A fluorinated stainless steel composite material, a method for preparing the same, and an application thereof

Abstract

The application provides a fluorinated stainless steel composite material and a preparation method and application thereof. The fluorinated stainless steel composite material comprises a stainless steel base material and a fluorinated layer. The fluorinated layer comprises a saturated concentration of fluorine elements. The saturated concentration is 3wt%-15wt%. The application utilizes the strong oxidizing property of fluorine which is higher than that of oxygen to form a fluorinated layer with a saturated fluorine concentration on the surface of the stainless steel, so as to prevent the process of iron reacting with water to form iron oxide.

Description

Technical Field

[0001] This application relates to the field of materials technology, specifically to a fluorinated stainless steel composite material, its preparation method, and its application. Background Technology

[0002] Compared to copper heat spreaders, stainless steel heat spreaders offer a range of advantages, including higher strength, lower cost, and better corrosion resistance. However, stainless steel heat spreaders still have drawbacks that limit their large-scale application. For example, the heat spreader operates under a vacuum. Under these conditions, stainless steel reacts with water and water vapor, releasing hydrogen gas. This hydrogen gas reduces the vacuum level inside the heat spreader, causing a rapid decline in heat homogenization and ultimately leading to its failure. Effectively resolving the hydrogen evolution reaction between stainless steel and water would rapidly promote the widespread use of stainless steel heat spreaders.

[0003] The reaction between stainless steel and water is mainly due to the reaction between the iron ions in the stainless steel and water.

[0004] Fe + H₂O = FeO + H₂↑

[0005] 2Fe + 3H₂O = Fe₂O₃ + 3H₂↑

[0006] 3Fe + 4H₂O = Fe₃O₄ + 4H₂↑

[0007] Traditional carbon steel can be treated with blackening to form an iron oxide film on its surface, thus preventing the hydrogen evolution reaction. However, stainless steel contains a large amount of alloying elements such as chromium and nickel, making it difficult to form a continuous and dense iron oxide film on its surface. Therefore, it is necessary to form a continuous and dense film on the surface of stainless steel to prevent the reaction between water and stainless steel. Summary of the Invention

[0008] To address the aforementioned problems, this application proposes a fluorinated stainless steel composite material. Utilizing the stronger oxidizing power of fluorine than oxygen, a fluorinated layer with a saturated fluorine concentration is pre-formed on the stainless steel surface, which can prevent the reaction of iron with water to form iron oxide, thus overcoming the deficiencies and defects mentioned in the prior art.

[0009] To achieve the above objectives, this application adopts the following technical solution:

[0010] The inventive point of this application is to provide a fluorinated stainless steel composite material, comprising a stainless steel substrate and a fluorinated layer; the fluorinated layer comprises fluorine element with a saturated concentration of 3wt% to 15wt%. Optionally, the fluorinated layer comprises: fluorine 3wt% to 15wt%, iron 50wt% to 70wt%, chromium 10wt% to 20wt%, nickel 6wt% to 12wt%, molybdenum 2wt% to 12wt%, manganese <2wt%, cobalt <2wt%, and carbon <0.08wt%.

[0011] Another inventive point of this application is to provide a method for preparing the fluorinated stainless steel composite material as described in any of the above.

[0012] Optionally, the preparation method includes: (1) subjecting stainless steel to high-temperature treatment to increase the stainless steel grain size and obtain a fluorinated precursor; (2) immersing the high-temperature treated fluorinated precursor in a saturated solution of ammonium fluoride or ammonium hydrogen fluoride to obtain a fluorinated intermediate with surface-adsorbed fluorides; (3) baking the fluorinated intermediate to obtain the fluorinated stainless steel composite material.

[0013] Optionally, the conditions for high-temperature treatment in step (1) are: temperature 900–1050°C, time 1–10 h. Optionally, the grain size of the stainless steel in step (1) is grade 1–3.

[0014] Optionally, the soaking conditions are as follows: the temperature is 10-30℃ and the time is 3-10h.

[0015] Optionally, the interval between steps (2) and (3) shall not exceed 4 hours.

