High-temperature oxidation film forming process for heat exchange tube of high-temperature gas cooled reactor steam generator

Abstract

The application provides a high-temperature oxidation film forming process for heat exchange tubes of a high-temperature gas cooled reactor steam generator, comprising the following steps: vacuum heat treatment of the heat exchange tubes in a vacuum furnace; cleaning treatment of the heat exchange tubes; oxidation treatment of the heat exchange tubes in an oxidation vacuum furnace; under the condition that the temperature in the furnace is not lower than 400 DEG C, oxygen in the vacuum furnace is pumped out, protective atmosphere is filled into the oxidation vacuum furnace, and then the heat exchange tubes are cooled. The application combines the chemical transformation characteristics of materials at different temperatures, designs and transforms the furnace body structure, and accurately controls the atmosphere during the cooling process after high-temperature oxidation film forming, so that the key technical indexes and comprehensive performance of the oxidation film are significantly improved, and the problems of instability, color difference and frequent rework of the oxidation film in the prior art are solved.

Description

Technical Field

[0001] This invention belongs to the field of material surface corrosion protection technology, specifically relating to a high-temperature oxidation film formation process for heat exchange tubes used in high-temperature gas-cooled reactor steam generators. Background Technology

[0002] High-temperature gas-cooled reactors (HTGRs) are one of the advanced fourth-generation nuclear power reactor technologies, boasting advantages such as high safety, high efficiency, good economy, and wide applicability. The spiral heat exchange tubes used in their steam generators are made of two materials: ASME SA-213T22 and Incoloy-800H. ASME SA-213T22 is a low-alloy steel, highly susceptible to corrosion during manufacturing, assembly, and storage. However, corrosion is unacceptable according to the cleanliness and wall thickness design requirements for the heat exchange tubes. Without effective protection, product quality will be severely compromised, potentially posing a safety hazard to the entire nuclear power system. Therefore, surface anti-corrosion treatment is necessary for the heat exchange tubes made of ASME SA-213T22. Because the spiral heat exchange tubes require subsequent assembly, the method of bagging and filling with inert gas for surface anti-corrosion cannot be used. Furthermore, due to the high surface cleanliness and complex structure of the spiral heat exchange tubes (Φ19*3mm, length greater than 60m, spiral shape), coating or electroplating anti-corrosion processes are also not feasible. Therefore, the only way to prevent corrosion is by forming a Fe3O4 oxide film on the surface of the spiral heat transfer tube under high temperature conditions.

[0003] However, in existing technologies, the high-temperature oxide film on the surface of the spiral heat exchanger tubes of steam generators still suffers from instability, color differences, and frequent rework. The main reason for this is the high proportion of Fe2O3 in the formed high-temperature oxide film. In the existing high-temperature film formation process, the heat exchanger tubes are heated to a certain temperature in a vacuum environment and then filled with pure oxygen, which forms a Fe3O4 oxide film on the surface of the heat exchanger tubes. However, during the cooling process (200-400℃), Fe3O4 will transform into low-temperature γ-Fe2O3 in an oxygen-rich environment (4Fe3O4+O2→6Fe2O3), resulting in reduced corrosion resistance and instability of the oxide film. Specifically, this manifests as a reddish-brown color after visual inspection according to GB / T 15519, oxide film corrosion after oxalic acid resistance testing, and red spots appearing after porosity and continuity tests with copper sulfate solution (CN 107177814A). Therefore, it is necessary to develop a better oxidation film formation process for heat exchange tubes used in high-temperature gas-cooled reactor steam generators to give them better and more durable corrosion protection. Summary of the Invention

[0004] To address the issues of instability, color variation, and frequent rework in the high-temperature oxide film on the surface of heat exchange tubes used in high-temperature gas-cooled reactor steam generators, this invention provides a superior high-temperature oxidation film formation process. Improvements are made to the cooling treatment of the heat exchange tubes after film formation, resulting in Fe3O4 as the main component of the oxide film on the heat exchange tube surface. Furthermore, the composition of the oxide film is uniform and stable, and the quality consistency of the oxide film is high across different batches, effectively improving the quality stability of the oxide film on the heat exchange tube surface.

