Aluminum material with a corrosion-resistant coating and method for manufacturing the aluminum material.

Abstract

Provided is an aluminum material provided with a corrosion-resistant film by steam treatment, which can exhibit excellent corrosion resistance. **Solution**: It relates to an aluminum material comprising a base material made of aluminum or an aluminum alloy and a corrosion-resistant film formed on at least a part of the base material. The corrosion-resistant film contains at least one kind of aluminum hydroxide oxide and an Al-based layered double hydroxide (Al-based LDH) represented below. And the Al-based LDH essentially contains an Al-based LDH whose anion component is nitrate ion. Also, in the present invention, the Al-based LDH is preferentially oriented on the (003) plane. TIFF2026049968000006.tif11149 M1 is a divalent metal element and is any one of Co, Zn, Mg, Cu, Mn, Fe, Ni, Cr, Cd, Ca, and M1 2+ is a divalent metal ion. The anion component A n- is OH - , CO3 2- , NO3 - , SO4 2- , F - , Cl - is at least any one of them.

Description

Technical Field

[0001] The present invention relates to an aluminum material having a corrosion-resistant coating with good corrosion resistance and a method for producing the same. More specifically, the present invention relates to an aluminum material comprising a base material made of aluminum or an aluminum alloy and a corrosion-resistant coating formed by steam treatment, and a surface treatment method capable of effectively forming a corrosion-resistant coating.

Background Art

[0002] Aluminum materials made of aluminum or aluminum alloys are widely used in the automotive, aircraft, and other general industrial fields because they are lightweight and can be strengthened by adding appropriate alloying elements and age hardening. In particular, in the automotive and aerospace equipment fields, various components made of aluminum materials are widely used to replace steel materials for the purpose of improving fuel efficiency by reducing weight.

[0003] When using aluminum materials for the above various applications, ensuring corrosion resistance is one of the necessary requirements. Aluminum forms a natural oxide film and becomes passivated when left in air. However, since the thickness of this natural oxide film is about several nanometers, it is prone to corrosion in an environment with extreme moisture, acid, or alkali. Therefore, surface treatment methods have been considered as a way to impart corrosion resistance to aluminum materials. Conventionally known surface treatment methods include chemical conversion treatments such as anodic oxidation treatment, plating treatment, zinc phosphate treatment, and chromate treatment (for example, Patent Document 1). In these various chemical conversion treatments, an alloy material to be treated is brought into contact with and immersed in a treatment solution containing acids such as H2SO4, alkalis, and heavy metal ions such as Cr to form a corrosion-resistant film on the alloy surface. However, in these surface treatments such as chemical conversion treatments, there are problems from the perspectives of the cost of procuring special treatment solutions and waste liquid treatment and the environmental load. Therefore, a surface treatment process applying water vapor has been attracting attention as a surface treatment method with a smaller environmental load than the above chemical conversion treatments. The present inventors have also disclosed a water vapor treatment in which water vapor within a predetermined temperature range is brought into contact with the alloy surface to improve the corrosion resistance of aluminum materials (for example, Patent Documents 2 and 3). This water vapor treatment is a surface treatment process that forms a film mainly composed of metal hydroxides on the surface of a base material made of an aluminum material and improves the corrosion resistance by the anticorrosive action of the film.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

[0005] Steam treatment of aluminum materials offers greater safety and environmental compatibility compared to conventional chemical treatments, particularly in terms of wastewater treatment. Furthermore, it allows for relatively easy application of corrosion resistance to alloy materials through the formation of a protective coating. In addition, because the coating is formed by a direct reaction between steam and the substrate surface, a uniform and highly adhesive coating can be created. Moreover, the coating exhibits high conformability to the substrate's shape, making it effective as a surface treatment for substrates with complex shapes.

[0006] Furthermore, the corrosion-resistant coating formed by steam treatment has distinctive characteristics in its composition. In addition to aluminum hydroxide oxide, the coating formed by steam treatment may contain aluminum-based layered double hydroxides (LAYED DDH).

[0007] Among the components of the corrosion-preventive coating on the aluminum material described above, aluminum hydroxide oxide, also known as boehmite, is a compound that can be included in corrosion-preventive coatings produced by surface treatments other than steam treatment. A characteristic of corrosion-preventive coatings produced by steam treatment is that they may contain Al-based layered double hydroxides. Layered double hydroxides are compounds formed in which trivalent Al ions are substituted at the divalent metal sites of a divalent metal (M1) hydroxide, and have a layered structure, as shown by the following structural formula.

[0008] [ka]

[0009] In the structural formula shown above, the metal ion (M1 2+is an ion of a divalent metal element M1. The metal element M1 is a metal derived from a solute element in the aluminum alloy that is the base material, or a metal introduced before or during the steam treatment. The metal element M1 includes, for example, cobalt (Co), zinc (Zn), magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), cadmium (Cd), calcium (Ca), and the like. Also, the anion (A n- ) in the above structural formula is a hydroxide ion (OH - ), carbonate ion (CO3 2- ), nitrate ion (NO3 - ), sulfate ion (SO4 2- ), fluoride ion (F - ), or chloride ion (Cl - ) at least one of these. These anions are usually supplied from the treatment atmosphere in the steam treatment. In the steam treatment of aluminum materials heretofore, pure water is often applied, but in that case, carbonate ions (CO3 2- ) derived from carbon dioxide in the air are supplied as anions.

