High temperature oxidation behavior and mechanism of FeXCr0.5Ti ferritic stainless steels

In order to investigate the oxidation mechanism of ferritic stainless steel during long-term oxidation at high temperature. The oxidation behavior of Fe-Cr-Ti ferritic stainless steels with 10.38 wt% Cr and 17.41 wt% Cr at 800 °C and 900 °C for 100 h was studied by a constant temperature weight gain method. The morphology and composition of the oxide film were characterized by SEM, EDS and XRD. The experimental results indicate that the oxygen element mainly diffuses inward at 800 °C for two stainless steels, and the oxide film is composed of (Cr1.3Fe0.7)O3 + MnCr2O4. When the temperature rises to 900 °C, metal element mainly diffuses outward, and Fe2O3 outer oxide layer and Fe rich Fe-Cr inner oxide layer are formed in Fe11Cr0.5Ti stainless steel; Cr2O3 + Cr rich M3O4 spinel oxide film is formed in Fe18Cr0.5Ti stainless steel, while the inner layer is composed of SiO2. The main reason for the significant decrease of oxidation resistance of Fe11Cr0.5Ti stainless steel is that the low content of Cr cannot form a Cr rich oxide layer to inhibit the outward diffusion of Fe element, and the stability of oxide film is poor to protect the matrix.


Introduction
In the last decades, with the rapid increase of stainless steel demand, nickel resources are becoming scarce, and the traditional austenitic stainless steel is gradually replaced by nickel-free ferritic stainless steel [1][2][3]. Ferritic stainless steel exhibits excellent corrosion resistance and high temperature oxidation resistance, with low cost, high thermal conductivity and small thermal expansion coefficient, and is therefore widely used in automobile, energy, home appliances and other fields. Hence, it has become an energy-saving green stainless steel material [4][5][6][7][8].
High temperature oxidation resistance is one of the important indexes of stainless steel in high temperature service [9,10]. Ferritic stainless steels with different chemical compositions are distinct from oxidation products and oxide film structures, and therefore the causes of high temperature failure are also different. In recent years, material researchers have done a lot of research work on the oxidation behavior of stainless steel. Vicente et al [11] studied the oxidation kinetics of EN1.4509 ferritic stainless steel at high temperature with different time. By comparing the surface morphologies of stainless steel after oxidation for 8 min and 24 h, it is found that the oxidation products grow in an octahedral shape, and the particle size of oxidation products increases with the extension of oxidation time. Serra et al [12] studied the growth kinetics and composition of the film formed on the surface of stainless steel after short-term oxidation at different temperatures. It is proved that the oxide film is mainly composed of Cr element on the surface of stainless steel after oxidation at 850°C for 2 h. The oxide film is mainly composed of Fe element above 900°C, and the contents of Ti, Si and other elements are low, and the oxidation rate rises with increasing the temperature. Salgado et al [13] conducted a comparative study on the oxidation behavior of AISI441 ferritic stainless steel at high temperature and different oxygen partial pressure for 50 h, and found that the growth of oxide film on the surface of stainless steel followed the parabolic law under all test conditions. The oxidation resistance of stainless steel in synthetic air is higher than that in 1 ppm O 2 +Ar atmosphere. In the synthetic air at 850°C, there is no continuous and dense oxide film on the surface of stainless steel, and the oxidation products are mainly composed of MnCr 2 O 4 spinel and TiO 2 . The content of Cr in oxide film increases with the increase of temperature. Yan et al [14] studied the oxidation behavior of 9Cr5Si ferritic heat-resistant steel after 90 h cyclic oxidation in air at high temperature, and found that the value of oxidation rate constant K p changed by an order of magnitude with the increase of temperature, and the oxidation rate increased significantly. The phase in the outer layer of the oxide film does not change and is composed of Cr 2 MnO 4 +Cr 2 O 3 phase.
Exploring the high temperature oxidation behavior of ferritic stainless steel for a long time is not only of great significance for theoretical research, but also provides reliable data support for its application in the manufacturing of hot end components of exhaust system. In this work, the oxidation behavior of two Fe-Cr-Ti ferritic stainless steels with different Cr contents was studied at 800°C and 900°C for 100 h so as to investigate the oxidation kinetics, analyze oxidation products and oxide film structure and reveal the oxidation mechanism.

