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A comparative study of hot corrosion resistance of HVOF sprayed NiCrBSi and Stellite-6 coated Ni-based superalloy at 900 °C
Materials Science and Engineering A 445–446 (2007) 210–218
A comparative study of hot corrosion resistance of HVOF sprayed NiCrBSi and Stellite-6 coated Ni-based superalloy at 900 ◦C T.S. Sidhu ∗ , S. Prakash, R.D. Agrawal Metallurgical & Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 13 June 2006; accepted 10 September 2006
Abstract A comparative study was carried out to evaluate the hot corrosion resistance of NiCrBSi and Stellite-6 coated nickel-based superalloy Superni 600 (15.5Cr–10Fe–0.5Mn–0.2C–balance Ni) in the molten salt environment of Na2 SO4 –60%V2 O5 salt mixture at 900 ◦ C under cyclic conditions. Hot corrosion experiments were performed for 50 cycles, each cycle consisting of 1 h heating in the laboratory tube furnace followed by 20 min cooling in the open air. The thermogravimetric technique was used to establish the kinetics of corrosion. The morphology, phase composition and element concentration of the corrosion products were detected using the combined techniques of X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive analysis (SEM/EDAX) and electron probe micro analyzer (EPMA). The hot corrosion resistance of NiCrBSi coating has been found to be better than that of Stellite-6 coating. The hot corrosion resistance of both the coatings has been attributed to the formation of oxides of chromium, silicon and nickel along with spinels of nickel–chromium and cobalt–chromium. These oxides seal/plug the pores and splat boundaries, and act as diffusion barriers to the inward diffusion of corroding species. © 2006 Published by Elsevier B.V. Keywords: NiCrBSi; Stellite-6; HVOF coating; Hot corrosion; Superalloy; Protective coatings
1. Introduction Due to depletion of high-grade fuels and for economic reasons, residual fuel oils are often used in boilers, refinery furnaces and gas turbines. Residual fuel oil contains sodium, vanadium and sulphur as impurities, which form compounds such as Na2 SO4 (melting point 884 ◦ C), V2 O5 (melting point 670 ◦ C), and complex vanadates by reactions in the combustion systems [1–3]. These compounds, known as ash, deposit on the surface of materials and induce accelerated oxidation (hot corrosion). Moreover, the vanadium compounds are also good oxidation catalysts and allow oxygen and other gases in the combustion atmosphere to diffuse rapidly to the metal surface and cause further oxidation [4]. A number of countermeasures are currently in use or under investigation to combat the hot corrosion such as using inhibitors, controlling the process parameters, designing suitable industrial alloys and depositing the protective coatings. The inhibitors like MgO, CeO2 , CaO, MnO2 , etc. have been
∗
Corresponding author. Tel.: +91 1332 285731; fax: +91 1332 285243. E-mail address: [email protected] (T.S. Sidhu).
0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2006.09.015
successfully used to decrease the extent of hot corrosion in the most aggressive environment of Na2 SO4 –60% V2 O5 at 900 ◦ C [5–6]. However, their use in actual industrial environment is limited due to the practical problems of injecting them along with the fuel in the combustion chamber. Further, controlling the various process parameters of the boiler and gas turbine such as air/fuel ratio, temperature, pressure, etc., can also be useful to some extent to combat the hot corrosion, but these parameters can be controlled only within certain limits. The corrosion resistance of the superalloys can be improved by adding fair amounts of Al and Cr, and small amounts of Y, Zr and Hf [7]. However, these elements (such as Al, Cr) can be added only up to a limited extent as their higher concentrations adversely affects the mechanical properties of the alloys [8,9]. The use of protective coatings is the most practical, reliable and economically viable method to resist the hot corrosion. These composite materials perform better under extreme conditions, the base material provides the required mechanical strength and the coatings protect them against wear, erosion or corrosion [10–12]. The composition and structure of the coatings are determined by the role that they have to play in the various material systems and service environments [13]. During service, the coatings are
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Table 1 Composition of the feedstock alloys, coating thickness, microhardness and porosity Feedstock alloys
Chemical composition (wt.%)
Shape
Average coating thickness (m)
Average porosity (%age)
Microhardness, Hv (Vickers hardness)
Particle size (m)
Weight (%age)
NiCrBSi powder (PA 101HV) Stellite-6 powder (Jet-Kote 7206)
Ni–15.3Cr–3.1B–4.8Si–4.2Fe–0.6C
Spherical
270
<1.5
810–850
+45, −45
1.02, 98.98
Co–28Cr–4.9W–2.7Fe–2.3Ni–1.1Si–1.2C
Spherical
265
<2
850–900
−53, +45, −45
0.22, 2.23, 97.55
expected to form slowly growing protective oxides which should not allow the corrosive species to diffuse into the coating and should also have resistance to cracking and spallation under mechanical and thermal stresses induced during operation of the components. Further, these coating should also serve as a reservoir for the elements expected to form protective oxides [13,14]. Therefore, the identification/development of suitable coatings is of great interest for higher temperature applications in boilers and gas turbines. The present investigation is in continuation to an earlier publication of the author [15] and is an attempt to evaluate the hot corrosion behaviour of NiCrBSi and Stellite-6 coated nickel-based superalloy, for applications of these coatings on the hot section components of boilers and gas turbines.
wheel cloth-polishing machine. A layer of Na2 SO4 –60% V2 O5 (wt.%) salt mixture was applied uniformly on the warm polished specimens with the help of a camel hairbrush. Amount of the salt coating was kept in the range of 3.0–5.0 mg/cm2 . During the hot corrosion runs, the weight of boats and specimens were measured together at the end of each cycle with the help of Electronic Balance Machine Model 06120 (Contech) with a sensitivity of 1 mg. The spalled scale was also included at the time of measurements of weight change to determine total rate of corrosion. XRD, SEM/EDAX, and EPMA techniques were used to analyse the corrosion products.