[0010] And, [M1 2+ 1-x Al 3+ x (OH)2] on the right side of the above structural formula of LDH is called a double hydroxide basic layer, and [A n- x / n ·yH2O] on the left side of the structural formula may be called an intermediate layer. In LDH, it has a stacked structure in which an intermediate layer having a negative charge is sandwiched between layers of a double hydroxide basic layer having a positive charge. This stacked structure is maintained in the film.

[0011] LDH possesses a unique function as a component of corrosion-preventive coatings due to its dynamic action in corrosive environments. Specifically, when other anions / molecules (guest substances) are in close proximity, LDH can maintain the structure of the double hydroxide base layer (host layer) while exchanging the intermediate layer between the host layers with the guest substance and incorporating it into the interior. This dynamic action is sometimes referred to as anion exchange capacity due to host-guest reactions. When the guest substance is a corrosive ion / molecule, this host-guest reaction ensures the corrosion resistance of the LDH-containing coating. [Overview of the project] [Problems that the invention aims to solve]

[0012] As described above, steam treatment of aluminum materials offers many advantages in terms of environmental impact and processing costs, and the corrosion-preventive effect of the resulting coating is also highly anticipated. However, considering the expanding range of applications for aluminum materials in the future, further improvements in corrosion resistance should be made to aluminum materials for which a corrosion-preventive coating is formed by steam treatment. In general, the corrosion resistance of a corrosion-preventive coating is often dependent on its thickness, and increasing the thickness to improve corrosion resistance requires increasing the processing time. The same applies to corrosion-preventive coatings formed by steam treatment, so a processing method that more efficiently increases the film thickness is needed.

[0013] The present invention was made against the background described above, and aims to provide an aluminum material having a corrosion-resistant coating formed by steam treatment that exhibits superior corrosion resistance compared to conventional materials. Furthermore, it provides a method for manufacturing such an aluminum material, which includes a surface treatment that can efficiently form a corrosion-resistant coating. [Means for solving the problem]

[0014] The inventors have been diligently studying how to improve the corrosion resistance of anticorrosion coatings by steam treatment, and have decided to investigate the optimization of the composition of Al-based layered double hydroxide (LDH) contained in the anticorrosion coating and the conditions for steam treatment to promote LDH generation. As described above, LDH is a double hydroxide base layer ([M1 2+ 1-x Al 3+ x (OH)2) and the mesoplex ([A n- x / n It has a layered structure of [·yH2O]), and while maintaining this structure, it interacts with anions such as corrosive ions in the corrosive environment and the anionic component (A) of the intermediate layer. n- It exhibits a corrosion-preventive effect by exchanging with nitrate ions (NO3). The inventors considered that the degree of anion exchange in Al-based LDH differs depending on the anionic component of the intermediate layer. And, as an anionic component that preferably promotes the anion exchange, they selected nitrate ions (NO3 - We discovered ) and conceived of a corrosion-preventive coating that uses Al-based LDH, which contains this as an essential component, as a constituent.

[0015] Furthermore, the inventors decided to investigate a steam treatment process for forming an anticorrosion coating containing Al-based LDH with nitrate ions as an essential anionic component. As a result, they found that a suitable aluminum material containing Al-based LDH can be obtained by performing a pretreatment that supplies nitrate ions separately from the supply of coating components in the steam treatment. They then discovered that the Al-based LDH in this suitable anticorrosion coating exhibits a unique orientation not seen before, leading to the present invention.

[0016] In other words, the present invention relates to an aluminum material comprising a substrate made of aluminum or an aluminum alloy and a corrosion-resistant coating formed on at least a part of the substrate, wherein the corrosion-resistant coating contains aluminum hydroxide oxide (AlO(OH)) and at least one Al-based layered double hydroxide represented by the following formula 1, and the Al-based layered double hydroxide is essentially the anionic component (A) in the following formula. n-This aluminum material is characterized by containing an Al-based layered double hydroxide in which nitrate ions are present, and the Al-based layered double hydroxide is preferentially oriented on the (003) plane.

[0017] [ka] (In the formula, M1 is a divalent metallic element and is at least one of the following: cobalt (Co), zinc (Zn), magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), cadmium (Cd), and calcium (Ca). M1 2+ It is a divalent metal ion. The anionic component is A n- is hydroxide ion (OH - ), carbonate ions (CO3 2- ), nitrate ion (NO3 - ), sulfate ions (SO4 2- ), fluoride ions (F - ), chloride ions (Cl - ) is at least one of the following.

[0018] The following describes in more detail an aluminum material having a predetermined anticorrosion coating according to the present invention and a method for producing the aluminum material, including a steam treatment to form the anticorrosion coating.

[0019] A. Structure of the aluminum material according to the present invention As described above, the aluminum material according to the present invention consists of a base material and a corrosion-resistant coating. Details of each component are as follows.

[0020] A-1. Base material The base material for the aluminum material according to the present invention is aluminum or an aluminum alloy. Aluminum is pure aluminum with a purity of 99.5% or higher, and an aluminum alloy is an alloy in which additive elements are added to aluminum. In an aluminum alloy, examples of additive elements that alloy with aluminum include at least one of the following elements: zinc (Zn), magnesium (Mg), silicon (Si), copper (Cu), manganese (Mn), lithium (Li), iron (Fe), nickel (Ni), silver (Ag), zirconium (Zr), and chromium (Cr). When an aluminum alloy is used as the base material, an aluminum alloy containing a total of 0.1% by mass or more and less than 30% by mass of these additive elements is preferred.

