High-Temperature Steam and Atmospheric Oxidation Characteristic of a Heat-Resistant SP2215 Steel

Abstract

The high-temperature oxidation performance of SP2215 has become an important issue when they were used as superheaters and reheaters exposed to two different high-temperature environments. In this study, the oxidation behavior of SP2215 steel was investigated under steam and an atmosphere of 650–800 °C for 240 h. The microstructural and chemical characterization of the samples were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), a glow discharge optical emission spectrometer (GD-OES), and atomic force microscope (AFM). The kinetic curves of oxidation revealed excellent oxidation resistance under both environments, but significant different oxidation characteristics, oxide film composition, and structure were obvious. In the steam experiment, selective intergranular oxidation was evident at relatively low temperatures, which was attributed to the preference absorption of supercritical water molecules at the grain boundary. Conversely, a double-layer structure of outer Fe₂O₃ and a small amount of Fe₃O₄ and inner Cr₂O₃ was formed uniformly at 800 °C. In the high-temperature atmosphere experiment, a protective chromium film was dominant at 650–700 °C, and a loose multicomponent oxide film was formed at 750–800 °C, primarily consisting of Cr₂O₃, spinel FeCr₂O₄, and CuO.

1. Introduction

In response to the urgent need to reduce coal consumption and mitigate pollutant emissions, ultra-supercritical (USC) fossil power plants have been recognized as a crucial strategy [1,2]. For the improvement of thermal efficiency and safety of energy production in USC coal-fired power plants, heat-resistant steels, such as those used in reheaters and superheaters, are essential [3,4,5,6]. Currently, the S30432 (18Cr9Ni3CuNbN) and TP310HCbN (25Cr20NiNbN) heat-resistant steels have been widely used in the heating surface of high-temperature superheater and high-temperature reheater systems. However, the rapid precipitation of M₂₃C₆ or primary MX and the high C/Nb ratio in S30432 steel will lead to high intergranular corrosion sensitivity [5,7,8]. For the TP310HCbN steel, the high Cr content ensures excellent corrosion resistance, but volume defects and lattice distortion around M₂₃C₆ at high temperatures result in low impact strength [9,10].

In response, an excellent austenitic heat-resistant steel SP2215 (22Cr15Ni3.5CuNbN) has been developed as a potential replacement for traditional materials. This steel exhibits remarkable high-temperature creep strength, with minimal reduction in impact toughness even after extensive aging. In comparison with conventional materials, SP2215 steel provides superior mechanical properties while utilizing less chromium (Cr) and nickel (Ni). Most studies have revealed that the superior properties of SP2215 steel under high temperatures can be attributed to the stabilization and strengthening effect of niobium (Nb) and nitrogen (N), as well as the presence of a Cu-rich phase [11,12,13,14]. Du et al. [12] found that the nano-precipitation formed by the precipitation of copper particles, niobium-rich carbonitides, and NbCrN phases enhances the materials’ high-temperature mechanical properties. This steel also has significant creep strength at high temperatures, and shows only a slight decrease in fatigue strength at a peak stress below 450 MPa at 700 °C [14]. Weiet al. [15] found that pitting occurs in SP2215 during long-term service under a steam temperature of 520 °C and a flue gas temperature of 1090 °C (>4000 h), which was attributed to the lack of protection by the passivation layer.

The foremost parameters which influence the general oxidation of the metal are the temperature and the nature of the oxidizing gas [16]. In USC power plants, superheaters and reheaters are exposed to two distinct service environments: the combustion gas side and the steam side. The inner side of the tube is subjected to high-pressure steam that is oxidizing, corrosive, and has a scouring effect, whereas the outer side of the tube is exposed to combustion gases and ashes. Therefore, service materials must possess excellent oxidation properties in both environments [17,18,19,20,21]. However, the current application of SP2215 steel is still in its early stage, and the high-temperature oxidation resistance in the two different environments has not yet been deeply understood. The oxidation resistance of an alloy is directly related to the nature of the oxide layer formed on its surface [22,23]. The high-temperature oxidation behavior of metals is influenced by many factors, such as the oxidation temperature and the atmosphere [6,24], surface treatment [25,26,27], and grain size [28]. Fujikawa et al. [29] reported that steel with a coarse grain tends to peel off easily during the cooling process at 850 °C. Li et al. [30] found a spalling oxidation layer on Super304H formed under steam at 605 °C due to the difference in the internal and external expansion coefficients, with oxidation products consisting of an inner layer of chromium oxide and an outer layer of iron oxide. Zielinski et al. [29] revealed that oxide layers differ completely with components and properties between HR3C and HR6W at high temperatures. Although extensive research has been conducted on the oxidation behavior of heat-resistant alloy materials, the oxidation behavior of SP2215 may vary in different environments at high temperatures.

