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 Sun et al., 2015). They must be subjected to high temperature oxidation and hot corrosion by O 2 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 Na 2 SO 4 , 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 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 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 preoxidation 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-Al 2 O 3 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 Na 2 SO 4 salt only once at 1050°C. The high temperature oxidation and corrosion behavior of aluminide coating in Na 2 SO 4 salt is investigated by corrosion kinetics, crosssectional microstructure, and composition distribution. Besides, the degradation of aluminide coating under this corrosion condition is discussed.

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 -4NH 4 Cl -xSi -(94-x)Al 2 O 3 , the composition of mixed powder was 76.0 wt.% Al 2 O 3 , 20.4 wt.% AlSi40, and 3.6 wt.% NH 4 Cl. 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.

Hot Corrosion Tests
The hot corrosion test was implemented by salt coating method at 1050°C. The specimens were preheated and coated with saturated Na 2 SO 4 solution. The average amount of salt coating was 0.5-1.0 mg/cm 2 . 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). Figure 1 shows the oxidation kinetics curve of as-received and pre-oxidation aluminide coating in Na 2 SO 4 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 2 shows XRD results of the specimens after hot corrosion in Na 2 SO 4 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 Al 2 O 3 , β-NiAl, γ՛-Ni 3 Al, Cr 3 Si, and Cr 2 O 3 is further confirmed using XRD, and NiAl 2 O 4 phase peaks are observed. After 24 h, the corrosion product generated on two types of coatings chiefly composed of Al 2 O 3 . For two types of coatings, the peaks of γ՛-Ni 3 Al 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՛-Ni 3 Al phase during hot corrosion test. And the oxide scale with poor protection containing Al 2 O 3 and NiAl 2 O 4 spinel phase is formed on two types of coatings. The NiAl 2 O 4 spinel phase can be found on both coatings, but the peak intensity of NiAl 2 O 4 phase on preoxidation aluminide coating is weaker than on as-received aluminide coating. Figure 3 shows the surface topography of as-received and preoxidation aluminide coating on K438 superalloy in Na 2 SO 4 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 Na 2 SO 4 . In Figure 3C, corrosion cavities in the oxide scale can be observed by the magnifying image. After hot corrosion covered by Na 2 SO 4 as shown in Figures 3D,F, there are two morphologies of Al 2 O 3 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 4 shows the cross-sectional microstructure of asreceived and pre-oxidation aluminide coating after hot corrosion in Na 2 SO 4 salt at 1050°C for 120 h. Apparently, the oxide scale of as-received aluminide coating is thicker than preoxidation 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 asreceived 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.  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.

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 The initial reducing substances come from Na 2 SO 4 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.
According to the corrosion kinetics curve, microstructure evolution, and element distribution, the pre-oxidation aluminide coating exhibits excellent hot corrosion resistance in Na 2 SO 4 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 Na 2 SO 4 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-Al 2 O 3 , and the protective α-Al 2 O 3 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 Al 2 O 3 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 Na 2 SO 4 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 Na 2 SO 4 (melting point is 884°C in air) is determined by the oxygen partial pressure and the activity of Na 2 O or SO 3 in the molten salt, according to the reaction 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 Al 2 O 3 scale and molten salt (Liu et al., 2016a;Yuwen and Zhou, 2016). The following reactions occur The interaction between outward diffusion of Ni 2+ and inward diffusion AlO − 2 encountered and reacted with each other to form the NiAl 2 O 4 (reaction Eq. 5). Therefore, XRD analysis shows the existence of NiAl 2 O 4 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 AlO − 2 concentration gradient in the oxide scale. According to the fluxing mechanism, AlO − 2 forms at the metal/scale interface and moves outward in the molten salts, and then converts to porous Al 2 O 3 particles where the oxygen partial pressure is high, as the Eq. 4 in the article. Frontiers in Materials | www.frontiersin.org June 2021 | Volume 8 | Article 679682 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 Na 2 SO 4 salt at 1050°C, the reaction of producing S 2 is as follows (Yuan and Wang, 2010) The released S 2 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 2Cr + S 2 g 2CrS (7) 4 3 Cr The results show that Cr 2 O 3 at the oxide scale is more likely formed by direct oxidation of Cr. It strongly proves the reason for Cr 2 O 3 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 ΔG θ it is related to the initial state and final state of reaction. In the experiment, the ΔG θ T values of various chemical reactions at 1050°C are analyzed by using HSC Chemistry 6.0 software, as shown in Figure 11. The ΔG θ T 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 Na 2 SO 4 , sulfides are found in the alloy and ii) the corrosion degree depends on the oxygen ions' amount formed in salt.

CONCLUSION
The aim of present work is to investigate the high temperature oxidation and corrosion behavior of Ni-based aluminide coating in Na 2 SO 4 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 asreceived 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, writingoriginal 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.