Experimental methods
A Fe-Cr-Ti ferritic stainless steel plate with 10.38 wt.% Cr and 17.41 wt.% Cr was used as an experimental material in this study, and the thickness of the plate was 1.2 mm and the chemical composition was given in table 1. The oxidation test sample was in a rectangular shape with the size of 60 mm×30 mm cut by a shear plate. In order to avoid the error caused by the collapse of oxide scale on the surface of the sample during the experiment, a cylindrical corundum crucible with a diameter of 50 mm was selected to hold the sample. The data weighed before and after the experiment were the total mass of the samples and crucible. Before the oxidation experiment, the sample and corundum crucible should be pretreated to ensure that there was no oxide scale on the surface of the sample and the weight of the crucible was constant during the whole oxidation process. Firstly, the sample was polished with 600 # , 1000 # , 1500 # , 2000 # SiC water sandpaper to remove the oxide scale on the surface. Then, the sample was ultrasonically cleaned in ethanol for 5 min and dried by a cold air blower. Finally, the sample was dried in a 150°C-200°C drying oven. After 1 h, the sample was taken out and put into a drying dish and cooled to room temperature for standby. The corundum crucible needed to be roasted repeatedly in a 1000°C resistance heating furnace to remove the moisture and impurities until its weight was constant, and then put it into a dryer for standby. A micrometer was used to accurately measure the surface size of the cooled sample, and the surface area was calculated. The total weight of sample and corundum crucible before and after high temperature test was weighed by a BS124S electronic analytical balance. The experimental equipment was SX-4-10 box resistance furnace. The temperature difference between the furnace and digital display temperature was ±1°C.
This experiment was a continuous constant temperature oxidation experiment, and the oxidation rate was measured by a discontinuous weighing mode. The oxidation temperature was 800°C and 900°C, the longest oxidation time was 100 h, and the weighing time was 5 h, 10 h, 15 h, 20 h, 30 h, 50 h, 75 h and 100 h. Three parallel samples were selected at each temperature and weighing time, and each sample was weighed three times to take the average value. After the oxidation test, the oxidation weight gain per unit area was calculated, the oxidation kinetics curve was drawn, and the oxidation kinetics model was analyzed.
The surface morphologies of the samples after high temperature oxidation were characterized by a GeminiSEM 300 field emission scanning electron microscope, and the surface element distribution was measured by EDS. An XRD-7000 x-ray diffractometer was utilized to analyze the phase of the oxide film on the surface of the sample after high temperature oxidation. The x-ray tube was irradiated by Kα of Cu target, the tube voltage was 40 kV, the tube current was 45 mA, 2θ equaled to 20°-100°, and the step size was 4 (°)/min.

Results and analysis
3.1. Qxidation kinetics analysis Figure 1 shows the oxidation kinetics curve of Fe11Cr0.5Ti and Fe18Cr0.5Ti stainless steels after continuous oxidation in air at 800°C and 900°C for 100 h. The oxidation kinetics curve of Fe11Cr0.5Ti ferritic stainless steel at 800°C is similar to parabolic law. The oxidation kinetics curve at 900°C is a parabola in the first 15 h, while in the last 85 h the oxidation weight gain is significantly accelerated, showing a linear characteristic. After 100 h continuous oxidation, the weight gain is 110.62 mg·cm −2 , which is 84 times of 1.12 mg·cm −2 at 800°C. This indicates that Fe11Cr0.5Ti stainless steel has good oxidation resistance at 800°C, abnormal oxidation occurs at 900°C, and oxidation weight gain increases significantly. The results show that the Fe18Cr0.5Ti stainless steel with high Cr content has no rapid oxidation phenomenon in the process of continuous oxidation at 800°C and 900°C, and the oxygen gain is 0.335 mg·cm −2 and 1.31 mg·cm −2 after oxidation for 100 h, respectively. The oxidation kinetics curve is parabolic. By comparing the oxidation kinetics curves of two ferritic stainless steels at the same temperature, it is found that the oxidation weight gain of Fe11Cr0.5Ti ferritic stainless steel at the end of oxidation is higher than that of Fe18Cr0.5Ti stainless steel, which indicates that the increase of Cr content can improve the high temperature oxidation resistance of Fe-Cr-Ti ferritic stainless steel in air. When the oxidation kinetics curve of metal materials conforms to the parabolic law, the growth process of oxide film is related to the diffusion of elements. Metal cations generated at the metal/oxide interface diffuse outward through the oxide layer, O 2 is reduced to O 2at the air/oxide interface, and O 2ions diffuse inward through the oxide/metal interface. Oxidation is controlled by cation diffusion and/or oxygen through the oxide film. The film has good oxidation resistance [15]. According to the Kofstad [16] and Arrhenius formula, the oxidation kinetics model is established as shown in equation (1), the relationship between the oxidation rate constant and the activation energy of steel grade is shown in equation (2), and the logarithm of the left and right sides of the equal sign of equations (1)-(2) is taken as shown in equations (3)-(4).
