JP7587802B2 - Surface treatment method for aluminum or magnesium material - Google Patents

Description

The present invention relates to a surface treatment method for a substrate made of an aluminum material or a magnesium material. More specifically, the present invention relates to a method for forming an anticorrosive film by contacting the surface of a substrate made of an aluminum material or a magnesium material with water vapor, and relates to a surface treatment method that can form an anticorrosive film with excellent corrosion resistance in a shorter time than conventional methods.

Because aluminum and magnesium materials are lightweight, they are widely used in automobiles, aircraft, and other industrial fields in general to reduce the weight of various parts, and their range of application is expanding. In particular, in the fields of automobiles and aerospace equipment, various parts made of aluminum and magnesium materials are widely used as replacements for steel materials in order to improve fuel efficiency through weight reduction.

Corrosion resistance is one of the concerns when aluminum and magnesium materials are used for the above-mentioned purposes. When aluminum is left in air, a natural oxide film is generated and the aluminum is passivated. However, since the thickness of this natural oxide film is about several nanometers, it is easily corroded in an extremely humid, acidic or alkaline environment. On the other hand, magnesium is a very active metal, and the surface is easily corroded by oxidation in the usage environment. Therefore, surface treatment methods for improving the corrosion resistance of these alloy materials have been studied. Currently, chemical conversion treatments such as anodizing, plating, zinc phosphate treatment, and chromate treatment are known depending on the usage environment (for example, Patent Document 1). In these various chemical conversion treatments, a corrosion-resistant film is formed on the alloy surface by contacting and immersing the alloy material to be treated in a treatment solution containing an acid or _alkali such as _H2SO4 and heavy metal ions such as Cr.

Surface treatment techniques such as the above-mentioned chemical conversion treatments have problems such as increased costs for procuring unique treatment solutions and treating waste liquids, as well as environmental impact. Therefore, surface treatment techniques that apply water vapor have been attracting attention as a surface treatment method that has a smaller environmental impact than the above-mentioned chemical conversion treatments. The present inventors have also disclosed a water vapor treatment in which water vapor in a specified temperature range is brought into contact with the alloy surface in order to improve the corrosion resistance of aluminum and magnesium materials (for example, Patent Document 2 and Patent Document 3). This water vapor treatment is a surface treatment technique that forms a film mainly composed of hydroxides of each metal on the surface of a substrate made of an aluminum or magnesium material, and improves corrosion resistance through the anticorrosive effect of the film.

Japanese Patent Application Publication No. 9-112521 International Publication No. 2017/135363 JP 2014-125639 A

Steam treatment of aluminum and magnesium materials is safer and more environmentally friendly than conventional chemical conversion treatments in terms of waste liquid treatment, and can relatively easily impart corrosion resistance to alloy materials by forming an anticorrosive film. In addition, in steam treatment, a film is formed by the direct reaction of water vapor with the substrate surface, so a uniform film with good adhesion can be formed. Furthermore, a film that can be formed with high conformability to the substrate shape can be formed, making it effective as a surface treatment for substrates with complex shapes.

However, considering the above-mentioned trend toward expanding the range of applications of aluminum and magnesium materials, there are also areas in which steam treatment can be improved. One area in which steam treatment can be improved is shortening the treatment time. The corrosion resistance of materials on which an anticorrosive film is formed is greatly affected by the thickness of the film. To ensure corrosion resistance even with steam treatment, it is necessary to form a thick anticorrosive film. In this regard, steam treatment is a treatment method that requires a relatively long time to form an anticorrosive film, and products that use this treatment are inferior in terms of productivity. Therefore, improvements to steam treatment that can efficiently form a thick anticorrosive film in a short time are desired.

The present invention was made against the background described above, and aims to provide a surface treatment method that can efficiently form an anticorrosive film in a short time on a substrate made of an aluminum or magnesium material.

The inventors conducted extensive research to solve the above problems and discovered that by subjecting a substrate to a humidified state by supplying water in advance and then subjecting it to steam treatment, the growth rate of the anticorrosive coating is significantly increased, allowing the anticorrosive coating to be formed efficiently in a short period of time.

That is, the present invention is a surface treatment method for forming an anticorrosive film on at least a portion of the surface of a substrate made of an aluminum material or a magnesium material, the surface treatment method comprising a humidification step of supplying moisture to at least a portion of the surface of the substrate, and a water vapor treatment step of contacting the substrate after the humidification step with water vapor to form an anticorrosive film on at least a portion of the surface of the substrate. The surface treatment method according to the present invention will be described in detail below with respect to the treatment steps.

