- Email: [email protected]
Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 °C
G Model
ARTICLE IN PRESS
APSUSC-27861; No. of Pages 10
Applied Surface Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C Q.X. Fan, S.M. Jiang ∗ , H.J. Yu, J. Gong, C. Sun Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, PR China
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 16 April 2014 Accepted 9 May 2014 Available online xxx Keywords: Aluminide coating Pack cementation process Hot corrosion Microstructure Superalloy
a b s t r a c t Two Co modified aluminide coatings with different Co contents were prepared by pack cementation process and above-the-pack process. The hot corrosion tests of the two coatings were performed in mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C, with a simple aluminide coating as the reference coating. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) with energy dispersive X-ray spectrometer (EDS) were used to characterize the coatings and the corrosion scales. Results indicate that the addition of Co improves the hot corrosion resistance of the simple aluminide coating in the mixed sulfate salts, for the sulfide as well as its eutectic of cobalt are more stable, and possess higher melting points than those of nickel. While in the mixed salt containing chloride, the coating with medium Co content possesses the best corrosion resistance, primarily because the nitrides formed in the deposition process deteriorate the corrosion resistance of the coating with highest Co content. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Salt deposits of sulfur, sodium, chlorine and potassium can develop on the surface of gas-turbine hardware, especially when the engines operate in locations where salt can be ingested, i.e., coastal areas and regions with high level of airborne pollutants [1]. These salts would lead to severe hot corrosion of the materials [2]. Usually hot corrosion is divided into two categories: type I (high temperature) hot corrosion and type II (low temperature) hot corrosion [3]. The development of these two forms is affected by various parameters such as: thermomechanical condition, contaminant composition and fluxing rate [4]. For the sulfate salts, the type I hot corrosion usually happens at temperatures between 815 ◦ C and 927 ◦ C, and the type II hot corrosion occurs in the 593–760 ◦ C range [3]. While for the salts containing chloride, the type I hot corrosion can take place at the temperature of 700 ◦ C, since it is above the fusing point of the eutectic salts containing Na2 SO4 and NaCl (about 620 ◦ C) [5]. Considering the economic cost and good oxidation resistance, aluminide diffusion coatings are commonly used to protect the nickel-based engine alloys from environmental erosion [6]. The diffusion coating can be deposited by hot dip aluminization, slurry
∗ Corresponding author. Tel.: +86 24 83978081; fax: +86 24 23891320. E-mail addresses: [email protected] (Q.X. Fan), [email protected], [email protected] (S.M. Jiang).
aluminization, pack cementation process and chemical vapor deposition (CVD) [7–10]. Among the above preparation methods, pack cementation process is used most widespread for single or binary deposition such as Co–Al, Cr–Al and Si–Al [11–13], while CVD is the best technique to prepare the diffusion aluminide coating. Firstly, it can be used for coating the long narrow internal air channels of vanes and blades [14,15]; Secondly, the coatings prepared by CVD method are inclusion free and possess homogeneous structures, since the specimens are separated from the powder mixture [14]. However, some elements might be difficult to be deposited by CVD, for the activity of the target element, chamber vacuum degree, and elemental compositions of the furnace have great influences on the deposition process. In above-the-pack process the samples are isolated from the powder mixture, so the coatings prepared by this method also possess good microstructure and properties. Because of this similarity, some researchers have called it as CVD method [16,17]. It has been widely acknowledged that the addition of beneficial elements like Cr [18,19], Pt [20,21], Si [22,23] and reactive elements [24–26] to the simple aluminide coatings can improve either the corrosion resistance or oxidation resistance of the coatings. Co-based alloy is supposed to possess better corrosion resistance than Ni-based alloy [27], and it’s of growing interest to investigate the effect of Co applied to simple aluminide coatings. Some researchers have found that the addition of Co could decrease the pores in the aluminide coating or NiCrAlY on the nickel-based alloys resulting in a higher oxidation resistance [28,29]. Other researchers
http://dx.doi.org/10.1016/j.apsusc.2014.05.043 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
2
have demonstrated that Co applied to the NiAl coating is beneficial to improve the corrosion resistance above 800 ◦ C [30,31]. However seldom research has been done at medium temperature 600–800 ◦ C. To investigate the influence of Co on corrosion resistance of the simple aluminide coating at medium temperature, two Co modified aluminide coating with different Co contents are prepared by a two step process, first depositing Co by pack cementation and then aluminizing by above-the-pack process. The hot corrosion behavior of the two Co modified aluminide coatings in comparison with the simple aluminide coating is performed in two different mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C. The influence of Co on the hot corrosion resistance and the microstructure as well as phase evolution of the coatings during the corrosion process will be studied in detail.
2. Experimental procedures The nominal composition of the nickel based alloy used as the substrate was Ni–10.48 Co–15.28 Cr–3.10 Al–4.40 Ti–5.13 W–2.08 Mo–0.19 Nb–0.31 Hf–0.08 B–0.085 C (wt.%). Cubical samples of 15 mm × 10 mm × 2 mm were ground down to 800 # SiC paper, prior to being ultrasonically washed within acetone, ethanol and deionized water successively. For preparation of the Coating 1, Co was deposited at 1000 ◦ C for 6 h and then aluminized at 1000 ◦ C for 5 h. The coating 2 was prepared by depositing Co at 850 ◦ C for 6 h and subsequently aluminized at 1000 ◦ C for 5 h. As a reference coating, the coating 3 was simple aluminide coating which prepared under the same aluminization condition as that of the coating 1 and coating 2. For pack cementation deposition of Co, the specimens were buried in the powder mixture made up of 480 g Al2 O3 , 320 g cobalt powder and 50 g NH4 Cl at 1000 ◦ C or 850 ◦ C for 6 h. After cooled down to room temperature in the furnace, the specimens were taken out and cleaned in ethanol and deionized water again for the subsequent aluminization. For the Al deposition, the specimens with or without Co coatings were hung above the powder mixture composed of 600 g FeAl powder and 40 g NH4 Cl at 1000 ◦ C for 5 h. For both the deposition of Co and Al, the furnace chamber was pumped to at least 100 Pa, and then Ar was filled in, following by pumping the furnace chamber below 100 Pa again. This process was repeated for three times to eliminate oxygen in the furnace, following by filling Ar in the furnace until the chamber pressure was up to 2.5 × 104 Pa. Cyclic hot corrosion tests of the three coatings were conducted in two different mixed salts in static air at 1 atm. The specimens were heated on a hot plate to about 150 ◦ C, and then coated a thin layer of salt mixture Na2 SO4 /K2 SO4 (3:1, w/w) or Na2 SO4 /NaCl (3:1, w/w) on the surface. The samples were measured before and after importing the salt mixture to ensure a salt supply of 1 mg/cm2 . Subsequently the samples were placed within crucibles and taken into the furnace which was heated up to 700 ◦ C. At regular interval (20 h), the specimens were taken out and cooled down to room temperature. Prior to being dried and weighed by electronic balance with a sensitivity of 10−5 g, the specimens were washed in boiling distilled water to obliterate the remaining salt. Afterwards a thin uniform fresh salt coating was brushed on the specimens again to continue the experiment. For each test, three parallel samples were tested to ensure experimental accuracy. Phase identification and morphologies of the coatings and corrosion scales are characterized by means of X-ray diffractometer (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). For cross-sectional microstructure observing, an electroless Ni layer was coated on the surface of the specimens and then mounted in a plastic resin. Additionally, transmission electron microscope (TEM) with EDS system was used for the phase identification of some specimens.
Fig. 1. XRD patterns of the two Co coatings deposited at 1000 ◦ C (a) and 850 ◦ C (b).