Where, Δm is the oxidation weight gain per unit area (mg·cm −2 ), n is the oxidation rate index, t is the oxidation time (h), K p is the oxidation rate constant (mg 2 ·cm −4 ·h −1 ), K 0 is the constant, Q is the oxidation activation energy (kJ·mol −1 ), T is the oxidation temperature (K), and R is the gas constant, which is 8.314 J·mol −1 ·K −1 .
Regression analysis was carried out on the experimental results in accordance with the parabolic law in the oxidation kinetics curve by using equations (3)-(4), and the n, K p and Q values of two stainless steels at different temperatures were obtained. The results are shown in figures 2, 3 and table 2. It can be seen that with the increase of temperature, the oxidation rate constant K p value of two stainless steel at 900°C is higher than that at 800°C, and the oxidation resistance decreases; at the same temperature, the higher the Cr content of stainless steel is, the closer the oxidation rate index n value approaches 2, the smaller the oxidation rate constant K p value is, the greater the oxidation activation energy Q value is, and the higher the high temperature oxidation resistance is. The oxidation kinetics curve is affected by many factors, such as the porosity of the oxide layer, the adhesion between the oxide layers, the defects on the surface of the oxide layer, etc, so the parabolic law only exists under ideal conditions [17]. The closer the oxidation rate index n approaches 2, the more the oxidation kinetics curve is close to parabola, and the better the oxidation resistance is.  Figure 4 shows the XRD analysis results of surface oxidation products of Fe11Cr0.5Ti and Fe18Cr0.5Ti stainless steels after continuous oxidation at different temperatures for 100 h. The results show that the oxide film of Fe11Cr0.5Ti stainless steel is thinner at 800°C, the diffraction peak of Fe-Cr matrix is obvious, and the main oxidation product is MnCr 2 O 4 +(Cr 1.3 Fe 0.7 )O 3 . While the oxide film is thicker at 900°C, the peak of Fe-Cr matrix disappears completely, and the oxidation product transforms into pure Fe oxide Fe 2 O 3 . This shows that the oxidation resistance of Fe11Cr0.5Ti stainless steel decreases sharply with the increase of temperature. When a thick Fe 2 O 3 oxide layer is formed on the surface of the sample, the x-ray cannot penetrate, but it is not dense and has no ability to protect the matrix. At high temperature, the adhesion of Fe 2 O 3 oxide layer is weak, and a  Steels The increase of Cr content does not change the element type of oxidation products, but changes the oxygen content. The ratio of the strongest peak of the product to the strongest peak of the matrix was 0.37, 0.17 and 0.76, respectively. On the other hand, the increase of Cr content will reduce the thickness of oxidized stainless steel surface and increase the oxidation resistance of stainless steel at 800°C; Fe18Cr0.5Ti stainless steel has better oxidation resistance at 900°C and below, and the oxidation resistance decreases with the increase of oxidation temperature. Figure 5 shows the surface morphology and element distribution of Fe11Cr0.5Ti and Fe18Cr0.5Ti stainless steels after continuous oxidation at different temperatures for 100 h, and table 3 shows the EDS analysis results of stainless steel surface oxidation products. Figures 5(a) and (b) are the surface morphology and element distribution of Fe11Cr0.5Ti stainless steel after oxidation at 800°C for 100 h. There are many fine oxide particles on the surface of the sample. The energy spectrum results show that the surface oxide layer is mainly rich in Cr and Fe elements, and the oxidation products are corundum type Cr-Fe oxide and nodular Cr-Mn oxide. In addition, large white granular oxides can be observed in the grain or at the grain boundary. These oxides are rich in Ti and mainly Ti oxides. Nodular Cr-Mn oxide is composed of spinel like Cr-Mn oxide particles, which cover the surface of the matrix to isolate the metal from the air and slow down the oxidation process. Figures 5(c) and (d) are the surface morphology and element distribution of Fe18Cr0.5Ti stainless steel after oxidation at 800°C for 100 h. Compared with Fe11Cr0.5Ti stainless steel, the oxide film is more compact, there are few nodular oxides with larger particles at the grain boundary, and the oxide particles in the grain boundary are smaller. The energy