Anti-corrosion film and method for producing same

Abstract

An anti-corrosion film 12, which is provided on a surface of a magnesium-lithium alloy, contains 20-40 mass% of phosphorus, 25 mass% or more of oxygen, 17 mass% or less of fluorine, 20-30 mass% of magnesium and 5 mass% or less of nitrogen. The anti-corrosion film has an amorphous structure. With respect to an alloy member comprising a magnesium-lithium alloy as a base material, thickness irregularity and film breakage when dropped are suppressed with respect to the anti-corrosion film that is provided on a surface of the base material, the appearance of the member is improved, and coating film separation of the coated film is suppressed.

Description

Corrosion-preventive film and method for manufacturing the same

[0001] This disclosure relates to a corrosion-preventive film formed on the surface of a substrate made of a magnesium alloy, and a method for manufacturing the corrosion-preventive film.

[0002] Magnesium-lithium alloys, which are primarily composed of magnesium and contain lithium, are lightweight and possess excellent mechanical strength. Therefore, their application to various products is being considered. However, because magnesium alloys are prone to corrosion, it has been necessary to form an anti-corrosion film on the surface of magnesium alloy substrates. Patent Document 1 discloses a method for manufacturing magnesium alloy products in which an anti-corrosion film is formed on the surface of the magnesium alloy by anodizing.

[0003] Japanese Patent Publication No. 2016-102236

[0004] However, when the anticorrosion film obtained by the manufacturing method described in Patent Document 1 is applied to a substrate made of magnesium-lithium alloy, uneven film thickness occurs, particularly on substrates injected by die casting, resulting in a problem where a good painted appearance cannot be obtained. While forming a thicker anticorrosion film can reduce the appearance defects caused by uneven film thickness, a thicker anticorrosion film leads to the problem of paint peeling due to cracking of the anticorrosion film when the product is dropped. Therefore, the present disclosure aims to suppress appearance defects caused by uneven film thickness and paint peeling due to cracking of the anticorrosion film formed on the surface of a magnesium-lithium alloy.

[0005] The first aspect of this disclosure is a corrosion-preventive film that covers and prevents corrosion of a substrate mainly composed of magnesium and containing lithium, characterized in that it contains 20% to 40% by mass of phosphorus, 25% or more by mass of oxygen, 17% or less by mass of fluorine, 20% to 30% by mass of magnesium, and 5% or less by mass of nitrogen, and has an amorphous structure.

[0006] The second aspect of this disclosure is a method for manufacturing an alloy member, comprising the steps of arranging an anode and a cathode in an electrolyte and applying a voltage between the anode and the cathode to form a corrosion-preventive film on the anode, wherein the anode is a substrate mainly composed of magnesium and containing lithium, and the electrolyte is located within a region enclosed by the coordinates (X, Y) of the following coordinates [1] to [5], where the fluoride ion concentration is X mol / L and the phosphate ion concentration is Y mol / L: [1] (1.9, 0.7) [2] (5.8, 0.7) [3] (10.0, 4.1) [4] (4.4, 4.1) [5] (1.9, 2.2)

[0007] The third aspect of this disclosure is an alloy member comprising a substrate mainly composed of magnesium and containing lithium, and a corrosion-resistant film provided on the substrate, wherein the corrosion-resistant film is the corrosion-resistant film according to the first disclosure.

[0008] According to this disclosure, an alloy member is provided in which unevenness in the thickness of the anticorrosion film applied to the surface of a magnesium-lithium alloy and cracking of the film when dropped are improved, resulting in an improved appearance and suppressed peeling of the coating.

[0009] This is a schematic cross-sectional view of an alloy member according to an embodiment of this disclosure. This is a flow diagram of the manufacturing process of an alloy member according to an embodiment of this disclosure. This is a schematic diagram of an anodizing apparatus for producing a corrosion-resistant film according to an embodiment of this disclosure. This is a schematic diagram of an imaging apparatus to which an alloy member according to an embodiment of this disclosure is applied. This is a diagram showing the XRD analysis results of the corrosion-resistant films of the examples and comparative examples. This is a schematic diagram illustrating the evaluation method for paint peeling in the examples and comparative examples.

[0010] The corrosion-preventive film of this disclosure is a corrosion-preventive film formed on a substrate made of a magnesium-lithium alloy, and is characterized by containing predetermined amounts of phosphorus, oxygen, fluorine, and magnesium, and having an amorphous structure. Furthermore, the alloy member of this disclosure is characterized by comprising a substrate made of a magnesium-lithium alloy and the corrosion-preventive film of this disclosure, and preferably the corrosion-preventive film is covered with a coating film. Embodiments of this disclosure will be described below.

