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Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 ˚C
Accepted Manuscript Title: Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 Authors: Hongyu Wang, Xiang Zhang, Zeng Xu, Han Wang, Changshun Zhu PII: DOI: Reference:
S0010-938X(18)30883-7 https://doi.org/10.1016/j.corsci.2018.11.026 CS 7782
To appear in: Received date: Revised date: Accepted date:
16 May 2018 21 November 2018 22 November 2018
Please cite this article as: Wang H, Zhang X, Xu Z, Wang H, Zhu C, Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 , Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.11.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 ˚C Hongyu Wang*, Xiang Zhang, Zeng Xu, Han Wang and Changshun Zhu School of Mechanical Engineering Jiangsu University, Zhenjiang City, Jiangsu Province, China, 212013
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E-mail address: [email protected] (Hongyu Wang)
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Graphical abstract
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Highlights
The dissolution capacity of molten salt at 1150 ˚C is much stronger.
The Al-Si coating has failed after hot corrosion for 20 h in mixed sulphate at 1150 ˚C.
The coating is not able to form a protective oxide scale in the process of hot corrosion.
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The large consumption of Al causes the outward diffusion of Cr and Si to react with molten
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salt.
the original anti-oxidation and corrosion mechanism of diffusion barrier is destroyed.
Abstract: Hot corrosion test of Al-Si coating was investigated at 1150 ˚C based on the results reported from previous work which was conducted at 1050 ˚C. It has been observed that the coating 1
failed after 20 h of hot corrosion. When the temperature of hot corrosion reached 1150 ˚C, the dissolution effect of the molten salt to Al2O3 scale was sharply enhanced. Therefore, during hot corrosion, Al-Si coating was not able to generate a protective oxide scale, and the original anti-oxidation and corrosion mechanism of diffusion barrier of Al-Si coating cannot be retained. Keywords: Metal coatings, molten salt, hot corrosion.
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1 Introduction Metallic bond coatings are widely used in thermal barrier coatings (TBCs) on the surface of turbine engine blade [1]. With an increase in the working temperature of gas turbine, the working
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temperature of metallic bond coating is more than 1050 ˚C and even reaches a temperature of 1200 ˚C [2-4]. Hot oxidation corrosion resistance are important evaluation index of metallic bond coating [5]. In the complex working conditions, the effect of hot corrosion is more severe than that of hot
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oxidation, with negative consequences on the safe and reliable operation of the thermal barrier
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coating (TBC) and aero engines.
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Many researchers have carried out extensive studies on the hot corrosion of metallic bond
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coating, but the temperature of hot corrosion is mainly concentrated in the range from 800 to 950 ˚C [6-12]. For example, Wang et al. [6] investigated the hot corrosion behaviour of NiCrAlY coating in
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mixed sulphate at 850 ˚C. The results showed that the coating started to spalling off after 20 h of hot corrosion, and after 80 h severe internal sulphidation occurred, which was accompanied by a loss in
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the protectiveness of coating. Therefore, the hot corrosion of metallic bond coating becomes very severe at high temperatures (800 to 950 ˚C). Subsequently, an improvement in the high temperature
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corrosion resistance has become a hot topic of research interest, and a further exploration at temperatures exceeding 1000 ˚C has never been carried out [13-19]. Recently, Wang et al. examined the hot corrosion behaviour and mechanism of NiCoCrAlY coating [20], aluminum rare
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earth coating [21] and Al-Si coating [22] at 1050 ˚C. The obtained results showed that the hot corrosion is more severe than the corrosion occurring at a conventional high temperature (800 to 950 ˚C). However, in order to develop a new type of metallic bond coating that can meet its working requirements, it is obviously necessary to understand the mechanism of hot corrosion of metallic bond coating occurring at the critical working temperature.
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Al-Si coating as a member of metallic bond coating, not only easily forms a stable protective Al2O3 scale at high temperature, but also can show the unique characteristics of in situ formation of internal diffusion barriers that can restrain the failure of coating during hot oxidation corrosion. In this investigation, based on our previous study [22], the hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 ˚C was further studied. The cross-sectional and surface microscopic images of the coating were observed and the phase composition of the eroded surfaces was characterized.
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Also, the elemental distribution of the coating was analysed along with studying the kinetics of hot corrosion. Moreover, the mechanism of hot corrosion under the subjected conditions was
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systematically discussed. 2 Experimental process
The matrix used in this study was Nickel-based superalloy GH4033, and its chemical
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composition is illustrated in Table 1 [20]. For this study, the matrix alloy was made into specimens
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with the dimensions of 6 mm × 4 mm × 4 mm using a wire-electrode cutting, and the surfaces and
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sides of the specimen were ground by 400-mesh abrasive paper, and then were cleaned in acetone
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ultrasonically.