spectrum analysis results show that the surface of the oxide film is mainly rich in Cr and Fe elements, and the grain boundary is mainly Cr-Mn oxides. The content of Cr is higher than that of Fe11Cr0.5Ti stainless steel. When the two stainless steels were continuously oxidized at 800°C, the surface oxide film did not peel off, and both had good oxidation resistance. Fe11Cr0.5Ti stainless steel was worse than Fe18Cr0.5Ti stainless steel,  After 100 h oxidation of Fe18Cr0.5Ti stainless steel, the oxide film is composed of compact Cr/Cr-Mn granular oxides, which is consistent with the results of XRD analysis, and the Fe18Cr0.5Ti stainless steel has good protection effect. It can be seen from figure 5 that with the increase of temperature, the high temperature oxidation resistance of the two stainless steels decreases, and the Cr/Mn spinel oxide formed on the surface of Fe18Cr0.5Ti stainless steel will aggregate and grow up, which will reduce the protection effect of oxide scale on the matrix, thus reducing the oxidation resistance. The distribution of cross-section elements in the oxide layer can reflect the oxidation resistance of stainless steel and explain the formation mechanism of oxide film. Figure 6 shows the cross-section morphologies and element distribution including O, Cr, Mn, Fe, Ti and Si of the oxide products of Fe11Cr0.5Ti and Fe18Cr0.5Ti ferritic stainless steels after constant temperature oxidation at different temperatures for 100 h. By comparing the color depth of each element in the picture, the distribution of each element and the change of concentration gradient can be directly analyzed, which provides a basis for determining the composition of the composite oxide film. Figure 7 shows the oxide layer thickness of two stainless steels after constant temperature oxidation at different temperatures for 100 h. It can be seen from figures 6(a), (b) and 7 that the composition of the crosssection oxide film of the two stainless steels at 800°C is similar, the outer layer is Cr-Mn oxides, and the inner layer is Cr rich oxides. The thickness of the oxide film of Fe11Cr0.5Ti stainless steel is 2.26±0.208 μm, which is greater than that of Fe18Cr0.5Ti stainless steel by 1.54±0.277 μm, indicating that the stainless steel matrix with low Cr content is oxidized thicker and has low oxidation resistance. In addition, the above-mentioned results are in consistent with the kinetics curves. From figures 6(c), (d) and 7, it is found that when the temperature increases to 900°C, the oxide layer thickness of Fe11Cr0.5Ti stainless steel is 740±36.06 μm, and obvious delamination phenomenon occurs. The outer layer is pure Fe oxide layer, the inner layer is Fe rich Fe/Cr oxide layer, and a lot of cracks exist at the junction of the inner and outer oxide layers. The oxide film on the surface of Fe18Cr0.5Ti stainless steel and its interface are uneven, wedge-shaped and embedded into the matrix with a large depth. The outer layer is composed of Cr 2 O 3 +Cr rich Cr/Mn oxides, and the inner layer is composed of SiO 2 oxide. The total thickness is 2.52±0.259 μm. The existence of SiO 2 fills the cavity left in the matrix after Cr 2+ /Cr 3+ and Mn 2+ /Mn 3+ diffusing to the oxide film/air interface, prevents the aggregation of the cavity, inhibits the outward diffusion of Fe ions in the matrix, and finally reduces the oxidation rate, resulting in the formation of dense oxide film on the surface. By comparing the thickness and element distribution of oxide film cross section of stainless steel under different conditions, it is found that the oxide film of two stainless steels at 800°C and Fe18Cr0.5Ti stainless steel at 900°C are Cr rich oxide layers with thin thickness and less changes, which are closely combined with the matrix and have good oxidation resistance. The oxidation resistance is consistent with the results of XRD analysis. At 900°C, the matrix of Fe11Cr0.5Ti stainless steel is seriously corroded at high temperature, and a loose Fe 2 O 3 oxide film with a thickness of 370 μm is formed on the outer layer, which seriously reduces the high temperature oxidation resistance. According to the energy spectrum, the inner oxide is supposed to be Fe 2 CrO 4 spinel oxide. Due to the different structures of the inner and outer oxides, the difference of linear expansion coefficient is large, which is easy to produce thermal stress and lead to the