[0016] Optionally, the baking temperature is 130–500℃ and the baking time is 0.5–2 hours.

[0017] Another inventive point of this application is to provide an application of the fluorinated stainless steel composite material as described above in a heat spreader.

[0018] Compared with the prior art, this application has the following advantages:

[0019] (1) This application forms a fluorinated layer with a saturated fluorine concentration on the surface of stainless steel. The fluorinated layer also contains chromium, nickel and molybdenum with a higher concentration than the substrate, which greatly reduces the content of iron on the surface. Moreover, these high concentrations of chromium, nickel and molybdenum can not only improve the corrosion resistance of stainless steel, but also form a coating on the substrate and form a continuous film with fluorine to prevent water vapor from corroding the stainless steel.

[0020] (2) This application first uses high temperature treatment to rearrange the composition and grains of stainless steel surface, increasing the proportion of chromium, nickel and molybdenum on the surface; at the same time, the high temperature treatment causes the stainless steel grains to undergo high temperature diffusion rearrangement, and the final grain size becomes larger; the larger the grain size, the greater the solubility of fluorine on the grain surface, and the more fully it combines with iron ions, thus laying the foundation for the next step of saturated fluorine penetration; then the surface fluorine penetration is carried out by immersing the high temperature treated fluoride precursor in a saturated solution of ammonium fluoride or ammonium bifluoride. The fluorine element in the solution can combine with iron quickly to react and generate a complex, reducing the content of free iron ions; then it works together with corrosion-resistant elements such as molybdenum, chromium, and nickel to generate a protective layer, further encapsulating the iron and forming a continuous protective layer; finally, it is baked to decompose the excess ammonium fluoride or ammonium bifluoride on the surface, and can also remove hydrogen on or inside the stainless steel surface, reducing the generation of hydrogen embrittlement. Attached Figure Description

[0021] Figure 1 An optical microscope image of the stainless steel surface after high-temperature fluorine treatment, provided in Test Example 1 of this application.

[0022] Figure 2 An optical microscope image of the fluorinated layer of the fluorinated stainless steel composite material provided in Test Example 1 of this application.

[0023] Figure 3 An optical microscope image of the fluorinated layer of the fluorinated stainless steel composite material provided in Test Example 2 of this application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this application clearer, a more detailed description is provided below. However, it should be understood that the description herein is merely for explaining this application and is not intended to limit its scope.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. All reagents and instruments used herein are commercially available, and the characterization methods involved can be found in relevant descriptions in the prior art, and will not be repeated here.

[0026] To further understand this application, the following detailed description is provided in conjunction with the preferred embodiments.

[0027] Example 1

[0028] This embodiment provides a fluorinated stainless steel composite material, including a stainless steel substrate and a fluorinated layer; the fluorinated layer includes fluorine element with a saturated concentration; the saturated concentration is 3wt% to 15wt%.

[0029] The fluorine element combines with the iron on the surface of stainless steel. Through full combination with the iron, a relatively dense protective layer can be formed, preventing external water and gases from corroding the stainless steel.

[0030] The fluorinated layer comprises: 3wt%–15wt% fluorine, 50wt%–70wt% iron, 10wt%–20wt% chromium, 6wt%–12wt% nickel, 2wt%–12wt% molybdenum, <2wt% manganese, <2wt% cobalt, and <0.08wt% carbon.

[0031] Compared to stainless steel substrates, fluorinated layers have higher chromium, nickel, and molybdenum content, which significantly reduces iron content. At the same time, chromium, nickel, and molybdenum can also form eutectic or alloy forms with some iron, further reducing the free iron content. Fluorine then combines with the remaining free iron, thereby preventing the free iron ions from reacting with water.

[0032] The thickness of the fluorinated layer is 10–25 μm, for example, it can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm or any value between any two values.

[0033] When the size is too small, the protective layer is not thick enough, the protective effect is weakened, and gaps are easily formed, allowing acidic substances to enter the stainless steel and cause corrosion of the substrate. When the size is too large, on the one hand, it reduces the subsequent hydrogen leakage, which causes the surface brittleness to increase rapidly, resulting in a decrease in the overall material toughness; on the other hand, excessive thickness will affect the heat transfer performance.

