JTTEE5 21:887–899 DOI: 10.1007/s11666-012-9786-6 1059-9630/$19.00 Ó ASM International S. Rao, L. Frederick, and A. McDonald (Submitted October 20, 2011; in revised form March 21, 2012) Corrosion of components in a recovery boiler is a major problem faced by the pulp and paper industry. The superheater tubes become severely corroded due to the presence of sulfidic gases in the boiler and molten salts which are deposited on the surface of the tubes. As a result, the boiler must be decommissioned for expensive maintenance and repairs. Yttria-stabilized zirconia (YSZ) coatings have been shown to provide corrosion resistance when applied on gas turbines operating at high temperatures. Air plasma-sprayed YSZ environmental barrier coatings on Type 309 stainless steel were exposed to three different corrosive environments: Test A—600 °C, salt vapors, flue gases, 168 h; Test B—600 °C, molten salt, air, 168 h; and Test C—600 °C, molten salt, flue gases, 168 h. Two different types of YSZ coatings—conventional YSZ and nanostructured YSZ—were tested to study their resistance to corrosion and molten salt penetration. The performances of both types of coatings were evaluated, and a comparative study was conducted. It was found that the nanostructured YSZ samples protected the stainless steel substrate better than their conventional counterparts. This superior performance was attributed to the presence of semi-molten nano-agglomerates present in the coating microstructure, which acted as collection points for the penetrating molten salts. Keywords corrosion, environmental barrier coating, nanostructured coating, recovery boiler, yttria-stabilized zirconia 1. Introduction Recovery boilers in the pulp and paper industry are large fuel-to-energy boilers designed to combust organic waste found in black liquor from the Kraft pulping process (Ref 1). The purpose of the Kraft pulping process is to remove the lignin from the cellulose fibers in wood and black liquor is a by-product of that process. Black liquor contains spent inorganic chemicals and organic waste. Although, recovery boilers are very similar to boilers that burn coal, municipal waste, and wood waste, a recovery boiler also functions as a chemical reactor (Ref 2). Its primary functions are; to convert the spent inorganic Portions of this article were previously published in S. Rao, A. McDonald, D. Singbeil, Resistance of nanostructured thermal barrier coatings to molten salts and high-temperature boiler flue gases, in: Proceedings of the 23rd Canadian Congress of Applied Mechanics, June 5-9, 2011 (Vancouver, BC, Canada), University of British Columbia, 2011, 4 pages on compact disk. S. Rao and A. McDonald, Department of Mechanical Engineering, University of Alberta, 4-9 Mechanical Engineering Building, Edmonton, AB T6G 2G8, Canada; and L. Frederick, FPInnovations, 3800 Wesbrook Mall, Vancouver, BC V6S 2L9, Canada. Contact e-mail: andre.mcdonald@ualberta.ca. Journal of Thermal Spray Technology cooking chemicals from the black liquor into forms suitable for reuse in the pulping process and to produce steam. Flue gas temperatures may reach 700 °C in the upper furnace (Ref 3). Combustion of black liquor fuel results in the formation of hot, sulfidic gases, which corrode the boiler surfaces and tubes. Corrosion of components in a recovery boiler is a major challenge faced by the pulp and paper industry. It limits the overall energy efficiency of the boiler, largely due to restrictions placed on maximum steam temperatures in the tubes to prevent molten salt corrosion of the superheater tubes. Several protection methods have been employed to combat corrosion of superheater tubes in boilers. These methods include application of Inconel Alloy 625 cladding, weld overlays, high-velocity oxy-fuel- (HVOF), and thermal-sprayed coatings, to name a few. Lee et al. (Ref 4), in a comprehensive study of waste-to-energy boilers, have shown that application of Inconel 625 cladding on superheater tubes did not provide corrosion resistance when the temperatures reached or exceeded 400-420 °C. Weld overlays may be a suitable alternative. However, Solomon (Ref 5) found that the application of weld overlays to the same area may cause embrittlement of the previously deposited overlay, which leads to the formation of cracks that will propagate into the coated tube. More recently, it has been found that cold-sprayed Ni-20Cr coatings were successful in providing protection to boiler steel exposed to corrosive molten salts at high temperatures (Ref 6). Extensive studies on the use of yttria-stabilized zirconia (YSZ)-based coatings have been conducted (Ref 7-11). Jones (Ref 7) has presented a thorough review of the use Volume 21(5) September 2012—887 Peer Reviewed Resistance of Nanostructured Environmental Barrier Coatings to the Movement of Molten Salts Peer Reviewed of zirconia-based thermal barrier coatings (TBCs) in engines to combat hot corrosion. It has been found that YSZ is suitable for applications where the corrodents are only sodium and sulfur. Further studies were conducted to show that in high-temperature applications, zirconia-based TBCs degraded rapidly in the presence of corrosive vanadium compounds (Ref 7, 8). While YSZ has garnered much research interest and found successful applications in engines (Ref 11-13), this material has also found environmental barrier coating (EBC) applications in other equipment where corrosion is problematic. Shankar et al. (Ref 9) have shown that plasma-sprayed YSZ did not degrade and corrosion attack was limited when it was exposed to molten LiCl-KCl medium at 873 K for extended time periods. Further study by Lee and Baik (Ref 10) has shown that YSZ possessed superior corrosion resistance against molten LiCl-Li2O salt at 650 °C due to the formation of a dense protective oxide layer of noncrystalline reaction products. These studies on the corrosion behavior of YSZ coatings have been useful to expand existing understanding of the performance of these coatings, and have focused on molten salts devoid of other compounds such as water. However, further study that involves studies of the impact of molten salts, coupled with flue gases (containing water vapor) from equipment such as boilers, is still needed. Conventional fused-and-crushed YSZ has been a ubiquitous standard feedstock powder for the top coat of plasma-sprayed thermal and EBC. However, nanostructured YSZ is a recently engineered material that has been developed as an alternative to conventional YSZ. Nanostructured materials are those which exhibit grain sizes that are less than 100 nm in at least one dimension (Ref 14). Nano-sized powder particles are spray-dried and agglomerated into porous micron-sized particles to facilitate deposition and avoid clogging of tubes in the air plasma spraying process. Coatings fabricated from the nanostructured powder possess microstructures that have a bimodal feature, consisting of a structure formed by resolidification of the agglomerates that are fully molten in the spray jet and the semi-molten agglomerates known as nano-zones (Ref 15). Improved material properties of coatings fabricated from nanostructured YSZ have been observed by several investigators. In particular, Wang et al. (Ref 16) has shown that plasma-sprayed nanostructured zirconia coatings possessed higher thermal shock and isothermal oxidation resistance than conventional zirconia coatings. It has also been found that nanostructured zirconia coatings have improved toughness, lower porosity, and, decreased numbers of microcracks after thermal cycling tests (Ref 16, 17). It has been suggested that the improved toughness and decreased numbers of microcracks was due to the presence of the nano-zones present in the coating, which arrested the growth of cracks (Ref 17). A coating in which vertical cracks do not propagate through to the substrate and are arrested by the nano-zones would prove beneficial if applied on superheater tubes in recovery boilers. This could potentially restrict penetration by corrosive salts and sulfidic gases present in the boiler. 888—Volume 21(5) September 2012 The objectives of this study are to characterize the performance of conventional YSZ and nanostructured YSZ coatings in different corrosive environments that are typically found in industrial recovery boilers. A qualitative comparison of the resistance to the movement of salts of the two types of coatings will be presented. 2. Experimental Method 2.1 Thermal Spraying All the coating samples consisted of a bond coat and a ceramic top coat. Ni-Cr-Al-Co-Y2O3 bond coat powder (Metco 461NS, Sulzer Metco, Inc., Westbury, NY, USA) was deposited using a plasma spray torch (3MBM ThermoSpray Gun, Sulzer Metco, Inc., Westbury, NY, USA). Conventional YSZ (ZrO2-8Y2O3, Metco 204NS-G, Sulzer Metco, Inc., Westbury, NY, USA) and nanostructured YSZ (ZrO2-7Y2O3, Nanox Powder S4007, Inframat Corporation, Farmington, CT, USA) were deposited as the top coats using the same plasma spray torch. The spray parameters for deposition of the bond coat and the top coat are shown in Table 1. The plasma gases were argon and hydrogen. A volumetric powder feeder (5MPE, Sulzer Metco, Inc., Westbury, NY, USA) with argon as the carrier gas was used. The