In this study, the oxidation behavior and mechanism of SP2215 when exposed to temperatures in the range of 650–800 °C in atmospheric and steam environments are discussed. The oxidation kinetics and the formation process of the oxide layer at different temperatures are investigated. The morphological properties and elemental composition of the oxidation products are also analyzed, and the oxidation mechanisms are discussed in detail. The research assesses the feasibility of using SP2215 in an extremely high-temperature environment and provides dependable data and a theoretical foundation for the future implementation of SP2215 in the next generation of USC units.

2. Materials and Methods

2.1. Materials

The materials used in this test were supplied by Yongxing Special Materials Technology Co., Ltd. (Hangzhou, China). The manufacture of these materials included multiple steps, including electric arc furnace smelting, argon oxygen decarburization refining, secondary refining in a ladle furnace, ingot casting, homogenizing, regrinding, billet opening, and hot rolling. After casting, the ingot was heated to 1240 °C and kept for 6 h to eliminate possible segregations that occur during solidification. Then, the ingot was forged at 1240 °C to obtain rough blanks. Subsequently, the surface of the blanks was ground, and the blanks were then held at 1240 °C for 5 h to examine the changes in microstructural and precipitates and then hot rolled into Φ110 mm billets. The chemical compositions of SP2215 were examined by using a Spectroscope (MAXx16-M, SPECTRO, Kleve, Germany), as listed in

Table 1. Chemical compositions of SP2215 (wt.%).

C	Si	Mn	P	Cr	Mo	Ni	Co	Cu	Nb	V	N	Fe
0.07	0.41	0.60	0.02	22.80	0.34	15.00	0.01	3.49	0.52	0.30	0.32	Bal.

.

Samples with dimensions of Φ12 mm × 3.0 mm dimensions were cut for testing, and ground using 200–2000 # SiC papers on a grinding machine (LaboPol-20, Struers, Copenhagen, Denmark) to produce clean surfaces with no preexisting oxide layer or other contaminants. The samples were cleaned with acetone in an ultrasonic bath and dried by cool air. After grinding, all the samples were weighted by using a precision balance (Quintix224-1CN, Shanghai Zhuojing Electronic Technology Co., Ltd., Shanghai, China) by Sartorius (precision up to 0.1 mg). The dimensions of each individual sample were measured with a standard caliper, with a precision of 0.02 mm. After the oxidation experiments, weight change, and weight change per surface area (ΔW) were measured or calculated.

2.2. Oxidation Experiments

The SP2215 samples were placed in corundum crucibles in the HMX-1800-20 atmosphere furnace and the PTF-1200X tube furnace. Atmospheric and steam high-temperature oxidation tests were performed in the atmosphere and tube furnaces, respectively. During the atmospheric oxidation test, fresh air was fed into the furnace via a compressor to ensure that a high temperature atmospheric environment was maintained throughout the test. The steam oxidation tests were conducted in pure steam at atmospheric pressure, and deionized water entered the tube furnace via a peristaltic pump to produce steam. Both experiments were performed at 650, 700, 750, and 800 °C, and the duration times were 24, 72, 144, 192, and 240 h. After the exposure time was reached, the samples were cooled in the furnace and were then taken out. The weight of the samples was measured directly after cooling.

