12 the amount of fluorine present in waste fuels is on the level of a few hundred mg/kg. The most important sources of fluorine are high-performance plastics known as fluoropolymers. These plastics can be found in cell phones, cable coating, silicon chips, coated cookware, athletic and extreme weather clothing, food handling implements, some medical equipment, and so forth.14 Despite the globally increasing trend of the use of fluoropolymers in a great variety of applications, and hence its increasing abundance in the waste flow, the literature related to possible corrosion problems in © 2015 American Chemical Society

waste-combusting or cocombusting atmospheres caused by fluorine-containing deposits is currently unavailable. The available literature on fluorine-related corrosion is connected to the conditions in nuclear reactors with an operating temperature range of 600−850 °C in which molten fluorides are used as reactor coolant, liquid fuel, or solvent.15−17 However, in this work, the max test temperature was 550 °C, and KF was not in a molten phase (KF Tm = 858 °C). Wang et al.15 investigated F-corrosion in nuclear reactors and reported that corrosion in molten fluorides (e.g., KF−NaF−LiF) mainly occurs by the dissolution of alloying elements into the melt, and increases in the following order: Ni, Co, Fe, Cr, Al, as reported by Olson et al.16 The weight-loss due to corrosion was observed to increase with increasing Cr-content in the steel, whereas Ni-based alloys were found to be more resistant to the attack from molten fluorides.16 Ouyang et al.17 investigated the effect of moisture on corrosion of Ni-based alloys in molten alkali fluoride (F−Li−Na−K) salt environments at 600 and 700 °C. Water vapor was suggested to react with the fluoride ion and generate HF gas. A portion of the HF gas may be dissolved into the molten salt and transformed into a liquid phase, which tends to accelerate the corrosion rate of metals. It was also reported that with increasing moisture and Cr-content, alloys are more prone to suffer from intergranular and pitting corrosion.17 In recent years, the utilization of renewable fuels such as biomass and waste and development of power generation systems with increased steam parameters has led to an increase in the research on water vapor effects on oxidation of steels. Superheater tubes are often made of Fe−Cr and Fe−Cr−Ni alloys. These materials withstand high-temperature corrosion relying on the formation of a protective Cr-rich oxide scale. This protective oxide usually consists of a corundum-type solid solution (Cr,Fe)2O3. The oxide properties depend on the Received: January 27, 2015 Revised: March 19, 2015 Published: April 6, 2015 2709

DOI: 10.1021/acs.energyfuels.5b00204 Energy Fuels 2015, 29, 2709−2718

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Energy & Fuels Table 1. Detailed Elemental Compositions of the Tested Steels (wt %), Analyzed with SEM 10CrMo9-10 AISI 347 Sanicro 28 (S28)

Fe

Cr

95.96 68.74 36.11

2.24 18.08 27.36

Ni 10.92 31.38

Mo

Mn

Si

C

1.00

0.45 0.92 1.15

0.25 0.46 0.46

0.07 0.04 0.01

3.51

Nb

P

S

0.81

0.01 0.02 0.01

0.01 0.01 0.01

Figure 1. Mean oxide layer thicknesses of steels exposed to KCl, KBr, and KF for 168 h in dry air and wet air at 450 and 550 °C.

The similarities in the behaviors of Br and F to the highly corrosive and already fairly well-explored Cl, their noticeable presence in waste fuels, and the lack of information on the potential harmful effects of those elements have been the main motivations for this research. This paper is a continuation of the studies on potassium halide (KCl, KBr, and KF)-induced high temperature corrosion, as reported in Part 126 of this work. In Part 1, the corrosion behavior of three different superheater materials (10CrMo9-10, AISI347, and Sanicro28) under KBr and KF salts in dry air was studied. The results were compared with the results from earlier tests with KCl from Enestam et al.27 In this paper, Part 2, further tests concerning the corrosivity of KBr, KF, and KCl in a water-vapor-rich atmosphere were performed. The corrosion behaviors of the test steels in both dry and wet air were compared.