I. Substrate The subject of the surface treatment method according to the present invention is a substrate made of an aluminum material or a magnesium material. In the present invention, the shape of the substrate is not particularly limited, and any shape such as a plate shape or a tube shape can be applied. There is also no limitation on the dimensions of the substrate. The meanings of aluminum material and magnesium material are as follows.

I-1. Aluminum material Aluminum material is aluminum or an aluminum alloy mainly composed of aluminum. Examples of additive elements that are alloyed with aluminum in an aluminum alloy include at least one of zinc (Zn), magnesium (Mg), silicon (Si), copper (Cu), manganese (Mn), lithium (Li), iron (Fe), nickel (Ni), silver (Ag), zirconium (Zr), and chromium (Cr). The aluminum alloy substrate of the present invention is preferably an aluminum alloy containing a total of 0.1% by mass or more and less than 50% by mass of these additive elements.

Specific examples of aluminum alloys include various aluminum alloys specified by the International Aluminum Alloy Name. For example, various precipitation hardening aluminum alloys such as 2000 series Al-Cu alloys, 4000 series Al-Si alloys, 5000 series Al-Mg alloys, 6000 series Al-Mg-Si alloys, 7000 series Al-Zn-Mg alloys or Al-Zn-Mg-Cu alloys are suitable. However, the present invention is not limited to these standardized alloy systems, and alloy systems with a wide range of compositions can be used.

I-2. The magnesium material constituting the magnesium material substrate is magnesium or a magnesium alloy containing magnesium as the main component. Examples of additive elements that are alloyed with magnesium in the magnesium alloy include aluminum (Al), zinc (Zn), calcium (Ca), manganese (Mn), zirconium (Zr), and rare earth elements (scandium (Sc), yttrium (Y), lanthanoids), etc. Many of the magnesium alloys in practical use contain aluminum as an additive element.

The content of additive elements in the magnesium alloy is preferably 1% by mass or more and 15% by mass or less for aluminum. As for other additive elements, zinc is preferably more than 0% by mass and 5% by mass or less. Calcium is preferably more than 0% by mass and 5% by mass or less, and manganese is preferably more than 0% by mass and 5% by mass or less. Furthermore, zirconium is preferably more than 0% by mass and 5% by mass or less, and rare earth elements are preferably more than 0% by mass and 5% by mass or less.

Specific examples of magnesium alloys as described above include alloys specified by ASTM standards. For example, AZ alloys (AZ31, AZ61, AZ91) with aluminum and zinc as additive elements, AZX alloys (AZX311, AZX611, AZX612, AZX615, AZX6112, AZX711, AZX811, AZX911, AZX1001) with aluminum, zinc, and calcium as additive elements, and AMX alloys (AMX301, AMX601, AMX701, AMX801, AMX901, AMX1001) with aluminum, manganese, and calcium as additive elements can be suitably applied to the present invention.

When the substrate made of the aluminum or magnesium material described above is subjected to the surface treatment method according to the present invention, it can be washed and dried as a pretreatment. In addition, for aluminum or magnesium alloys that have age-hardening properties based on their composition, a solution heat treatment and, if necessary, an ageing heat treatment may be performed before the surface treatment.

II. Supplying moisture to the substrate (humidification process)
The surface treatment method according to the present invention includes a new pretreatment step in comparison with the conventional surface treatment method using steam treatment of a substrate made of an aluminum material or a magnesium material. Specifically, a humidification step of supplying moisture to the substrate surface is added before the steam treatment. From the study of the present inventors, by contacting the substrate in a moisture-supplied state with steam, the growth rate of the anticorrosive film is significantly improved, and a homogeneous anticorrosive film can be formed in a short time. The reason why the growth of the anticorrosive film on the substrate to which moisture has been applied is thus accelerated is not clear. However, the present inventors consider that the reason is due to the water film on the substrate surface formed by the supply of moisture. In the present invention, the dissolution of metal ions derived from the substrate into the water film is promoted, and the reaction between the metal ions and hydroxide ions derived from the steam occurs quickly, accelerating the film growth.