2. Experimental
3.1. Corrosion kinetics
2.1. Deposition and characterisation of the coatings
The weight gain plots for the hot corroded bare and NiCrBSi and Stellite-6 coated Superni 600 are shown in Fig. 1. It can be inferred from the plots that the weight gain values for the coated Superni 600 are smaller than those for bare Superni 600. The NiCrBSi coating provides relatively higher protection than that provided by Stellite-6 coating. Both the coatings deposited on Superni 600 follow the parabolic behaviour up to the total 50 cycles of study as can be inferred from the square of weight change (mg2 /cm4 ) versus number of cycle plots shown in Fig. 2, whereas the bare Superni 600 shows visible deviation from the parabolic rate law. The parabolic rate constants (kp in 10−10 g2 cm−4 s−1 ) for the bare Superni 600 is calculated as 13.7, whereas for NiCrBSi and Stellite-6 coated Superni 600,
The nickel-based superalloy Superni 600 (15.5Cr–10Fe– 0.5Mn–0.2C–balance Ni) was used as a substrate material. The specimens with dimensions of approximately 20 mm × 15 mm × 5 mm were ground with SiC papers down to 180 grit and subsequently grit blasted with alumina powders (Grit 45) before spraying of the coatings by HVOF Process. Commercially available NiCrBSi and Stellite-6 powders were used as feedstock alloys and the details are given in Table 1. The coatings were sprayed at M/S Metallizing Equipment Co. Pvt. Ltd., Jodhpur (India) using Hipojet-2100 high velocity oxy-fuel thermal spray system. The spray parameters employed for Hipojet-2100 system were oxygen flow rate, 250 LPM; fuel (LPG) flow rate, 60 LPM; air flow rate, 900 LPM; spray distance, about 20 cm; fuel pressure, 6 kg/cm2 ; oxygen pressure, 8 kg/cm2 ; air pressure, 6 kg/cm2 . The specimens were cooled with compressed air jets during and after spraying. The details regarding characterisation of the coatings and the corrosion products have been reported elsewhere [15,16]. In brief, the average thickness of the coatings, porosity and microhardness values of the coatings are given in Table 1.
3. Results and discussions
2.2. Hot corrosion tests Experiments were performed in the molten salt (Na2 SO4 – 60% V2 O5 ) environment for 50 cycles under cyclic conditions. Each cycle consisted of one hour heating at 900 ◦ C in a silicon carbide tube furnace followed by 20 min cooling in the open air. The specimens were polished down to 1 m alumina on a
Fig. 1. Weight gain vs. number of cycles plot for the coated and uncoated Superni 600 subjected to hot corrosion for 50 cycles in Na2 SO4 –60% V2 O5 environment at 900 ◦ C.
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site up to which the oxygen has penetrated into the bare/coated specimens. The average scale thickness as measured from these micrographs for the bare, NiCrBSi coated and Stellite-6 coated Superni 600 is found to be as 65, 120 and 305 m, respectively. 3.3. X-ray diffraction analysis
Fig. 2. (Weight gain/area)2 vs. number of cycles plot for the coated and uncoated alloy Superni 600 subjected to hot corrosion for 50 cycles in Na2 SO4 –60% V2 O5 environment at 900 ◦ C.
values of kp are 4.84 and 8.15, respectively. It can be inferred from the corrosion kinetics that the HVOF coatings under study provide the necessary protection against the hot corrosion. The macrographs for the HVOF sprayed NiCrBSi and Stellite-6 coated superalloy hot corroded in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles are shown in Fig. 3. Initially, the scale formed on NiCrBSi coated Superni 600 was shining with dark black appearance. Subsequently, the colour of the scale became reddish brown (rust colour) after few cycles. Thereafter, a compact and dense continuous scale gradually developed on the NiCrBSi coated superalloy and no spallation of the scale was noticed even at the corners or edges. The colour of the oxide scale formed on the Stellite-6 coated Superni 600 was black after the first cycle, which turned to dark grey during seventh to ninth cycles, and subsequently showed the formation of silver grey patches on the dark greenish grey background. The Stellite-6 coated Superni 600 has shown some spalling and peeling of the scale from sixth cycle onwards, whereas a compact, regular and adherent oxide scale is formed on the NiCrBSi coated Superni 600.
The XRD profiles for the scale of bare and coated Superni 600 after exposure to molten salt environment for 50 cycles are shown in Fig. 5. The major and minor phases present in the surface scale of the specimens are given in Table 2. The presence of MnO2 phase in the corroded Stellite-6 coated alloy indicates minor diffusion of Mn from the substrate. 3.4. SEM/EDAX analysis 3.4.1. Surface analysis SEM micrographs along with EDAX analysis at some selected sites of interest of the corroded bare and coated Superni 600 are shown in Fig. 6. The fragile and irregular scale, rich in NiO, is formed on the surface of bare Superni 600 due to the fluxing action of the molten salt (Figs. 4a and 6a), which is porous due to reprecipitation of oxides. Small amounts of Cr2 O3 , Fe2 O3 , V2 O5 and MnO are also found to be present in the scale. A compact and adherent surface scale is formed on NiCrBSi coated Superni 600 in which irregular size globules are packed together. These globules are found to be mainly rich in SiO2 , while the oxides of Cr, Fe, Ni and V are also found to be present in small amounts (Fig. 6b). Whereas a homogeneous and continuous surface scale is developed on Stellite-6 coated Superni 600, which has CoO and Cr2 O3 as the main phases along with minor quantity of NiO, Fe2 O3 , MnO and WO3 (Fig. 6c). The existence of MnO in the surface scale of Stellite-6 coated Superni 600 indicates the diffusion of Mn from the substrate to the uppermost part of the scale during hot corrosion of the specimen.
3.2. Scale thickness measurement The BSE images across the smallest cross-section of corroded bare and coated Superni 600 are shown in Fig. 4. The scale thickness was taken as the distance from the surface to the
3.4.2. Cross-section analysis EDAX analysis was carried out at different point of interest along the cross-section of the hot corroded NiCrBSi coated Superni 600 and the results are given in Fig. 7. The BSE
Fig. 3. Macrographs of the coated Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) NiCrBSi coated and (b) Stellite-6 coated.
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Fig. 4. SEM back scattered images for the bare and HVOF coated Superni 600 superalloy subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) Bare superalloy; (b) NiCrBSi coated; (c) Stellite-6 coated.
Fig. 5. X-ray diffraction patterns for the bare and coated superalloy Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles.