[0011] <Alloy Member> Figure 1 is a schematic diagram of the alloy member of this disclosure, and is a cross-sectional view taken when cut in the lamination direction. The alloy member 10 of this embodiment comprises a base material 11, an anticorrosion film 12 provided on the base material 11, and a coating film 13 provided on the anticorrosion film 12. The use of the alloy member of this embodiment is not particularly limited, and it can be used as a structural material such as an exterior member, interior member, and sliding member of equipment having parts. Depending on the application, two or more films such as a primer and a topcoat layer may be provided on the anticorrosion film 12. Examples of coating films 13 include a heat-shielding film, a light-shielding film, and an anti-reflective film that have a heat-shielding function.

[0012] The inventors of this disclosure first found that by performing the anodic oxidation treatment to form a corrosion-preventive film 12 on the surface of the substrate 11 in an electrolyte containing fluorine, ammonium, and phosphoric acid (described later), the voltage used for film formation can be reduced. Furthermore, they found that by setting the composition range so that the formed corrosion-preventive film 12 has an amorphous structure, cracking and unevenness of the film thickness of the corrosion-preventive film 12 can be reduced, and the film can be made to address the issues of peeling and poor appearance of the coating film 13 applied to the surface of the corrosion-preventive film 12.

[0013] On the other hand, in conventional anodic oxidation treatment and micro-arc oxidation treatment described in Patent Document 1, the corrosion protection film 12 can be made amorphous by increasing the voltage, and uneven film thickness can be suppressed. However, films formed at high voltage have many voids and cracks and do not become dense films. As a result, cracking of the corrosion protection film 12 may occur due to impacts such as when the product is dropped, causing the coating to peel off.

[0014] As described above, we found that by lowering the voltage during anodizing and creating an amorphous structure with elements within a desired range for the protective film, it is possible to achieve both reduced peeling due to cracking of the protective film and a good appearance with reduced film thickness unevenness.

[0015] [Base Material] The base material 11 is made of a magnesium-lithium alloy (hereinafter referred to as Mg-Li alloy) whose main component is magnesium (Mg) and which contains lithium (Li). In this specification, the main component is the element with the largest total mass among the elements contained when the material is composed of multiple elements. Or, when the material is composed of multiple compounds, it is the compound with the largest total mass among the compounds contained.

[0016] Among Mg-Li alloys, it is preferable that the sum of the Mg and Li content is 90% by mass or more for use as the base material 11. When the sum of the Mg and Li content is 90% by mass or more, it becomes lighter than magnesium alloys that do not contain Li. Mg-Li alloys are lightweight metallic materials and have superior vibration damping properties and specific strength compared to magnesium alloys that do not contain Li. Superior vibration damping means that vibrations are quickly dampened by rapidly converting vibration energy into thermal energy. Specific strength is the tensile strength per unit density, and the higher the specific strength, the lighter the component can be.

[0017] The Mg-Li alloy may contain aluminum (Al) and zirconium (Zr) in addition to Mg and Li, and may also contain germanium (Ge) and / or beryllium (Be). Furthermore, in addition to the elements mentioned above, it is possible to contain at least one element selected from the group consisting of zinc (Zn), calcium (Ca), silicon (Si), and manganese (Mg), with the remainder being unavoidable impurities, magnesium, and lithium. Examples of unavoidable impurities include iron (Fe), cobalt (Co), and nickel (Ni).

[0018] The Li content in the Mg-Li alloy is preferably in the range of 0.5% by mass to 15% by mass. A Li content of 0.5% by mass or more allows for a lightweight alloy, while a Li content of 15% by mass or less provides sufficient vibration damping. Preferably, it is between 8% by mass and 14% by mass. Even more preferably, it is in the range where the α phase and β phase coexist, which is between 5% by mass and 11% by mass. Within this range, the corrosion resistance of the base material 11 is high.

[0019] The Al content in Mg-Li alloys is preferably in the range of 1% by mass to 8% by mass. In Mg-Li alloys, Al plays a role in increasing the fracture strength of the alloy. Therefore, when the Al content is within the above range, the mechanical strength of the Mg-Li alloy can be sufficiently high compared to when Al is not present. This is thought to be because Al and Mg react, and their compound, MgAl2, precipitates, thereby increasing the mechanical strength. A more preferable content is in the range of 4% by mass to 7% by mass.

