Patent Application
20240360563
The present invention relates to a hexavalent chromium-free water-based surface treatment solution, a surface-treated metal, and a surface treatment method.
A conventionally known surface treatment method includes surface-treating a metal with a hexavalent chromium ion-containing water-based surface treatment solution to improve the corrosion resistance of the metal.
However, in view of recent trends in environmental regulations, some restrictions may apply to the use of such a hexavalent chromium ion-containing water-based surface treatment solution.
Thus, various hexavalent chromium-free water-based surface treatment solutions (containing no hexavalent chromium ions) have been developed. For example, a known hexavalent chromium-free water-based surface treatment solution for use in the surface treatment of zinc-based plated steel is an acidic aqueous solution containing a trivalent chromium compound, a cobalt compound, an organic acid with a chelating ability, phosphoric acid, and sulfuric acid (see Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-194553
Unfortunately, such a hexavalent chromium-free water-based surface treatment solution contains a large amount of acid components and thus may cause an intense etching reaction on the surface of zinc-based plated steel, which will make it impossible to form a colorless coating due to deposition of a large amount of trivalent chromium ions. Moreover, when formed on the coating formed by the surface treatment with such a hexavalent chromium-free water-based surface treatment solution, a paint film will have a low adhesion to the coating.
It is an object of the present invention to provide a hexavalent chromium-free water-based surface treatment solution that can provide metals with improved corrosion resistance and improved adhesion to paint film and can form a colorless coating.
An aspect of the present invention is directed to a hexavalent chromium-free water-based surface treatment solution for use in surface treatment of a metal, the hexavalent chromium-free water-based surface treatment solution including: trivalent chromium ions; phosphate ions; sulfate ions; and colloidal silica and having a molar ratio of the phosphate ions to the trivalent chromium ions of 1.9 or more and 3.2 or less and a molar ratio of the sulfate ions to the trivalent chromium ions of 0.10 or more and 0.30 or less.
The hexavalent chromium-free water-based surface treatment solution may have a content of the trivalent chromium ions of 0.5 mass % or more and 10.0 mass % or less.
The hexavalent chromium-free water-based surface treatment solution may have a molar ratio of the colloidal silica in terms of SiO2 to the trivalent chromium ions of 0.30 or more and 2.00 or less.
The hexavalent chromium-free water-based surface treatment solution may further contain nickel ions and may have a content of the nickel ions of 0.04 mass % or more and 4.00 mass % or less.
The hexavalent chromium-free water-based surface treatment solution may further contain polyacrylic acid and may have a content of the polyacrylic acid of 0.5 mass % or more and 30.0 mass % or less.
The hexavalent chromium-free water-based surface treatment solution may further contain an oxazoline group-containing polymer and may have a content of the oxazoline group-containing polymer of 0.5 mass % or more and 5.0 mass % or less.
The hexavalent chromium-free water-based surface treatment solution may have a pH of 1.0 or more and 4.0 or less.
Another aspect of the present invention is directed to a surface-treated metal including: a metal; and a coating resulting from surface treatment of the metal with the hexavalent chromium-free water-based surface treatment solution.
The surface-treated metal may further include a paint film on the coating.
Another aspect of the present invention is directed to a surface treatment method including: surface-treating a metal with the hexavalent chromium-free water-based surface treatment solution to form a coating.
The present invention provides a hexavalent chromium-free water-based surface treatment solution that can provide metals with improved corrosion resistance and improved adhesion to paint film and can form a colorless coating.
Hereinafter, embodiments of the present invention will be described.
An embodiment of the present invention is directed to a hexavalent chromium-free water-based surface treatment solution that is for use in the surface treatment of a metal, in other words, for use in surface-treating a metal to produce a surface-treated metal having a coating.
The metal is typically, but not limited to, iron, zinc, aluminum, or any combination of two or more of those metals. Specifically, the metal may be an alloy or a plated metal material.
The hexavalent chromium-free water-based surface treatment solution of this embodiment contains trivalent chromium ions (Cr3+), phosphate ions (PO43−), sulfate ions (SO42−), and colloidal silica (SiO2).
In the surface treatment solution, the molar ratio (PO43−/Cr3+) of the phosphate ions to the trivalent chromium ions is 1.9 or more and 3.2 or less, preferably 2.2 or more and 3.0 or less. With a molar ratio of the phosphate ions to the trivalent chromium ions of less than 1.9 or more than 3.2, the surface treatment solution may provide the metal with lower corrosion resistance and lower adhesion to paint film.
