Analysis of initial oxidation process of 2205 duplex stainless steel in closed vessel at high temperature

Analysis of initial oxidation process of 2205 duplex stainless steel in closed vessel at high temperature

The initial formation process of oxide film on 2205 duplex stainless steel in closed vessel at 1050 ℃ was studied. The surface oxide morphology, structure and composition of 2205 duplex stainless steel were analyzed by metallographic microscope, SEM, Raman spectrometer and glow discharge spectrometer. The results show that at the initial stage of oxidation (less than 3 min), the oxidation degree and oxide type of ferrite and austenite are obviously different. Chromium rich oxide is mainly formed on the surface of ferrite, while iron oxide is mainly formed on the surface of austenite. The oxidation resistance of ferrite is better than that of austenite at the initial stage of oxidation. With the increase of oxidation time (more than 5 min), the chromium rich and iron rich oxides on the surface of the two phases change obviously, that is, the iron oxide on the surface of the ferrite phase increases and the oxidation rate accelerates; However, with the increase of chromium rich oxides on the surface of austenite phase, the oxidation rate of austenite phase gradually slows down. During the oxidation process, Cr diffuses from subsurface layer to surface layer, and finally a certain thickness of chromium rich oxide is formed in the surface layer. In the closed environment, it is easy to form granular nitrogen / oxide on the surface of 2205 duplex stainless steel, and it is preferentially formed on the surface of ferrite phase; After oxidation for a certain period of time, this type of compound began to precipitate on the surface of austenite phase. With the extension of oxidation time, the amount of this type of compound increased until it distributed on the whole surface of the sample.

Duplex stainless steel overcomes the respective disadvantages of ferritic steel and austenitic steel, and has both advantages, such as high strength, pitting corrosion resistance, intergranular corrosion resistance of ferritic stainless steel, and weldability and toughness of austenitic stainless steel [1]. Due to the difference in the content and distribution of elements in austenite and ferrite of duplex stainless steel, the electrode potential between the two phases is different, which shows that one phase is corroded or oxidized before the other phase, which will eventually cause the failure of the whole material [2]. There are many researches on the selective corrosion of duplex stainless steel. For example, in the mixed solution of 2mol/L H2SO4 and 0.5mol/l HCl, the ferrite phase is corroded first [3,4], while in the nitric acid system, the austenite phase is corroded first [5].
Generally, the structure and composition of oxide scale produced in hot rolling process of duplex stainless steel are more complex than that of ferrite stainless steel and austenitic stainless steel with single phase. The oxidation degree, oxide type and structure of each phase on the surface of duplex stainless steel are also different, which not only affects the surface quality of products, but also has adverse effects on subsequent pickling effect [6,7], Therefore, it is very important to clarify the formation process and oxide film structure of duplex stainless steel for improving the surface quality of duplex stainless steel. However, in the process of high temperature oxidation, selective oxidation also exists in duplex stainless steel. When s32101 and s32304 stainless steels are oxidized in air at 300500600 and 660 ℃, the austenite phase in s32101 stainless steel is oxidized first, while s32304 stainless steel is ferrite phase, which is mainly due to the different contents of Cr and Mn in each phase of these two kinds of steels [8]. However, in the mixture of propane and air at 1000 ℃, a continuous SiO2 enrichment layer was formed on the ferrite of s32101 stainless steel [9]. Sánchez-Tovar et al. [10] studied the surface color change of ferrite and austenite of alloy 900 (UNS 1.4462) alloy during oxidation at 700 ~ 900 ℃ in air. The results show that in the initial stage, Fe rich oxide is formed on the surface of ferrite phase, which is orange red, while CR rich oxide is formed on the surface of austenite phase, which is blue. Although the oxidation process of ferrite and austenite phases in duplex stainless steel has been analyzed, the initial oxidation process of each phase surface, oxide composition and the influence mechanism of each element are not very clear. In addition, the effect of oxidation medium on the oxidation process and oxide composition and structure of stainless steel is obvious. It is of great significance to further understand the composition and structure of oxide film on the surface of duplex stainless steel in different environments.
Dual phase stainless steel is often used in high temperature closed environment, and the oxidation behavior of each phase on the surface under this oxidation condition is rarely studied. In this paper, 2205 duplex stainless steel was placed in a closed vessel for high temperature oxidation at different time to analyze the composition, structure and formation process of ferrite and austenite surface oxides and the diffusion process of elements in the oxidation process.

