Fabrication of Co-Based Cladding Layer by Microbeam Plasma and Its Corrosion Mechanism to Molten Salt

Corrosion of the molten salts Na2SO4 and NaCl has become one of the major factors in the failure of steel components in boilers and engines. In this study, CoNiCrAlY cobalt-based cladding layers with different NiCr-Cr3C2 ratios were prepared by microbeam plasma cladding technology. The influence of the NiCr-Cr3C2 content on the microstructure, mechanical properties, and molten salt corrosion resistance of CoNiCrAlY was investigated. The CoNiCrAlY with a 25 wt.% NiCr-Cr3C2 (NC25) cladding layer possessed the highest microhardness (348.2 HV0.3) and the smallest coefficient of friction (0.4751), exhibiting great overall mechanical properties. The generation of protective oxides Cr2O3, Al2O3, and spinel phase (Ni,Co)Cr2O4 is promoted by the addition of 25 wt.% NiCr-Cr3C2, which significantly reduces the corrosion of the cladding layer, and this effect is much more obvious at 950 °C than that at 750 °C. Furthermore, its corrosion mechanism was clarified. From the findings emerge a viable solution for the design and development of new high-temperature corrosion-resistant coatings.


Introduction
Inconel 625 is a typical nickel-based high-temperature alloy that is widely used in high-temperature environments such as industrial gas turbines.However, with the rapid development of modern industry, components are subjected to increasingly demanding service conditions.At a high temperature of about 750 • C, steel components can wear severely because of softening and oxidation, and are often corroded in reaction with combustion gas and deposited salts, which can lead to material failure and significant economic losses [1][2][3].Among various approaches, the most cost-effective and efficient approach to solve this problem is to coat them with wear-and corrosion-resistant coatings.The coating provides protection against wear and corrosion, extending the service life of the material, and the substrate provides the necessary mechanical strength [4][5][6].
The material and structural design of the cladding layer is important for protecting the substrate material.CoNiCrAlY coatings have recently received considerable attention because of their excellent properties such as oxidation and corrosion resistance, thanks to the reasonable elemental composition [7][8][9].The high oxidation and corrosion resistance of NiCr-Cr 3 C 2 , along with a similar thermal expansion coefficient to the substrate Inconel 625, are extensively used to fabricate wear-and corrosion-resistant coatings on steel components used in the hot sections of various boilers and engines [10][11][12][13].In order to improve the particle erosion resistance of CoNiCrAlY cladding layer, different contents of NiCr-Cr 3 C 2 were added in this study, and the influence law of the additive amount on the wear and molten salt corrosion resistance of the cladding layer was systematically investigated.Critically, the functioning of the metal components is significantly hindered by the attacks of molten salts, such as Na 2 SO 4 and NaCl, caused by gas burning, leading to corrosion and failure of the components [14,15].Xia et al. found that Na 2 SO 4 is a dominant force in corrosion, but the addition of even small amounts of NaCl in Na 2 SO 4 is much more harmful than only either NaCl or Na 2 SO 4 [16].Wang et al. found that chlorine produced by the reaction of NaCl molten salt and O 2 is one of the main thermal corrosion factors for components [17].Thermal corrosion is primarily categorized into low-temperature (650~800 • C) and high-temperature (850~950 • C) thermal corrosion, and the peak corrosion rates of NiCrAl alloys are about 700 • C and 900 • C, respectively [18,19].However, the higher amount of Co contained in the CoNiCrAlY powder used in this paper would theoretically lead to a higher corrosion resistance of the cladding layer, and, therefore, the corrosion experiments were set at the slightly higher temperatures of 750 • C and 950 • C.
The present study uses microbeam plasma cladding technology to prepare cobaltbased cladding layers with good mechanical properties and molten salt corrosion resistance.The optimal composition of the cladding layer was determined, high-temperature molten salt corrosion experiments were conducted, and the corrosion mechanism was clarified.The findings can aid in determining a viable solution for the design and development of high-temperature corrosion-resistant coatings that will extend the service life of equipment and improve production efficiency.

Materials and Methods
The CoNiCrAlY powder used in this experiment has particle sizes of 170~325 mesh, predominantly consisting of γ/γ and β-NiAl phases, with the chemical composition presented in Table 1.The NiCr-Cr 3 C 2 powder has particle sizes of 325~500 mesh, comprising 25 wt.%NiCr and 75 wt.%Cr 3 C 2 phases.Microbeam plasma cladding technology was utilized to prepared the cobalt-based cladding layers on Inconel 625 substrates.The cladding layer without additives is denoted as CoNiCrAlY, while layers with 5, 10, 15, 20, 25, and 30 wt.% of NiCr-Cr 3 C 2 are denoted as NC5, NC10, NC15, NC20, NC25, and NC30, respectively.The powder compositions and proportions are outlined in Table 2.The mechanical alloying process via ball milling was conducted with a rotation rate of 500 RPM, a ball-to-material ratio of 10:1, and alternating positive and negative rotations for two hours.After ball milling, the powder was compacted and flattened by placing it into a mold with PVA, producing a rectangular powder strip measuring 50 mm × 4 mm × 2 mm.Subsequently, the strip was dried at 120 • C for two hours in an oven for further utilization.The substrates used were Inconel 625 plates, and the surface was polished with sandpaper to remove the oxide film.The grease was washed off, and the surface was cleaned with alcohol and oven-dried at 80 • C for 1 h.
The experiment utilized a transTIG 800 welding power supply manufactured by Fronius Company (Wels, Austria), and the built-in Plasma Module 10 was employed as a microbeam plasma arc generator.The transTIG is a fully digitized TIG DC power source with a welding current range of 0.5 A to 80 A [20].The plasma and shielding gas used in the experiment was 99.99% pure argon gas.To facilitate welding, the prepared powder strip was positioned on the central axis of the substrate surface.After experimentation and optimization, the welding process's parameters were set as shown in Table 3.This work employed a field emission scanning electron microscope (SEM, JSM-7800F, JEOL Ltd., Tokyo, Japan) with X-ray energy dispersive spectroscopy (EDS) to observe the microstructure, element distribution, and grain size.In addition, X-ray diffraction (XRD, D8 Advance, Bruker AXS, Karlsruhe, Germany) using Cu-K α radiation was applied to detect the phase structure of cladding layers, as well as whether a phase change occurs after wear and molten salt corrosion.Rietveld whole-spectrum fitting analysis, a crystal structure refinement method, was performed using EXPGUI GSAS 1.00 software to quantitatively analyze the phase composition of the cladding layer.
The microhardness of the cladding layer was measured using a Vickers hardness tester (HV-1000A, Huayin Test Instrument Co. Ltd., Laizhou, China).The friction and wear experiments on the cladding layer were conducted at room temperature using the UMT-2 reciprocal sliding friction and wear testing apparatus provided by The Bruker Company (Ettlingen, Germany).The tribological test parameters are shown in Table 4.A white light interferometer 3D surface profiler was used to measure the wear scars' depth and width to calculate the cladding layer's wear rate.The wear rate R is calculated using the following formula: where the V is the wear volume (mm 3 ), the F is the load (N), and the L is the wear distance (m).High-temperature molten salt corrosion tests were conducted in this experiment.Multiple rectangular samples, measuring 20 mm × 10 mm × 2 mm, were cut from the cladding layer, using wire-cutting technology, polished, cleaned, and dried to obtain more accurate corrosion data.To simulate the service conditions of the cladding layer, a corrosive mixture of 75 wt.%Na 2 SO 4 + 25 wt.%NaCl was selected.The salt mixture was applied on the sample surface at 3 mg/cm 2 , and molten salt corrosion experiments were conducted at 750 • C and 950 • C. The samples were taken out every 5 h to calculate the cladding layer's corrosion weight loss per unit area, thereby evaluating the molten salt corrosion resistance.After 50 h of continuous molten salt corrosion experiments, the corrosion products were analyzed to investigate the corrosion resistance mechanism of the cladding layer.