The molar ratio (SO42−/Cr3+) of the sulfate ions to the trivalent chromium ions is 0.10 or more and 0.30 or less, preferably 0.15 or more and 0.25 or less. With a molar ratio of the sulfate ions to the trivalent chromium ions of less than 0.10, the hexavalent chromium-free water-based surface treatment solution may form a colored coating and have reduced storage stability. With a molar ratio of the sulfate ions to the trivalent chromium ions of more than 0.30, the hexavalent chromium-free water-based surface treatment solution may provide the metal with lower corrosion resistance and lower adhesion to paint film.
The hexavalent chromium-free water-based surface treatment solution of this embodiment preferably has a trivalent chromium ion content of 0.5 mass % or more and 10.0 mass % or less, preferably 1.5 mass % or more and 7.0 mass % or less. With a trivalent chromium ion content of 0.5 mass % or more, the hexavalent chromium-free water-based surface treatment solution of this embodiment will provide the metal with improved corrosion resistance. With a trivalent chromium ion content of 10.0 mass % or less, the hexavalent chromium-free water-based surface treatment solution of this embodiment will provide the metal with improved adhesion to paint film.
The molar ratio (SiO2/Cr3+) of the colloidal silica in terms of SiO2 to the trivalent chromium ions is preferably 0.30 or more and 2.00 or less, more preferably 0.35 or more and 1.50 or less. With a molar ratio of the colloidal silica in terms of SiO2 to the trivalent chromium ions of 0.30 or more and 2.00 or less, the surface treatment solution will provide the metal with improved corrosion resistance.
The water content of the hexavalent chromium-free water-based surface treatment solution of this embodiment is typically, but not limited to, 10 mass % or more and 99 mass % or less.
The hexavalent chromium-free water-based surface treatment solution of this embodiment may further contain nickel ions (Ni2+). The surface treatment solution with this feature will provide the metal with improved corrosion resistance.
The hexavalent chromium-free water-based surface treatment solution of this embodiment preferably has a nickel ion content of 0.04 mass % or more and 4.00 mass % or less, more preferably 0.08 mass % or more and 3.00 mass % or less. With a nickel ion content of 0.04 mass % or more and 4.00 mass % or less, the hexavalent chromium-free water-based surface treatment solution of this embodiment will provide the metal with improved corrosion resistance.
The hexavalent chromium-free water-based surface treatment solution of this embodiment may further contain polyacrylic acid. The surface treatment solution with this feature will provide the metal with improved adhesion to paint film.
The hexavalent chromium-free water-based surface treatment solution of this embodiment preferably has a polyacrylic acid content of 0.5 mass % or more and 30.0 mass % or less, more preferably 1.0 mass % or more and 15.0 mass % or less. With a polyacrylic acid content of 0.5 mass % or more and 30.0 mass % or less, the hexavalent chromium-free water-based surface treatment solution of this embodiment will provide the metal with improved adhesion to paint film.
The hexavalent chromium-free water-based surface treatment solution of this embodiment may further contain an oxazoline group-containing polymer when it contains polyacrylic acid. The surface treatment solution with this feature will provide the metal with improved corrosion resistance. In this case, the oxazoline group-containing polymer functions as a cross-linking agent for the polyacrylic acid.
The oxazoline group-containing polymer may be a commercially available product, such as EPOCROS series (manufactured by Nippon Shokubai Co., Ltd.).
The hexavalent chromium-free water-based surface treatment solution of this embodiment preferably has an oxazoline group-containing polymer content of 0.5 mass % or more and 5.0 mass % or less, more preferably 0.8 mass % or more and 3.0 mass % or less. With an oxazoline group-containing polymer content of 0.5 mass % or more and 5.0 mass % or less, the hexavalent chromium-free water-based surface treatment solution of this embodiment will provide the metal with improved corrosion resistance.
The hexavalent chromium-free water-based surface treatment solution of this embodiment preferably has a pH of 1.0 or more and 4.0 or less, more preferably 1.3 or more and 3.0 or less. With a pH of 1.0 or more and 4.0 or less, the hexavalent chromium-free water-based surface treatment solution of this embodiment will have improved storage stability.
The source of the trivalent chromium ions in the hexavalent chromium-free water-based surface treatment solution of this embodiment is typically, but not limited to, chromium phosphate, chromium nitrate, chromium acetate, chromium chloride, or any other trivalent chromium salt.
The source of the sulfate ions in the hexavalent chromium-free water-based surface treatment solution of this embodiment is typically, but not limited to, sulfuric acid or a metal sulfate, such as nickel sulfate.
The source of the phosphate ions in the hexavalent chromium-free water-based surface treatment solution of this embodiment is typically, but not limited to, phosphoric acid or a metal phosphate, such as chromium phosphate.
