Aluminum Material or Aluminum Alloy Material Provided With Coating Film, Method for Producing Same, and Aqueous Surface Treatment Agent

TECHNICAL FIELD

[0001]The present invention relates to: an aluminum material or aluminum alloy material that includes a surface treatment coating having excellent thermal corrosion resistance; and a method of producing the same. The present invention also relates to: an aqueous surface treatment agent that can form a surface treatment coating having excellent thermal corrosion resistance on or over a surface of an aluminum material or aluminum alloy material.

BACKGROUND ART

[0002]In a wide range of fields of aircraft materials, building materials, automobile components, and the like, aluminum materials and aluminum alloy materials that are surface-treated with a chromium ion-containing surface treatment agent and have a coating have been used for the purpose of providing corrosion resistance.

[0003]For example, Patent Document 1 discloses a chemical conversion treatment liquid for metal materials, which contains a component (A) composed of a water-soluble trivalent chromium compound, a component (B) composed of at least one selected from water-soluble titanium compounds and water-soluble zirconium compounds, a component (C) composed of a water-soluble nitrate compound, a component (D) composed of a water-soluble aluminum compound, and a component (E) composed of a fluorine compound, and is controlled to have a pH in a range of 2.3 to 5.0.

[0004]Patent Document 2 discloses a chemical conversion treatment liquid that contains a specific trivalent chromium compound, a specific zirconium compound, and a specific dicarboxylic acid compound in prescribed amounts.

[0005]Further, Patent Document 3 discloses a surface treatment agent for aluminum or aluminum alloys, which contains a trivalent chromium-containing ion (A), at least one ion (B) selected from titanium-containing ions and zirconium-containing ions, a zinc-containing ion (C), a free fluorine ion (D), and a nitrate ion (E).

RELATED ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0009]Aluminum materials and aluminum alloy materials may be exposed to a high-temperature environment depending on their use. However, the coatings formed on aluminum materials and aluminum alloy materials using the surface treatment agents disclosed in Patent Documents 1 and 2 have room for improvement in terms of deterioration of the corrosion resistance caused by exposure to a high-temperature environment (thermal corrosion resistance). Further, even when the surface treatment agent disclosed in Patent Document 3 is used, the thermal corrosion resistance is still not deemed to be sufficient although it is improved.

[0010]The present invention provides an aluminum material or aluminum alloy material that includes a surface treatment coating having excellent thermal corrosion resistance. Another object of the present invention is to provide a surface treatment agent that can form the surface treatment coating.

Means for Solving the Problems

[0011]The present inventors intensively studied to solve the above-described problems, and consequently discovered that an aluminum material or aluminum alloy material which has, on or over a surface thereof, a coating containing chromium, zirconium, zinc, and carbon and having characteristic infrared reflection peaks, exhibits excellent corrosion resistance as well as excellent thermal corrosion resistance, thereby completing the present invention.

Effects of the Invention

[0024]According to the present invention, the following can be provided: an aluminum material or aluminum alloy material that includes a surface treatment coating having excellent thermal corrosion resistance; and a surface treatment agent that can form the surface treatment coating.

MODE FOR CARRYING OUT THE INVENTION

(1) Aluminum Material or Aluminum Alloy Material

[0030]The aluminum material or aluminum alloy material is not particularly limited as long as it is a metal material that contains aluminum. Particularly, an aluminum die-cast material to which it is difficult to impart corrosion resistance due to a thick surface oxide film and segregation of an alloy component is effective as a material on which the surface treatment coating of the present embodiment is to be formed. The use of the aluminum material or aluminum alloy material having the surface treatment coating is not particularly limited; however, the aluminum material or aluminum alloy material is preferably used as a component that is exposed to a high-temperature environment depending on the situation of use, such as an engine peripheral part or an ECU housing, since it is unlikely to cause deterioration of the thermal corrosion resistance. The aluminum material or aluminum alloy material having the surface treatment coating can be provided with an extended life by an improvement in the thermal corrosion resistance, and this can achieve effective use of resources.