Microstructures
Figure 1 presents the X-ray diffraction (XRD) patterns of NC5~NC30 cladding layers.The main phases in the cladding layer include the γ/γ phases, β-NiAl, and the following three types of chromium carbide: Cr 3 C 2 , Cr 7 C 3 , and Cr 23 C 6 .After the Rietveld fitting and quantitative analysis, the phase content proportions in each cladding layer were calculated, as shown in Table 5.It is evident that the Cr 3 C 2 and β-NiAl contents in the NC25 cladding layer are the highest, indicating a higher bonding density and stronger resistance to deformation.Consequently, it is inferred that NC25 has the best overall mechanical properties.
products were analyzed to investigate the corrosion resistance mechanism of the cladding layer.

Figure 1 presents the X-ray diffraction (XRD) patterns of NC5~NC30 cladding layers
The main phases in the cladding layer include the γ/γ′ phases, β-NiAl, and the following three types of chromium carbide: Cr3C2, Cr7C3, and Cr23C6.After the Rietveld fitting and quantitative analysis, the phase content proportions in each cladding layer were calcu lated, as shown in Table 5.It is evident that the Cr3C2 and β-NiAl contents in the NC25 cladding layer are the highest, indicating a higher bonding density and stronger resistance to deformation.Consequently, it is inferred that NC25 has the best overall mechanica properties.Figure 2 presents the microstructure and elemental distribution of the NC25 cladding layer.The spherical gray phase is primarily composed of Fe, Co, and Ni, which share the same crystal structure at elevated temperatures.According to the XRD analysis results in Figure 1, a γ/γ′ solid solution formed in the Ni.The black particulate matter contained in the spherical gray phase is characterized by dispersed aluminum enrichment.During ox idation or corrosion, the incoming oxygen is consumed by these aluminum-rich areas resulting in the formation of Al2O3, which provides protective properties.Combining the XRD analysis results in Figure 1, the grain boundary is identified as the β phase, which is primarily a solid solution of Ni and Al.
The magnified microstructure of the intergranular regions is exhibited in Figure 2b and the EDS spectrum of the white phase is shown in Figure 2c.Combined with the XRD  Figure 2 presents the microstructure and elemental distribution of the NC25 cladding layer.The spherical gray phase is primarily composed of Fe, Co, and Ni, which share the same crystal structure at elevated temperatures.According to the XRD analysis results in Figure 1, a γ/γ solid solution formed in the Ni.The black particulate matter contained in the spherical gray phase is characterized by dispersed aluminum enrichment.During oxidation or corrosion, the incoming oxygen is consumed by these aluminum-rich areas, resulting in the formation of Al 2 O 3 , which provides protective properties.Combining the XRD analysis results in Figure 1, the grain boundary is identified as the β phase, which is primarily a solid solution of Ni and Al.
The magnified microstructure of the intergranular regions is exhibited in Figure 2b, and the EDS spectrum of the white phase is shown in Figure 2c.Combined with the XRD analysis results in Figure 1, the white phase is mainly composed of chromium carbide particles.Moreover, it is assumed that in addition to the added Cr 3 C 2 , new phases Cr 7 C 3 and Cr 23 C 6 are generated within the chromium carbide particles.The wear resistance is enhanced by these high-hardness chromium carbide particles.Furthermore, their intimate association with the β phase matrix contributes to the overall performance of the cladding layer, particularly in improving corrosion resistance.
Materials 2024, 17, x FOR PEER REVIEW 5 of 22 analysis results in Figure 1, the white phase is mainly composed of chromium carbide particles.Moreover, it is assumed that in addition to the added Cr3C2, new phases Cr7C3 and Cr23C6 are generated within the chromium carbide particles.The wear resistance is enhanced by these high-hardness chromium carbide particles.Furthermore, their intimate association with the β phase matrix contributes to the overall performance of the cladding layer, particularly in improving corrosion resistance.Figure 3 illustrates the microstructures of the NC5~NC30 cladding layers.The phase compositions of each sample were similar.Combining the above phase composition analyses, it can be seen that with the increase in NiCr-Cr3C2 contents, the width of the β phase at the grain boundaries increased, and the size of the gray grains increased and then decreased.The change in the NiCr-Cr3C2 contents affected the distribution of chemical elements within the molten pool, thereby influencing grain growth and morphology.analysis results in Figure 1, the white phase is mainly composed of chromium carbide particles.Moreover, it is assumed that in addition to the added Cr3C2, new phases Cr7C3 and Cr23C6 are generated within the chromium carbide particles.The wear resistance is enhanced by these high-hardness chromium carbide particles.Furthermore, their intimate association with the β phase matrix contributes to the overall performance of the cladding layer, particularly in improving corrosion resistance.Figure 3 illustrates the microstructures of the NC5~NC30 cladding layers.The phase compositions of each sample were similar.Combining the above phase composition analyses, it can be seen that with the increase in NiCr-Cr3C2 contents, the width of the β phase at the grain boundaries increased, and the size of the gray grains increased and then decreased.The change in the NiCr-Cr3C2 contents affected the distribution of chemical elements within the molten pool, thereby influencing grain growth and morphology.

Hardness and Tribological Properties
The microhardness curve in Figure 4 represents a top-down analysis of the CoNiCrAlY and NC5~NC30 cladding layer sections, revealing that the hardness distribution of the cladding layers were relatively uniform.The average hardness of NC25 was the highest, reaching 348.2 HV 0.3 , an increase of 17.8% compared to the substrate, while the hardness of the NC30 cladding layer significantly decreased.The added NiCr-Cr 3 C 2 undergoes decarburization during the cladding process.As the NiCr-Cr 3 C 2 content increases, the carbon content in the alloy rises, the decarburization phenomenon is suppressed, and the Cr 3 C 2 content gradually increases, improving the cladding layer's hardness.However, an excess of NiCr-Cr 3 C 2 will promote the decomposition of the Cr 3 C 2 phase, reducing the hardness of the cladding layer.