stand-off distance, the distance between the torch and the substrate, is shown in Table 1. A robot (HP20, Motoman, Inc., West Carrollton, OH, USA), operating at 400 mm/s, was used to deposit the coatings. The size distribution of the Ni-Cr-Al-Co-Y2O3 powder that was used as the bond coat material was between 22 and 150 lm ( 150 + 22 lm). The conventional and nanostructured YSZ powders were sieved with a Ro-Tap siever (RX-29-CAN, W.S. Tyler, Mentor, OH, USA) to obtain a size distribution of 20-90 lm ( 90 + 20 lm). All coatings were deposited onto Type 309 stainless steel substrates with dimensions of 25 9 20 9 2 mm3. All substrates were grit blasted with #24 alumina grit before the bond coat was applied. In addition to the coated samples, uncoated Type 309 stainless steel samples were also included in the test to compare the extent of corrosion. Table 1 Plasma spraying parameters Spray parameter Primary gas Secondary gas Ar flow rate H2 flow rate Ar pressure H2 pressure Powder carrier gas pressure Powder carrier gas flow rate Standoff distance Current Voltage Bond coat (Ni-Cr-Al-Co-Y2O3) Top coat (YSZ) Argon Hydrogen 90 SCFH 20 SCFH 75 psig 100 psig 60 psig 14 SCFH 140 mm (5.5 in) 500 A 60 V Argon Hydrogen 90 SCFH 30 SCFH 75 psig 100 psig 60 psig 14 SCFH 64 mm (2.5 in) 500 A 60 V Journal of Thermal Spray Technology Once all the samples were sprayed with the respective thermal-sprayed coatings, they were sonicated for 2 min and cleaned using Versaclean (Fisherbrand Versaclean, Thermo-Fisher Scientific, Fairlawn, NJ, USA), de-ionized water, dry, filtered, reagent grade, anhydrous ethanol. Triplicate sets of each coating sample and one uncoated (control) substrate sample were exposed to three different environments at 600 ± 3 °C. The following test exposures were held for 168 h:    Test A: Exposure to salt vapors and flue gases. Test B: Exposure to molten salt and air. Test C: Exposure to molten salt and flue gases. To differentiate the effects of salt vapors from condensed salt deposits, in Test A, the coated coupons were exposed to salt vapors only and were not coated initially with the salt. The salt vapors were introduced by placing the standard salt mixture, in crystalline form, in an alumina pot adjacent to the coupons in the furnace. The salt was a synthetic mixture of reagent grade chemicals (10.2 wt.% KCl, 11.5 wt.% Na2CO3, 73.9 wt.% Na2SO4, and 4.4 wt.% K2SO4) used to simulate upper boiler tube deposits. The mixture was first dissolved in distilled water and a near-saturated solution was formed with a concentration of 192 g/L. This salt solution was sprayed on to the samples using an artistÕs air brush (Iwata HP-BC 1 Plus, Iwata Medea, Inc., Portland, OR, USA). The air pressure used was 20 pounds per square inch gage (psig), and the stand-off distance was fixed at 190 mm. A jig was manufactured to keep the substrate perpendicular to the axis of the air brush. The apparatus is shown in Fig. 1. While the solution was being sprayed, the substrates were maintained at a temperature of approximately 130 °C. This allowed the water from the solution to evaporate immediately, leaving a layer of solid salt on the coated sample. In order to ensure that all the samples had Fig. 1 Salt spray apparatus Journal of Thermal Spray Technology approximately the same amount of salt, they were weighed before and after application of the salt coating. On average, each sample had approximately 18.6 ± 2.5 mg/cm2 (n = 10) of salt on their surface. The salt was removed from one extra test sample and differential thermal analysis (DTA) was conducted. The DTA analysis confirmed that the salt had a single first melting point of around 520 °C. The first melting point is the temperature at which the material first begins to melt and below which there is no liquid phase in the salt (Ref 2). All three corrosion tests were conducted simultaneously in three different horizontal tube furnaces (Lindberg Model 55347, Lindberg/MPH, Riverside, MI, USA). All samples were arranged in an alumina crucible. These crucibles were placed on top of a D-shaped alumina support and then inserted into a nominal 70- mm ID mullite tube. The simulated boiler flue gas for Tests A and C consisted of 5 vol.% O2, 10 vol.% CO2, 20 vol.% H2O, balance N2, and the total flow rate was maintained at 200 standard cubic centimeters per minute (SCCM). The volume flow rate of air in Test B was also maintained at 200 SCCM. Mass flow controllers (MFCs) (MKS, Andover, MA, USA) ranging from 20 to 1000 SCCM regulated the flow of the oxygen, carbon dioxide, and nitrogen gases. Water vapor was maintained at 20 vol.