chromia content, where oxides with a high Cr/Fe ratio are more protective.18,19 However, it was reported that in the presence of water vapor, such protective oxide is depleted in Cr, leading to a poorly protective Fe-rich oxide, thereby reducing the expected performance life of the alloys.20 Asteman et al.21,22 investigated the oxidation of 304L steel in O2+10%/40%H2O environments at 500−800 °C. They proposed that chromium vaporizes in the form of chromium oxide hydroxide (CrO(OH)2) via reaction 1, which leads to chromium depletion from the protective oxide, resulting in the formation of a poorly protective Fe-rich oxide and thus an increased oxidation rate. 2Cr2O3(s) + 3O2 (g) + 4H 2O(g) = 4CrO2 (OH)2 (g) (1) 23

Pettersson et al. investigated corrosion of 304L steel in KCl +5%O2 and KCl+5%O2+40%H2O environments at 600 °C. A greater corrosivity of the exposures in KCl+5%O2+40%H2O compared to KCl+5%O2 was observed, and they suggested that this was mainly due to the joint effect of chromium depletion by evaporation of CrO(OH)2 via reaction 1 and by formation of K2CrO4 via reaction 2.

2. MATERIALS AND METHODS Three different superheater steel materials were used in the experiments: a low-alloy steel (10CrMo9-10), an austenitic stainless steel (AISI 347), and a high-alloy austenitic stainless steel (Sanicro 28). Table 1 presents the detailed steel compositions. The steel samples had the dimensions of 20 × 20 × 5 mm (length × width × thickness). Before the experiments, the steel samples were ground with 1000 grid SiC paper, washed in ethanol, and then preoxidized for 24 h at 200 °C. After the pretreatment, the steel samples were covered with 0.25g of KCl, KBr or KF (2.5 g/cm3) with a particle size of 30− 100 μm, placed in a horizontal tube furnace, and heat-treated at 450 and 550 °C in wet air containing 30 vol % water vapor for 168 h (7 days). After the heat treatment, the samples were removed from the furnace, cooled to room temperature, and cast in a resin. The samples were then cut in the middle to reveal the cross-section, which were polished with kerosene and cleaned before being analyzed with a scanning electron microscope (operated at 20 kV) equipped with an energydispersive X-ray detector (SEM/EDX). The oxide layer thickness and the thickness distribution were measured at some 10 000 points over 4 mm of a representative

8KCl(s) + 2Cr2O3(s) + 3O2 (g) + 4H 2O(g) = 4K 2CrO4 (s) + 8HCl(g)

(2) 24,25

However, Lehmusto et al. reported an opposite effect of water vapor on corrosion of 304L,24 10CrMo, and Alloy 62525 in the presence of KCl at 500−600 °C. It was observed that thinner oxide layers were formed under wet conditions than under dry conditions. Two explanations were suggested: (1) water causes the oxide layer to be denser and thus thinner; (2) KCl might form volatile species such as HCl in the presence of water, which would leave with the surrounding gas flow and thus decrease the amount of reactive chloride in the system.25 The first explanation is a methodological artifact of the experimental method, which only measures the thickness of the oxide layer, whereas the second explanation is connected to the reaction mechanisms. No conclusive evidence for either explanation was given. 2710


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Figure 2. SEM and X-ray maps of 10CrMo9-10 exposed to (a) KCl, (b) KBr, and (c) KF at 550 °C in wet air for 168 h.