3. Results and discussion 3.1. The microstructure of the coatings before oxidation Figs. 1 and 2 present the XRD patterns and cross-sectional images of the two Co coatings deposited at 1000 ◦ C and 850 ◦ C respectively. As can be seen in Figs. 1 and 2, the outer layer of the two Co coatings consists of singular ␥-(Co,Ni) with thicknesses about 8 m and 3 m separately. The ␥-(Co,Ni) is also the prime phase in the transitional layer of the two Co coatings. Some dotted and strip-like grey phases rich in Ti are scattered in the 1000 ◦ C Co coating, while no such Ti rich phases are found in the 850 ◦ C Co coating. Fig. 2(c) shows the TEM morphology of the transitional zone of the Co coating deposited at 1000 ◦ C, and it can be seen that the grey dotted phase is TiN, while the strip-like phase is the mixed phase of TiN and TiC (C0.7 N0.3 Ti). The cross-sectional images of the coating 1, coating 2 and coating 3 are presented in Fig. 3 respectively. For the coating 1, the outer layer is composed of single -(Ni,Co)Al (abbreviated as  in the following paragraph). The inner layer consists of Cr(W) rich phases and stripe like Ti rich phases which mainly form during the process of Co deposition. The coating 2 and coating 3 also possess a doublelayered structure, an outer layer with single  phase and an inner Cr(W) rich phase layer. The elemental compositions at the near surface of the three coatings are 49.75 Al–36.47 Co–11.62 Ni–1.32 Cr–0.85 Fe (at.%), 50.23 Al–18.06 Co–30.06 Ni–1.09 Cr–0.58 Fe (at.%) and 52.23 Al–3.19 Co–43.20 Ni–0.81 Cr–0.58 Fe (at.%) respectively detected by EDS. It can be seen that the Al content and Ni content in the outer layer are greater in the order of the coating 1, coating 2 and coating 3, while the Co and Cr contents become less from the coating 1 to the coating 3. 3.2. Mass change curves Fig. 4 presents the mass change curves of the bare alloy, coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C respectively. Since the three parallel samples of each coatings have small mass gains with little difference in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 , so the error bars are not shown in Fig. 4(a). It can be seen that the mass gain of the bare alloy increases to 0.065 mg/cm2 after hot corrosion for 20 h, and then it decreases gradually to about −0.131 mg/cm2 after hot corrosion for 700 h. The coating 1 and coating 2 have smooth mass change curves, the maximum mass gains of the two coatings are 0.138 mg/cm2 and
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
3
Fig. 2. The cross-sectional images of the Co coating deposited at 1000 ◦ C (a) and 850 ◦ C (b); (c) TEM images of the transitional layer of the Co coating deposited at 1000 ◦ C.
0.224 mg/cm2 separately. The coating 3 has a much larger mass gain than the coating 1 and coating 2 with a maximum mass gain of 1.378 mg/cm2 ten times that of the coating 1. In Fig. 4(b), the mass gain of the bare alloy reaches 1.06 mg/cm2 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl for 100 h, and then it decreases quickly afterwards. All the three coatings go through big corrosion rates at the initial 120 h, and then transit into a stable period keeping extremely low corrosion rates subsequently. The maximum mass gains of the coating 1, coating 2 and coating 3 are 1.32 mg/cm2 , 1.24 mg/cm2 , 1.76 mg/cm2 respectively. Note that the mass gain of the coating 2 is larger than that of the coating 1 in the first 200 h, but after that it becomes a little lower than that of the coating 1. The coating 3 has a larger mass gain than the coating 1 and coating 2. 3.3. Corrosion scale Fig. 5 presents the XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 380 h and 700 h separately. For the coating 1,  is the only phase detected by XRD after hot corrosion for 700 h, indicating only a little Al is consumed to form the thin continuous alumina scale protecting the substrate from corrosion, which is confirmed in Fig. 7. As for the coating 2, 
phase is the domain phase throughout the whole corrosion test. The diffraction peaks of ␥ -(Ni,Co)3 Al (abbreviated as ␥ in the following paragraph) as well as Al2 O3 are detected after hot corrosion for 380 h, and remain relatively weak as the corrosion continues to 700 h. The  phase is dominant in the coating 3 after hot corrosion for 380 h, but the ␥ phase replacing  becomes the domain phase in the coating when the corrosion proceeds to 700 h. The surface and cross-sectional images of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h are shown in Figs. 6 and 7. As can be seen in Fig. 6, a dense and continuous alumina scale is formed on the surface of the coating 1 and coating 2, while the alumina scale on the coating 1 possesses smaller and denser particles than that on the coating 2. Crack and spallation occur on the surface of the coating 3, indicating large stress is generated in the alumina scale. In Fig. 7, the coating 1 and coating 2 are covered by a extremely thin layer of alumina, beneath which the Al contents are about 48.0 at.% and 46.42 at.% respectively according to the EDS analysis. Large amounts of  phase remains and no sulfide or subscale is found in both of the coatings, indicating the external continuous alumina scales barrier the inward diffusion of corrosive elements effectively in the mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C. While for the coating 3, the alumina layer with non uniform thickness is formed on the
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 4
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 3. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c).
surface, which is consistent with the typical corrosion morphology of the type II hot corrosion characterized in the form of pitting. Some small dotted sulfides emerge beneath the alumina scale. The Al content is 29.0 at.% in the outer layer, which reveals that the beneficial element Al is consumed more than that in the coating 1 and coating 2. In combined with the mass change curves of the three coatings presented in Fig. 4(a), it can be concluded that the corrosion resistance is worse in the order of the coating 1, coating 2 and coating 3, indicating the addition of Co is beneficial for the improvement of the corrosion resistance in the mixed sulfate salt. Fig. 8 shows the XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h. It can be seen that  phase is still the domain phase in the three coatings, although the diffraction peaks of ␥ phase are detected with strong intensity. The cross-sectional images of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C are presented in Fig. 9. All the three coatings are covered by a continuous layer of alumina scale. The  phase is abundant in the outer layer, but internal oxide of Al and sulfides mainly of Cr as
well as Ti are generated beneath the adherent alumina scale. The corrosion morphologies of the three coatings in the mixed salt containing NaCl are different from that in the mixed sulfate salt, since the presence of NaCl decreases the melting point of the mixed salt and the type I hot corrosion occurs under this condition. Note that the internal oxidation and sulfidation are slightest in the coating 2 and worst in the coating 3 among the three coatings. Combined with the mass change curves shown in Fig. 4(b), it can be inferred that the coating 2 with medium Co content possesses the best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C, followed by the coating 1 and coating 3. 3.4. Discussion 3.4.1. The degradation process of the coatings The type II hot corrosion usually happens above the melting point of the corrosive salt, resulting from the formation of low melting point sulfate eutectics such as Na2 SO4 –NiSO4 (melting point 671 ◦ C), Na2 SO4 –CoSO4 (575 ◦ C), Ni–Ni3 S2 (635 ◦ C), Co–Co4 S3 (880 ◦ C) [32–34]. These molten eutectics would dissolve
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
5
Fig. 4. The mass change curves of the bare alloy, coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 (a) and 75 wt.% Na2 SO4 + 25 wt.% NaCl (b) at 700 ◦ C.
alloy elements and prevent the formation of continuous protective Cr2 O3 or Al2 O3 films, thus lead to further corrosion of the alloys. Since CoSO4 are more stable than NiSO4 , and Na2 SO4 –CoSO4 has a lower melting point than that of Na2 SO4 –NiSO4 , so it is deemed that the Co-based alloy possesses worse low temperature hot corrosion resistance than the Ni-based alloy [32,35]. However, for the corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 (melting point 823 ◦ C [34]), experimental results demonstrate the coating 1 with the highest Co content possesses the lowest corrosion rate, following by the coating 2 and coating 3, indicating that the addition of Co improves the corrosion resistance of the simple aluminide coating in mixed sulfate salt at 700 ◦ C. Luthra [36] has proposed that the formation of liquid eutectic salt Na2 SO4 –CoSO4 could be restrained completely by the formation of stable oxide ␣-Al2 O3 and bits of CoAl2 O4 , when the Al content in the Co–Al alloy is higher than 15 wt.%. The Al content on the surface of all the three coatings is about 50 at.% (about 30 wt.%), much higher than 15 wt.%. When all the three coatings are exposed in the corrosive environment, the oxidation of Al would be oxidized more quickly than other elements like Co and Ni. And soon a protective continuous alumina layer would be formed after the transient oxidation process. As can be seen in Fig. 5, no oxide or sulfide of Co or Ni is found by XRD in all the three coatings. The surface images shown in Fig. 6 also indicate that a continuous alumina scale is formed on the surface of the coating 1 and coating 2. Spallation
Fig. 5. The XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 380 h (a) and 700 h (b).
