Influence of Pre-Oxidation on High Temperature Oxidation and Corrosion Behavior of Ni-Based Aluminide Coating in Na2SO4 Salt at 1050°C

Aluminide coating is prepared on K438 Ni-based superalloy by pack cementation. The hot corrosion test of as-received and pre-oxidation aluminide coating in Na₂SO₄ salt are carried out at 1050°C. The coating morphologies, phase composition, and corrosion products are characterized by XRD, SEM, EDS, and TEM. The conclusion is that the pre-oxidation aluminide coating has excellent hot corrosion properties. After hot corrosion, the oxide scale of pre-oxidation aluminide coating is the thinnest and maintains good integrity. However, the oxide scale of as-received aluminide coating fluctuates greatly and there are corrosion cavities. CrS is formed in the alloy immediately below the oxide scale for the two coating systems.

Introduction

Ni-based superalloy has excellent creep resistance, high rupture strength, and corrosion resistance, so it is a perfect material for key components of aeroengines (He et al., 2015; Sun et al., 2015). They must be subjected to high temperature oxidation and hot corrosion by O₂ or other corrosive substances, for example, sulfur (Kosieniak et al., 2012; Yang et al., 2020). In heavy air pollution or coastal working environment, the metallic sulfates, such as Na₂SO₄, will be deposited on the surface of key components and accelerate their degradation (Eliaz et al., 2002; Wang et al., 2011). But the high temperature oxidation resistance of Ni-based superalloy is not good due to insufficient supply of Cr and Al at high temperature, which shortens the working life of blades. With the Cr or Al elements’ depletion, the oxidation rate is accelerated and voids begin to form in the oxide scale, which provides a channel for the invasion of external corrosion particles (Liu et al., 2015; Yuan et al., 2015; Liu et al., 2016b). Therefore, it is necessary to apply protective coating on the surface of Ni-based superalloy to improve its high temperature properties.

The aluminide coating is usually used as the protective coating of Ni-based superalloy (Li et al., 2020). Aluminide coating has the following advantages: i) low cost and easy processing; ii) continuous, adhesive, and slow-growing oxide scale can be formed at a high temperature; and iii) excellent oxidation properties (Hou and Priimak, 2005; Hou and McCarty, 2006). Due to the rising engine-inlet temperature, the protective coatings have to be able to serve in a more aggressive environment because the aluminide coating is easy to generate void, crack, and spallation during long-term servicing. The high temperature oxidation and corrosion resistance of aluminide coating can be enhanced by adding many elements (Montero et al., 2013; Zhao and Zhou, 2014; Yang et al., 2016; Huang et al., 2020). The common methods for preparing aluminide coating are thermal spray, pack cementation, chemical vapor deposition, and hot-dipping (Hu et al., 2006; Zhang et al., 2012; Cheng et al., 2013; Shirvani et al., 2013; Xu et al., 2015; Sitek et al., 2016). Comparatively, the pack cementation process is simple and the cost is low, so it is suitable for specimens with higher requirements for bonding strength of the coating (Yuwen and Zhou, 2016; Yu et al., 2019).

Moreover, previous studies have found that the alloy is oxidized to form an excellent and dense oxide scale after pre-oxidation treatment, which hinders the oxidation and corrosion of alloy during the working, and the preparation and implementation are relatively simple (Matsumoto et al., 2006; Bao et al., 2008). The pre-oxidation treatment is beneficial to improve the oxidation and corrosion resistance of materials. Chen et al. (2008) studied the property of TBC pre-oxidation treatment. The results showed that α-Al₂O₃ film could be quickly formed on the bonding coating after TBC pre-oxidation treatment, which improves the high temperature oxidation properties and thermal cycle life of TBCs. Liu et al. (2010) found that the pre-oxidation scale can effectively reduce the corrosion rate of DZ68 alloy. The corrosion rate increases with the destruction of oxide scale, and the corrosion product fall off obviously.

Therefore, the pre-oxidation aluminide coating is prepared combined with the advantages of pre-oxidation. In addition, there are few studies on hot corrosion at the same pre-oxidation and corrosion temperature. In the study, the hot corrosion test is carried out by coating with Na₂SO₄ salt only once at 1050°C. The high temperature oxidation and corrosion behavior of aluminide coating in Na₂SO₄ salt is investigated by corrosion kinetics, cross-sectional microstructure, and composition distribution. Besides, the degradation of aluminide coating under this corrosion condition is discussed.

