Improved hot corrosion resistance of Dy-Co-modified aluminide coating by pack cementation process on nickel base superalloys

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Improved hot corrosion resistance of Dy-Co-modified aluminide coating by pack cementation process on nickel base superalloys Pei Yuwen, Chungen Zhou ∗ Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Key Laboratory of Aerospace Materials and Performance (Ministry of Education), Beijing 100191, China

a r t i c l e

i n f o

Article history: Received 17 June 2016 Received in revised form 9 September 2016 Accepted 10 September 2016 Available online xxx Keywords: A. Metal coatings A. Superalloys B. TEM C. Hot corrosion

a b s t r a c t Dy-Co-modified aluminide coating was deposited onto Ni-based superalloy DZ125 using pack cementation method. The microstructure and hot corrosion behavior of the coating were investigated. The results show that as-deposited coating has a two-layer structure. The outer layer of the coating is composed mainly of Al0.9 Ni1.1 with dispersed Dy2 Hf2 O7 . The mass gain of the coating is 0.23 mg/cm2 after exposure to 75 wt.% Na2 SO4 + 25 wt.% NaCl at 900 ◦ C for 100 h. The corrosion product formed on the coating is characterized as Al2 O3 . Role of Dy in the improvement of the hot corrosion resistance has been discussed. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Because of good high temperature mechanical strength and creep resistance, nickel base superalloys are often used for the hot section components of advanced gas turbine engines [1–8]. However, nickel base superalloys must be coated at elevated temperatures above 900 ◦ C because their oxidation and hot corrosion resistance are poor when exposed to the aggressive environment. The aluminide diffusion coatings have attracted increasing attention on account of low cost and easy processing. They provide good oxidation resistance for Ni-based superalloys due to the continuous formation of protective Al2 O3 scale [9–11]. However, cavities and spallation are prone to generate in aluminide coatings during cyclic oxidation process and the hot corrosion resistance is poor [12,13]. Recently, many researches have focused on cobalt-modified aluminide coatings, which could effectively increase oxidation and hot corrosion resistance of Ni-based superalloys [14–17]. However, the cracking and spallation of oxide scale appeared when the thermal cycling times were beyond 21 at 1050 ◦ C, indicating that the cyclic oxidation resistance of Co-modified aluminide coating was poor [18]. Considerable researches have shown that adding a small amount of elements such as Pt, Si, Y, Ce, Hf and Dy or their oxides can facilitate the formation of protective oxide scale and improve the adhesion between the oxide scale and the coating [19–26]. Zhao

et al. [23] investigated the effect of Y2 O3 content in the pack on microstructure and hot corrosion resistance of Y-Co-modified aluminide coating. The result demonstrated that Y2 O3 addition could accelerate the diffusion of Co, thus improving the hot corrosion resistance of the coating. The study by Liu [24] proved that the hot corrosion resistance of the Y-Ce-Co-modified aluminide coating was superior to normal Co-modified aluminide coating due to the synergistic effect of Y and Ce. Minor Dy modified NiAl alloys and coatings have recently exhibited good potential in enhancing the cyclic oxidation resistance [25,27,28]. Research showed that Dy improved the oxidation resistance of NiAl-31Cr-3Mo superalloy by forming continuous sole protective Al2 O3 scale [29,30]. Guo et al. [31] investigated the effect of Dy on oxide scale adhesion of NiAl coatings at 1200 ◦ C and found that the presence of Dy prevented sulfur segregation and void formation at the interface of the oxide scale and the coating. Therefore, the oxide scale on the NiAlDy coating remained adherent and virtually no spallation occurred even after prolonging cyclic oxidation. However, the hot corrosion resistance of the Co-Al-Dy coating has not been studied so far. In the present study, Dy-Co-modified aluminide coating on nickel based superalloys was prepared using a pack cementation method. The microstructure and hot corrosion behavior of the coating were investigated. Role of Dy in the improvement of the hot corrosion resistance was also discussed.

∗ Corresponding author. E-mail address: [email protected] (C. Zhou). http://dx.doi.org/10.1016/j.corsci.2016.09.011 0010-938X/© 2016 Elsevier Ltd. All rights reserved.