happens on the surface of the coating 3, but the prime phase is still ␣-Al2 O3 in the flaking zone. The oxides of Co or Ni formed at the transient stage might be deoxidized by the more active element Al around or react with Al2 O3 by forming CoAl2 O4 . It reveals that the oxidation of Ni and Co is suppressed on the surface of all the three coatings. Without the formation of CoO and NiO, the corrosive fusing eutectic sulfates Na2 SO4 –NiSO4 and Na2 SO4 –CoSO4 could not be formed either. The sulfates CoSO4 and NiSO4 are formed by the following reactions: CoO + SO3 = CoSO4
(1)
NiO + SO3 = NiSO4
(2)
Thus the corrosion rate of the coatings decreases a lot. However with the consumption of oxygen by the formation of alumina, the activity of sulfur increases. SO4 2− = SO3 + O2−
(3)
And it would diffuse into the coating and react with the elements like Ni, Co and Cr forming sulfides, as the corrosion proceeds [37]. As can be seen in Fig. 7(c), bits of sulfides are found beneath the alumina scale in the coating 3. Since the sulfide and its eutectic of Co
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 6
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 6. The surface images of the coating 1 (a 400×, b 3000×), coating 2 (c 400×, d 3000×) and coating 3 (e 400×, f 3000×) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
7
Fig. 7. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h.
Fig. 8. The XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h.
(Co–Co4 S3 ) are more stable than those of Ni (Ni–Ni3 S2 ) [38,39], thus Co replacing Ni in the coatings alleviates the sulfidation process. Besides, the addition of Co is said to reduce the oxygen permeability in the NiAl alloy and make the external alumina scale more protective [1]. Consequently, the corrosion resistance is enhanced by the addition of Co. When the chloride NaCl is added into the mixed salts, the melting point of the mixed salts decreases to 620 ◦ C and the type I hot corrosion happens under this condition. As can be seen in Fig. 9, the corrosion of the three coatings is much severer compared with that in mixed sulfate salts, and the internal oxidation as well as sulfidation happens in the outer layer of all the three coatings. Since the mixed salts are liquid at 700 ◦ C and the activity of the corrosive elements like sulfur and oxygen ion are much higher than they in the mixed sulfate salts, thus leading to higher sulfidation and oxidation rates. Moreover, the presence of chloride would lead to the self-sustaining chlorination/oxidation reaction [40,41]. The Cl− ion could diffuse into the coatings to form volatile products which would lead to the formation of void and pits at grain boundaries and provide easy path for the flow of corrosive elements and oxygen [37]. Experimental results demonstrate that the coating 2 with a medium Co content possesses the best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl, following by the
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 8
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 9. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h.
coating 1, and the coating 3. It may be attributed to the two contrary effects exerted by the addition of Co in the coatings: on one hand the sulfide and eutectic of Co are more stable than those of Ni, resulting in a higher corrosion resistance in the coating with higher Co content; on the other hand the chloride of cobalt CoCl2 is easier to be formed than that of nickel according to their Gibbs free energy. The presence of chlorides especially the volatile ones could accelerate chlorination/oxidation reaction. Since CoCl2 (735 ◦ C) is solid at 700 ◦ C, so the former aspect would play a more important role than the latter under this condition. Therefore the addition of Co is beneficial for the improvement of the corrosion resistance in the mixed salts of 75 wt.% Na2 SO4 + 25 wt.% NaCl. However, the coating 2 is more corrosion resistant than the coating 1, which may be primarily attributed to the nitrides/carbides formed in the coating 1, since they are easier to be oxidized or sulfidized than other alloy phases [42]. As for the corrosion in the mixed sulfate salt, the nitrides do not deteriorate the corrosion resistance of the coating 1 during the whole test, because the protective alumina scale decreases the inward diffusion rate greatly. As can be seen in Fig. 7, no sulfide is formed inside the coating. The nitrides/carbides would
accelerate the degradation of the coating when the sulfur or oxide diffuses deep in the coating and contact with them. 3.4.2. The effect of Co on corrosion resistance According to the above analysis, Co applied to aluminide coatings possess better hot corrosion resistance than the simple aluminide coating in the mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C when the liquid eutectics Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 could not appear on the surface. The formation of Na2 SO4 –CoSO4 is reported to be the main reason for the detrimental effect of Co on the corrosion resistance of aluminide coating reported by Deodeshmukh et al. [43,44]. They have found that the addition of Co into the Ni–Al–Pt alloys decreases the type II hot corrosion resistance, while the detrimental effect reduces with the increase of the Al content in the alloys [43]. Also they have prepared two Pt modified aluminide coatings on the Ni-based as well as Co-based superalloy separately, and the type II hot corrosion test demonstrates that the coating on the Ni-based superalloy has better corrosion resistance than that on the Co-based superalloy [44]. The different conclusions may be attributed to that
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
a much milder corrosive environment is applied in this paper, since only 1 mg/cm2 corrosive salts are brushed on the samples, and no corrosive gas of O2 –0.1%SO2 is used. Besides, the elemental compositions and microstructures of the three coatings are different from those reported in the above two literatures. The adherent alumina scales effectively hinder the emerging of the formation of the eutectic Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 on the surface of the three coatings, thus the detrimental effect of Co on the hot corrosion resistance is avoided. While Co–Co4 S3 possesses a higher melting point than Ni–Ni3 S2 , the addition of Co decreases the sulfidation process improving the corrosion resistance. In a comprehensive view, the addition of Co into the simple aluminide coating is beneficial for the enhancement of the hot corrosion resistance in mild corrosive environment at 700 ◦ C, while it might exert adverse effect when the coating works in severe environment especially with high Pso3 . 4. Conclusion Two Co modified aluminide coatings with different Co contents were prepared by a combined method of pack cementation and above-the-pack process. The two coatings together with the simple aluminide coating possess a double-layered structure, an outer layer with single -(Ni,Co)Al phase and an inter layer scattered with Cr(W) rich phases. Hot corrosion test in the mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 demonstrates that the addition of Co increases the hot corrosion resistance of the coatings greatly. The continuous alumina layers on the surface of the three coatings restrain the formation of the corrosive fusing eutectic Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 completely. However, the activity of sulfur increases with the consumption of oxygen ion, and it would diffuse into the coatings to form sulfides. Since the sulfide and its eutectic of cobalt are more stable than those of nickel, the coating with higher Co content possesses better corrosion resistance. While the coating 2 with medium Co content possesses best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl, because on one hand, the stability and melting point of the sulfide as well as eutectic of cobalt are higher than those of nickel, which means Co is helpful for the enhancement of the corrosion resistance; on the other hand, the chloride of cobalt is easier to be formed than that of nickel which would advance the corrosion process. However, the coating 1 has lower corrosion resistance than the coating 2 primarily due to the nitrides formed during the deposition process. Acknowledgements This work was supported by Natural Science Foundation of China under Grant Nos. 51001106 and 51301180 as well as National Key Basic Research Program of China (973 Program, No. 2012CB625100). References [1] M.N. Task, B. Gleeson, F.S. Pettit, G.H. Meier, Compositional effects on the Type I hot corrosion of -NiAl alloys, Surface and Coatings Technology 206 (2011) 1552–1557. [2] Y.N. Wu, A. Yamaguchi, H. Murakami, S. Kuroda, Role of iridium in hot corrosion resistance of Pt-Ir modified aluminide coatings with Na2 SO4 -NaCl salt at 1173 K, Materials Transactions 47 (2006) 1918–1921. [3] M.R. Khajavi, M.H. Shariat, A. Pasha, Comparison of hot corrosion protection of coatings on IN-738LC, Surface Engineering 20 (2004) 469–473. [4] N. Eliaz, G. Shemesh, R. Latanision, Hot corrosion in gas turbine components, Engineering Failure Analysis 9 (2002) 31–43. [5] J. Ma, S.M. Jiang, J. Gong, C. Sun, Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 ◦ C, Corrosion Science 70 (2013) 29–36.