[0005] The technical solution adopted in this invention is a high-temperature oxidation film formation process for heat exchange tubes in a high-temperature gas-cooled reactor steam generator, comprising the following steps:

[0006] Step 1: Perform vacuum heat treatment on the heat exchange tubes;

[0007] Step 2: Clean the heat exchange tubes;

[0008] Step 3: Perform oxidation treatment on the heat exchange tubes in an oxidation vacuum furnace;

[0009] Step 4: Under the condition that the temperature inside the furnace is not lower than 400℃, remove the oxygen from the vacuum furnace, then fill the oxidation vacuum furnace with a protective atmosphere, and then cool the heat exchange tubes.

[0010] During the oxidation of the iron-based alloy matrix, iron and oxygen can form three oxides: FeO, Fe3O4, and Fe2O3. The oxide states are shown in the Fe-O phase diagram. Figure 1 FeO is unstable at low temperatures, while at high temperatures, iron ions easily diffuse through the numerous cation vacancies within FeO, accelerating the oxidation reaction at the oxide / oxygen interface. Therefore, the presence of a large amount of FeO reduces the antioxidant properties of the oxide film. Fe2O3 has two structural types: low-temperature γ-Fe2O3 and high-temperature α-Fe2O3. γ-Fe2O3 undergoes an allotropic transformation at 400–500℃ to become α-Fe2O3. α-Fe2O3 has an orthorhombic hexahedral crystal structure, exists over a wide temperature range, exhibits good high-temperature stability and antioxidant properties, begins to decompose above 1100℃, and completely decomposes above 1565℃. Fe3O4 is a magnetic oxide with a complex spinel-type cubic structure, and its structure is very stable from room temperature to its melting point (1538℃). Because Fe3O4 has fewer defects than FeO, ions have difficulty passing through it, preventing further oxidation reactions. Therefore, Fe3O4 is not only stable, but also has the best antioxidant properties among the three oxides.

[0011] Preferably, in step one, the vacuum heat treatment refers to heating the heat exchange tube to 590–640°C and holding it at that temperature for 20–30 minutes under a pressure not exceeding 0.01 Pa, more preferably heating it to 620–640°C and holding it for 25–30 minutes. This process is a normal heat treatment stress relief process for the heat exchange tube. Without the high-temperature oxidation film formation process and subsequent anti-corrosion treatments such as oil sealing of the present invention, the preparation of the spiral heat exchange tube for the high-temperature gas-cooled reactor steam generator in the prior art CN 114632837 A would be completed at this point. However, unlike the prior art CN 107177814A, the present invention first performs vacuum heat treatment on the heat exchange tube, and then performs cleaning treatment on it. The reason is that the stress relief treatment (i.e., the vacuum heat treatment in step one) requires the clamping of complex tooling, and if the subsequent high-temperature oxidation treatment is performed in the same vacuum furnace after the treatment, it will cause more damage to the oxide film on the surface of the heat exchange tube. Therefore, the present invention chooses to perform vacuum heat treatment and high-temperature oxidation treatment in two separate furnaces. Specifically, after vacuum heat treatment in a traditional vacuum furnace, the tooling is removed, the heat exchange tubes are taken out, and then the heat exchange tubes are cleaned by surface sandblasting / shot peening. Subsequently, without tooling, they are placed in an oxidation vacuum furnace for high-temperature oxidation treatment. This improved process results in a more complete oxide film on the surface of the heat exchange tubes, eliminating a large number of scratches caused by tooling.

[0012] Preferably, in step two, the cleaning process refers to shot peening or sandblasting the heat exchange tubes to remove residual impurities from their surface. Generally, shot peening or sandblasting is the first step. Specifically, glass / ceramic shot / sand of approximately 200 mesh can be used to perform shot peening / sandblasting on the heat exchange tubes to achieve the required cleanliness and roughness. After shot peening or sandblasting, residual sand particles and dust on the surface of the heat exchange tubes are blown away.