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Table 1 Nominal chemical compositions of DZ125 Ni-based superalloy (in wt.%). Ni

Co

Cr

W

Al

Ta

Mo

Hf

Ti

60.3

10.0

8.9

7.0

5.1

3.8

2.0

1.5

1.0

Table 2 Compositions of Co-Al-Dy compound pack mixtures. Pack

Co

Al

Dy2 O3

NaF

Al2 O3

Mass fraction of composition (%)

20

7.6

1

4

67.4

2. Experiment 2.1. Materials The nickel-based superalloy DZ125 was used in the present study as the substrate and its nominal composition was shown in Table 1. The specimens prepared for pack cementation were cut from the alloy buttons with a dimension of approximately 12 mm in diameter and 3 mm in thickness. All surfaces of the specimens were ground by SiC papers from 150-grit to 800-grit. Subsequently, the samples were ultrasonically cleaned in alcohol and acetone prior to coating preparation. 2.2. Coating preparation The pack cementation method was utilized to form the Dy-Comodified aluminide coating. The pack mixtures were composed of Co, Al, Dy2 O3 , NaF and Al2 O3 powders, and the compositions of Co, Al and Dy used for co-deposition were listed in Table 2. The average particle size of all the powders with the purity exceeding 99.5% was less than 100 meshes. Each powder was accurately weighed in proportion and stirred evenly in the mortar. The specimens were buried in the pack powders which were placed in cylindrical alumina retort with a dimension of 20 mm diameter and 35 mm length. The cylindrical alumina retort was then covered and sealed with an alumina lid using silica sol bonder. After that, the alumina crucibles containing the mixed powders and specimens were put into a tubular resistance furnace whose temperature was program-controlled. According to the preset program, the furnace temperature was raised to 1050 ◦ C at a rate of 5 ◦ C/min. The specimens were held at 1050 ◦ C for 10 h and then cooled down to the room temperature. During the whole process, argon gas was indispensable for protecting the substrate from being oxidized. 2.3. Hot corrosion testing Hot corrosion test of the Co-Al-Dy coating was performed in air at 900 ◦ C in an open-ended tube furnace for 100 h to evaluate the hot corrosion resistance. Before high temperature hot corrosion test, the specimens were placed on a hot iron plate and all surfaces of the specimens were coated by a mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl. The salt supply was 3 mg/cm2 . The specimens coated with salt layer were placed into the alumina crucibles and were weighed together as the original weight. The samples were taken out at specified time intervals (1, 3, 5, 10, 20, 40, 60, 80 and 100 h), cooled down to ambient temperature and then weighed by an electronic balance (Model BS 224S Sartorious, Germany) with a precision of 0.1 mg. Three measurements for weight gain at each time were taken and averaged. After measuring all samples at each exposure time, they were put into the furnace again to go on with the experiment. The residual salt in the surface of the coating was cleaned by the deionized water and alcohol when the test was over.

Fig. 1. Surface morphology of the Co-Al-Dy coating co-deposited on DZ125 superalloy at 1050 ◦ C for 10 h.

2.4. Analyzing methods In order to recognize the effects of Dy on the hot corrosion property of the Co-modified aluminide coating, scanning electron microscopy (SEM) (Model FEI Quanta 600, USA) with EDS system and electron probe micro-analyzer (EPMA) (Model JEOL JXA-8230) with WDS system were utilized to characterize the composition and microstructure of Dy-Co-modified aluminide coating and corrosion product. Coated specimens and corroded specimens were analyzed by X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan) using Cu K␣ radiation to investigate the surface phase composition. The thin film sample for TEM observation was prepared by ion milling method and observed by TEM (JEOL JEM 2010) equipped with EDS to analyze the precipitates in the coating. 3. Results and discussion 3.1. Coating characterization Fig. 1 shows the surface morphology of the coated specimen prepared by pack cementation. It can be seen that the coating surface is flat and consists of polygonal network of grain boundary ridges, which is typical characterization of outwardly growing aluminide coating. The cross-sectional microstructure and major elemental concentration profiles are shown in Fig. 2. From Fig. 2(a), the coating has a visible two-layer structure, including a uniform outer layer of approximately 21 ␮m and a diffusion zone of about 19.5 ␮m. The diffusion layer above the substrate illustrates that the formation of the coating is primarily due to the outward diffusion of Ni [32–34]. As shown in Fig. 2(b), the compositions of the outer layer and the diffusion zone are acquired by wavelength dispersive spectrometer. Five different positions in the same distance from the coating surface were detected by WDS and the average value was taken. The composition of the outer layer analyzed by WDS is 23Al, 9.9Co, 62Ni, 5Cr and 0.021Dy (in wt.%). The composition of the diffusion zone detected by WDS is 13.2Al, 9.3Co, 44.6Ni, 3.8Mo, 10Cr, 14W, 3.7Ta, 0.83Ti and 0.015Dy (in wt.%). The pattern of the XRD in Fig. 3 shows that the outer layer is consisted mainly of Al0.9 Ni1.1 . Meanwhile, a small amount of Al2 O3 is determined, proving that the outward diffusion of Ni contributes to the formation of the coating. Since the formation of the coating is controlled by the outward diffusion of Ni, Al2 O3 particles as