[0034] Example 2

[0035] This embodiment provides a method for preparing a fluorinated stainless steel composite material. The fluorinated stainless steel composite material is described in the same way as the fluorinated stainless steel composite material in Example 1, and will not be repeated here.

[0036] The preparation method includes: (1) subjecting stainless steel to high-temperature treatment to increase the stainless steel grain size and obtain a fluorinated precursor; (2) immersing the high-temperature treated fluorinated precursor in a saturated solution of ammonium fluoride or ammonium hydrogen fluoride to obtain a fluorinated intermediate with surface adsorbed fluorides; (3) baking the fluorinated intermediate to obtain the fluorinated stainless steel composite material.

[0037] Stainless steel includes at least one of 304, 304L, 316, and 316L, preferably 316.

[0038] The purpose of high-temperature treatment is to rearrange the surface composition and grains of stainless steel. For example, iron has the worst high-temperature resistance; at high temperatures, iron atoms will penetrate into the interior of the stainless steel, thus reducing their content on the surface. Meanwhile, chromium, nickel, and molybdenum have the lowest high-temperature resistance. After high-temperature treatment, the chromium, nickel, and molybdenum content on the surface can be significantly higher than the original proportions, thereby improving the surface's corrosion resistance. Taking stainless steel 316 as an example, after high-temperature treatment, the molybdenum content on the stainless steel surface is 2wt% to 3wt% higher than the original composition of stainless steel 316.

[0039] High-temperature treatment causes the stainless steel grains to undergo high-temperature diffusion rearrangement, which also increases the grain size on the stainless steel surface. The larger the grain size, the greater the solubility of fluorine on the grain surface, and the more fully it combines with iron ions. After high-temperature treatment, the grain size of the stainless steel surface is grade 1 to 3, i.e., coarse grains.

[0040] The conditions for high-temperature treatment in step (1) are: temperature 900~1050℃, time 1~10h.

[0041] This high-temperature treatment needs to be carried out in a non-oxidizing atmosphere, preferably an inert atmosphere, to prevent oxidation of the stainless steel, which would affect the subsequent penetration of fluorine.

[0042] Non-oxidizing atmospheres include inert atmospheres and / or reducing atmospheres.

[0043] Inert atmospheres include at least one of helium atmosphere, neon atmosphere, argon atmosphere, krypton atmosphere, and xenon atmosphere; reducing atmospheres include at least one of hydrogen atmosphere, hydrogen sulfide atmosphere, and carbon monoxide atmosphere.

[0044] Then, fluorine is infiltrated, specifically by immersing the high-temperature treated fluorinated precursor in a saturated solution of ammonium fluoride or ammonium hydrogen fluoride to obtain a fluorinated intermediate with surface-adsorbed fluorides.

[0045] Soaking conditions: temperature 10–30℃, time 3–10 hours. During the soaking process, maintain the soaking temperature to avoid decomposition of fluorides due to temperature rise.

[0046] Then bake, but the interval between steps (2) and (3) should not exceed 4 hours. That is, after the fluorine is infiltrated, it needs to be baked within 4 hours. If the time is too long, some hydrogen will enter the stainless steel and cause problems such as "hydrogen embrittlement".

[0047] The baking temperature is 130-500℃, and the time is 0.5-2 hours.

[0048] Baking is performed to decompose ammonium fluoride or ammonium bifluoride, preventing excess ammonium fluoride or ammonium bifluoride from remaining on the stainless steel surface and affecting its properties. At the same time, it can remove hydrogen adsorbed on the stainless steel surface in a timely manner, thereby preventing hydrogen embrittlement from occurring during the use of the stainless steel heat spreader.

[0049] This embodiment also provides an application of the fluorinated stainless steel composite material as described above in a heat spreader.