AISI 347 with all the three salts are similar in both dry and wet atmospheres. In dry air at 550 °C (Figure 1), the corrosivity of the three test salts was in most cases similar for each specific test steel. The sole exception was KCl showing extreme corrosivity to Sanicro 28. Among the 10CrMo9-10 cases in wet air at 550 °C, KBr showed much higher corrosivity than KCl and KF. In the cases of the two austenitic steels (AISI 347 and Sanicro 28) in wet air at 550 °C, KF appeared to be more corrosive than KCl and KBr. The more aggressive behavior of KF toward high-Cr austenitic stainless steel was most likely due to the detrimental reaction of Cr with fluorine-containing salt, as indicated by Olson et al.16 Olson et al.16 mentioned that weight loss due to corrosion under molten fluoride salt (F−Li− Na−K) was correlated with the Cr-content in the alloys, as Cr was expected to be selectively attacked due to the highly negative Gibb’s free energy of its fluorides. In general, the corrosion resistance of AISI 347 was shown to be the best among the three tested steels at 450 and 550 °C. 3.2. Cases in Wet Air. Due to the similarities observed in the oxide layer structures at 450 and 550 °C in wet air, only the cases at 550 °C in wet air are discussed on the basis of the SEM/X-ray maps and the oxide layer thickness distribution curves. 3.2.1. Low Alloy Steel 10CrMo9-10. The SEM and X-ray maps of 10CrMo9-10 exposed to KCl, KBr, and KF at 550 °C in wet air are shown in Figure 2. With KCl or KF (Figure 2a,c), the oxide layer that formed on 10CrMo9-10 was very even,

region of the cross-section using a panorama SEM technique, which has been well-described in detail in earlier publications.28−31 The corrosion attack was expressed as the mean thickness of the oxide layer. The distributions of various chemical elements in the oxide layer, steel, and salt deposit were identified by EDX.

3. RESULTS AND DISCUSSION 3.1. Mean Oxide Layer Thickness. The mean oxide layer thickness values for tests done in dry air (from Part 1) and wet air are summarized in Figure 1. The results marked with the symbol “x” represent duplicate samples showing the reproducibility of the tests. The corrosion layers developed on all three tested steels were generally thicker at the higher test temperature (550 °C) in both dry and wet air, especially for the iron-based steel 10CrMo9-10. At 450 °C, the effect of water vapor was not obvious. At 550 °C, the influence of water vapor became significant in some cases, but the trend was not consistent. For instance, in the case of 10CrMo9-10 with KBr, the mean oxide layer thicknesses increased significantly from 57 μm in dry air to 233 μm in wet air, while with KCl and KF, the oxide layers were thinner in the presence of water vapor. In the case of Sanicro 28 with KCl, a very thick oxide layer (113 μm) was formed in dry air, while in wet air, an oxide layer just 2 μm thick was formed. The exposure of Sanicro 28 with KF, however, showed increased oxide layer growth in wet air. The results for 2711


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Figure 3. Panorama SEM image of 10CrMo9-10 exposed to KBr at 550 °C in wet air for 168 h.

internal oxidation is typical for Fe−Cr alloys32 and Ni−Cr alloys33 in a water-vapor-containing atmosphere. Exposure of AISI 347 to KF in wet air (Figure 5c) caused more severe corrosion, with local deep pitting. K, but no F, was observed together with Fe-, Cr-, and Ni-oxides throughout the pits. The exact K-compounds are difficult to determine; however, K2CrO4 is most likely present. The absence of F in the oxide scale indicates that it left probably as HF, according to reaction 2a. However, reaction 2a seems not to occur in the cases of KCl and KBr in wet air, because K was not observed in the oxide layer (Figure 5a,b).

compact, and had good adherence to the steel surface. The oxide layer mainly contained Fe-oxide and traces of Cr-oxide. Typically, Fe was distributed throughout the oxide layer, with enrichment in the outer part; Cr was distributed only in the inner oxide layer just above the steel surface. Internal oxidation along the grain boundaries was observed on 10CrMo9-10 exposed to KCl (Figure 2a). With KBr in wet air at 550 °C, 10CrMo9-10 suffered from disastrous corrosion attack (Figure 3). The oxide layer contained two varying layers. The outermost layer was composed of porous Fe-oxide, which grew around the salt particles; the inner layer was a very dense and even Fe- and Crcontaining oxide layer (Figure 2b and Figure 3). The oxide layer thickness distribution curves over a 4 mm width of cross-section from tests with different salts at 550 °C are shown in Figure 4. For both KCl and KF, the curves are

8KX(s) + 2Cr2O3(s) + 3O2 (g) + 4H 2O(g) = 4K 2CrO4 (s) + 8HX(g)

(2a)