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Fig. 3. XRD pattern of surface of Co-Al-Dy coating deposited on DZ125 superalloy at 1050 ◦ C for 10 h.

Dy2 Hf2 O7 precipitate is not detected by XRD because the content of Dy2 Hf2 O7 precipitate is very low. Fig. 4(d) shows the SAED pattern of the out layer of the coating. The two sides of the diffraction parallelogram for the coating have a d-spacing of 2.034 Å and 2.035 Å which can be identified to be the (110) and (101) reflections of Al0.9 Ni1.1 with Pm–3 m structure. These results prove that the out layer of the coating is consisted mainly of Al0.9 Ni1.1 with dispersed Dy2 Hf2 O7 . 3.2. Hot corrosion behavior

Fig. 2. Cross-sectional SEM image (a) and major elemental concentration profile (b) of the Co-Al-Dy coated specimen held at 1050 ◦ C for 10 h.

the inert filler are wrapped in the NiAl coating [14]. There are no Co-containing phases and Dy-containing phases detected by the XRD. However, the EPMA major elemental concentration profile (Fig. 2(b)) shows that Co is finely dispersed in the coating. The result indicates that Co may exist in the form of solid solution and a part of Ni may be replaced by Co in Al0.9 Ni1.1 phase. Dy may appear in the form of precipitate in the coating due to the low solubility of Dy in NiAl. Fig. 4(a) presents the TEM image of the outer layer of DyCo-modified aluminide coating. It can be seen that there is precipitate within the grains and along the grain boundaries. The EDS analysis reveals that the chemical compositions of phases A and B are 45.94O-22.42Hf-14.07Dy-7.82Ni-5.61Al-4.13Co (at.%) and 47.16O-24.96Hf-16.56Dy-5.03Ni-3.62Al-2.66Co (at.%), respectively. Fig. 4(b) and (c) present SAED patterns of the phases A and B in Dy-Co-modified aluminide coating, respectively. Based on the SAED pattern of Fig. 4(b), the two sides of the diffraction parallelogram for the phase A with a d-spacing of 3.011 Å and 2.609 Å can be identified to be the (111) and (200). In the meantime, the two edges of the parallelogram for the phase B with a d-spacing of 3.011 Å and 1.845 Å can be determined to be the (111) and (220). It can be concluded that the precipitated A and B are both Dy2 Hf2 O7 phase with Fm–3 m structure. Dy2 Hf2 O7 may be generated by the reaction of Dy2 O3 and HfO2 during the high temperature processing. However,