2.3. Microstructure and Characterizations

The oxide films of SP2215 were characterized to analyze oxidation behaviors. X-ray diffraction (XRD, D8 ADVANCE DAVINCI, BRUKER, Karlsruhe, Germany) was conducted using Cu Kα (0.1547 nm) radiation in a 2θ range from 10° to 90° with a scanning rate of 6°/min. Scanning electron microscopy (SEM, FEI Quanta FEG 250, FEI, Hillsboro, OR, USA) with energy dispersive spectrometry (EDS) was also performed. To prepare the samples for transmission electron microscopy (TEM, Talos F200x, Thermo Fisher Scientific, Waltham, MA, USA), the hot-rolled samples were cut and mechanically ground to a thickness of 40 µm. Small discs with a diameter of 3 mm were cut and then thinned by twin-jet electropolishing (TJ100-BE, Thermo Fisher Scientific, Waltham, MA, USA) at −30 °C and 40 V. The electrolyte was a mixture of 20% perchloric acid and anhydrous alcohol. The crystal structure and the second phase were analyzed by TEM. X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, Kratos, Manchester, UK) was utilized to examine the oxide composition and content. The distribution of Fe, Cr, Cu, O, and C in the oxide layers after 240 h of exposure of the sample was measured using glow discharge optical emission spectrometry (GD-OES, GDA 750HP, Spectruma Analytik GmbH, Hof, Germany), with a light-source power of 200 W and a sputtering rate of 2 μm·min⁻¹. The samples’ chemical composition and surface roughness were analyzed using EDS and atomic force microscopy (AFM, Dimension 3100, Vecco, Plainview, NY, USA), respectively. Cross-sectional comparisons of the oxide layers, exposed for 240 h in both environments at 800 °C, were performed using EDS.

3. Results

3.1. SP2215 Microstructure

The microstructure of SP2215 steel after hot-rolling treatment is shown in Figure 1, revealing a typical austenitic morphology, with an average grain size of 20 approximately µm. The precipitate phase particles were uniformly distributed in the matrix, whereas the different spherical sizes of these precipitated phases indicated the existence of multiple compositions. According to the EDS results in

Table 2. Compositions of the particles indicated by the red cross symbol in Figure 1 (wt%).

Position	C	N	Cr	Ni	Cu	Nb	Fe	V
A	7.26	5.69	24.17	0.54	-	44.55	5.46	2.14
B	2.78	-	25.77	14.27	3.11	-	54.07	-
C	12.1	7.24	20.59	2.32	-	28.12	12.85	1.31
D	3.93	-	22.79	14.76	3.44	-	55.08	-

, the small particles located at the grain boundary were detected as Cr₂₃C₆ phase, whereas the particles of uneven size enriched with Nb and C or N were distributed among the grains and grain boundaries. In addition, annealing twins present in the steel signified good mechanical properties.

To observe finer microtopography, TEM was used, and the results are shown in Figure 2. A gray particle (marked by the wire frame) was detected, and the diffraction patterns and EDS results showed that the selected precipitate is rich in Nb and possesses face-centered cubic structures. In light of previous studies, these sediments were determined to be the primary carbon nitride MX phase [14]. The smaller carbon nitride formed during solution aging and was considered an important precipitated phase, improving the alloy’s high-temperature strength. On the contrary, the larger size of MX contributed to crack formation, reducing the strength of the alloy. According to the EDS result, the Cu element distributed uniformly in the matrix. It was reported that the Cu-rich precipitates were in a completely coherent form after high-temperature aging, which effectively improves the thermal strength of materials [14,31,32,33].

3.2. Oxidation Weight Gain of the SP2215

Table 3. Weight gain per area ΔW of SP2215 in the steam and atmosphere.

	24 h	72 h	144 h	192 h	240 h
S-650	0.058	0.087	0.087	0.097	0.127
S-700	0.084	0.130	0.158	0.186	0.186
S-750	0.119	0.209	0.228	0.248	0.278
S-800	0.138	0.265	0.285	0.305	0.314
A-650	0.086	0.115	0.172	0.200	0.229
A-700	0.120	0.179	0.210	0.240	0.270
A-750	0.179	0.298	0.447	0.626	0.715
A-800	0.501	0.943	1.562	1.827	2.004

Note: S is the steam environment, and A is the atmospheric environment.

lists the values of weight gain per area ∆W of SP2215, which were collected periodically by weighing the specimens. The mass–gain curves in the steam and atmospheric environments for four temperature series of 650, 700, 750, and 800 °C are shown in Figure 3. As expected, the oxidation weight gains of SP2215 increased with the oxidation time or temperature in both environments. At lower temperatures, mass gains in both environments were small. However, at higher temperatures (750–800 °C), oxidation mass gain in the atmosphere showed a large increase, whereas the weight gain in the steam was more stable. Moreover, the mass gain of the samples in the steam was smaller than that in the atmosphere in each temperature series after 240 h of exposure.