[0020] The total content of germanium (Ge) and beryllium (Be) in Mg-Li alloys is in the range of 0.02% by mass or more and 0.4% by mass or less. In Mg-Li alloys, Ge and Be play a role in improving corrosion resistance by partially replacing Al. As mentioned above, in Mg-Li alloys containing Al, the mechanical strength increases due to the reaction of Al and Mg, but at that time, lithium-rich grain boundaries segregate in the matrix phase, making it susceptible to corrosion. However, by partially substituting Al with elements with smaller atomic radii than Al, such as Ge and Be, Ge and Be are actively positioned at the grain boundaries in place of Li, and the segregation of Li at the grain boundaries can be suppressed. Therefore, corrosion resistance can be improved. The content of Ge alone is preferably in the range of 0.01% by mass or more and 0.4% by mass or less. More preferably, it is in the range of 0.01% by mass or less and 0.2% by mass or less. Furthermore, the Be content is preferably in the range of 0.02% by mass or more and 0.1% by mass or less. More preferably, it is in the range of 0.01% by mass or more and 0.05% by mass or less.

[0021] The Zr content in the Mg-Li alloy is preferably in the range of 0.6% by mass or more and 3.0% by mass or less. This is because it prevents the particle size of the base material 11 from becoming coarser.

[0022] In Mg-Li alloys, Zn, Ca, Si, and manganese (Mn) can increase the strength of the base material 11. The sum of the content of these elements is preferably in the range of 0.01% by mass or more and 5% by mass or less. If Zn is present, it is preferably present at 3% by mass or less. More preferably at 0.2% by mass or more and 3% by mass or less. If Mn is present at 0.3% by mass or less. More preferably at 0.1% by mass or more and 0.3% by mass or less. If Si is present at 0.2% by mass or less. More preferably at 0.1% by mass or more and 0.2% by mass or less. If Ca is present at 3.0% by mass or less. More preferably at 0.1% by mass or more and 1.0% by mass or less.

[0023] The raw materials for the Mg-Li alloy are not particularly limited. Commercially available options include, for example, "LZ91," "LAZ771," and "LAZ941" manufactured by Anli Materials Technology Co., Ltd.

[0024] The thickness of the base material 11 is not particularly limited, but from the viewpoint of ensuring sufficient rigidity, it is preferable that it be thicker than the thickness of the anticorrosion film 12.

[0025] [Corrosion Protection Film] The corrosion protection film 12 is provided on the substrate 11. The average thickness of the corrosion protection film 12 is 5 μm or more and 40 μm or less. If the thickness of the corrosion protection film 12 is 40 μm or less, peeling of the coating due to cracking of the corrosion protection film 12 when the product is dropped is suppressed. Also, if the thickness of the corrosion protection film 12 is 5 μm or more, unevenness in appearance due to unevenness in film thickness is suppressed.

[0026] Preferably, the corrosion protection film 12 contains at least Mg, phosphorus (P), fluorine (F), and oxygen (O). Furthermore, the content of each element is preferably 20% to 40% by mass for P, 25% or more by mass for O, 17% or less by mass for F, 20% to 30% by mass for Mg, and 5% or less by mass for nitrogen. By setting the content of each element in the corrosion protection film 12 within this range, the corrosion protection film 12 can be made amorphous, resulting in a dense film that suppresses both cracking and uneven film thickness. On the other hand, if the content of each element in the corrosion protection film 12 falls outside the above range, crystallization of the corrosion protection film 12 is more likely to occur, making it difficult to form an amorphous structure, which can lead to cracking and uneven film thickness. The reason why film cracking can be suppressed when the anticorrosion film 12 has an amorphous structure is that the melting point in its component range is lowered, allowing it to melt at a lower temperature. This reduces the temperature difference until the molten material solidifies, decreasing the shrinkage rate, which is expected to reduce cracks and residual stress, thus making it less prone to cracking. This can also be inferred from the fact that the anticorrosion film 12 of this embodiment can be formed at a lower voltage.

[0027] A preferred composition is one in which the mass ratio of F content to Mg content is 0.05 to 0.70, the mass ratio of P content is 0.8 to 1.41, and the mass ratio of oxygen content is 1.5 to 2.2.

[0028] <Method for Manufacturing the Anticorrosion Film> Figure 2 is a flowchart of the manufacturing process for the alloy member 10 of this embodiment, and Figure 3 is a schematic diagram of an anodizing apparatus for producing an anticorrosion film 12 by anodizing the base material 11.