Fig. 5 shows the hot corrosion kinetics of the Co-Al-Dy coated specimen and the reference Co-Al coated specimen with 75 wt.% Na2 SO4 + 25 wt.% NaCl salt deposition in air at 900 ◦ C. For the CoAl reference coating, the mass change increases dramatically to 0.32 mg/cm2 in the initial 20 h [14]. By contrast, the mass gain of the Co-Al-Dy coating slowly rises up to 0.21 mg/cm2 in the first 20 h. The mass changes of the two coated specimens are almost constant after 20 h and gradually increase to 0.23 and 0.33 mg/cm2 [14] after hot corrosion at 900 ◦ C for 100 h, respectively. The corrosion kinetics indicates that the mass change of the Co-Al-Dy coated specimen is smaller. As a result, the addition of Dy improves the hot corrosion resistance of Co-modified aluminide coating. 3.3. Characterization of corrosion product The surface morphology for the corroded sample is presented in Fig. 6. It can be seen that the Co-Al-Dy coating has dense and compact surface and there are round particles uniformly distributed on the surface after hot corrosion for 100 h. Moreover, these particles contain two kinds of elements which are Al and O elements based on the EDS results. The scale may be composed of ␣-Al2 O3 . The XRD pattern of the corroded specimen in Fig. 7 shows that ␣-Al2 O3 phase forms on the coating. However, because the Al2 O3 scale is very thin, the diffraction peaks of Al0.9 Ni1.1 beneath the scale are also detected in the spectrum with high intensity. In addition, there exist the minor NiO phase and AlNi3 phase. Occurrence of AlNi3 phase is due to depletion of Al in the coating. Fig. 8 presents the cross-sectional microstructure for the Co-AlDy coated specimen after hot corrosion at 900 ◦ C for 100 h. From this figure, the surface of the coating is covered with an oxide scale and oxide protrusions working as “pegs” are developed into the coating. The oxide scale is continuous and compact and no voids

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Fig. 4. (a) TEM image of the outer layer of Dy-Co-modified aluminide coating and SAED patterns of (b) Dy2 Hf2 O7 , (c) Dy2 Hf2 O7 and (d) Al0.9 Ni1.1 .

Fig. 5. The corrosion kinetics of Co-Al-Dy and Co-Al coated specimen at 900 ◦ C for 100 h. Data of Co-Al coated specimen was obtained from Qiao et al. [14].

and spallation are observed. Therefore, the addition of Dy improves the adhesion between the oxide scale and the coating. Fig. 9 shows the cross-sectional morphology and corresponding element mapping of the corroded sample. Area A and area B are corresponded to the corrosion product and the coating, respectively.

Fig. 6. Surface morphology of the Co-Al-Dy coating after hot corrosion test with a salt deposit of 3 mg/cm2 at 900 ◦ C for 100 h.

The main elements in area A are Al and O, indicating that the corrosion product formed above the coating is ␣-Al2 O3 . A great quantity of Ni is discovered in the coating due to the outward diffusion of Ni. The distribution of Co is similar to that of Ni in the coating, which

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Fig. 7. XRD pattern of surface of the Co-Al-Dy coating after hot corrosion with a salt deposit of 3 mg/cm2 at 900 ◦ C for 100 h.

powerfully demonstrates the formation of solid solution combining with the results of XRD. Minute quantity of sulfur is sporadically distributed beneath the oxide scale, which are likely to be present in the form of sulfides. No sulfur is detected on the Al2 O3 /coating interface. The element of Dy is dispersed mainly in the outside of Al2 O3 scale. 3.4. Discussion Hot corrosion can be regarded as an accelerated oxidation induced by the deposited salts, such as sulphate and chloride [35]. The melting point of the mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl is about 645 ◦ C and the mixed salt is molten at 900 ◦ C. During the hot corrosion process, the reaction of Al2 O3 and salts may result in the dissolution of the protective Al2 O3 scale. The reactions are as follows [36,37]. 1 Na2 SO4 → Na2 O+SO2 + O2 2 Al2 O3 +Na2 O →

+ 2AlO− 2 +2Na

2−

→ Al2 O3 + O

1 O2 + 2NaCl → 2NaAlO2 + Cl2 2

efficiently. Choi et al. [39] showed that the presence of Co in NiAl coating could promote the formation of Al2 O3 scale even though the concentration of Al was kept at a lower level due to the socalled the third element effect. In addition, the study by Beltran and Shores [40] showed that the diffusion rate of sulfur in Ni was two orders of magnitude higher than that of sulfur in Co at 1000 ◦ C. Therefore, the addition of Co promotes the formation of Al2 O3 scale and retards the sulphuration/oxidation cyclic reactions by reducing the internal diffusion of sulphur in the process of hot corrosion, which improves hot corrosion resistance. In the present study, the beneficial effect of Dy addition can be attributed to the formation of Dy2 Hf2 O7 precipitate. According to TEM results above, a small amount of Dy2 Hf2 O7 precipitate is generated in the out layer of the coating. The possible reactions during the pack cementation process are as follows [41,42]: Al(s)+3NaF(l) → AlF3 (g)+3Na(g)