[0021] The base material can be any aluminum alloy specified by the International Aluminum Alloy Name (INA). Typical aluminum alloys that are suitable include precipitation-hardening types such as the 2000 series Al-Cu alloy, the 4000 series Al-Si alloy, the 5000 series Al-Mg alloy, the 6000 series Al-Mg-Si alloy, and the 7000 series Al-Zn-Mg or Al-Zn-Mg-Cu alloy. However, the application is not limited to these standardized alloy systems, and a wide range of alloy compositions can be used.

[0022] In this invention, the shape of the substrate is not particularly limited, and any shape, such as plate-shaped or tubular, can be used. There are also no restrictions on the dimensions of the substrate. Because steam treatment offers a high degree of freedom in selecting the shape and dimensions of the substrate, even substrates with complex shapes or large dimensions can be coated with a corrosion-resistant film.

[0023] A-2. Composition of the anticorrosion coating The aluminum material of the present invention comprises a corrosion-resistant coating formed by steam treatment. The corrosion-resistant coating in the present invention includes aluminum hydroxide oxide and Al-based layered double hydroxide (Al-based LDH) as essential components.

[0024] A-2-1. Aluminum hydroxide oxide Aluminum hydroxide oxide (boehmite) is an aluminum compound denoted as γ-AlO(OH) or simply AlO(OH). Aluminum hydroxide oxide has good chemical stability and corrosion-preventive properties.

[0025] A-2-2. Al-based layered double hydroxides (Al-based LDH) The corrosion-preventive coating for aluminum materials of the present invention contains Al-based LDH along with the above-mentioned aluminum hydroxide oxide. The presence of Al-based LDH is not necessarily essential for conventional corrosion-preventive coatings produced by steam treatment, but it is an essential component in the present invention. Al-based LDH is represented by the structural formula in Chemical Formula 2 above, and is a double hydroxide base layer ([M1 2+ 1-x Al 3+ x (OH)2) and the mesoplex ([A n- x / n It is a complex compound consisting of [·yH2O]) and has a layered structure. In the structural formula, M1 is a divalent metallic element and is at least one of Co, Zn, Mg, Cu, Mn, Fe, Ni, Cr, Cd, and Ca. Anionic component (A n- ) is, OH - CO3 2- NO3 - SO4 2- F - Cl - It is at least one of the following.

[0026] The LDH in the corrosion-preventive coating in this invention is at least one LDH selected from the group of metal M and the group of anionic components described above, and may contain multiple types of LDH. However, the corrosion-preventive coating has an anionic component of nitrate ion (NO3) as an essential LDH. - It contains LDH, which is a corrosive ion in a corrosive environment (Cl). The anions in LDH are corrosive ions in a corrosive environment (Cl - F -It undergoes an exchange reaction with other anionic components (such as nitrates), thereby exhibiting its function as a corrosion-preventive coating. The anionic component that particularly readily exhibits this exchange reaction is the nitrate ion. Conversely, other anionic components such as carbonate ions and sulfate ions tend to remain stably within the LDH, making the aforementioned exchange reaction less likely to proceed. For these reasons, the present invention requires the presence of an LDH with nitrate ions as its anionic component.

[0027] Furthermore, the corrosion-preventive coating in this invention may contain LDH with nitrate ions as the anionic component, but may also contain LDH with other anions (such as carbonate ions) as the anionic component. In particular, the presence of other anionic components is permissible when the peak intensity of the (003) plane of LDH in the XRD diffraction pattern of the corrosion-preventive coating meets the preferred conditions described later.

[0028] Furthermore, there are no particular restrictions on the metal M and its ions in LDH. In particular, since metal M is introduced through multiple routes, such as as an additive element when the substrate is an aluminum alloy, or through treatments the substrate undergoes before steam treatment, there may be multiple LDHs with different metal Ms. Preferred metal Ms are magnesium, cobalt, zinc, and nickel.

[0029] A-2-3. Other components contained in the anticorrosion coating The corrosion-preventive coating for aluminum materials according to the present invention has aluminum hydroxide oxide and Al-based LDH as essential components, and may consist only of these, but may also contain other substances. For example, it may contain additive elements of the aluminum alloy base material, or compounds thereof (intermetallic compounds, oxides, hydroxides, etc.). It may also contain compounds (oxides, hydroxides, hydrates, salts) derived from components of water vapor that come into contact with the base material for film formation. The presence of these other components is permissible if in trace amounts (10% by mass or less).

[0030] A-3. Orientation of the anticorrosion coating The corrosion-preventive coating on aluminum materials of the present invention exhibits strong orientation on specific crystal planes due to Al-based LDH containing nitrate ions as an anionic component. The interlayer distance of Al-based LDH varies depending on the anionic component. In the present invention, nitrate ions are essential as the anionic component of Al-based LDH, and LDH containing nitrate ions preferentially orients to the (003) plane.

[0031] The orientation of LDH in the anticorrosion coating can be confirmed and characterized from the profile obtained when an aluminum material is subjected to X-ray diffraction (XRD) on the anticorrosion coating. Specifically, in the X-ray diffraction profile, the diffraction peak intensity due to (003) reflection of Al-based LDH is higher than the peak intensity of other crystal planes of LDH. Furthermore, although the diffraction profile of the anticorrosion coating also shows diffraction peaks of aluminum hydroxide oxide in addition to LDH, the diffraction peak of the (003) plane of LDH has a higher peak intensity than any of the diffraction peaks of aluminum hydroxide oxide. In other words, in the present invention, the diffraction peak intensity of the (003) plane of LDH is the highest in the diffraction profile of the anticorrosion coating.