Upon calculation, the average oxidation rates of SP2215 after 240 h of oxidation at 650, 700, 750, and 800 °C were 5.32 × 10⁻³, 8.19 × 10⁻³, 1.15 × 10⁻², and 1.31 × 10⁻² g·m⁻²·h⁻¹ in the steam environment and 9.825 × 10⁻³, 1.105 × 10⁻², 2.947 × 10⁻², and 8.351 × 10⁻² g·m⁻²·h⁻¹ in the atmospheric environment, respectively. According to GB/T 13303-91 standards [34], the average oxidation rates under these conditions were less than 0.1 g·m⁻²·h⁻¹, indicating that, at the 650–800 °C temperatures used in the steam and atmospheric environments, SP2215 belongs in the complete oxidation-resistant category.

3.3. Oxidation Kinetics

On the basis of the weight gain curves shown in Figure 3, the oxidation kinetics of SP2215, as described in Equation (1) [35,36], follow a parabolic trend with a rate constant of 2, with diffusion-controlled layer growth by a constant diffusion coefficient, in both environments examined in this study.

$${\left(∆W\right)}^2=k_p\times t$$

(1)

where ∆W is the weight gain per unit area (g·cm⁻²), t is the oxidation time (h), and k_p is the oxidation rate constant (g²·cm⁻⁴·h⁻¹). The linearity of (ΔW)² versus t of the SP2215 samples in the atmosphere and steam are shown in Figure 4a,b, respectively. The kinetic coefficient k_p follows the Arrhenius relationship, as shown as follows:

$$k_p=k_0{e}^{\frac{-Q}{RT}}$$

(2)

where Q is the activation energy (kJ·mol⁻¹), R is the universal gas constant (8.314 J·K⁻¹·mol⁻¹), T is the temperature in Kelvin (K), and k₀ is the pre-exponential constant. Taking the logarithm of both sides of Equation (2) yields.

$$\mathrm{l}\mathrm{n}{k}_p=-\frac{Q}{R}\times \frac{1}{T}+\mathrm{l}\mathrm{n}{k}_0$$

(3)

Regression analyses were performed using Equation (3), using 1/T as the independent variable shown in Figure 4c,d. This process yielded values of k₀, followed by Q. The rate constants and activation energies derived from this analysis are listed in

Table 4. Parabolic oxidation rate constants in the steam and atmosphere.

	ln(k0)	Q (kJ/mol)
Steam	16.49 ± 1.63	116 ± 13.4
Atmosphere	0.60 ± 6.75	242 ± 55.8

. The results show that the values of k_p increased with the temperature in both oxidation environments. This result accords with the general condition that the oxidation rate constant increases with temperature. In addition, the k_p values of SP2215 in the atmospheric oxidation environment were larger than those in the steam oxidation environment at four series of temperatures. Hence, the steam environment controls the oxidation rate of the material surface, which may be affected by different reaction mechanisms in various environments.

3.4. Surface Morphology of the Oxide Layers

The surface morphologies and element mapping of the original SP2215 and oxide films formed in the steam are shown in Figure 5. According to the EDS result, the elements were uniformly distributed in the matrix. Grain boundaries were evident after oxidation at 650–700 °C, as shown in Figure 5b,c. According to the elemental distribution maps in Figure 5 and the XRD results in Figure 10, the burr oxide at the grain boundary was Cr₂O₃, and the spinel oxide within the grain was Fe₂O₃. When the steam temperature increased to 750 °C, the burr oxide disappeared, and the spinel oxide covered the inside of the grain. The distribution of the oxide and the grain was the same, and the crystal boundary between the oxide film and the heat-resistant steel matrix was evident. When the steam temperature increased to 800 °C, the oxide film completely covered the substrate, and the surface density was high and relatively flat. This smooth surface was conducive to the release of thermal stress and growth stress in the oxide film, thereby reducing the tendency of crack formation in the oxide film. The EDS analysis showed that Cr and Mn were diffused from one grain boundary to another, and Fe followed a similar diffusion pathway from one grain boundary to another.