(5)

(2)

Co(s)+2NaF(l) → CoF2 (g)+2Na(g)

(6)

Dy2 O3 (s)+6NaF(l) → 2[Dy](s)+3Na2 O(s)+3F2 (g)

(7)

[Dy](s)+AlF3 (g) → DyF3 (g)+[Al](s)

(8)

2[Dy](s) + 3CoF2 (g) → 2DyF3 (g) + 3[Co](s)

(9)

(3)

The alkalinity of the salt/oxidation scale interface increases again due to the released O2− . As a result, it promotes the fluxing of the alumina scale. As the self-cycling process proceeds, more and more Al from the coating will be consumed. Finally, the resultant Al2 O3 scale is porous and thus could not provide effective protection. When the pure Na2 SO4 salt deposit is partially replaced by NaCl and changed into the salt mixture deposit, the hot corrosion process is greatly accelerated through oxychlorination and chlorination/oxidation cyclic reactions, causing the further dissolution of the protective oxide scale [38]: Al2 O3 +

Fig. 8. Cross-sectional microstructure of the Co-Al-Dy coated specimen after hot corrosion test with a salt deposit of 3 mg/cm2 at 900 ◦ C for 100 h.

(1)

As the hot corrosion proceeds, concentration gradient of AlO2 − between the salt/oxide scale interface and the salt/air interface can be generated, thus the AlO2 − will migrate from the salt/oxidation scale interface to the salt/air interface. The alkalinity of melting salt is low in the position of the salt/air interface where Al2 O3 will re-precipitate by the following reaction: 2AlO− 2

5

(4)

The formation of a protective Al2 O3 layer or reduction of the internal diffusion of sulfur could increase the hot corrosion resistance

The resultant [Al] reacts with nickel atoms which diffuse towards to the surface of nickel base superalloy to yield NiAl phase. Meanwhile, a small amount of Hf diffuses to the coating surface from the substrate. Since the formation of the coating is controlled by the outward diffusion of Ni, Al2 O3 particles as the inert filler are wrapped in the NiAl coating. The values of the standard Gibbs free energy of formation of Al2 O3 , Dy2 O3 and HfO2 calculated per mole of O2 at 1050 ◦ C are −836.014 kJ, −984.799 kJ and −875.944 kJ, respectively [43]. Dy2 O3 and HfO2 are more thermodynamically stable than Al2 O3 at 1050 ◦ C. Therefore, it is likely that Hf and Dy can react with the wrapped Al2 O3 in the coating by the following reactions [26]: Al2 O3 +2Dy → Dy2 O3 +2Al

(10)

3 3 Al2 O3 + Hf → HfO2 +2Al 2 2

(11)

The standard Gibbs free energies of reactions (10) and (11) at 1050 ◦ C are −223.178 kJ/mol and −59.896 kJ/mol, respectively [43].

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Fig. 9. EPMA elemental mappings for Co-Al-Dy coating after hot corrosion with a salt deposit of 3 mg/cm2 at 900 ◦ C for 100 h.

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The standard Gibbs free energies of the reactions above are negative, indicating that these two reactions are likely to occur during the pack cementation process. It can be inferred that Dy2 Hf2 O7 is formed through the reaction of Dy2 O3 and HfO2 during high temperature process: Dy2 O3 +2HfO2 → Dy2 Hf2 O7

(12)