The preparation of Al-Si coating on the surface of sample was carried out by following the
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powder pack cementation method [23]. The pack mixture was 20 wt% Al, 10 wt% Si, 68 wt% Al2O3, and 2 wt% NH4Cl [24]. During coating, the samples were placed in an infiltration container
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made of 45 steel, and the powder mixture was filled around it. The infiltration container were capped sealed and then placed in a tubular resistance furnace. The parameters of temperature were
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as follows: heating for 4 h at 850 ˚C and then continuing the heating for 8 h at 1050 ˚C. Following this, the containers were cooled in the furnace. The hot corrosion tests were performed in a tubular resistance furnace [21]. Before this, the
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surfaces and sides of the coated specimens were polished with 400-mesh sandpaper, cleaned in an anhydrous ethanol ultrasonically and then dried. The weight of samples was measured. Subsequently, one surface with a dimension of 6 mm × 4 mm was coated by the salt of 75 wt% Na2SO4 + 25 wt% K2SO4 with 2 mg·cm-2. After that, the samples were kept in an alumina crucible and placed in a tubular resistance furnace to carry out the hot corrosion at 1150 ˚C. The hot corrosion time was varied as follows: 5, 8, 10, 20, 30, 40, 50, 60, 70 and 80 h. The corroded 3
samples were cooled and boiled in de-ionized water for 20 min to eliminate the remains, dried and then the weight was measured. Finally, the weight gain of the corroded samples was calculated. The cross-section and surface images of the coating were observed using CMY210 optical microscope (OM, China) and Mp-94260CS scanning electron microscope (SEM, Japan). The elemental compositions of the coating were analysed by energy dispersive spectrometer (EDS) equipped on SEM. The phase compositions of the coating were characterized by D8 ADVANCE
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X-ray diffraction (XRD, Germany). Before observing the cross-sectional morphologies, the surface of sample was coated by nickel-plating, meanwhile, the samples should be etched.
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3 Results 3.1 The morphology of initial coating
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Figure 1 shows the results of initial coating, in which (a) is the cross-sectional image; (b) is the EDS energy spectrum; (c) is the XRD pattern. Fig. 1a depicts that the thickness of coating is about
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300 μm. According to its morphology, the entire coating is mainly composed of inner and outer
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layers. From Fig.1b and Fig.1c, the outer layer mainly contains Ni-Al and Cr(Ni)-Si compounds,
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whereas the inner layer contains Ni-Al and Cr-Si compounds.
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3.2 Hot corrosion kinetics
Figure 2 displays the changes in the weight of Al-Si coating from 0 to 80 h of hot corrosion.
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According to weight-gain/loss characteristics, the weight-gain rates are calculated by Eq. 1, and the results are marked at the corresponding position in Fig. 2. Meanwhile, for comparison, the changes
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in the weight at 1050 ˚C [22] are plotted in Fig. 2. The changes in the weight can be roughly composed of four parts, i.e. a slow weight-gain from
0 to 8 h, a rapid weight-gain from 8 to 20 h, a relatively fast weight-gain from 20 to 70 h, and a
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rapid weight-loss from 70 to 80 h. During hot corrosion from 0 to 8 h, the Al-Si coating shows a slow weight-gain at both temperatures, and its weight-gain rates are only 0.037 mg/(cm2·h). Except for this stage, the changes in the weight at 1150 ˚C are almost completely different from that at 1050 ˚C. During hot corrosion from 8 to 20 h, the rate of weight-gain at 1150 ˚C is 0.313 mg/(cm2·h), while at 1050 ˚C it is still relatively low. During hot corrosion from 20 to 70 h, the weight-gain is slightly slower as compared to the previous stage, but it still maintains a high rate of 4
weight-gain (0.089 mg/(cm2·h)). After 70 h, the weight-gain reaches the maximum, i.e. 8.478 mg/cm2, and then drastically reduces. At 75 h, the weight of coating is less than that of initial coating. However, the weight-gain at 1050 ˚C is much lower reaching a maximum of 2.462 mg/cm2. Moreover, the process of weight-loss at 1050 ˚C is extremely slow, and after 100 h the weight of coating is still higher as compared to substrate coating. Therefore, the hot corrosion of Al-Si coating at 1150 ˚C is much more severe as compared to at 1050 ˚C.
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3.3 The morphology of the coatings
Figure 3 shows the results of hot corrosion for 8 h, in which (a) is the cross-sectional image; (b)
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is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d) is the XRD spectrum. As can be seen from Fig. 3a that the thickness of corrosion layer on the surface of coating is about 5 μm which is distributed with many particles. The results of EDS analyses show
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that the composition of these particles is consistent with the composition of the coating, which
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mainly contains Ni and Al. Moreover, the thickness of outer layer of Ni-Al under the corrosion
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layer is about 20 μm, and outer layer of Ni-Al has a little Cr-Si compounds, while the inner layer of Ni-Al has a large amount of Cr-Si compounds. From Fig. 3b, the surface of coating contains a large
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number of island-like bumps, whereas the area around island-like bumps is relatively flat consisting
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mainly of Al, O and Ni (the weight faction of Ni is 43.58%). Combined with the cross-sectional morphology, the island-like bumps are the mentioned particles. High-magnification morphology of the island-like bumps (Fig. 3c), shows that their surfaces are covered with needle-like, slice-like and
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a small amount of granular particles mainly composed of Al and O (> 90% based on EDS analysis). The needle-like and slice-like particles correspond to the metastable θ-Al2O3 while the granular
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particles are the stable α-Al2O3. Meanwhile, it could be revealed that the surface of coating mainly contains NiAl and Al2O3 as illustrated in Fig. 3d. The intensity of Al2O3 diffraction peak is relatively low, further confirming that the surface of coating does not generate a continuous Al2O3
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scale. The obtained analysis results indicate that the molten salt has a strong dissolution effect on the Al2O3 scale at over 1000 ˚C [20, 21]. Therefore, during hot corrosion from 0 to 8 h, the oxide scale is continuously formed and dissolved and due to which a slow increase in the weight has been observed.
