Effect of water vapor on the oxidation behavior of

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Mar 23, 2018 - [6] E. Sadeghimeresht, H. Hooshyar, N. Markocsan, S. Joshi, P. Nylén ... [7] M. Oksa, P. Auerkari, J. Salonen, T. Varis, Fuel Process. Technol.
Received: 11 December 2017 DOI: 10.1002/maco.201710005

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Accepted: 23 March 2018

ARTICLE

Effect of water vapor on the oxidation behavior of HVAF-sprayed NiCr and NiCrAlY coatings Esmaeil Sadeghimeresht1 Jesper Liske2

| Johan Eklund2 | Julien Phother Simon2 |

| Nicolaie Markocsan1 | Shrikant Joshi1

1 Department

of Engineering Science, University West, 46153 Trollhättan, Sweden 2 Department

of Environmental Inorganic Chemistry, Chalmers University of Technology, 41296 Gothenburg, Sweden Correspondence Esmaeil Sadeghimeresht, Department of Engineering Science, University West, 46153 Trollhättan, Sweden. Email: [email protected]

Isothermal oxidation behavior of NiCr and NiCrAlY coatings deposited onto low alloy 16Mo3 steel by high-velocity air fuel (HVAF) process was investigated in 5% O2 + 20% H2O + N2 at 600 °C for 168 h. Whereas NiCrAlY showed lower mass gain compared to NiCr, both coatings succeeded in maintaining the integrity with the substrate during the exposure without any breakaway oxidation. A thin Cr-rich oxide scale (Cr2O3) formed on NiCr, and a thin mixed oxide scale (Al2O3 with NiCr2O4) formed on NiCrAlY significantly increasing the oxidation protection in the presence of water vapor. KEYWORDS NiCr, NiCrAlY, oxidation protection, thermal spray coating, water vapor

1 | INTRODUCTION Oxidation of load-bearing components such as water wall and superheater tubes has been shown as a major technical challenge in biomass-/waste-fired boilers.[1–3] It extremely restricts the boiler's functionality and leads to significant economic losses owing to the long downtimes required for service and overhaul.[4,5] Toward enhancing the durability of such components, and thereby increasing the thermal/ electrical efficiency, implementing a dense and adherent coating that lasts longer in the boiler's corrosive environment could be always in a high demand.[6] The composition and microstructure of such coatings can be peculiarly engineered to provide superior oxidation protection in a given environment and temperature.[7] Ni-based coatings alloyed with ∼20 wt% Cr[8,9] are widely used in protecting components against oxidation in dry environment as a protective oxide scale of Cr2O3[10] could maintain the coating and substrate from further oxidation. However, the corrosive environment in many industries, in particular waste and biomass-fired boilers, is characterized by Materials and Corrosion. 2018;1–10.

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high concentration of water vapor.[11] The oxidation is particularly severe in the presence of water vapor (pH2O = 0.02–0.04 atm[12] is reported to be sufficient) as well as oxygen, where formation of a volatile, unstable and non-protective CrO2(OH)2(g) (see Eq. [1]) depletes the oxide in Cr and suppresses the inherent protective nature of the oxide.[13,14] It is worth noting that sizeable literature on the role of H2O (g) is available; however, the exact effect of water vapor in the extent of degradation is still a controversial issue. 3 Cr2 O3 ðsÞ þ 2H2 O þ O2 ðgÞ → 2CrO2 ðOHÞ2 ðgÞ 2

ð1Þ

It is well-known that the microstructure of the coatings must be dense, defect-free, with a sufficient reservoir of protective scale-forming elements such as Cr or/and Al to serve as an efficient and durable barrier in a corrosive environment.[6,15] Prior coating efforts have been focused on the oxidation behavior of Ni-based coatings deposited by commonly used thermal spraying techniques such as highvelocity oxy fuel (HVOF)[7,16] and atmospheric plasma © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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spraying (APS).[17] However, the coating's features associated with these techniques, for example, splat boundaries, pores and in-situ formed oxides,[18] lead to the formation of a discontinuous oxide scale which provide motivation to seek even more efficient protective barriers. High-velocity air fuel (HVAF) spraying technique has being increasingly employed to produce coatings with better protective characteristics.[19] In HVAF, compressed air (∼9 bar) and a fuel (usually propane) are mixed in the combustion chamber of spray gun. The feedstock powder is axially injected into the chamber by a carrier gas (e.g., N2). Due to the thermal energy produced by combustion, the gas jet exiting from the nozzle (located in front of the combustion chamber) is heated up to a temperature of ∼1800 °C[20,21] having a velocity of around 700–1200 m s−1.[21] The relatively low particle temperature and dwell time compared to plasma spraying and HVOF guarantee that there is almost no in-situ oxygen pick-up, phase transformation, or elemental depletion of powder feedstock during spraying.[22] Therefore, the protective scale-forming elements such as Cr or/and Al are not consumed during the spraying process to form oxides and preserved for oxidation protection. The few studies have been performed on HVAF-sprayed Ni-based coatings confirm that the coatings present excellent oxidation behavior in hightemperature dry environments.[23,24] Given the limited data available in wet environment, particularly for the coatings sprayed with the HVAF technique, laboratory testing was performed in 5% O2 + 20% H2O + N2 at 600 °C for 168 h. The environment was selected in order to get a better understanding of the complex oxidation reactions that occur during field exposures, such as in biomass boilers, in a more simplified environment. The results of more complex laboratory exposures (5% O2 + 20% H2O + N2 with addition of KCl salt) and actual boiler exposures, which are currently being performed, will be reported in separate papers.