(X = F, Cl, Br) The oxide layer thickness distribution curves presented in Figure 6 confirmed the observations of the SEM images in Figure 5. The curves for KBr and KCl are similarly narrow, indicating a thin and even oxide layer formation. The curve for KF has a sharp peak in a low thickness range similar to the curves for KCl and KBr, suggesting that the most frequently occurring oxide layer was very thin and even. However, this KF curve also has a long tailing in a wide data range (10−150 μm), confirming the varying thickness values due to the locally occurring pitting corrosion (Figure 7). The deepest pitting is about 50 μm in depth. Such deep pitting corrosion can cause serious local damage to the steel. For such a case with an uneven oxide layer, the average thickness value does not correctly reflect the real corrosion behavior. Therefore, it is important to consider this kind of corrosion separately. 3.2.3. High-Cr and -Ni Austenitic Steel Sanicro 28. The SEM and X-ray maps of Sanicro 28 exposed to KCl, KBr, and KF at 550 °C in wet air are shown in Figure 8. Sanicro 28, with a high Cr and Ni content, showed very good corrosion resistance to KBr and KCl in wet air at 550 °C; a very thin and even Fe- and Cr-containing oxide layer was formed. Due to the depletion of Fe and Cr from the steel surface, a Ni-rich metallic layer was observed on the steel surface (Figure 8a,b). Similar to the three cases observed for AISI 347 in wet air (Figure 5a−c), Sanicro 28 also suffered from more corrosion with KF (Figure 8c) than with KCl and KBr (Figure 8a,b). Locally occurring pitting corrosion was also observed on Sanicro 28 after exposures with KF. The pitting oxides contained Fe, Cr, Ni, and K. The exact K-compounds are difficult to determine, but K2CrO4 was most likely formed. In

Figure 4. Oxide layer thickness distribution curves of 10CrMo9-10 exposed to KCl, KBr, and KF at 550 °C in wet air for 168 h.

very narrow, indicating a thin and even oxide layer formation. In contrast, the distribution curve for KBr is very wide and flat with a thickness range of 70−400 μm, indicating a thick and uneven oxide layer formation. 3.2.2. Austenitic Stainless Steel AISI 347. The SEM and Xray maps of AISI 347 exposed to KCl, KBr, and KF at 550 °C in wet air are shown in Figure 5. Internal oxidation was generally observed on AISI 347 with both KCl and KBr in wet air (Figure 5a,b). The internal oxide layer consisted of Fe-, Cr-, and Nioxides, whereas the outer scale contained only Fe-oxide. The protectiveness of the original chromia film is lost, probably due to the formation of volatile chromium oxide hydroxide (CrO(OH)2), according to reaction 1.21 Therefore, this left only the nonprotective external scale of Fe-oxide, beneath which at the scale/metal interface the oxygen partial pressure is sufficiently high for metal to be internally oxidized. This kind of 2712


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Figure 5. SEM and X-ray maps of AISI 347 exposed to (a) KCl, (b) KBr, and (c) KF at 550 °C in wet air for 168 h.

wet air, KF seems selectively more detrimental to Cr2O3 by reaction 2a than do KCl and KBr. The oxide layer thickness distribution curves of Sanicro 28 with the three salts (Figure 9) are quite similar to corresponding curves presented earlier for AISI 347 (Figure 6). After exposure to KF, Sanicro 28 (Figure 10) suffered more local pitting corrosion than AISI 347 (Figure 7). The deepest pitting depth for Sanicro 28 was 40 μm. 3.3. Cases with Significant Differences in Dry and Wet Air. The influence of water vapor on corrosion behavior was not consistent. In this section, cases with significant differences between dry air (from Part 1) and wet air are discussed.

Figure 6. Oxide layer thickness distribution curves of AISI 347 exposed to KCl, KBr, and KF at 550 °C in wet air for 168 h.

Figure 7. SEM cross section of AISI 347 exposed to KF at 550 °C in wet air for 168 h. 2713


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Figure 8. SEM and X-ray maps of Sanicro 28 exposed to (a) KCl, (b) KBr, and (c) KF at 550 °C in wet air for 168 h.