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Figure 4 shows the results of hot corrosion for 20 h, in which (a) is the cross-sectional image; (b) is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d) is the XRD spectrum. It is evident from Fig. 4a that the surface of coating forms a corrosion layer and the thickness is about 10 μm. Meanwhile, the outer layer of Ni-Al is significantly enlarged from 20 to 60 μm, whereas the amount of Cr-Si compounds on the inner layer of Ni-Al is reduced. It can be seen that Cr and Si have spread out in the coating during this stage. Combined with Fig. 4d, it
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can be noted that with an increase in the time of hot corrosion, a portion of Cr in the coating is diffused outwardly. This phenomenon is different from the inward diffusion of Cr and Si in the Al-Si coating towards the formation of an inner diffusion barrier during hot oxidation [24] or
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conventional hot corrosion [25]. It could be considered that Cr and Ni are also involved in the reaction of hot corrosion, resulting in a rapid weight-gain of Al-Si coating. As shown in Fig. 4b, the surface of coating began to exhibit exfoliation features in some areas. High-magnification image
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(Fig. 4c) reveals that the corrosion products are mainly globular particles, consisting of a stable
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α-Al2O3 according to EDS analysis. These exfoliation features explain the appearance of a decrease
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of weight-gain rate from 0.313 to 0.089 mg/(cm2·h). Based on the above results, it can be confirmed
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that although the surface of coating is covered with Al2O3 scale, it has a poor resistance to the molten salt.
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Figure 5 shows the results of hot corrosion for 70 h, in which (a) is the cross-sectional image; (b) is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d)
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is the XRD spectrum. Fig. 5a reveals that the thickness of corrosion layer increased to about 200 μm. The black corrosion area mainly consists of Ni, Cr, Al, Ti, O and S. The gray area under the
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corrosion layer is dominated by Ni with a small amount of Al, while the white area near the substrate is a Cr-rich area. Meanwhile, a large number of voids are found out below the white Cr-rich area and EDS analysis results show that it contains a large number of O and S, and the Cr-Si
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compounds in the coating almost completely disappeared. These morphological features, in conjunction to EDS analysis results and XRD patterns (Fig. 5d) indicate that almost all the elements in the coating are involved in the hot corrosion reaction, and the internal oxidation and internal sulphidation penetrate into the substrate, leading to the complete failure of the coating. Moreover, it could be noted from Fig. 5b that the surface of coating exhibits some features such as the exfoliation of corrosion products, which explains the rate of weight-gain is not high at this stage 6
(Fig. 2), despite the corrosion of coating is already severe. With a further observation of high-magnification image (Fig. 5c), the appearance of narrow flakes and a small amount of fine particles is evident and EDS analysis shows that the main elements of narrow flakes are Ni and O. Combined with Fig. 5d, it could be observed that the coating is mainly composed of corrosion products containing substrate elements of Ni, Cr, Ti with O and S. 4 Discussion
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According to the results as shown in Fig. 2, the changes of weight at 1150 ˚C can roughly be
relatively fast weight-gain stage and (d) rapid weight-loss stage.
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divided into four stages (Fig. 6): (a) slow weight-gain stage, (b) rapid weight-gain stage, (c)
The melting point of 75 wt% Na2SO4 + 25 wt% K2SO4 is approximately 823 ˚C [26], the boiling point of Na2SO4 and K2SO4 is 1404 ˚C and 1689 ˚C, respectively. However, the temperature
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of hot corrosion is 1150 ˚C and therefore the mixed sulphate is not evaporated. During hot corrosion,
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the molten salt could be decomposed as shown in Eq. 2 [27]
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Al-Si coating mainly contains Ni-Al and Cr-Si compounds. During hot corrosion from 0 to 8 h,
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Al-Si coating contains a large amount of antioxidant element Al which spontaneously undergoes selective oxidation as shown in Eq. 3 [28]. Meanwhile, as O2 is consumed, the concentration of O2-
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increases, resulting in the dissolution of Al2O3 scale according to Eq. 4 [29]. Moreover, the free energy of the reaction represented by Eq. 4 at 1150 ˚C is -56.132 KJ/mol, which is much lower than
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that at 1050 ˚C (-21.942 KJ/mol) and at 850 ˚C (-20.462 KJ/mol). It is considered that Al2O3 scale is quickly dissolved once it is formed and as a result, the surface of coating does not form a dense
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Al2O3 scale in the first 8 h of hot corrosion. Inevitably, molten salt is heterogeneous during the experimental process, and thus the
dissolution effect of molten salt is irregular. Therefore, the primary Al2O3 scale on the surface of
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coating is not completely dissolved and remains on the surface of coating as island-like bumps (Fig. 2b). To a certain degree, these island-like bumps reduce the corrosion effect of the molten salt on the coating, and Al-Si coating is completely covered by Al2O3 scale at some point from 8 to 20 h. However, due to the strong dissolution effect of molten salt at 1150 ˚C, a large amount of Al will be consumed in order to generate a protective alumina scale. Figure 7 depicts the distribution of Al element along the cross-section of coating after hot corrosion for 0 and 20 h. It is obvious that the 7
Al content is much lower than that before hot corrosion, being lower than a half near the surface of coating. Combined with Fig. 4d, it could be noted that the Al content is not enough to form a protective Al2O3 scale, and thus the Ni element in the outer Ni-Al phase of Al-Si coating is involved in the hot corrosion reaction, as shown in Eq. 5 [30]. Meanwhile, the CrS phase and spinel products were appeared, indicating that Cr and Si in Cr-Si compounds of the inner layer have diffused outwards. This phenomenon is completely different from the inward diffusion of Cr and Si towards
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the formation of an inner diffusion barrier during the conventional hot corrosion process [25]. Figure 8 presents the distribution of Cr element along the cross-section of Al-Si coating after
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hot corrosion for 0 and 20 h. After 20 h, the inner layer has almost no Cr, while the Cr content of outer layer is high. Fig. 9 depicts the corresponding distribution of Si along the cross-section of Al-Si coating after hot corrosion for 0 and 20 h. After 20 h, the inner layer has almost no Si and the
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Si content of outer layer is also low.