[0032] Furthermore, in cases where the anionic component of LDH does not contain nitrate ions, or where nitrate ions are present but many other anions are also present, the ion exchange effect by nitrate ions is less likely to be exerted, and the preferred orientation of the (003) plane described above is less likely to occur. In such cases, the diffraction peak intensity of the (003) plane of LDH decreases, and the diffraction peaks of other crystal planes become higher. In addition, the diffraction peak of the (003) plane of LDH becomes broader.

[0033] Furthermore, in the XRD diffraction profile of the anticorrosive coating when the X-ray source is Cukα rays, the angles (2θ) at which the diffraction peak of Al-based LDH appears are around 9.0°~12.0° ((003) plane), around 18.0°~24.0° ((006) plane), and around 27.0°~36.0° ((012) plane), etc. In addition, the diffraction peak of aluminum hydroxide oxide appears around 2θ = 14.0°~14.7° ((020) plane), around 28.0°~28.7° ((021) plane), and around 48.8°~49.5° ((015) plane), etc.

[0034] Furthermore, to explain in more detail the characteristics of the X-ray diffraction profile of the corrosion-resistant coating of the present invention, the peak intensity of the diffraction peak of the (003) plane of the Al-based layered double hydroxide is I LDH Let the peak intensity of the diffraction peak of the (020) plane of aluminum hydroxide oxide be I AlO(OH) In this case, the peak intensity ratio of the two is I LDH / I AlO(OH) The score is 3.0 or higher.

[0035] Peak intensity ratio I LDH / I AlO(OH) This value can be attributed to the amount of Al-based LDH produced and the density of the corrosion-preventive coating. In the corrosion-preventive coating of the present invention, aluminum hydroxide oxide is formed on a substrate (aluminum or aluminum alloy), and its surface is coated with Al-based LDH. Peak intensity ratio I LDH / I AlO(OH) The increase suggests an increase in the production of Al-based LDH and a resulting effective coating state of aluminum hydroxide oxide. And the peak intensity ratio I LDH / I AlO(OH) Corrosion-preventive coatings with a peak intensity ratio of 3.0 or higher exhibit better corrosion resistance than conventional corrosion-preventive coatings treated with steam. LDH / I AlO(OH) Regarding this, a corrosion-resistant coating with a peak intensity ratio of 5.0 or higher is more preferable, and one with a peak intensity ratio of 7.0 or higher is particularly preferable. LDH / I AlO(OH) There is no specific upper limit, but around 25 is expected. Also, I LDHThe peak to obtain is the (003) reflection peak intensity of the LDH, and the peak near 2θ = 9.0° to 12.0° is referenced. Then, I AlO(OH) The peak required to obtain the desired result is the (020) reflection peak of aluminum hydroxide oxide, so a peak near 2θ = 14.0° to 14.7° is referenced.

[0036] A-4. Other components of the anticorrosion coating The thickness of the corrosion-preventive coating formed by the surface treatment method according to the present invention is preferably 10 μm to 50 μm. This is because a coating less than 10 μm thick has little corrosion protection effect on the substrate, and if it exceeds 50 μm thick, cracking and peeling may occur in the coating due to stress and thermal shock, which may actually result in inferior corrosion resistance. A coating thickness of 10 μm, the lower limit, requires a long time with conventional surface treatment methods, but it can be achieved in a short time with the present invention.

[0037] B. Method for producing aluminum material according to the present invention Next, the method for producing aluminum material according to the present invention will be described in detail, particularly the steam treatment and its pretreatment. As previously stated, the corrosion-preventive coating in the present invention contains Al-based LDH, and essentially contains LDH whose anionic component is nitrate ions. The anionic component of Al-based LDH can be introduced from the steam source during the steam treatment. However, according to the inventors' research, in order to adjust the composition and promote the growth of Al-based LDH, it is preferable to bring a solution containing the necessary metal M and anionic component into contact with the substrate surface before the steam treatment, and then perform the steam treatment in that state, rather than adjusting the steam source. This is thought to be because direct contact with the substrate surface increases the frequency of collisions between the substrate and the reactants, making the reaction proceed more easily. However, in order to form the corrosion-preventive coating of the present invention, pretreatment with a solution containing nitrate ions, which are essential anionic components in the present invention, is necessary.

[0038] In other words, the method for manufacturing an aluminum material according to the present invention is a method for manufacturing an aluminum material that includes a steam treatment step in which a corrosion-preventive film is formed by contacting at least a part of the surface of a substrate made of aluminum or an aluminum alloy with steam, wherein the method includes a pretreatment step in which a divalent metal nitrate compound solution is supplied to at least a part of the surface of the substrate before the steam treatment step, and the steam treatment is performed on the substrate after the pretreatment step. The pretreatment step and steam treatment step described above will be explained below in relation to the method for manufacturing an aluminum material according to the present invention.

[0039] B-1. Pretreatment process The pretreatment step is a process to promote the generation and growth of Al-based LDH, in which the anionic component is nitrate ions, when forming a corrosion-preventive film by steam treatment. In this pretreatment step, a nitrate solution of a divalent metal (metal M) is brought into contact with the substrate surface. The divalent metal is preferably one of the metals shown in the description of the composition of Al-based LDH above (Co, Zn, Mg, Cu, Mn, Fe, Ni, Cr, Cd, Ca), and particularly preferably Mg. The solution brought into contact with the substrate in the pretreatment step is particularly preferably an aqueous magnesium nitrate solution.