After hot corrosion, Al2 O3 film can form at the coating surface and then Dy2 Hf2 O7 phase may be incorporated into Al2 O3 film. The Dy2 Hf2 O7 precipitate at the grain boundaries may block these regions as diffusion paths for the other large atoms like Mo and W, thus suppressing the penetration of alloying elements into the film and improving the adhesion of the scale [44]. In addition, as can be seen from Fig. 8, the oxide protrusions working as “pegs” are developed into the coating and pin the oxide scale. No voids and spallation are found. So the addition of trace Dy may encourage the formation of “oxide pegs” on the coating by promoting the nucleation and formation of Al2 O3 and prevent the generation of voids at the scale/coating interface [28,45]. Both of these mechanisms are beneficial for the improvement of the oxide adherence and thus increase hot corrosion resistance. Restraining the internal diffusion of sulphur in the coating is important for the improvement in the hot corrosion resistance. It is generally known that the segregation of sulfur at grain boundaries of NiAl causes the formation of the voids and reduces the adhesion of the oxide scale [46]. The poor oxide scale adhesion could lead to the early spallation of the alumina scale. The reactive element Dy can inhibit sulfur adverse effect, and the mechanism may be that Dy has a strong chemical reactivity and a very low solubility in NiAl. Dy precipitates at grain boundaries and grains in the form of Dy2 Hf2 O7 phase. The precipitated Dy2 Hf2 O7 phase may catch sulfur and suppress sulfur segregation at the interface of the coating and the corrosion product [47]. Therefore, the addition of Dy improves the hot corrosion resistance of the coating by inhibiting cavity formation and increasing the adhesion of the oxide scale. 4. Conclusion (1) Dy-Co-modified aluminide coating prepared by pack cementation method has a two-layer structure of the out layer and diffusion zone. The out layer is consisted mainly of Al0.9 Ni1.1 phase with minor Dy2 Hf2 O7 precipitate. (2) Dy-Co-modified aluminide coating exhibits a significant improvement in hot corrosion resistance as compared to the Co-Al coating. Dy element is beneficial for the improvement of the hot corrosion resistance. (3) The addition of Dy improves the adhesion of Al2 O3 layer on the coating by retarding the outward diffusion of alloying elements and promoting the formation of “oxide pegs”. In addition, Dy2 Hf2 O7 particles prevent the sulfur segregation at the interface of the coating and the corrosion product. Acknowledgement This project is supported by the National Natural Science Foundation of China under the contract of 51371021 and 51431003, and the Aviation Science Foundation of 2014ZE51053. References [1] B.B. Seth, The utility gas turbine perspective, in: T.M. Pollock (Ed.), Superalloy 2000, TMS, Warredak, 2000, pp. 3–15. [2] C. Fu, C.H. Wang, Z.M. Ren, G.H. Cao, Comparison of microstructure and oxidation behavior between Pt-free and Pt-modified ␦-Ni2 Si coatings on Ni-based superalloys, Corros. Sci. 98 (2015) 211–222. [3] Y.H. Zhou, L. Wang, G. Wang, D.L. Jin, W. Hao, X.F. Zhao, J. Zhang, P. Xiao, Influence of substrate composition on the oxidation performance of nickel

[4] [5]

[6]

[7]

[8]

[9]

[10]

[11] [12]

[13] [14] [15]

[16]

[17]

[18]

[19]

[20]

[21] [22] [23]

[24]

[25] [26]

[27]

[28]

[29]

[30]

[31] [32]

[33]