The surface morphology and element mapping of the original SP2215 and oxides formed at 650–800 °C for 240 h are shown in Figure 6, where oxide particles were distributed on the base scale and no cracking and spalling were present. The EDS mapping analysis revealed that the oxide film contained Fe, Cr, Mn, Ni, O, and few Cu elements. At low temperatures (650–700 °C), a thinner compact oxide film, which comprised fine granular oxides, was formed. As the temperature increased, the oxidized particles became progressively larger and coarser with agglomeration. After oxidation at 800 °C, the surface was completely covered by iron oxide, and the surface oxide was relatively loose between the oxides. Furthermore, the proportion of Fe and Cr in the oxides decreased, whereas the proportion of Cu of increased, indicating that Cu accelerated diffusion and began to enrich at 800 °C.

3.5. GD-OES of the Oxide Layers

The GD-OES depth profiles after 240 h of oxidation in the steam at 650–800 °C are shown in Figure 7. The test results indicated that the diffusion mechanism of Fe, Cr, Mn, and O in the oxide film varied between different temperatures. At 600 °C, the outermost layer of the oxide film mainly consisted of Fe oxides and carbides, and the inner layer contained Cr oxides. At 700 °C, the content of Cr increased rapidly, and the Cr and Fe oxides were distributed in medium proportions in the oxide layer. At 750 °C, the outer layer of the oxide film was mainly Fe oxide, and Cr was almost only present inside the iron oxide film. At 800 °C, the oxide layer structure changed from being monolayer to being bilayer, in which the outer layer was Fe-rich and the inner layer was Cr-rich. This characteristic played a key role in inhibiting the inward diffusion of O and the outward diffusion of Cr. Mn was enriched in the middle of the inner and outer layers, whereas Cu was not found in the oxide layer in the steam environment.

As shown in Figure 8, the oxide layer of SP2215 formed in the atmosphere at 650–800 °C and had different depth distribution results of elements (Cr, Fe, Mn, Cu, C, and O), mainly in the content and distribution of Cr, Fe, and O. At 650 °C, Cr, Fe, O, Cu, and C were enriched on the surface. The thickness of the oxide layer was about 0.5 μm. Given that the free energy of Cr was less than that of Fe, a thin protective film of Cr oxide quickly formed on the surface of the material, hindering the further occurrence of the reaction. As the temperature increased (700–750 °C), the distribution of elements in the oxide layers changed, and the content of Cr and Cu increased obviously when the oxide layer became thicker. The Cr precipitation rate became faster, and a Cr-rich layer was thus formed on the matrix surface, with a Cr content of 50%–60%. At 800 °C, the Cr content in the oxide layer decreased, which may be affected by a large increase in the Fe oxide content. Generally, the iron content gradually increased with the temperature. Moreover, the oxide layers in all temperature series showed varying levels of Cu, especially at 800 °C (Figure 8d), where the Cu content in the oxide layer was as high as 10%. The distribution positions of the Cu-rich layer and the Cr₂O₃ oxide layer partially overlapped.

3.6. Cross-Sectional Microstructure

The cross-sectional morphologies and element mapping of the original SP2215 and the oxide layers in the steam and atmosphere environments at 800 °C after 240 h of oxidation are shown in Figure 9a, Figure 9b, and Figure 9c, respectively. A significant oxidation layer was not observed in the original sample and the observation section is relatively smooth than that after high-temperature oxidation. As for the steam environment, Figure 9b shows a heterogeneous thin scale containing cracking and oxidation pits. The oxide layers formed at high temperatures (800 °C) contained mainly more corrosion cavities. This may provide a channel for the diffusion of O in the oxide film, and the antioxidant performance may be decreased. A thick, uniform, and continuous oxide layer in the atmosphere formed on the steel surface, as shown in Figure 9c. The EDS mapping indicates that the oxide layers on the steel mainly contained Cr and Fe oxides at 800 °C. Oxygen diffused through the voids into the substrate causing local internal oxidation, which loosened the oxide layer. A small amount of Cu oxides were also found on the outer surface of the oxide layers.