(2) Surface Treatment Coating

[0031]The surface treatment coating of the present embodiment is formed on or over a surface of the aluminum material or aluminum alloy material, and contains chromium, zirconium, zinc, and carbon. The surface treatment coating of the present embodiment may consist of only these elements, or may contain other components. The components and the composition (contents) of the coating will now be described in detail.

(2-1) Chromium

[0032]The form of chromium contained in the surface treatment coating may be, but is not particularly limited to, metallic chromium, a chromium oxide such as hexavalent chromium oxide or trivalent chromium oxide, or a chromium compound bound to a component contained in a surface treatment agent. Chromium may be contained in any one of, or two or more of these forms. The content of chromium in the surface treatment coating is usually in a range of 5 mg/m² to 100 mg/m², preferably in a range of 10 mg/m² to 50 mg/m², more preferably in a range of 10 mg/m² to 30 mg/m², in terms of metallic chromium. The content of chromium in the surface treatment coating is measured using an X-ray fluorescence spectrometer.

(2-2) Zirconium

[0033]The form of zirconium contained in the surface treatment coating may be, but is not particularly limited to, metallic zirconium, zirconium oxide, or a zirconium compound bound to a component contained in a surface treatment agent. Zirconium may be contained in any one of, or two or more of these forms. The content of zirconium in the surface treatment coating is usually in a range of 2 mg/m² to 150 mg/m², preferably in a range of 5 mg/m² to 100 mg/m², more preferably in a range of 5 mg/m² to 50 mg/m², in terms of metallic zirconium. The content of zirconium in the surface treatment coating is measured using an X-ray fluorescence spectrometer.

(2-3) Zinc

[0034]The form of zinc contained in the surface treatment coating may be, but is not particularly limited to, metallic zinc, zinc oxide, or a zinc compound bound to a component contained in a surface treatment agent. Zinc may be contained in any one of, or two or more of these forms.

(2-4) Content Ratio of Zinc with Respect to Total Content of Chromium, Zirconium, and Zinc

[0035]Taking a total content of chromium, zirconium, and zinc that are contained in the surface treatment coating as 100% by atom, usually, the content ratio of zinc (Zn/(Cr+Zr+Zn)) is preferably in a range of 2% by atom to 60% by atom, more preferably in a range of 10% by atom to 40% by atom, still more preferably in a range of 10% by atom to 30% by atom.

[0036]The content of each component in the surface treatment coating for determination of the content ratio of zinc with respect to a total content of chromium, zirconium, and zinc (Zn/(Cr+Zr+Zn)) is measured by repeatedly performing the measurement of element amount based on sputtering and X-ray photoelectron spectroscopy (XPS) multiple times using an X-ray photoelectron spectrometer. It is noted here that the depth position in XPS is controlled with the distance at which SiO₂ is sputtered by Ar ions. Specifically, sputtering is performed for 0.25 minutes (sputtering rate: 11.0 nm/min in terms of SiO₂), and the sample surface is irradiated with a monochromatic Al-Kα ray in an analysis diameter of 100 μm, after which the resulting photoelectrons are measured. This process is repeated to measure the contents of chromium, zirconium, and zinc obtained at each depth in the thickness direction of the surface treatment coating. Taking a total content of these elements as 100% by atom, the content ratio of zinc (Zn/(Cr+Zr+Zn)) is calculated.

(2-5) Carbon

[0037]All or some of the carbon atoms contained in the surface treatment coating are derived from an organic compound having a hydroxy group and a carboxyl group that is blended in a surface treatment agent. The content of carbon in the surface treatment coating is usually in a range of 2 mg/m² to 20 mg/m², preferably in a range of 3 mg/m² to 15 mg/m², more preferably in a range of 4 mg/m² to 12 mg/m². The content of carbon in the surface treatment coating is measured using a total organic carbon analyzer.