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coatings were also sprayed on the flat faces of the cut cylindrical pieces to ensure that the specimens were coated on all sides to study the oxidation kinetics. Prior to spraying, the substrates were grit blasted with alumina particles (63 ± 10 μm) for cleaning and roughening the surfaces to ensure good bonding. The HVAF spray parameters given in Table 1 were chosen based on preliminary coating trials conducted to obtain the least porous microstructure. All coatings were sprayed to a thickness of around 250 µm. The surface of the coatings was analyzed using a 3D focus-variation optical microscope (InfiniteFocusSL, Alicona GmBH, Austria). The coatings deposited with thermal spraying processes reveal relatively high profile roughness parameters and include some features like unmelted or semimelted particles, and surface pores. Therefore, all surfaces of the coatings (as seen in Figure 1) were polished with 0.2 µm SiC suspension to a roughness of Ra < 0.1 µm prior to oxidation exposure in order to eliminate the complex effect of the coating's surface roughness on the oxidation behavior. Figure 1 shows the presence of few semi-melted particles over the coatings’ surface (round features) and zones where the particles were fully deformed or splashed (flat regions). The exposure was performed in a tube furnace fitted with a 44 mm diameter silica-glass tube. The experiments were carried out in a 5% O2 + 20% H2O + N2 environment, at ambient pressure and flow rate of 3 cm3 s−1 (equal to 180 ml min−1). The water-vapor content was regulated by setting the dew point of the gas to 60.4 °C using a reflux condenser. The flow rate of the gas was measured by a BIOS DC2 Flow Calibrator. The samples were placed in separate alumina boats, one tilted sample (60°) in each. A flow of N2 over the specimens was maintained throughout the exposure to avoid any corrosion during cooling. The alumina boats were positioned on an alumina tray in the furnace with the samples parallel to the flow direction. Three samples of each coating chemistry were exposed isothermally at 600 °C

2 | EXPERIMENTAL PROCEDURE A commercially available low carbon steel – 16Mo3 (in wt%; 0.01Cr-0.3Mo-0.5Mn-0.3Si-0.15C-Bal. Fe), a widely used boiler tube material, was employed as the substrate material. The alloy was sourced in the form of rods with 25-mm diameter. Two commercially available gas-atomized powders supplied by H. C. Starck GmbH, that is, NiCr (in wt%; 21.3Cr-0.1O, Bal. Ni) and NiCrAlY (in wt%; 21.2Cr-7.3Al0.6Y-0.2O-Bal. Ni), each with particle size distribution of 45 ± 22 μm, were used to spray onto the 16Mo3 substrate. A M3™-HVAF gun (Uniquecoat, Oilville, VA, USA) was utilized to produce the coatings. A 16Mo3 rod (length = 500 mm) was placed in a horizontal rotating mandrel for spraying the coating on the cylindrical surface. After spraying, the rod was cut into pieces of 5 mm length and

TABLE 1 HVAF process parameters used to spray the NiCr and NiCrAlY coatings Variables Nozzle type

3L2G

Air pressure, MPa

0.8

Fuel 1 pressure-Propane, MPa

0.7

Fuel 2 pressure-Propane, MPa

0.7

Carrier gas pressure-N2, MPa

0.4

Feed rate, g min−1

150

Pass velocity, m min−1 −1

50

Pass spacing, mm rev

5

Spray distance, mm

300

Number of passes

8

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FIGURE 1 3-D surface topography of the as-sprayed coatings before polishing, a) NiCr, and b) NiCrAlY. Round features represent the semimelted particles while flat regions (at relatively low height) represent the fully deformed particles [Color figure can be viewed at wileyonlinelibrary.com]