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The results as shown in Fig. 8 and 9 not only confirm that Cr and Si in the Cr-Si compounds
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diffused outward in the first 20 h of corrosion, but also indicate that a large amount of Si is consumed. This can be explained taking into account that Si is easier to be oxidized than Cr [31],
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and thus Si at first reacts with O2 to form SiO2, as shown in Eq. 6. A part of Cr also reacts with O2
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to form Cr2O3 according to Eq. 7 and the generated Cr2O3 further reacts with NiO to form the spinel (Eq. 8), and the remaining Cr reacts with Ni3S2 towards the formation of CrS (Eq. 9). Hence, due to the looseness of SiO2 and the formation of spinel, the damage of protective Al2O3 scale is
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accelerated. Although the surface of coating is covered with Al2O3 scale after 20 h of hot corrosion,
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the oxide scale shows exfoliation features (Fig. 4b). Therefore, at this time the Al2O3 scale has no protective effect.
Subsequently, owing to that there is no dense protective oxide scale, and Ni, Cr, Al and even
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Ti from the substrate react with O and S to form the mixed corrosion products (Fig. 5). Nevertheless, these corrosion products are very easy to exfoliate, so the rate of weight-gain from 20 to 70 h is not faster than that from 0 to 20 h. When the corrosion layer reaches a certain thickness, a complete exfoliation takes place resulting in a rapid weight-loss (Fig. 2). 5 Conclusions
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(1) The process of hot corrosion at 1150 ˚C mainly consists of four stages based on the weight-gain/loss characteristics: slow weight-gain, rapid weight-gain, relatively fast weight-gain and rapid weight-loss. Except for the first 8 h of hot corrosion, the changes in weight observed at 1150 ˚C are almost completely different from that at 1050 ˚C. The hot corrosion at 1150 ˚C is more severe than that at 1050 ˚C. (2) After 20 h of hot corrosion, the Al-Si coating not only fails to generate a dense protective
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oxide scale, but also consumes a large amount of antioxidant element Al. At this time, Cr and Ni in the coating begin to participate in the hot corrosion reaction. Based on this result, the Al-Si coating
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has failed after 20 h of hot corrosion.
(3) The free energy of molten salt to dissolve Al2O3 scale at 1150 ˚C is -56.132 kJ/mol, which is much lower than that at 1050 ˚C (-21.942 kJ/mol) and at 850 ˚C (-20.462 kJ/mol). This result
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indicates that the molten salt has a strong dissolution on the Al2O3 scale at 1150 ˚C.
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(4) During the hot corrosion, due to the large consumption of anti-oxidative element Al in the
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early stage of hot corrosion, the Cr and Si in the Cr-Si compounds diffuse outwards and participate
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in the reaction of hot corrosion. This phenomenon destroys the mechanism by which Cr and Si are
Acknowledgements
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diffused inwards to form an inner diffusion barrier in the process of conventional hot corrosion.
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This work was supported by Natural Science Foundation of China under grant numbers 51405204; and Colleges and Universities Graduate Innovation Practice Program of Jiangsu
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Province under grant numbers SJLX16_0439. References
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Appendices
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Table 1 Nominal chemical composition of GH4033 alloy.
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Element Cr Al Ti Fe B Si Ni
weight fraction (wt.%) 19-22 0.60-1.00 2.40-2.80 4.00 (max) 0.01 (max) 0.35 (max) Balance
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Figure 1 Cross-sectional OM image (a), EDS line scan (b) and XRD energy spectrum (c) of the Al-Si coating.
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Figure 2 The weight changes of the coating in the mixed sulphate at 1150 ˚C and 1050 ˚C.
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Figure 3 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
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A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 8 h.
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Figure 4 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
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A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 20 h.
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Figure 5 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
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A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 70 h.
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Figure 6 The schematic image of hot corrosion process at 1150 ˚C.
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Figure 7 The EDS line scan of Al element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
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Figure 8 The EDS line scan of Cr element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
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Figure 9 The EDS line scan of Si element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
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Eq. 1 k=
ΔMass gain ΔTime
Eq. 2 2SO 2-=2SO +O +2O 24 2 2 Eq. 3 4 Al+3O =2 Al O 2 2 3
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Eq. 4 Al O +O 2-=2 AlO 2 3 2 Eq. 5 2SO 2-+9 Ni=6 NiO+Ni S +2O 24 3 2
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Eq. 6 Si+O =SiO 2 2
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Eq. 7 4Cr+3O =2Cr O 2 2 3
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Eq. 8 Cr O +NiO=NiCr O 2 3 2 4
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Eq. 9 2Cr+Ni S =2CrS 3Ni 3 2
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S0010-938X(18)30883-7 https://doi.org/10.1016/j.corsci.2018.11.026 CS 7782
To appear in: Received date: Revised date: Accepted date:
16 May 2018 21 November 2018 22 November 2018
Please cite this article as: Wang H, Zhang X, Xu Z, Wang H, Zhu C, Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 , Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.11.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 ˚C Hongyu Wang*, Xiang Zhang, Zeng Xu, Han Wang and Changshun Zhu School of Mechanical Engineering Jiangsu University, Zhenjiang City, Jiangsu Province, China, 212013
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E-mail address: [email protected] (Hongyu Wang)
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Graphical abstract
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Highlights
The dissolution capacity of molten salt at 1150 ˚C is much stronger.