9
[6] D. He, H. Guan, X. Sun, X. Jiang, Manufacturing, structure and high temperature corrosion of palladium-modified aluminide coatings on nickel-base superalloy M38, Thin Solid Films 376 (2000) 144–151. [7] W.-J. Cheng, C.-J. Wang, Study of microstructure and phase evolution of hotdipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction, Applied Surface Science 257 (2011) 4663–4668. [8] K. Shirvani, M. Saremi, A. Nishikata, T. Tsuru, The role of silicon on microstructure and high temperature performance of aluminide coating on superalloy In-738LC, Materials Transactions-JIM 43 (2002) 2622–2628. [9] C. Houngninou, S. Chevalier, J.P. Larpin, Synthesis and characterisation of pack cemented aluminide coatings on metals, Applied Surface Science 236 (2004) 256–269. [10] Z. Xiang, S. Rose, P. Datta, Vapour phase codeposition of Al and Si to form diffusion coatings on ␥-TiAl, Materials Science and Engineering: A 356 (2003) 181–189. [11] M. Qiao, C. Zhou, Codeposition of Co and Al on nickel base superalloys by pack cementation process, Surface and Coatings Technology 206 (2012) 2899–2904. [12] W.D. Costa, B. Gleeson, D. Young, Co-deposited chromium-aluminide coatings. III. Origins of non-equilibrium effects, Surface and Coatings Technology 88 (1997) 165–171. [13] Z.D. Xiang, P.K. Datta, Codeposition of Al and Si on nickel base superalloys by pack cementation process, Materials Science and Engineering: A 356 (2003) 136–144. [14] H. Rafiee, H. Arabi, S. Rastegari, Effects of temperature and Al-concentration on formation mechanism of an aluminide coating applied on superalloy IN738LC through a single step low activity gas diffusion process, Journal of Alloys and Compounds 505 (2010) 206–212. [15] A. Firouzi, K. Shirvani, The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111, Corrosion Science 52 (2010) 3579–3585. [16] J. Lu, S. Zhu, F. Wang, Cyclic oxidation and hot corrosion behavior of Y/Crmodified aluminide coatings prepared by a hybrid slurry/pack cementation process, Oxidation of Metals 76 (2011) 67–82. [17] B. Gleeson, W.H. Cheung, W.D. Costa, D.J. Young, The hot-corrosion behavior of novel CO-deposited chromium-modified aluminide coatings, Oxidation of Metals 38 (1992) 407–424. [18] W. Wu, A. Rahmel, M. Schorr, Role of platinum in the Na2 SO4 -induced hot corrosion resistance of aluminum diffusion coatings, Oxidation of Metals 22 (1984) 59–81. [19] D.K. Das, Microstructure and high temperature oxidation behavior of Ptmodified aluminide bond coats on Ni-base superalloys, Progress in Materials Science 58 (2013) 151–182. [20] P. Dai, Q. Wu, Y. Ma, S. Li, S. Gong, The effect of silicon on the oxidation behavior of NiAlHf coating system, Applied Surface Science 271 (2013) 311–316. [21] H.-H. Liu, W.-J. Cheng, C.-J. Wang, The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al-Si coated 430 stainless steels in air-NaCl(g) atmosphere, Applied Surface Science 257 (2011) 10645–10652. [22] B.M. Warnes, Reactive element modified chemical vapor deposition low activity platinum aluminide coatings, Surface and Coatings Technology 146–147 (2001) 7–12. [23] R. Bianco, R.A. Rapp, Pack cementation aluminide coatings on superalloys: codeposition of Cr and reactive elements, Journal of the Electrochemical Society 140 (1993) 1181–1190. [24] D.R. Chang, J.J. Grisik, Metallic coating powder containing Al and Hf, Google Patents, 1976. [25] J. Stringer, Hot corrosion of high-temperature alloys, Annual Review of Materials Science 7 (1977) 477–509. [26] J.R. Rairden III, High temperature oxidation resistant dispersion strengthened nickel-chromium alloys, Google Patents, 1978. [27] D. Seo, K. Ogawa, Y. Suzuki, K. Ichimura, T. Shoji, S. Murata, Comparative study on oxidation behavior of selected MCrAlY coatings by elemental concentration profile analysis, Applied Surface Science 255 (2008) 2581–2590. [28] M. Srivastava, J.N. Balaraju, B. Ravisankar, C. Anandan, V.K. William Grips, High temperature oxidation and corrosion behaviour of Ni/Ni–Co–Al composite coatings, Applied Surface Science 263 (2012) 597–607. [29] Q.X. Fan, S.M. Jiang, D.L. Wu, J. Gong, C. Sun, Preparation and hot corrosion behaviour of two Co modified NiAl coatings on a Ni-based superalloy, Corrosion Science 76 (2013) 373–381. [30] D. Wortman, R. Fryxell, K. Luthra, P. Bergman, Mechanism of low temperature hot corrosion: Burner rig studies, Thin Solid Films 64 (1979) 281–288. [31] D. Coutsouradis, A. Davin, M. Lamberigts, Cobalt-based superalloys for applications in gas turbines, Materials Science and Engineering 88 (1987) 11–19. [32] N. Simms, A. Encinas-Oropesa, J. Nicholls, Hot corrosion of coated and uncoated single crystal gas turbine materials, Materials and Corrosion 59 (2008) 476–483. [33] K.L. Luthra, O.H. Leblanc Jr., Low temperature hot corrosion of Co-Cr-Al alloys, Materials Science and Engineering 87 (1987) 329–335. [34] K. Chiang, F. Pettit, G. Meier, Low temperature hot corrosion, NASA, Technical Memorandum (1983) 519–530. [35] Shengtai Shih, Yunshu Zhang, Xuanming Li, Sub-melting point hot corrosion of alloys and coatings, Materials Science and Engineering: A 120 (1989) 277–282. [36] K.L. Luthra, Kinetics of the low temperature hot corrosion of Co-Cr-Al alloys, Journal of the Electrochemical Society 132 (1985) 1293–1298. [37] G.S. Mahobia, N. Paulose, V. Singh, Hot corrosion behavior of superalloy IN718 at 550 and 650 ◦ C, Journal of Materials Engineering and Performance 22 (2013) 2418–2435.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
[38] D.L. Ellis, Hot Corrosion of the B2 Nickel Aluminides, 1993. [39] G. Goward, Protective coatings – purpose, role, and design, Materials Science and Technology 2 (1986) 194–200. [40] S.M. Jiang, H.Q. Li, J. Ma, C.Z. Xu, J. Gong, C. Sun, High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating II: Oxidation and hot corrosion, Corrosion Science 52 (2010) 2316–2322. [41] D. McKee, D. Shores, K. Luthra, The effect of SO and NaCl on high temperature hot corrosion, Journal of the Electrochemical Society 125 (1978) 411.
