Metallic material with graphite coating and method for manufacturing the same

Abstract

To provide a metal material having a long term corrosion suppression effect and a manufacturing method of the same.SOLUTION: A metal material having a graphite coating according to an aspect of the invention includes a metal substrate 10, and a coating 20 that is formed on the substrate 10, has a flat part 211 arranged along a surface of the substrate 10 and an anchor part 212 extending from a peripheral edge of the flat part 211 to an inside of the substrate 10, and is composed of a graphite crystal 21.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a metallic material having a graphite coating and a method for producing the same. [Background technology]

[0002] Most metal materials react with oxygen in their operating environment, forming an oxide film on their surface. The presence of this film prevents the metal from oxidizing. However, in actual operating environments, the oxide film is often destroyed by deteriorating factors such as acid rain, ultraviolet rays, and chlorine in the air, causing corrosion of the metal material.

[0003] To prevent such corrosion, treatments such as plating of metal materials and painting with anticorrosive paints have long been used.

[0004] Furthermore, in recent years, attention has been focused on the gas barrier properties, chemical, thermal and mechanical stability, and strength of graphene or graphite, and they have been used to coat the surfaces of metal materials.

[0005] For example, Patent Document 1 describes a nanoparticle having a diameter of 1 to 40 μm, a thickness of 30 nm or less (excluding 0 nm), and a surface area of ​​40 to 1500 m 2 It is disclosed that a steel sheet is coated with graphene oxide in which functional groups are substituted at the edges of graphene having a molecular weight of 0.5g / g in an amount of 0 to 5% (excluding 0%) of the graphene weight.

[0006] In addition to such treatments aimed at preventing corrosion of metallic materials, various techniques have been reported for forming a layer or film of a carbon material on the surface of a metallic material. For example, Patent Document 2 discloses a method of bringing a carbon material into contact with the surface of at least a part of the steel structure of a steel material, placing the steel material in a furnace while maintaining this contact state, and heating the steel material at a temperature equal to or higher than the eutectic point of cast iron, thereby dissolving and diffusing carbon from the carbon material into the steel structure, thereby producing an iron-based composite material having at least a steel structure layer, a cast iron structure layer, and a carburized structure layer obtained by carburizing the steel structure between the steel structure layer and the cast iron structure layer. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2016-504262 [Patent Document 2] Patent No. 4420015 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the corrosion prevention method of coating the surface of a metal material with a carbon material has a problem in that the corrosion inhibition effect lasts only for a short period of time due to insufficient adhesive strength between the metal material and the carbon material.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a corrosion-protective structure for metallic materials that has a corrosion-inhibiting effect that lasts for a long period of time. [Means for solving the problem]

[0010] The present inventors have conducted various studies in order to achieve the above-mentioned object, and have found that by controlling the conditions for forming a graphite coating on a metal substrate, the resulting graphite coating can be composed of graphite crystals having flat portions arranged along the surface of the substrate and anchor portions extending from the periphery of the flat portions into the interior of the substrate, and that the corrosion inhibition effect can be maintained for a long period of time, which led to the completion of the present invention.

[0011] That is, one aspect of the present invention for solving the above-mentioned problems is a metal material having a graphite coating, comprising: a metal substrate; and a coating formed on the substrate and made of graphite crystals, the graphite crystals having a flat portion disposed along the surface of the substrate and an anchor portion extending from the periphery of the flat portion into the interior of the substrate.