(±3 °C) for 168 h. The highest temperature in typical biomass and waste boilers on superheater tubes is reported as ∼450 and 550 °C, respectively, during the course of operation (>10 000 h).[25] The selection of the test temperature (600 °C) was to accelerate the dominant corrosion mechanisms in a shorter period of exposure (168 h). Just before and after the exposure, each sample was individually weighed using a Sartorius™ balance (Cubis MSA3.6P0TRDM, Sartorius, Germany) with microgram resolution. The balance was calibrated using its internal calibration function and periodically with standard weights. A Leica EM TIC 3X triple ion-beam cutter (Leica Microsystems, Helsinki, Finland) equipped with an argon ion gun and operated at 6 kV for sputtering was used to prepare broad and smooth cross sections in the millimeter range. The board ion beam (BIB) cross sectioning is an advanced technique to produce accurate cross sections with limited artifacts and distortion through the oxidation product. The cross-section and surface morphology of all as-sprayed and exposed coatings was characterized using a QUANTA-200 FEG (FEI, the Netherlands) SEM equipped with X-ray energy dispersive spectroscopy (EDS). The morphology and microstructure of the as-sprayed and exposed coatings were studied using backscattered electron (BSE) signal in the SEM. The phases present in the coating before and after oxidation tests were identified by an X-ray diffractometer (D5000, Siemens, Germany), equipped for grazing incidence analysis with CuKα radiation (λ = 0.154 nm) operating with a fixed incident angle of 1° and diffraction angle (2θ) between 25° and 80°. The porosity content of the coatings was determined by image analysis (IA) technique using ImageJ software[26] by converting the SEM micrographs of the polished coatings with horizontal field width of 100 μm into binary images, and quantifying the percentages based on the gray scale contrast. Micro-hardness measurements were performed on the polished cross-sections of the coatings according to ASTM E384 standard with a Vickers indenter (HV) (Shimadzu,

HMV-2, Tokyo, Japan) using a load of 300 g and a dwell time of 15 s.[27] Hardness of each coating was determined as an average of ten measurements performed on the sample.

3 | RESULTS AND DISCUSSION 3.1 | Characterization of the feedstock powders Figure 2 shows the particle size distribution, spherical morphology, cross-section, and EDS point analysis of the NiCr and NiCrAlY powders. Very few satellite particles around large particles and some small pores as typical characteristics of the morphology of gas-atomized powders[28] could be seen in both images. The EDS analysis showed that the composition of the powders was close to their nominal compositions given in section 2. It is reported that the narrow size distribution of the particles used in this study (45 ± 22 μm), seen in Figure 2, leads to more uniform particle momentums and particle trajectories at injector exit which consequently result in lower scatter of particle velocity and temperature, thus more uniform and dense coatings at impact on the substrate.[29]

3.2 | Microstructure of polished coatings Figure 3a,b shows the cross sections of the polished NiCr and NiCrAlY coatings sprayed by the HVAF process. The porosity measured on the cross-sectional SEM images using image analysis method was 0.6 ± 0.1 and 1.3 ± 0.2 vol% for NiCr and NiCrAlY, respectively. The coatings produced had less porosity to the substrate than the coatings produced with the other spraying techniques such as HVOF and APS, see Table 2. The hardness of the coatings was 391 ± 19 and 486 ± 26 HV0.3 for the NiCr and NiCrAlY coatings, respectively. Generally, the hardness value is affected by material composition and microstructure. While composition

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FIGURE 2

Morphology (SEM: SE mode), cross-section (SEM: BSE mode) with EDS point analysis and particle size distribution of the feedstock powders, a) NiCr, and b) NiCrAlY [Color figure can be viewed at wileyonlinelibrary.com]

was identical for the coatings sprayed with different thermal spray processes (see Table 2), the main reason for the hardness variation can be attributed to the coating microstructure. The higher hardness of the HVAF coatings produced in this study may be attributed to their lower content of porosity compared to the HVOF and APS coatings. A few fine pores distributed within the HVAF coatings confirmed the combined effect of softening of the sprayed particles at high temperature and high in-flight particle velocity in HVAF.[30–32] A few particles that were either semi-molten or not fully plastically deformed to form splats were also noted. There also appeared to be good interlocking between the coating and substrate, based on the sound

FIGURE 3

integrity of the interface. The different local contrast observed in the NiCrAlY coating cross-section (Figure 3b) corresponds to the presence of different phases.[33] The brighter phase represents the γ phase enriched in Ni, while the gray phase corresponds to the β phase enriched in Al. The HVAF process was able to form the very fine β phase with a uniform distribution within the γ phase to ensure homogeneous diffusion of Al to the surface protective layer during exposure in an oxidizing environment. Figure 3 showed that the NiCr and NiCrAlY coatings sprayed by HVAF could be potentially good candidates in term of microstructure for oxidation protection applications. The protective scale forming elements such as Cr or/and Al are