[0050] Example 3

[0051] Based on the content of this application, the preparation methods of the fluorinated stainless steel composite material in Example 1 and Example 2 are described in detail below:

[0052] Experimental Example 1

[0053] (1) In a helium atmosphere, 316 stainless steel is subjected to high-temperature treatment at 950°C for 7 hours to increase the stainless steel grain size and obtain a fluorinated precursor with a grain size of grade 2; (2) The fluorinated precursor is immersed in a saturated solution of ammonium fluoride at a temperature of 25°C, i.e., all surfaces of the fluorinated precursor are submerged in the solution for 2 hours to obtain a fluorinated intermediate with surface adsorbed fluoride; (3) After immersion, wait for 1.5 hours and bake the fluorinated intermediate at 200°C for 2 hours to obtain a fluorinated stainless steel composite material.

[0054] like Figure 1 As shown, the stainless steel surface after high-temperature treatment has significantly larger grains compared to the untreated surface in Comparative Example 1.

[0055] like Figure 2 As shown, the fluorinated layer is a continuous and dense film with a relatively smooth and flat surface; the thickness of the fluorinated layer is 15 μm.

[0056] The elemental content of two random regions of the fluorinated stainless steel composite material was detected, and the results are shown in Table 1.

[0057] Table 1

[0058]

[0059] As shown in Table 1, the content of each element in the two regions is not much different, indicating that the elements in the fluorinated layer prepared in this application are evenly distributed and can completely, continuously and densely coat the stainless steel surface, thereby avoiding the side reaction of water and iron.

[0060] Experimental Example 2

[0061] (1) In a helium atmosphere, 316 stainless steel is subjected to high-temperature treatment at 1020℃ for 5 hours to increase the stainless steel grain size and obtain a fluorinated precursor with a grain size of grade 1; (2) The fluorinated precursor is immersed in a saturated solution of ammonium fluoride and the temperature is maintained at 25℃, that is, all surfaces of the fluorinated precursor are submerged in the solution for 5 hours to obtain a fluorinated intermediate with surface adsorbed fluoride; (3) After immersion, the fluorinated intermediate is baked immediately at 450℃ for 1 hour to obtain a fluorinated stainless steel composite material.

[0062] like Figure 3 As shown, the fluorinated layer of this application has a smooth and flat surface; the thickness of the fluorinated layer is 22 μm.

[0063] The elemental content of the fluorinated stainless steel composite material was tested in two random areas, and the results are shown in Table 2.

[0064] Table 2

[0065]

[0066]

[0067] As shown in Table 2, the elemental contents are similar between the two regions, indicating that the fluorinated layer contains the same amount of components at all locations.

[0068] Comparative Example 1

[0069] It is basically the same as Experiment 1, except that step (1) is missing, that is: the 316 stainless steel is not subjected to high temperature treatment, and steps (2) and (3) are carried out directly.

[0070] Because no high-temperature treatment was performed, the grain size of the fluorinated precursor did not change, resulting in limited subsequent fluorine penetration. Consequently, an effective protective fluorinated layer could not be obtained, and the fluorine content in the fluorinated layer was only 0.1%. That is, the elemental content of the fluorinated layer of the fluorinated stainless steel composite material obtained in this comparative example is: F 0.1%, Cr 16.5%, Mn 1.16%, Fe 68.07%, Co 1.25%, Ni 10.4%, and Mo 2.52%.

[0071] Comparative Example 2

[0072] It is basically the same as Experimental Example 1, except that step (2) is missing, that is, the fluorinated precursor is not soaked, and step (3) is carried out after step (1).

[0073] Since this comparative example was not immersed in a saturated solution of ammonium fluoride, there was no fluorine penetration. However, during the high-temperature treatment stage, the chromium, nickel, and molybdenum content on the surface was much higher than the original content ratio, namely: Cr 18.1%, Mn 1.45%, Fe 58.08%, Co 1.29%, Ni 11.4%, and Mo 9.68%.

[0074] Comparative Example 3

[0075] The experiment is basically the same as in Experiment 1, except that step (2) is changed, that is, the fluorinated precursor is soaked in water. Steps (1) and (3) are the same as in Experiment 1.

[0076] Almost identical to Comparative Example 2, there was still no fluorine penetration; however, during the high-temperature treatment stage, the chromium, nickel, and molybdenum content on the surface was much higher than the original proportions.