[0013] Preferably, in step three, the oxidation treatment refers to: filling the oxidation vacuum furnace with pure oxygen until the furnace pressure is 0.08–0.1 MPa, and holding the heat exchange tube at 540–560°C for 40–50 minutes. Specifically, according to the Fe-O phase diagram ( Figure 1 It can be seen that the oxide film formed on the iron surface below 570℃ is mainly composed of Fe3O4 and Fe2O3. This oxide film has a relatively dense structure and excellent oxidation resistance. When the temperature is higher than 570℃, the FeO content in the oxide film increases significantly, leading to a sharp decrease in the oxidation resistance of the oxide film. Therefore, the optimal high-temperature oxidation treatment temperature for heat exchange tubes is 540–560℃. Regarding the selection of furnace pressure, experiments have shown that when the oxygen pressure is less than 0.08 MPa, the oxide film formed on the surface of the heat exchange tube has poor density and defects. When the oxygen pressure is between 0.09 and 0.1 MPa, the main component of the oxide film formed is Fe3O4, and the film layer is uniform, dense, and complete, with good adhesion to the substrate.

[0014] In the existing technology CN 107177814A, after the heat exchange tube undergoes high-temperature oxidation treatment, it is directly cooled to room temperature in a vacuum furnace before being removed and subsequently oil-sealed. However, the inventors discovered that when the heat exchange tube is cooled to 400–200°C, the Fe3O4 in the film layer will transform into low-temperature γ-Fe2O3 in an aerobic environment (4Fe3O4 + O2 → 6Fe2O3), resulting in a reduced anti-corrosion effect and instability of the oxide film. Specifically, this manifests as a reddish-brown appearance after visual inspection according to GB / T 15519, corrosion of the oxide film after oxalic acid resistance testing, or red spots after porosity and continuity tests with copper sulfate solution. Therefore, the inventors believe it is necessary to improve the cooling treatment of the heat exchange tube after film formation. Considering the conditions and actual process environment for the conversion of Fe3O4 to γ-Fe2O3, the conversion can be avoided by reducing the oxygen content within the conversion temperature range or rapidly passing through the conversion temperature range during the cooling process. However, due to the limitations of the vacuum furnace cooling conditions, only the method of reducing the oxygen content within the conversion temperature range can be used, i.e., removing the oxygen from the vacuum furnace above 400℃. Based on this, after numerous experiments, the inventors proposed improving the cooling process by controlling the atmosphere. Specifically, under the condition that the furnace temperature is not lower than 400℃, the oxygen in the oxidation vacuum furnace is removed, and then a protective atmosphere is introduced into the oxidation vacuum furnace, followed by cooling of the heat exchange tubes.

[0015] Preferably, in step four, the removal of oxygen from the oxidation vacuum furnace refers to: first, evacuating the oxidation vacuum furnace to a low vacuum level of 800–1000 Pa, and then evacuating it to a high vacuum level of 0.01–0.05 Pa. This staged evacuation method helps to achieve a higher vacuum level in the oxidation vacuum furnace, relatively thoroughly removing residual oxygen and preventing the conversion of Fe3O4 to γ-Fe2O3.

[0016] Preferably, in step four, a protective atmosphere is introduced into the oxidation vacuum furnace until the furnace pressure reaches 0.1–0.2 MPa. The protective atmosphere is an inert gas. More specifically, the inert gas is selected from argon, helium, and nitrogen, with argon being more preferred. The introduced protective atmosphere carries away heat from the furnace through the furnace's fan and heat exchange system, thus accelerating cooling.

[0017] Preferably, in step four, the cooling treatment refers to cooling the heat exchange tube until its temperature does not exceed 150°C, and then removing the heat exchange tube. (Referring to the Fe-O phase diagram...) Figure 1If the furnace exit temperature of the heat exchange tube exceeds 150℃, the Fe3O4 in the surface oxide film may still transform into γ-Fe2O3 when it comes into contact with external oxygen, and the possibility increases with increasing temperature. Therefore, to avoid this situation, the furnace exit temperature should not exceed 150℃. For example, the heat exchange tube should be cooled to room temperature before being removed.