There is no particular limitation on the method of supplying moisture to the substrate surface. One method of supplying moisture is to immerse the substrate in a treatment liquid containing water. This humidification process by immersion is completed by immersing the entire substrate or a part of it in the treatment liquid and then lifting the substrate. There is no particular restriction on the immersion time. The substrate may be immersed in the necessary area of the treatment liquid and immediately lifted up, or may be immersed for 1 second to 3 minutes before being lifted up.

The humidification step can also be carried out by spraying or dripping a treatment liquid containing water onto the substrate. In these cases, the amount of water to be sprayed or dripped onto the substrate is not particularly limited. For example, it is sufficient to apply about 50 μL to 5 mL of treatment liquid per 1 ^cm2 of the region to which water is to be supplied.

In these humidification processes, moisture may be supplied to the entire surface of the substrate, or it may be supplied partially, or to one or both sides.

The treatment liquid, which is the moisture supplied in the humidification process, essentially contains water. The treatment liquid may consist essentially of water, in which case industrial water or tap water can be used, but it is preferable to use pure water. When using pure water, it is preferable to use ion-exchanged water, distilled water, or ultrapure water with an electrical conductivity of 1 mS/m or less.

The treatment liquid for the humidification step can be the same as the water or an aqueous solution containing an appropriate salt that serves as the water vapor source for the water vapor that is brought into contact with the substrate in the subsequent water vapor treatment step. The aqueous solution in this case will be described in detail later, but an aqueous solution containing at least one of carbonates, nitrates, sulfates, and fluorides can be used. In particular, when a method of immersing the substrate in a treatment liquid is adopted for the humidification step, the substrate can be immersed in the treatment liquid, and then the treatment liquid is heated to convert it into water vapor, thereby performing water vapor treatment. This method of sharing the treatment liquid and water vapor source for the humidification step can simplify the structure of the surface treatment device compared to when they are prepared separately.

However, even if the same water or aqueous solution as the water vapor source can be used, the treatment liquid supplied to the substrate in the humidification process must not be in a vapor state. If the treatment liquid is in a vapor state, the formation of a coating will begin, and the object of the present invention cannot be achieved. In other words, it is preferable that the treatment liquid supplied to the substrate in the humidification process is at a temperature of 5°C or higher and 95°C or lower.

III. Steam treatment step : By carrying out a steam treatment in which steam is brought into contact with the substrate that has been subjected to the moisture supply step as described above, a corrosion-resistant film is formed on at least a part of the substrate surface. The temperature of the steam brought into contact with the substrate in the steam treatment step for forming the corrosion-resistant film is preferably 100°C or higher and 350°C or lower. If the temperature of the steam exceeds 350°C, the corrosion-resistant film may become porous or crack. If the temperature of the steam is lower than 100°C, the rate at which the corrosion-resistant film is formed decreases, and the corrosion-resistant film is not formed sufficiently. The temperature of the steam is more preferably 130°C to 250°C. In addition, the treatment time of the steam treatment is preferably 0.1 hours or higher. If the treatment time is less than 0.1 hours, it is difficult to form a corrosion-resistant film of sufficient thickness even if the humidification treatment of the present invention is performed. There is no particular need to limit the upper limit, but considering the purpose of shortening the time for forming the corrosion-resistant film and improving productivity, it is preferably 5 hours or less, and more preferably 2 hours or less.

The water vapor to be brought into contact with the substrate is generated by heating and vaporizing water. As the water used as the water vapor source, industrial water or tap water can be used, and pure water is preferred. An aqueous solution containing an appropriate salt can also be used. When using pure water, it is preferred to use ion-exchanged water, distilled water, or ultrapure water with an electrical conductivity of 1 mS/m or less. As an aqueous solution containing salt, the vapor of an aqueous solution of carbonate, nitrate, sulfate, or fluoride salt can be used. These salts include salts of alkali metals (lithium, sodium, potassium, etc.) (sodium carbonate, sodium nitrate, etc.), salts of alkaline earth metals (calcium, strontium, barium, etc.) (calcium carbonate, calcium nitrate, etc.), salts of precious metals, salts of common metals, etc. An aqueous solution of one or a combination of these salts can be used.

The steam pressure is preferably in the range of 0.1 to 10 MPa. The pressure is more preferably 0.2 to 5 MPa. When pressurized steam is used, a two-phase equilibrium state of saturated steam and subcritical water is reached, making it possible to promote reactivity in forming a corrosion-resistant film. By maintaining a constant steam pressure during treatment, a uniform corrosion-resistant film can be formed.