(2-6) Peaks in Infrared Spectrum

[0038]An infrared spectrum of the surface treatment coating is obtained by measuring the surface treatment coating by a specular reflection method of Fourier transform infrared spectroscopy (FT-IR). The infrared spectrum obtained by measuring the surface treatment coating of the present embodiment has peaks in specific wavelength ranges. A peak derived from a hydroxy group appears at 3,600 cm⁻¹ to 3,000 cm⁻¹, and a peak derived from a carboxyl group appears at 1,750 cm⁻¹ to 1,700 cm⁻¹.

[0039]The infrared spectrum can be obtained using a Fourier transform infrared spectrophotometer under the following measurement conditions: measurement wave number=4,000 cm⁻¹ to 400 cm⁻¹, resolution=4 cm⁻¹, and 128 scans. As a background, an aluminum or aluminum alloy material that does not have the surface treatment coating is used.

[0040]Thus far, the aluminum material or aluminum alloy material having a surface treatment coating according to the present embodiment have been described and, particularly, the thermal corrosion resistance can be improved by the features that the content of zinc in the surface treatment coating is in the above-described range, and that an infrared spectrum measured for the surface treatment coating has peaks in the above-described wavelength ranges.

(3) Surface Treatment Agent

[0041]The surface treatment agent of the present embodiment is a surface treatment agent that is used for a surface treatment of an aluminum or aluminum alloy material. The surface treatment agent can be utilized as a chemical conversion treatment agent.

[0042]The surface treatment agent contains: a chromium-containing ion (A); a zirconium-containing ion (B); a zinc-containing ion (C); and an organic compound (D) having a hydroxy group and a carboxyl group. The surface treatment agent may be prepared by blending only supply sources of these ions (A) to (C) and the organic compound (D) in an aqueous medium, or may be prepared by blending other components in addition thereto. These components, the composition (contents), and the liquid properties will now be described in detail.

(3-1) Aqueous Medium

[0043]The aqueous medium is not particularly limited as long as it is water or a mixture of water and a water-miscible organic solvent (a mixture containing not less than 50% by volume of water based on the volume of the aqueous medium). The water-miscible organic solvent is not particularly limited as long as it is miscible with water, and examples thereof include: ketone-based solvents, such as acetone and methyl ethyl ketone; amide-based solvents, such as N,N-dimethylformamide and dimethylacetamide; alcohol-based solvents, such as methanol, ethanol, and isopropanol; ether-based solvents, such as ethylene glycol monobutyl ether and ethylene glycol monohexyl ether; and pyrrolidone-based solvents, such as 1-methyl-2-pyrrolidone and 1-ethyl-2-pyrrolidone. These water-miscible organic solvents may be mixed with water singly, or in combination of two or more thereof.

(3-2) Chromium-Containing Ion (A)

[0044]In the surface treatment agent, the supply source of the chromium-containing ion (A) is not particularly limited as long as it is capable of providing the ion (A) when mixed in an aqueous medium. Examples of the supply source include chromium fluoride, chromium nitrate, chromium sulfate, and chromium phosphate. These supply sources may be used singly, or in combination of two or more thereof. The content of the ion (A) in the surface treatment agent is not particularly limited; however, it is usually in a range of 5 to 1,000 mg/L, preferably in a range of 50 to 500 mg/L, more preferably in a range of 100 to 200 mg/L, in terms of chromium-equivalent mass concentration. The ion (A) may be an ion containing a trivalent chromium.