[0018] Meanwhile, to better complement the improved cooling method described above, the inventors also modified the structure of the existing vacuum furnace. The improved oxidation vacuum furnace includes a pump device and a cooling device; the pump device includes a screw pump, which is connected to the inner cavity of the oxidation vacuum furnace through a dedicated high-temperature oxygen venting channel. Compared to traditional vacuum furnaces, the oxidation vacuum furnace provided by this invention, in addition to the existing pump device including a Roots pump and a diffusion pump, also adds a screw pump capable of withstanding high temperatures in the presence of oxygen, and a dedicated high-temperature oxygen venting channel connecting the screw pump to the inner cavity of the oxidation vacuum furnace, for safely removing oxygen from the furnace under high-temperature conditions. More specifically, based on the oxidation vacuum furnace, step four includes: first, using a screw pump to evacuate the oxidation vacuum furnace to a low vacuum level of 800–1000 Pa, and then using a Roots pump and a diffusion pump to evacuate the oxidation vacuum furnace to a high vacuum level of 0.01–0.05 Pa; then, turning off the screw pump, Roots pump, and diffusion pump, and filling the vacuum furnace with a protective atmosphere until the furnace pressure reaches 0.1–0.2 MPa; finally, using a cooling device, such as a circulating fan, to cool the heat exchange tubes until their temperature does not exceed 150°C, and then removing the heat exchange tubes. Evacuating the furnace to a certain degree using a screw pump before turning on the Roots pump and diffusion pump allows for a higher vacuum level, which helps to completely remove residual oxygen and prevent the conversion of Fe3O4 to γ-Fe2O3. On the other hand, since the diffusion pump contains flammable oil, directly removing high-temperature oxygen through the diffusion pump path poses an explosion risk. Therefore, this invention specifically modifies the traditional vacuum furnace by adding a screw pump connected to the furnace cavity via a dedicated high-temperature oxygen removal channel. This pump removes most of the high-temperature oxygen from the furnace, preventing it from passing through the diffusion pump in large quantities. The diffusion pump is then used to remove the remaining oxygen in the furnace once it has dropped to a lower level, ensuring safety.

[0019] Preferably, step five is also included: inspecting the oxide film on the heat exchange tubes. If the inspection fails, steps one through four are repeated until the inspection passes. Specifically, after the heat exchange tubes have cooled to below 150°C, the furnace door is opened to discharge the material, and an oxide film quality inspection is performed. The routine inspection includes: oxide film appearance and surface quality inspection, oxalic acid resistance test, porosity and continuity inspection, and oxide film composition analysis.

[0020] Preferably, the heat exchange tubes that have undergone the above-mentioned high-temperature oxidation film formation treatment are then subjected to oil sealing treatment. Specifically, the heat exchange tubes are immersed in mineral oil heated to 60–150°C for at least 20 minutes. The mineral oil can be completely vaporized at 200°C.

[0021] This invention also provides heat exchange tubes for high-temperature gas-cooled reactor steam generators processed by the above-described process. Inspection revealed that the oxide film on the surface of the processed heat exchange tubes was uniform and exhibited good overall consistency. After undergoing an oxalic acid resistance test according to GB / T 15519-2002 standard, the oxide film at the oxalic acid-impregnated areas showed no change compared to other areas, meeting the standard for a high-quality film. When a 3% copper sulfate solution was applied to the oxide film surface and allowed to stand for 30 seconds, no red spots or patches appeared, indicating that the oxide film was free of pores and damage, and possessed good porosity and continuity. After oil sealing, the samples only began to show slight surface rust after 4 hours of neutral salt spray testing at 35±2℃, and the rust diffusion remained slow even after 8 hours of testing, demonstrating good corrosion resistance.

[0022] The beneficial effects of this invention are as follows: By combining the chemical transformation characteristics of materials at different temperatures and through the design and modification of the furnace structure, this invention can precisely control the atmosphere during the cooling process after high-temperature oxidation film formation, which significantly improves the key technical indicators and comprehensive performance of the oxide film and solves the problems of instability, color difference and rework in the oxide film of the prior art. Attached Figure Description

[0023] Figure 1 This is the Fe-O phase diagram.

[0024] Figure 2 This is an inspection image of the appearance and surface quality of the oxide film on the heat exchanger tube after high-temperature oxidation film formation treatment in Example 2 of the present invention.

[0025] Figure 3 The image shows the oxalic acid resistance test results of the heat exchange tube after high-temperature oxidation film formation treatment in Example 2 of the present invention.

[0026] Figure 4 The graph shows the test results of porosity and continuity of the heat exchange tube after high-temperature oxidation film formation treatment in Example 2 of the present invention.

[0027] Figure 5 This is an external view of the heat exchange tube after high-temperature oxidation film formation treatment and oil sealing treatment in Embodiment 2 of the present invention.