There are no particular limitations on the method of contacting the substrate with water vapor. Water vapor treatment may be performed by exposing the substrate to water vapor in a closed space such as a reactor or container. As a specific method, the substrate may be placed in a container together with water, and exposed to a water vapor atmosphere generated by controlling the temperature and pressure. Water vapor may also be sprayed directly onto the substrate.

Depending on the type of aluminum and magnesium material used as the substrate, the above steam treatment can not only form an anticorrosive film, but also increase the hardness (strength) of the substrate. This increase in hardness is due to age hardening caused by the heat of the steam when the substrate is a precipitation-hardened alloy, as described below.

The humidification process and water vapor treatment process described above need to be performed at least once, and may be performed repeatedly. By performing the process repeatedly, a corrosion protection film consisting of multiple film layers can be formed. Furthermore, each process may be performed under different conditions. By performing the process repeatedly and under different conditions, multiple corrosion protection films with different compositions can be formed.

IV. Formation of Anticorrosive Film by Steam Treatment By the above-mentioned steam treatment, an anticorrosive film is formed on at least a part of the surface of the substrate made of an aluminum material or a magnesium material. The composition of the anticorrosive film varies depending on the material constituting the substrate.

(a) Anticorrosive Film for Aluminum Material When the substrate is an aluminum material, the anticorrosive film formed by steam treatment essentially contains aluminum oxide hydroxide. Aluminum oxide hydroxide is also called boehmite and is an aluminum compound expressed as γ-AlO(OH) or simply AlO(OH). Aluminum oxide hydroxide has chemical stability and has a high anticorrosive effect. The composition of the anticorrosive film essentially contains aluminum oxide hydroxide, and may be composed of only aluminum oxide hydroxide. However, the film may contain substances other than aluminum oxide hydroxide. For example, it may contain trace amounts of additive elements of the aluminum alloy substrate or compounds thereof (intermetallic compounds, oxides, hydroxides, etc.). In addition, compounds (oxides, hydroxides, hydrates, salts) derived from components in the steam that comes into contact with the substrate to form the film may also be included.

In addition, the prior art states that, in addition to aluminum hydroxide oxide, an Al-based layered double hydroxide (hereinafter sometimes referred to as LDH) is optionally included as a component of the corrosion protection film of an aluminum material by steam treatment. LDH is a compound in which a hydroxide of a divalent metal (M) derived from an additive element in an aluminum alloy is combined with a double hydroxide in which ions of a trivalent metal, Al, are substituted for the divalent metal site, forming a layered structure, and is represented by the following chemical formula:

（式中、Ｍ１は２価の金属元素であり、コバルト（Ｃｏ）、亜鉛（Ｚｎ）、マグネシウム（Ｍｇ）、銅（Ｃｕ）、マンガン（Ｍｎ）、鉄（Ｆｅ）、ニッケル（Ｎｉ）、クロム（Ｃｒ）、カドミウム（Ｃｄ）、カルシウム（Ｃａ）のいずれかである。陽イオンＭ１^２＋は２価の金属イオンである。陰イオンであるＡ^ｎ－は、水酸化物イオン（ＯＨ^－）、炭酸イオン（ＣＯ_３ ^２－）、硝酸イオン（ＮＯ_３ ^－）、硫酸イオン（ＳＯ_４ ^２－）、フッ素イオン（Ｆ^－）、塩素イオン（Ｃｌ^－）の少なくともいずれかである。）

(In the formula, M1 is a divalent metal element and is any one of cobalt (Co), zinc (Zn), magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), nickel (Ni), chromium (Cr), cadmium (Cd), and calcium (Ca). The cation M1 ²⁺ is a divalent metal ion. The anion A ^n- is at least any one of a hydroxide ion (OH ^- ), a carbonate ion (CO ₃ ^2- ), a nitrate ion (NO ₃ ^- ), a sulfate ion (SO ₄ ^2- ), a fluorine ion (F ^- ), and a chloride ion (Cl ^- ).)

In the structural formula shown in Chemical Formula 1, [M1 ²⁺ _1-x Al ³⁺ _x (OH) ₂ ] in the formula is called the double hydroxide basic layer, and [A ^n- _x/ n.yH ₂ O] in the formula is sometimes called the intermediate layer. Al-based layered double hydroxides have a layered structure in which a negatively charged intermediate layer is sandwiched between positively charged double hydroxide basic layers, and this structure is maintained in the corrosion protective coating.

