【Special Materials Knowledge】Study on Long-term Uniform Corrosion of 316Ti Stainless Steel under High Temperature and Pressure

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Sino-German research on the long-term corrosion of 316Ti stainless steel. Austenitic stainless steels are widely used in the modern aerospace industry and nuclear power sector. Among them, austenitic stainless steel 316Ti is the key material for the main pipelines in pressurized water reactor nuclear power plants. 316Ti austenitic stainless steel is primarily used in pressurized water reactor pressure stabilizing bellows, nozzle heat shields, upper charging pump casings, and nuclear submarine steam generators. The addition of titanium to 316Ti can improve the ductility and high-temperature mechanical properties of stainless steel after irradiation. Due to the strong bonding ability between titanium and carbon, titanium first binds with the carbon in stainless steel, reducing the tendency for chromium depletion at grain boundaries; this thereby **improves the resistance of stainless steel to intergranular corrosion. Research on stainless steel 316Ti at home and abroad focuses mainly on stress corrosion cracking, high-temperature creep fracture behavior, corrosion resistance at room temperature, crack propagation in high-temperature water, the influence of various elemental compositions on intergranular corrosion, and changes in properties after irradiation. However, there are no reports on its uniform corrosion performance in high-temperature aqueous environments. The main pipe materials for the AP1000 nuclear power plants currently under construction and those to be built in the future in our country will inevitably be 316Ti stainless steel, and the chemical uniform corrosion resistance of 316Ti in high-temperature water is fundamental to evaluating its service performance. To thoroughly study the uniform corrosion behavior of 316Ti stainless steel in the primary side environment of pressurized water reactors, it was subjected to a 1680-hour static autoclave high-temperature and high-pressure water corrosion test. Microscopic analysis was conducted on the oxide films formed at different stages, and the uniform corrosion rate was quantitatively evaluated. Research methods for 316Ti 1.1 Experimental materials The 316Ti stainless steel used in the experiments was produced in China, and its chemical composition is shown in Table 1. http://image2.135editor.com/cache/remote/aHR0cHM6Ly9tbWJpei5xcGljLmNuL21tYml6X3BuZy9pYVFiU0tpYkV1VEoxcXUyOUZQeVlUT3BYcHIyNGtOZmZqNlNmZ0tvVjY0VlN6cUhuSmdEb1duM2ZYbGcyY0loaWNDckVwT3JJMmVuM0FNZTBBZUVkV3Jwdy8wP3d4X2ZtdD1wbmc= The nominal dimensions of the test sample are 25mm*20mm*2mm; a hole is present in the center of the sample for hanging it, with a diameter of 2.5mm. The surface of the sample was polished using 400#, 800#, 1200#, and 2000# SIC sandpapers in sequence, followed by ultrasonic cleaning in anhydrous ethanol. After drying, it was used for testing, and its weight was measured using an electronic balance with a precision of 0.1mg. To reduce the random errors in the experiments, a large number of parallel samples were used; initially, 16 parallel samples were placed into the autoclave. The experiment was conducted in 4 cycles of 336 h, 816 h, 1152 h, and 1680 h respectively; weighing was performed after each cycle, and a sample was taken for analysis. The test medium simulates the water chemistry environment of a pressurized water reactor primary loop: 1000 ppm boric acid + 2.2 ppm lithium hydroxide, with the solution prepared using ultrapure water (resistivity of 18.2 μΩ·cm). 1.2 Test apparatus The test apparatus is a 20L static high-pressure reactor; the reactor body is made of 316Ti stainless steel, and the volume of boron-lithium water added each time is 13L. 1.3 Test operation parameters: The test temperature is 300°C, the test pressure is the saturated vapor pressure of water vapor at 8.7 MPa, and the pH value of the solution’s water temperature is 6.5. The first time it is placed in the kettle, the environment is one with saturated oxygen; thereafter, after each water change, nitrogen at 0.25 MPa is used to remove oxygen three times, ensuring that the dissolved oxygen level remains below 0.05 ppm. Results and analysis of 316Ti 2.1 Corrosion resistance analysis Figure 1 shows the corrosion weight gain curve for 316Ti stainless steel, while Figure 2 presents the corrosion weight gain rate curve for the same material. Both the corrosion weight gain value and the corrosion weight gain rate value are negative; therefore, it is actually a loss of weight due to corrosion. Experiments show that under the conditions of boron-lithium water in a pressurized water reactor primary loop at 300°C, the corrosion weight loss reaches a maximum value of 2.7 mg/dm2 after 1152 hours of corrosion ； At 1680 hours, the corrosion weight loss decreased to 1.39 mg/dm2 ； At 336 h, the weight loss rate was a maximum of 3.16*0.0001 mg/(dm2·h) ； At 1680 hours, the weight loss rate decreased to 9.78*0.00001 mg/(dm2·h). It can be shown that at the beginning, the protective property of the oxide film is poor; most of the formed oxide film dissolves, resulting in significant weight loss. As time passes, the oxide film becomes increasingly dense and its protective properties improve, thereby reducing the rate of corrosion-induced weight loss. This indicates that 316Ti stainless steel has good resistance to long-term uniform corrosion. 