3.7. Composition of Oxide Layers

The XRD patterns of SP2215 after oxidation in the service environments at 650–800 °C and of the original material are shown in Figure 10. The original material shows only austenite peak and iron chromium oxide was undetected. The detection of the matrix phase in the metal implies that the analysis covers the entire oxide layer. Analysis of patterns reveals the dominant Cr₂O₃ scale on all sample surfaces, even at lower temperatures of oxidation. The intensity of Cr₂O₃ gradually increased with the oxidation temperature in both environments, implying the gradual thickening of the Cr₂O₃ scale. Correspondingly, in the atmospheric oxidation environment, the peak strength of the alloy matrix declined after oxidation due to the gradual thickening of the oxide layer. In addition, FeCr₂O₄ was observed after oxidation at high temperatures in the atmospheric environment, whereas Fe₂O₃ and Fe₃O₄ were observed after oxidation at similarly high temperatures in the steam environment.

To further investigate the oxidation process of SP2215 in the steam and atmospheric environments, the high-resolution Fe2p, Cr2p, and O1s peaks of the XPS spectra at 800 °C were characterized, as shown in Figure 11. In Figure 11a, the broad spectrum of Fe 2p was due to chemical shift, with 2p_3/2 (710.1 and 711.4 eV) and 2p_1/2 (723.7 and 725.0 eV) attributable to Fe²⁺ and Fe³⁺, respectively. In addition, the satellite peak (716.1 and 719.4 eV) in the center further verified the presence of Fe²⁺ and Fe³⁺ [37]. The content of Fe²⁺ to Fe³⁺ was about 1:2, indicating that Fe₂O₃ and Fe₃O₄ were formed in the passivation film at 800 °C. Similarly, as shown in Figure 11d, the spin–orbit–split spectrum of Fe2p comprised 2p_3/2 (709.8 and 711.6 eV), 2p_1/2 (723.4 and 725.2 eV), and the satellite peak (715.8 and 719.6 eV) in the center, which could obviously prove the presence of Fe²⁺ and Fe³⁺ in the material [37]. The content of Fe²⁺ to Fe³⁺ was about 1:1. In combination with Cr2p and O1s peaks, the Fe₂O₃ and FeCr₂O₄ phases were present in the oxidized SP2215 passivation film at 800 °C [38].

Cr were detected in both environments. Noticeably, the peaks located at 586.4 eV and 576.6 eV could be associated with the characteristic signal of Cr 2p_1/2 and 2p_3/2, respectively (Figure 11b). These peaks indicated the presence of Cr₂O₃ bond [39]. The FeCr₂O₄ produced in the atmosphere was composed of equal proportions of FeO and Cr₂O₃ (Figure 11e). Therefore, the oxidation film in both environments contained Cr₂O₃.

In the steam environment at 800 °C (Figure 11c), the O1s indicated the four deconvoluted peaks (~530.2, ~531.7, ~532.7, and ~533.8 eV) associated with O²⁻, C–O, C=O, and H₂O (adsorbed) bonds, respectively [37,40,41,42]. The most noticeable ingredient was the proportion of O²⁻ in the oxidation film. The O²⁻ peak was attributed to the oxides in the SP2215 oxidation film. Furthermore, O1s split into four peaks at 800 °C in the atmospheric environment (Figure 11f). Particularly, the spectra of O1s displayed two peaks at ~528.5 eV and ~531.0 eV, which were associated with CuO and Cu₂O, respectively [43]. In conjunction with the SEM energy spectrum results (Figure 6d), CuO and Cu₂O were present in the obtained material surface.