[0040] The nitric acid concentration of the nitrate solution used in the pretreatment step is preferably between 10 mM and 2.0 M. If the concentration is below 10 mM, the collision frequency of the reaction decreases, making it difficult to form a film thickness effective for improving corrosion resistance. If the concentration is above 2.0 M, the film thickness tends to become too thick, which may adversely affect adhesion. Furthermore, the pH of the nitrate solution is preferably between 8.5 and 11.5. This is because if the pH is below 8.5, the formation of Al-based LDH becomes difficult, and if the pH is above 11.5, the amount of nitrate ions, which are anionic components, incorporated into the LDH decreases. To adjust the pH to this preferred range, an alkali such as sodium hydroxide may be added to the nitrate solution.

[0041] Furthermore, the nitrate solution for the pretreatment step may contain metal salts of other anions (such as carbonate ions and sulfate ions). However, if the metal salts of other anions are in excess, it may inhibit the formation and growth of Al-based LDH, which uses nitrate ions as its anionic component. Therefore, it is preferable that the amount of metal salts of other anions included be 10 mol% or less.

[0042] The method for supplying a divalent metal nitrate solution to the substrate surface is not particularly limited. Methods of supplying the nitrate solution include spraying or dropping it onto the substrate. In these cases, the amount of solution sprayed or dropped onto the substrate is not particularly limited. For example, for a 1 cm² area where a corrosion-resistant film is to be formed... 2 The substrate can be coated with a nitrate solution of approximately 50 μL to 5 mL per unit. Alternatively, the substrate may be immersed in the nitrate solution. The immersion pretreatment step is completed by immersing the entire or a portion of the substrate in the treatment solution and then removing the substrate. There is no specific time limit for immersion. The required area of ​​the substrate may be immersed in the treatment solution and immediately removed, or it may be immersed for approximately 1 to 180 seconds before being removed. The temperature of the nitrate solution in the pretreatment step is preferably between 20°C and 50°C.

[0043] The pretreatment process forms a liquid film of nitrate solution on the substrate surface. Nitrate ions and divalent metal ions are uniformly distributed in this liquid film, and by performing the subsequent steam treatment in this state, an Al-based LDH with nitrate ions as the anionic component is formed.

[0044] B-2. Steam Treatment Process The steam treatment is a process in which a corrosion-preventive film is formed by bringing steam into contact with the substrate while a nitrate solution is attached to the substrate surface as a result of the above pretreatment step. The temperature of the steam brought into contact with the substrate during the steam treatment is preferably between 120°C and 250°C. If the steam temperature exceeds 250°C, the thermal decomposition of the generated Al-based LDH is more likely to proceed. Furthermore, there is a risk of porosity and cracking of the corrosion-preventive film. If the steam temperature is below 120°C, the formation rate of the corrosion-preventive film decreases, making it difficult to form a corrosion-preventive film of sufficient thickness. A steam temperature of 140°C to 190°C is more preferable. The steam treatment time is preferably 5 hours or more. While the method of the present invention allows for the formation of a corrosion-preventive film at an efficient rate, it is still difficult to form a corrosion-preventive film of suitable thickness in less than 5 hours. There is no particular upper limit, but considering the objectives of shortening the time for corrosion-preventive film formation and improving productivity, 24 hours or less is preferred, and 12 hours or less is more preferable.

[0045] The water vapor that comes into contact with the substrate is generated by heating and vaporizing a liquid that serves as the water vapor source. As with conventional methods, water can be used as the water vapor source. The water used as the water vapor source can be pure water, industrial water, or tap water.

[0046] The steam pressure is preferably in the range of 0.1 to 10 MPa. More preferably, the pressure is 0.2 to 5 MPa. When pressurized steam is applied, a two-phase equilibrium state is reached between saturated steam and subcritical water, which promotes the reactivity for the formation of the corrosion-preventive film. By maintaining a constant steam pressure during treatment, a uniform corrosion-preventive film can be formed.

[0047] Furthermore, in order to form an anticorrosive coating containing LDH at a high film formation rate, it is preferable to use an aqueous solution of ammonium salt as the water vapor source. Water vapor treatment is often carried out in a closed space such as an autoclave. The aqueous solution of ammonium salt has a relatively low boiling point and can be vaporized without leaving a liquid phase, thereby increasing the pressure of the closed atmosphere. In addition, ammonium salt has a buffering effect and can maintain the pH of the atmosphere around weakly basic. Since these conditions are suitable for the growth of Al-based LDH, the film formation rate and density of the anticorrosive coating are favorable. In this respect, if only water is used as the water vapor source, a liquid phase (liquid water) may be partially present on the substrate, the atmospheric pressure will be somewhat low, and there may be areas where LDH is not formed. Preferred solutes for the ammonium salt solution include ammonium nitrate, ammonium chloride, and aqueous ammonia. Furthermore, the ammonium ion concentration of the ammonium salt solution used as the water vapor source is preferably 0.1 M to 1.0 M.

[0048] There are no particular limitations on the method of bringing water vapor into contact with the substrate. Water vapor treatment may be performed by exposing the substrate to be treated to water vapor in a closed space such as a predetermined reactor or container. Specifically, treatment can be carried out by placing the substrate in a container with water and exposing the substrate to a water vapor atmosphere generated by controlling the temperature and pressure. Alternatively, treatment may be carried out by directly spraying water vapor onto the material to be treated.