In the corrosion protection film for aluminum materials according to the present invention, the above-mentioned LDH is included as an optional component and is not an essential component. The effect of improving corrosion resistance by steam treatment does not require the presence of LDH in the film, and it is considered that the effect is achieved by the formation of aluminum oxide hydroxide alone.

(b) Anticorrosive Film of Magnesium Material When an anticorrosive film is formed on a substrate made of a magnesium material by steam treatment, the anticorrosive film essentially contains magnesium hydroxide (Mg(OH) ₂ ). Magnesium hydroxide is also chemically stable and is useful as an anticorrosive film. The anticorrosive film may be composed of only magnesium hydroxide. The anticorrosive film is mainly composed of magnesium hydroxide, but also allows various additive elements other than magnesium contained in the magnesium alloy of the substrate as inevitable impurities, such as magnesium oxide (MgO), magnesium carbonate ( _MgCO3 ), and magnesium oxide hydroxide ( _Mg5O (OH) ₈ ). In addition, the anticorrosive film may also contain compounds (oxides, hydroxides, hydrates, salts) derived from components in the steam that comes into contact with the substrate to form the anticorrosive film.

Furthermore, the anticorrosive coating for magnesium materials essentially contains magnesium hydroxide (Mg(OH) ₂ ), but may also optionally contain Mg—Al-based layered double hydroxide (Mg—Al-based layered double hydroxide). Mg—Al-based layered double hydroxide is a hydroxide represented by the following chemical formula 2.

（式中、陰イオンであるＡ^ｎ－は、水酸化物イオン（ＯＨ^－）、炭酸イオン（ＣＯ_３ ^２－）、硝酸イオン（ＮＯ_３ ^－）、硫酸イオン（ＳＯ_４ ^２－）、フッ素イオン（Ｆ^－）、塩素イオン（Ｃｌ^－）の少なくともいずれかである。）

(In the formula, the anion A ^n- is at least one of a hydroxide ion (OH ^- ), a carbonate ion (CO ₃ ^2- ), a nitrate ion (NO ₃ ^- ), a sulfate ion (SO ₄ ^2- ), a fluorine ion (F ^- ), and a chloride ion (Cl ^- ).)

The Mg—Al-based layered double hydroxide is composed of metal ions (magnesium ions (Mg ²⁺ ) and aluminum ions (Al ³⁺ )) supplied from the substrate (magnesium alloy) and anions (A ⁿ⁻ ) supplied from water vapor that comes into contact with the substrate during film formation.

In the corrosion protection coating for magnesium materials according to the present invention, the above-mentioned Mg-Al-based layered double hydroxide is included as an optional component and is not an essential component. The effect of improving corrosion resistance by steam treatment can be achieved by the formation of magnesium oxide hydroxide alone.

The structure of the corrosion protection coating on the aluminum and magnesium materials described above can be examined based on the profile obtained by X-ray diffraction analysis (XRD), for example. The amount of each component present can also be estimated based on the peak intensity of each component.

The thickness of the anticorrosive coating formed by the surface treatment method of the present invention is preferably 10 μm to 100 μm. A coating less than 10 μm thick has poor anticorrosive effect on the substrate, and a coating thicker than 100 μm may crack or peel due to stress or thermal shock, resulting in poor corrosion resistance. A coating thickness of 5 μm, which is the lower limit, requires a long time with conventional surface treatment methods, but can be achieved in a short time with the present invention.

As described above, the present invention is capable of forming a thicker and more uniform anticorrosive coating on a substrate made of aluminum or magnesium material in a shorter time than conventional methods, and is useful as a surface treatment method that improves corrosion resistance and has excellent productivity.

FIG. 1 is a diagram for explaining the configuration of a steam curing apparatus. 2 is a SEM photograph showing the surface morphology of the anticorrosive coating formed on the surface of a substrate made of the aluminum alloy of Example 1. 1 shows XRD profiles of the surfaces of aluminum alloy substrates on which anticorrosive coatings of Example 1 and Comparative Example 1 are formed. 3 shows polarization curves of aluminum alloy substrates on which the anticorrosive coatings of Example 1 and Comparative Example 1 are formed. 4 is a SEM photograph showing the surface morphology of the anticorrosive coatings formed on the magnesium alloy substrates of Examples 3 and 4 and Comparative Examples 2 and 4. SEM photographs showing cross sections of magnesium alloy substrates on which anticorrosive coatings of Example 4 and Comparative Example 4 were formed. 1 shows XRD profiles of the surfaces of magnesium alloy substrates on which anticorrosive coatings of Examples 3 and 4 and Comparative Examples 3 and 4 are formed. 4 shows polarization curves of magnesium alloy substrates on which the anticorrosive coatings of Example 2 and Comparative Example 2 are formed. 4 shows polarization curves of magnesium alloy substrates on which anticorrosive coatings of Example 4 and Comparative Example 4 are formed.