Experimental

Specimen Preparation

K438 Ni-based superalloy has good high temperature oxidation and corrosion resistance (Yu et al., 2019). The chemical constituents are listed in Table 1. The specimens of 17 mm diameter and 3 mm thickness were cut by wire. The specimens’ surface was polished with 1,000-mesh sandpaper, then ultrasonically washed in anhydrous ethanol for 15 min and dried. By the equation of pack cementation mixed powder, 2Al - 4NH₄Cl - xSi - (94-x)Al₂O₃, the composition of mixed powder was 76.0 wt.% Al₂O₃, 20.4 wt.% AlSi40, and 3.6 wt.% NH₄Cl. The appropriate powder was weighed by balance and mixed evenly with a glass rod. Then the specimens were embedded in mixed powder, and then the crucible is placed in a tube furnace and heat-treated in Ar gas at 1000°C for 10 h until a stable aluminide coating was transformed. Then the specimen surface was polished with 1,000-mesh sandpaper until the surface of specimens showed a metallic luster. Then some specimens were pre-oxidized in a muffle furnace at 1050°C for 1 h. The residual specimens were as-received.

TABLE 1

TABLE 1. Chemical composition (in wt. %)of K438 Ni-based superalloy.

Hot Corrosion Tests

The hot corrosion test was implemented by salt coating method at 1050°C. The specimens were preheated and coated with saturated Na₂SO₄ solution. The average amount of salt coating was 0.5–1.0 mg/cm². The above salt-coated specimens were put into a muffle furnace for hot corrosion test. The specimens were taken out for static cooling for 24 h as a cycle, weighing and observing the macroscopic morphology. Then five cycles were carried out in static air at 1050°C until the test time reached 120 h.

The analytical balance with precision of 0.1 mg was used during the test. The relationship between the mass change of specimens and corrosion time was recorded, and the corrosion kinetics curve was drawn. The surface and cross-sectional morphology of coating was observed and analyzed before and after corrosion by using a scanning electron microscope (SEM, FEI Quanta 6427) with energy dispersive spectrometer (EDS, Oxford INCA X-sight 6427), and the phase of coating was analyzed by using an X-ray diffractometer (XRD, BRUKER D8 Advance) with Cu Kα radiation (λ = 0.15405 nm) in the range 2θ = 20–90° with a step size of 0.02° and a counting time of 1 s per step to investigate the phase composition in the outermost layer of coating. The oxide scale was researched by using a transmission electron microscope (TEM, Tecnai G2 TF30), and the microstructure of oxide scale was determined. TEM specimens were prepared by focused ion beam (FIB).

Results

Figure 1 shows the oxidation kinetics curve of as-received and pre-oxidation aluminide coating in Na₂SO₄ salt at 1050°C. The oxide scale formation leads to the mass change and the mass loss caused by oxide scale exfoliation or dissolution are obtained, and the corresponding conclusions are given in the experiment. During the first 24 h, the mass of as-received aluminide coating increased rapidly due to the growth of oxide scale. After 48 h of hot corrosion, the mass of as-received aluminide coating decreased rapidly, indicating that the oxide scale peeled off or dissolved. Obviously, the pre-oxidation aluminide coating has excellent corrosion properties, and the specimens’ mass increases slowly and steadily after 48 h.

FIGURE 1

FIGURE 1. mass change curves of aluminide coating during hot corrosion test in Na₂SO₄ salt at 1050°C.

Figure 2 shows XRD results of the specimens after hot corrosion in Na₂SO₄ salt at 1050°C for 24 h and 120 h, respectively. The position and intensity of X-ray diffraction pattern can be found obviously. The presence of main phases on the coating surface of Al₂O₃, β-NiAl, γ՛-Ni₃Al, Cr₃Si, and Cr₂O₃ is further confirmed using XRD, and NiAl₂O₄ phase peaks are observed. After 24 h, the corrosion product generated on two types of coatings chiefly composed of Al₂O₃. For two types of coatings, the peaks of γ՛-Ni₃Al phase can be observed after hot corrosion for 120 h, indicating the consumption of aluminum in the coating leads to phase transformation of β-NiAl to γ՛-Ni₃Al phase during hot corrosion test. And the oxide scale with poor protection containing Al₂O₃ and NiAl₂O₄ spinel phase is formed on two types of coatings. The NiAl₂O₄ spinel phase can be found on both coatings, but the peak intensity of NiAl₂O₄ phase on pre-oxidation aluminide coating is weaker than on as-received aluminide coating.