[0049] The pretreatment and steam treatment steps described above can be performed as a set, and at least one set is sufficient, but multiple sets may be repeated. By repeating the process, a corrosion-resistant coating consisting of multiple layers of film can be formed. Furthermore, the treatment conditions in each set may differ as long as they are within the preferred range described above. By repeating the process under different conditions, it may be possible to form multiple layers of corrosion-resistant coatings with different compositions.

[0050] When subjecting a substrate made of the aluminum material described above to the surface treatment method according to the present invention, it may be appropriately washed and dried as a pretreatment. Furthermore, for aluminum alloys that have age-hardening properties based on their composition, solution heat treatment and, if necessary, age heat treatment may be performed before the surface treatment. [Effects of the Invention]

[0051] As described above, the corrosion-preventive coating for aluminum materials according to the present invention contains Al-based LDH, and essentially contains Al-based LDH having nitrate ions as an anionic component. As a result, the aluminum material according to the present invention has even better corrosion resistance than conventional aluminum materials having a corrosion-preventive coating obtained by steam treatment. [Brief explanation of the drawing]

[0052] [Figure 1] A diagram illustrating the general configuration of a steam treatment device. [Figure 2] SEM images showing the surface morphology of the anticorrosion coating formed on the aluminum alloy substrate surface of Example 1 and Example 2. [Figure 3] XRD profiles (water vapor temperature 160°C) on the surface of aluminum alloy substrates on which corrosion-resistant coatings were formed for Examples 1 and 2 and the Comparative Example. [Figure 4] XRD profiles (water vapor temperature 180°C) on the surface of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 1 and 2 and the Comparative Example. [Figure 5] XRD profiles (water vapor temperature 200°C) on the surface of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 1 and 2 and the Comparative Example. [Figure 6] Polarization curves of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 1 and 2 and the Comparative Example (water vapor temperature 160°C). [Figure 7] Polarization curves of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 1 and 2 and the Comparative Example (water vapor temperature 180°C). [Figure 8]Polarization curves (water vapor temperature 200°C) of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 1 and 2 and the Comparative Example. [Figure 9] SEM images showing the surface morphology of the anticorrosion coating formed on the aluminum alloy substrate surface of Examples 3 and 4. [Figure 10] XRD profiles (water vapor temperature 180°C) on the surface of aluminum alloy substrates on which the corrosion-resistant coatings of Examples 3 and 4 were formed. [Figure 11] Polarization curves of aluminum alloy substrates with corrosion-resistant coatings formed in Examples 3 and 4. [Modes for carrying out the invention]

[0053] First Embodiment The embodiments of the present invention will be described below. In this embodiment, a 7000 series aluminum alloy was used as the base material, and after pretreatment under different conditions, a water vapor treatment was performed to form a corrosion-resistant coating.

[0054] Example 1 A sheet of Al-5.6 mass%Zn-2.6 mass%Mg-Cu alloy (7075 alloy), a 7000 series aluminum alloy, was prepared as a substrate (sample) (20 x 20 mm, 3 mm thick). This substrate was surface polished (#400, #1200, #2000) and then ultrasonically cleaned for 5 minutes.

[0055] The substrates that had undergone the above pretreatment were subjected to pretreatment with a nitrate solution and steam treatment. In this embodiment, the steam treatment apparatus shown in Figure 1 was used, and after pretreatment in this apparatus, the apparatus was sealed and steam treatment was performed. The steam curing apparatus in Figure 1 is a horizontal autoclave in which a sealed container made of Teflon® is housed. A steam source is injected into the bottom of the sealed container, and a sample stand is placed on it. In the pretreatment step, after injecting the steam source into the sealed container, the sample stand was set and the substrate was placed on it. Then, the nitrate solution was dropped onto the surface of the substrate. The nitrate solution used was a 1M aqueous solution of magnesium nitrate, adjusted to pH 10 with ammonia water. For the pretreatment, 300 μL of this magnesium nitrate aqueous solution was dropped onto the surface of the substrate.

[0056] After the above pretreatment, the sealed container was covered and sealed, and the autoclave was sealed. Then, the autoclave was heated in an electric furnace and steam treatment was performed. In this example, 20 mL of pure water was injected as the steam source. The heating temperatures for the steam treatment were 160°C, 180°C, and 200°C. The steam treatment time was 12 hours.

[0057] Example 2 In this example, a corrosion-preventive coating was formed by steam treatment, in which the steam source was changed from pure water to an ammonium nitrate solution.

[0058] In this example, an aluminum alloy plate material that had been pre-treated in the same manner as in Example 1 was used as the base material, and pre-treatment was performed by dropping a magnesium nitrate solution onto it in the same manner as in Example 1. Then, 20 mL of 0.5 M ammonium nitrate solution was injected into a steam treatment device as a steam source, and steam treatment was performed. In this example as well, the heating temperatures for steam treatment were 160°C, 180°C, and 200°C, and the treatment time was 12 hours.

[0059] Comparative Example For comparison with Examples 1 and 2, a corrosion-resistant coating was formed by conventional steam treatment. The substrate was set up using the same test apparatus as in Examples 1 and 2, and steam treatment was performed using pure water as the steam source without any pretreatment.

[0060] For the aluminum material samples treated with steam in Examples 1 and 2 and the Comparative Example, morphological observation of the corrosion-preventive coating was performed using SEM and XRD analysis was conducted, and anodic polarization tests were performed to evaluate corrosion resistance.