3.8. AFM Topography

Detailed three-dimensional AFM topography and the average value of the surface roughness parameter (Ra, nm) are displayed in Figure 12 and Figure 13. They reveal the evolution of the geometric characteristics of the surface oxidation product of alloy SP2215 when exposed to steam and atmosphere at 650, 700, 750, and 800 °C. Figure 12a–d depict the transition of burr oxides in the steam. The surface roughness increased significantly at first from 650 to 700 °C. This phenomenon could be attributed to the rapid formation and the predominantly longitudinal growth of the metal oxide, which agrees well with Figure 12a,b. From 700 to 750 °C, the surface roughness overtly decreased, which was likely due to the gradual formation of Fe-rich protective external layer scale. The AFM topographies (Figure 13a–d) clearly depict a transition of granular oxides in the atmospheric environment. As the temperature increased from 650 to 700 °C, an intuitive growth in the surface roughness was observed, and then it remained almost unchanged. In all probabilities, as the temperature increased, the oxide particles gradually became larger, coarser, and aggregate from 650 to 700 °C. As shown in Figure 13c,d, the surface of the gathered oxide particles had material attachment, which could potentially be affected by iron ions diffused from the matrix.

4. Discussion

According to the results of the oxidation dynamics curves, SP2215 steel demonstrated excellent oxidation resistance at 650–800 °C in the steam and atmosphere. However, the oxidation characteristics in the two environments were extremely different, and the composition of oxidation products also varied gradually with temperature. To elucidate the oxidation mechanism of SP2215 steel, the corresponding mechanism diagram is summarized in Figure 14. The evolution of the oxidation process with temperature under steam and atmosphere is as follows.

First, in the steam environment, intergranular selective oxidation was evident, especially at relatively low temperatures (Figure 5). A thicker layer of Cr oxide accumulated on the grain boundaries, whereas thinner Fe oxide was found within the grains at 650 °C. This phenomenon could be attributed to the special oxidation environment. In the steam environment, the supercritical water molecules worked instead of oxygen. The supercritical water molecules decomposed to generate oxygen anions, which further combined with metal atoms to generate oxide crystal nucleuses [44]. On the one hand, on the basis of the oxidation kinetic curves (Figure 3), the oxidation rate in the steam was slower than that in the atmospheric environment. This may suggest a slower diffusion rate of supercritical water molecules through the interface. Meantime, the energy at grain boundaries is higher than those within the grains. The limited supercritical water molecules would likely absorb preferentially at more active sites, that is, the grain boundaries. On the other hand, the alloy element of Cr would be enriched and precipitate in the grain boundaries at a lower oxidation temperature. The quick diffusion path of Cr through the grain boundaries easily formed. Therefore, Cr oxidized vertically along the grain boundaries, and the Cr₂O₃ rapidly formed at the grain boundaries. Simultaneously, Fe₂O₃ slowly formed at the less active sites, that is, within the grains. Here, Cr₂O₃ was longitudinally deposited at the grain boundaries, whereas Fe₂O₃ transversely diffused within the grains.

When the temperature reached 700 °C, the diffusion rate of Cr along the grain boundaries increased, and the reaction between O and Cr accelerated; thus, the difference in intergranular oxidation intensified. Selective oxidation was more evident (Figure 5c). Oxidation reaction further intensified with the increase in temperature. At 750 °C, due to the consumption of Cr, the oxidation rate of Cr declined, whereas that of iron continued to accelerate with enough content. The accumulation of oxide production in grains exceeded the amount in grain boundaries, which opposes the morphological characteristics at lower temperatures. At 800 °C, Fe oxide was dominant and formed a double-oxide structure of an inner-layer Cr oxide and outer-layer iron oxide in the grain boundaries. Surface roughness was also significantly reduced, as shown in Figure 12. At the same time, the high temperature prompted the reaction to produce massive Fe₃O₄ (Equation (4)). As the consumption of the supercritical water molecules and the accumulation of hydrogen in the oxidation process lowered the potential at the interface, the double-layer structure became more stable, obstructing the diffusion of metal ions, thereby increasing the material’s oxidation resistance.

$$3\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)={\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)$$

(4)