The Al-Si coating has failed after hot corrosion for 20 h in mixed sulphate at 1150 ˚C.
The coating is not able to form a protective oxide scale in the process of hot corrosion.
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The large consumption of Al causes the outward diffusion of Cr and Si to react with molten
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salt.
the original anti-oxidation and corrosion mechanism of diffusion barrier is destroyed.
Abstract: Hot corrosion test of Al-Si coating was investigated at 1150 ˚C based on the results reported from previous work which was conducted at 1050 ˚C. It has been observed that the coating 1
failed after 20 h of hot corrosion. When the temperature of hot corrosion reached 1150 ˚C, the dissolution effect of the molten salt to Al2O3 scale was sharply enhanced. Therefore, during hot corrosion, Al-Si coating was not able to generate a protective oxide scale, and the original anti-oxidation and corrosion mechanism of diffusion barrier of Al-Si coating cannot be retained. Keywords: Metal coatings, molten salt, hot corrosion.
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1 Introduction Metallic bond coatings are widely used in thermal barrier coatings (TBCs) on the surface of turbine engine blade [1]. With an increase in the working temperature of gas turbine, the working
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temperature of metallic bond coating is more than 1050 ˚C and even reaches a temperature of 1200 ˚C [2-4]. Hot oxidation corrosion resistance are important evaluation index of metallic bond coating [5]. In the complex working conditions, the effect of hot corrosion is more severe than that of hot
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oxidation, with negative consequences on the safe and reliable operation of the thermal barrier
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coating (TBC) and aero engines.
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Many researchers have carried out extensive studies on the hot corrosion of metallic bond
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coating, but the temperature of hot corrosion is mainly concentrated in the range from 800 to 950 ˚C [6-12]. For example, Wang et al. [6] investigated the hot corrosion behaviour of NiCrAlY coating in
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mixed sulphate at 850 ˚C. The results showed that the coating started to spalling off after 20 h of hot corrosion, and after 80 h severe internal sulphidation occurred, which was accompanied by a loss in
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the protectiveness of coating. Therefore, the hot corrosion of metallic bond coating becomes very severe at high temperatures (800 to 950 ˚C). Subsequently, an improvement in the high temperature
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corrosion resistance has become a hot topic of research interest, and a further exploration at temperatures exceeding 1000 ˚C has never been carried out [13-19]. Recently, Wang et al. examined the hot corrosion behaviour and mechanism of NiCoCrAlY coating [20], aluminum rare
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earth coating [21] and Al-Si coating [22] at 1050 ˚C. The obtained results showed that the hot corrosion is more severe than the corrosion occurring at a conventional high temperature (800 to 950 ˚C). However, in order to develop a new type of metallic bond coating that can meet its working requirements, it is obviously necessary to understand the mechanism of hot corrosion of metallic bond coating occurring at the critical working temperature.
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Al-Si coating as a member of metallic bond coating, not only easily forms a stable protective Al2O3 scale at high temperature, but also can show the unique characteristics of in situ formation of internal diffusion barriers that can restrain the failure of coating during hot oxidation corrosion. In this investigation, based on our previous study [22], the hot corrosion behaviour of Al-Si coating in mixed sulphate at 1150 ˚C was further studied. The cross-sectional and surface microscopic images of the coating were observed and the phase composition of the eroded surfaces was characterized.
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Also, the elemental distribution of the coating was analysed along with studying the kinetics of hot corrosion. Moreover, the mechanism of hot corrosion under the subjected conditions was
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systematically discussed. 2 Experimental process
The matrix used in this study was Nickel-based superalloy GH4033, and its chemical
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composition is illustrated in Table 1 [20]. For this study, the matrix alloy was made into specimens
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with the dimensions of 6 mm × 4 mm × 4 mm using a wire-electrode cutting, and the surfaces and
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sides of the specimen were ground by 400-mesh abrasive paper, and then were cleaned in acetone
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ultrasonically.
The preparation of Al-Si coating on the surface of sample was carried out by following the
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powder pack cementation method [23]. The pack mixture was 20 wt% Al, 10 wt% Si, 68 wt% Al2O3, and 2 wt% NH4Cl [24]. During coating, the samples were placed in an infiltration container
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made of 45 steel, and the powder mixture was filled around it. The infiltration container were capped sealed and then placed in a tubular resistance furnace. The parameters of temperature were
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as follows: heating for 4 h at 850 ˚C and then continuing the heating for 8 h at 1050 ˚C. Following this, the containers were cooled in the furnace. The hot corrosion tests were performed in a tubular resistance furnace [21]. Before this, the
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surfaces and sides of the coated specimens were polished with 400-mesh sandpaper, cleaned in an anhydrous ethanol ultrasonically and then dried. The weight of samples was measured. Subsequently, one surface with a dimension of 6 mm × 4 mm was coated by the salt of 75 wt% Na2SO4 + 25 wt% K2SO4 with 2 mg·cm-2. After that, the samples were kept in an alumina crucible and placed in a tubular resistance furnace to carry out the hot corrosion at 1150 ˚C. The hot corrosion time was varied as follows: 5, 8, 10, 20, 30, 40, 50, 60, 70 and 80 h. The corroded 3
samples were cooled and boiled in de-ionized water for 20 min to eliminate the remains, dried and then the weight was measured. Finally, the weight gain of the corroded samples was calculated. The cross-section and surface images of the coating were observed using CMY210 optical microscope (OM, China) and Mp-94260CS scanning electron microscope (SEM, Japan). The elemental compositions of the coating were analysed by energy dispersive spectrometer (EDS) equipped on SEM. The phase compositions of the coating were characterized by D8 ADVANCE
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X-ray diffraction (XRD, Germany). Before observing the cross-sectional morphologies, the surface of sample was coated by nickel-plating, meanwhile, the samples should be etched.