Experimental method

The experimental material is 2205 duplex stainless steel hot rolled plate produced by TISCO, with a thickness of 5 mm. The main chemical compositions (mass fraction,%) are Cr 22.36, Ni 22.36, Mo 3.18, Mn 1.37, Si 0.65, n 0.15, P 0.014, C 0.02 and Fe. The size of the sample is 14 mm × 14 mm × 5 mm. The samples were first treated in a tube furnace at 1050 ℃ (at this temperature, the two-phase ratio of 2205 duplex stainless steel is closest to 1 : 1) for 90 min. Before the oxidation test, the six surfaces of the sample were polished with 1200# sandpaper to be bright, then mechanically polished, and cleaned with alcohol for standby. The sample is placed in a closed container as shown in Fig. 1, which is made of 304 stainless steel tube. The container is sealed by smashing and crimping at both ends. The inner atmosphere is air, and the oxygen partial pressure will decrease with the oxidation time. The rapid oxidation of 2205 dual phase steel in air can be avoided by oxidizing the sample in the closed vessel, so as to slow down the oxidation rate. The samples were kept at 1050 ℃ for 1, 2, 3, 5, 10, 20 and 60 min, and then quickly cooled to room temperature for observation and characterization.

Fig.1 Closed container and sample placement
The samples after high temperature oxidation for different time were observed by 11px-dc optical microscope and Zeiss evo18 scanning electron microscope (SEM). The surface oxide layer was analyzed by EDS attached to SEM. The structure and composition of the oxide were characterized by x-plora confocal Raman spectrometer, GDA750 glow discharge spectrometer (GDS) was used to measure the element contents of the samples from the surface layer to the substrate after high temperature oxidation.

Results and discussion

Analysis of initial formation process of oxide film

Figure 2 shows the microstructure of 2205 duplex stainless steel hot rolled plate after solution treatment at 1050 ℃. It can be seen that the microstructure of the sample after solution treatment is mainly composed of dark ferrite (α)  And light colored austenite (γ)  Austenite is distributed in ferrite in island shape, and no other precipitates are observed.

Fig.2 Optical micrograph of 2205 duplex stainless steel after solution treatment at 1050 ℃
Figure 3 shows the macro morphology of the upper surface of the sample before oxidation at 1050 ℃ and after oxidation for 1, 2, 3, 5, 10, 20 and 60 min respectively. It can be seen that with the extension of oxidation time, the color of sample surface gradually becomes dark, from light yellow to rust red to dark brown, and the surface becomes more and more rough. That is, with the extension of oxidation time, the degree of oxidation becomes more and more serious.

Fig.3 Macromorphologies of the upper surface of 2205 duplex stainless steel before (a) and after oxidation for 1 min (b), 2 min (c), 3 min (d), 5 min (e), 10 min (f), 20 min (g) and 60 min (h)
Fig. 4 shows the microstructure of 2205 duplex stainless steel after oxidation at 1050 ℃ for 1, 2, 3 and 5 min. The results show that the oxidation resistance of ferrite is obviously different from that of austenite. FIG. 4A shows the surface oxidized for 1 min. the color of austenite surface is dark, indicating that oxidation has occurred, while the color of ferrite remains unchanged. It can be seen that the oxidation resistance of austenite phase is weaker than that of ferrite phase at the initial stage of oxidation. When the oxidation time is increased to 2 min (Fig. 4b), the surface color of austenite and ferrite are further changed, the color of austenite is rust red, the color of ferrite is light blue, and the change of surface oxidation color should correspond to the type of oxide formed [10]. When the oxidation time is extended to 3 min (Fig. 4C), the oxidation degree of two phases in duplex stainless steel changes obviously compared with that of 2 min, the oxidation rate of ferrite phase is accelerated, and the color is similar to that of austenite oxidation for 1 min, while the oxidation rate of austenite is relatively stable. When the oxidation time reaches 5 min (Fig. 4D), the oxidation rate of ferrite and austenite tends to be flat, but fine and white granular substances appear on the surface of ferrite.