Hardness and Tribological Properties
The microhardness curve in Figure 4 represents a top-down analysis of the CoNiCrAlY and NC5~NC30 cladding layer sections, revealing that the hardness distribution of the cladding layers were relatively uniform.The average hardness of NC25 was the highest, reaching 348.2 HV0.3, an increase of 17.8% compared to the substrate, while the hardness of the NC30 cladding layer significantly decreased.The added NiCr-Cr3C2 undergoes decarburization during the cladding process.As the NiCr-Cr3C2 content increases, the carbon content in the alloy rises, the decarburization phenomenon is suppressed, and the Cr3C2 content gradually increases, improving the cladding layer's hardness.However, an excess of NiCr-Cr3C2 will promote the decomposition of the Cr3C2 phase, reducing the hardness of the cladding layer.The average coefficient of friction (COF) and wear rate for each sample are illustrated in Figure 5a, highlighting the similarities observed between these two parameters.The 2D images of wear scar morphology are shown in Figure 5b.It can be seen that the wear resistance of NC5~NC20 is inferior to that of the CoNiCrAlY substrate.The friction coefficient of NC25 was the lowest, and the width and depth of the worn surface were also the smallest, indicating the best wear resistance.However, the wear rate and friction coefficient of NC30 were higher than those of the substrate, suggesting that the content of hardphase NiCr-Cr3C2 needs to be controlled to achieve optimal hardness and wear resistance.The average coefficient of friction (COF) and wear rate for each sample are illustrated in Figure 5a, highlighting the similarities observed between these two parameters.The 2D images of wear scar morphology are shown in Figure 5b.It can be seen that the wear resistance of NC5~NC20 is inferior to that of the CoNiCrAlY substrate.The friction coefficient of NC25 was the lowest, and the width and depth of the worn surface were also the smallest, indicating the best wear resistance.However, the wear rate and friction coefficient of NC30 were higher than those of the substrate, suggesting that the content of hard-phase NiCr-Cr 3 C 2 needs to be controlled to achieve optimal hardness and wear resistance.

Hardness and Tribological Properties
The microhardness curve in Figure 4 represents a top-down analysis of the CoNiCrAlY and NC5~NC30 cladding layer sections, revealing that the hardness distribution of the cladding layers were relatively uniform.The average hardness of NC25 was the highest, reaching 348.2 HV0.3, an increase of 17.8% compared to the substrate, while the hardness of the NC30 cladding layer significantly decreased.The added NiCr-Cr3C2 undergoes decarburization during the cladding process.As the NiCr-Cr3C2 content increases, the carbon content in the alloy rises, the decarburization phenomenon is suppressed, and the Cr3C2 content gradually increases, improving the cladding layer's hardness.However, an excess of NiCr-Cr3C2 will promote the decomposition of the Cr3C2 phase, reducing the hardness of the cladding layer.The average coefficient of friction (COF) and wear rate for each sample are illustrated in Figure 5a, highlighting the similarities observed between these two parameters.The 2D images of wear scar morphology are shown in Figure 5b.It can be seen that the wear resistance of NC5~NC20 is inferior to that of the CoNiCrAlY substrate.The friction coefficient of NC25 was the lowest, and the width and depth of the worn surface were also the smallest, indicating the best wear resistance.However, the wear rate and friction coefficient of NC30 were higher than those of the substrate, suggesting that the content of hardphase NiCr-Cr3C2 needs to be controlled to achieve optimal hardness and wear resistance.Figure 6 shows the worn surface morphologies of CoNiCrAlY and NC5~NC30 cladding layer.Samples mainly exhibit adhesive wear, and more white wear debris can be observed in Figure 6b-e.Figure 6f shows that the amount of white wear debris significantly decreased for NC25.In Figure 6g, the size of the white wear debris continues to increase, which may indicate the aggregation and growth of abrasive particles during wear.Figure 6 shows the worn surface morphologies of CoNiCrAlY and NC5~NC30 cladding layer.Samples mainly exhibit adhesive wear, and more white wear debris can be observed in Figure 6b-e.Figure 6f shows that the amount of white wear debris significantly decreased for NC25.In Figure 6g, the size of the white wear debris continues to increase, which may indicate the aggregation and growth of abrasive particles during wear.).The main difference is that with the increase in NiCr-Cr 3 C 2 content, the metal oxide content first decreased, then increased, and finally decreased, with a peak at NC25.Moreover, it can be demonstrated that more spinel phase (Ni,Co)Cr 2 O 4 is formed on the surface of NC25.These oxides form a protective film on the surface of the cladding layer, effectively reducing external friction and wear.Moreover, Cr 2 O 3 has a protective effect and provides some lubrication [21].These factors can significantly improve the wear resistance of the cladding layer.The XRD patterns of the CoNiCrAlY and NC5~NC30 cladding layers after wear are shown in Figure 7. Their phase compositions are similar, and the primary components detected include γ/γ′ and three types of chromium carbide (Cr3C2, Cr7C3, and Cr23C6), as well as oxides (NiO, Cr2O3, Al2O3, and (Ni,Co)Cr2O4).The main difference is that with the increase in NiCr-Cr3C2 content, the metal oxide content first decreased, then increased, and finally decreased, with a peak at NC25.Moreover, it can be demonstrated that more spinel phase (Ni,Co)Cr2O4 is formed on the surface of NC25.These oxides form a protective film on the surface of the cladding layer, effectively reducing external friction and wear.Moreover, Cr2O3 has a protective effect and provides some lubrication [21].These factors can significantly improve the wear resistance of the cladding layer.Figure 9 shows the surface morphology and elemental composition of the CoNiCrAlY cladding layer after molten salt corrosion at 750 °C.After 50 h of corrosion, parts of the oxide layer on the surface of the coating began to spall.The surface is mainly  Figure 10 illustrates the surface morphology and elemental composition of the NC25 cladding layer after molten salt corrosion at 750 °C.Compared with the CoNiCrAlY cladding layer at the same temperature, there is a diffuse distribution of Ni and a small amount of Co oxides on the surface, as well as some aggregated and spherical Cr2O3 and (Ni,Co)Cr2O4.The formation of Cr2O3 undergoes decarburization and oxidation of carbides in a complex and specific process.Cr atoms have formed carbides, so they cannot independently diffuse to the coating surface to react with oxygen.Oxygen atoms diffuse from the surface oxide layer to the interface between the oxide and carbide phases, making the carbide phase gradually convert into the oxide phase.
Generally, Ni and Co elements are rapidly oxidized to NiO and CoO, which cover the slowly forming Cr2O3, and a more stable spinel phase (Ni,Co)Cr2O4 is created through the solid-phase reaction.The reaction processes are represented in Equations ( 7) and ( 8).
Cr2O3 + NiO = NiCr2O4 (7) It has been previously shown that (Ni,Co)Cr2O4 can grow stably on the Cr2O3 substrate, which may be due to the good adhesion and stability of the oxide film formed during corrosion.
The released chlorine gas and oxygen diffuse inward after the surface oxide film ruptures, reacting with the base metal to form the chlorides CrCl 2 , NiCl 2 , and AlCl 3 .The volatile chlorides diffuse outward through the pores and voids in the coating, and upon contact with oxygen, oxides like Cr 2 O 3 are formed and loosely deposited on the surface [22].Cr 2 O 3 continue to be corroded by chloride ions in the molten salt, thus crack and spall gradually.In the initial stages of corrosion, Cr 2 O 3 is consumed by Na 2 SO 4 , forming Cr 2 (SO 4 ) 3 and Na 2 O.In an alkaline environment, Cr 2 O 3 reacts with Na 2 O to form Na 2 CrO 4 , accelerating the corrosion of the oxide layer.Additionally, the formed SO 3 reacts with surface CoO to generate the low-melting-point eutectic compound Na 2 SO 4 -CoSO 4 , causing pitting corrosion and making the surface oxide film more porous and looser [23].
Figure 10 illustrates the surface morphology and elemental composition of the NC25 cladding layer after molten salt corrosion at 750 • C. Compared with the CoNiCrAlY cladding layer at the same temperature, there is a diffuse distribution of Ni and a small amount of Co oxides on the surface, as well as some aggregated and spherical Cr 2 O 3 and (Ni,Co)Cr 2 O 4 .The formation of Cr 2 O 3 undergoes decarburization and oxidation of carbides in a complex and specific process.Cr atoms have formed carbides, so they cannot independently diffuse to the coating surface to react with oxygen.Oxygen atoms diffuse from the surface oxide layer to the interface between the oxide and carbide phases, making the carbide phase gradually convert into the oxide phase.