[42] 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, Corrosion Science 53 (2011) 1990–1998. [43] V. Deodeshmukh, B. Gleeson, Environmental and compositional effects on the hot-corrosion behavior of Ni-based alloys and coatings, in: Corrosion 2008, NACE International, New Orleans, LA. [44] V. Deodeshmukh, Hot corrosion behavior of Pt-modified Ni- and Co-based alloys and coatings, Ph.D. Dissertation, Iowa State University, 2007.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
ARTICLE IN PRESS
APSUSC-27861; No. of Pages 10
Applied Surface Science xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C Q.X. Fan, S.M. Jiang ∗ , H.J. Yu, J. Gong, C. Sun Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, PR China
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 16 April 2014 Accepted 9 May 2014 Available online xxx Keywords: Aluminide coating Pack cementation process Hot corrosion Microstructure Superalloy
a b s t r a c t Two Co modified aluminide coatings with different Co contents were prepared by pack cementation process and above-the-pack process. The hot corrosion tests of the two coatings were performed in mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C, with a simple aluminide coating as the reference coating. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) with energy dispersive X-ray spectrometer (EDS) were used to characterize the coatings and the corrosion scales. Results indicate that the addition of Co improves the hot corrosion resistance of the simple aluminide coating in the mixed sulfate salts, for the sulfide as well as its eutectic of cobalt are more stable, and possess higher melting points than those of nickel. While in the mixed salt containing chloride, the coating with medium Co content possesses the best corrosion resistance, primarily because the nitrides formed in the deposition process deteriorate the corrosion resistance of the coating with highest Co content. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Salt deposits of sulfur, sodium, chlorine and potassium can develop on the surface of gas-turbine hardware, especially when the engines operate in locations where salt can be ingested, i.e., coastal areas and regions with high level of airborne pollutants [1]. These salts would lead to severe hot corrosion of the materials [2]. Usually hot corrosion is divided into two categories: type I (high temperature) hot corrosion and type II (low temperature) hot corrosion [3]. The development of these two forms is affected by various parameters such as: thermomechanical condition, contaminant composition and fluxing rate [4]. For the sulfate salts, the type I hot corrosion usually happens at temperatures between 815 ◦ C and 927 ◦ C, and the type II hot corrosion occurs in the 593–760 ◦ C range [3]. While for the salts containing chloride, the type I hot corrosion can take place at the temperature of 700 ◦ C, since it is above the fusing point of the eutectic salts containing Na2 SO4 and NaCl (about 620 ◦ C) [5]. Considering the economic cost and good oxidation resistance, aluminide diffusion coatings are commonly used to protect the nickel-based engine alloys from environmental erosion [6]. The diffusion coating can be deposited by hot dip aluminization, slurry
∗ Corresponding author. Tel.: +86 24 83978081; fax: +86 24 23891320. E-mail addresses: [email protected] (Q.X. Fan), [email protected], [email protected] (S.M. Jiang).
aluminization, pack cementation process and chemical vapor deposition (CVD) [7–10]. Among the above preparation methods, pack cementation process is used most widespread for single or binary deposition such as Co–Al, Cr–Al and Si–Al [11–13], while CVD is the best technique to prepare the diffusion aluminide coating. Firstly, it can be used for coating the long narrow internal air channels of vanes and blades [14,15]; Secondly, the coatings prepared by CVD method are inclusion free and possess homogeneous structures, since the specimens are separated from the powder mixture [14]. However, some elements might be difficult to be deposited by CVD, for the activity of the target element, chamber vacuum degree, and elemental compositions of the furnace have great influences on the deposition process. In above-the-pack process the samples are isolated from the powder mixture, so the coatings prepared by this method also possess good microstructure and properties. Because of this similarity, some researchers have called it as CVD method [16,17]. It has been widely acknowledged that the addition of beneficial elements like Cr [18,19], Pt [20,21], Si [22,23] and reactive elements [24–26] to the simple aluminide coatings can improve either the corrosion resistance or oxidation resistance of the coatings. Co-based alloy is supposed to possess better corrosion resistance than Ni-based alloy [27], and it’s of growing interest to investigate the effect of Co applied to simple aluminide coatings. Some researchers have found that the addition of Co could decrease the pores in the aluminide coating or NiCrAlY on the nickel-based alloys resulting in a higher oxidation resistance [28,29]. Other researchers
http://dx.doi.org/10.1016/j.apsusc.2014.05.043 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
2
have demonstrated that Co applied to the NiAl coating is beneficial to improve the corrosion resistance above 800 ◦ C [30,31]. However seldom research has been done at medium temperature 600–800 ◦ C. To investigate the influence of Co on corrosion resistance of the simple aluminide coating at medium temperature, two Co modified aluminide coating with different Co contents are prepared by a two step process, first depositing Co by pack cementation and then aluminizing by above-the-pack process. The hot corrosion behavior of the two Co modified aluminide coatings in comparison with the simple aluminide coating is performed in two different mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C. The influence of Co on the hot corrosion resistance and the microstructure as well as phase evolution of the coatings during the corrosion process will be studied in detail.
2. Experimental procedures The nominal composition of the nickel based alloy used as the substrate was Ni–10.48 Co–15.28 Cr–3.10 Al–4.40 Ti–5.13 W–2.08 Mo–0.19 Nb–0.31 Hf–0.08 B–0.085 C (wt.%). Cubical samples of 15 mm × 10 mm × 2 mm were ground down to 800 # SiC paper, prior to being ultrasonically washed within acetone, ethanol and deionized water successively. For preparation of the Coating 1, Co was deposited at 1000 ◦ C for 6 h and then aluminized at 1000 ◦ C for 5 h. The coating 2 was prepared by depositing Co at 850 ◦ C for 6 h and subsequently aluminized at 1000 ◦ C for 5 h. As a reference coating, the coating 3 was simple aluminide coating which prepared under the same aluminization condition as that of the coating 1 and coating 2. For pack cementation deposition of Co, the specimens were buried in the powder mixture made up of 480 g Al2 O3 , 320 g cobalt powder and 50 g NH4 Cl at 1000 ◦ C or 850 ◦ C for 6 h. After cooled down to room temperature in the furnace, the specimens were taken out and cleaned in ethanol and deionized water again for the subsequent aluminization. For the Al deposition, the specimens with or without Co coatings were hung above the powder mixture composed of 600 g FeAl powder and 40 g NH4 Cl at 1000 ◦ C for 5 h. For both the deposition of Co and Al, the furnace chamber was pumped to at least 100 Pa, and then Ar was filled in, following by pumping the furnace chamber below 100 Pa again. This process was repeated for three times to eliminate oxygen in the furnace, following by filling Ar in the furnace until the chamber pressure was up to 2.5 × 104 Pa. Cyclic hot corrosion tests of the three coatings were conducted in two different mixed salts in static air at 1 atm. The specimens were heated on a hot plate to about 150 ◦ C, and then coated a thin layer of salt mixture Na2 SO4 /K2 SO4 (3:1, w/w) or Na2 SO4 /NaCl (3:1, w/w) on the surface. The samples were measured before and after importing the salt mixture to ensure a salt supply of 1 mg/cm2 . Subsequently the samples were placed within crucibles and taken into the furnace which was heated up to 700 ◦ C. At regular interval (20 h), the specimens were taken out and cooled down to room temperature. Prior to being dried and weighed by electronic balance with a sensitivity of 10−5 g, the specimens were washed in boiling distilled water to obliterate the remaining salt. Afterwards a thin uniform fresh salt coating was brushed on the specimens again to continue the experiment. For each test, three parallel samples were tested to ensure experimental accuracy. Phase identification and morphologies of the coatings and corrosion scales are characterized by means of X-ray diffractometer (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). For cross-sectional microstructure observing, an electroless Ni layer was coated on the surface of the specimens and then mounted in a plastic resin. Additionally, transmission electron microscope (TEM) with EDS system was used for the phase identification of some specimens.
Fig. 1. XRD patterns of the two Co coatings deposited at 1000 ◦ C (a) and 850 ◦ C (b).