(3-3) Zirconium-Containing Ion (B)

[0045]In the surface treatment agent, the supply source of the zirconium-containing ion (B) is not particularly limited as long as it is capable of providing the ion (B) when mixed in an aqueous medium. Examples of the supply source include zirconium sulfate, zirconium oxysulfate, zirconium ammonium sulfate, zirconium nitrate, zirconium oxynitrate, zirconium ammonium nitrate, hexafluorozirconic acid, hexafluorozirconium complex salts, zirconium acetate, zirconium lactate, zirconium tetraacetylacetonate, zirconium tributoxyacetylacetonate, tetra-n-butoxy zirconium, and tetra-n-propoxy zirconium. These supply sources may be used singly, or in combination of two or more thereof. The content of the ion (B) in the surface treatment agent is not particularly limited; however, it is usually in a range of 5 to 1,000 mg/L, preferably in a range of 30 to 300 mg/L, more preferably in a range of 70 to 200 mg/L, in terms of zirconium-equivalent mass concentration.

(3-4) Zinc-Containing Ion (C)

[0046]In the surface treatment agent, the supply source of the zinc-containing ion (C) is not particularly limited as long as it is capable of providing the ion (C) when mixed in an aqueous medium. Examples of the supply source include metallic zinc, zinc oxide, zinc carbonate, zinc nitrate, zinc chloride, zinc sulfate, zinc fluoride, zinc iodide, zinc dihydrogen phosphate, and zinc acetylacetonate. These supply sources may be used singly, or in combination of two or more thereof. The content of the ion (C) in the surface treatment agent is not particularly limited; however, it is usually in a range of 500 to 10,000 mg/L, preferably in a range of 1,000 to 5,000 mg/L, more preferably in a range of 1,500 to 2,000 mg/L, in terms of zinc-equivalent mass concentration.

(3-5) Organic Compound (D) Having Hydroxy Group and Carboxyl Group

[0047]The organic compound (D) having a hydroxy group and a carboxyl group is not particularly limited as long as it can be mixed in an aqueous medium. Examples of the organic compound (D) include organic acids, such as gluconic acid, heptogluconic acid, galactonic acid, mannonic acid, glucaric acid, galactaric acid, mannaric acid, arabinoic acid, fructuronic acid, glucuronic acid, iduronic acid, galacturonic acid, mannuronic acid, and guluronic acid, and salts of these organic acids (e.g., sodium salts and potassium salts). These organic compounds may be used singly, or in combination of two or more thereof. The amount of the organic compound having a hydroxy group and a carboxyl group in the surface treatment agent is not particularly limited; however, it is usually in a range of 2 to 2,000 mg/L, preferably in a range of 10 to 1,000 mg/L, more preferably in a range of 100 to 300 mg/L.

(3-6) Free Fluorine Ion (E)

[0048]The surface treatment agent may also contain a free fluorine ion (E). In the surface treatment agent, a supply source of the free fluorine ion (E) is not particularly limited as long as it is capable of providing the free fluorine ion (E) when mixed in an aqueous medium. Examples of the supply source include hydrofluoric acid, ammonium fluoride, chromium fluoride, hexafluorotitanic acid, hexafluorotitanium complex salts, hexafluorozirconic acid, hexafluorozirconium complex salts, magnesium fluoride, aluminum fluoride, hexafluorosilicic acid, sodium fluoride, potassium fluoride, zinc fluoride, fluoroboric acid, sodium borofluoride, and ammonium borofluoride. These supply sources may be used singly, or in combination of two or more thereof. The free fluorine ion (E) may be provided by the same compound as the above-described supply source of the ion (A), (B) and/or (C), or may be provided by a different compound. In the surface treatment agent, the fluorine-equivalent mass concentration of the free fluorine ion (E) is not particularly limited; however, it is preferably 70 to 200 mg/L, more preferably 80 to 150 mg/L.

[0049]In the present specification, the free fluorine ion concentration means a value measured at a temperature at which the surface treatment agent is brought into contact with or over a surface of an aluminum material or aluminum alloy material. The free fluorine ion concentration is a value measured in accordance with a well-known method, and can be measured using, for example, a commercially available ion counter.