[0029] As shown in Figure 2, the alloy member 10 of this embodiment is prepared by (1) preparing an electrolyte solution and (2) pickling the substrate in parallel, and (3) performing an anodizing treatment, in which the substrate 11 is used as the anode to perform anodizing treatment and form a corrosion-resistant film 12. After that, in (4) painting, a coating film 13 is applied to the surface of the corrosion-resistant film 12. Depending on the cleanliness of the substrate to be coated, it is possible to omit (2) pickling the substrate. Step (3) will be described in detail below.

[0030] The anodic oxidation apparatus 30 shown in Figure 3 consists of an outer tank 32 that holds and temperature-controls the electrolyte 31 and an inner tank 33 that performs the electrical reaction. The outer tank 32 is equipped with a temperature control mechanism 34 that maintains a constant temperature of the electrolyte 31. The temperature of the electrolyte 31 can be set from a low temperature where the components do not condense to a high temperature where the components do not decompose. The optimal temperature is around 25°C, which does not require much energy to adjust the liquid temperature. Although the apparatus is described as a two-tank system, it is also possible to produce the corrosion-resistant film 12 with a single-tank system.

[0031] The outer tank 32 and the inner tank 33 are connected by a magnetic pump 35, and the electrolyte 31 is circulated between them. In the formation of the anticorrosion film 12, exchange of the electrolyte on the surface of the substrate 11 is important, but since the anodic oxidation reaction is an exothermic reaction, the electrolyte 31 on the surface undergoes self-convection, resulting in active exchange of the electrolyte. Therefore, the circulation of the electrolyte by the pump 35 is more for the removal of by-products generated in the electrolyte 31 than for exchange of the electrolyte on the surface. In the anodic oxidation of the Mg-Li alloy surface, the anticorrosion film 12 is formed, and Li from the Mg-Li alloy is released into the solution as ions. These released lithium ions react with fluoride ions and phosphate ions, which are components of the electrolyte, to produce sparingly soluble salts. These sparingly soluble salts may remain in the electrolyte 31 as fine particles, potentially causing the electrolyte 31 to become cloudy. Therefore, a bag filter 36 is provided at the liquid outlet of the inner tank 33 to remove the fine particles generated in the electrolyte 31. The filter 36 is preferably capable of removing fine particles of 10 μm or larger. Furthermore, the filter may also be installed within the piping system connected to the pump 35.

[0032] Next, an electrical circuit capable of forming a corrosion-resistant film 12 is formed. A carbon electrode that functions as a cathode 37 is placed inside the inner tank 33. The cathode material is not particularly limited as long as it is a material that is stable and conductive to the electrolyte 31. For example, platinum, stainless steel, and titanium can also be used.

[0033] Next, the anode 38, made of Mg-Li alloy, is set by being sandwiched in the conductive holding jig 39. Although a native oxide film is formed on the surface of the anode 38, it does not need to be removed beforehand as it will be replaced by a fluoride film, phosphate film, or fluorine phosphate film during the anodizing process. If oil stains are present on the surface of the anode 38 due to machining or other processes, it is necessary to go through a pre-cleaning process to remove the oil stains.

[0034] The material of the conductive holding jig 39 is a metal that has been pre-anodicized at a high voltage, using a metal with a higher anodic oxidation voltage than the anode 38 used during anodizing. While pure Mg, AZ31, and AZ91 can be used, the material is not limited to these.

[0035] The conductive holding jig 39, which sandwiches the anode 38, is connected to the DC stabilized power supply 40 with wires so that it becomes the anode and the carbon plate becomes the cathode 37. Furthermore, the cathode 37 and the conductive holding jig 39, which are connected by wires, are immersed in the inner tank 33 to establish an anodic oxide film formation circuit. Then, a voltage is applied between the anode 38 and the cathode 37 to form an anticorrosion film 12.

[0036] Finally, the timing for stopping the current is determined by the cumulative current value applied to the Mg-Li alloy. To give a specific example, the amount of electricity required to grow the corrosion protection film 12 to a thickness of 1 μm is 100 cm². 2 The required energy is 42.5 coulombs per unit. The desired film thickness can be obtained by supplying the necessary amount of electricity and then switching off the power. Furthermore, if the current value during energization is stable, the desired film thickness can also be obtained by controlling the energization time.