Back-scattered SEM micrographs of cross-sections and EDS point analysis of the as-sprayed coatings prepared by broad ion beam (BIB) milling technique, a) NiCr, b) NiCrAlY

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TABLE 2 Coating properties obtained using the HVAF process in this study in comparison with the HVOF and APS coatings in earlier studies NiCr

Porosity (vol%) a

Hardness

NiCrAlY

HVAF (this study)

HVOF

APS

HVAF (this study)

HVOF

0.6 ± 0.1

2.2[34]

2.22–4.45[35]

1.3 ± 0.2

3.2[36]

391 ± 19 (HV0.3)

298 (HV0.2)

[37]

284 (HV0.2)

[37]

486 ± 26 (HV0.3)

350–400 (HV0.3)

APS 2.88–4.42[35] [38]

300–375 (HV0.1)[39]

a

The applied load in Vickers microhardness measurement is given in ().

not depleted during the HVAF spraying process[22] and preserved for oxidation protection (see EDS point analysis in Figure 3a,b). This is a consequence of the negligible in situ oxide pick up or phase transformation during HVAF spraying.[40]

3.3 | Phase constitution of as-sprayed coatings The XRD patterns of the feedstock powders and polished NiCr and NiCrAlY coatings are shown in Figure 4. Three primary peaks of the powders (2θ[°] ≈ 44.5, 51.5, and 76) correspond to the austenitic Ni-(Cr/Al) solid solution phase. The polished coatings retained the solid solution phase of the feedstock powders. No significant phase transformation or oxide formation (e.g., NiO, Cr2O3, or Al2O3) was observed in the as-sprayed NiCr and NiCrAlY coatings as the XRD could not detect them. The presence of such phases might be below

the detection limit in XRD. Typically, the in situ oxides formed during thermal spraying are attributed to the particles being exposed to high temperature in ambient air environment in-flight during the spraying process[41] and/or because of pre-existing oxygen in the feedstock material.[42] The influence of the above is minimal in the investigated samples, as the dwell time in the HVAF process is short and the temperature (2200 °C) and APS (>5000 °C).[21,44,45] The XRD patterns of the as-sprayed coatings (Figure 4) show that the main peaks shifted toward slightly lower 2θ angles which might be due to the expansion effect of the Cr atom in the lattice of Ni as a result of the rapid cooling of (Ni, Cr) super-saturation solution during gas atomization process.[18] The peak broadening of the as-sprayed coatings compared to the powders is due to three primary factors of (i) presence of micro-strain due to the plastic deformation (peening effect) during the HVAF spraying process[22]; (ii) reduction in crystallite size; and (iii) instrumental broadening due to beam size, sample to detector distance, air scatter, etc.[46]

3.4 | Mass change measurement

FIGURE 4

XRD patterns of the feedstock powders and corresponding polished coatings, a) NiCr, and b) NiCrAlY [Color figure can be viewed at wileyonlinelibrary.com]

The mass gain for the NiCr and NiCrAlY coatings was 0.16 (±0.01) and 0.09 (±0.005) mg cm−2, respectively, indicating oxide formation on the surface. While the values showed that both coatings were protective in the given environment, a relatively lower mass gain was observed in the NiCrAlY coating compared to the NiCr coating. It is most probably due to the formation of a thin, dense, and protective Cr2O3 on NiCr and Al2O3 on NiCrAlY. Although the oxidation study was performed in only one exposure period of 168 h, it is well-known that a high oxidation rate could be reported at the beginning of exposure which is due to the discontinuity in formed oxide films (Cr2O3 or Al2O3) in the coating.[47] On increasing the exposure time, the oxide scale gradually covers the entire surface and becomes continuous, thereby isolating the coating from the oxidation environment at high temperature. It was reported that the major mass gain of the thermal spray coatings during the early stages of the exposure might be attributed to the rapid formation of oxides not only on the surface but also at the splat boundaries or/and

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within interconnected pores due to the high diffusion of oxygen.[17,48] In a previous work performed in ambient air (in the absence of water vapor),[44] the above two coatings had shown much better corrosion protection behavior, as reflected in their lower recorded mass gains (0.014 ± 0.004 and 0.008 ± 0.002 mg cm−2 for NiCr and NiCrAlY, respectively, after 168 h of exposure at 600 °C). The results obtained in the present study confirmed the detrimental effect of water vapor on the corrosion performance of the coatings. Moreover, it is also pertinent to mention that the mass gain of a NiCr bulk alloy (Ni22Cr) exposed in water vapor-containing (30% H2O + 3% O2) and ambient air environments at 650 °C after 96 h was recorded as 0.08 and 0.13 mg cm−2, respectively,[49] confirming the beneficial effect of the HVAF coatings to control the high temperature corrosion.