Cross-Sectional Morphology Analysis at 750 °C
Figure 11 shows the cross-sectional morphology and elemental analysis results of the CoNiCrAlY cladding layer after molten salt corrosion at 750 °C.From the cross-sectional morphology, it can be seen that the spongy loose surface pitting depth is about 50 µm.This is mainly due to the chemical reaction between Na2SO4 and the surface of the cladding layer in the early stage of molten salt corrosion, forming low-density NiO and Cr2O3 oxide films that cannot provide good protection.As the reaction deepens and metal chlorides continue to evaporate, the corrosion reaction gradually penetrates into the cladding layer.At this point, the (CrO4) 2− formed, as in Equation ( 3), gradually diffuses to the surface of the cladding layer and reacts again with the oxidant in the molten salt, forming relatively loose Cr2O3.Oxygen channels are formed by this process on the surface, allowing the molten salt to penetrate the cladding layer more smoothly, thereby accelerating the corrosion of the cladding layer by molten salt.From the map scan results, it can be seen that during the molten salt corrosion, the elemental content inside the cladding layer remains relatively stable, proving that the interactions among elements in the CoNiCrAlY cladding layer can effectively resist molten salt corrosion.Meanwhile, the main protective oxide, Al2O3, effectively prevents the corrosion by molten salt.
It has been previously shown that (Ni,Co)Cr 2 O 4 can grow stably on the Cr 2 O 3 substrate, which may be due to the good adhesion and stability of the oxide film formed during corrosion.

Cross-Sectional Morphology Analysis at 750 • C
Figure 11 shows the cross-sectional morphology and elemental analysis results of the CoNiCrAlY cladding layer after molten salt corrosion at 750 • C. From the crosssectional morphology, it can be seen that the spongy loose surface pitting depth is about 50 µm.This is mainly due to the chemical reaction between Na 2 SO 4 and the surface of the cladding layer in the early stage of molten salt corrosion, forming low-density NiO and Cr 2 O 3 oxide films that cannot provide good protection.As the reaction deepens and metal chlorides continue to evaporate, the corrosion reaction gradually penetrates into the cladding layer.At this point, the (CrO 4 ) 2− formed, as in Equation (3), gradually diffuses to the surface of the cladding layer and reacts again with the oxidant in the molten salt, forming relatively loose Cr 2 O 3 .Oxygen channels are formed by this process on the surface, allowing the molten salt to penetrate the cladding layer more smoothly, thereby accelerating the corrosion of the cladding layer by molten salt.From the map scan results, it can be seen that during the molten salt corrosion, the elemental content inside the cladding layer remains relatively stable, proving that the interactions among elements in the CoNiCrAlY cladding layer can effectively resist molten salt corro-sion.Meanwhile, the main protective oxide, Al 2 O 3 , effectively prevents the corrosion by molten salt.The cross-sectional morphology and elemental analysis results for the NC25 cladding layer after molten salt corrosion at 750 °C are shown in Figure 12.From the cross-sectional morphology, it can be seen that the density of the surface oxide film increased, and no obvious extended pits are observed.This may be due to the sufficient oxidation reaction at high temperature, making the NiO and Cr2O3 oxide films more complete and denser.From the map scan results, a distinct layering phenomenon can be detected in the cladding layer, as follows: the outermost layer is mainly Cr2O3, the second layer is a mixed layer of NiO and Al2O3, and the rest is the matrix.This is due to the peeling of the surface Cr2O3 oxide film, and already a small amount of Al is consumed rapidly, forming an aluminumpoor zone.When the aluminum-poor zone increases to a certain thickness, the diffusion rate of Al cannot keep up with the consumption rate and diffuse to the surface, thus forming Al2O3 in the second layer.
Comparing the XRD patterns, surface microstructures, and elemental analysis results at 750 °C, it can be seen that after adding 25% NiCr-Cr3C2 the surface condition of the cladding layer after molten salt corrosion is improved, the number of pits is significantly reduced, and the corrosion resistance is enhanced.The cross-sectional morphology and elemental analysis results for the NC25 cladding layer after molten salt corrosion at 750 • C are shown in Figure 12.From the cross-sectional morphology, it can be seen that the density of the surface oxide film increased, and no obvious extended pits are observed.This may be due to the sufficient oxidation reaction at high temperature, making the NiO and Cr 2 O 3 oxide films more complete and denser.From the map scan results, a distinct layering phenomenon can be detected in the cladding layer, as follows: the outermost layer is mainly Cr 2 O 3 , the second layer is a mixed layer of NiO and Al 2 O 3 , and the rest is the matrix.This is due to the peeling of the surface Cr 2 O 3 oxide film, and already a small amount of Al is consumed rapidly, forming an aluminum-poor zone.When the aluminum-poor zone increases to a certain thickness, the diffusion rate of Al cannot keep up with the consumption rate and diffuse to the surface, thus forming Al 2 O 3 in the second layer.
Comparing the XRD patterns, surface microstructures, and elemental analysis results at 750 • C, it can be seen that after adding 25% NiCr-Cr 3 C 2 the surface condition of the cladding layer after molten salt corrosion is improved, the number of pits is significantly reduced, and the corrosion resistance is enhanced.