[0037] Furthermore, the current setting can be determined by the surface area of ​​the anode 38 and the current density. For example, 100 cm² 2 The current density on the anode 38 surface is 5 A / 100 cm². 2 When performing anodizing, the set current is 5A. If the current density is low, the anodizing process takes longer, resulting in lower productivity. Conversely, if the current density is high, the anodizing process takes shorter, improving productivity. However, if it is too high, voids and cracks in the formed anodized film may grow larger, potentially reducing its crack resistance. Therefore, the current density is set to 1A / 100cm².2 10 A / 100 cm or less 2 is preferable. By the above method, the anticorrosive film 12 of the present embodiment can be obtained.

[0038] The manufacturing method of the Mg-Li alloy which is the base material 11 as the anode 38 is not particularly limited, and examples thereof include casting, thixoforming, and die casting. The base material 11 obtained by rapidly cooling molten metal has a tendency for the surface structure to vary in-plane depending on the formed shape, the direction of the molten metal flow with respect to the mold, the temperature difference, etc. The in-plane distribution of this surface structure also causes variations in the film thickness of the anticorrosive film 12, but with the above manufacturing method, it is possible to provide an alloy member with excellent appearance even for such an anode 38.

[0039] The electrolytic solution 31 used for anodic oxidation is a liquid containing fluoride ions, ammonium molecules, and phosphate ions. As substances containing fluoride ions and ammonium molecules, ammonium bifluoride, ammonium fluoride, hydrogen fluoride, ammonia, etc. can be used. As substances containing phosphate ions, for example, phosphoric acid, ammonium phosphate trihydrate (including diammonium hydrogen phosphate and diammonium phosphate) can be used. To most easily prepare the electrolytic solution 31, it is a method of dissolving a desired amount of ammonium fluoride and ammonium phosphate trihydrate in pure water. Note that this electrolytic solution is prepared while paying attention to make the pH 5.7 or more and 7.5 or less so that high-concentration hydrogen fluoride is not generated. Note that ammonium phosphate trihydrate has low solubility. Therefore, as a method of increasing the concentration of phosphate ions more, phosphoric acid may be added to the electrolytic solution to make the pH acidic, and then ammonium phosphate trihydrate may be added and dissolved.

[0040] The electrolytic solution 31 is in a concentration range where the coordinates (X, Y) when the fluoride ions in the liquid are X mol / L and the phosphate ions are Y mol / L are within the region surrounded by the following coordinates [1] to [5]. [1] (1.9, 0.7) [2] (5.8, 0.7) [3] (10.0, 4.1) [4] (4.4, 4.1) [5] (1.9, 2.2)

[0041] Further, at the above concentration, the ratio of the phosphate ion concentration to the sum of the fluoride ion concentration and the phosphate ion concentration is preferably 11% or more and 54% or less. Here, the phosphate ion concentration indicated is the concentration when the phosphoric acid added to the liquid is 100% dissociated and ionized (PO4 3- ). In the actual electrolytic solution, phosphate ions exist in the form of H2PO4 - and HPO4 2- .

[0042] <Device> The alloy member of the present disclosure can be applied to a housing in a device including a housing and components provided in the housing. Hereinafter, as an example of a device to which the alloy member of the present disclosure is applied, a single-lens reflex digital camera will be described as an example, but the device is not limited thereto and may be a smartphone or a compact digital camera.

[0043] FIG. 4 shows the configuration of a single-lens reflex digital camera 50, which is an example of an imaging device of the device. In FIG. 4, a camera body 52 and a lens barrel 51 as an optical device are coupled. The lens barrel 51 is a so-called interchangeable lens that is detachable from the camera body 52.

[0044] Light from the subject passes through an optical system composed of a plurality of lenses 53, 55, etc., which are examples of components arranged on the optical axis of the imaging optical system in the housing of the lens barrel 51, and is received by the imaging element 60 to be photographed. Here, the lens 55 is supported by an inner cylinder 54 and is movably supported with respect to the outer cylinder of the lens barrel 51 for focusing and zooming.

[0045] During the observation period before shooting, light from the subject is reflected by the main mirror 57, which is an example of a component inside the housing 65 of the camera body 52. ​​After passing through the prism 61, the image is projected onto the photographer's screen through the viewfinder lens 62. The main mirror 57 is, for example, a half-mirror, and the light that passes through the main mirror 57 is reflected by the sub-mirror 58 towards the AF (autofocus) unit 63. This reflected light is used, for example, for distance measurement. The main mirror 57 is attached and supported by the main mirror holder 66 by adhesive or other means. During shooting, the main mirror 57 and sub-mirror 58 are moved out of the optical path via a drive mechanism (not shown), the shutter 59 is opened, and the image of the light incident from the lens barrel 51 is projected onto the image sensor 60. The aperture 56 is configured to change the brightness and depth of field during shooting by changing the aperture area.