7

aluminide coating prepared by pack cementation, Corros. Sci. 110 (2016) 284–295. W.Y. Gong, L.J. Zhang, D.Z. Yao, C.G. Zhou, Diffusivities and atomic mobilities in fcc Ni-Pt alloys, Scr. Mater. 61 (2009) 100–103. A. Rauf, Q. Yu, L. Jin, C. Zhou, Microstructure and thermal properties of nanostructured lanthana-doped yttria-stabilized zirconia thermal barrier coatings by air plasma spraying, Scr. Mater. 66 (2012) 109–112. C.G. Zhou, C.L. Wang, Y.X. Song, Evaluation of cyclic oxidation of thermal barrier coatings exposed to NaCl vapor by finite element method, Mater. Sci. Eng. A 490 (2008) 351–358. C.G. Zhou, J.S. Yu, S.K. Gong, H.B. Xu, Influence of water vapor on the high temperature oxidation behavior of thermal barrier coatings, Mater. Sci. Eng. A 348 (2003) 327–332. H.B. Guo, S.K. Gong, C.G. Zhou, H.B. Xu, Investigation on hot-fatigue behaviors of gradient thermal barrier coatings by EB-PVD, Surf. Coat Technol. 148 (2001) 110–116. F.H. Latief, E.M. Sherif, K. Kakehi, Role of aluminide coating on oxidation resistance of Ni-based single crystal superalloy at 900◦ C, Int. J. Electrochem. Sci. 10 (2015) 1873–1882. Si. J. Park, S.M. Seo, Y.S. Yoo, Hi-W. Jeong, H.J. Jang, Effects of Al and Ta on the high temperature oxidation of Ni-based superalloys, Corros. Sci. 90 (2015) 305–312. R. Burgel, Coating service experience with industrial gas turbines, Mater. Sci. Technol. 2 (1986) 302–308. K. Shirvani, M. Saremi, A. Nishikata, T. Tsuru, Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2 SO4 -20 wt.% NaCl melt at 750 ◦ C, Corros. Sci. 45 (2003) 1011–1021. J. Brenneman, J. Wei, Z. Sun, L. Liu, G. Zou, Y. Zhou, Oxidation behavior of GTD111 Ni-based superalloy at 900 ◦ C in air, Corros. Sci. 100 (2015) 267–274. M. Qiao, C.G. Zhou, Hot corrosion behavior of Co modified NiAl coating on nickel base superalloys, Corros. Sci. 63 (2012) 239–245. Q.X. Fan, S.M. Jiang, D.L. Wu, Preparation and hot corrosion behaviour of two Co modified NiAl coatings on a Ni-based superalloy, Corros. Sci. 76 (2013) 373–381. Q.X. Fan, S.M. Jiang, H.J. Yu, J. Gong, C. Sun, Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. 311 (2014) 214–223. H.Y. He, Z.J. Liu, W. Wang, C.G. Zhou, Microstructure and hot corrosion behavior of Co-Si modified aluminide coating on nickel based superalloys, Corros. Sci. 100 (2015) 466–473. H.X. Li, M. Qiao, C.G. Zhou, Formation and cyclic oxidation resistance of Hf-Co-modified aluminide coatings on nickel base superalloys, Mater. Chem. Phys. 143 (2014) 915–920. J.J. Zang, P. Song, J. Feng, X.P. Xiong, R. Chen, G.L. Liu, J.S. Lu, Oxidation behaviour of the nickel-based superalloy DZ125 hot-dipped with Al coatings doped by Si, Corros. Sci. (2016), http://dx.doi.org/10.1016/j.corsci.2016.07. 020. R.D. Liu, S.M. Jiang, H.J. Yu, J. Gong, C. Sun, Preparation and hot corrosion behaviour of Pt modified AlSiY coating on a Ni-based superalloy, Corros. Sci. 104 (2016) 162–172. D.P. Whittle, J. Stringer, Improvement in properties: additives in oxidation resistance, Philos. Trans. R. Soc. Lond. A 295 (1980) 309–329. B.A. Pint, Experimental observations in support of the dynamic segregation theory to explain the reactive element effect, Oxid. Met. 45 (1996) 1–37. X.S. Zhao, C.G. Zhou, Effect of Y2 O3 content in the pack on microstructure and hot corrosion resistance of Y-Co-modified aluminide coating, Corros. Sci. 86 (2014) 223–230. Z.J. Liu, X.S. Zhao, C.G. Zhou, Improved hot corrosion resistance of Y-Ce-Co-modified aluminide coating on nickel base superalloys by pack cementation process, Corros. Sci. 92 (2015) 148–154. G.Y. Zhang, H. Zhang, J.T. Guo, Improvement of cyclic oxidation resistance of a NiAl-based alloy modified by Dy, Surf. Coat. Technol. 201 (2006) 2270–2275. H.B. Guo, Y.J. Cui, H. Peng, S.K. Gong, Improved cyclic oxidation resistance of electron beam physical vapor deposited nano-oxide dispersed ␤-NiAl coatings for Hf-containing superalloy, Corros. Sci. 52 (2010) 1440–1446. H.B. Guo, X.Y. Wang, S.K. Gong, Effects of Dy on the microstructure and spallation failure of the alumina scales grown on NiAl, Int. J. Mod. Phys. B 24 (2010) 3149–3154. H.B. Guo, S.X. Wang, X.Y. Wang, S.K. Gong, Cyclic oxidation behaviors of EB-PVD Dy doped ␤-NiAl coatings at 1100 ◦ C, Int. J. Mod. Phys. B 24 (2010) 3143–3148. G.Y. Zhang, H. Zhang, Z.S. Wang, J.T. Guo, L.Z. Zhou, Oxidation behavior of NiAl-30.9Cr-3Mo-0.1Dy alloy at high temperature, Rare Metal Mater. Eng. 201 (2006) 2270–2275. Karin M. Carling, Emily A. Carter, Effects of segregating elements on the adhesive strength and structure of the ␣-Al2 O3 /␤-NiAl interface, Acta Mater. 55 (2007) 719–723. H.B. Guo, T. Zhang, S.X. Wang, Effect of Dy on oxide scale adhesion of NiAl coatings at 1200 ◦ C, Corros. Sci. 53 (2011) 2228–2232. Z.D. Xiang, J.S. Burnell-Gray, P.K. Datta, Conditions for co-deposition of Al and Cr on Ni base superalloys by pack cementation process, Surf. Eng. 17 (2001) 287–294. Z.D. Xiang, J.S. Burnell-Gray, P.K. Datta, Aluminide coating formation on nickel base superalloys by pack cementation process, J. Mater. Sci. 36 (2001) 5673–5682.