Molten Salt Corrosion Kinetics at 750 • C
The corrosion weight loss curves of the CoNiCrAlY and NC25 cladding layers at 750 • C are shown in Figure 13.In the range of 0~5 h, the cladding layer quickly lost weight, and then the weight loss rate gradually slowed down.During the corrosion process, the average corrosion weight loss rate of the CoNiCrAlY cladding layer was 1.558 mg/cm 2 , while the average corrosion weight loss rate of the NC25 cladding layer was 1.049 mg/cm 2 .This indicates that the molten salt corrosion resistance of the CoNiCrAlY cladding layer was significantly improved by the addition of 25        The low-temperature hot corrosion mechanism at 750 • C can be summarized as follows: The melting point of the 75 wt.%Na 2 SO 4 + 25 wt.%NaCl mixed salt is about 620 • C; thus, it is in a molten state during the experiment [24].According to the acidbase dissolution mechanism, because of the different acidity and alkalinity of molten salt, protective oxides Cr 2 O 3 and Al 2 O 3 are dissolved in different ways [25].For the sulfate system, the following reaction occurs at 750 • C: The SO 3 generated, as per Equation ( 4), is consumed by CoO on the surface, forming a low-melting-point eutectic compound Na 2 SO 4 -CoSO 4 , whose melting point is only about 565 • C, existing as a liquid molten salt film at 750 • C [23].In this molten salt film, SO 3 migrates inward as (S 2 O 7 ) 2− , and Co 2+ , SO 4 2− , and dissolved SO 2 migrate outward, causing pits and making the surface oxide film more porous and looser [26].Moreover, beneficial elements are dissolved by these low-melting-point eutectics, and the formation or repair of Cr 2 O 3 or Al 2 O 3 oxide films is hindered, causing cobalt-based alloys to corrode rapidly.
During the corrosion process, although no significant sulfides and pitting phenomena appear inside the NC25 cladding layer, a small number of internal oxidation pores form under the oxide film.The excellent corrosion resistance of the NC25 cladding layer is due to the rapid formation of Cr     Figure 16 shows the surface morphology and elemental composition of the NC25 cladding layer after molten salt corrosion at 950 °C.Instead of obvious corrosion holes on the surface, a large number of spinel phases were generated at 950 °C; thus, the corrosion resistance of the NC25 cladding layer is further confirmed.The chemical composition of this spinel phase was detected to be mainly O, Cr, Co, and Ni, and through further composition analysis, it can be judged to be (Ni,Co)Cr2O4.The (Ni,Co)Cr2O4 phase has high thermal stability and good corrosion resistance, effectively resisting the erosion of corrosive substances in high-temperature environments.The (Ni,Co)Cr2O4 phase is partly distributed in spherical aggregates (Figure 16a) with some unformed spinel structures on the spheres, and mostly dispersed on the surface of the cladding layer (Figure 16b), indicating that the cladding layer has relatively uniform corrosion resistance.
Comparing the XRD patterns, surface microstructures, and elemental analysis results at 950 °C, it can be found that the CoNiCrAlY cladding layer is more likely to be corroded at high temperatures.In contrast, the NC25 cladding layer has a more stable phase composition and corrosion resistance at high temperatures, supporting its performance in practical applications.Figure 16 shows the surface morphology and elemental composition of the NC25 cladding layer after molten salt corrosion at 950 • C. Instead of obvious corrosion holes on the surface, a large number of spinel phases were generated at 950 • C; thus, the corrosion resistance of the NC25 cladding layer is further confirmed.The chemical composition of this spinel phase was detected to be mainly O, Cr, Co, and Ni, and through further composition analysis, it can be judged to be (Ni,Co)Cr 2 O 4 .The (Ni,Co)Cr 2 O 4 phase has high thermal stability and good corrosion resistance, effectively resisting the erosion of corrosive substances in high-temperature environments.The (Ni,Co)Cr 2 O 4 phase is partly distributed in spherical aggregates (Figure 16a) with some unformed spinel structures on the spheres, and mostly dispersed on the surface of the cladding layer (Figure 16b), indicating that the cladding layer has relatively uniform corrosion resistance.
Comparing the XRD patterns, surface microstructures, and elemental analysis results at 950 • C, it can be found that the CoNiCrAlY cladding layer is more likely to be corroded at high temperatures.In contrast, the NC25 cladding layer has a more stable phase composition and corrosion resistance at high temperatures, supporting its performance in practical applications.

Cross-Sectional Morphology Analysis at 950 • C
Figure 17 shows the cross-sectional morphology and elemental analysis results of the CoNiCrAlY cladding layer after molten salt corrosion at 950 • C. Compared with its state at 750 • C, the cross-sectional pits are significantly smaller, indicating that the CoNiCrAlY has better molten salt corrosion resistance at higher temperatures because of reasonable elemental design.From the map scan results, it can be seen that the upper layer consists mainly of Ni, O, and Cr elements, which are from the NiO and Cr 2 O 3 oxide films generated during corrosion that have not completely peeled off.A small amount of Al element is detected in the middle of the cladding layer in the form of Al 2 O 3 .The lower layer significantly increased in S content, in addition to Ni and Cr, and is the region heavily corroded by Na 2 SO 4 .The reason for the internal sulfurization may be that the loose Cr 2 O 3 oxide film generated during corrosion continues to dissolve and fracture at high temperatures, and the film-forming elements Cr and Al are heavily consumed, resulting in the rapid diffusion of S along the pores into the interior of the substrate, forming sulfides such as CrS [27].