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Table 2 Major and minor phases identified by XRD analysis of the hot corroded bare and coated Superni 600 Description
Major phases
Minor phases
Uncoated Superni 600 superalloy NiCrBSi coated Stellite-6 coated
NiO, Fe2 O3 , NiCr2 O4 , Ni(VO3 )2 , FeV, and FeV2 O4 SiO2 , Cr2 O3 , NiCr2 O4 , Fe2 O3 , and Ni(VO3 )2 CoO, Cr2 O3 , CoCr2 O4 , NiCr2 O4 , Fe2 O3 , and NiO
CrVO4 – MnO2
image shows the formation of a continuous, adherent and compact oxide. The EDAX analysis of the substrate near the coating–substrate interface (point 1) shows the absence of oxygen and other corrosive species indicating the excellent protection behaviour of the coating. EDAX analysis (point 2) shows the presence of alumina inclusion at the coating–substrate interface. Except the top surface of about 100 m, the remaining portion of the coating appears to have a structure similar to as-sprayed conditions. EDAX analysis also shows the absence of oxygen in this region (point 3). Top surface of the coating showed drastic changes in the structure. The featureless white appearance of the as-coated coating is transformed into a new grey phase with the formation of irregular grain shaped structure underneath. The dark black phase in the upper part of the scale (point 6) is found to be rich in Si, Ni and O, suggesting the formation of oxides of Si and Ni. The white grains (point 5) underneath the upper part of the scale are Ni-rich splats, which are in an un-oxidised state
due to absence of oxygen at this point. EDAX analysis shows that the light grey phase formed at the boundaries of these splats (point 4) consists of mainly Si, Ni, Cr and oxygen. The wt.% of Si increases at this point to 35% while wt.% of Ni decreases to 25% (being 4.8 wt.% Si and 72 wt.% Ni in the coating alloy). The presence of oxygen, about 36 wt.%, indicates that this light grey phase is rich in silicon oxide. 3.4.3. EPMA analysis BSE images and EPMA elemental mappings for the NiCrBSi and Stellite-6 coated Superni 600 after cyclic hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles are shown in Figs. 8 and 9, respectively. X-ray mappings for the corroded NiCrBSi coated Superni 600 (Fig. 8) indicate the formation of a scale consisting of mainly silicon, nickel and chromium. Iron and sodium are also present throughout the scale. Silicon forms a thick band in the upper-
Fig. 6. SEM/EDAX analysis showing elemental composition (wt.%) for the bare and coated Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) bare Superni 600; (b) NiCrBSi coated; (c) Stellite-6 coated.
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Fig. 7. Oxide scale morphology and variations of elemental composition across the cross-section of NiCrBSi coated Superni 600 hot corroded in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles.
most part of the scale, whereas chromium forms a thick band in the subscale area just below the top scale thereby leaving a chromium-depleted layer underneath. Nickel-rich splats are present in the subscale containing chromium and silicon at the splat boundaries. Iron shows a relatively higher concentration near the scale–substrate interface indicating its diffusion from the substrate to the coating. Manganese also diffused up to the surface of the scale. The presence of vanadium and sulphur below the top scale indicates that they have penetrated along the splat boundaries. The topmost layer of the scale formed on Stellite-6 coated Superni 600 mainly consists of chromium and oxygen, which indicates that this thin layer is rich in chromium oxide (Fig. 9). Some amounts of Fe and Co also exist in this region and form their oxides. Just under this layer, there is an intermediate band enriched in cobalt-rich splats which are oxidized mostly at the boundaries. In the remaining part, the scale has a lamellar splat structure. The presence of oxygen, chromium and silicon indicates that the oxides of chromium and silicon are formed mostly at the splat boundaries and the cobalt-rich splats are found to be in the un-reacted state. Sulphur penetrates through the scale and reaches to the vicinity of scale–substrate interface. 4. Discussions In general, both the coatings under study showed protective behaviour in the given molten salt environment at 900 ◦ C under
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cyclic conditions and performed better than the bare superalloy. The results of the present investigation show that due to selective oxidation property of chromium and silicon, Cr2 O3 and SiO2 formed along the boundaries of nickel-rich and cobalt-rich splats, and in pores have blocked the passages and enabled the coatings to develop barriers against the penetration and diffusion of corrosive species. Additionally, very low porosity and the flat splat structure of the coatings have also contributed in developing hot corrosion resistance at higher temperatures, since corrosive species mostly propagate along the splat boundaries and through the pores and voids. Due to dense and flat splat structure of the coatings, the distance from the coating surface to coating-substrate interface along splat boundaries is highly increased which enables the coatings to develop resistance against hot corrosion. Fig. 1 shows that the weight of bare superalloy increases continuously, whereas weight gain of the NiCrBSi and Stellite6 coated specimens is relatively high during the first eight to nine cycles of hot corrosion, but subsequently increase in weight is gradual. The initial high oxidation rate of the coated specimens has been ascribed to the rapid formation of oxides at the splat boundaries and within open pores due to the penetration of the oxidizing species. During the subsequent cycles, as discussed above, the formation of oxides have blocked the pores and splat boundaries, and acted as diffusion barriers to the inward diffusion of oxidizing species. The formation of oxides at the splat boundaries is confirmed by EPMA analysis (Fig. 9). As a consequence, the growth of the oxides becomes limited mainly to the surface of the specimens. Therefore, the steady state of oxidation has reached with the progress of exposure time. Both the coatings deposited on Superni 600 follow parabolic rate law for the total 50 cycles of study. Therefore it can be inferred that both the coatings provide necessary protection to the substrate. In terms of weight gains, the NiCrBSi coating shows better resistance to hot corrosion than that of Stellite-6 coating. In the NiCrBSi coated alloy, only the upper part of the coating has oxidised up to 100–110 m from the top surface and the remaining portion appears similar to the structure of as-sprayed coating, whereas Stellite-6 coating partially oxidised up to the coating–substrate interface along the splat boundaries. Therefore, it is concluded that the NiCrBSi coating provides better protection to the substrate superalloys in the given molten salt environment as compared to Stellite-6 coating. The formation of two thick oxide layers in the scale, the uppermost one rich in silicon oxide and the sub-layer rich in chromium oxide have contributed for the better hot corrosion resistance of this coating as can be seen in the EPMA analysis shown in Fig. 8. The XRD analysis also reveals the presence of SiO2 and Cr2 O3 as the main protective phases along with spinel NiCr2 O4 in the surface scale of the hot corroded NiCrBSi coated superalloys. The EDAX analysis further supports the formation of these phases. The surface scale formed on NiCrBSi coated superalloy is dense and compact, and shows no indication of any spallation, sputtering or peeling off during the course of study (Fig. 6b). The BSE image also shows that the scale formed on the NiCrBSi coated Superni 600 is adherent and the substrate is not affected by internal oxidation (Fig. 4b). This is further confirmed by the
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Fig. 8. Composition image (BSEI) and X-ray mappings along the cross-section of the NiCrBSi coated Superni 600 superalloy subjected to hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles.