The source of the nickel ions in the hexavalent chromium-free water-based surface treatment solution of this embodiment is typically, but not limited to, nickel carbonate, nickel sulfate, or any other nickel salt.
The hexavalent chromium-free water-based surface treatment solution of this embodiment may further contain components other than those described above (additional components).
Examples of such additional components include cross-linking agents for polyacrylic acid, which are not oxazoline group-containing polymers, leveling agents, defoaming agents, and pH adjusting agents.
Another embodiment of the present invention is directed to a surface-treated metal including: a metal; and a coating resulting from the surface treatment of the metal with the hexavalent chromium-free water-based surface treatment solution according to an embodiment of the present invention.
The metal may be in any suitable shape, such as a sheet or plate.
The metal sheet may be, for example, a zinc-plated steel sheet or a zinc alloy-plated steel sheet.
Examples of the zinc alloy-plated steel sheet include zinc-nickel plated steel sheets, zinc-iron plated steel sheets, zinc-chromium plated steel sheets, zinc-aluminum plated steel sheets, zinc-titanium plated steel sheets, zinc-magnesium plated steel sheets, zinc-aluminum-magnesium plated steel sheets, and zinc-manganese plated steel sheets.
The plating may be, for example, electroplating, hot-dip plating, or vapor deposition plating.
The surface-treated metal of this embodiment may further have a paint film on the coating.
The paint film may be formed using any suitable paint, such as an epoxy paint, an acrylic paint, a polyester paint, a urethane paint, or a fluoride paint.
The paint film may be a monolayer film or a multilayer film.
The monolayer film may be formed using a one-coat paint.
The one-coat paint may be, for example, an acrylic melamine paint.
The multilayer film may be formed using a primer and a topcoat.
Examples of the primer include epoxy-polyester primers and phenolic primers.
Examples of the topcoat include polyester topcoats.
Another embodiment of the present invention is directed to a surface treatment method including: surface-treating a metal with the hexavalent chromium-free water-based surface treatment solution according to an embodiment of the present invention to form a coating (the step of forming a coating).
For example, the step of forming a coating may include applying the hexavalent chromium-free water-based surface treatment solution according to an embodiment of the present invention to a surface of a metal; and then drying the surface treatment solution.
The hexavalent chromium-free water-based surface treatment solution according to an embodiment of the present invention may be applied to the metal by any suitable method, such as roll coater coating, brush coating, roller coating, bar coater coating, or flow coating.
The surface treatment solution may be dried using a known method.
The surface treatment solution is preferably dried such that the surface of the metal reaches a temperature (peak metal temperature) of 60° C. or more and 90° C. or less.
The step of forming a coating may include drying the hexavalent chromium-free water-based surface treatment solution being applied to the surface of the metal. For example, the hexavalent chromium-free water-based surface treatment solution according to an embodiment of the present invention may be dried by being applied to the surface of the metal having been preheated.
In the step of forming a coating, the coating formed after the drying is preferably in an amount of 0.1 mg/m2 or more and 500 mg/m2 or less, more preferably 1 mg/m2 or more and 250 mg/m2 or less.
The surface treatment method of this embodiment may further include forming a paint film on the coating (the step of forming a paint film).
For example, the step of forming a paint film may include applying a paint to the coating and then drying and baking the paint film.
The paint may be a one-coat paint. Alternatively, a primer and a topcoat may be used.
In a case where a primer and a topcoat are used, the primer may be applied and then dried and baked, which may be followed by applying and then drying and baking the topcoat, or the primer and the topcoat may be sequentially applied and then simultaneously dried and baked.
Hereinafter, examples of the present invention will be described, which are not intended to limit the present invention.
Chromium phosphate (Cr(H1.5PO4)2), phosphoric acid (H3PO4), sulfuric acid (H2SO4), colloidal silica (SiO2), nickel carbonate (NiCO3), polyacrylic acid (PA), an oxazoline group-containing polymer (OX), a pH adjusting agent, and ion-exchanged water were mixed to satisfy the properties shown in Table 1 and stirred to form a hexavalent chromium-free water-based surface treatment solution.
The details of the raw materials used are given below.
A hexavalent chromium-free water-based surface treatment solution was prepared in the same manner as shown above, except that chromium nitrate (Cr(NO3)3) was used instead of chromium phosphate.
The details of the raw material used are given below. Chromium nitrate: Liquid chromium nitrate (manufactured by Nippon Chemical Industrial Co., Ltd.)
The details of the steel sheets used are given below.
The color of the coating formed on the steel surface was visually evaluated.