[0028] Figure 6 The image shows the salt spray test results of the heat exchange tube after high-temperature oxidation film formation treatment in Example 2 of the present invention. Detailed Implementation

[0029] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0030] Example 1:

[0031] An oxidation vacuum furnace for high-temperature oxidation treatment of heat exchange tubes in a high-temperature gas-cooled reactor steam generator is disclosed. Compared to conventional vacuum furnaces in the prior art, the oxidation vacuum furnace incorporates a screw pump capable of withstanding high temperatures in the presence of oxygen for evacuating gases (such as oxygen) from the furnace. Specifically, the pump system includes a screw pump, a Roots pump, and a diffusion pump. The screw pump is connected to the inner cavity of the oxidation vacuum furnace via a dedicated high-temperature oxygen venting channel. In one specific embodiment, under the condition that the furnace temperature is not lower than 400°C, the oxidation vacuum furnace is evacuated to a low vacuum using the screw pump until the furnace pressure is 800-1000 Pa. The gas evacuated from the inner cavity of the oxidation vacuum furnace is discharged through the high-temperature oxygen venting channel. After closing the high-temperature oxygen venting channel connecting the screw pump to the inner cavity of the furnace, the vacuum furnace is evacuated to a high vacuum using the Roots pump and diffusion pump until the furnace pressure is 0.01-0.05 Pa. The screw pump, Roots pump, and diffusion pump are then closed, and a protective atmosphere is introduced into the vacuum furnace until the furnace pressure is 0.1-0.2 MPa.

[0032] The oxidation vacuum furnace provided by this invention also includes a cooling device, specifically a circulating fan, for cooling objects inside the vacuum furnace, such as heat exchange tubes. In one specific embodiment, the circulating fan is turned on to cool the heat exchange tubes inside the vacuum furnace, ensuring that the temperature of the heat exchange tubes does not exceed 150°C, before the heat exchange tubes are removed. The oxidation vacuum furnace should have other basic structures well known to those skilled in the art, including a heat exchange system, a vacuum system, and a control system, to achieve a series of operations such as vacuum heat treatment, high-temperature oxidation treatment, and cooling treatment of the heat exchange tubes of the high-temperature gas-cooled reactor steam generator.

[0033] Example 2:

[0034] This embodiment utilizes the oxidation vacuum furnace provided in Embodiment 1 for treating heat exchange tubes of a high-temperature gas-cooled reactor steam generator to perform high-temperature oxidation film formation treatment on the heat exchange tubes. The specific process is as follows:

[0035] Step 1: Perform vacuum heat treatment on the heat exchange tube in a vacuum furnace: Turn on the vacuum system of the vacuum furnace and evacuate the vacuum level inside the furnace to 0.01 Pa; turn on the temperature control system of the vacuum furnace, heat the heat exchange tube to 630°C and hold it at that temperature for 30 minutes, then remove the heat exchange tube.

[0036] Step 2: Cleaning the heat exchange tubes: Sandblast the heat exchange tubes with sand of about 200 mesh, and then blow away the residual sand particles and dust on the surface of the heat exchange tubes; hoist the cleaned heat exchange tubes into the oxidation fixture, connect the workpiece thermocouple, and put them into the oxidation vacuum furnace.

[0037] Step 3: Oxidize the heat exchange tube in an oxidation vacuum furnace; fill the oxidation vacuum furnace with pure oxygen until the pressure inside the furnace is 0.09 MPa, and keep the heat exchange tube at 550°C for 50 minutes.

[0038] Step 4: With the furnace temperature not lower than 400℃, first use a screw pump to evacuate the oxidation vacuum furnace to a low vacuum until the furnace pressure is 1000Pa, then use a Roots pump and a diffusion pump to evacuate the oxidation vacuum furnace to a high vacuum until the furnace pressure is 0.05Pa; turn off the screw pump, Roots pump, and diffusion pump, and fill the oxidation vacuum furnace with argon gas until the furnace pressure is 0.15MPa; use a circulating fan to cool the heat exchange tubes until the heat exchange tube temperature is not higher than 150℃, and then remove the heat exchange tubes.