Cross-Sectional Morphology Analysis at 950 °C
Figure 17 shows the cross-sectional morphology and elemental analysis results of the CoNiCrAlY cladding layer after molten salt corrosion at 950 °C.Compared with its state at 750 °C, the cross-sectional pits are significantly smaller, indicating that the CoNiCrAlY has better molten salt corrosion resistance at higher temperatures because of reasonable elemental design.From the map scan results, it can be seen that the upper layer consists mainly of Ni, O, and Cr elements, which are from the NiO and Cr2O3 oxide films generated during corrosion that have not completely peeled off.A small amount of Al element is detected in the middle of the cladding layer in the form of Al2O3.The lower layer significantly increased in S content, in addition to Ni and Cr, and is the region heavily corroded by Na2SO4.The reason for the internal sulfurization may be that the loose Cr2O3 oxide film generated during corrosion continues to dissolve and fracture at high temperatures, and the film-forming elements Cr and Al are heavily consumed, resulting in the rapid diffusion of S along the pores into the interior of the substrate, forming sulfides such as CrS [27].
The cross-sectional morphology and elemental analysis results of the NC25 cladding layer after molten salt corrosion at 950 °C are shown in Figure 18.It can be seen that the upper layer is mainly composed of Al2O3 and Cr2O3, and the internal γ/γ′ phase is well protected.Compared with the CoNiCrAlY, the S content inside the NC25 decreases, and internal sulfurization is reduced.This may be attributed to the fact that Na2SO4 is already heavily consumed when dissolving surface Cr2O3, forming a large amount of spinel phase, the corrosion effect of Na2SO4 is reduced and the inward diffusion of S is relieved.The cross-sectional morphology and elemental analysis results of the NC25 cladding layer after molten salt corrosion at 950 • C are shown in Figure 18.It can be seen that the upper layer is mainly composed of Al 2 O 3 and Cr 2 O 3 , and the internal γ/γ phase is well protected.Compared with the CoNiCrAlY, the S content inside the NC25 decreases, and internal sulfurization is reduced.This may be attributed to the fact that Na 2 SO 4 is already heavily consumed when dissolving surface Cr 2 O 3 , forming a large amount of spinel phase, the corrosion effect of Na 2 SO 4 is reduced and the inward diffusion of S is relieved.19.It can be seen that the weight loss rate was relatively slow in the initial corrosion period of 0~20 h, Cr and Al are slowly oxidized on the surface, and the oxide film gradually generated.The stress was raised and some local cracks were produced, but there was no loss in the overall mechanical integrity of the sample.Meanwhile, the acidity and alkalinity of Na 2 SO 4 changed with the molten salt environment.In the mid-corrosion period of 20~35 h, the surface oxide film formed in large amounts and then dissolved by Na 2 SO 4 , and the film-forming elements Cr and Al below the oxide filmmatrix interface were heavily consumed.These lead to penetration of S into the cladding layer, where the weight loss rate is significantly accelerated and corrosion is deepened.In the late corrosion period of 35~50 h, the surface oxide film peels off on a large scale, S continues to penetrate downward to corrode the substrate, and sulfides form in the interior.The sample was severely corroded, and the overall mechanical integrity was damaged.The weight loss of the CoNiCrAlY and NC25 cladding layers showed a similar trend as the corrosion proceeded.However, the weight loss was more severe in the CoNiCrAlY, with an average corrosion weight loss rate of 9.428 mg/cm 2 , while the NC25 was corroded relatively lightly at a rate of 3.242 mg/cm 2 .The CoNiCrAlY was more easily damaged during corrosion, while the NC25 was relatively stable, indicating that the molten salt corrosion resistance of the cladding layer at 950 • C can be significantly improved by the addition of 25 wt.%NiCr-Cr 3 C 2 .
Moreover, compared with the corrosion results at 750 • C, the molten salt corrosion resistance of the substrate significantly improved by 25 wt.%NiCr-Cr 3 C 2 , indicating that the cladding layer with this composition can perform better at 950 • C.These data and information will serve as an important reference for research and in the application of corrosion-resistant cladding layers.
atively lightly at a rate of 3.242 mg/cm 2 .The CoNiCrAlY was more easily damaged during corrosion, while the NC25 was relatively stable, indicating that the molten salt corrosion resistance of the cladding layer at 950 °C can be significantly improved by the addition of 25 wt.%NiCr-Cr3C2.
Moreover, compared with the corrosion results at 750 °C, the molten salt corrosion resistance of the substrate significantly improved by 25 wt.%NiCr-Cr3C2, indicating that the cladding layer with this composition can perform better at 950 °C.These data and information will serve as an important reference for research and in the application of corrosion-resistant cladding layers.By comparing the molten salt corrosion results of CoNiCrAlY and NC25 cladding layers at 950 °C, it can be seen that in addition to oxide films, the substrate can be effectively protected by the (Ni,Co)Cr2O4 phase formed during corrosion [28].As shown in Figure 20 The high-temperature hot corrosion mechanism at 950 °C can be summarized as follows: because of the surface tension and capillary effects, after the molten salt penetrates the porous oxide layer, smaller pores are filled first, leading to very fast corrosion speed, with almost no continuous oxide layer between the oxide layer and the substrate.At high temperatures, the diffusion speed of gases and ions is faster.Molten Na2SO4 is dissolved in alkaline solution, according to Equation (4), at 950 °C, and Cr is first oxidized by O2 produced by molten salt decomposition to Cr2O3.As the equilibrium, as shown by Equation (2), shifts to the right, the partial pressure of S in the oxide film increases, with S2O7 2− participating in the reaction at the interface between the oxide film and the molten salt, as the main oxidant, as follows: S 2− penetrates the cladding layer, reacting with metals or metal oxides to form sulfides.As the reaction progresses, the alkaline Na2O in the molten salt continue to increases, and Cr2O3 dissolves into the molten salt [30].The reaction process is represented by Equation (12), as follows: It is indicated in the Rapp-Goto molten salt model that if there is a negative gradient in the dissolution of protective metal oxides in the molten salt, hot corrosion may continue [31].This means that certain metal oxides gradually dissolve and precipitate in the molten salt, resulting in the metal substrate is exposed to and corroded by the molten salt.The equilibrium of Equation ( 12) shifts to the right attributed to the high content of Cr2O3 in the NC25 cladding layer, more (Cr2O4) 2− is generated, and then the spinel phase (Ni,Co)Cr2O4.This process causes the alkalinity of the molten salt to gradually exceed the initial value of corrosion and stabilize at a certain level.During this process, the negative gradient in the dissolution of protective metal oxides gradually disappears and reaches saturation.Oxygen in the environment can reach the interface between the substrate and the oxide film as the old oxide film peels off; new dense Al2O3 and Cr2O3 films are generated, and further corrosion is prevented.An illustration of the corrosion process is shown in Figure 21.It is shown in the above tests that distinct layering can be observed in the corroded cladding layer.The upper layer is NiO and some loose Cr2O3 layers; the middle layer is mostly Al2O3, Cr2O3, and spinel phase (Ni,Co)Cr2O4; and the lower layer is the substrate and sulfides.Cr and Al play crucial roles in the corrosion resistance of the cladding layer, closely related to their characteristic of readily reacting with oxygen to form oxides.Because of the strong affinity of Cr and Al with oxygen in the initial stage of oxidation, they rapidly combine with oxygen to form oxides, leading to a significant increase in the weight of the cladding layer.However, as the oxidation process continues, further oxygen The high-temperature hot corrosion mechanism at 950 • C can be summarized as follows: because of the surface tension and capillary effects, after the molten salt penetrates the porous oxide layer, smaller pores are filled first, leading to very fast corrosion speed, with almost no continuous oxide layer between the oxide layer and the substrate.At high temperatures, the diffusion speed of gases and ions is faster.Molten Na 2 SO 4 is dissolved in alkaline solution, according to Equation ( 4 S 2− penetrates the cladding layer, reacting with metals or metal oxides to form sulfides.As the reaction progresses, the alkaline Na 2 O in the molten salt continue to increases, and Cr 2 O 3 dissolves into the molten salt [30].The reaction process is represented by Equation ( 12), as follows: It is indicated in the Rapp-Goto molten salt model that if there is a negative gradient in the dissolution of protective metal oxides in the molten salt, hot corrosion may continue [31].This means that certain metal oxides gradually dissolve and precipitate in the molten salt, resulting in the metal substrate is exposed to and corroded by the molten salt.The equilibrium of Equation ( 12) shifts to the right attributed to the high content of Cr 2 O 3 in the NC25 cladding layer, more (Cr 2 O 4 ) 2− is generated, and then the spinel phase (Ni,Co)Cr 2 O 4 .This process causes the alkalinity of the molten salt to gradually exceed the initial value of corrosion and stabilize at a certain level.During this process, the negative gradient in the dissolution of protective metal oxides gradually disappears and reaches saturation.Oxygen in the environment can reach the interface between the substrate and the oxide film as the old oxide film peels off; new dense Al 2 O 3 and Cr 2 O 3 films are generated, and further corrosion is prevented.An illustration of the corrosion process is shown in Figure 21.penetration is effectively prevented by these oxides.Some of the Cr elements are converted into volatile Cr2O3 during oxidation, which slows down the oxidative weight gain of the cladding layer and enables the cladding layer to remain stable for a longer period of time.The more stable Al2O3 film plays a major role in the cladding layer, and its relatively slow growth kinetics make it difficult for oxygen to diffuse through it.Therefore, the Al2O3 film can maintain substantial inertness even in high-temperature and oxidative atmospheres.This excellent oxidation resistance allows the NC25 cladding layer to maintain a stable performance even in harsh environments, and its service life is extended.