3.5 | Phase constitution of exposed coatings The XRD and SEM/EDS analysis of the cross-section of the oxidized samples were also performed to study the composition of the formed oxide scales and correlate the oxidation mechanisms to coating features discussed previously, as seen in Figures 5–7. According to the XRD analysis shown in Figure 5, the NiCr coating formed a layer of Cr2O3, whereas a mixed layer of Al2O3, and NiCr2O4 formed on the NiCrAlY coating after the exposure. The thickness of the oxidation product formed on both coatings was very thin since the primary phases of the coatings (the main three peaks seen in Figure 4) were also detected even after the exposure (see

FIGURE 5

XRD patterns of the exposed coatings in 5% O2 + 20% H2O + N2 for 168 h at 600 °C, NiCr and NiCrAlY [Color figure can be viewed at wileyonlinelibrary.com]

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Figure 5). No sign of NiO was observed after the exposure, confirming that this oxide most probably reacted with the more stable oxides, for example, Cr2O3 and formed NiCr2O4.

3.6 | Microstructure of formed oxide scales 3.6.1 | NiCr coating As can be seen in Figure 6a, no trace of internal oxide through individual splats or splat boundaries could be observed, confirming that water vapor could not significantly damage the coating. Figure 6b shows the NiCr coating/substrate interface after exposure where no obvious trace of corrosive agent diffusion was observed. Figure 6c along with the EDS elemental mapping analysis clearly shows that the formed protective thin layer (less than 1 μm) of oxide remained intact on the surface of the NiCr coating even after exposure. Based on the EDS analysis, the oxide scale consisting of a continuous mixed layer of Cr-rich oxide could be identified on the surface. Formation of the oxide scale did not create a significant Cr-depletion zone. After 168 h of exposure at 600 °C, no sign of coating or oxide spallation was observed. There is limited information regarding the oxidation of NiCr alloys in wet environment in literature, especially in view of the extensive use of such materials in many corrosive combustion systems such as biomass and waste incineration, or the anticipated applications in ultra-supercritical boilers being recently developed for power plants. Although it was suggested that Ni-containing oxides might be unaffected by the presence of water vapor,[50,51] it was shown elsewhere that in the presence of water vapor, an increase in the oxidation rate of a NiCr-based coating with addition of Al could be detected.[52] It was further shown that the degree of material degradation due to water vapor depends on the partial pressure of oxygen in the test environment. At low pO2, formation of less-protective oxides such as NiO could be reduced and, thereby, the Cr content required to form the protective Cr-rich scale would be lower.[53] Furthermore, there is a good evidence from NiCr that scale formation involves dominant inward oxygen diffusion, combined with significant outward Al diffusion, the diffusion pathways being on splat boundaries. As the NiCr coating in the present study had a Cr content of ∼21 wt%, a thin and protective oxide scale rich in Cr was expected as also reported in the literature.[54] The fact that the scale neither spalls nor grows is amply indicative of its protective nature. It seems that with 21% Cr, the depletion in Cr is relatively small and breakaway is avoided.[55]

3.6.2 | NiCrAlY coating As seen in Figure 7a–c, the NiCrAlY coating microstructure seems to be unaffected by the wet environment. Moreover, a

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FIGURE 6 Cross-sectional SEM micrographs of the NiCr coating oxidized in 5% O2 + 20% H2O + N2 at 600 °C for 168 h, a) overall view, b) coating/substrate interface, c) formed oxide scale/coating interface and the EDS elemental mapping analysis after exposure [Color figure can be viewed at wileyonlinelibrary.com]

very thin continuous layer (less than 1 μm) of aluminum oxide entirely covering the surface of the NiCrAlY coating formed. Some Al signals could be detected in EDS elemental mapping analysis, beneath the continuous oxide scale and within the coating, which were most probably attributed to the Al2O3 scale formed at the splat boundaries. The oxide scale formed on NiCrAlY showed a similar thickness as that formed on the NiCr sample in Figure 7c. The formed Al2O3 scale on NiCrAlY was uniform without any degradation within the splat boundaries. Similar to the NiCr sample in Figure 6b, no obvious trace of oxygen diffusion was observed adjacent to the NiCrAlY coating/substrate interface after exposure (Figure 7b). There was also no significant Al/Cr depletion

beneath the formed surface oxide after exposure. Thus, the coating microstructure was almost unaffected after the exposure (cf Figures 6 and 7). It is worth noting that, in a previous work,[56] a NiCrAlY coating had similarly showed lower mass gain compared to a NiCr coating exposed to dry ambient air environment (0.014 ± 0.004 and 0.008 ± 0.002 mg cm−2 for NiCr and NiCrAlY, respectively, after 168 h of dry exposure at 600 °C). Together with the present result, this is suggestive of better performance of NiCrAlY coatings in both dry and wet environments. As also already mentioned, the mass gain of both coatings was higher in dry environment compared to the present wet condition (5% O2 + 20% H2O + N2) which might be due either to the presence of water vapor or lower oxygen