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Fig. 9. Composition image (BSEI) and X-ray mappings along the cross-section of the Stellite-6 coated Superni 600 superalloy subjected to hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles.
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cross-sectional EDAX analysis for the NiCrBSi coated Superni 600 (Fig. 7), which shows the absence of oxygen and other corrosive species in the substrate near the coating–substrate interface. Wang et al. [17] have reported that the addition of Si and B can promote the selective oxidation of the protective scale-forming elements resulting in the formation of a continuous scale in the initial corrosion stage and improve the adherence of the outer scale to the coating in the subsequent hot corrosion process. Therefore, the behaviour of the NiCrBSi coating is in good agreement with the findings of Wang et al. [17]. The elemental mapping for O of hot corroded Stellite-6 coated Superni 600 (Fig. 9) shows the absence of oxygen underneath the coating-substrate interface, indicating that the substrate is in the un-reacted state, thereby, suggesting the protective behaviour of the Stellite-6 coating. The hot corrosion resistance of Stellite-6 coating and its tendency to act like a diffusion barrier to the degrading species can be attributed to the formation of oxides of chromium and silicon at the boundaries of Co-rich splats as revealed by EPMA analysis (Fig. 9), and to the formation of surface oxides of mainly cobalt and chromium along with the spinels of cobalt–chromium and nickel–chromium as revealed by XRD analysis and supported by EDAX analysis. As proposed by Luthra [18] the formation of spinel CoCr2 O4 blocks the diffusion activities through the cobalt oxide (CoO) thereby suppressing the further formation of this oxide. He further opined that increase in the growth of CoCr2 O4 and Cr2 O3 , in competition with CoO and Co3 O4 formation, increases the corrosion resistance of alloys. The formation of most of these phases has also been reported by Sidhu and Prakash [19] while studying the plasma sprayed Stellite-6 coating in the same molten salt environment for the same number of cycles at the same temperature. Some spalling of the oxide scale of Stellite-6 coated Superni 600, as observed during cooling periods of the thermal cycles, is due to different values of thermal expansion coefficients of the coatings, substrate and oxides [20–22]. 5. Conclusions 1. The HVOF sprayed NiCrBSi and Stellite-6 coatings improve the hot corrosion resistance of the Superni 600 in the given conditions. The hot corrosion resistance of the coatings has been attributed to the formation of oxides of silicon and chromium and spinels of cobalt–chromium and nickel–chromium at the surface of the coatings, and at the splat boundaries. 2. The oxide scale has preferentially formed at the splat boundaries due to oxidation of the active elements of the coatings.
The Ni- and Co-rich splats of the coatings mostly remain in the un-oxidised state. 3. The better hot corrosion resistance of NiCrBSi coating as compared to that of Stellite-6 coating is due to the presence of silicon and boron, which promote the selective oxidation of the protective scale-forming elements, and also improve the adherence of the outer scale to the coating. 4. Both the coatings have shown higher rate of hot corrosion during initial cycles of exposure and thereafter the corrosion rate decreases and finally stabilises. Initially, the oxygen permeates inward along the splat boundaries and pores, and causes rapid oxidation. Subsequently, these oxides plug/seal all possible diffusion paths in the coatings, thereby block or slow down the penetration of aggressive species. The corrosion is then confined mainly to the surface of the coatings, thus resulting in the corrosion rate to reach a steady state 5. The spalling observed in the case of Stellite-6 coated Superni 600 is due to different values of thermal expansion coefficients of the coatings, the substrate and the oxides. References [1] W.T. Reid, External Corrosion and Deposits—Boilers and Gas Turbines, Elsevier, New York, 1971, p. 115. [2] P.A. Alexander, R.A. Marsden, in: D. Johnson, Litter (Eds.), Proceeding of the Conference on Mechanism of Corrosion by Fuel Impurities, Butterworths, London, 1963, p. 542. [3] K.L. Luthra, H.S. Spacil, J. Electrochem. Soc. 129 (1982) 649. [4] K. Natesan, Corrosion 32 (9) (1976) 364. [5] S.N. Tiwari, S. Prakash, Mater. Sci. Technol. 14 (1998) 467. [6] Gitanjaly, S. Prakash, S. Singh, Br. Corros. J. 37 (1) (2002) 56. [7] F. Wang, H. Lou, L. Bai, W. Wu, Mater. Sci. Eng. A: Struct. 121 (1989) 387. [8] N. Eliaz, G. Shemesh, R.M. Latanision, Eng. Fail. Anal. 9 (2002) 31. [9] M.J. Pomeroy, Mater. Design 26 (2005) 223. [10] M.H. Li, X.F. Sun, J.G. Li, Z.Y. Zhang, T. Jin, H.R. Guan, Z.Q. Hu, Oxide Met. 59 (5–6) (2003) 591. [11] P.S. Sidky, M.G. Hocking, Br. Corros. J. 34 (3) (1999) 171. [12] P.S. Liu, K.M. Liang, S.R. Gu, Corros. Sci. 43 (2001) 1217. [13] M.G. Hocking, Surf. Coat. Technol. 62 (1–3) (1993) 460. [14] I. Gurappa, Mater. Sci. Technol. 19 (2003) 178. [15] T.S. Sidhu, S. Prakash, R.D. Agrawal, Surf. Coat. Technol. 201 (1-2) (2006) 273. [16] T.S. Sidhu, S. Prakash, R.D. Agrawal, Thin Solid Films, 515 (1-2) (2006) 95. [17] Q.M. Wang, Y.U. Wu, P.L. Ke, H.T. Cao, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 186 (3) (2004) 389. [18] K.L. Luthra, J. Electrochem. Soc. 132 (6) (1985) 1293. [19] B.S. Sidhu, S. Prakash, J. Mater. Process. Technol. 172 (1) (2006) 52. [20] P. Niranatlumpong, C.B. Ponton, H.E. Evans, Oxide Met. 53 (3–4) (2000) 241. [21] H.E. Evans, P. Taylor, Oxide Met. 55 (1–2) (2001) 17. [22] B.S. Sidhu, S. Prakash, Oxide Met. 63 (3–4) (2005) 241.