[0012] Another aspect of the present invention is a method for producing the aforementioned metal material having a graphite coating, including: preparing a metal substrate; housing the substrate in a heating device and heating it in an inert atmosphere to a film-forming temperature at which the amount of carbon in solid solution in the metal constituting the substrate becomes larger than room temperature; while maintaining the temperature of the substrate at the film-forming temperature, flowing a carbon-containing gas through the heating device for 10 minutes or more to form a carbon film on the surface of the substrate by chemical vapor deposition; and slowly cooling the substrate in the inert atmosphere to 750°C. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a corrosion-protective structure for metal materials that has a corrosion-inhibiting effect that lasts for a long period of time. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view showing the structure of a metal material having a graphite coating according to one aspect of the present invention. [Figure 2] Enlarged view of the substrate-coating interface in Figure 1 [Figure 3] Electron microscope photograph of the vicinity of the substrate-coating interface in a metal material having a graphite coating according to one aspect of the present invention. [Figure 4] Electron microscope photographs of a cross section of a metal material according to an embodiment of the present invention ((a) and (b): TEM images, (c): SEM image) [Figure 5] Electron microscope photographs of the front and back surfaces of the graphite coating in the metal material according to the embodiment of the present invention, and Raman spectra measured on each surface. [Figure 6] Graph showing the relationship between the depth from the surface and the etching rate for metal materials according to examples and comparative examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Each aspect of the present invention will be described in detail below based on one embodiment, but the present invention is not limited to this embodiment.

[0016] [Graphite-coated metal material] A metallic material having a graphite coating according to one aspect of the present invention (hereinafter, sometimes simply referred to as the "metallic material according to the first aspect" or "metallic material 1") includes a metallic substrate 10 and a coating 20 formed on the substrate 10, as shown in a cross section of the metallic material in FIG. 1. As shown in FIG. 2, the coating 20 is composed of graphite crystals 21 each having a flat portion 211 disposed along the surface of the substrate 10 and an anchor portion 212 extending from the periphery of the flat portion 211 into the interior of the substrate 10. As shown in FIG. 2, the substrate 10 may contain a near-interface layer 11 with a high carbon content near the interface with the coating 20. When a cross section of the metallic material according to the first aspect is observed with a scanning electron microscope (SEM), the presence of the near-interface layer 11 can be confirmed by the appearance of a mottled pattern of light and dark shading, as shown in FIG. 3.

[0017] The substrate 10 is made of metal. Its shape and material can be selected appropriately depending on the application. As the material of the substrate 10, one containing iron or nickel as the main component is preferable, since the amount of carbon dissolved in solid solution at high temperatures is larger than at room temperature. Among these, one containing iron as the main component is more preferable, since embrittlement due to the formation of a carbon-containing phase is less likely to occur. Examples of materials containing iron as the main component include those containing essentially only iron as the metal component, such as pure iron, mild steel, hard steel, and cast iron, as well as alloys containing 50% by mass or more of iron. When the substrate 10 contains essentially only iron as the metal component, the iron content is preferably 93% by mass or more, more preferably 98% by mass or more, and even more preferably consists of iron and unavoidable impurities.

[0018] The coating 20 is formed on and covers the substrate 10, and functions to prevent the substrate 10 from coming into direct contact with deteriorating factors such as acid rain, ultraviolet rays, and chlorine present in the air. In view of this function of the coating 20, the coating 20 does not necessarily have to be formed on the entire surface of the substrate 10, but it is sufficient that the coating 20 is formed at least in areas that may come into contact with deteriorating factors.

[0019] The coating 20 is composed of graphite particles 21. The graphite particles 21 have flat portions 211 arranged along the surface of the substrate 10 and anchor portions 212 extending from the periphery of the flat portions 211 into the interior of the substrate 10. The graphite particles 21 have anchor portions 212 in addition to the flat portions 211, which improves the adhesion strength of the coating 20 to the substrate 10 and prevents peeling of the coating 20 while the metal material 1 is in use, and the resulting loss of the corrosion-inhibiting effect. Note that the graphite particles 21 may also have anchor portions 212 extending into the interior of the substrate 10 from a location other than the periphery of the flat portions 211.

[0020] Here, it is confirmed by the following method that the coating 20 is composed of graphite particles 21 having flat portions 211 and anchor portions 212.

[0021] First, the metal material 1 is cut perpendicularly to the surface on which the coating 20 is formed to obtain a thin section. A focused ion beam (FIB) device can be used for cutting.

[0022] Next, the cut surface of the obtained thin piece is observed with a transmission electron microscope (TEM) or a scanning electron microscope (SEM) to obtain an image of the vicinity of the interface between the substrate 10 and the coating 20.