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FIGURE 7 Cross-sectional SEM micrographs of the NiCrAlY coating oxidized in 5% O2 + 20% H2O + N2 at 600 °C for 168 h, a) overall view, b) coating/substrate interface, c) formed oxide scale/coating interface and the EDS elemental mapping analysis after exposure [Color figure can be viewed at wileyonlinelibrary.com]

partial pressure in the present wet environment. Galerie et al.[57] have reported low rate of NiO growth when H2O is the only oxidant present due to slow phase boundary reaction for the insertion of oxide ions into the scale. In any case, the concentration of Cr needed to form an external scale is expected to decrease with decreasing growth rate of NiO, in agreement with the experimental findings. While it was previously found that water vapor addition enhances the growth rate of the Cr-rich scale,[53] it was shown in this study that water vapor increased the oxidation rate. Considering simultaneous effect of oxygen and water vapor, high pO2 could be more beneficial as it encourages formation of the protective oxide scale, whereas adding water vapor interrupts the protectiveness of the formed oxide scale. A particularly interesting comparison of the results from this work could be with Fe-based materials alloyed with Cr, which undergo breakaway oxidation in 5% O2 + 20% H2O + N2,[58] while Ni-based coatings, for example, NiCr and NiCrAlY in this study, did not. There is no reason but to assume that there is no volatilization for

the latter coatings, while in the former alloys, breakaway oxidation was triggered by the evaporation of CrO2(OH)2. Moreover, the interdiffusion coefficient in bcc FeCr is larger than that in fcc NiCr.[59] However, comparison of the results indicates that H2O accelerates chromia growth on FeCr but not NiCr or NiCrAlY (mass gains after 168 h at 600 °C in wet O2 was ∼1.4 mg cm−2 for Fe25Cr while it was ∼0.16 and 0.09 mg cm−2 for the NiCr and NiCrAlY coatings, respectively in this study). The reason for this difference might be that Fe rapidly incorporates into the formed chromia scale in water vapor, but there is not a significant effect of Ni on the scales on NiCr or NiCrAlY at 600 °C. To sum up, both coatings were observed to be successful in maintaining its adhesion with the substrate over the period of exposure. Moreover, it was clear from the cross-sectional EDS that oxygen diffusion was not substantial enough to penetrate through the coatings and reach the substrate. This highlights the beneficial protective capability of the dense HVAF coatings, since the substrate

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did not suffer any oxidation. Furthermore, the oxide scales formed over the coatings showed high spallation resistance, which is desirable to ensure effectiveness of the coatings as the oxidation behavior of an alloy is defined mainly by scale spallation resistance.[9] Therefore, the HVAF coatings, with chemistry promoting formation of a protective oxide scale, could be useful in imparting oxidation resistance in the test conditions that simulated a wet oxidizing environment.

4 | CONCLUSIONS The aim of this work was to study the oxidation behavior of the HVAF-sprayed NiCr and NiCrAlY coatings in 5% O2 + 20% H2O + N2 at 600 °C for 168 h. 1) A thin Cr-rich oxide scale (Cr2O3) formed on the NiCr coating, whereas a thin Al2O3 mixed with NiCr2O4 formed on the NiCrAlY coating. 2) While there was a sign of internal oxidation in the NiCrAlY coating, no breakaway oxidation occurred in both exposed coatings in the presence of water vapor. 3) The water vapor effect was insignificant in such a rather low pO2 environment due to the large Cr or Al reservoir, and the continued growth of Cr-rich oxide or Al2O3. 4) Both coatings found to be successful in maintaining the integrity with their respective substrates throughout the exposure to the environment of the study, without any significant spalling of the oxide scales. 5) The HVAF-sprayed NiCr and NiCrAlY coatings may be recommended as a proper chemistry to be used for boilers (as well as other similar high-temperature applications) as far as oxidation resistance in water vapor is concerned. 6) Further studies on shorter exposures need to be performed on the coatings to understand oxidation mechanism in the presence of water vapor. Investigations in more complex environments are required to more accurately assess the coatings performance in boiler applications.

