JPH0941155A - Chromating bath excellent in bath stability and chromated metallic sheet produced thereby - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve stability of a chromating bath by using specified amounts of a hexavalent chromium reducing substance, Cr and an inorganic acid and also a vinyl based organic copolymer aqueous emulsion as components of the bath and adjusting the pH of the bath to a specified value. SOLUTION: This chromating bath contains a 0.001 to 0.3mol/l hexavalent chromium reducing substance, 0.02 to 0.35mol/l total Cr and a 0.15 to 0.9mol/l inorganic acid wherein the ratio of Cr(III)/Cr(VI) is 25/75 to 75/25, the pH of the bath is <=2 as a whole and the hexavalent chromium reducing substance is at least one substance selected from hydroxyl group-containing vinyl based monomers each of which is copolymerized with another monomer to form a vinyl based organic copolymer aqueous emulsion, hydroxyl group-containing additives and ester based additives each of which is hydrolyzed at a pH of <=2 to form an alcohol. When the pH of the chromating bath is adjusted to <=2, carboxyl groups in the bath are hardly dissociated and, therefore, the bath is not gelatinized. Thus, any gelation or coagulation of the chromating bath is inhibited from occurring over a long period to enable stable production of a surface-chromated metallic sheet.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate treatment bath excellent in treatment bath stability, and a chromate treated metal plate produced by using the bath.

[0002]

2. Description of the Related Art Chromate treatment has hitherto been known as rust-preventing treatment for steel sheets, zinc-based plated steel sheets, aluminum-based plated steel sheets, aluminum sheets or aluminum alloy sheets. There are an electrolytic chromate treatment containing trivalent chromium as a main component, a reactive chromate treatment, and a coating chromate treatment containing hexavalent chromium and dried without washing after coating. In recent years, chromate-treated metal plates have been widely used for home appliances, building materials, automobiles and the like, and various performances have been demanded by consumers. Examples include uniformity of appearance, anti-fingerprint property, adhesion to paint, corrosion resistance of flat plate material when used bare, corrosion resistance at processed part and scratched part, and chromium elution resistance at alkali degreasing. Regarding the chromium elution resistance of coating type chromate containing a large amount of hexavalent chromium, from the standpoint of environmental protection aimed at reducing environmental load, we believe that it is an important issue for the material supplier to further improve. To be

As a technique for improving the chromium elution resistance of the chromate film, for example, as disclosed in Japanese Patent Laid-Open No. 3-215683, heating the chromate film is performed at a plate temperature of 300.
It is known to carry out the method at a relatively high temperature of ℃. However, according to this method, hexavalent chromium is reduced to trivalent chromium during drying, and hexavalent chromium hardly remains in the film. Therefore, the corrosion resistance in the processed portion and the scratched portion is significantly inferior to that of the chromate containing soluble hexavalent chromium. In JP-A-5-287548 and JP-A-4-358082, hexavalent chromium is reduced and fixed by adding a resin such as polyacrylic acid or a reducing agent such as alcohol, and trivalent chromium is added to the resin. The method of cross-linking is adopted. This method improves the corrosion resistance due to the barrier effect of the chromate film. However, since the self-repairing action of hexavalent chromium hardly functions at the damaged portion of the chromate film such as the processed portion and the scratched portion, the corrosion resistance problem remains in the processed portion and the scratched portion.

In Japanese Patent Laid-Open No. 61-23767, a copolymer emulsion of an acrylic acid ester-based monomer and an acrylic acid-based monomer and a coexisting film with a hexavalent chromate are achieved. As a result, the hexavalent chromium has a self-repairing function in the damaged portion such as the processed portion and the scratched portion to solve the problem of corrosion resistance in the processed portion and the scratched portion. However, since the carboxyl group of polyacrylic acid is hydrophilic,
Since the water absorption rate of the coating film becomes high, the problem of optimizing the addition amount remains in consideration of the secondary adhesion of the coating film and the alkaline degreasing property.

In order to overcome this, Japanese Patent Laid-Open No. 5-230
In Japanese Patent No. 666, a hydroxyl group-containing vinyl monomer and a carboxyl group-containing vinyl monomer are copolymerized in a vinyl-based organic copolymer aqueous emulsion to improve secondary adhesion and alkaline degreasing performance. However, in this method, since the emulsion contains a large amount of hydroxyl groups, the hydroxyl groups in the emulsion serve as reducing agents for hexavalent chromium, and finally gelation occurs.