[0061] [Observation of the surface morphology of the anticorrosion coating using SEM] The surface morphology of the corrosion-preventive coatings was observed using a scanning electron microscope (SEM). Figure 2 is an SEM image (×30000) showing the surface morphology of the corrosion-preventive coatings formed on aluminum materials in Example 1 and Example 2 after steam treatment (160°C). In the corrosion-preventive coating of Example 1, which was steam-treated using an ammonium nitrate solution as the steam source, a uniform and dense film, presumably made of LDH, was formed. On the other hand, in the corrosion-preventive coating of Example 2, which was steam-treated using pure water, areas without LDH (white areas in the photograph) were observed. These morphologies of the corrosion-preventive coatings were also observed at other steam treatment temperatures.

[0062] [Analysis by X-ray diffraction (XRD)] Next, XRD analysis was performed on the surfaces of the aluminum materials on which the corrosion-preventive coatings of Examples 1 and 2 and Comparative Example 1 were formed. XRD was performed using a Cu-Kα X-ray source with a voltage of 40kV and a current of 30mA.

[0063] Figures 3, 4, and 5 show the XRD profiles of each aluminum material when the steam treatment temperature was set to 160°C, 180°C, and 200°C. From Figure 3, it can be confirmed that a corrosion-preventive coating containing aluminum hydroxide oxide (AlO(OH)) and Al-based LDH was formed in both Examples 1 and 2. Furthermore, the (003) reflection peak is extremely high, indicating that Al-based LDH is preferentially oriented on this surface. In particular, the corrosion-preventive coating of Example 2 shows a particularly strong peak intensity of the diffraction peak originating from Al-based LDH. Here, for Examples 1 and 2, the peak intensity ratio I LDH / I AlO(OH) When measured, in Example 1, I LDH / I AlO(OH) = 8.3, and in Example 2, I LDH / I AlO(OH) The result was 16.

[0064] On the other hand, in the aluminum material subjected to steam treatment (steam source: pure water) without the pretreatment of Comparative Example 3, diffraction peaks of aluminum hydroxide oxide were observed, but almost no diffraction peaks of Al-based LDH were observed (I LDH / I AlO(OH) (≒0).

[0065] The XRD results above confirmed that pretreatment with a nitrate solution is extremely effective in generating Al-based LDH. Furthermore, it was confirmed that applying an ammonium salt solution as the steam source in the steam treatment is preferable to further increase the amount of Al-based LDH generated and to form a dense, highly effective corrosion-preventive coating (Example 2).

[0066] [Anodic Polarization Measurement] Next, the polarization curves of the aluminum materials with the anticorrosion coatings formed in Example 1, Example 2, and Comparative Example 1 were measured to evaluate their corrosion resistance. For polarization measurement, a 5 wt% NaCl aqueous solution was used as the electrolyte. After bubbling the solution with nitrogen before measurement, the polarization curve was measured using a potentiometer / galvanostat (VersaSTAT4, Princeton Applied Research).

[0067] Figures 6, 7, and 8 show the anodic polarization curves of each aluminum material when the steam treatment temperature was set to 160°C, 180°C, and 200°C. For comparison, the polarization curve of an aluminum alloy (7075 alloy) substrate without corrosion-resistant film formation is also shown in each figure. From these figures, it was confirmed that the aluminum material that underwent the pretreatment of Example 2 followed by steam treatment with an ammonium salt solution had the lowest anodic current and the best corrosion resistance.

[0068] Second EmbodimentIn this embodiment, the composition of the treatment solution in the pretreatment step before steam treatment was investigated. Here, two types of treatment solutions were used for the pretreatment step: one containing sodium carbonate (Na2CO3) as the nitrate solution dropped onto the substrate in the pretreatment step, and another containing only magnesium nitrate solution, as in the first embodiment. Steam treatment was then performed, and the corrosion resistance of the formed anticorrosion film was investigated.

[0069] Using the same steam treatment apparatus as in the first embodiment, a steam source (ammonium nitrate solution) was injected into a Teflon® sealed container, and after setting the sample stage and substrate, the treatment solution was added dropwise. The treatment solutions used were the same 1M magnesium nitrate aqueous solution as in the first embodiment (Example 3) and a mixed solution of 1M magnesium nitrate aqueous solution and 1M sodium carbonate (Example 4). Both treatment solutions were adjusted to pH 10 with ammonia water. In the pretreatment step, 300 μL of the treatment solution was added dropwise to the substrate surface.

[0070] The substrate after pretreatment was subjected to steam treatment using a 0.5 M ammonium nitrate solution as the steam source, similar to the first embodiment. The heating temperatures for the steam treatment were 160°C, 180°C, 200°C, and 240°C. The steam treatment time was 12 hours.

[0071] [Observation of the surface morphology of the anticorrosion coating using SEM] Figure 9 shows the results of SEM observation of the surface morphology (water vapor temperature 180°C) of the anticorrosion coatings in Examples 3 and 4. The surface morphology was similar, and compared to the anticorrosion coating using pure water as the water vapor source (comparative example of the first embodiment), the coverage rate by LDH was good. However, a denser coating was formed in Example 3, where only nitrate ions were supplied, compared to Example 4, where carbonate ions were supplied in the pretreatment.