FIGURE 2

FIGURE 2. XRD results of aluminide coating after hot corrosion for different time in Na₂SO₄ salt at 1050°C.

Figure 3 shows the surface topography of as-received and pre-oxidation aluminide coating on K438 superalloy in Na₂SO₄ salt at 1050°C after 120 h. Figures 3C–F are the local area of Figures 3A,B in high magnification, respectively. The surface morphology illustrated that serious oxide spallation is found above the as-received aluminide coating after hot corrosion covered by Na₂SO₄. In Figure 3C, corrosion cavities in the oxide scale can be observed by the magnifying image. After hot corrosion covered by Na₂SO₄ as shown in Figures 3D,F, there are two morphologies of Al₂O₃ on the pre-oxidation aluminide coating surface: nodule-shaped and needle-shaped. It means that there are still metastable oxides. The oxide scale spallation of pre-oxidation aluminide coating is less compared to as-received aluminide coating.

FIGURE 3

FIGURE 3. Surface morphologies of the aluminide coating after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C for (A,C,E) as-received and (B,D,F) pre-oxidation.

Figure 4 shows the cross-sectional microstructure of as-received and pre-oxidation aluminide coating after hot corrosion in Na₂SO₄ salt at 1050°C for 120 h. Apparently, the oxide scale of as-received aluminide coating is thicker than pre-oxidation aluminide coating, which is consistent with the mass change curve shown in Figure 1. Voids and cracks are found in the oxide scale of as-received aluminide coating after hot corrosion as shown in Figure 4C. The oxide scale of as-received aluminide coating has great fluctuation, indicating that the oxide grows inward during dissolution. The EDS results are shown in Table 2; points 1 and 2 in Figure 4A are obtained to be γ՛-Ni3Al and β-NiAl in this order. On the contrary, the oxide scale formed by the pre-oxidation aluminide coating is the thinnest and maintains good integrity.

FIGURE 4

FIGURE 4. Cross-sectional microstructures of the aluminide coating after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C for (A,C) as-received and (B,D) pre-oxidation.

TABLE 2

TABLE 2. The chemical compositions (in at. %) of points in Figure 4 by EDS.

Furthermore, in order to obtain the layer distribution information and mapping distribution image of corrosion specimens accurately, the line scanning and mapping scanning analysis methods of EDS are used. Figure 5 shows that Al and O are mainly distributed on the surface of specimens, while the distribution of Cr is not uniform and there are some isolated regions rich in Cr. Similarly, from the line scan in Figure 6 and Figure 7, there is signal S in the nickel-based alloy, indicating that the substrate has been sulfurized. The signal of S and Cr is consistent, indicating the formation of CrS.

FIGURE 5

FIGURE 5. EDS mapping of the aluminide coating cross section after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C for (A) as-received and (B) pre-oxidation.

FIGURE 6

FIGURE 6. Line scan of the as-received aluminide coating cross section after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C.

FIGURE 7

FIGURE 7. Line scan of the pre-oxidation aluminide coating cross section after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C.

Discussion

During the initial stage of hot corrosion, electrons transfer from metal atoms to the reducing substances in the deposit, and the aluminide coating begins to be oxidized. The equation is as follows

$\frac{4}{3}Al+O_2\left(g\right)\mathrm{ = }\frac{2}{3}A{l}_2{O}_3(1)$

The initial reducing substances come from Na₂SO₄ and oxygen in the gaseous environment in Figure 8. The oxidation rate of aluminum on the coating surface is accelerated at the initial stage due to the increase of oxygen partial pressure. Then oxidation products are formed on the coating surface under the deposited salt, which is similar to the products obtained by the gas-coating reaction when there is no deposited salt on the surface of specimens. However, since sulfur also enters the specimens from the deposited salt, the difference still exists.

FIGURE 8

FIGURE 8. Schematic diagram of aluminide coating oxygen consumption during the initial stage of hot corrosion.

According to the corrosion kinetics curve, microstructure evolution, and element distribution, the pre-oxidation aluminide coating exhibits excellent hot corrosion resistance in Na₂SO₄ salt. The above results show that the oxide scale formed by pre-oxidation treatment can properly protect the specimens eroded by molten salt. The hot corrosion performance of pre-oxidation specimens in Na₂SO₄ salt at 1050°C for 120 h is obviously better than as-received specimens, and there is still a continuous pre-oxidation layer on the surface of specimens after hot corrosion. Meanwhile, even if sulfur exists in the high temperature environment, the oxide scale still has high stability. It may be due to the preformed continuous protective α-Al₂O₃, and the protective α-Al₂O₃ availably reduces the inward diffusion of sulfur. Therefore, the oxide scale slows the sulfuration of specimens and improves the oxidation property of specimens.