3. Results and discussion 3.1. The microstructure of the coatings before oxidation Figs. 1 and 2 present the XRD patterns and cross-sectional images of the two Co coatings deposited at 1000 ◦ C and 850 ◦ C respectively. As can be seen in Figs. 1 and 2, the outer layer of the two Co coatings consists of singular ␥-(Co,Ni) with thicknesses about 8 m and 3 m separately. The ␥-(Co,Ni) is also the prime phase in the transitional layer of the two Co coatings. Some dotted and strip-like grey phases rich in Ti are scattered in the 1000 ◦ C Co coating, while no such Ti rich phases are found in the 850 ◦ C Co coating. Fig. 2(c) shows the TEM morphology of the transitional zone of the Co coating deposited at 1000 ◦ C, and it can be seen that the grey dotted phase is TiN, while the strip-like phase is the mixed phase of TiN and TiC (C0.7 N0.3 Ti). The cross-sectional images of the coating 1, coating 2 and coating 3 are presented in Fig. 3 respectively. For the coating 1, the outer layer is composed of single -(Ni,Co)Al (abbreviated as  in the following paragraph). The inner layer consists of Cr(W) rich phases and stripe like Ti rich phases which mainly form during the process of Co deposition. The coating 2 and coating 3 also possess a doublelayered structure, an outer layer with single  phase and an inner Cr(W) rich phase layer. The elemental compositions at the near surface of the three coatings are 49.75 Al–36.47 Co–11.62 Ni–1.32 Cr–0.85 Fe (at.%), 50.23 Al–18.06 Co–30.06 Ni–1.09 Cr–0.58 Fe (at.%) and 52.23 Al–3.19 Co–43.20 Ni–0.81 Cr–0.58 Fe (at.%) respectively detected by EDS. It can be seen that the Al content and Ni content in the outer layer are greater in the order of the coating 1, coating 2 and coating 3, while the Co and Cr contents become less from the coating 1 to the coating 3. 3.2. Mass change curves Fig. 4 presents the mass change curves of the bare alloy, coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C respectively. Since the three parallel samples of each coatings have small mass gains with little difference in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 , so the error bars are not shown in Fig. 4(a). It can be seen that the mass gain of the bare alloy increases to 0.065 mg/cm2 after hot corrosion for 20 h, and then it decreases gradually to about −0.131 mg/cm2 after hot corrosion for 700 h. The coating 1 and coating 2 have smooth mass change curves, the maximum mass gains of the two coatings are 0.138 mg/cm2 and
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
3
Fig. 2. The cross-sectional images of the Co coating deposited at 1000 ◦ C (a) and 850 ◦ C (b); (c) TEM images of the transitional layer of the Co coating deposited at 1000 ◦ C.
0.224 mg/cm2 separately. The coating 3 has a much larger mass gain than the coating 1 and coating 2 with a maximum mass gain of 1.378 mg/cm2 ten times that of the coating 1. In Fig. 4(b), the mass gain of the bare alloy reaches 1.06 mg/cm2 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl for 100 h, and then it decreases quickly afterwards. All the three coatings go through big corrosion rates at the initial 120 h, and then transit into a stable period keeping extremely low corrosion rates subsequently. The maximum mass gains of the coating 1, coating 2 and coating 3 are 1.32 mg/cm2 , 1.24 mg/cm2 , 1.76 mg/cm2 respectively. Note that the mass gain of the coating 2 is larger than that of the coating 1 in the first 200 h, but after that it becomes a little lower than that of the coating 1. The coating 3 has a larger mass gain than the coating 1 and coating 2. 3.3. Corrosion scale Fig. 5 presents the XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 380 h and 700 h separately. For the coating 1,  is the only phase detected by XRD after hot corrosion for 700 h, indicating only a little Al is consumed to form the thin continuous alumina scale protecting the substrate from corrosion, which is confirmed in Fig. 7. As for the coating 2, 
phase is the domain phase throughout the whole corrosion test. The diffraction peaks of ␥ -(Ni,Co)3 Al (abbreviated as ␥ in the following paragraph) as well as Al2 O3 are detected after hot corrosion for 380 h, and remain relatively weak as the corrosion continues to 700 h. The  phase is dominant in the coating 3 after hot corrosion for 380 h, but the ␥ phase replacing  becomes the domain phase in the coating when the corrosion proceeds to 700 h. The surface and cross-sectional images of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h are shown in Figs. 6 and 7. As can be seen in Fig. 6, a dense and continuous alumina scale is formed on the surface of the coating 1 and coating 2, while the alumina scale on the coating 1 possesses smaller and denser particles than that on the coating 2. Crack and spallation occur on the surface of the coating 3, indicating large stress is generated in the alumina scale. In Fig. 7, the coating 1 and coating 2 are covered by a extremely thin layer of alumina, beneath which the Al contents are about 48.0 at.% and 46.42 at.% respectively according to the EDS analysis. Large amounts of  phase remains and no sulfide or subscale is found in both of the coatings, indicating the external continuous alumina scales barrier the inward diffusion of corrosive elements effectively in the mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C. While for the coating 3, the alumina layer with non uniform thickness is formed on the
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 4
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 3. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c).
surface, which is consistent with the typical corrosion morphology of the type II hot corrosion characterized in the form of pitting. Some small dotted sulfides emerge beneath the alumina scale. The Al content is 29.0 at.% in the outer layer, which reveals that the beneficial element Al is consumed more than that in the coating 1 and coating 2. In combined with the mass change curves of the three coatings presented in Fig. 4(a), it can be concluded that the corrosion resistance is worse in the order of the coating 1, coating 2 and coating 3, indicating the addition of Co is beneficial for the improvement of the corrosion resistance in the mixed sulfate salt. Fig. 8 shows the XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h. It can be seen that  phase is still the domain phase in the three coatings, although the diffraction peaks of ␥ phase are detected with strong intensity. The cross-sectional images of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C are presented in Fig. 9. All the three coatings are covered by a continuous layer of alumina scale. The  phase is abundant in the outer layer, but internal oxide of Al and sulfides mainly of Cr as
well as Ti are generated beneath the adherent alumina scale. The corrosion morphologies of the three coatings in the mixed salt containing NaCl are different from that in the mixed sulfate salt, since the presence of NaCl decreases the melting point of the mixed salt and the type I hot corrosion occurs under this condition. Note that the internal oxidation and sulfidation are slightest in the coating 2 and worst in the coating 3 among the three coatings. Combined with the mass change curves shown in Fig. 4(b), it can be inferred that the coating 2 with medium Co content possesses the best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C, followed by the coating 1 and coating 3. 3.4. Discussion 3.4.1. The degradation process of the coatings The type II hot corrosion usually happens above the melting point of the corrosive salt, resulting from the formation of low melting point sulfate eutectics such as Na2 SO4 –NiSO4 (melting point 671 ◦ C), Na2 SO4 –CoSO4 (575 ◦ C), Ni–Ni3 S2 (635 ◦ C), Co–Co4 S3 (880 ◦ C) [32–34]. These molten eutectics would dissolve
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
5
Fig. 4. The mass change curves of the bare alloy, coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 (a) and 75 wt.% Na2 SO4 + 25 wt.% NaCl (b) at 700 ◦ C.
alloy elements and prevent the formation of continuous protective Cr2 O3 or Al2 O3 films, thus lead to further corrosion of the alloys. Since CoSO4 are more stable than NiSO4 , and Na2 SO4 –CoSO4 has a lower melting point than that of Na2 SO4 –NiSO4 , so it is deemed that the Co-based alloy possesses worse low temperature hot corrosion resistance than the Ni-based alloy [32,35]. However, for the corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 (melting point 823 ◦ C [34]), experimental results demonstrate the coating 1 with the highest Co content possesses the lowest corrosion rate, following by the coating 2 and coating 3, indicating that the addition of Co improves the corrosion resistance of the simple aluminide coating in mixed sulfate salt at 700 ◦ C. Luthra [36] has proposed that the formation of liquid eutectic salt Na2 SO4 –CoSO4 could be restrained completely by the formation of stable oxide ␣-Al2 O3 and bits of CoAl2 O4 , when the Al content in the Co–Al alloy is higher than 15 wt.%. The Al content on the surface of all the three coatings is about 50 at.% (about 30 wt.%), much higher than 15 wt.%. When all the three coatings are exposed in the corrosive environment, the oxidation of Al would be oxidized more quickly than other elements like Co and Ni. And soon a protective continuous alumina layer would be formed after the transient oxidation process. As can be seen in Fig. 5, no oxide or sulfide of Co or Ni is found by XRD in all the three coatings. The surface images shown in Fig. 6 also indicate that a continuous alumina scale is formed on the surface of the coating 1 and coating 2. Spallation
Fig. 5. The XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 380 h (a) and 700 h (b).
happens on the surface of the coating 3, but the prime phase is still ␣-Al2 O3 in the flaking zone. The oxides of Co or Ni formed at the transient stage might be deoxidized by the more active element Al around or react with Al2 O3 by forming CoAl2 O4 . It reveals that the oxidation of Ni and Co is suppressed on the surface of all the three coatings. Without the formation of CoO and NiO, the corrosive fusing eutectic sulfates Na2 SO4 –NiSO4 and Na2 SO4 –CoSO4 could not be formed either. The sulfates CoSO4 and NiSO4 are formed by the following reactions: CoO + SO3 = CoSO4
(1)
NiO + SO3 = NiSO4
(2)
Thus the corrosion rate of the coatings decreases a lot. However with the consumption of oxygen by the formation of alumina, the activity of sulfur increases. SO4 2− = SO3 + O2−
(3)
And it would diffuse into the coating and react with the elements like Ni, Co and Cr forming sulfides, as the corrosion proceeds [37]. As can be seen in Fig. 7(c), bits of sulfides are found beneath the alumina scale in the coating 3. Since the sulfide and its eutectic of Co
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 6
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 6. The surface images of the coating 1 (a 400×, b 3000×), coating 2 (c 400×, d 3000×) and coating 3 (e 400×, f 3000×) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
7
Fig. 7. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 at 700 ◦ C for 700 h.