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[34] G.W. Goward, D.H. Boone, Mechanisms of formation of diffusion aluminide coatings on nickel-base superalloys, Oxid. Met. 3 (1971) 475–495. [35] N. Birks, G.H. Meier, F.S. Pettit, Introduction to the High-Temperature Oxidation of Metals, 2nd edn., University Press, Cambridge, New York, 2006. [36] K. Zhang, M.M. Liu, S.L. Liu, C. Sun, F.H. Wang, Hot corrosion behaviour of a cobalt-base super-alloy K40S with and without NiCrAlYSi coating, Corros. Sci. 53 (2011) 1990–1998. [37] H. Huang, C. Liu, L.Y. Ni, C.G. Zhou, Evaluation of microstructural evolution of thermal barrier coatings exposed to Na2 SO4 using impedance spectroscopy, Corros. Sci. 53 (2011) 1369–1374. [38] Z.B. Bao, Q.M. Wang, W.Z. Li, X. Liu, J. Gong, T.Y. Xiong, C. Sun, Preparation, hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy, Corros. Sci. 51 (2009) 860–867. [39] S.C. Choi, H.J. Cho, D.B. Lee, Effect of Cr, Co and Ti additions on the high temperature oxidation behaviour of Ni3 Al, Oxid. Met. 46 (1996) 109–127. [40] A.M. Beltran, D.A. Shores, in: C.T. Sims, W.C. Hagel (Eds.), “Hot Corrosion”, The Superalloys, 1972, Wiley-Interscience, New York, 2016, pp. 317–339 (Chapter 11).

[41] T. Qi, X.P. Guo, Effect of Y2 O3 on Microstructure of silicide coatings on Nb-silicide-based ultrahigh temperature alloy, J. Mater. Eng. 1 (2010) 12–15. [42] L.R. Xiao, Z.G. Cai, D.Q. Yi, L. Ying, H.Q. Liu, D.Y. Hua, Morphology, structure and formation mechanism of silicide coating by pack cementation process, Trans. Nonferrous Met. Soc. China 16 (2006) 239–244. [43] Dalun Ye, Jianhua Hu, Practical Inorganic Thermodynamic Data Manual Micro-trained Influx, Metallurgical Industry Press, Beijing China, 2002. [44] W.J. Quadakkers, H. Halzbrechen, K.G. Brief, H. Beske, Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys, Oxid. Met. 32 (1989) 67–88. [45] S.X. Wang, Ultra-high Temperature Oxidation Behaviors of NiAlDy Coatings [D], Beihang University, 2009. [46] J.A. Haynes, B.A. Pint, K.L. More, Y. Zang, I.G. Wright, Influence of sulfur platinum, and hafnium on the oxidation behavior of CVD NiAl bond coatings, Oxid. Met. 58 (2002) 513–544. [47] Y. Ikeda, K. Nii, M. Yata, Y2 O3 dispersion effect on Al2O3 protective coating examined on the basis of five models, ISIJ Int. 33 (1993) 298–306.

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