Further, in JP-A-61-23767 and JP-A-5-230666, a monomer copolymerized in a vinyl-based organic copolymer aqueous emulsion has a low p-value.
Also included are monomers such as methyl acrylate that hydrolyze in H to form alcohols. The alcohol produced by hydrolysis reduces hexavalent chromium to produce trivalent chromium, and therefore the treatment bath may gel in the long term. Considering the long-term stability of the treatment bath, it is necessary to pay attention to the application of acrylic acid ester which is easily hydrolyzed at low pH.

[0007]

DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a chromate treatment bath having excellent treatment bath stability and a chromate metal plate produced by using the chromate treatment bath by solving the above problems. Has an aim.

[0008]

The present invention provides a secondary adhesion,
Various vinyl-based organic copolymer aqueous emulsion resins, additives, etc. are added and adjusted to various pHs in order to provide a stable processing bath for producing chromate-treated metal plates having functions such as self-repairing function. The bath stability of the chromate treatment bath was examined. Furthermore, the composition of the gel produced due to poor bath stability was also analyzed. As a result, a hydroxyl group-containing monomer copolymerized in the vinyl-based organic copolymer aqueous emulsion, which can reduce hexavalent chromium in the chromate bath, a hydroxyl group-containing additive, and p
Limit the total concentration when at least one or more of the ester-based additives that hydrolyze to produce alcohol at H2 or less is contained. Adjust the hexavalent and trivalent chromium concentrations and the pH of the treatment bath. Vinyl-based organic copolymerization To optimize the composition of (meth) acrylic acid ester copolymerized in an aqueous emulsion. By the above, it has been found and reached that the objective chromate bath stability can be achieved.
It was also found that when the ratio of hydroxyl groups / carboxyl groups on the surface of the coating film formed by applying the treatment bath to a metal plate is within the optimum range, the coating film primary and secondary adhesion performance is excellent.

The gist of the present invention is that (a) the hexavalent chromium reducing substance is 0.001 mol / l or more and 0.30 mol / l or more.
Hereafter, (b) the total Cr concentration is 0.02 mol / l or more,
35 mol / l or less, (c) Cr (III) / Cr (VI) =
25/75 or more and 75/25 or less, (d) the inorganic acid concentration is 0.15 mol / l or more and less than 0.9 mol / l, (e)
A chromate treatment bath having a bath composition consisting of a vinyl-based organic copolymer aqueous emulsion and having an overall bath pH of 2 or less and excellent in stability. Then, the chromate-treated metal plate is excellent in coating film primary and secondary adhesion performance in which the ratio of hydroxyl groups / carboxyl groups on the surface of the chromate film formed by coating and drying the treatment bath is 2.5 or less.

Hereinafter, the present invention will be described in detail. Gelation occurs when the hydroxyl group-containing vinyl organic copolymer emulsion resin is added to the hexavalent and trivalent chromium mixed aqueous solution. The mechanism is considered as follows. The hexavalent chromium oxidizes the hydroxyl group in the resin into an aldehyde group and a carboxyl group. The trivalent chromium cations generated at this time form a salt with the carboxyl anion to form a crosslinked structure, and a gel is generated. However 6 values and 3
If inorganic acid such as phosphoric acid or sulfuric acid is added to the mixed solution of valent chromium in advance and the treatment bath is adjusted to pH 2 or less, the gelation rate decreases or the gelation rate decreases although the oxidation rate of hydroxyl groups increases. It does not change. It is considered that the carboxyl group is hardly dissociated at such a low pH.

Further, the composition of trivalent chromium in the gel was analyzed by X-ray photoelectron spectroscopy (XPS). As a result, more chromium hydroxide was formed in the bath after gelation than before gelation. I found out that 3 in the chromate bath
It is considered that divalent chromium, hexavalent chromium, phosphoric acid and the like form a dinuclear complex, and OH ⁻ is coordinated therein. Therefore, it was considered that the crosslinked structure forming the gel was formed by the resin and the chromium binuclear complex being crosslinked by hydrogen bonds or the like. In other words, the hydroxyl group-containing emulsion resin is not the direct cause of gelation, but hexavalent chromium
It was concluded that the direct cause was the reduction to valent chromium, the increase in pH, and the increase in chromium hydroxide. Therefore, in order to suppress gelation of the chromate treatment bath, the pH is adjusted so that chromium hydroxide does not increase, and the concentrations of hexavalent and trivalent chromium and the concentrations of all organic substances capable of reducing hexavalent chromium are set to the optimum ranges. Need to be restricted.