Second, in the atmospheric environment, circumstances differed, and intergranular oxidation did not occur. At relatively low temperatures (650–700 °C), the oxygen was adsorbed uniformly on the surface by van der Waals force, which could promote the diffusion of metal elements outside the matrix. The probability of oxidation at the whole interface was equal, whereas Cr₂O₃ would be preferentially generated on the surface due to ${∆\mathrm{G}}_{{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3}$< ${∆\mathrm{G}}_{{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3}$ during the oxidation process. Despite the availability of quick diffusion path of Cr through the grain boundary, Cr oxide would still grow horizontally along the matrix–oxide interface to form a protective Cr₂O₃ layer. Under these circumstances, the lower temperature resulted in Cr forming a thin oxide film on the surface, preventing further oxidation of the substrate. Moreover, oxygen diffused outside the surface toward the matrix–oxide film interface, resulting in only a small amount of iron in the oxidation process forming Fe₂O₃. The following reactions occur at this stage:

$$4\mathrm{C}\mathrm{r}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)={2\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)$$

(5)

$$4\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)={2\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)$$

(6)

As the temperature increased (750 °C), the reaction rate significantly accelerated. Given the total Cr content of 22.8% in SP2215 steel, Cr could not sufficiently form a dense and continuous oxidation film in a short time [45]. Therefore, Fe and Cr oxidation occurred concurrently instead of being dominated by Cr, creating gaps in the oxide products at higher temperatures, which laid the foundation for the migration and further oxidation of other elements. As shown in Figure 8, with the increase in temperature, the Cr oxide film thickened significantly, and the entire oxide layer contained part of Fe oxide. Oxygen diffused within the oxide film and reacted with Fe and Cr, which can be expressed as follows:

$${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)=\mathrm{F}\mathrm{e}{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_4\left(\mathrm{s}\right)$$

(7)

With the temperature continually increasing (800 °C), the diffusion rate of Fe, Cr, and O from the matrix constantly accelerated, and the rapid growth of Fe–Cr spinel oxide led to the formation of pores in the oxide film, which further intensified the reaction [23]. Cu was distributed in the outer layer at 650–750 °C in a granular form, which did not significantly affect the denseness of the oxide film. However, as shown in Figure 8 and Figure 9, at temperatures up to 800 °C, higher levels of Cu oxide were evenly distributed in the oxide layer, which may result in a looser oxide film.

5. Conclusions

This study aims to investigate the oxidation resistance and the oxide behavior of a new heat-resistant steel SP2215 in steam and atmosphere environments at 650–800 °C for 240 h. The experimental results led to the following conclusions.

• The isothermal oxidation kinetics of SP2215 in the steam and atmospheric environments at 650–800 °C agrees with the parabolic law and belongs to the oxidation resistance category. The oxidation activation energy of SP2215 is estimated to be 242 ± 55.8·kJ·mol⁻¹ and 116 ± 13.4 kJ·mol⁻¹ in the atmospheric and steam environments, respectively.
• In the steam environment, supercritical water molecules preferentially absorb at active grain boundaries, prompting Cr to diffuse rapidly through the grain boundary. As a result, Cr₂O₃ is rapidly deposited longitudinally at the grain boundary. Simultaneously, Fe₂O₃ is transversely diffused slowly within the grain. This process results in intergranular selective oxidation. With the increase in temperature, the phenomenon initially intensifies, then reverses with the depletion of Cr. Finally, Fe oxide predominates, forming a double oxide structure with an inner layer of Cr oxide and an outer layer of iron oxide in the grain boundary.
• In the atmospheric environment, oxygen is adsorbed uniformly on the surface. Given ${∆\mathrm{G}}_{{\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_3}$ < ${∆\mathrm{G}}_{{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3}$ and the quick diffusion path of Cr through the grain boundary, Cr oxide is preferentially generated and grows horizontally along the matrix–oxide interface to form a protective Cr₂O₃ layer. At the same time, only a small amount of iron participates in the oxidation process to form Fe₂O₃. With the increase in temperature, Cr is consumed gradually, and the quantity of iron oxide within the oxide layer increases correspondingly.
• We focused on the oxidation behavior of SP2215 in this study, but the detailed heat transfer characteristics under A-USC conditions are still unclear and methods for calculating metal temperatures with high accuracy are lacking [46]. Long-term testing verification (>1000 h) is needed for SP2215. In addition, different types of oxidation gas, such as SO₂, are needed to explore its high-temperature oxidation behavior.