Fig.4 Surface color distributions of 2205 duplex stainless steel after oxidation at 1050 ℃ for 1 min (a), 2 min (b), 3 min (c) and 5 min (d)

Composition and structure of oxide film

The above analysis shows that with the extension of oxidation time, the surface oxidation color of the two phases changes gradually, indicating that various types of oxides are formed on the surface of each phase during the oxidation process. Therefore, the surface morphology of the samples is further observed by SEM. Figure 5 shows the scanning backscatter diagram and EDS analysis results of the samples after oxidation at 1050 ℃ for 1, 5, 10 and 20 min (Table 1). SEM images show that the oxidation rate of austenite phase is relatively stable after oxidation for 1 ~ 20 min; However, with the oxidation time increasing to 20 min, white particles are formed on the surface of ferrite in addition to self oxidation, and the size and quantity of particles increase gradually until the whole ferrite surface is covered. With the further prolongation of oxidation time, fine white granular materials are gradually formed on the surface of austenite. EDS analysis results (Table 1) show that n mainly exists in the white granular material on the surface of ferrite phase, and the Cr content on the surface of ferrite phase is significantly higher than that on the surface of austenite phase at the initial stage of oxidation, which is the reason why the oxidation resistance of ferrite phase is higher than that of austenite phase at the initial stage of oxidation. In a word, the composition of oxides formed on the surface of ferrite and austenite phase changes with the oxidation time, which may be due to different element contents in the matrix, different oxygen activity, different outward diffusion rate and the change of environmental oxygen partial pressure.

Fig.5 Surface morphologies of 2205 duplex stainless steel after oxidation at 1050 ℃ for 1 min (a), 5 min (b), 10 min (c) and 20 min (d)
Table.1 EDS results of oxide scale on 2205 duplex stainless steel oxidized at 1050 ℃ for different time
(mass fraction/%)

In order to further analyze the structure and composition of white granular materials, the surface of samples oxidized at 1050 ℃ for 60 min was analyzed. Figure 6A shows the scanning backscatter diagram of the sample after oxidation for 60min. It can be seen that the surface of ferrite phase and austenite phase has been basically covered by white particles, so it is difficult to distinguish austenite phase and ferrite phase. However, some areas with small and sparse white particles in the figure should belong to the original austenite position. Fig. 6C is the EDS analysis result of white particles. It can be seen that the white particles mainly contain n, O and Cr.

Fig.6 SEM surface images (a) and the magnified image of area I in Fig.6a (b) of 2205 duplex stainless steel after oxidation at 1050 ℃ for 60 min, and corresponding EDS analysis result of white grains in Fig.6b (c)
Figure 7 shows the results of Raman testing on the surface of ferrite and austenite after oxidation at 1050 ℃ for 1, 5, 10 and 20 min. The results show that the structures of Fe rich and Cr rich oxides formed on the surfaces of the two phases are obviously different after different oxidation time. Fig. 7a shows that the types of oxides formed in the initial oxidation stage of ferrite and austenite phases are different. The oxides formed on the surface of austenite phase are mainly Fe containing oxides, such as 225242293409 and 612 cm-1 characteristic peaks corresponding to α- The characteristic peaks of Fe2O3 [11], 505 and 660cm-1 correspond to γ- Fe2O3 [12], where α- The 293cm-1 characteristic peak of Fe2O3 is the strongest. The oxides formed on the ferrite surface are mainly chromium containing oxides, in which the weak characteristic peak at 551cm-1 corresponds to Cr2O3 [13], and the strong characteristic peak at 683cm-1 corresponds to FeCr2O4 [14]. Due to the compact CR rich oxide, the oxidation of ferrite is not obvious in terms of surface oxidation degree, while the oxidation degree of austenite is more serious due to the formation of Fe2O3 type oxide on the surface. The results of Raman spectrum analysis of the sample surface after oxidation for 5 min are shown in Fig. 7b. With the extension of oxidation time, the surface oxides of austenite are similar to those in Fig. 7a, which are still dominated by Fe rich oxides, i.e α- Fe2O3 and γ- Fe2O3. However, the oxide on the surface of ferrite appears to be the main component α- Fe2O3; From the surface oxidation color, the oxidation degree of ferrite is heavier than that of austenite. With the oxidation time further extended to 10 min (as shown in Fig. 7C), the surface oxidation of the two phases is obvious, and the oxides on the ferrite surface are mainly formed by the characteristic peaks at 222 and 289 cm-1 α- Comparison of characteristic peaks of fe2o3505 and 641cm-1 γ- Cr2O3 [13] of the characteristic peaks of Fe2o3351 and 598cm-1, FeCr2O4 of the characteristic peak of 685cm-1 and CrN [15] of the characteristic peak of 660cm-1, in which the proportion of chromium oxide and nitride increases; The oxide on austenite surface is mainly composed of the characteristic peaks at 222242 and 288cm-1 α- The strongest characteristic peak of fe2o3404cm-1 and Cr2O3 of 609cm-1 were observed. In a word, with the increase of oxidation time, the proportion of Cr rich oxides on austenite surface increases, that is, the corresponding oxidation rate gradually slows down. When the oxidation time continues to 20 min (Fig. 7D), the 655 cm-1 characteristic peak of ferrite surface corresponds to CrN [15], the 609 cm-1 characteristic peak corresponds to Cr2O3 [13], and the 395 cm-1 characteristic peak corresponds to CrN [15] γ- Fe2O3. Microstructure of 222 and 288 cm-1 austenite surface α- The characteristic peaks of Fe2O3 and Cr2O3 [13] at 400 and 596cm-1 are very strong, and the characteristic peak of CrN at 655cm-1 also appears. Therefore, in the early oxidation stage of 2205 duplex stainless steel, Cr rich oxides are formed on the surface of ferrite phase, while Fe rich oxides are formed on the surface of austenite phase, which shows that the oxidation resistance of ferrite surface is strong [16]; With the progress of oxidation, Fe2O3 type oxides on the surface of ferrite phase increase, but the oxidation resistance of ferrite phase is weaker than that of austenite phase; With the increase of oxidation time, the amount of nitrogen-containing and oxygen-containing compounds formed on the surface of ferrite phase increased, and finally formed on the surface of austenite phase and distributed on the surface of the whole sample.