A comparative study of hot corrosion resistance of HVOF sprayed NiCrBSi and Stellite-6 coated Ni-based superalloy at 900 ◦C T.S. Sidhu ∗ , S. Prakash, R.D. Agrawal Metallurgical & Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India Received 13 June 2006; accepted 10 September 2006
Abstract A comparative study was carried out to evaluate the hot corrosion resistance of NiCrBSi and Stellite-6 coated nickel-based superalloy Superni 600 (15.5Cr–10Fe–0.5Mn–0.2C–balance Ni) in the molten salt environment of Na2 SO4 –60%V2 O5 salt mixture at 900 ◦ C under cyclic conditions. Hot corrosion experiments were performed for 50 cycles, each cycle consisting of 1 h heating in the laboratory tube furnace followed by 20 min cooling in the open air. The thermogravimetric technique was used to establish the kinetics of corrosion. The morphology, phase composition and element concentration of the corrosion products were detected using the combined techniques of X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive analysis (SEM/EDAX) and electron probe micro analyzer (EPMA). The hot corrosion resistance of NiCrBSi coating has been found to be better than that of Stellite-6 coating. The hot corrosion resistance of both the coatings has been attributed to the formation of oxides of chromium, silicon and nickel along with spinels of nickel–chromium and cobalt–chromium. These oxides seal/plug the pores and splat boundaries, and act as diffusion barriers to the inward diffusion of corroding species. © 2006 Published by Elsevier B.V. Keywords: NiCrBSi; Stellite-6; HVOF coating; Hot corrosion; Superalloy; Protective coatings
1. Introduction Due to depletion of high-grade fuels and for economic reasons, residual fuel oils are often used in boilers, refinery furnaces and gas turbines. Residual fuel oil contains sodium, vanadium and sulphur as impurities, which form compounds such as Na2 SO4 (melting point 884 ◦ C), V2 O5 (melting point 670 ◦ C), and complex vanadates by reactions in the combustion systems [1–3]. These compounds, known as ash, deposit on the surface of materials and induce accelerated oxidation (hot corrosion). Moreover, the vanadium compounds are also good oxidation catalysts and allow oxygen and other gases in the combustion atmosphere to diffuse rapidly to the metal surface and cause further oxidation [4]. A number of countermeasures are currently in use or under investigation to combat the hot corrosion such as using inhibitors, controlling the process parameters, designing suitable industrial alloys and depositing the protective coatings. The inhibitors like MgO, CeO2 , CaO, MnO2 , etc. have been
∗
Corresponding author. Tel.: +91 1332 285731; fax: +91 1332 285243. E-mail address: [email protected] (T.S. Sidhu).
0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2006.09.015
successfully used to decrease the extent of hot corrosion in the most aggressive environment of Na2 SO4 –60% V2 O5 at 900 ◦ C [5–6]. However, their use in actual industrial environment is limited due to the practical problems of injecting them along with the fuel in the combustion chamber. Further, controlling the various process parameters of the boiler and gas turbine such as air/fuel ratio, temperature, pressure, etc., can also be useful to some extent to combat the hot corrosion, but these parameters can be controlled only within certain limits. The corrosion resistance of the superalloys can be improved by adding fair amounts of Al and Cr, and small amounts of Y, Zr and Hf [7]. However, these elements (such as Al, Cr) can be added only up to a limited extent as their higher concentrations adversely affects the mechanical properties of the alloys [8,9]. The use of protective coatings is the most practical, reliable and economically viable method to resist the hot corrosion. These composite materials perform better under extreme conditions, the base material provides the required mechanical strength and the coatings protect them against wear, erosion or corrosion [10–12]. The composition and structure of the coatings are determined by the role that they have to play in the various material systems and service environments [13]. During service, the coatings are
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Table 1 Composition of the feedstock alloys, coating thickness, microhardness and porosity Feedstock alloys
Chemical composition (wt.%)
Shape
Average coating thickness (m)
Average porosity (%age)
Microhardness, Hv (Vickers hardness)
Particle size (m)
Weight (%age)
NiCrBSi powder (PA 101HV) Stellite-6 powder (Jet-Kote 7206)
Ni–15.3Cr–3.1B–4.8Si–4.2Fe–0.6C
Spherical
270
<1.5
810–850
+45, −45
1.02, 98.98
Co–28Cr–4.9W–2.7Fe–2.3Ni–1.1Si–1.2C
Spherical
265
<2
850–900
−53, +45, −45
0.22, 2.23, 97.55
expected to form slowly growing protective oxides which should not allow the corrosive species to diffuse into the coating and should also have resistance to cracking and spallation under mechanical and thermal stresses induced during operation of the components. Further, these coating should also serve as a reservoir for the elements expected to form protective oxides [13,14]. Therefore, the identification/development of suitable coatings is of great interest for higher temperature applications in boilers and gas turbines. The present investigation is in continuation to an earlier publication of the author [15] and is an attempt to evaluate the hot corrosion behaviour of NiCrBSi and Stellite-6 coated nickel-based superalloy, for applications of these coatings on the hot section components of boilers and gas turbines.
wheel cloth-polishing machine. A layer of Na2 SO4 –60% V2 O5 (wt.%) salt mixture was applied uniformly on the warm polished specimens with the help of a camel hairbrush. Amount of the salt coating was kept in the range of 3.0–5.0 mg/cm2 . During the hot corrosion runs, the weight of boats and specimens were measured together at the end of each cycle with the help of Electronic Balance Machine Model 06120 (Contech) with a sensitivity of 1 mg. The spalled scale was also included at the time of measurements of weight change to determine total rate of corrosion. XRD, SEM/EDAX, and EPMA techniques were used to analyse the corrosion products.