[0050]In the surface treatment agent of the present embodiment, a variety of metal components and additives may be incorporated within a range that does not impair the effects of the present invention. Examples of the metal components include vanadium, molybdenum, tungsten, manganese, cerium, magnesium, calcium, cobalt, nickel, strontium, lithium, niobium, yttrium, and bismuth. Examples of the additives include formyl group-containing compounds, benzoyl group-containing compounds, amino group-containing compounds, imino group-containing compounds, cyano group-containing compounds, azo group-containing compounds, thiol group-containing compounds, sulfo group-containing compounds, nitro group-containing compounds, amidino group-containing compounds, urethane bond-containing compounds, and aromatic ring-containing compounds. These metal components and additives may be used singly, or in combination of two or more thereof. These metal components and additives are incorporated within a range that does not impair the effects of the present invention; therefore, the content thereof is at most several % by mass with respect to a total amount of the surface treatment agent.

(3-7) pH of Surface Treatment Agent

[0051]The pH of the surface treatment agent of the present embodiment is not particularly limited; however, it is preferably 3.0 to 6.0, more preferably 4.0 to 5.0. In the present specification, the pH means a value measured at a temperature at which the surface treatment agent is brought into contact with or over a surface of an aluminum material or aluminum alloy material. The pH is a value measured in accordance with a well-known method, and can be measured using, for example, a commercially available pH meter.

[0052]Thus far, the composition of the surface treatment agent of the present embodiment has been described, and another aspect of the present invention is a surface treatment agent used for a surface treatment of an aluminum or aluminum alloy material, which surface treatment agent is prepared by blending a supply source of the chromium-containing ion (A), a supply source of the zirconium-containing ion (B), a supply source of the zinc-containing ion (C), and the organic compound (D) having a hydroxy group and a carboxyl group in an aqueous medium. As each of the supply sources, one or more compounds may be used.

(4-1) Method of Producing Surface Treatment Agent

[0053]The surface treatment agent of the present embodiment can be obtained by blending with stirring appropriate amounts of a supply source of the chromium-containing ion (A), a supply source of the zirconium-containing ion (B), a supply source of the zinc-containing ion (C), and the organic compound (D) having a hydroxy group and a carboxyl group in an aqueous medium. In the production, solid supply sources may be added to the aqueous medium, or the solid supply sources may be dissolved in the aqueous medium in advance and then added as an aqueous medium solution. The pH range of the surface treatment agent is as described above, and the pH is preferably adjusted with a pH modifier, such as nitric acid, sulfuric acid, hydrofluoric acid, ammonium bicarbonate, aqueous ammonia, sodium bicarbonate, or sodium hydroxide; however, the pH modifier is not limited to these components. Such a pH modifier may be used singly, or in combination of two or more thereof.

(4-2) Method of Producing Aluminum or Aluminum Alloy Material Having Surface Treatment Coating

[0054]A method of producing an aluminum or aluminum alloy material having a surface treatment coating formed by the surface treatment agent of the present embodiment includes the step of bringing the surface treatment agent of the present embodiment into contact with or over a surface of the aluminum or aluminum alloy material. By this step, a surface treatment coating is formed on or over the surface of the aluminum or aluminum alloy material. Pretreatment steps, such as the washing step and the pickling step, may also be performed prior to the contact step. The water-washing step may be performed after each step, and the drying step may be performed after each water-washing step.

(4-3) Aluminum or Aluminum Alloy Material

[0055]The aluminum or aluminum alloy material to be treated with the surface treatment agent is not particularly limited, and the surface treatment agent is particularly effective on aluminum die-cast materials which have a thick surface oxide film and on which an alloy component is segregated. The use of the aluminum or aluminum alloy material is not particularly limited, and examples thereof include engine peripheral devices and ECU housings.

(4-4) Washing Step

[0056]In the production method of the present embodiment, the washing step of bringing a known cleaning agent into contact with or over the surface of the aluminum or aluminum alloy material is preferably performed prior to the contact step. A washing method is not particularly limited, and examples thereof include solvent degreasing and alkali degreasing.