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3 Results 3.1 The morphology of initial coating
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Figure 1 shows the results of initial coating, in which (a) is the cross-sectional image; (b) is the EDS energy spectrum; (c) is the XRD pattern. Fig. 1a depicts that the thickness of coating is about
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300 μm. According to its morphology, the entire coating is mainly composed of inner and outer
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layers. From Fig.1b and Fig.1c, the outer layer mainly contains Ni-Al and Cr(Ni)-Si compounds,
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whereas the inner layer contains Ni-Al and Cr-Si compounds.
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3.2 Hot corrosion kinetics
Figure 2 displays the changes in the weight of Al-Si coating from 0 to 80 h of hot corrosion.
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According to weight-gain/loss characteristics, the weight-gain rates are calculated by Eq. 1, and the results are marked at the corresponding position in Fig. 2. Meanwhile, for comparison, the changes
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in the weight at 1050 ˚C [22] are plotted in Fig. 2. The changes in the weight can be roughly composed of four parts, i.e. a slow weight-gain from
0 to 8 h, a rapid weight-gain from 8 to 20 h, a relatively fast weight-gain from 20 to 70 h, and a
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rapid weight-loss from 70 to 80 h. During hot corrosion from 0 to 8 h, the Al-Si coating shows a slow weight-gain at both temperatures, and its weight-gain rates are only 0.037 mg/(cm2·h). Except for this stage, the changes in the weight at 1150 ˚C are almost completely different from that at 1050 ˚C. During hot corrosion from 8 to 20 h, the rate of weight-gain at 1150 ˚C is 0.313 mg/(cm2·h), while at 1050 ˚C it is still relatively low. During hot corrosion from 20 to 70 h, the weight-gain is slightly slower as compared to the previous stage, but it still maintains a high rate of 4
weight-gain (0.089 mg/(cm2·h)). After 70 h, the weight-gain reaches the maximum, i.e. 8.478 mg/cm2, and then drastically reduces. At 75 h, the weight of coating is less than that of initial coating. However, the weight-gain at 1050 ˚C is much lower reaching a maximum of 2.462 mg/cm2. Moreover, the process of weight-loss at 1050 ˚C is extremely slow, and after 100 h the weight of coating is still higher as compared to substrate coating. Therefore, the hot corrosion of Al-Si coating at 1150 ˚C is much more severe as compared to at 1050 ˚C.
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3.3 The morphology of the coatings
Figure 3 shows the results of hot corrosion for 8 h, in which (a) is the cross-sectional image; (b)
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is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d) is the XRD spectrum. As can be seen from Fig. 3a that the thickness of corrosion layer on the surface of coating is about 5 μm which is distributed with many particles. The results of EDS analyses show
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that the composition of these particles is consistent with the composition of the coating, which
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mainly contains Ni and Al. Moreover, the thickness of outer layer of Ni-Al under the corrosion
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layer is about 20 μm, and outer layer of Ni-Al has a little Cr-Si compounds, while the inner layer of Ni-Al has a large amount of Cr-Si compounds. From Fig. 3b, the surface of coating contains a large
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number of island-like bumps, whereas the area around island-like bumps is relatively flat consisting
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mainly of Al, O and Ni (the weight faction of Ni is 43.58%). Combined with the cross-sectional morphology, the island-like bumps are the mentioned particles. High-magnification morphology of the island-like bumps (Fig. 3c), shows that their surfaces are covered with needle-like, slice-like and
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a small amount of granular particles mainly composed of Al and O (> 90% based on EDS analysis). The needle-like and slice-like particles correspond to the metastable θ-Al2O3 while the granular
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particles are the stable α-Al2O3. Meanwhile, it could be revealed that the surface of coating mainly contains NiAl and Al2O3 as illustrated in Fig. 3d. The intensity of Al2O3 diffraction peak is relatively low, further confirming that the surface of coating does not generate a continuous Al2O3
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scale. The obtained analysis results indicate that the molten salt has a strong dissolution effect on the Al2O3 scale at over 1000 ˚C [20, 21]. Therefore, during hot corrosion from 0 to 8 h, the oxide scale is continuously formed and dissolved and due to which a slow increase in the weight has been observed.