ACKNOWLEDGMENTS Financial support of the Knowledge Foundation for the SCoPe project (RUN 2016-0201) and Västra Götalandsregionen (VGR) for the PROSAM project (RUN 2016-01489) are highly acknowledged. The Swedish High Temperature Corrosion Centre (HTC) at Chalmers University of Technology is also appreciated for the help in the performing the exposure and XRD/BIB/SEM/EDS analysis. The authors would like to thank Mr. Jonas Olsson, Mr. Stefan Björklund, and Mr. Kenneth Andersson for their valuable help and advice in processing and characterization of the HVAF coatings in this study. The authors would also like to appreciate Mr.

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Joakim Bonér (Mytolerans AB, Sweden) for his help to perform the 3D microscopy.

ORCID Esmaeil Sadeghimeresht 7663-9631

http://orcid.org/0000-0002-

REFERENCES [1] M. Montgomery, T. Vilhelmsen, S. A. Jensen, Mater. Corros. 2008, 59, 783. [2] M. Schütze, J. Quadakkers, Mater. Corros. 2011, 62, 475. [3] S. C. Okoro, S. Kiamehr, M. Montgomery, F. J. Frandsen, K. Pantleon, Mater. Corros. 2016, 68, 499. [4] K. Szymański, A. Hernas, G. Moskal, H. Myalska, Surf. Coat. Technol. 2015, 268, 153. [5] S. C. Okoro, M. Montgomery, F. J. Frandsen, K. Pantleon, Energy Fuels 2015, 29, 5802. [6] E. Sadeghimeresht, H. Hooshyar, N. Markocsan, S. Joshi, P. Nylén, Oxid. Met. 2016, 86, 299. [7] M. Oksa, P. Auerkari, J. Salonen, T. Varis, Fuel Process. Technol. 2014, 125, 236. [8] G. Kaushal, H. Singh, S. Prakash, Metall. Mater. Trans. A 2011, 42, 1836. [9] H. Singh, D. Puri, S. Prakash, R. Maiti, Mater. Sci. Eng. A 2007, 464, 110. [10] M. Kumar, H. Singh, N. Singh, Arch. Metall. Mater. 2013, 58, 523–528. [11] K. Segerdahl, J.-E. Svensson, L.-G. Johansson, Mater. Corros. 2002, 53, 247. [12] D. J. Young, High Temperature Oxidation and Corrosion of Metals, 2nd ed., Elsevier, Amsterdam 2016. pp. 549. [13] H. Asteman, J.-E. Svensson, L.-G. Johansson, Corros. Sci. 2002, 44, 2635. [14] X. Peng, J. Yan, Y. Zhou, F. Wang, Acta Mater. 2005, 53, 5079. [15] E. Sadeghimeresht, L. Reddy, T. Hussain, N. Markocsan, S. Joshi, Corros. Sci. 2018, 132, 170. [16] M. Oksa, J. Metsäjoki, J. Kärki, J. Therm. Spray Technol. 2014, 24, 194. [17] P. Niranatlumpong, C. B. Ponton, H. E. Evans, Oxid. Met. 2000, 53, 241. [18] B. Song, Z. Pala, K. T. Voisey, T. Hussain, Surf. Coat. Technol. 2017, 318, 224. [19] A. A. Verstak, G. Kusinski, International Thermal Spray Conference and Exposition (ITSC) 2012, American Society for Metals, OH, USA 2012. [20] Z. Zeng, N. Sakoda, T. Tajiri, S. Kuroda, Surf. Coat. Technol. 2008, 203, 284. [21] E. Sadeghimeresht, N. Markocsan, P. Nylén, S. Björklund, Appl. Surf. Sci. 2016, 369, 470. [22] E. Sadeghimeresht, N. Markocsan, P. Nylén, Surf. Coat. Technol. 2016, 304, 606. [23] G.-J. Yang, X.-D. Xiang, L.-K. Xing, D.-J. Li, C.-J. Li, C.-X. Li, J. Therm. Spray Technol. 2012, 21, 391. [24] A. P. Wang, Z. M. Wang, J. Zhang, J. Q. Wang, J. Alloys Compd. 2007, 440, 225.