[0072] [Analysis by X-ray diffraction (XRD)] XRD analysis was performed on the surfaces of aluminum materials on which the anticorrosion coatings of Examples 3 and 4 were formed. The measurement conditions were the same as in the first embodiment. Figure 10 shows the XRD profile of the aluminum material after treatment at a water vapor temperature of 180°C. From Figure 10, it can be confirmed that an anticorrosion coating containing aluminum hydroxide oxide and Al-based LDH was formed in all cases. In addition, preferential orientation of Al-based LDH to the (003) plane was observed in this embodiment as well. However, the peak intensity of the diffraction peak originating from Al-based LDH was clearly higher in Example 3. For these, the peak intensity ratio I LDH / I AlO(OH) When measured, in Example 3, I LDH / I AlO(OH) = 7.2, and in Example 4, I LDH / I AlO(OH) The result was 3.5.

[0073] [Anodic Polarization Measurement] Next, the polarization curves of the aluminum materials with the corrosion-preventive coatings formed in Examples 3 and 4 were measured to evaluate their corrosion resistance. The polarization measurement conditions were the same as in the first embodiment. Figure 11 shows the polarization curves of the aluminum material in Example 3 (water vapor temperature: 160°C, 180°C, 200°C, 240°C) and the aluminum material in Example 4 (water vapor temperature: 180°C). The corrosion potential and corrosion current density measured based on these polarization curves, as well as the current density around -0.4V, are as follows.

[0074] [Table 1]

[0075] Table 1 shows that the aluminum material of Example 4 (water vapor temperature 180°C), in which a carbonate solution was mixed into the treatment solution during pretreatment, had a higher corrosion current density and a higher current density around -0.4V than the material of Example 3, which was pretreated with only a nitrate solution at all water vapor temperatures. Furthermore, the corrosion potential of the aluminum material of Example 4 was the lowest. Therefore, it was confirmed that the aluminum material of Example 4 had inferior corrosion resistance to the aluminum material of Example 3.

[0076] Considering the results of the second embodiment described above, it is confirmed that the corrosion resistance of the protective coating decreases when carbonate ions are mixed into the treatment solution in the pretreatment step. Despite no significant difference in surface morphology observed by SEM of the coating, the reason for the decreased corrosion resistance in Example 4 is thought to be the anionic component of LDH in the protective coating. It is thought that the protective coating of Example 4 contains a large amount of LDH with carbonate ions as its anionic component. As mentioned above, carbonate ions in LDH have poor exchange activity with corrosive ions, and this is thought to be related to the difference in corrosion resistance. [Industrial applicability]

[0077] As described above, the present invention relates to an aluminum material having a corrosion-resistant coating on a substrate made of aluminum or an aluminum alloy that exhibits better corrosion resistance than conventional materials. Furthermore, the method for manufacturing the aluminum material according to the present invention includes a steam treatment step that efficiently forms the aforementioned good corrosion-resistant coating. The present invention is useful as a constituent material for various aluminum material components, whose range of application is expanding, and as a surface treatment method for various aluminum materials.

Claims

1. In an aluminum material comprising a base material made of aluminum or an aluminum alloy and a corrosion-resistant coating formed on at least a part of the base material, The aforementioned corrosion-preventive coating contains aluminum hydroxide oxide (AlO(OH)) and at least one Al-based layered double hydroxide represented by the following formula 1. The aforementioned Al-based layered double hydroxide must contain the anionic component (A) in the following formula. ｎ－ ) contains an Al-based layered double hydroxide in which nitrate ions are present. An aluminum material characterized in that the Al-based layered double hydroxide is preferentially oriented on the (003) plane. 【Chemistry 1】 (In the formula, M1 is a divalent metallic element and is at least one of the following: cobalt (Co), zinc (Zn), magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), cadmium (Cd), and calcium (Ca). M1 ２＋ It is a divalent metal ion. A, which is an anion component ｎ－ is at least one of hydroxide ions (OH － ), carbonate ions (CO ３ ２－ ), nitrate ions (NO ３ － ), sulfate ions (SO ４ ２－ ), fluoride ions (F － ), and chloride ions (Cl － ).)

2. The aluminum material according to claim 1, wherein, in the profile obtained by X-ray diffraction of the aluminum material through a corrosion-resistant coating, the peak intensity of the diffraction peak of the (003) plane of the Al-based layered double hydroxide is higher than the peak intensity of the other diffraction peaks of the Al-based layered double hydroxide.

3. In the profile obtained by X-ray diffraction of an aluminum material through a corrosion-resistant coating, the peak intensity of the diffraction peak of the (003) plane of the Al-based layered double hydroxide is I ＬＤＨ The peak intensity of the diffraction peak of the (020) plane of aluminum hydroxide oxide is set to I ＡｌＯ(ＯＨ) In this case, the ratio of the peak intensities of the two, I ＬＤＨ / I ＡｌＯ(ＯＨ) The aluminum material according to claim 1 or claim 2, wherein the ratio is 3.0 or higher.

4. A method for manufacturing an aluminum material, comprising a steam treatment step in which a corrosion-preventive coating is formed by bringing water vapor into contact with at least a portion of the surface of a substrate made of aluminum or an aluminum alloy, Prior to the steam step, the process includes a pretreatment step in which a divalent metal nitrate compound solution is supplied to at least a portion of the surface of the substrate. A method for producing an aluminum material, characterized by performing a steam treatment on the substrate after the aforementioned pretreatment step.

5. The method for producing an aluminum material according to claim 4, wherein the steam treatment step is a process of bringing steam at a temperature of 120°C to 250°C into contact with the substrate.

6. The method for producing an aluminum material according to claim 4 or 5, wherein the steam treatment step involves treating the ammonium salt solution in a steam atmosphere using a steam source.

Images