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Figure 4 shows the results of hot corrosion for 20 h, in which (a) is the cross-sectional image; (b) is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d) is the XRD spectrum. It is evident from Fig. 4a that the surface of coating forms a corrosion layer and the thickness is about 10 μm. Meanwhile, the outer layer of Ni-Al is significantly enlarged from 20 to 60 μm, whereas the amount of Cr-Si compounds on the inner layer of Ni-Al is reduced. It can be seen that Cr and Si have spread out in the coating during this stage. Combined with Fig. 4d, it
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can be noted that with an increase in the time of hot corrosion, a portion of Cr in the coating is diffused outwardly. This phenomenon is different from the inward diffusion of Cr and Si in the Al-Si coating towards the formation of an inner diffusion barrier during hot oxidation [24] or
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conventional hot corrosion [25]. It could be considered that Cr and Ni are also involved in the reaction of hot corrosion, resulting in a rapid weight-gain of Al-Si coating. As shown in Fig. 4b, the surface of coating began to exhibit exfoliation features in some areas. High-magnification image
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(Fig. 4c) reveals that the corrosion products are mainly globular particles, consisting of a stable
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α-Al2O3 according to EDS analysis. These exfoliation features explain the appearance of a decrease
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of weight-gain rate from 0.313 to 0.089 mg/(cm2·h). Based on the above results, it can be confirmed
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that although the surface of coating is covered with Al2O3 scale, it has a poor resistance to the molten salt.
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Figure 5 shows the results of hot corrosion for 70 h, in which (a) is the cross-sectional image; (b) is the low-magnification image of the surface; (c) is the high-magnification image of A-zone; (d)
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is the XRD spectrum. Fig. 5a reveals that the thickness of corrosion layer increased to about 200 μm. The black corrosion area mainly consists of Ni, Cr, Al, Ti, O and S. The gray area under the
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corrosion layer is dominated by Ni with a small amount of Al, while the white area near the substrate is a Cr-rich area. Meanwhile, a large number of voids are found out below the white Cr-rich area and EDS analysis results show that it contains a large number of O and S, and the Cr-Si
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compounds in the coating almost completely disappeared. These morphological features, in conjunction to EDS analysis results and XRD patterns (Fig. 5d) indicate that almost all the elements in the coating are involved in the hot corrosion reaction, and the internal oxidation and internal sulphidation penetrate into the substrate, leading to the complete failure of the coating. Moreover, it could be noted from Fig. 5b that the surface of coating exhibits some features such as the exfoliation of corrosion products, which explains the rate of weight-gain is not high at this stage 6
(Fig. 2), despite the corrosion of coating is already severe. With a further observation of high-magnification image (Fig. 5c), the appearance of narrow flakes and a small amount of fine particles is evident and EDS analysis shows that the main elements of narrow flakes are Ni and O. Combined with Fig. 5d, it could be observed that the coating is mainly composed of corrosion products containing substrate elements of Ni, Cr, Ti with O and S. 4 Discussion
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According to the results as shown in Fig. 2, the changes of weight at 1150 ˚C can roughly be
relatively fast weight-gain stage and (d) rapid weight-loss stage.
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divided into four stages (Fig. 6): (a) slow weight-gain stage, (b) rapid weight-gain stage, (c)
The melting point of 75 wt% Na2SO4 + 25 wt% K2SO4 is approximately 823 ˚C [26], the boiling point of Na2SO4 and K2SO4 is 1404 ˚C and 1689 ˚C, respectively. However, the temperature
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of hot corrosion is 1150 ˚C and therefore the mixed sulphate is not evaporated. During hot corrosion,
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the molten salt could be decomposed as shown in Eq. 2 [27]
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Al-Si coating mainly contains Ni-Al and Cr-Si compounds. During hot corrosion from 0 to 8 h,
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Al-Si coating contains a large amount of antioxidant element Al which spontaneously undergoes selective oxidation as shown in Eq. 3 [28]. Meanwhile, as O2 is consumed, the concentration of O2-
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increases, resulting in the dissolution of Al2O3 scale according to Eq. 4 [29]. Moreover, the free energy of the reaction represented by Eq. 4 at 1150 ˚C is -56.132 KJ/mol, which is much lower than
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that at 1050 ˚C (-21.942 KJ/mol) and at 850 ˚C (-20.462 KJ/mol). It is considered that Al2O3 scale is quickly dissolved once it is formed and as a result, the surface of coating does not form a dense
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Al2O3 scale in the first 8 h of hot corrosion. Inevitably, molten salt is heterogeneous during the experimental process, and thus the
dissolution effect of molten salt is irregular. Therefore, the primary Al2O3 scale on the surface of
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coating is not completely dissolved and remains on the surface of coating as island-like bumps (Fig. 2b). To a certain degree, these island-like bumps reduce the corrosion effect of the molten salt on the coating, and Al-Si coating is completely covered by Al2O3 scale at some point from 8 to 20 h. However, due to the strong dissolution effect of molten salt at 1150 ˚C, a large amount of Al will be consumed in order to generate a protective alumina scale. Figure 7 depicts the distribution of Al element along the cross-section of coating after hot corrosion for 0 and 20 h. It is obvious that the 7
Al content is much lower than that before hot corrosion, being lower than a half near the surface of coating. Combined with Fig. 4d, it could be noted that the Al content is not enough to form a protective Al2O3 scale, and thus the Ni element in the outer Ni-Al phase of Al-Si coating is involved in the hot corrosion reaction, as shown in Eq. 5 [30]. Meanwhile, the CrS phase and spinel products were appeared, indicating that Cr and Si in Cr-Si compounds of the inner layer have diffused outwards. This phenomenon is completely different from the inward diffusion of Cr and Si towards
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the formation of an inner diffusion barrier during the conventional hot corrosion process [25]. Figure 8 presents the distribution of Cr element along the cross-section of Al-Si coating after
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hot corrosion for 0 and 20 h. After 20 h, the inner layer has almost no Cr, while the Cr content of outer layer is high. Fig. 9 depicts the corresponding distribution of Si along the cross-section of Al-Si coating after hot corrosion for 0 and 20 h. After 20 h, the inner layer has almost no Si and the
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Si content of outer layer is also low.