In general, there are two ways to be invaded by molten salt at the oxide-coating interface. First, the surface of specimens is completely coated with molten salt before the complete oxide scale is formed because the formation of continuous alumina takes time. The molten salts would spread and the specimens are invaded during the initial stage of hot corrosion. Thus, the oxide scale formed by pre-oxidation treatment can effectively prevent molten salt entering the specimens directly. So, the oxide-coating interface is comparatively smooth. The second method of molten salt invading oxide-coating interface is to generate cracks and voids in the oxide scale. The growth and dissolution of oxides occur at the same time during the hot corrosion process. The mass gain of pre-oxidation specimens in the first 24 h is smaller than as-received specimens, indicating that it takes a long time to form a complete Al₂O₃ scale. As a result, the molten salts remain on the surface of coating. The molten salts would further promote the serious dissolution of oxide scale, resulting in a jagged morphology (see Figures 4A,B).

Due to the reaction between Na₂SO₄ and specimens, it is important that the sediments become alkaline or acidic than the original deposited state, and the thermodynamically stable phase diagram is shown in Figure 9. The change in the composition of these sediments affects the corrosion products’ layer. When the deposited salt passes through the corrosion products’ layer and comes in contact with the specimens, the degradation process would accelerate corrosion. As shown in Figure 9, the composition of Na₂SO₄ (melting point is 884°C in air) is determined by the oxygen partial pressure and the activity of Na₂O or SO₃ in the molten salt, according to the reaction

$\begin{array}{c}N{a}_{2}S{O}_4=S{O}_3\left(g\right)+N{a}_{2}O \end{array}(2)$

FIGURE 9

FIGURE 9. Thermodynamically stable phase diagram of Na-O-S system at constant temperature.

The alkaline melting occurs when the oxygen ion in the molten salt reacts with oxides to form solute. Bornstein and DeCrescente studies showed that hot corrosion can be caused by alkaline melting (Bornstein et al., 1973; Bornstein et al., 1989). During hot corrosion, the protective oxide scale is dissolved because of the reaction between Al₂O₃ scale and molten salt (Liu et al., 2016a; Yuwen and Zhou, 2016). The following reactions occur

$\begin{array}{c}A{l}_2{O}_3+N{a}_{2}O=2NaAl{O}_2 \end{array}(3)$

$\begin{array}{c}2Al{O}_2^{-}=A{l}_2{O}_3+O^{2-} \end{array}(4)$

$\begin{array}{c}N{i}^{2+}+2Al{O}_2^{-}=NiA{l}_2{O}_4 \end{array}(5)$

The interaction between outward diffusion of $\mathrm{N}{\mathrm{i}}^{2+}$ and inward diffusion $\mathrm{Al}{\mathrm{O}}_{\mathrm{2}}^{\mathrm{-}}$ encountered and reacted with each other to form the NiAl₂O₄ (reaction Eq. 5). Therefore, XRD analysis shows the existence of NiAl₂O₄ spinel (Figure 2). Corrosion cavities can be clearly observed by the cross-sectional TEM morphology of oxide scale (Figure 10A). There are three morphologies of corrosion cavities as shown in Figure 10B. The corrosion cavities can be divided into the following three stages: nucleation (occurrence), expansion (development), and deep cavity (result). Therefore, the possible causes of corrosion cavities in the oxide scale can be reasonably inferred. The reaction Eq. 3 forms $\mathrm{Al}{\mathrm{O}}_{\mathrm{2}}^{\mathrm{-}}$ concentration gradient in the oxide scale. According to the fluxing mechanism, $\mathrm{Al}{\mathrm{O}}_{\mathrm{2}}^{\mathrm{-}}$ forms at the metal/scale interface and moves outward in the molten salts, and then converts to porous Al₂O₃ particles where the oxygen partial pressure is high, as the Eq. 4 in the article.

FIGURE 10

FIGURE 10. (A) Cross-sectional TEM morphology of oxide scale formed on the as-received aluminide coating after hot corrosion for 120 h in Na₂SO₄ salt at 1050°C; (B) the schematic diagram of corrosion cavity morphology.