According to JIS Z 2371, a salt spray test (SST) was performed on the steel sheet with the coating for 240 hours, which was followed by visual evaluation of the ratio of the surface area covered with white rust (white rust ratio). The corrosion resistance (SST) of the steel sheet with the coating was evaluated according to the criteria below.
An epoxy polyester primer NSC5610NC PRIMER (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the coating on the steel sheet using a bar coater such that a primer film with a dry thickness of 5 μm could be formed. The primer was then dried and baked at a peak metal temperature of 215° C. Next, a polyester topcoat S/C 490HQ 1C4661 (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the primer film using a bar coater such that a topcoat film with a dry thickness of 15 μm could be formed. The topcoat was then dried and baked at a peak metal temperature of 230° C., so that a paint film (paint film 1) was formed.
A paint film (paint film 2) was formed in the same manner as in the formation of paint film 1, except that a phenolic primer N Kinocoat 38 (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the coating on the steel sheet using a bar coater such that a primer film with a dry thickness of 5 μm could be formed and the primer was then dried and baked at a peak metal temperature of 220° C.
A one-coat acrylic melamine paint SUPERLAC 100 (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was applied to the coating on the steel sheet using a bar coater such that a paint film with a dry thickness of 20 μm could be formed. The one-coat paint was then dried and baked at 150° C. for 20 minutes to form a paint film (paint film 3).
In the environment at 20° C., the steel sheet with the paint film was bent at 180° using two 0.35 mm-thick aluminum sheets as spacers for between folded parts. The bent portion of the steel sheet with the paint film was subjected to adhesive tape peeling three times. The adhesive tape peeling test was performed according to JIS Z 0237:2009. What percentage of the paint film was peeled off was determined by observation with a 20× loupe. The paint film adhesion (2TT) was evaluated according to the criteria below.
Paint film adhesion (0TT) was evaluated in the same manner as in the evaluation of paint film adhesion (2TT), except that no spacer was used while the steel sheet was bent at 180°.
According to JIS Z 2371, the paint film on the steel sheet was cross-cut, and then the steel sheet was subjected to a salt spray test (SST) for 1,000 hours. The widths of paint blisters from the cross-cut lines on one side were measured and averaged. The corrosion resistance (SST) of the steel sheet with the paint film was evaluated based on the average blister width according to the criteria below. Acceptable: The average blister width is at most 0.45 mm. Unacceptable: The average blister width is more than 0.45 mm.
According to JIS K 5621, the paint film on the steel sheet was cross-cut, and then the steel sheet was subjected to a combined cyclic corrosion test (CCT) for 2,000 hours. The widths of paint blisters from the cross-cut lines on one side were measured and averaged. The corrosion resistance (CCT) of the steel sheet with the paint film was evaluated based on the average blister width according to the criteria below. Acceptable: The average blister width is at most 0.3 mm. Unacceptable: The average blister width is more than 0.3 mm.
Table 2 shows the properties of each of the hexavalent chromium-free water-based surface treatment solutions and the results of evaluation of each of the steel sheets having undergone the surface treatments (primary and secondary anti-corrosion treatments). In this regard, the hexavalent chromium-free water-based surface treatment solutions of Comparative Examples 2 and 5 had unacceptable storage stability, and thus the evaluation of the corrosion resistance was omitted for them.
Table 2 indicates that the hexavalent chromium-free water-based surface treatment solutions of Examples 1 to 16 successfully formed colorless coatings and that the steel sheets surface-treated with the hexavalent chromium-free water-based surface treatment solutions of Examples 1 to 16 had higher corrosion resistance and higher adhesion to the paint film.
In contrast, the steel sheet surface-treated with the hexavalent chromium-free water-based surface treatment solution of Comparative Example 1, in which the PO43−/Cr3+ ratio was 4, had lower corrosion resistance and lower adhesion to the paint film. The steel sheet surface-treated with the hexavalent chromium-free water-based surface treatment solution of Comparative Example 3, which contained no PO43−, had lower corrosion resistance and lower adhesion to the paint film. The steel sheet surface-treated with the hexavalent chromium-free water-based surface treatment solution of Comparative Example 4, in which the SO42−/Cr3+ ratio was 0.4, had lower corrosion resistance and lower adhesion to the paint film. The hexavalent chromium-free water-based surface treatment solution of Comparative Example 5, in which the SO42−/Cr3+ ratio was 0.06, formed a colored coating. The steel sheet surface-treated with the hexavalent chromium-free water-based surface treatment solution of Comparative Example 6, which contained no SiO2, had lower corrosion resistance and lower adhesion to the paint film.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-130449 | Aug 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/023602 | 6/13/2022 | WO |