First embodiment : Hereinafter, an embodiment of the present invention will be described with examples and comparative examples. In this embodiment, a substrate made of an aluminum material (aluminum alloy), an Al-5.6 mass% Zn-2.6 mass% Mg-Cu alloy (7075 alloy), which is a 7000 series aluminum alloy, was subjected to a surface treatment to form an anticorrosive coating.

Example 1 : An aluminum alloy plate (70 x 30 mm, thickness 3.0 mm) was prepared as a substrate (sample), washed and dried. Then, 500 μL of pure water was sprayed per 1 ^cm2 as a treatment liquid onto the surface of the substrate.

Next, a steam treatment was performed by contacting the moisture-supplied substrate with steam, forming a corrosion-resistant film on the substrate surface. For the steam treatment, a steam curing device as shown in Figure 1 was used. The steam curing device in Figure 1 is a horizontal autoclave, and pure water (20 ml) is injected into the lower part as a steam source. The upper part of the device is designed to allow multiple samples (substrates) to be hung. For the steam treatment, the substrate was quickly placed in the steam curing device after moisture was supplied, and the treatment was started. The steam treatment was performed at a temperature of 200°C, for a treatment time of 6 hours, and with the temperature and pressure maintained at 1.0 MPa (Example 1).

Comparative Example 1 : As a comparative example, the substrate after drying was subjected to a water vapor treatment without supplying moisture. The conditions of the water vapor treatment were the same as those of Example 1.

Various evaluations were carried out on the substrates of this embodiment and the comparative example manufactured by the above process.

[Observation of the surface morphology of the anticorrosive coating by SEM]
The surface morphology of the anticorrosive coating was observed by a scanning electron microscope (SEM). Figure 2 is an SEM photograph (x30,000) showing the surface morphology of the anticorrosive coating formed after the steam treatment of the aluminum alloy substrates of Example 1 and Comparative Example 1. It can be seen from Figure 1 that the anticorrosive coating formed by the steam treatment after the supply of moisture in Example 1 uniformly and densely covers the substrate surface, even though the treatment time is the same as that of the anticorrosive coating of Comparative Example 1 without the supply of moisture.

[Analysis by X-ray diffraction (XRD)]
Next, the surfaces of the substrates on which the anticorrosive coatings of Example 1 and Comparative Example 1 were formed were analyzed by X-ray diffraction (XRD). X-ray diffraction (XRD) can identify the structure of the anticorrosive coating based on the obtained profile (X-ray diffraction pattern). XRD was measured using a Cu-Kα X-ray source at a voltage of 40 kV and a current of 30 mA.

Figure 3 shows XRD profiles of the surfaces of the substrates on which the anticorrosive coatings of the present embodiment and the comparative example are formed. From Figure 3, it can be seen that an anticorrosive coating containing aluminum oxide hydroxide (AlO(OH)) and an Al-based layered double hydroxide is formed on the surface of each substrate. However, the anticorrosive coating of Example 1 has stronger peak intensities of aluminum oxide hydroxide and Al-based layered double hydroxide compared to the anticorrosive coating of Comparative Example 1. In other words, it was confirmed that a thicker anticorrosive coating was formed in Example 1, in which water was supplied before the water steam treatment, than in Comparative Example 1, even though the steam treatment time was the same.

[Polarization measurements]
Next, the corrosion resistance was evaluated by measuring the polarization curves of the substrates on which the anticorrosive coatings of Example 1 and Comparative Example 1 were formed. For the polarization measurement, a 5 wt % NaCl aqueous solution was used as the electrolyte, and the solution was bubbled with nitrogen before the measurement, and then the polarization curve was measured using a potentio/galvanostat (VersaSTAT4, manufactured by Princeton Applied Research).