[0023] Next, in the obtained electron microscope image, focusing on coating 20, graphite particles 21 are identified, which are recognized as portions in which multiple layers each having a thickness of about 0.3 nm are stacked. It is confirmed whether or not these graphite particles 21 have a portion (flat portion 211) having a layered structure along the surface of substrate 10 as shown in Figure 2, and whether or not they have a portion (anchor portion 212) extending from the end of the portion having the layered structure into the interior of substrate 10 as shown in the same figure.

[0024] When the presence of these is confirmed, it is determined that the coating 20 is composed of graphite particles 21 having flat portions 211 and anchor portions 212. In light of the fact that the coating 20 is composed of graphite particles 21 of similar shapes, if even one graphite particle 21 having such a shape is confirmed, it is presumed that the coating 20 contains many graphite particles 21 of similar shapes, and therefore it can be said that the coating has sufficient adhesive strength to the substrate 10. However, since there is a possibility that graphite particles 21 of the above-mentioned shape are generated accidentally, in order to guarantee sufficient adhesive strength to the substrate 10, when 10 or more graphite particles are observed, it is preferable that more than half of them are graphite particles 21 having flat portions 211 and anchor portions 212.

[0025] The surface of the coating 20 has a Raman spectrum with a ratio of the D band peak intensity to the G band peak intensity, I D / I G In the Raman spectrum, the G band peak is due to the regular six-membered rings that constitute graphene or graphite, and the D band peak is due to defects. D / I G A small value of I means that the coating 20 is a high-quality film with few defects. Since the destruction of the coating 20 starts from defects, a coating 20 with few defects will have high durability. In order to obtain a more durable coating 20, the intensity ratio I D / I G The value is more preferably 0.05 or less, and even more preferably 0.01 or less.

[0026] The back surface of the coating 20 is the part that comes into contact with the substrate 10, and is therefore required to have an anchor effect. The presence of defects in the graphene or graphite that constitutes the back surface of the coating 20 enables a stronger bond with the substrate 10. As mentioned above, the presence of defects in the coating 20 can be confirmed by the peak of the D band in the Raman spectrum. In this case, the ratio I of the peak intensity of the D band to the peak intensity of the G band is D / I Gis preferably 0.1 or more.

[0027] There are no particular limitations on the method for measuring Raman spectra, and known devices and methods can be used. However, since the peak positions of each band in the obtained spectrum depend on the number of graphene layers and the laser wavelength used for excitation, it is preferable to use the analysis software attached to the measurement device when identifying the peaks. When single-layer graphene is measured with an excitation wavelength of 532 nm, the G-band and D-band peaks are at 1582 cm -1 , 1350cm -1 Therefore, when a Raman spectrum measured at an excitation wavelength of the same order has a single peak near each of the wavenumbers, the intensity of each peak can be taken as the peak intensity of the G band and the D band, even if it is difficult to use analytical software.

[0028] The Raman spectrum of the back surface of the coating 20 is obtained by immersing the metal material 1 relating to the first side in an acid or alkali solution capable of dissolving the substrate 10 to remove the substrate 10, and then measuring the remaining coating 20 using the method described above.

[0029] It is more preferable that coating 20 has the above-mentioned Raman spectrum intensity ratio, and that the interlayer distance of graphite particles 21 constituting coating 20 is 0.3 nm, which is equal to that of an ideal bulk material. This further improves the durability of coating 20.

[0030] In the metal material 1 according to the first aspect described above, the coating 20 formed on the surface of the substrate 10 is composed of graphite particles 21 having flat portions 211 as well as anchor portions 212, and therefore the action of the anchor portions 212 makes the coating 20 less likely to peel off from the substrate 10. As a result, the coating structure is maintained for a long period of time, and corrosion resistance is significantly improved.