2. Experimental
3.1. Corrosion kinetics
2.1. Deposition and characterisation of the coatings
The weight gain plots for the hot corroded bare and NiCrBSi and Stellite-6 coated Superni 600 are shown in Fig. 1. It can be inferred from the plots that the weight gain values for the coated Superni 600 are smaller than those for bare Superni 600. The NiCrBSi coating provides relatively higher protection than that provided by Stellite-6 coating. Both the coatings deposited on Superni 600 follow the parabolic behaviour up to the total 50 cycles of study as can be inferred from the square of weight change (mg2 /cm4 ) versus number of cycle plots shown in Fig. 2, whereas the bare Superni 600 shows visible deviation from the parabolic rate law. The parabolic rate constants (kp in 10−10 g2 cm−4 s−1 ) for the bare Superni 600 is calculated as 13.7, whereas for NiCrBSi and Stellite-6 coated Superni 600,
The nickel-based superalloy Superni 600 (15.5Cr–10Fe– 0.5Mn–0.2C–balance Ni) was used as a substrate material. The specimens with dimensions of approximately 20 mm × 15 mm × 5 mm were ground with SiC papers down to 180 grit and subsequently grit blasted with alumina powders (Grit 45) before spraying of the coatings by HVOF Process. Commercially available NiCrBSi and Stellite-6 powders were used as feedstock alloys and the details are given in Table 1. The coatings were sprayed at M/S Metallizing Equipment Co. Pvt. Ltd., Jodhpur (India) using Hipojet-2100 high velocity oxy-fuel thermal spray system. The spray parameters employed for Hipojet-2100 system were oxygen flow rate, 250 LPM; fuel (LPG) flow rate, 60 LPM; air flow rate, 900 LPM; spray distance, about 20 cm; fuel pressure, 6 kg/cm2 ; oxygen pressure, 8 kg/cm2 ; air pressure, 6 kg/cm2 . The specimens were cooled with compressed air jets during and after spraying. The details regarding characterisation of the coatings and the corrosion products have been reported elsewhere [15,16]. In brief, the average thickness of the coatings, porosity and microhardness values of the coatings are given in Table 1.
3. Results and discussions
2.2. Hot corrosion tests Experiments were performed in the molten salt (Na2 SO4 – 60% V2 O5 ) environment for 50 cycles under cyclic conditions. Each cycle consisted of one hour heating at 900 ◦ C in a silicon carbide tube furnace followed by 20 min cooling in the open air. The specimens were polished down to 1 m alumina on a
Fig. 1. Weight gain vs. number of cycles plot for the coated and uncoated Superni 600 subjected to hot corrosion for 50 cycles in Na2 SO4 –60% V2 O5 environment at 900 ◦ C.
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site up to which the oxygen has penetrated into the bare/coated specimens. The average scale thickness as measured from these micrographs for the bare, NiCrBSi coated and Stellite-6 coated Superni 600 is found to be as 65, 120 and 305 m, respectively. 3.3. X-ray diffraction analysis
Fig. 2. (Weight gain/area)2 vs. number of cycles plot for the coated and uncoated alloy Superni 600 subjected to hot corrosion for 50 cycles in Na2 SO4 –60% V2 O5 environment at 900 ◦ C.
values of kp are 4.84 and 8.15, respectively. It can be inferred from the corrosion kinetics that the HVOF coatings under study provide the necessary protection against the hot corrosion. The macrographs for the HVOF sprayed NiCrBSi and Stellite-6 coated superalloy hot corroded in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles are shown in Fig. 3. Initially, the scale formed on NiCrBSi coated Superni 600 was shining with dark black appearance. Subsequently, the colour of the scale became reddish brown (rust colour) after few cycles. Thereafter, a compact and dense continuous scale gradually developed on the NiCrBSi coated superalloy and no spallation of the scale was noticed even at the corners or edges. The colour of the oxide scale formed on the Stellite-6 coated Superni 600 was black after the first cycle, which turned to dark grey during seventh to ninth cycles, and subsequently showed the formation of silver grey patches on the dark greenish grey background. The Stellite-6 coated Superni 600 has shown some spalling and peeling of the scale from sixth cycle onwards, whereas a compact, regular and adherent oxide scale is formed on the NiCrBSi coated Superni 600.
The XRD profiles for the scale of bare and coated Superni 600 after exposure to molten salt environment for 50 cycles are shown in Fig. 5. The major and minor phases present in the surface scale of the specimens are given in Table 2. The presence of MnO2 phase in the corroded Stellite-6 coated alloy indicates minor diffusion of Mn from the substrate. 3.4. SEM/EDAX analysis 3.4.1. Surface analysis SEM micrographs along with EDAX analysis at some selected sites of interest of the corroded bare and coated Superni 600 are shown in Fig. 6. The fragile and irregular scale, rich in NiO, is formed on the surface of bare Superni 600 due to the fluxing action of the molten salt (Figs. 4a and 6a), which is porous due to reprecipitation of oxides. Small amounts of Cr2 O3 , Fe2 O3 , V2 O5 and MnO are also found to be present in the scale. A compact and adherent surface scale is formed on NiCrBSi coated Superni 600 in which irregular size globules are packed together. These globules are found to be mainly rich in SiO2 , while the oxides of Cr, Fe, Ni and V are also found to be present in small amounts (Fig. 6b). Whereas a homogeneous and continuous surface scale is developed on Stellite-6 coated Superni 600, which has CoO and Cr2 O3 as the main phases along with minor quantity of NiO, Fe2 O3 , MnO and WO3 (Fig. 6c). The existence of MnO in the surface scale of Stellite-6 coated Superni 600 indicates the diffusion of Mn from the substrate to the uppermost part of the scale during hot corrosion of the specimen.
3.2. Scale thickness measurement The BSE images across the smallest cross-section of corroded bare and coated Superni 600 are shown in Fig. 4. The scale thickness was taken as the distance from the surface to the
3.4.2. Cross-section analysis EDAX analysis was carried out at different point of interest along the cross-section of the hot corroded NiCrBSi coated Superni 600 and the results are given in Fig. 7. The BSE
Fig. 3. Macrographs of the coated Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) NiCrBSi coated and (b) Stellite-6 coated.
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Fig. 4. SEM back scattered images for the bare and HVOF coated Superni 600 superalloy subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) Bare superalloy; (b) NiCrBSi coated; (c) Stellite-6 coated.
Fig. 5. X-ray diffraction patterns for the bare and coated superalloy Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles.