2.2 Observation of the oxide film morphology After 14 days of corrosion in high-temperature and high-pressure boron-lithium water under simulated pressurized water reactor primary loop conditions, the surface of 316Ti stainless steel changed from a bright metallic luster to a bluish-black color, with the oxide film exhibiting uniformity in color. After 34 days of high-temperature corrosion, the color of the surface of 316Ti stainless steel deepened to brown; after 48 days, its color was essentially the same as that after 34 days, remaining brown; after 70 days of high-temperature corrosion, its color turned dark brown. This is consistent with the results of the weight loss curve: at the beginning, the weight loss is significant, and the color changes are also noticeable ； Later, due to the gradual formation of a protective oxide film, the color deepened further to dark brown. http://image2.135editor.com/cache/remote/aHR0cHM6Ly9tbWJpei5xcGljLmNuL21tYml6X2pwZy9pYVFiU0tpYkV1VEoxcXUyOUZQeVlUT3BYcHIyNGtOZmZqcW05cjlPaWFwaGpTc3VNRklxVlJpYzd6dU16RmtDRk81UmJJbWpES2ZaUWZKQzRocHVFaWJXTWFRLzA/d3hfZm10PWpwZWc= After being exposed to the corrosion caused by boron-lithium water at high temperature and pressure of 300°C in a 14-day saturated oxygen simulation pressurized water reactor, no dense oxide layer was formed on the surface of 316Ti stainless steel; EDX spectroscopy was performed on the entire surface (3(c)), and the analysis results are shown in Table 2 (Figure 3(d)). The atomic ratio of oxygen atoms to metal atoms is 23:75, indicating a very low oxygen content, which proves that the oxide layer is thin and that some of the signals originate from the base metal. The atomic ratio of (Cr+Ni) to Fe is 0.553, which is higher than the 0.514 of the matrix, indicating a greater degree of dissolution of iron atoms in the oxide film. Energy spectrum analysis was performed on the larger particles (Figure 3(e)) (Figure 3(f)); the ratio of oxygen atoms to metal atoms was 50:49, and the contents of Cr and Ni were very low, indicating that the large particles are mainly iron oxide particles. The atomic ratio of Cr+Ni to Fe in the large particles is 0.073, which is much lower than that of the matrix, which is 0.465; this also confirms that the large particles are iron-rich and chromium-nickel-poor oxides. The Ti content in the large oxide particles is also relatively high. This iron-rich, chromium- and nickel-poor oxide has very poor corrosion resistance, and it dissolves first in high-temperature and high-pressure primary loop environments. After 70 days of corrosion in a high-temperature and high-pressure boron-lithium water environment at 300°C (Figure 4), two oxide layers formed on the surface of 316Ti stainless steel (Figure 4(a)). The innermost layer was a fine and dense oxide layer, with particle diameters ranging from 200 to 300 nm. Above this fine and dense oxide layer, larger oxide particles with diameters of 0.5 to 1.6 μm were distributed sparsely; these particles had cubic or polygonal shapes. EDX spectroscopic analysis was performed on the large particles (Figure 4(b)) (Figure 4(c)); the atomic ratio of oxygen atoms to metal atoms was 44:55, and the atomic ratio of (Cr+Ni) to Fe was 0.278, which is lower than the value of 0.465 for the matrix. This indicates that the large particles are primarily iron-rich and chromium-nickel-poor oxide particles. The atomic ratio of (Cr+Ni) to Fe in these particles is higher than 0.073 for the oxide particles formed after 14 days of corrosion, meaning their corrosion resistance is also higher than that of the oxide particles produced after 14 days. Energy spectrum analysis of the spaces between the large particles (Figure 4(d)) (Figure 4(e)) showed that the percentage of oxygen atoms decreased to 32%, while the proportion of metal atoms was 67%. The atomic ratio of (Cr+Ni) to Fe was 0.544, which is higher than the value of 0.514 for the matrix; the contents of Cr and Ni were high. The small oxide particles were primarily chromium- and nickel-rich and iron-poor oxides. The thin layer of chromium- and nickel-rich, iron-poor oxides is highly dense, offering excellent corrosion resistance; it prevents oxygen atoms from diffusing into the metal matrix and metal ions from migrating outward. 316Ti stainless steel exhibits good resistance to high-temperature corrosion over the long term due to the formation of a dense oxide film. Conclusions for 316Ti: a. The corrosion weight loss of stainless steel 316Ti in the environment of a simulated pressurized water reactor primary loop increases rapidly at first and then gradually decreases. After 70 days of corrosion in a high-temperature, high-pressure boron-lithium water environment, its uniform corrosion weight loss rate decreased to 9.78*0.00001 mg/(dm2·h), indicating that 316Ti stainless steel possesses good resistance to long-term uniform corrosion. In the high-temperature and high-pressure boron-lithium water environment of a simulated pressurized water reactor primary loop, b.316Ti stainless steel initially only experienced the dissolution of its oxide film, without the formation of a dense protective oxide layer. After 70 days of high-temperature oxidation, a dual-layer oxide film structure was formed on the surface of 316Ti stainless steel. The layer close to the substrate was a dense oxide film with fine oxide particles whose diameter ranged from 200 to 300 nm; it was an oxide layer rich in chromium and nickel but poor in iron ； The outer oxide particles are coarse, with diameters ranging from 0.5 to 1.6 μm; moreover, the density of this oxide layer is not as high as that of the inner, finer oxide layer. It is an iron-rich and chromium/nickel-poor oxide layer, and its Ti content is also higher than that of the matrix. The improved long-term uniform corrosion resistance of c.316Ti stainless steel is due to the formation of a fine and dense oxide film rich in chromium and nickel and poor in iron.