Fig. 8. The XRD patterns of the coating 1, coating 2 and coating 3 after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h.
(Co–Co4 S3 ) are more stable than those of Ni (Ni–Ni3 S2 ) [38,39], thus Co replacing Ni in the coatings alleviates the sulfidation process. Besides, the addition of Co is said to reduce the oxygen permeability in the NiAl alloy and make the external alumina scale more protective [1]. Consequently, the corrosion resistance is enhanced by the addition of Co. When the chloride NaCl is added into the mixed salts, the melting point of the mixed salts decreases to 620 ◦ C and the type I hot corrosion happens under this condition. As can be seen in Fig. 9, the corrosion of the three coatings is much severer compared with that in mixed sulfate salts, and the internal oxidation as well as sulfidation happens in the outer layer of all the three coatings. Since the mixed salts are liquid at 700 ◦ C and the activity of the corrosive elements like sulfur and oxygen ion are much higher than they in the mixed sulfate salts, thus leading to higher sulfidation and oxidation rates. Moreover, the presence of chloride would lead to the self-sustaining chlorination/oxidation reaction [40,41]. The Cl− ion could diffuse into the coatings to form volatile products which would lead to the formation of void and pits at grain boundaries and provide easy path for the flow of corrosive elements and oxygen [37]. Experimental results demonstrate that the coating 2 with a medium Co content possesses the best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl, following by the
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 8
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
Fig. 9. The cross-sectional images of the coating 1 (a), coating 2 (b) and coating 3 (c) after hot corrosion in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C for 380 h.
coating 1, and the coating 3. It may be attributed to the two contrary effects exerted by the addition of Co in the coatings: on one hand the sulfide and eutectic of Co are more stable than those of Ni, resulting in a higher corrosion resistance in the coating with higher Co content; on the other hand the chloride of cobalt CoCl2 is easier to be formed than that of nickel according to their Gibbs free energy. The presence of chlorides especially the volatile ones could accelerate chlorination/oxidation reaction. Since CoCl2 (735 ◦ C) is solid at 700 ◦ C, so the former aspect would play a more important role than the latter under this condition. Therefore the addition of Co is beneficial for the improvement of the corrosion resistance in the mixed salts of 75 wt.% Na2 SO4 + 25 wt.% NaCl. However, the coating 2 is more corrosion resistant than the coating 1, which may be primarily attributed to the nitrides/carbides formed in the coating 1, since they are easier to be oxidized or sulfidized than other alloy phases [42]. As for the corrosion in the mixed sulfate salt, the nitrides do not deteriorate the corrosion resistance of the coating 1 during the whole test, because the protective alumina scale decreases the inward diffusion rate greatly. As can be seen in Fig. 7, no sulfide is formed inside the coating. The nitrides/carbides would
accelerate the degradation of the coating when the sulfur or oxide diffuses deep in the coating and contact with them. 3.4.2. The effect of Co on corrosion resistance According to the above analysis, Co applied to aluminide coatings possess better hot corrosion resistance than the simple aluminide coating in the mixed salts of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 and 75 wt.% Na2 SO4 + 25 wt.% NaCl at 700 ◦ C when the liquid eutectics Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 could not appear on the surface. The formation of Na2 SO4 –CoSO4 is reported to be the main reason for the detrimental effect of Co on the corrosion resistance of aluminide coating reported by Deodeshmukh et al. [43,44]. They have found that the addition of Co into the Ni–Al–Pt alloys decreases the type II hot corrosion resistance, while the detrimental effect reduces with the increase of the Al content in the alloys [43]. Also they have prepared two Pt modified aluminide coatings on the Ni-based as well as Co-based superalloy separately, and the type II hot corrosion test demonstrates that the coating on the Ni-based superalloy has better corrosion resistance than that on the Co-based superalloy [44]. The different conclusions may be attributed to that
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
a much milder corrosive environment is applied in this paper, since only 1 mg/cm2 corrosive salts are brushed on the samples, and no corrosive gas of O2 –0.1%SO2 is used. Besides, the elemental compositions and microstructures of the three coatings are different from those reported in the above two literatures. The adherent alumina scales effectively hinder the emerging of the formation of the eutectic Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 on the surface of the three coatings, thus the detrimental effect of Co on the hot corrosion resistance is avoided. While Co–Co4 S3 possesses a higher melting point than Ni–Ni3 S2 , the addition of Co decreases the sulfidation process improving the corrosion resistance. In a comprehensive view, the addition of Co into the simple aluminide coating is beneficial for the enhancement of the hot corrosion resistance in mild corrosive environment at 700 ◦ C, while it might exert adverse effect when the coating works in severe environment especially with high Pso3 . 4. Conclusion Two Co modified aluminide coatings with different Co contents were prepared by a combined method of pack cementation and above-the-pack process. The two coatings together with the simple aluminide coating possess a double-layered structure, an outer layer with single -(Ni,Co)Al phase and an inter layer scattered with Cr(W) rich phases. Hot corrosion test in the mixed salt of 75 wt.% Na2 SO4 + 25 wt.% K2 SO4 demonstrates that the addition of Co increases the hot corrosion resistance of the coatings greatly. The continuous alumina layers on the surface of the three coatings restrain the formation of the corrosive fusing eutectic Na2 SO4 –CoSO4 and Na2 SO4 –NiSO4 completely. However, the activity of sulfur increases with the consumption of oxygen ion, and it would diffuse into the coatings to form sulfides. Since the sulfide and its eutectic of cobalt are more stable than those of nickel, the coating with higher Co content possesses better corrosion resistance. While the coating 2 with medium Co content possesses best corrosion resistance in mixed salt of 75 wt.% Na2 SO4 + 25 wt.% NaCl, because on one hand, the stability and melting point of the sulfide as well as eutectic of cobalt are higher than those of nickel, which means Co is helpful for the enhancement of the corrosion resistance; on the other hand, the chloride of cobalt is easier to be formed than that of nickel which would advance the corrosion process. However, the coating 1 has lower corrosion resistance than the coating 2 primarily due to the nitrides formed during the deposition process. Acknowledgements This work was supported by Natural Science Foundation of China under Grant Nos. 51001106 and 51301180 as well as National Key Basic Research Program of China (973 Program, No. 2012CB625100). References [1] M.N. Task, B. Gleeson, F.S. Pettit, G.H. Meier, Compositional effects on the Type I hot corrosion of -NiAl alloys, Surface and Coatings Technology 206 (2011) 1552–1557. [2] Y.N. Wu, A. Yamaguchi, H. Murakami, S. Kuroda, Role of iridium in hot corrosion resistance of Pt-Ir modified aluminide coatings with Na2 SO4 -NaCl salt at 1173 K, Materials Transactions 47 (2006) 1918–1921. [3] M.R. Khajavi, M.H. Shariat, A. Pasha, Comparison of hot corrosion protection of coatings on IN-738LC, Surface Engineering 20 (2004) 469–473. [4] N. Eliaz, G. Shemesh, R. Latanision, Hot corrosion in gas turbine components, Engineering Failure Analysis 9 (2002) 31–43. [5] J. Ma, S.M. Jiang, J. Gong, C. Sun, Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 ◦ C, Corrosion Science 70 (2013) 29–36.