[0039] Step 5: Inspect the oxide film on the heat exchanger tubes. If the inspection fails, repeat steps 1 through 4 until the inspection is passed. The inspection includes: oxide film appearance and surface quality inspection, oxalic acid resistance test, porosity and continuity inspection, and oxide film composition analysis.

[0040] Upon inspection, the oxide film on the surface of the heat exchanger tube obtained after high-temperature oxidation film formation treatment was uniform and showed good overall consistency. Figure 2 After undergoing an oxalic acid resistance test according to GB / T 15519-2002 standard, the oxide film on the oxalic acid-impregnated areas showed no change compared to other areas, meeting the standard for a high-quality film. Figure 3 A 3% copper sulfate solution was applied dropwise to the oxide film surface and allowed to stand for 30 seconds. No red spots or patches appeared, indicating that the oxide film was free of pores and damage, and had good porosity and continuity. Figure 4 ).

[0041] The heat exchange tubes that have undergone the above-mentioned high-temperature oxidation film formation treatment are then subjected to oil sealing treatment: the heat exchange tubes are immersed in mineral oil heated to 60-150°C for 20 minutes. The mineral oil can be completely vaporized at 200°C. Figure 5 After being sealed with oil, the samples only began to show slight surface rust after 4 hours of neutral salt spray testing at 35±2℃. Even after 8 hours, the rust spread slowly, indicating good corrosion resistance. Figure 6).

[0042] Example 3:

[0043] This embodiment utilizes the oxidation vacuum furnace provided in Embodiment 1 for treating heat exchange tubes of a high-temperature gas-cooled reactor steam generator to perform high-temperature oxidation film formation treatment on the heat exchange tubes. The specific process is as follows:

[0044] Step 1: Same as Example 2.

[0045] Step 2: Same as Example 2.

[0046] Step 3: Same as Example 2.

[0047] Step 4: With the furnace temperature not lower than 400℃, use a screw pump to evacuate the oxidation vacuum furnace to a low vacuum until the furnace pressure is 1000Pa; turn off the screw pump and fill the oxidation vacuum furnace with argon gas until the furnace pressure is 0.15MPa; use a circulating fan to cool the heat exchange tubes until the heat exchange tube temperature is not higher than 150℃, and then remove the heat exchange tubes.

[0048] Step 5: Same as Example 2.

[0049] Upon inspection, the oxide film on the surface of the heat exchanger tube obtained by high-temperature oxidation film formation treatment was generally uniform, but occasional red oxide spots were observed. After oxalic acid resistance test according to GB / T 15519-2002 standard, the oxide film in the oxalic acid-impregnated area showed very few red oxide spots compared with the oxalic acid-impregnated area in Example 2. After applying a 3% copper sulfate solution to the oxide film surface and letting it stand for 30 seconds, occasional red spots or red patches were observed, indicating that the oxide film had no pores or damage, and the porosity and continuity were slightly lower than those of the heat exchanger tube treated in Example 2.

[0050] The heat exchange tubes that had undergone the above-mentioned high-temperature oxidation film formation treatment were then subjected to oil sealing treatment: the heat exchange tubes were immersed in mineral oil heated to 60–150°C for 20 minutes, and the mineral oil could be completely vaporized at 200°C. After the oil-sealed samples were tested in a neutral salt spray test at 35±2°C for 4 hours, slight surface rust began to appear. However, after 7 hours of testing, the rust spread accelerated, and the corrosion resistance was slightly lower than that of the heat exchange tubes treated in Example 2.

[0051] Comparative Example 1:

[0052] This embodiment utilizes the oxidation vacuum furnace provided in Embodiment 1 for treating heat exchange tubes of a high-temperature gas-cooled reactor steam generator to perform high-temperature oxidation film formation treatment on the heat exchange tubes. The specific process is as follows:

[0053] Step 1: Same as Example 2.

[0054] Step 2: Same as Example 2.

[0055] Step 3: Same as Example 2.

[0056] Step 4: Fill the oxidation vacuum furnace with argon gas to create a furnace with 0.09 MPa oxygen and 0.06 MPa argon gas. Use a circulating fan to cool the heat exchange tubes until their temperature does not exceed 150°C, then remove the heat exchange tubes.

[0057] Step 5: Same as Example 2.