[0031] [Method of manufacturing a metal material having a graphite coating] A method for producing a graphite-coated metal material according to another aspect of the present invention (hereinafter sometimes simply referred to as the "production method according to the second aspect") includes preparing a metal substrate, placing the substrate in a heating device and heating it in an inert atmosphere to a film-forming temperature at which the amount of carbon dissolved in the metal constituting the substrate becomes greater than room temperature, flowing a carbon-containing gas through the heating device for 10 minutes or more while maintaining the temperature of the substrate at the film-forming temperature to form a carbon film on the surface of the substrate by chemical vapor deposition (CVD), and slowly cooling the substrate to 750°C in the inert atmosphere. These processing steps are described in detail below.

[0032] First, a metal substrate is prepared and placed in a heating device, and after the inside of the device is filled with an inert atmosphere, the substrate is heated. The heating device to be used is not limited as long as it has gas introduction and exhaust paths and can selectively heat the substrate.

[0033] The inert atmosphere may be a rare gas atmosphere such as argon or a nitrogen atmosphere, provided that a rare gas atmosphere is used when the metal constituting the substrate reacts with nitrogen during heating.

[0034] The atmosphere in which the substrate is heated may be an inert gas atmosphere containing a reducing gas instead of an inert atmosphere. By using an atmosphere containing a reducing gas during heating, oxides present on the surface of the substrate are reduced and removed to reveal a clean surface, and a carbon film formed by chemical vapor deposition, which will be described later, is directly attached to the surface. This further improves the adhesion between the substrate and the graphite coating. Examples of usable reducing gases include hydrogen and the like.

[0035] The substrate is heated until it reaches a film-forming temperature at which the amount of carbon dissolved in the metal constituting the substrate becomes greater than room temperature. Examples of the film-forming temperature include 700°C to 1150°C for a substrate primarily composed of iron, and 800°C to 1320°C for a substrate primarily composed of nickel. The rate of temperature rise during heating is not particularly limited, and may be, for example, 5°C / min to 500°C / min.

[0036] Next, while maintaining the substrate temperature at the film-forming temperature, a carbon-containing gas is passed through the heating apparatus for 10 minutes or more, exposing carbon to the substrate surface by chemical vapor deposition. During this process, carbon atoms diffuse from the gas phase into the substrate, dissolving in the metal constituting the substrate. However, because the substrate is at the film-forming temperature, the amount of carbon atoms available for dissolution is greater than at room temperature. Furthermore, the long holding time at the film-forming temperature (10 minutes or more) and the continuous supply of the carbon-containing gas, which is the source material for the carbon film, during this time enable the continuous formation of the carbon film, enabling the supply of a sufficient amount of carbon atoms into the substrate. These factors combine to result in the dissolution of carbon in the metal located near the interface with the carbon film in an amount exceeding the solubility limit at room temperature. This carbon then forms a flat portion and an anchor portion upon slow cooling, as described below.

[0037] The carbon-containing gas flowing through the heat treatment device is not particularly limited as long as it can form a carbon film on the surface of the substrate. Examples include hydrocarbons such as methane and acetylene. Gases generated by heating and sublimating solid organic materials such as polystyrene and fluorene may also be used.

[0038] The carbon-containing gas is passed for 10 minutes or more. This allows a sufficient amount of carbon to diffuse into the substrate, enabling the formation of anchor portions during slow cooling, as described below. The carbon-containing gas is passed for a period of 20 minutes or more, preferably 50 minutes or more.

[0039] Next, the atmosphere in the heat treatment device is changed to an inert atmosphere, and the substrate is slowly cooled to 750°C in this atmosphere. By slowly cooling from the film formation temperature, the amount of carbon dissolved in the metal in the substrate decreases to a level close to equilibrium, and the carbon that is no longer dissolved precipitates as a heterogeneous phase. It is presumed that, because the atmosphere in the heat treatment device is an inert atmosphere, the carbon film formed on the substrate surface does not disappear but remains as a coating, and by connecting with the carbon precipitated as a heterogeneous phase, a flat portion and an anchor portion are formed. The temperature range for slow cooling is preferably from the film formation temperature to 730°C, more preferably from the film formation temperature to 700°C, in order to precipitate a sufficient amount of carbon.

[0040] Here, "slow cooling" in this specification means cooling the object at a rate slower than the cooling rate when the object is left in a room temperature environment without being sprayed with a refrigerant.