Figure 4 shows the polarization curves of the substrates on which the anticorrosive coatings of the present embodiment and the comparative example are formed. For comparison, Figure 4 also shows the polarization curves of an aluminum alloy (7075 alloy) substrate without the formation of an anticorrosive coating. From Figure 4, it can be seen that Example 1, in which the anticorrosive coating was formed by steam treatment, exhibits a significant decrease in corrosion current density compared to a substrate without an anticorrosive coating, and the effect of improving corrosion resistance due to the anticorrosive coating is confirmed. Furthermore, comparing Example 1 with Comparative Example 1, it can be seen that the corrosion current density is decreased in Example 1 and the corrosion potential becomes nobler. From this result, it can be seen that Example 1, in which moisture was supplied before the steam treatment, has improved corrosion resistance compared to Comparative Example 1, in which moisture was not supplied. It is believed that this improvement in corrosion resistance is due to the formation of a thicker coating due to the moisture supply.

Second embodiment : In this embodiment, a substrate made of AZ31 alloy (Mg-3% Al-1% Zn) was used as the magnesium material (alloy), and a surface treatment was performed to form an anticorrosive film on the substrate surface.

A magnesium alloy plate (70×30 mm, 3.0 mm thick) was prepared as a substrate (sample), washed and dried, and then 100 μL of pure water was dropped per 1 ^cm2 onto the surface of the substrate as a treatment liquid.

Next, the substrate to which moisture had been supplied was brought into contact with water vapor to perform water vapor treatment, and an anticorrosive film was formed on the substrate surface. As in the first embodiment, the water vapor treatment was performed using the steam curing device shown in FIG. 1. The water vapor treatment was performed at a temperature of 140°C for a treatment time of 1 hour (Example 2), 3 hours (Example 3), and 5 hours (Example 4), with the temperature and pressure maintained at 1 MPa.

Comparative Examples 2 to 4 : As comparative examples, the substrate after drying was subjected to steam treatment without supplying moisture. The conditions of the steam treatment were a steam temperature of 140° C. and a treatment time of 1 hour (Comparative Example 2), 3 hours (Comparative Example 3), and 5 hours (Comparative Example 4).

Various evaluations were carried out on the substrates of this embodiment and the comparative example manufactured by the above process.

[Observation of the surface morphology of the anticorrosive coating by SEM]
Fig. 5 is a SEM photograph (x5000) showing the surface morphology of the anticorrosive coating formed on the surface of the magnesium alloy substrate after each water vapor treatment time. As can be seen from Fig. 5, the anticorrosive coating of the comparative example has many microcracks on its surface, and is therefore considered to have insufficient corrosion resistance. In contrast, the anticorrosive coating of the present embodiment covers the substrate surface homogeneously and densely, as compared to the comparative example with the same treatment time, and it can be confirmed that the coating is formed stably.

Next, the substrate on which the anticorrosive coating was formed was cut perpendicular to the anticorrosive coating, and the cross section was polished and then observed with an SEM to measure the film thickness. Figure 6 is an SEM photograph (x5000) showing the cross section of the substrate on which the anticorrosive coating of Example 4 and Comparative Example 4 (steam treatment time: 5 hours) was formed. From Figure 6, it can be seen that the anticorrosive coating of Example 4 (film thickness 12.5 μm) is thicker than that of Comparative Example 4 (film thickness 8 μm) at the same treatment time. From these observation results, it was confirmed that the humidification process by supplying moisture accelerates the film formation rate during steam treatment.

[Analysis by X-ray diffraction (XRD)]
Next, the surfaces of the substrates on which the anticorrosive coatings of this embodiment and the comparative example were formed were analyzed by X-ray diffraction (XRD). Fig. 7 shows XRD profiles of the substrates on which the anticorrosive coatings of Example 3 (treatment time 3 hours), Example 4 (treatment time 5 hours), Comparative Example 3 (treatment time 3 hours), and Comparative Example 4 (treatment time 5 hours) were formed. From Fig. 7, it was confirmed that an anticorrosive coating containing magnesium hydroxide and Mg-Al-based layered double hydroxide was formed on the surface of each substrate. In Examples 3 and 4, the peak intensity of the Mg-Al-based layered double hydroxide was strong.