[0077] Comparative Example 4

[0078] It is basically the same as Experiment 1, except that step (3) is missing, that is, no baking is performed after soaking.

[0079] The fluoride layer has the same elemental content as in Example 1. However, hydrogen embrittlement is more likely to occur in stainless steel. In addition, untreated ammonium fluoride on the surface can cause potential safety problems. Therefore, baking is recommended.

[0080] Performance testing

[0081] The state of iron in the fluorinated layer was tested. When free iron is present, it readily reacts with water. The test method used was the blue spot test, specifically: 5 grams of potassium ferricyanide (K3[Fe[CN]6]) was added to 1 ml of 98% sulfuric acid and 5 ml of 36% hydrochloric acid, along with an appropriate amount of distilled water to prepare a 100 ml solution (prepared fresh before use). The solution was directly applied or dropped onto the surfaces of the test examples and comparative examples, and the appearance of blue spots was observed, i.e., whether blue spots appeared and the time it took for them to appear. It should be noted that this test should be performed after the acid-washed and passivated surfaces have basically dried. The test liquid should be thoroughly rinsed off after the test.

[0082] The basic principle of the blue dot test is that if the surface passivation film is imperfect or there is ferrous ion contamination, ferrous ions will be present, and the following reaction will occur.

[0083] 3Fe2 + +2[Fe[CN6]]-=Fe3[Fe[CN]6]2 precipitate is a dark blue precipitate, which can be used to check whether passivation treatment has been performed and to check the passivation effect.

[0084] Results: In Comparative Example 1, the blue spots appeared on the fluorinated stainless steel composite material within 10 minutes, indicating the presence of a certain amount of free iron. In Comparative Examples 2 and 3, the blue spots appeared within 18 minutes, indicating that the amount of free iron on the stainless steel surface decreased after high-temperature baking; however, this treatment method could not significantly improve the corrosion resistance of the material. In contrast, in Test Examples 1-4 of this application, the fluorinated layer did not show blue spots after 24 hours, indicating that the amount of free iron in the alloy layer was almost non-existent, and the infiltrated fluorine could form a continuous film with high concentrations of chromium, nickel, and molybdenum, thus exhibiting good corrosion resistance. The blue spot time in Comparative Example 4 was the same as in the test examples, but due to brittleness and safety issues, its application scenarios were limited.

[0085] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A fluorinated stainless steel composite material, characterized in that, It includes a stainless steel substrate and a fluorinated layer; the fluorinated layer contains fluorine at a saturated concentration. The saturation concentration is 3wt%~15wt%; The preparation method of the fluorinated stainless steel composite material includes: (1) High-temperature treatment of stainless steel increases the stainless steel grain size and yields fluorinated precursor; (2) The fluorinated precursor after high-temperature treatment is immersed in a saturated solution of ammonium fluoride or ammonium hydrogen fluoride to obtain a fluorinated intermediate with surface adsorbed fluoride. (3) The fluorinated intermediate is baked to obtain the fluorinated stainless steel composite material; The conditions for the high-temperature treatment in step (1) are: temperature 900~1050℃, time 1~10 h; The baking temperature is 130~500℃, and the time is 0.5h~2h.

2. The fluorinated stainless steel composite material according to claim 1, characterized in that, The fluorinated layer comprises: 3wt%~15wt% fluorine, 50wt%~70wt% iron, 10wt%~20wt% chromium, 6wt%~12wt% nickel, 2wt%~12wt% molybdenum, <2wt% manganese, <2wt% cobalt, and <0.08wt% carbon.

3. The fluorinated stainless steel composite material according to claim 1, characterized in that, In step (1), the grain size of the stainless steel is grade 1 to 4.

4. The fluorinated stainless steel composite material according to claim 1, characterized in that, Soaking conditions: temperature 10~30℃, time 3~10h.

5. The fluorinated stainless steel composite material according to claim 1, characterized in that, The interval between steps (2) and (3) shall not exceed 4 hours.

6. The application of the fluorinated stainless steel composite material according to claim 1 in a heat exchanger.

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