[0041] The slow cooling is preferably carried out under conditions in which the average cooling rate from the film formation temperature to 750°C is 200°C / min or less. This allows a sufficient amount of anchors to be formed, improving the adhesive strength between the substrate and the coating. Here, the average cooling rate refers to the value ΔT / t obtained by dividing the difference ΔT between the film formation temperature and 750°C by the time t (min) required to reach 750°C from the film formation rate. The average cooling rate is more preferably 100°C / min or less, even more preferably 20°C / min or less, and particularly preferably 10°C / min or less.

[0042] According to the manufacturing method of the second aspect including the above-mentioned processing operations, it is possible to efficiently obtain the metallic material of the first aspect, i.e., a metallic material comprising a metallic substrate and a coating formed on the substrate, the coating being composed of graphite crystals having flat portions arranged along the surface of the substrate and anchor portions extending from the periphery of the flat portions into the interior of the substrate. [Example]

[0043] Hereinafter, each embodiment of the present invention will be described in more detail based on examples, but the present invention is not limited to these examples in any way.

[0044] [Example] First, a 10 mm × 10 mm × 0.2 mm cold-rolled steel plate (SPCC: Steel Plate Cold Commercial) was prepared as a substrate. This steel plate was placed in a CVD apparatus, and an argon-hydrogen mixed gas (hydrogen content 2%) was circulated at a flow rate of 10 sccm, creating an inert gas atmosphere containing a reducing gas. Next, while maintaining the gas type and flow rate, the substrate was heated to 1050 ° C and held for 30 minutes to clean the substrate surface. Next, the gas circulating in the apparatus was changed to 100 sccm of methane-argon mixed gas (methane content 2%), and the gas temperature was then raised to 1100 ° C and held for 120 minutes to form a carbon film by the CVD method. Next, the gas circulating in the apparatus was changed to 50 sccm of argon gas, and the substrate was cooled to 727 ° C over 20 minutes. The average cooling rate at this time was 20 ° C / min. Next, the furnace was cooled to 100° C. or less while maintaining the flow rate of argon gas, to obtain a metal material according to the example.

[0045] [Comparative Example] The cold-rolled steel sheet prepared in the example was used as it was for the metal material of the comparative example.

[0046] <Evaluation> (Graphite coating shape confirmation) The graphite coating and the shape of the graphite particles constituting it were confirmed for the metal material according to the example using the method described above. As a result, it was confirmed that the graphite coating was composed of graphite particles having flat portions and anchor portions. An electron microscope photograph of the cross section of the metal material according to the example is shown in FIG. 4. In the figure, (a) and (b) are TEM images, and (c) is an SEM image. FIG. 5(a) shows an electron microscope photograph of the surface of a graphite particle, and FIG. 5(c) shows an electron microscope photograph of the back surface of the graphite particle after the metal material was dissolved in hydrochloric acid and removed.

[0047] (Raman spectrum measurement of graphite coating) The Raman spectrum of the graphite-coated surface of the metal material according to the example was measured. As a result, as shown in FIG. 5(b), -1 A sharp peak due to the G band was observed near the peak intensity ratio I D / I G The value was 0, below the measurement limit. On the other hand, the Raman spectrum of the graphite-coated backside was also measured. As a result, as shown in Figure 5 (d), a peak at 1350 cm -1 Around 1580cm -1 Sharp peaks due to the D band and G band were observed around this region.