Discussion
In the early stage of high temperature oxidation, metal elements on the surface of stainless steel will react with oxygen element in the air to form corresponding oxides, but the type of oxides in the later stage of oxidation depends on the activity and content of metal elements. Cr 2 O 3 , Mn 2 O 3 , Fe 2 O 3 and other oxides will be formed in the initial oxidation stage of Fe-Cr-Ti ferritic stainless steel. Because of the element diffusion rate Cr>Mn>Fe and the high content of Cr, Cr will be selectively oxidized to form a dense Cr 2 O 3 protection film, and other metal elements are mainly internally oxidized or formed in the inner layer of the external oxidation layer. According to figures 8(a) and (b), the element depth distribution of Fe11Cr0.5Ti and Fe18Cr0.5Ti stainless steels after oxidation at 800°C for 100 h shows that the oxidation is mainly due to the inward diffusion of oxygen anions, and Cr/Mn elements are enriched from the surface to about 0.65 μm and 0.8 μm, respectively. In the range of 0.65 μm-2.2 μm and 0.8 μm-1.8 μm, Mn element content decreases and Cr/Fe elements content increase. The results show that the outer layer of the oxide film formed at 800°C is composed of (Cr 1.3 Fe 0.7 )O 3 +MnCr 2 O 4 , and the inner layer is composed of (Cr 1.3 Fe 0.7 )O 3 . With the increase of Cr content, the dense Cr 2 O 3 protection film will be formed preferentially, the growth rate of the oxide film will be reduced, and finally a thin oxide layer will be formed, which is consistent with the results in figure 6. In the initial stage of oxidation, oxygen element diffuses inward and reacts with surface metal elements to form This indicates that Cr element diffuses outward to form Cr 2 O 3 film in the initial stage of oxidation, and a certain number of small cavities are formed at the interface of matrix/oxide film. However, due to the low content of Cr element, Cr 2 O 3 oxide cannot diffuse from the matrix to the surface in time to fill the cavities, and the oxide film is gradually loose, which cannot inhibit the diffusion of Fe element to the interface of oxide film/air, resulting in a large number of voids. The adhesion between the original Cr 2 O 3 oxide scale and the matrix decreases and finally the oxide scale peels off. The Fe element reacts with the O element between the oxide film/air interface to form Fe/Cr oxides and a Fe rich oxide film with poor protection. With the increase of oxidation time, the oxidation rate increases, the unstable oxidation occurs, the pure Fe oxide film is formed on the surface, and the oxygen resistance decreases significantly. Due to the significant difference in the growth rate of internal and external oxide films, the interface between oxide layer and matrix moves fast, the internal oxidation zone/matrix front cannot be established, so no SiO 2 oxide layer is formed after 100 h continuous oxidation of Fe11Cr0.5Ti stainless steel. It can be seen from figure 8(d) that the outer layer of the oxide film is rich in Cr and Mn, in which the content of Cr is greater than that of Mn; the inner layer is a Si rich oxide layer with a thickness of about 0.3 μm. The results show that due to the high Cr concentration on the surface of Fe18Cr0.5Ti stainless steel, the critical Cr concentration required to form a continuous and dense Cr 2 O 3 oxide film can be met. In the initial stage of oxidation, Cr and a small amount of Mn rapidly diffuse outward to form a layer of Figure 7. Oxide scale thickness of two stainless steels oxidized at 800°C and 900°C for 100 h. Cr 2 O 3 +MnCr 2 O 4 +Mn 1.5 Cr 1.5 O 4 oxide film and improve the adhesion between the oxide film and the matrix. The growth mode of the oxide film is an embedded mode. With the increase of oxidation time, SiO 2 enters into the cavities caused by the outward diffusion of metal cations, which not only increases the adhesion between the matrix and the oxide film but also improves the oxidation resistance.
The high temperature oxidation resistance of stainless steel is still related to the critical strain e 0 of oxide film during its fracture, as shown in equations Where K IC is the fracture strength, C is the radius of the physical defects at the maximum strain of the interface, r is the interface undulation height, and f is the geometric factor. From equation (7), we can get that the critical