9
[6] D. He, H. Guan, X. Sun, X. Jiang, Manufacturing, structure and high temperature corrosion of palladium-modified aluminide coatings on nickel-base superalloy M38, Thin Solid Films 376 (2000) 144–151. [7] W.-J. Cheng, C.-J. Wang, Study of microstructure and phase evolution of hotdipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction, Applied Surface Science 257 (2011) 4663–4668. [8] K. Shirvani, M. Saremi, A. Nishikata, T. Tsuru, The role of silicon on microstructure and high temperature performance of aluminide coating on superalloy In-738LC, Materials Transactions-JIM 43 (2002) 2622–2628. [9] C. Houngninou, S. Chevalier, J.P. Larpin, Synthesis and characterisation of pack cemented aluminide coatings on metals, Applied Surface Science 236 (2004) 256–269. [10] Z. Xiang, S. Rose, P. Datta, Vapour phase codeposition of Al and Si to form diffusion coatings on ␥-TiAl, Materials Science and Engineering: A 356 (2003) 181–189. [11] M. Qiao, C. Zhou, Codeposition of Co and Al on nickel base superalloys by pack cementation process, Surface and Coatings Technology 206 (2012) 2899–2904. [12] W.D. Costa, B. Gleeson, D. Young, Co-deposited chromium-aluminide coatings. III. Origins of non-equilibrium effects, Surface and Coatings Technology 88 (1997) 165–171. [13] Z.D. Xiang, P.K. Datta, Codeposition of Al and Si on nickel base superalloys by pack cementation process, Materials Science and Engineering: A 356 (2003) 136–144. [14] H. Rafiee, H. Arabi, S. Rastegari, Effects of temperature and Al-concentration on formation mechanism of an aluminide coating applied on superalloy IN738LC through a single step low activity gas diffusion process, Journal of Alloys and Compounds 505 (2010) 206–212. [15] A. Firouzi, K. Shirvani, The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111, Corrosion Science 52 (2010) 3579–3585. [16] J. Lu, S. Zhu, F. Wang, Cyclic oxidation and hot corrosion behavior of Y/Crmodified aluminide coatings prepared by a hybrid slurry/pack cementation process, Oxidation of Metals 76 (2011) 67–82. [17] B. Gleeson, W.H. Cheung, W.D. Costa, D.J. Young, The hot-corrosion behavior of novel CO-deposited chromium-modified aluminide coatings, Oxidation of Metals 38 (1992) 407–424. [18] W. Wu, A. Rahmel, M. Schorr, Role of platinum in the Na2 SO4 -induced hot corrosion resistance of aluminum diffusion coatings, Oxidation of Metals 22 (1984) 59–81. [19] D.K. Das, Microstructure and high temperature oxidation behavior of Ptmodified aluminide bond coats on Ni-base superalloys, Progress in Materials Science 58 (2013) 151–182. [20] P. Dai, Q. Wu, Y. Ma, S. Li, S. Gong, The effect of silicon on the oxidation behavior of NiAlHf coating system, Applied Surface Science 271 (2013) 311–316. [21] H.-H. Liu, W.-J. Cheng, C.-J. Wang, The mechanism of oxide whisker growth and hot corrosion of hot-dipped Al-Si coated 430 stainless steels in air-NaCl(g) atmosphere, Applied Surface Science 257 (2011) 10645–10652. [22] B.M. Warnes, Reactive element modified chemical vapor deposition low activity platinum aluminide coatings, Surface and Coatings Technology 146–147 (2001) 7–12. [23] R. Bianco, R.A. Rapp, Pack cementation aluminide coatings on superalloys: codeposition of Cr and reactive elements, Journal of the Electrochemical Society 140 (1993) 1181–1190. [24] D.R. Chang, J.J. Grisik, Metallic coating powder containing Al and Hf, Google Patents, 1976. [25] J. Stringer, Hot corrosion of high-temperature alloys, Annual Review of Materials Science 7 (1977) 477–509. [26] J.R. Rairden III, High temperature oxidation resistant dispersion strengthened nickel-chromium alloys, Google Patents, 1978. [27] D. Seo, K. Ogawa, Y. Suzuki, K. Ichimura, T. Shoji, S. Murata, Comparative study on oxidation behavior of selected MCrAlY coatings by elemental concentration profile analysis, Applied Surface Science 255 (2008) 2581–2590. [28] M. Srivastava, J.N. Balaraju, B. Ravisankar, C. Anandan, V.K. William Grips, High temperature oxidation and corrosion behaviour of Ni/Ni–Co–Al composite coatings, Applied Surface Science 263 (2012) 597–607. [29] Q.X. Fan, S.M. Jiang, D.L. Wu, J. Gong, C. Sun, Preparation and hot corrosion behaviour of two Co modified NiAl coatings on a Ni-based superalloy, Corrosion Science 76 (2013) 373–381. [30] D. Wortman, R. Fryxell, K. Luthra, P. Bergman, Mechanism of low temperature hot corrosion: Burner rig studies, Thin Solid Films 64 (1979) 281–288. [31] D. Coutsouradis, A. Davin, M. Lamberigts, Cobalt-based superalloys for applications in gas turbines, Materials Science and Engineering 88 (1987) 11–19. [32] N. Simms, A. Encinas-Oropesa, J. Nicholls, Hot corrosion of coated and uncoated single crystal gas turbine materials, Materials and Corrosion 59 (2008) 476–483. [33] K.L. Luthra, O.H. Leblanc Jr., Low temperature hot corrosion of Co-Cr-Al alloys, Materials Science and Engineering 87 (1987) 329–335. [34] K. Chiang, F. Pettit, G. Meier, Low temperature hot corrosion, NASA, Technical Memorandum (1983) 519–530. [35] Shengtai Shih, Yunshu Zhang, Xuanming Li, Sub-melting point hot corrosion of alloys and coatings, Materials Science and Engineering: A 120 (1989) 277–282. [36] K.L. Luthra, Kinetics of the low temperature hot corrosion of Co-Cr-Al alloys, Journal of the Electrochemical Society 132 (1985) 1293–1298. [37] G.S. Mahobia, N. Paulose, V. Singh, Hot corrosion behavior of superalloy IN718 at 550 and 650 ◦ C, Journal of Materials Engineering and Performance 22 (2013) 2418–2435.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043
G Model APSUSC-27861; No. of Pages 10 10
ARTICLE IN PRESS Q.X. Fan et al. / Applied Surface Science xxx (2014) xxx–xxx
[38] D.L. Ellis, Hot Corrosion of the B2 Nickel Aluminides, 1993. [39] G. Goward, Protective coatings – purpose, role, and design, Materials Science and Technology 2 (1986) 194–200. [40] S.M. Jiang, H.Q. Li, J. Ma, C.Z. Xu, J. Gong, C. Sun, High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating II: Oxidation and hot corrosion, Corrosion Science 52 (2010) 2316–2322. [41] D. McKee, D. Shores, K. Luthra, The effect of SO and NaCl on high temperature hot corrosion, Journal of the Electrochemical Society 125 (1978) 411.
[42] 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, Corrosion Science 53 (2011) 1990–1998. [43] V. Deodeshmukh, B. Gleeson, Environmental and compositional effects on the hot-corrosion behavior of Ni-based alloys and coatings, in: Corrosion 2008, NACE International, New Orleans, LA. [44] V. Deodeshmukh, Hot corrosion behavior of Pt-modified Ni- and Co-based alloys and coatings, Ph.D. Dissertation, Iowa State University, 2007.
Please cite this article in press as: Q.X. Fan, et al., Microstructure and hot corrosion behaviors of two Co modified aluminide coatings on a Ni-based superalloy at 700 ◦ C, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.043