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Table 2 Major and minor phases identified by XRD analysis of the hot corroded bare and coated Superni 600 Description
Major phases
Minor phases
Uncoated Superni 600 superalloy NiCrBSi coated Stellite-6 coated
NiO, Fe2 O3 , NiCr2 O4 , Ni(VO3 )2 , FeV, and FeV2 O4 SiO2 , Cr2 O3 , NiCr2 O4 , Fe2 O3 , and Ni(VO3 )2 CoO, Cr2 O3 , CoCr2 O4 , NiCr2 O4 , Fe2 O3 , and NiO
CrVO4 – MnO2
image shows the formation of a continuous, adherent and compact oxide. The EDAX analysis of the substrate near the coating–substrate interface (point 1) shows the absence of oxygen and other corrosive species indicating the excellent protection behaviour of the coating. EDAX analysis (point 2) shows the presence of alumina inclusion at the coating–substrate interface. Except the top surface of about 100 m, the remaining portion of the coating appears to have a structure similar to as-sprayed conditions. EDAX analysis also shows the absence of oxygen in this region (point 3). Top surface of the coating showed drastic changes in the structure. The featureless white appearance of the as-coated coating is transformed into a new grey phase with the formation of irregular grain shaped structure underneath. The dark black phase in the upper part of the scale (point 6) is found to be rich in Si, Ni and O, suggesting the formation of oxides of Si and Ni. The white grains (point 5) underneath the upper part of the scale are Ni-rich splats, which are in an un-oxidised state
due to absence of oxygen at this point. EDAX analysis shows that the light grey phase formed at the boundaries of these splats (point 4) consists of mainly Si, Ni, Cr and oxygen. The wt.% of Si increases at this point to 35% while wt.% of Ni decreases to 25% (being 4.8 wt.% Si and 72 wt.% Ni in the coating alloy). The presence of oxygen, about 36 wt.%, indicates that this light grey phase is rich in silicon oxide. 3.4.3. EPMA analysis BSE images and EPMA elemental mappings for the NiCrBSi and Stellite-6 coated Superni 600 after cyclic hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles are shown in Figs. 8 and 9, respectively. X-ray mappings for the corroded NiCrBSi coated Superni 600 (Fig. 8) indicate the formation of a scale consisting of mainly silicon, nickel and chromium. Iron and sodium are also present throughout the scale. Silicon forms a thick band in the upper-
Fig. 6. SEM/EDAX analysis showing elemental composition (wt.%) for the bare and coated Superni 600 subjected to hot corrosion in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles: (a) bare Superni 600; (b) NiCrBSi coated; (c) Stellite-6 coated.
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Fig. 7. Oxide scale morphology and variations of elemental composition across the cross-section of NiCrBSi coated Superni 600 hot corroded in Na2 SO4 –60% V2 O5 environment at 900 ◦ C for 50 cycles.
most part of the scale, whereas chromium forms a thick band in the subscale area just below the top scale thereby leaving a chromium-depleted layer underneath. Nickel-rich splats are present in the subscale containing chromium and silicon at the splat boundaries. Iron shows a relatively higher concentration near the scale–substrate interface indicating its diffusion from the substrate to the coating. Manganese also diffused up to the surface of the scale. The presence of vanadium and sulphur below the top scale indicates that they have penetrated along the splat boundaries. The topmost layer of the scale formed on Stellite-6 coated Superni 600 mainly consists of chromium and oxygen, which indicates that this thin layer is rich in chromium oxide (Fig. 9). Some amounts of Fe and Co also exist in this region and form their oxides. Just under this layer, there is an intermediate band enriched in cobalt-rich splats which are oxidized mostly at the boundaries. In the remaining part, the scale has a lamellar splat structure. The presence of oxygen, chromium and silicon indicates that the oxides of chromium and silicon are formed mostly at the splat boundaries and the cobalt-rich splats are found to be in the un-reacted state. Sulphur penetrates through the scale and reaches to the vicinity of scale–substrate interface. 4. Discussions In general, both the coatings under study showed protective behaviour in the given molten salt environment at 900 ◦ C under
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cyclic conditions and performed better than the bare superalloy. The results of the present investigation show that due to selective oxidation property of chromium and silicon, Cr2 O3 and SiO2 formed along the boundaries of nickel-rich and cobalt-rich splats, and in pores have blocked the passages and enabled the coatings to develop barriers against the penetration and diffusion of corrosive species. Additionally, very low porosity and the flat splat structure of the coatings have also contributed in developing hot corrosion resistance at higher temperatures, since corrosive species mostly propagate along the splat boundaries and through the pores and voids. Due to dense and flat splat structure of the coatings, the distance from the coating surface to coating-substrate interface along splat boundaries is highly increased which enables the coatings to develop resistance against hot corrosion. Fig. 1 shows that the weight of bare superalloy increases continuously, whereas weight gain of the NiCrBSi and Stellite6 coated specimens is relatively high during the first eight to nine cycles of hot corrosion, but subsequently increase in weight is gradual. The initial high oxidation rate of the coated specimens has been ascribed to the rapid formation of oxides at the splat boundaries and within open pores due to the penetration of the oxidizing species. During the subsequent cycles, as discussed above, the formation of oxides have blocked the pores and splat boundaries, and acted as diffusion barriers to the inward diffusion of oxidizing species. The formation of oxides at the splat boundaries is confirmed by EPMA analysis (Fig. 9). As a consequence, the growth of the oxides becomes limited mainly to the surface of the specimens. Therefore, the steady state of oxidation has reached with the progress of exposure time. Both the coatings deposited on Superni 600 follow parabolic rate law for the total 50 cycles of study. Therefore it can be inferred that both the coatings provide necessary protection to the substrate. In terms of weight gains, the NiCrBSi coating shows better resistance to hot corrosion than that of Stellite-6 coating. In the NiCrBSi coated alloy, only the upper part of the coating has oxidised up to 100–110 m from the top surface and the remaining portion appears similar to the structure of as-sprayed coating, whereas Stellite-6 coating partially oxidised up to the coating–substrate interface along the splat boundaries. Therefore, it is concluded that the NiCrBSi coating provides better protection to the substrate superalloys in the given molten salt environment as compared to Stellite-6 coating. The formation of two thick oxide layers in the scale, the uppermost one rich in silicon oxide and the sub-layer rich in chromium oxide have contributed for the better hot corrosion resistance of this coating as can be seen in the EPMA analysis shown in Fig. 8. The XRD analysis also reveals the presence of SiO2 and Cr2 O3 as the main protective phases along with spinel NiCr2 O4 in the surface scale of the hot corroded NiCrBSi coated superalloys. The EDAX analysis further supports the formation of these phases. The surface scale formed on NiCrBSi coated superalloy is dense and compact, and shows no indication of any spallation, sputtering or peeling off during the course of study (Fig. 6b). The BSE image also shows that the scale formed on the NiCrBSi coated Superni 600 is adherent and the substrate is not affected by internal oxidation (Fig. 4b). This is further confirmed by the
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Fig. 8. Composition image (BSEI) and X-ray mappings along the cross-section of the NiCrBSi coated Superni 600 superalloy subjected to hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles.