[0058] Upon inspection, the oxide film on the surface of the heat exchanger tube obtained by high-temperature oxidation film formation treatment showed red oxide spots and poor overall consistency. After oxalic acid resistance test according to GB / T 15519-2002 standard, the oxide film in the oxalic acid-impregnated area showed corrosion compared with other oxalic acid-impregnated areas. After applying a 3% copper sulfate solution to the oxide film surface and letting it stand for 30 seconds, reddish marks appeared, indicating that there were fine pores and damage in the oxide film, and the porosity and continuity were lower than those of the heat exchanger tube treated in Example 2.

[0059] The heat exchange tubes that had undergone the above-mentioned high-temperature oxidation film formation treatment were then subjected to oil sealing treatment: the heat exchange tubes were immersed in mineral oil heated to 60–150°C for 20 minutes, and the mineral oil could be completely vaporized at 200°C. After the oil-sealed samples were tested in a neutral salt spray test at 35±2°C for 1 hour, slight surface rust began to appear on the surface. After 4 hours of testing, the rust spread at a faster rate, and the corrosion resistance was lower than that of the heat exchange tubes treated in Example 2.

[0060] As can be seen from Examples 2-3 and Comparative Example 1, atmosphere control during the cooling process after high-temperature oxidation film formation has a significant impact on the key technical indicators and overall performance of the oxide film. Compared to Example 3, Example 2 comprehensively utilizes screw pumps, Roots pumps, and diffusion pumps to perform segmented, multi-level oxygen removal from the vacuum furnace in a "low vacuum followed by high vacuum" manner. This resulted in the furnace pressure ultimately reaching 0.05 Pa. The complete removal of oxygen effectively prevents the conversion of Fe3O4 to γ-Fe2O3 within the oxide film during the cooling process, solving the problems of instability, color difference, and frequent rework in existing oxide films.

[0061] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope of the present invention.

Claims

1. A high-temperature oxidation film formation process for heat exchanger tubes in a high-temperature gas-cooled reactor steam generator, characterized in that, Includes the following steps: Step 1: Perform vacuum heat treatment on the heat exchange tubes; Step 2: Clean the heat exchange tubes; Step 3: Perform oxidation treatment on the heat exchange tubes in an oxidation vacuum furnace; Step 4: Under the condition that the temperature inside the furnace is not lower than 400℃, remove the oxygen from the oxidation vacuum furnace, then fill the oxidation vacuum furnace with a protective atmosphere, and then cool the heat exchange tubes. In step four, the oxidation vacuum furnace has a pump device and a cooling device; the pump device includes a screw pump, which is connected to the inner cavity of the oxidation vacuum furnace through a high-temperature oxygen venting channel; the removal of oxygen from the oxidation vacuum furnace refers to: first, evacuating the oxidation vacuum furnace to a low vacuum until the furnace pressure is 800~1000 Pa, and then evacuating the oxidation vacuum furnace to a high vacuum until the furnace pressure is 0.01~0.05 Pa; the cooling treatment refers to: cooling the heat exchange tubes until the temperature of the heat exchange tubes does not exceed 150℃, and then removing the heat exchange tubes.

2. The process as described in claim 1, characterized in that, In step four, a protective atmosphere is introduced into the oxidation vacuum furnace until the pressure inside the furnace is 0.1~0.2 MPa. The protective atmosphere is an inert gas.

3. The process as described in claim 1, characterized in that, In step one, the vacuum heat treatment refers to heating the heat exchange tube to 590~640℃ under a pressure of no more than 0.01 Pa and then holding it at that temperature for 20~30 min.

4. The process as described in claim 1, characterized in that, In step two, the cleaning process refers to removing residual impurities from the surface of the heat exchange tube after shot peening or sandblasting.

5. The process as described in claim 1, characterized in that, In step three, the oxidation treatment refers to: filling the vacuum furnace with pure oxygen until the furnace pressure is 0.08~0.1 MPa, and keeping the heat exchange tube at 540~560℃ for 40~50 min.

6. The process as described in claim 1, characterized in that, It also includes step five: inspecting the oxide film on the heat exchange tube. If the inspection fails, repeat steps one through four until the inspection passes.

7. A heat exchange tube for a high-temperature gas-cooled reactor steam generator, produced by the process described in any one of claims 1 to 6.

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