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[25] J. Koppejan, S. Van Loo, The Handbook of Biomass Combustion and Co-Firing, Routledge, London 2012. [26] C. A. Schneider, W. S. Rasband, K. W. Eliceiri, Nat. Methods 2012, 9, 671. [27] ASTM E384-11e1, Standard Test Method for Knoop and Vickers Hardness of Materials, ASTM International, West Conshohocken, PA, 2011, www.astm.org [28] T. Pettersson, Characterization of Metal Powders Produced by Two Gas Atomizing Methods for Thermal Spraying Applications, KTH Royal Institute of Technology, Stockholm 2016. [29] P. Fauchais, G. Montavon, G. Bertrand, J. Therm. Spray Technol. 2010, 19, 56. [30] G.-J. Yang, X.-D. Xiang, L.-K. Xing, D.-J. Li, C.-J. Li, C.-X. Li, J. Therm. Spray Technol. 2012, 21, 391. [31] S. Matthews, B. James, M. Hyland, Corros. Sci. 2009, 51, 1172. [32] S. Matthews, B. James, M. Hyland, Corros. Sci. 2013, 70, 203. [33] J. C. Pereira, J. C. Zambrano, M. J. Tobar, A. Yañez, V. Amigó, Surf. Coat. Technol. 2015, 270, 243. [34] N. F. Ak, C. Tekmen, I. Ozdemir, H. S. Soykan, E. Celik, Surf. Coat. Technol. 2003, 174-175, 1070. [35] H. Singh, D. Puri, S. Prakash, Surf. Coat. Technol. 2005, 192, 27. [36] F. H. Yuan, Z. X. Chen, Z. W. Huang, Z. G. Wang, S. J. Zhu, Corros. Sci. 2008, 50, 1608. [37] V. Higuera, F. J. Belzunce, A. Carriles, S. Poveda, J. Mater. Sci. 2002, 37, 649. [38] G. Zhang, A.-F. Kanta, W.-Y. Li, H. Liao, C. Coddet, Mater. Des. 2009, 30, 622. [39] S. F. Chen, S. Y. Liu, Y. Wang, X. G. Sun, Z. W. Zou, X. W. Li, C. H. Wang, J. Therm. Spray Technol. 2014, 23, 809. [40] E. Sadeghimeresht, N. Markocsan, P. Nylén, J. Therm. Spray Technol. 2017, 26, 243. [41] M. H. Enayati, F. Karimzadeh, M. Tavoosi, B. Movahedi, A. Tahvilian, J. Therm. Spray Technol. 2010, 20, 440. [42] S. T. Bluni, A. R. Marder, Corrosion 1996, 52, 213. [43] E. Sadeghimeresht, N. Markocsan, S. Joshi, Surf. Coat. Technol. 2017, 317, 17. [44] E. Sadeghimeresht, N. Markocsan, P. Nylén, J. Therm. Spray Technol. 2016, 25, 1604.

SADEGHIMERESHT

ET AL.

[45] N. Rana, R. Jayaganthan, S. Prakash, Trans. Indian Inst. Met. 2014, 67, 393. [46] B. B. He, Two-Dimensional X-Ray Diffraction, John Wiley & Sons, New York 2011. [47] L. Intiso, L.-G. Johansson, S. Canovic, S. Bellini, J.-E. Svensson, M. Halvarsson, Oxid. Met. 2012, 77, 209. [48] H. Choi, B. Yoon, H. Kim, C. Lee, Surf. Coat. Technol. 2002, 150, 297. [49] N. Mu, K. Y. Jung, N. M. Yanar, G. H. Meier, F. S. Pettit, G. R. Holcomb, Oxid. Met. 2012, 78, 221. [50] N. Hussain, K. A. Shahid, I. H. Khan, S. Rahman, Oxid. Met. 1994, 41, 251. [51] N. Hussain, A. H. Qureshi, K. A. Shahid, N. A. Chughtai, F. A. Khalid, Oxid. Met. 2004, 61, 355. [52] C. Zhou, J. Yu, S. Gong, H. Xu, Surf. Coat. Technol. 2002, 161, 86. [53] E. Essuman, G. H. Meier, J. Żurek, M. Hänsel, T. Norby, L. Singheiser, W. J. Quadakkers, Corros. Sci. 2008, 50, 1753. [54] J. E. Croll, G. R. Wallwork, Oxid. Met. 1969, 1, 55. [55] N. K. Othman, N. Othman, J. Zhang, D. J. Young, Corros. Sci. 2009, 51, 3039. [56] E. Sadeghimeresht, N. Markocsan, M. Huhtakangas, S. Joshi, Surf. Coat. Technol. 2017, 316, 10. [57] A. Galerie, Y. Wouters, M. Caillet, Mater. Sci. Forum 2001, 369372, 231. [58] B. Pujilaksono, T. Jonsson, H. Heidari, M. Halvarsson, J.-E. Svensson, L.-G. Johansson, Oxid. Met. 2011, 75, 183. [59] Q. Zhang, J.-C. Zhao, J. Alloys Compd. 2014, 604, 142.

How to cite this article: Sadeghimeresht E, Eklund J, Phother Simon J, Liske J, Markocsan N, Joshi S. Effect of water vapor on the oxidation behavior of HVAF-sprayed NiCr and NiCrAlY coatings. Materials and Corrosion. 2018;1–10. https://doi.org/10.1002/maco.201710005