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Fig. 9. Composition image (BSEI) and X-ray mappings along the cross-section of the Stellite-6 coated Superni 600 superalloy subjected to hot corrosion at 900 ◦ C in Na2 SO4 –60% V2 O5 environment for 50 cycles.
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cross-sectional EDAX analysis for the NiCrBSi coated Superni 600 (Fig. 7), which shows the absence of oxygen and other corrosive species in the substrate near the coating–substrate interface. Wang et al. [17] have reported that the addition of Si and B can promote the selective oxidation of the protective scale-forming elements resulting in the formation of a continuous scale in the initial corrosion stage and improve the adherence of the outer scale to the coating in the subsequent hot corrosion process. Therefore, the behaviour of the NiCrBSi coating is in good agreement with the findings of Wang et al. [17]. The elemental mapping for O of hot corroded Stellite-6 coated Superni 600 (Fig. 9) shows the absence of oxygen underneath the coating-substrate interface, indicating that the substrate is in the un-reacted state, thereby, suggesting the protective behaviour of the Stellite-6 coating. The hot corrosion resistance of Stellite-6 coating and its tendency to act like a diffusion barrier to the degrading species can be attributed to the formation of oxides of chromium and silicon at the boundaries of Co-rich splats as revealed by EPMA analysis (Fig. 9), and to the formation of surface oxides of mainly cobalt and chromium along with the spinels of cobalt–chromium and nickel–chromium as revealed by XRD analysis and supported by EDAX analysis. As proposed by Luthra [18] the formation of spinel CoCr2 O4 blocks the diffusion activities through the cobalt oxide (CoO) thereby suppressing the further formation of this oxide. He further opined that increase in the growth of CoCr2 O4 and Cr2 O3 , in competition with CoO and Co3 O4 formation, increases the corrosion resistance of alloys. The formation of most of these phases has also been reported by Sidhu and Prakash [19] while studying the plasma sprayed Stellite-6 coating in the same molten salt environment for the same number of cycles at the same temperature. Some spalling of the oxide scale of Stellite-6 coated Superni 600, as observed during cooling periods of the thermal cycles, is due to different values of thermal expansion coefficients of the coatings, substrate and oxides [20–22]. 5. Conclusions 1. The HVOF sprayed NiCrBSi and Stellite-6 coatings improve the hot corrosion resistance of the Superni 600 in the given conditions. The hot corrosion resistance of the coatings has been attributed to the formation of oxides of silicon and chromium and spinels of cobalt–chromium and nickel–chromium at the surface of the coatings, and at the splat boundaries. 2. The oxide scale has preferentially formed at the splat boundaries due to oxidation of the active elements of the coatings.
The Ni- and Co-rich splats of the coatings mostly remain in the un-oxidised state. 3. The better hot corrosion resistance of NiCrBSi coating as compared to that of Stellite-6 coating is due to the presence of silicon and boron, which promote the selective oxidation of the protective scale-forming elements, and also improve the adherence of the outer scale to the coating. 4. Both the coatings have shown higher rate of hot corrosion during initial cycles of exposure and thereafter the corrosion rate decreases and finally stabilises. Initially, the oxygen permeates inward along the splat boundaries and pores, and causes rapid oxidation. Subsequently, these oxides plug/seal all possible diffusion paths in the coatings, thereby block or slow down the penetration of aggressive species. The corrosion is then confined mainly to the surface of the coatings, thus resulting in the corrosion rate to reach a steady state 5. The spalling observed in the case of Stellite-6 coated Superni 600 is due to different values of thermal expansion coefficients of the coatings, the substrate and the oxides. References [1] W.T. Reid, External Corrosion and Deposits—Boilers and Gas Turbines, Elsevier, New York, 1971, p. 115. [2] P.A. Alexander, R.A. Marsden, in: D. Johnson, Litter (Eds.), Proceeding of the Conference on Mechanism of Corrosion by Fuel Impurities, Butterworths, London, 1963, p. 542. [3] K.L. Luthra, H.S. Spacil, J. Electrochem. Soc. 129 (1982) 649. [4] K. Natesan, Corrosion 32 (9) (1976) 364. [5] S.N. Tiwari, S. Prakash, Mater. Sci. Technol. 14 (1998) 467. [6] Gitanjaly, S. Prakash, S. Singh, Br. Corros. J. 37 (1) (2002) 56. [7] F. Wang, H. Lou, L. Bai, W. Wu, Mater. Sci. Eng. A: Struct. 121 (1989) 387. [8] N. Eliaz, G. Shemesh, R.M. Latanision, Eng. Fail. Anal. 9 (2002) 31. [9] M.J. Pomeroy, Mater. Design 26 (2005) 223. [10] M.H. Li, X.F. Sun, J.G. Li, Z.Y. Zhang, T. Jin, H.R. Guan, Z.Q. Hu, Oxide Met. 59 (5–6) (2003) 591. [11] P.S. Sidky, M.G. Hocking, Br. Corros. J. 34 (3) (1999) 171. [12] P.S. Liu, K.M. Liang, S.R. Gu, Corros. Sci. 43 (2001) 1217. [13] M.G. Hocking, Surf. Coat. Technol. 62 (1–3) (1993) 460. [14] I. Gurappa, Mater. Sci. Technol. 19 (2003) 178. [15] T.S. Sidhu, S. Prakash, R.D. Agrawal, Surf. Coat. Technol. 201 (1-2) (2006) 273. [16] T.S. Sidhu, S. Prakash, R.D. Agrawal, Thin Solid Films, 515 (1-2) (2006) 95. [17] Q.M. Wang, Y.U. Wu, P.L. Ke, H.T. Cao, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 186 (3) (2004) 389. [18] K.L. Luthra, J. Electrochem. Soc. 132 (6) (1985) 1293. [19] B.S. Sidhu, S. Prakash, J. Mater. Process. Technol. 172 (1) (2006) 52. [20] P. Niranatlumpong, C.B. Ponton, H.E. Evans, Oxide Met. 53 (3–4) (2000) 241. [21] H.E. Evans, P. Taylor, Oxide Met. 55 (1–2) (2001) 17. [22] B.S. Sidhu, S. Prakash, Oxide Met. 63 (3–4) (2005) 241.