[0048] (Corrosion resistance test) Corrosion resistance tests were conducted on the metal materials according to the examples and comparative examples using the following method. First, the thickness of each metal material was measured with a micrometer to obtain the initial thickness. Next, a 10% hydrochloric acid solution was prepared, and each metal material was entirely immersed in it. Next, after a certain time had elapsed since the start of immersion, each metal material was removed from the hydrochloric acid solution and washed. The thickness of each metal material was calculated from the mass measurement results using a precision electronic balance, and each metal material after thickness calculation was immersed again in the hydrochloric acid solution multiple times. Next, the difference (μm) between the thickness measured after the nth hydrochloric acid immersion (where n is a natural number) and the thickness measured after the (n-1)th hydrochloric acid immersion was divided by the nth immersion time (h) to calculate the etching rate (μm / h) for each immersion. Here, the thickness measured after the 0th hydrochloric acid immersion was taken as the initial thickness. Next, the difference between the thickness measured after the nth hydrochloric acid immersion and the initial thickness was calculated, and this was taken as the depth (μm) from the surface for each immersion. The results calculated for each run were then plotted on a graph with the etching rate on the vertical axis and the depth from the surface on the horizontal axis to confirm the relationship between the etching depth and the etching rate. The results are shown in Figure 6.

[0049] As can be seen from FIG. 6, the etching rate of the metal material according to the example, which has a graphite coating composed of graphite particles with anchor portions on its surface, was lower up to a depth of approximately 100 μm from the surface than the comparative example, which does not have such a coating. The decrease in etching rate was particularly significant in the region of 20 μm or less from the surface. This is presumably due to the fact that the adhesion strength of the graphite coating to the substrate is increased by the action of the anchor portions, making the graphite coating less likely to peel off from the substrate. Furthermore, from FIG. 4, the distance from the surface of the graphite coating to the tip of the anchor portions is estimated to be approximately 10 μm, so it can be seen that the etching rate of the metal material according to the example is suppressed up to a depth exceeding the thickness of the graphite coating. This is presumably due to the fact that the carbon-rich near-interface layer formed in the substrate by carbon diffusion during carbon film formation contributes to corrosion suppression. [Industrial Applicability]

[0050] According to the present invention, it is possible to provide a corrosion-resistant structure for metal materials that exhibits a long-lasting corrosion-inhibiting effect. Therefore, the present invention is useful in that it can ensure the reliability of materials by inhibiting corrosion for a long period of time in infrastructure structures, machines, parts, vehicles, etc. made of metal materials, and can also significantly reduce the costs of repairs and renewals. Furthermore, the graphite coating of the present invention also has the effect of suppressing hydrogen diffusion into the metal substrate, making the present invention useful in that it can suppress hydrogen embrittlement when a hydrogen storage tank is made of stainless steel. [Explanation of symbols]

[0051] 1 Metal materials 10 Base 11 Near-interface layer 20 Covering 21 Graphite particles 211 Flat area 212 Anchor part

Claims

1. a metallic substrate, and A coating formed on the substrate and composed of graphite crystals having flat portions disposed along the surface of the substrate and anchor portions extending from the periphery of the flat portions into the interior of the substrate. A metallic material having a graphite coating, comprising:

2. The coating has a G band peak intensity I in the Raman spectrum measured on the surface. Ｇ D band peak intensity I Ｄ Ratio I Ｄ / I Ｇ 2. The graphite-coated metal material according to claim 1, wherein the σ is 0.1 or less.

3. 3. The graphite-coated metallic material according to claim 1, wherein the substrate contains 50% by mass or more of iron.

4. 4. The graphite-coated metal material according to claim 1, wherein the substrate contains 93% by mass or more of iron.

5. Providing a metallic substrate; placing the substrate in a heating device and heating it in an inert atmosphere to a film formation temperature at which the amount of carbon solid-solubilized in the metal constituting the substrate becomes greater than that at room temperature; While maintaining the temperature of the substrate at the film formation temperature, a carbon-containing gas is passed through the heating device for 10 minutes or more to form a carbon film on the surface of the substrate by chemical vapor deposition; and Slowly cooling the substrate to 750°C in an inert atmosphere. A method for producing a metal material having a graphite coating according to any one of claims 1 to 4, comprising:

6. 6. The method for producing a graphite-coated metal material according to claim 5, wherein the slow cooling is carried out under conditions such that an average temperature drop rate is 200°C / min or less.

7. 7. The method for producing a metal material having a graphite coating according to claim 5 or 6, wherein the temperature is raised to the film formation temperature in an inert gas atmosphere containing a reducing gas, and then the atmosphere is changed to form the carbon film.

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