% by introducing 40 SCCM of H2O into the furnace with the aid of a peristaltic pump (IPC High Precision multichannel dispenser, ISMATEC SA, Glattbrugg, Switzerland). 2.3 Coating Preparation for Analysis After completing the corrosion tests, the samples were removed from the furnace and mounted in epoxy by using vacuum impregnation. All sample preparation processes such as cutting, grinding, and polishing were done dry, Fig. 2 SEM image of the cross section of the Type 309 steel substrate for Test A—600 °C, salt vapors, flue gases, 168-h exposure Volume 21(5) September 2012—889 Peer Reviewed 2.2 Corrosion Testing Peer Reviewed without the use of coolants or lubricants. The exposed and mounted samples were cut in half by using a metal-bonded diamond blade. In order to minimize sample damage, an automated precision cutting machine (Accutom-50, Struers A/S, Ballerup, Denmark) was used. Samples were plane ground using 400 and 1200 silicon carbide (SiC) grit paper. After grinding, 5 lm alumina powder on napped cloth was used for final polishing. After polishing, the sample mounts were held outside the desiccator for several days to permit salt in the pores of the sample to migrate to the surface for easier detection. It was important to ensure that the corrosion scales and salts were present during analysis of the cross sections of the coating with a scanning electron microscope (SEM). This was done to permit qualitative determination of the extent of salt penetration into the coating system. Energy Fig. 3 (a) SEM image of the cross section (arrows point to salt crystals) and (b) EDS spectrum of the salt crystals in the conventional YSZ top coat for Test A—600 °C, salt vapors, flue gases, 168-h exposure 890—Volume 21(5) September 2012 Journal of Thermal Spray Technology 3. Results 3.1 SEM/EDS Images of Samples Exposed to Salt Vapors and Flue Gases (Test A) The high operating temperatures typical of the Kraft process in recovery boilers will produce salt vapors in the boiler flue gases. Figure 2 shows an SEM image of the cross section of Type 309 steel used in Test A. It can be clearly seen that the corrosion scale has spalled (peeled) from the bare steel sample. Figure 3(a) shows an SEM image of the cross section of the conventional YSZ-based EBC after the same test. Salt crystals, shown by white arrows, can be seen in the bond coat of this coating. This is confirmed by the EDS spectrum presented in Fig. 3(b). The EDS spectrum shows the presence of elements, such as Na and K, which were present in the corroding salt. Images of the cross section of the nanostructured (nano)based YSZ EBC coating used in this test are shown in Fig. 4. No salt crystals were found in the cross section of the nano-based EBC coatings. coating (see Fig. 6a), the crystals were more prone to crystallize in areas of the coating that contained semimolten nanoparticles in the nano-based YSZ top coat. This is evident from Fig. 7(b), which shows a high-magnification SEM image of the cross section of a nanostructured YSZ top coating. The presence of the semi-molten nanoparticles also confirms that the coating possessed nanostructured characteristics and a bimodal microstructure. Figure 7(c) is an EDS spectrum of the salt crystal shown in Fig. 7(b), and it confirms the presence of the 3.2 SEM/EDS Images of Samples Exposed to Molten Salt and Air (Test B) Molten salts will be present on the surfaces of industrial recovery boilers. To separate the effects of molten salts and the flue gases, this test exposed the samples to molten salts in the presence of air flowing at 200 SCCM (per Test B). As shown in Fig. 5, the corrosion scale has spalled from the bare steel sample, similar to that shown in Fig. 2. Salt crystals can be seen in the image and are shown by the white arrows. The substrate below the scale also appears to have been attacked. Figure 6(a) shows the cross section of a conventional YSZ top coat. Salt crystals can be seen in cracks present in the top coat and are shown by white arrows. A spot scan of the salt crystals was performed. The EDS spectrum resulting from this scan is shown in Fig. 6(b). It confirms the presence of Na, S, K, O, and a small amount of Cl. Figure 6(c) shows a high magnification image of a salt crystal. This crystal was found at the interface between the steel substrate and the bond coat over which the conventional YSZ based top coat was deposited. The EDS spectrum of this section of the coating-substrate interface is shown in Fig. 6(d). The elements present in the corroding salt such as Na, K, S, and Cl were observed in this spectrum, which indicates that the salt penetrated into and through