(4-5) Pickling Step

[0057]In the production method of the present embodiment, the pickling step of bringing a known pickling agent into contact with or over the surface of the aluminum or aluminum alloy material may be performed prior to the contact step. The pickling agent is not particularly limited, and examples thereof include nitric acid and hydrofluoric acid.

(4-6) Contact Step

[0058]In the contact step of the production method of the present embodiment, the contact temperature and the contact time are not particularly limited; however, usually, the surface treatment agent is brought into contact with or over the surface of the aluminum or aluminum alloy material at 30 to 80° C., preferably at 40 to 60° C., for 10 to 1,200 seconds. After this step, if necessary, the aluminum or aluminum alloy material may be washed with water and then with deionized water, and subsequently dried. The drying temperature is not particularly limited; however, it is preferably 15 to 100° C. A method of bringing the surface treatment agent into contact with or over the surface of the aluminum or aluminum alloy material is not particularly limited, and examples thereof include an immersion method, a spray method, and a flow-coating method.

(Post-Treatment Step)

[0059]The aluminum or aluminum alloy material having a surface treatment coating, which is produced by the above-described production method, may be further post-treated with hot water, a rust inhibitor, a post-treatment agent, a pH modifier, a coupling agent, and the like. After this post-treatment, the aluminum or aluminum alloy material may be washed with water and then dried, or may be dried without being washed with water.

[0060]The aluminum or aluminum alloy material having a surface treatment coating has excellent corrosion resistance even without the coating step of coating the surface treatment coating, and maintains excellent corrosion resistance (thermal corrosion resistance) even when the surface treatment coating is exposed to a high temperature; however, the coating step may be performed as well.

[0061]The coating step is not particularly limited and can be performed by, for example, a coating method such as aqueous coating, solvent coating, powder coating, anionic electrodeposition coating, or cationic electrodeposition coating, using a known paint composition.

EXAMPLES

Examples and Comparative Examples of the Present Invention Will Now be Described

[0062]It is noted here, however, that the present invention is not limited to the below-described Examples by any means.

<Aluminum Material>

Aluminum Die-Cast Material (JIS ADC12)

<Surface Treatment Agents>

[0063]Surface treatment agents of Examples and Comparative Examples were obtained by adding and mixing the raw materials shown in Table 1 in water such that the respective concentrations shown in Table 1 were attained. As a pH modifier, aqueous ammonia or nitric acid was used. The pH and the free fluorine ion concentration (denoted as “FF” in Table 1) were measured using a portable ion-pH meter [IM-32P (manufactured by DKK-TOA Corporation), pH electrode: GST-2729C (manufactured by DKK-TOA Corporation), ion electrode: fluoride ion composite electrode F-2021 (manufactured by DKK-TOA Corporation)].

<Treatment Method>

[0064]Specifically, the above-described aluminum die-cast material was immersed in an alkaline degreasing agent [20 g/L aqueous solution of FINE CLEANER 315E (manufactured by Nihon Parkerizing Co., Ltd.)] at 50° C. for 2 minutes, and the surface thereof was cleaned by rinsing with tap water. Subsequently, the contact step was performed on or over the surface of the aluminum die-cast material by immersing the aluminum die-cast material in each of the above-obtained surface treatment agents at the respective contact temperature shown in Table 1. Thereafter, the aluminum die-cast material was washed with running tap water (at normal temperature for 30 seconds) and then with running deionized water (at normal temperature for 30 seconds), after which the aluminum die-cast material was dried using an air blower (at normal temperature for 30 seconds), whereby aluminum die-cast materials having a surface treatment coating (test pieces 1 to 39) were each obtained.

<Measurement of Surface Treatment Coating>

[0065]For each of the aluminum die-cast materials having a surface treatment coating that were obtained by the above-described surface treatment method, the content of each component in the surface treatment coating and the infrared peaks of the surface treatment coatings were measured by the below-described respective methods. The measurement results are shown in Table 2.