Conclusions
The effects of the NiCr-Cr3C2 content on the microstructures, mechanical properties, and high temperature corrosion resistance of CoNiCrAlY cladding layers fabricated by is shown in the above tests that distinct layering can be observed in the corroded cladding layer.The upper layer is NiO and some loose Cr 2 O 3 layers; the middle layer is mostly Al 2 O 3 , Cr 2 O 3 , and spinel phase (Ni,Co)Cr 2 O 4 ; and the lower layer is the substrate and sulfides.Cr and Al play crucial roles in the corrosion resistance of the cladding layer, closely related to their characteristic of readily reacting with oxygen to form oxides.Because of the strong affinity of Cr and Al with oxygen in the initial stage of oxidation, they rapidly combine with oxygen to form oxides, leading to a significant increase in the weight of the cladding layer.However, as the oxidation process continues, further oxygen penetration is effectively prevented by these oxides.Some of the Cr elements are converted into volatile Cr 2 O 3 during oxidation, which slows down the oxidative weight gain of the cladding layer and enables the cladding layer to remain stable for a longer period of time.The more stable Al 2 O 3 film plays a major role in the cladding layer, and its relatively slow growth kinetics make it difficult for oxygen to diffuse through it.Therefore, the Al 2 O 3 film can maintain substantial inertness even in high-temperature and oxidative atmospheres.This excellent oxidation resistance allows the NC25 cladding layer to maintain a stable performance even in harsh environments, and its service life is extended.

Conclusions
The effects of the NiCr-Cr 3 C 2 content on the microstructures, mechanical properties, and high temperature corrosion resistance of CoNiCrAlY cladding layers fabricated by microbeam plasma technology were investigated, and its corrosion mechanism was clarified in this paper.The main conclusions are as follows: 1.
With the addition of NiCr-Cr 3 C 2 , the layer grain size firstly increases and then decreases, and the dispersion and granular width of the β phase are promoted.

2.
The NC25 cladding layer had the highest average hardness at 348.2 HV 0.3 , as well as the lowest average friction coefficient and wear rate at 0.

Figure 2 .
Figure 2. (a) Mapping scan results of NC25; (b) microstructural morphologies of the intergranular regions; (c) EDS spectrum of R D .

Figure 3
Figure 3 illustrates the microstructures of the NC5~NC30 cladding layers.The phase compositions of each sample were similar.Combining the above phase composition analyses, it can be seen that with the increase in NiCr-Cr 3 C 2 contents, the width of the β phase at the grain boundaries increased, and the size of the gray grains increased and then decreased.The change in the NiCr-Cr 3 C 2 contents affected the distribution of chemical elements within the molten pool, thereby influencing grain growth and morphology.

3. 3 .
Molten Salt Corrosion Behavior at 750 °C 3.3.1.Surface Phase and Morphology Analysis at 750 °C A 50 h corrosion test was conducted utilizing a mixture of 75 wt.%Na2SO4 and 25 wt.%NaCl.The XRD patterns of the CoNiCrAlY and NC25 coatings with the best comprehensive mechanical properties under molten salt corrosion at 750 °C are shown in Figure 8.It can be observed that the γ/γ′ phase still exists in the CoNiCrAlY coating.However, the β phase and chromium carbide can no longer be detected because the β phase has been oxidized to Al2O3, and the chromium carbide has been oxidized to Cr2O3.The γ/γ′ phase on the corrosion surface decreased after adding 25% NiCr-Cr3C2, while Cr2O3, NiO, and the spinel phase (Ni,Co)Cr2O4 significantly increased.This indicates that the formation of the surface oxide film and a new spinel phase is promoted for the addition of NiCr-Cr3C2.

Figure 8 .
Figure 8.The XRD pattern of molten salt corrosion products at 750 • C.

Figure 9 3 ) 22 Figure 9 .
Figure 9 shows the surface morphology and elemental composition of the CoNiCrAlY cladding layer after molten salt corrosion at 750 • C.After 50 h of corrosion, parts of the oxide layer on the surface of the coating began to spall.The surface is mainly composed of NiO, CoO, a small amount of Al 2 O 3 , and some sulfates.The interior of the coating is filled with pitting corrosion holes, exhibiting a spongy appearance, which is mainly due to the corrosive effect of molten sulfates.Initially, Cr 2 O 3 forms on the surface of the coating due to the combined action of oxygen and Na 2 SO 4 and is subsequently dissolved by NaCl, as follows: 4Cr + O 2 + SO 4 2− = 2Cr 2 O 3 + S 2− (2) 2Cr 2 O 3 + 5O 2 + 8Cl − = 4(CrO 4 ) 2− + 4Cl 2 ↑ (3) Materials 2024, 17, x FOR PEER REVIEW 10 of 22

Figure 9 .
Figure 9. (a) The surface morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 750 • C; (b) EDS spectrum of R A .Molten Na 2 SO 4 decomposes into Na 2 O and SO 3 , and the NaCl in the molten salt is oxidized to produce chlorine gas, as follows: Na 2 SO 4 = Na 2 O + SO 3 (4)

Figure 10 .
Figure 10.(a,b) Surface morphology of the NC25 cladding layer after molten salt corrosion at 750 °C; (c) EDS spectrum of RA; (d) EDS spectrum of RB.

Figure 10 .
Figure 10.(a,b) Surface morphology of the NC25 cladding layer after molten salt corrosion at 750 • C; (c) EDS spectrum of R A ; (d) EDS spectrum of R B .Generally, Ni and Co elements are rapidly oxidized to NiO and CoO, which cover the slowly forming Cr 2 O 3 , and a more stable spinel phase (Ni,Co)Cr 2 O 4 is created through the solid-phase reaction.The reaction processes are represented in Equations (7) and (8).Cr 2 O 3 + NiO = NiCr 2 O 4 (7)

Materials 2024 , 22 Figure 11 .
Figure 11.The cross-sectional morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 750 °C and the EDS map scan results.

Figure 11 .
Figure 11.The cross-sectional morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 750 • C and the EDS map scan results.

Figure 12 .
Figure 12.The cross-sectional morphology of the NC25 cladding layer after molten salt corrosion at 750 °C and EDS map scan results.3.3.3.Molten Salt Corrosion Kinetics at 750 °C The corrosion weight loss curves of the CoNiCrAlY and NC25 cladding layers at 750 °C are shown in Figure13.In the range of 0~5 h, the cladding layer quickly lost weight, and then the weight loss rate gradually slowed down.During the corrosion process, the average corrosion weight loss rate of the CoNiCrAlY cladding layer was 1.558 mg/cm 2 , while the average corrosion weight loss rate of the NC25 cladding layer was 1.049 mg/cm 2 .This indicates that the molten salt corrosion resistance of the CoNiCrAlY cladding layer was significantly improved by the addition of 25 wt.%NiCr-Cr3C2.

Figure 12 .
Figure 12.The cross-sectional morphology of the NC25 cladding layer after molten salt corrosion at 750 • C and EDS map scan results.

Figure 12 .
Figure 12.The cross-sectional morphology of the NC25 cladding layer after molten salt corrosion at 750 °C and EDS map scan results.
2 O 3 and (Ni,Co)Cr 2 O 4 , effectively preventing the formation of low-melting-point liquid Na 2 SO 4 -CoSO 4 and forming a dense oxide film in the molten salt environment, improving the corrosion and oxidation resistance of the cladding layer.Furthermore, the protective film of Al 2 O 3 is generated mainly through the Al elements in β-NiAl; thus, the content of β-NiAl and the consumption rate of the Al elements to some extent determine the corrosion resistance of the cladding layer.These factors together caused the cladding layer with 25 wt.%NiCr-Cr 3 C 2 to exhibit good corrosion resistance in the molten salt environment at 750 • C.