Shores and Mohanty (2004) found that the deposited salt is distributed in the corrosion cavities of oxide scale in most cases, and the surface tension effect plays an important role in the distribution of deposited salt. It is reasonable to assume that the deposited salt on the surface of specimens is a continuous film at the early stage of hot corrosion. However, the salt is more likely to be distributed in the corrosion cavities of unprotected oxides at the hot corrosion expanding stage. The corrosion may be uneven on the whole surface of specimens. As shown in Figure 6 and Figure 7, CrS is formed in the alloy immediately below the oxide scale. Upon coating with Na₂SO₄ salt at 1050°C, the reaction of producing S₂ is as follows (Yuan and Wang, 2010)

$\begin{array}{c}2S{O}_4^{2-}=2O^{2-}+3O_2\left(g\right)+S_2\left(g\right)\end{array}(6)$

The released S₂ reacts with the substrate to form CrS. The affinity of Cr-S is stronger than Ni-S (Li et al., 2006). The reaction is as follows

$\begin{array}{c}2Cr+S_2\left(g\right)=2CrS\end{array}(7)$

$\begin{array}{c}\frac{4}{3}Cr+O_2\left(g\right)=\frac{2}{3}C{r}_2{O}_3\end{array}(8)$

The results show that Cr₂O₃ at the oxide scale is more likely formed by direct oxidation of Cr. It strongly proves the reason for Cr₂O₃ occurrence in the oxide scale, as shown in Figure 2.

There are complex chemical reactions during hot corrosion. The main causes of hot corrosion are explored by calculating the thermodynamic Gibbs free energy of each reaction. The standard Gibbs free energy reaction is $\Delta G_T^{\theta }=\Delta H_{298}^{\theta }+{\displaystyle \underset{298}{\overset{\theta }{\int }}\Delta C_{p}dT-T\Delta S_{298}^{\theta }}-T{\displaystyle \underset{298}{\overset{\theta }{\int }}\Delta C_{p}dlnT}$; it is related to the initial state and final state of reaction. In the experiment, the $\Delta G_T^{\theta }$ values of various chemical reactions at 1050°C are analyzed by using HSC Chemistry 6.0 software, as shown in Figure 11. The $\Delta G_T^{\theta }$ value of chemical reactions is negative, so these reactions occur spontaneously. Figure 12 shows the schematic diagram of two coatings’ hot corrosion process to describe clearly the coating degradation process. The alkaline melting described above has several obvious characteristics, which are as follows: i) due to the release of sulfur from Na₂SO₄, sulfides are found in the alloy and ii) the corrosion degree depends on the oxygen ions’ amount formed in salt.

FIGURE 11

FIGURE 11. Standard Gibbs free energy changes (ΔGθ) of forming compounds as a function of temperature.

FIGURE 12

FIGURE 12. A schematic illustration of corrosion mechanism of aluminide coating for (A) as-received and (B) pre-oxidation.

Conclusion

The aim of present work is to investigate the high temperature oxidation and corrosion behavior of Ni-based aluminide coating in Na₂SO₄ salt. The following conclusions can be drawn from the above study:

(1) The mass change curve of two coating systems indicates that the pre-oxidation aluminide coating has excellent hot corrosion properties. The mass of as-received aluminide coating decreases rapidly, but the pre-oxidation aluminide coating mass increases slowly and steadily after 48 h.

(2) After hot corrosion, the oxide scale of pre-oxidation aluminide coating is the thinnest and maintains good integrity. But the oxide scale of as-received aluminide coating has great fluctuation, indicating that the oxide grows inward during dissolution.

(3) CrS is formed in the alloy immediately below the oxide scale due to the affinity of Cr-S being stronger than Ni-S.

(4) As corrosion cavities are observed in the oxide scale of as-received aluminide coating, the salt is more likely to be distributed in the corrosion cavities of unprotected oxides during hot corrosion.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author Contributions

QL: conceptualization, methodology, investigation, writing-original draft, and data curation. PS: funding acquisition, data curation, validation, writing - review and editing, and supervision. DZ, ZL: writing-review and editing, and supervision. RZ, CH: experiment and data curation. JL: conceptualization and supervision.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 52071168), the Yunnan Province Science Technology Major Project (No. 2019ZE001), the Rare and Precious Metal Materials Genome Engineering Project of Yunnan Province (No. 202002AB080001).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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