Fig.7 Raman spectra of the oxides formed on γ-Fe and α-Fe of 2205 duplex stainless steel after oxidation at 1050 ℃ for 1 min (a), 5 min (b), 10 min (c) and 20 min (d)
Figure 8 shows the GDS analysis results of 2205 duplex stainless steel after oxidation for 10 and 60 min. It can be seen that Fe, Cr and Mn oxides are mainly formed on the surface after oxidation. With the increase of oxidation time, Cr, Mn and Si become more enriched, which indicates that in the early stage of oxidation, the partial pressure of oxygen is higher, and the oxidation of Fe and Cr mainly occurs; As the oxidation proceeds, the oxygen partial pressure decreases continuously in the closed container, and the elements with higher oxygen activity such as Cr, Mn and Si are more likely to be oxidized. Finally, a certain thickness of Cr rich oxide layer with a small amount of Mn and Si is formed on the surface.

Fig.8 GDS profiles of main elements of 2205 duplex stainless steel after oxidation at 1050 ℃ for 10 min (a) and 60 min (b)

Conclusion

• (1) In the initial stage of oxidation of 2205 duplex stainless steel in closed air at 1050 ℃, the main oxide types formed on the surface of ferrite phase and austenite phase are obviously different. CR rich oxide is formed on the surface of ferrite phase, while Fe rich oxide is formed on the surface of austenite phase. The oxidation resistance of ferrite phase is higher than that of austenite phase.
• (2) In the early stage of oxidation, the partial pressure of oxygen is high, and the oxidation of Fe and Cr mainly occurs; As the oxidation proceeds, the oxygen partial pressure decreases continuously in the closed container, and the elements with higher oxygen activity such as Cr, Mn and Si are more likely to be oxidized. Finally, a certain thickness of Cr rich oxide layer with a small amount of Mn and Si is formed on the surface.
• (3) In this closed environment, with the progress of oxidation, the oxygen consumption is serious, the partial pressure of oxygen decreases, the partial pressure of nitrogen increases, the oxidation reaction gradually slows down, and the nitriding reaction begins to accelerate. In 2205 duplex stainless steel, the surface of ferrite phase with higher CR preferentially reacts with nitrogen to form granular compounds containing nitrogen and oxygen. With the further prolongation of the oxide, the nitriding reaction on the surface of austenite phase is strengthened with the weakening of the oxidation, thus the nitrogen and oxygen type particles are formed and gradually spread throughout the whole sample.

Author: Yue LI, Jian WANG, Yong ZHANG, Jingang BAI, Yadi HU, Yongfeng QIAO, Caili ZHANG, Peide HAN

Source: China 2205 Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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• Duplex Stainless Steel in Closed Container at High Temperature. Journal of Chinese Society for Corrosion and protection, 2018, 38(3): 296-302. DOI: 10.11902/1005.4537.2017.066