3.3.1. 10CrMo9-10 under KBr. 10CrMo9-10 with KBr at 550 °C showed much higher corrosion in wet air than in dry air, based on the oxide layer structures (Figure 11 and Figure 3) and the thickness distribution curves (Figure 12). A significant development of porous iron oxide growing around salt particles was observed in wet air (Figure 3). In the presence of water vapor, HBr was possibly released according to reaction 3a. Zhuang et al.34 reported that HBr is capable of diffusing through the oxide layer to the metal/scale interface and reacting with Fe to form FeBr2 through reaction 4a. When the temperature is above T4 (the temperature at which the vapor pressure reaches 10−4 atm34,35), FeBr2 can experience vaporization (reaction 5) and oxidation (reaction 6). The bromine gas formed according to reaction 6 is either released into the atmosphere or diffuses back to the metal/scale

Figure 9. Oxide layer thickness distribution curves of Sanicro 28 exposed to KCl, KBr, and KF at 550 °C in wet air for 168 h.

Figure 10. SEM cross section of Sanicro 28 exposed to KF at 550 °C in wet air for 168 h. 2714


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Figure 11. SEM and X-ray maps of 10CrMo9-10 exposed to KBr at 550 °C in dry air for 168 h.

FeBr2(s) = FeBr2(g)

(5)

4FeBr2(g) + 3O2 (g) = 2Fe2O3(s) + 4Br2(g)

(6)

4FeBr2(s) + 4HBr(g) + O2 (g) = 4FeBr3(s) + 2H 2O(g)

(7)

Figure 12. Oxide layer thickness distribution curves of 10CrMo9-10 exposed to KBr at 550 °C in dry and wet air for 168 h.

3.3.2. 10CrMo9-10 under KF. In the case of 10CrMo9-10 exposed to KF at 550 °C (Figure 13 and Figure 2c), the influence of water vapor was the opposite of that observed in the cases of 10CrMo9-10 with KBr in dry and wet air. Figure 13 shows that the oxide layer formed in dry air at 550 °C was very thick and uneven, which is also visible as a wide and flat oxide layer thickness distribution curve (Figure 14). The scale had a

interface where it reacts again with the steel. The same bromine corrosion and regeneration cycles may occur via FeBr3, which is possibly formed by reaction 7.34 At 550 °C, the significant development of a porous iron oxide growing around the salt particles in Figure 3 was most likely formed through vaporization and oxidation of FeBr3, because FeBr3 has a very low T4 (156 °C). In dry air, FeBr2 (with T4 of 509 °C) was most likely formed according to reactions 3b and 4b, and it became slightly volatile at 550 °C. As a consequence, only small amounts of porous iron oxides were locally formed around the salt particles (Figure 11).

Figure 14. Oxide layer thickness distribution curves of 10CrMo9-10 exposed to KF at 550 °C in dry and wet air for 168 h.

2KBr(s) + Fe2O3(s) + H 2O(g) = K 2Fe2O4 (s) + 2HBr(g)

multilayered structure and consisted mainly of Fe-oxide and a small amount of Cr-oxide. Furthermore, K was distributed in the oxide layer in the same regions as Fe and O, indicating the presence of a K−Fe-oxide. The quantification analysis of the Kcontaining area (an example marked with the red square in Figure 13) showed the presence of O (55 atom %), K (5 atom %), and Fe (40 atom %). Therefore, the K-containing area was

(3a) 4KBr(s) + 2Fe2O3(s) + O2 (g) = 2K 2Fe2O4 (s) + 2Br2(g) (3b)

2HBr(g) + Fe(s) = FeBr2(s) + H 2(g)

(4a)

Br2(g) + Fe(s) = FeBr2(s)

(4b)

Figure 13. SEM and X-ray maps of 10CrMo9-10 exposed to KF at 550 °C in dry air for 168 h. 2715


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Figure 15. SEM and X-ray maps of Sanicro 28 exposed to KCl at 550 °C in dry air for 168 h.