Figure 15
Figure 15 shows the surface morphology and elemental composition of the CoNiCrAlY cladding layer after molten salt corrosion at 950 • C. It can be seen that at 950 • C, the surface of the cladding layer became more porous with more corrosion holes extending inward under the action of chloride ions.The outer surface oxide film is mainly composed of NiO with a small amount of Co and Al, and most of Cr 2 O 3 has peeled off, indicating that the surface of the CoNiCrAlY cladding layer is heavily oxidized under high-temperature molten salt corrosion.

Figure 14 .
Figure 14.The XRD patterns of molten salt corrosion products at 950 °C.

Figure 15 .
Figure 15.(a) The surface morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 950 °C; (b) EDS spectrum of RA.

Figure 15 .
Figure 15.(a) The surface morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 950 • C; (b) EDS spectrum of R A .

Figure 16 .
Figure 16.(a,b) Surface morphology of the NC25 cladding layer after molten salt corrosion at 950 °C; (c) EDS spectrum of RA; (d) EDS spectrum of RB.

Figure 16 . 22 Figure 17 .
Figure 16.(a,b) Surface morphology of the NC25 cladding layer after molten salt corrosion at 950 • C; (c) EDS spectrum of R A ; (d) EDS spectrum of R B .Materials 2024, 17, x FOR PEER REVIEW 17 of 22

Figure 17 .
Figure 17.The cross-sectional morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 950 °C and EDS map scan results.Figure 17.The cross-sectional morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 950 • C and EDS map scan results.

Figure 17 .
Figure 17.The cross-sectional morphology of the CoNiCrAlY cladding layer after molten salt corrosion at 950 °C and EDS map scan results.

Figure 18 .
Figure 18.The cross-sectional morphology of the NC25 cladding layer after molten salt corrosion at 950 °C and EDS map scan results.3.4.3.Molten Salt Corrosion Kinetics at 950 °C The corrosion weight loss curves of the CoNiCrAlY and NC25 cladding layers at 950 °C are shown in Figure 19.It can be seen that the weight loss rate was relatively slow in

Figure 18 .
Figure 18.The cross-sectional morphology of the NC25 cladding layer after molten salt corrosion at 950 • C and EDS map scan results.3.4.3.Molten Salt Corrosion Kinetics at 950 • C The corrosion weight loss curves of the CoNiCrAlY and NC25 cladding layers at 950 • C are shown in Figure19.It can be seen that the weight loss rate was relatively slow in the initial corrosion period of 0~20 h, Cr and Al are slowly oxidized on the surface, and the oxide film gradually generated.The stress was raised and some local cracks were produced, but there was no loss in the overall mechanical integrity of the sample.Meanwhile, the acidity and alkalinity of Na 2 SO 4 changed with the molten salt environment.In the mid-corrosion period of 20~35 h, the surface oxide film formed in large amounts and then dissolved by Na 2 SO 4 , and the film-forming elements Cr and Al below the oxide filmmatrix interface were heavily consumed.These lead to penetration of S into the cladding layer, where the weight loss rate is significantly accelerated and corrosion is deepened.In the late corrosion period of 35~50 h, the surface oxide film peels off on a large scale, S continues to penetrate downward to corrode the substrate, and sulfides form in the interior.The sample was severely corroded, and the overall mechanical integrity was damaged.The weight loss of the CoNiCrAlY and NC25 cladding layers showed a similar trend as the corrosion proceeded.However, the weight loss was more severe in the CoNiCrAlY, with an average corrosion weight loss rate of 9.428 mg/cm 2 , while the NC25 was corroded relatively lightly at a rate of 3.242 mg/cm 2 .The CoNiCrAlY was more easily damaged during corrosion, while the NC25 was relatively stable, indicating that the molten salt corrosion resistance of the cladding layer at 950 • C can be significantly improved by the addition of 25 wt.%NiCr-Cr 3 C 2 .Moreover, compared with the corrosion results at 750 • C, the molten salt corrosion resistance of the substrate significantly improved by 25 wt.%NiCr-Cr 3 C 2 , indicating that the cladding layer with this composition can perform better at 950 • C.These data and
, taking NiCr2O4 as an example, it is a typical Fd3m spinel structure, with the crystal cell composed of tetrahedral clusters [NiO4] and octahedral clusters [CrO6].The (Ni,Co)Cr2O4 phase is structurally stable at high temperature, with the formation enthalpy ∆H at 950 °C of NiCr2O4 being −1212.23 kJ/mol and that of CoCr2O4 being −1263.16kJ/mol [29].

Figure 19 .
Figure 19.Corrosion weight loss curves of CoNiCrAlY and NC25 at 950 • C. By comparing the molten salt corrosion results of CoNiCrAlY and NC25 cladding layers at 950 • C, it can be seen that in addition to oxide films, the substrate can be effectively protected by the (Ni,Co)Cr 2 O 4 phase formed during corrosion [28].As shown in Figure 20, taking NiCr 2 O 4 as an example, it is a typical Fd3m spinel structure, with the crystal cell composed of tetrahedral clusters [NiO 4 ] and octahedral clusters [CrO 6 ].The (Ni,Co)Cr 2 O 4 phase is structurally stable at high temperature, with the formation enthalpy ∆H at 950 • C of NiCr 2 O 4 being −1212.23 kJ/mol and that of CoCr 2 O 4 being −1263.16kJ/mol [29].

Figure 20 .
Figure 20.Schematic diagram of the NiCr2O4 crystal structure.
), at 950 • C, and Cr is first oxidized by O 2 produced by molten salt decomposition to Cr 2 O 3 .As the equilibrium, as shown by Equation (2), shifts to the right, the partial pressure of S in the oxide film increases, with S 2 O 7 2− participating in the reaction at the interface between the oxide film and the molten salt, as the main oxidant, as follows :

Figure 21 .
Figure 21.Illustration of the molten salt corrosion process.

Figure 21 .
Figure 21.Illustration of the molten salt corrosion process.
4751 and 0.4528 × 10 −6 mm 3 /N•m, respectively.The wear mechanism is the surface of the cladding layer undergoing oxidative wear during friction with the formation of a dense protective Al 2 O 3 film, together with Cr 2 O 3 and (Ni,Co)Cr 2 O 4 .Additionally, Cr 2 O 3 has certain lubricative function, which improves the layer's tribological performance.3.Under molten salt corrosion at 750 • C, the cladding layer exhibited typical lowtemperature hot corrosion characteristics, with pitting holes extending inward on the surface due to the rapid corrosion by Na 2 SO 4 .The rapid formation of Cr 2 O 3 and (Ni,Co)Cr 2 O 4 in the NC25 cladding layer effectively inhibited the formation of low-melting-point eutectic Na 2 SO 4 -CoSO 4 , which reduced the average weight loss rate by 32.7%.4.Under molten salt corrosion at 950 • C, the cladding layer exhibited typical hightemperature hot corrosion characteristics, with local Al depletion and sulfide formation.The high content of Cr 2 O 3 in the NC25 cladding layer promoted the formation of the spinel phase (Ni,Co)Cr 2 O 4 , inhibiting the corrosion effect of Na 2 SO 4 and ion diffusion, significantly improving the stability of the oxide film, which reduced the average weight loss rate by 65.6%.Moreover, at 950 • C, the improvement in the corrosion resistance of the NC25 cladding layer compared with the matrix was far greater than at 750 • C, indicating it is more suitable for service at 950 • C.