<Contents of Metals>

[0066]The contents of metals (chromium and zirconium) were measured using an X-ray fluorescence spectrometer [ZSX Primus IV (manufactured by Rigaku Corporation)].

<Content Ratio of Zinc with Respect to Total Content of Chromium, Zirconium, and Zinc>

[0067]The content of each component in the surface treatment coating for determination of the content ratio of zinc with respect to a total content of chromium, zirconium, and zinc (Zn/(Cr+Zr+Zn)) was measured by repeatedly performing the measurement of element amount based on sputtering and XPS multiple times using an X-ray photoelectron spectrometer [PHI5000 VersaProbe III (manufactured by ULVAC-PHI, Inc.)]. It is noted here that the depth position in XPS was controlled with the distance at which SiO₂ was sputtered by Ar ions. Specifically, sputtering was performed for 0.25 minutes (sputtering rate: 11.0 nm/min in terms of SiO₂), and the sample surface was irradiated with a monochromatic Al-Kα ray in an analysis diameter of 100 μm, after which the resulting photoelectrons were measured. This process was repeated to measure the contents of chromium, zirconium, and zinc obtained at each depth in the thickness direction of the surface treatment coating. Taking a total content of these elements as 100% by atom, the content ratio of zinc (Zn/(Cr+Zr+Zn)) was calculated.

<Content of Carbon>

[0068]The content of carbon (total organic carbon adhered amount) was measured using a total organic carbon analyzer [TOC-L (manufactured by Shimadzu Corporation)].

<Infrared Peaks>

[0069]An infrared spectrum of the surface treatment coating was obtained by measuring the surface treatment coating by a specular reflection method of Fourier transform infrared spectroscopy (FT-IR). Specifically, the infrared spectrum was obtained using a Fourier transform infrared spectrophotometer [ALPHA (manufactured by Bruker AXS GmbH)] under the following measurement conditions: measurement wave number=4,000 cm⁻¹ to 400 cm⁻¹, resolution=4 cm⁻¹, and 128 scans. As a background, the aluminum die-cast material that did not have a surface treatment coating was used. It was verified whether or not the thus obtained infrared spectrum had a peak in a wavelength range of 3,600 cm⁻¹ to 3,000 cm⁻¹ and a peak in a wavelength range of 1,750 cm⁻¹ to 1,700 cm⁻¹.

<Evaluation of Test Pieces>

[0070]Further, for the above-obtained test pieces, tests were conducted as described below to evaluate the corrosion resistance and the post-heating corrosion resistance (thermal corrosion resistance) of each aluminum die-cast material having a surface treatment coating. The results thereof are shown in Table 3.

<<Evaluation Methods>>

<Corrosion Resistance>

[0071]The test pieces 1 to 39 were each subjected to a 360-hour neutral salt spray test (JIS Z2371:2015). After each test piece was dried, the ratio of white rust generated on the surface of each test piece was visually measured. The ratio of white rust is a ratio of the area of generated white rust with respect to the area of the observed part. The evaluation criteria were as follows. The evaluation results are shown in Table 3.

<Evaluation Criteria>

<Thermal Corrosion Resistance>

[0077]The test pieces were each heated in an electric oven (at 150° C. for 6 hours) and then subjected to a 240-hour neutral salt spray test (JIS Z2371:2015). After each test piece was dried, the ratio of white rust generated on the surface of each test piece was visually measured. The ratio of white rust is a ratio of the area of generated white rust with respect to the area of the observed part. The evaluation criteria were as follows. The evaluation results are shown in Table 3.

<Evaluation Criteria>

[0083]The present invention has been described above in detail referring to concrete Examples thereof; however, it is obvious to those skilled in the art that various modifications and changes can be made without departing from the spirit and the scope of the present invention.