[0046] The alloy member 10 of this disclosure can be used in at least a portion of the housings 64 and 65, and in this case, it is positioned so that the coating film 13 is located on at least a portion of the outer circumference of the housings 64 and 65. Since the alloy member 10 of this disclosure suppresses peeling of the coating film due to cracking of the coating film 13, it is possible to provide a lighter imaging device with suppressed peeling of the coating film compared to conventional devices.

[0047] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0048] (Example 1) First, an electrolyte solution 31 was prepared by dissolving ammonium fluoride at a ratio of 146.7 g / L and triammonium phosphate trihydrate at a ratio of 146.4 g / L in pure water. The prepared electrolyte solution 31 was poured into the outer tank 32 of the anodic oxidation apparatus 30 shown in Figure 3, and the magnetic pump 35 was started. After the electrolyte solution 31 in the inner tank 33 overflowed and liquid circulation began, the temperature control mechanism 34 was started to stabilize the liquid temperature at 20°C. Furthermore, a carbon plate was immersed in the inner tank 33 as the cathode 37 and connected to the cathode of the power supply 40 with a wire.

[0049] As the substrate 11 made of an Mg-Li alloy to be anodized, a die-cast substrate of "Ares" (composition: Mg - 9% Li - 4% Al - 1% Zn, manufactured by Anritsu Material Technology Co., Ltd.) was prepared. The size was a thin plate of 150 mm × 100 mm × 1 mm. The die-casting was performed using "LMI450M" manufactured by Sodick Co., Ltd.

[0050] Next, a conduction holding jig 39 made of AZ31 material was prepared. The conduction holding jig 39 has a fixed hinge at the tip of an AZ31 round bar and has an operating hinge at a position 107 mm therefrom. The fixed hinge and the operating hinge are structured to be shrunk by a Viton O-ring. This conduction holding jig 39 was previously anodized in the above electrolytic solution 31 until no current flowed at a voltage of 100 V. This is to make it easier to transmit power to the conduction holding jig 39. The above substrate 11 was sandwiched as the anode 38 between the conduction holding jigs 39 for which the pretreatment was completed, connected to the anode of the DC stabilized power supply 40 with a conducting wire, and immersed and arranged in the inner tank 33 at a position facing the cathode 37.

[0051] The DC stabilized power supply 40 used "PAT160 - 100TMX" manufactured by Kikusui Electronics Industry Co., Ltd. The input current was 15.25 A (surface area: 305 cm 2 , current density: 5 A / 100 cm 2 ). The target film thickness was 20 μm, and the input current was set to 2593 coulombs. When the specified coulomb amount was reached, the current of the DC stabilized power supply 40 was stopped.

[0052] The anode 38 and the conduction holding jig 39 after the anodizing treatment were taken out from the inner tank 33, washed with pure water to thoroughly wash away the electrolytic solution 31 adhering to the surface, and then dried in a clean oven at 60°C.

[0053] Finally, the surface of the anticorrosion film 12 was spray-coated using a one-component baking primer (product name: Panaco SMG) and an acrylic paint (product name: Armor Top) manufactured by Musashi Paint Co., Ltd. For the primer, thinner of product number Z-2854 (product name: Panaco MG Thinner) was used, diluted in a ratio of 2:1 (undiluted:thinner), and spray-coated to a thickness of 20 μm. After spraying, it was dried in the air for 10 minutes, and then dried in an oven at 160°C for 20 minutes. On the surface coated with primer, paint diluted with thinner of product number Z-2253 (product name: Armor Top Thinner) in a ratio of 2:1 (undiluted:thinner) was spray-coated to a thickness of 15 μm. After spraying, it was dried in the air for 10 minutes, and then dried in an oven at 160°C for 20 minutes to obtain an alloy member 10 having a painted film 13.

[0054] (Examples 2 to 11) As shown in Table 1, alloy members were manufactured in the same manner as in Example 1, except that the concentrations of fluoride ions and phosphate ions were adjusted by changing the composition and concentration of the materials constituting the electrolyte 31, and the input current value was changed. The voltage during the anodizing treatment was in the range of 60 to 70 V.

[0055] (Comparative Examples 1 to 7) As shown in Table 1, alloy members were manufactured in the same manner as in Example 1, except that the concentration of the electrolyte 31 and the input current were changed. The voltage during the anodizing treatment of Comparative Examples 1 to 3 and Comparative Examples 6 and 7 was in the range of 70 to 80 V, and the voltage during the anodizing treatment of Comparative Examples 4 and 5 was in the range of 230 to 250 V.