[Polarization measurement]
Next, the polarization curves of the substrates on which the anticorrosive coatings according to the present embodiment and the comparative example were formed were measured to evaluate their corrosion resistance. FIG. 8 shows the polarization curves of the substrates on which the anticorrosive coatings of Example 2 (treatment time 1 hour) and Comparative Example 2 (treatment time 1 hour) were formed. FIG. 9 shows the polarization curves of the substrates on which the anticorrosive coatings of Example 4 (treatment time 5 hours) and Comparative Example 4 (treatment time 5 hours) were formed. FIG. 8 also shows the polarization curve of a magnesium alloy (AZ31 alloy) substrate without steam treatment for comparison. From FIG. 8, the substrates on which the anticorrosive coatings were formed by the steam treatment showed a significant decrease in corrosion current density compared to the substrates that were not subjected to the steam treatment, confirming the effect of improving the corrosion resistance by the anticorrosive coating.

In addition, comparing Example 2 with Comparative Example 2, it can be seen that in Example 2, the corrosion current density decreases and the corrosion potential becomes nobler. Example 2 and Comparative Example 2 are samples formed by steam treatment for one hour, and even in the short period of one hour, the difference between the two is clear. The usefulness of supplying moisture before steam treatment can be confirmed.

Referring to Figure 9, it can be seen that the five-hour steam treatment reduced the corrosion current density in both Example 4 and Comparative Example 4, and that the growth of the film improved the corrosion resistance. Moreover, Example 4, in which moisture was supplied before the steam treatment, showed a further reduction in the corrosion current density and a noble corrosion potential compared to Comparative Example 4, in which only the steam treatment was performed, and it can be confirmed that a film with higher corrosion resistance was formed.

Third embodiment : In this embodiment, an aluminum alloy substrate was used, and moisture was supplied in the humidifying step by immersing the substrate in a treatment liquid, and then a water vapor treatment was performed to form an anticorrosive coating.

In this embodiment, a rotating mechanism equipped with a motor was added to the hanging section of the sample in the steam curing apparatus of FIG. 1, allowing the sample to be raised and lowered. Then, as in the first embodiment, an aluminum alloy (7075 alloy) substrate was washed, dried, and set in the apparatus. The substrate was then lowered to immerse the entire substrate in the treatment liquid (pure water) at the bottom of the apparatus, and then immediately raised to pull up the substrate and perform steam treatment. The conditions for the steam treatment were the same as in the first embodiment: temperature 200°C, treatment time 6 hours, and pressure 1.0 MPa.

After the steam treatment according to this embodiment, the substrate on which the film was formed was removed and its appearance was observed. The substrate surface had the same appearance as the aluminum alloy substrate of the first embodiment. When XRD analysis was performed, clear diffraction peaks of aluminum oxide hydroxide (AlO(OH)) and Al-based layered double hydroxide were observed, as in the first embodiment. This confirmed that immersing the substrate in the treatment liquid is also an effective way of supplying moisture.

As explained above, the present invention is capable of forming a thicker and more uniform anticorrosive coating on a substrate made of an aluminum alloy or magnesium alloy in a shorter time than conventional methods, and is useful as a surface treatment method that improves corrosion resistance and has excellent productivity. In addition, it is expected to be used as a surface treatment method for various aluminum and magnesium material components, the range of applications of which is expanding.

Claims (4)

A surface treatment method for forming an anticorrosive coating on at least a part of a surface of a substrate made of a magnesium material , comprising the steps of:
a humidifying step of supplying a treatment liquid made of pure water to at least a part of a surface of the substrate to humidify the substrate;
and a water steam treatment step of contacting the substrate after the humidification step with water steam to form a corrosion protection film on at least a part of the surface of the substrate .
The anticorrosive coating formed by the water vapor treatment contains magnesium hydroxide (Mg(OH) ₂₎ and, optionally, an Mg-Al-based layered double hydroxide represented by the following formula 2 .

(In the formula, the anion A ^n- is at least one of a hydroxide ion (OH ^- ), a carbonate ion (CO ₃ ^2- ), a nitrate ion (NO ₃ ^- ), a sulfate ion (SO ₄ ^2- ), a fluorine ion (F ^- ), and a chloride ion (Cl ^- ).) The surface treatment method according to claim 1, wherein the humidification step is a step of immersing the substrate in a treatment liquid consisting of pure water. The surface treatment method according to claim 1, wherein the humidification step is a step of spraying or dripping a treatment liquid consisting of pure water onto the substrate. 4. The surface treatment method according to claim 1, wherein the water vapor treatment step is a treatment in which water vapor having a temperature of 100° C. to 350° C. is brought into contact with the base material.

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