the conventional EBC to the coating-substrate interface. A typical cross section of the nanostructured YSZbased EBC coating is shown in Fig. 7(a). Although salt crystals were found in the YSZ top coat, none were found in the bond coat or at the interface between the coating and the steel substrate. Unlike the conventional YSZ top coat, where salt crystals were found in cracks in the Journal of Thermal Spray Technology Fig. 4 SEM images of the cross section of the nanostructured YSZ coating for Test A—600 °C, salt vapors, flue gases, 168-h exposure (white contaminants are alumina from the dry polishing process) Fig. 5 SEM image of the cross section (arrows point to salt crystals) of the Type 309 steel substrate for Test B—600 °C, molten salt, air, 168-h exposure Volume 21(5) September 2012—891 Peer Reviewed dispersive spectroscopy (EDS) was used to confirm the composition of the salt crystals. Peer Reviewed Fig. 6 (a) SEM image of the cross section (arrows point to salt crystals present in cracks), (b) EDS spectrum of the salt crystals shown in (a), (c) SEM image of the cross section (salt crystal at the interface of the bond coat and substrate) and (d) EDS spectrum of the top coat shown in (c) of the conventional YSZ top coat for Test B—600 °C, molten salt, air, 168-h exposure component elements present in the salt solution (Na, S, K, Cl) used in the corrosion test. 3.3 SEM/EDS Images of Samples Exposed to Molten Salt and Flue Gases (Test C) The coated samples were exposed to molten salts and flue gases to study the combined effect of the salts and gases on penetration of the corroding medium into the coating system. Test C involved exposing the samples to 892—Volume 21(5) September 2012 molten salts in the presence of flue gases. Figure 8 shows the corrosion scale that has peeled from the bare steel substrate. In this case, evidence of localized corrosion attack of the steel is present, which is indicative of intergranular corrosion (Ref 18). Similar to Test B, salt crystals were found mainly in cracks in the conventional based YSZ top coat. This can be seen in Fig. 9(a) where the salt crystals are shown by white arrows. These crystals were also found in the Journal of Thermal Spray Technology Peer Reviewed Fig. 6 continued interface between the substrate and the bond coat over which the conventional YSZ was deposited. This is emphasized in Fig. 9(b), which shows a salt crystal at the interface of bond coat and steel substrate. A spot scan was conducted on this crystal and the resulting EDS spectrum is shown in Fig. 9(c). From the spectrum, it is clear the crystal shown is a sodium chloride (NaCl) crystal. It was likely formed due to the melting and/or dissociation of KCl, Na2CO3, and Na2SO4, which produced free ions of Na and Cl to recombine during cooling to produce NaCl. Journal of Thermal Spray Technology The cross-sectional image of the nanostructured-based YSZ EBC from this test is shown in Fig. 10(a). Similar to Test B (samples exposed to molten salt and air), it can be clearly seen that the crystals tend to crystallize in the nano-zone areas of the EBC. The same phenomenon can also be seen in Fig. 10(b), which shows salt crystallizing in the semi-molten nano-particle area and oozing out. A spot scan was performed on the crystal shown in Fig. 10(b) and the resulting EDS spectrum is shown in Fig. 10(c). The spectrum confirms that the crystal shown is a sodium Volume 21(5) September 2012—893 Peer Reviewed Fig. 7 (a) SEM image of the cross section (arrows point to salt crystals), (b) SEM image of the cross section showing salt crystallizing in nano zones (arrows point to salt crystals), and (c) EDS spectrum of the salt crystals shown in (b) of the nanostructured YSZ top coat for Test B—600 °C, molten salt, air, 168-h exposure 894—Volume 21(5) September 2012 Journal of Thermal Spray Technology 4. Discussion 4.1 SEM/EDS Results All three tests showed the presence of salt crystals in the bond coat near the steel substrate that was protected by the conventional YSZ top coating. No salt was detected in the bond coat of any sample protected by the nanostructured YSZ coating. Semi-molten nano-particle agglomerates were embedded in the microstructure of the nanostructured YSZ coatings. It has been hypothesized that these semi-molten nano particles act as crack arresters and hence, the nano-coatings are tougher than their conventional counterparts (Ref 17). This results in fewer and smaller cracks in the nanostructured YSZ coating. In this study, it was found that the salt crystals were found primarily in the area of these nanozones in the nanostructured coating