[0056] <Evaluation> (Film thickness of the anticorrosion film) The film thickness of the anticorrosion film on the alloy members of the examples and comparative examples was measured. The film thickness was evaluated using the eddy current type film thickness gauge "SWT-9000" (probe: NF-0.6) manufactured by Sanko Electronics Laboratory Co., Ltd. Film thickness was measured at five arbitrary points on the uniform surface and the average value was calculated. The results are shown in Table 2.

[0057] (Component Ratio of the Corrosion Inhibitor Film) To evaluate the component ratio of the corrosion inhibitor film, elemental analysis was performed using EDS (Energy Dispersive X-ray Spectrometer). The EDS elemental analysis was performed using a ZEISS "SIGMA500" (FE-SEM) instrument. The elements targeted for elemental analysis by EDS were Mg, P, F, O, Al, and Ca. The analysis conditions were an acceleration voltage of 10 kV and a work distance of 8.5 mm to 9 mm. The measurement was performed by EDS elemental composition analysis from the surface side of the corrosion inhibitor film after anodizing treatment. The average value of the entire image area at a magnification of 500x was taken as the component ratio of the film. The results are shown in Table 2.

[0058] (Crystal structure of the corrosion protection film) To analyze the crystal structure of the corrosion protection film, measurements were taken using X-ray diffraction at a temperature of 25°C. The X-ray diffractometer used was Rigaku's "Ultima IV". A Cu tube was used, and the measurement wavelength λ was set to 1.5418 Å. The tube voltage was 40 kV and the tube current was 40 mA.

[0059] First, diffraction patterns were acquired using the 2θ-θ method for the range where 2θ is between 20° and 60°. The step size was 0.02°, and the scan speed was 2° / min (two integrations). Next, background noise was removed from the acquired diffraction patterns. Then, each peak in the background-removed diffraction pattern was identified.

[0060] Next, the α-phase and β-phase peaks of the Mg-Li alloy were extracted. Since these peaks originate from the substrate, we focused on identifying the peaks of the phosphoric acid and fluorine-derived films. The results are shown in Figure 5. As shown by the dashed lines, comparative examples 1 to 3 show peaks originating from the corrosion protection film (Mg2PO4F), indicating crystallization. On the other hand, the peaks of examples 1 to 8 are shown by solid lines, and no peaks originating from the corrosion protection film were detected. Furthermore, a broad waveform was observed from 20° to 40°, and since no crystal peaks originating from the corrosion protection film appeared, it was identified as an amorphous structure. The results of identifying the amorphous and crystalline structures in this way are shown in Table 2.

[0061] (Evaluation of paint film peeling) Figure 6 shows an overview of the evaluation method for paint film peeling. The alloy members 10 of the examples and comparative examples were placed on the jig 71, and a weight 72 was dropped onto the painted surface of the alloy member 10 to check for paint film peeling. The jig 71 has an outer diameter of 60 mm, with concentric holes with an inner diameter of 30 mm, and a thickness of 10 mm. The weight 72 is cylindrical with a mass of 1000 g and an outer diameter of 18 mm, and has a spherical tip. The center of the weight 72 and the center of the jig 71 were aligned, and the weight was dropped vertically onto the alloy member 10 from a height H of 300 mm. The results are shown in Table 2. Samples with no peeling were marked with ○ (good), samples with partial peeling in a dotted manner were marked with △ (acceptable), and samples with complete peeling across a surface were marked with × (unacceptable).

[0062] (Evaluation of Appearance Unevenness) Appearance unevenness was judged by whether the unevenness was visible to the naked eye on the alloy component 10 after painting. The thicker parts of the anodized film were white, and the thinner parts were gray, resulting in variations in shade due to film thickness unevenness. If the level of this shade difference is small, it will be hidden after painting and will not be visible as appearance unevenness, but if the shade difference is large, it will be visible even after painting and the product cannot be used. The results are shown in Table 2.

[0063]

[0064]

[0065] As described above, in the anticorrosion film of this embodiment, where the mass ratio of F to Mg is 0.05 to 0.70 and the mass ratio of P to Mg is 0.8 to 1.41, it is possible to suppress both coating peeling and appearance unevenness. In Comparative Examples 1 to 3, the component range is outside that of this embodiment, so the anticorrosion film crystallizes, resulting in either coating peeling or appearance unevenness, or both. In Comparative Examples 4 and 5, the anticorrosion film has an amorphous structure due to the high voltage of the anodizing treatment, but the film does not densify at high voltage, resulting in coating peeling due to film cracking. In Comparative Examples 6 and 7, the ratio of F to P is heavily skewed to one side, resulting in film unevenness on the high-concentration P side and film cracking on the high-concentration F side.