composed mainly of Fe-oxide and a small amount of potassium ferrate (K2Fe2O4), which might have been formed through reaction 8. However, this kind of K−Fe-oxide was not observed in the cases of 10CrMo9-10 under KCl and KBr in either dry or wet air. 4KF(s) + 2Fe2O3(s) + O2 (g) = 2K 2Fe2O4 (s) + 2F2(g) (8)

In the case of 10CrMo9-10 with KF in wet air at 550 °C, a much thinner, yet still significant, and even oxide layer was formed (Figure 2c). It is clearly visible in a narrow and sharp peaked thickness distribution curve (Figure 14). K was not observed in the oxide layer here as it had been observed in the case of exposures in dry air. 3.3.3. Sanicro 28 under KCl. Sanicro 28 under KCl showed significantly different corrosion behavior in dry air (Figure 15) and wet air (Figure 8a). As shown earlier in wet air (Figure 8a), the resistance of Sanicro 28 to KCl was very good. However, an extreme corrosion attack was observed in dry air (Figure 15). Fe and Cr were enriched in the uppermost layer of the porous oxide scale, which grew around the salt particles, indicating the vaporization of iron and chromium chlorides, followed by subsequent oxidation and condensation.27 K, but no Cl, was also present in the uppermost layer in the same region as Cr and O, suggesting the formation of potassium chromate. This confirms the reaction between KCl and Cr2O3, thereby destroying the protectiveness of the chromia scale and allowing subsequent accelerated oxidation. The inner oxide layer consisted of Fe-, Cr- and Ni-oxides. Duplicate tests of Sanicro 28 under KCl in dry air showed a similar severe corrosion attack. The Cr and Ni content in Sanicro 28 is the highest among the three test steels, but its corrosion resistance was significantly deteriorated when exposed to KCl in dry air. The oxide layer thickness distribution curves (Figure 16) also confirm the differences in dry and wet air. 3.3.4. AISI 347 under KCl or KBr. At 550 °C, AISI 347 showed similar corrosion behavior under KCl and KBr in both dry air (Figure 5a,b) and wet air (Figure 17a,b). The differences in the corrosion behaviors did not result from the salt but rather from the gas atmosphere. Potassium chromate was formed and distributed in the Fe− Cr−Ni-oxide layer in dry air (Figure 17a,b) but not in wet air (Figure 5a,b). The scale formed on AISI 347 under KCl or KBr in wet air (Figure 5a,b) had a better adherence to the steel surface and had a more compact structure than the scale formed in dry air (Figure 17a,b). In addition, internal oxidation

Figure 16. Oxide layer thickness distribution curves of Sanicro 28 exposed to KCl at 550 °C in dry and wet air for 168 h.

occurred only in wet air. As discussed earlier, the absence of chromium in the outer scale was most likely due to the vaporization of the chromium oxide in the form of chromium oxide hydroxide (CrO(OH)2).21 It seems that in dry air, the salt (KCl or KBr) is the trigger to the destruction of the protectiveness of Cr2O3 via reaction 9, whereas in wet air, the water vapor is the trigger via reaction 1. 8KX(s) + 2Cr2O3(s) + 5O2 (g) = 4K 2CrO4 (s) + 4X 2(g) (9)

(X = Cl, Br) Figure 18 shows the differences between dry and wet air in the oxide layer thickness distributions. Narrower and sharper curves for wet air than for dry air indicate thinner and more even oxide layer formation.

4. CONCLUSIONS In the first paper of this work, Part 1,26 the high-temperature corrosion behavior of three different superheater steels (10CrMo9-10, AISI347, and Sanicro28) exposed to KBr and KF salts in dry air was studied. The results were compared to an earlier published paper27 in which the same steels were exposed to KCl. In this work, Part 2, a continuation of the studies with the same steels exposed to KCl, KBr, and KF in wet air (30 vol % H2O) at 450 and 550 °C was carried out. In addition, the corrosion behaviors of the test steels in both dry and wet air were compared. The differences in the structures and thicknesses of the formed oxide layers depended not only on the gas atmosphere and temperature but also on the composition of the steel and the type of potassium halide salt deposit. 2716


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Figure 17. SEM and X-ray maps of AISI 347 exposed to (a) KCl and (b) KBr at 550 °C in dry air for 168 h.