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The results as shown in Fig. 8 and 9 not only confirm that Cr and Si in the Cr-Si compounds
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diffused outward in the first 20 h of corrosion, but also indicate that a large amount of Si is consumed. This can be explained taking into account that Si is easier to be oxidized than Cr [31],
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and thus Si at first reacts with O2 to form SiO2, as shown in Eq. 6. A part of Cr also reacts with O2
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to form Cr2O3 according to Eq. 7 and the generated Cr2O3 further reacts with NiO to form the spinel (Eq. 8), and the remaining Cr reacts with Ni3S2 towards the formation of CrS (Eq. 9). Hence, due to the looseness of SiO2 and the formation of spinel, the damage of protective Al2O3 scale is
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accelerated. Although the surface of coating is covered with Al2O3 scale after 20 h of hot corrosion,
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the oxide scale shows exfoliation features (Fig. 4b). Therefore, at this time the Al2O3 scale has no protective effect.
Subsequently, owing to that there is no dense protective oxide scale, and Ni, Cr, Al and even
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Ti from the substrate react with O and S to form the mixed corrosion products (Fig. 5). Nevertheless, these corrosion products are very easy to exfoliate, so the rate of weight-gain from 20 to 70 h is not faster than that from 0 to 20 h. When the corrosion layer reaches a certain thickness, a complete exfoliation takes place resulting in a rapid weight-loss (Fig. 2). 5 Conclusions
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(1) The process of hot corrosion at 1150 ˚C mainly consists of four stages based on the weight-gain/loss characteristics: slow weight-gain, rapid weight-gain, relatively fast weight-gain and rapid weight-loss. Except for the first 8 h of hot corrosion, the changes in weight observed at 1150 ˚C are almost completely different from that at 1050 ˚C. The hot corrosion at 1150 ˚C is more severe than that at 1050 ˚C. (2) After 20 h of hot corrosion, the Al-Si coating not only fails to generate a dense protective
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oxide scale, but also consumes a large amount of antioxidant element Al. At this time, Cr and Ni in the coating begin to participate in the hot corrosion reaction. Based on this result, the Al-Si coating
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has failed after 20 h of hot corrosion.
(3) The free energy of molten salt to dissolve Al2O3 scale at 1150 ˚C is -56.132 kJ/mol, which is much lower than that at 1050 ˚C (-21.942 kJ/mol) and at 850 ˚C (-20.462 kJ/mol). This result
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indicates that the molten salt has a strong dissolution on the Al2O3 scale at 1150 ˚C.
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(4) During the hot corrosion, due to the large consumption of anti-oxidative element Al in the
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early stage of hot corrosion, the Cr and Si in the Cr-Si compounds diffuse outwards and participate
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in the reaction of hot corrosion. This phenomenon destroys the mechanism by which Cr and Si are
Acknowledgements
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diffused inwards to form an inner diffusion barrier in the process of conventional hot corrosion.
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This work was supported by Natural Science Foundation of China under grant numbers 51405204; and Colleges and Universities Graduate Innovation Practice Program of Jiangsu
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Province under grant numbers SJLX16_0439. References
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Appendices
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Table 1 Nominal chemical composition of GH4033 alloy.
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PT
Element Cr Al Ti Fe B Si Ni
weight fraction (wt.%) 19-22 0.60-1.00 2.40-2.80 4.00 (max) 0.01 (max) 0.35 (max) Balance
A
Figure 1 Cross-sectional OM image (a), EDS line scan (b) and XRD energy spectrum (c) of the Al-Si coating.
12
13
A ED
PT
CC E
IP T
SC R
U
N
A
M
SC R
IP T
Figure 2 The weight changes of the coating in the mixed sulphate at 1150 ˚C and 1050 ˚C.
U
Figure 3 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
A
CC E
PT
ED
M
A
N
A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 8 h.
14
Figure 4 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
M
A
N
U
SC R
IP T
A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 20 h.
ED
Figure 5 Cross-sectional image (a), low-magnification image of the surface (b), high-magnification image of
A
CC E
PT
A-zone (c) and XRD spectrum (d) of the coating after hot corrosion for 70 h.
15
16
A ED
PT
CC E
IP T
SC R
U
N
A
M
A
N
U
SC R
IP T
Figure 6 The schematic image of hot corrosion process at 1150 ˚C.
A
CC E
PT
ED
M
Figure 7 The EDS line scan of Al element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
17
SC R
IP T
Figure 8 The EDS line scan of Cr element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
A
CC E
PT
ED
M
A
N
U
Figure 9 The EDS line scan of Si element in the coating after hot corrosion for, (a) 0 h and (b) 20 h.
18
Eq. 1 k=
ΔMass gain ΔTime
Eq. 2 2SO 2-=2SO +O +2O 24 2 2 Eq. 3 4 Al+3O =2 Al O 2 2 3
IP T
Eq. 4 Al O +O 2-=2 AlO 2 3 2 Eq. 5 2SO 2-+9 Ni=6 NiO+Ni S +2O 24 3 2
SC R
Eq. 6 Si+O =SiO 2 2
U
Eq. 7 4Cr+3O =2Cr O 2 2 3
N
Eq. 8 Cr O +NiO=NiCr O 2 3 2 4
A
CC E
PT
ED
M
A
Eq. 9 2Cr+Ni S =2CrS 3Ni 3 2
19