while the crystals appear to have originated from large cracks in the conventional YSZ coating. Figure 7(a), (b) and 10(a), (c) show the presence of salt crystals in the nano-zone areas. Figure 6(a) and 9(a) show salt crystals present in large cracks in the conventional YSZ top coat. A possible explanation for this phenomenon may be that these nanozones act as collection points for the molten salt that penetrate through the network of small cracks and pores in the coating. Once the samples cool, the salts crystallize in these nanozone areas. On the other hand, given that there are no nanoparticles present in the conventional YSZ coating, the molten salt continues to penetrate through the inter-connected cracks in the conventional coating until it reaches the denser bond coatsubstrate interface. This explains the presence of salt crystals near the surface of the substrate that is protected by the conventional YSZ top coating. A similar mechanism of penetration through a finer crack network in the bond coat may be the reason for the observed salt crystals at the bond coat-substrate interface. However, the exact mechanism is unclear and will require further investigation. Salts crystallizing in cracks in the conventional YSZ top coat may also have detrimental effects on the mechanical strength of the coating. A possible analogy that can be made here is that of the weathering of rocks due to salt crystallization. Salt crystal growth can exert tremendous pressure and is known to loosen individual minerals in a rock. This phenomenon is particularly effective in porous, granular rocks such as sandstone (Ref 19). As these crystals grow in the cracks, they wedge the rock apart. Similarly, as these crystals grow in the cracks of the conventional YSZ top coat, they may cause more stresses in the coating. In the case of the nanostructured YSZ-based EBC, it was found that the salt crystals are present primarily in the nano-zone areas. This can be confirmed from observations of Fig. 11, which shows that a majority of the salt crystals are present in the nanozone areas. It can also Journal of Thermal Spray Technology be seen that the amount of salt crystals present in the cracks of the coating is negligible. This could result in lower stresses in the nanostructured YSZ coating as the cracks are not being wedged apart due to the growing crystals. The bare steel samples in all the tests have been attacked violently and to a greater degree in comparison to the coated samples (see Fig. 2, 5, and 8). Interestingly, the bare steel exposed in Test C (Fig. 8) has experienced intergranular corrosion. This was not observed in Test B (Fig. 5) and Test A (Fig. 2). This is due to the presence of chlorine in an environment containing water vapor, which is the case in Test C where a molten salt mixture was exposed to simulated boiler flue gases. Austenitic stainless steels that are heated into or slowly cooled through the 500-800 °C temperature range and exposed to a corrosive environment are susceptible to intergranular corrosion (Ref 18). Water vapor, which acts as an electrolyte that aids the corrosion process was not present in Test B (molten salts and dry air). Hence, the stainless steel in this case did not exhibit intergranular corrosion. Although the stainless steel samples behaved differently in the tests that involved molten salts and flue gases when compared to the tests that involved molten salts and air, no such differences in behavior were observed for any of the coated samples. 4.2 XRD XRD analysis was conducted to characterize the coatings further. Metastable tetragonal phase (t0 -phase) is the most desirable phase for the integrity of YSZ plasmasprayed coatings (Ref 20). The metastable tetragonal phase is identified at the 72-75° region of the 2h co-ordinate on the XRD profiles of YSZ. The (004) and (220) Fig. 8 SEM image of the cross section of the Type 309 steel substrate for Test C—600 °C, molten salt, flue gases, 168-h exposure Volume 21(5) September 2012—895 Peer Reviewed sulfate (Na2SO4) crystal. No salt crystals were found in the bond coat or at the bond coat-substrate interface of the nanostructured YSZ EBC. Peer Reviewed Fig. 9 (a) SEM image of the cross section (arrows point to salt crystals), (b) SEM image of the cross section showing a salt crystal at the interface of the bond coat and the substrate (arrows point to salt crystals), and (c) EDS spectrum of the salt crystal shown in (b) of the conventional YSZ top coat for Test C—600 °C, molten salt, flue gases, 168-h exposure 896—Volume 21(5) September 2012 Journal of Thermal Spray Technology Peer Reviewed Fig. 10 (a) SEM image of the cross section (arrows point to salt crystals), (b) SEM image of the cross section showing salt crystallizing in nano zones (arrows point to salt crystals), and (c) EDS spectrum of the salt crystals shown in (b) of the nanostructured YSZ top coat for Test C—600 °C, molten salt, flue gases, 168-h exposure Journal of Thermal Spray Technology Volume 21(5) September 2012—897 Fig. 12 XRD pattern of (a) conventional YSZ top coat and (b) nanostructured YSZ top coat for Test B—600 °C, molten salt, air, 168 h Fig. 13 XRD pattern of (a) conventional YSZ top coat and (b) nanostructured YSZ top coat for Test C—600 °C, molten salt, flue gases, 168-h exposure Peer Reviewed Fig. 11 SEM image of the cross section (arrows point to salt crystals) of the nanostructured YSZ top coat for Test C—600 °C, molten salt, flue gases, 168-h exposure peaks in this region is associated with the metastable t0 phase (Ref 20-23). Previous studies (Ref 20-22) conducted on phase changes in YSZ coatings after annealing to temperatures above 1000 °C have shown that the t0 phase decomposes into the equilibrium tetragonal (t), cubic (c), and monoclinic (m) phases. In this present study, the samples were heated to 600 °C for 168 h. Hence, it was expected that the metastable t0 phase would be retained in the YSZ top coatings. The XRD patterns of the coating samples from Tests B and C are shown in Fig. 12 and 13. It was found that the (004) and (220) peaks were still present in the XRD patterns of all the coating samples, indicating that the t0 -phase was retained after exposure to the hot corrosive environment. These results suggest improved performance of the YSZ coatings in this study since Fang et al. (Ref 24) have shown significant tetragonal to monoclinic phase transformation in room temperature tests involving partially-stabilized zirconia held in HF + HCl solution for a maximum duration of 650 h. Longer duration tests would be necessary to study the effect of the corrosive environment of the recovery boiler on the phase transformation of YSZ. 898—Volume 21(5) September 2012 Journal of Thermal Spray Technology Plasma-sprayed EBC based on conventional and nanostructured YSZ top coats were exposed to corrosive molten salts and simulated recovery boiler flue gases. It was found that the nanostructured-based YSZ EBC provided improved protection to the underlying steel substrate in comparison to the conventional-based YSZ EBC. This was attributed to the presence of semi-molten nanoparticles in the coating and the collection of salt material in the nanozones. Conventional YSZ coatings do not have these semi-molten particles in their microstructure and hence, salt penetrated through the cracks to the bond coat-substrate interface. The non-transformable tetragonal (t0 ) phase, which is the most desirable phase for YSZ coatings, has been retained in the coatings after the molten salt corrosion tests in a high-temperature environment. This behavior was expected since the boiler temperature was not sufficiently high for transformation of the t0 phase into the cubic (c) and equilibrium tetragonal (t) phases. This study has shown that further study will be required on the impact of the aggressive molten salt-flue gas mixture on the EBC and on the transport of salts through the coating-substrate system. Longer duration tests will be required to verify if the nanostructured YSZ coating remains impenetrable to the salts and continues to provide adequate protection to the bond coat and substrate. Acknowledgments The authors gratefully acknowledge the assistance of Mr. Douglas Singbeil with useful discussions. Funding for this project was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Government of Alberta Small Equipment Grants Program (SEGP), and the Canada Foundation for Innovation (CFI). References 1. D. Singbeil, Corrosion in Recovery Boilers, ASM Handb., 2006, 13C, p 762-802 2. T. Adams, W. Frederick, T. Grace, M. Hupa, K. Lisa, A. Jones, and H. Tran, Upper Furnace Deposition and Plugging, Kraft Recovery Boilers, Tappi Press, Atlanta, GA, 1997, p 247-282 3. M. Kawaji, X.H. Shen, H. Tran, S. Esaki, and C. Dees, Prediction of Heat Transfer in the Kraft Recovery Boiler Superheater Region, Tappi J., 1995, 78(10), p 214-221 4. S. Lee, N. Themelis, and M. Castaldi, High-Temperature Corrosion in Waste-to-Energy Boilers, J. Therm. Spray Technol., 2007, 16(1), p 104-110 Journal of Thermal Spray Technology 5. N. Solomon, Erosion-Resistant Coatings for Fluidized Bed Boilers, Mater. Perform., 1998, 37(2), p 38-43 6. N. Bala, H. Singh, and S. 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