salt particles, while the inner layer consisted of a very thick, dense, and even Fe- and Cr-containing oxide layer. In the cases with KCl and KF only one thinner and very even layer was formed, with a mean thickness of 43 and 21 μm, respectively. • In wet air at 550 °C, AISI 347 and Sanicro 28 showed very good corrosion resistance to KCl and KBr, but no resistance to KF, with locally deep pitting corrosion. The deepest pitting depth was 50 μm for AISI 347 and 40 μm for Sanicro 28. • In the cases of AISI 347 with KCl or KBr at 550 °C, internal oxidation occurred only in wet air. The absence of chromium in the outer scale was most likely due to the vaporization of the chromium oxide in the form of chromium oxide hydroxide (CrO(OH)2). • In wet air, water seems to be the trigger, while the salt (KCl or KBr) was the trigger in dry air, to destroying the protectiveness of the chromia scale. Generally, AISI 347 was found to corrode the least, although Sanicro 28 in some cases seemed to perform better.

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Figure 18. Oxide layer thickness distribution curves of AISI 347 exposed to (a) KCl and (b) KBr at 550 °C in dry and wet air for 168 h.

AUTHOR INFORMATION

Corresponding Author

*E-mail: hwu@abo.fi.

• The corrosion layer thicknesses increased with temperature in both dry and wet air on all three test steels, especially in the case of the iron-based steel 10CrMo910. • At 450 °C, the effect of water vapor was not obvious; at 550 °C, the water vapor effect became significant in some cases, but the trend was not consistent. • In wet air at 550 °C, 10CrMo9-10 suffered from disastrous corrosion after exposure to KBr. The oxide layer consisted of two varying layers. The outer layer was composed of a porous iron oxide, also being around the

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been carried out within CLIFF (2014−2017) as part of the activities of the Åbo Akademi Process Chemistry Centre. Other research partners are VTT, Lappeenranta University of Technology, Aalto University and Tampere University of Technology. Support from the National Technology Agency of Finland (Tekes), Andritz Oy, Valmet Technologies Oy, Foster Wheeler Energia Oy, UPM-Kymmene 2717


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(26) Wu, H.; Yrjas, P.; Hupa, M. Laboratory Studies of PotassiumHalide-Induced High-Temperature Corrosion of Superheater Steels. Part 1: Exposures in Dry Air. Energy Fuels 2015, 29, 1186−1195. (27) Enestam, S.; Bankiewicz, D.; Tuiremo, J.; Mäkelä, K.; Hupa, M. Fuel 2013, 104, 294−306. (28) M. Westén-Karlsson Assessment of a Laboratory Method for Studying High Temperature Corrosion Caused by Alkali Salts, Licentiate Thesis, Åbo Akademi University, Åbo, Finland, 2008. (29) Skrifvars, B. -.; Backman, R.; Hupa, M.; Salmenoja, K.; Vakkilainen, E. Corros. Sci. 2008, 50, 1274−1282. (30) Bankiewicz, D.; Enestam, S.; Yrjas, P.; Hupa, M. Fuel Process. Technol. 2013, 105, 89−97. (31) Bankiewicz, D.; Yrjas, P.; Hupa, M. Energy Fuels 2009, 23, 3469−3474. (32) Essuman, E.; Meier, G. H.; Ż urek, J.; Hänsel, M.; Singheiser, L.; Quadakkers, W. J. Scr. Mater. 2007, 57, 845−848. (33) Essuman, E.; Meier, G. H.; Zurek, J.; Hänsel, M.; Norby, T.; Singheiser, L.; Quadakkers, W. J. Corros. Sci. 2008, 50, 1753−1760. (34) Zhuang, Y.; Chen, C.; Timpe, R.; Pavlish, J. Fuel 2009, 88, 1692−1697. (35) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. Prog. Energy Combust. Sci. 2000, 26, 283−298.

Oyj, Clyde Bergemann GmbH, International Paper Inc. 3motion Oy and Top Analytica Oy Ab is gratefully acknowledged.

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