[0066] This disclosure is not limited to the embodiments described above, and various modifications and alterations are possible without departing from the spirit and scope of this disclosure. Accordingly, the following claims are attached to make the scope of this disclosure public.

[0067] This application claims priority based on Japanese Patent Application No. 2024-201050, filed on 18 November 2024, and Japanese Patent Application No. 2025-171633, filed on 10 October 2025, and all of the contents of those applications are incorporated herein by reference.

[0068] 10: Alloy component 11: Base material 12: Anticorrosion film 13: Paint film 31: Electrolyte 37: Cathode 38: Anode

Claims

1. A corrosion-preventive film that covers and protects the surface of a substrate mainly composed of magnesium and containing lithium, characterized in that it contains 20% to 40% by mass of phosphorus, 25% or more by mass of oxygen, 17% or less by mass of fluorine, 20% to 30% by mass of magnesium, and 5% or less by mass of nitrogen, and has an amorphous structure.

2. The corrosion-preventive film according to claim 1, characterized in that its thickness is 5 μm or more and 40 μm or less.

3. The corrosion-preventive film according to claim 1 or 2, characterized in that the mass ratio of fluorine content to magnesium content is 0.05 or more and 0.70 or less, the mass ratio of phosphorus content is 0.8 or more and 1.41 or less, and the mass ratio of oxygen content is 1.5 or more and 2.2 or less.

4. A method for manufacturing an alloy member, comprising the steps of: arranging an anode and a cathode in an electrolyte; and applying a voltage between the anode and the cathode to form a corrosion-preventive film on the anode, wherein the anode is a substrate mainly composed of magnesium and containing lithium, and the electrolyte is located within a region enclosed by the coordinates (X, Y) of the following coordinates [1] to [5], where the fluoride ion concentration is X mol / L and the phosphate ion concentration is Y mol / L. [1] (1.9, 0.7) [2] (5.8, 0.7) [3] (10.0, 4.1) [4] (4.4, 4.1) [5] (1.9, 2.2) 5. The method for manufacturing an alloy member according to claim 4, characterized in that the ratio of the phosphate ion concentration to the sum of the fluoride ion concentration and the phosphate ion concentration in the electrolyte is in the range of 11% to 54%.

6. An alloy member comprising a base material mainly composed of magnesium and containing lithium, and a corrosion-resistant film provided on the base material, wherein the corrosion-resistant film is the corrosion-resistant film described in any one of claims 1 to 3.

7. The alloy member according to claim 6, characterized in that the sum of the magnesium content and the lithium content in the base material is 90% by mass or more.

8. The alloy member according to claim 6 or 7, characterized in that the lithium content in the substrate is 0.5% by mass or more and 15% by mass or less.

9. The alloy member according to any one of claims 6 to 8, characterized in that the base material contains aluminum in a range of 1% by mass or more and 8% by mass or less.

10. The alloy member according to any one of claims 6 to 9, characterized in that the base material contains at least one of germanium and beryllium in an amount of 0.02% by mass or more and 0.4% by mass or less.

11. The alloy member according to claim 10, characterized in that the germanium content in the substrate is 0.01% by mass or more and 0.4% by mass or less, and the beryllium content is 0.02% by mass or more and 0.1% by mass or less.

12. The alloy member according to any one of claims 6 to 11, characterized in that the base material contains zirconium in an amount of 0.6% by mass or more and 3.0% by mass or less.

13. The alloy member according to any one of claims 6 to 12, characterized in that the base material contains at least one element selected from zinc, calcium, silicon, and manganese in an amount of 0.01% by mass or more and 5% by mass or less.

14. The alloy member according to 13, characterized in that the zinc content is 3% by mass or less, the calcium content is 3.0% by mass or less, the silicon content is 0.2% by mass or less, the manganese content is 0.3% by mass or less, and the remainder is unavoidable impurities, magnesium, and lithium.

15. The alloy member according to any one of claims 6 to 14, characterized in that the anticorrosion film is covered with a paint film.

16. An apparatus comprising a housing and a component provided inside the housing, wherein the housing includes an alloy member as described in any one of claims 6 to 15.

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