Further, when an acrylic acid ester monomer in which an alkyl group derived from an alcohol component is a methyl group, an ethyl group, or a propyl group is copolymerized with a vinyl-based organic copolymer aqueous emulsion, when the pH of the chromate treatment bath is 2 or less, Much of these are hydrolyzed, and methyl alcohol, ethyl alcohol, and propyl alcohol, which can reduce hexavalent chromium, are easily produced. That is, hydrolysis is more likely to occur as the molecular weight of the alcohol component of the ester group is smaller. Therefore, it is necessary to limit the concentration of these acrylic acid ester monomers or to replace them with other non-hydrolyzable monomers without use.

The concentration of hexavalent chromium reducing organic matter is 0.00
It is an essential condition that it is 1 mol / l or more and 0.30 mol / l or less. If this concentration exceeds 0.30 mol / l, the amount of hexavalent chromium reducing substance is too much, and therefore a large amount of 3
Valence chromium is generated, the stability of the treatment bath is significantly lowered, and gelation occurs. Further, at a low concentration of less than 0.001 mol / l, the adhesion performance in the hydroxyl group-containing vinyl-based monomer copolymerized with the vinyl-based organic copolymer aqueous emulsion,
The hydroxyl group-containing additive and the ester-based additive that hydrolyzes to produce alcohol at pH 2 or less cannot sufficiently exhibit the original functions expected after the film formation such as anti-oxidation and toughness improvement.

The total chromium concentration is preferably 0.02 mol / l or more and 0.35 mol / l or less. More preferably, it is 0.03 mol / l or more and 0.30 mol / l or less. When the total chromium concentration is less than 0.02 mol / l,
Since there is little hexavalent chromium, the corrosion resistance of the scratched part due to the self-repairing function cannot be expected. On the other hand, 0.35 mol / l
If it exceeds, chromium cannot be completely captured by the resin, and the chromium elution resistance deteriorates.

The ratio of hexavalent / trivalent chromium is 25/75 or more 7
It is preferably 5/25 or less. It is more preferably 30/70 or more and 70/30 or less. Hexavalent / trivalent chromium ratio is 2
If it is less than 5/75, the amount of chromium hydroxide increases due to a slight increase in pH, and the treatment bath becomes unstable and gels. On the other hand, if it exceeds 75/25, most of the added vinyl organic copolymer emulsions will oxidize the hydroxyl groups to the carboxyl groups, making it impossible to secure the secondary adhesion after coating. Hexavalent and trivalent chromium can be prepared by previously partially reducing chromic anhydride with starch, alcohol or the like.

The concentration of the inorganic acid is preferably 0.15 mol / l or more and less than 0.9 mol / l. 0.15 mol / l
If it is less than this, the pH becomes high, chromium hydroxide is generated, and the treatment bath gels. If it is 0.9 mol / l or more, the chromium compound will deliquesce, and a dry film cannot be formed. Inorganic acids include sulfuric acid, polyphosphoric acid, nitric acid, phosphoric acid and the like. The pH of the entire treatment bath is preferably 2 or less. The pH is more preferably 1.5 or less. Above pH 2, chromium hydroxide increases and the treatment bath becomes unstable,
Gel.

Hexavalent chromium is reduced to trivalent chromium by oxidizing a hydroxyl group-containing organic substance. Therefore,
The hexavalent chromium reducing substance that limits the addition amount is a vinyl organic copolymer emulsion or oligomer in which a hydroxyl group-containing vinyl monomer is copolymerized, an unreacted residual monomer, or an ester type interface containing a hydroxyl group or hydrolyzing at a pH of 2 or less. It is classified into additives such as activators, antioxidants and plasticizers. The hydroxyl group-containing vinyl monomer includes 2-hydroxyethyl (meth) acrylate, hydroxylpropyl (meth) acrylate, 3-hydroxybutyl (meth) acrylate, 2,2-bis (hydroxymethyl) ethyl acrylate, ( (Meth) acrylic acid hydroxyesters such as 2,3-hydroxypropyl (meth) acrylic acid and 3-chloro-2-hydroxypropyl (meth) acrylic acid,
There are monomers containing a hydroxyl group such as allyl alcohol and N-methylol acrylamides.

Examples of the plasticizer include dimethyl phthalate, diethyl phthalate, glycerin monooleate, methyl acetylricinoleate, butyl acetylricinoleate, and diethylenephthalate. These are copolymerized for the purpose of improving the secondary adhesion of the chromate film. As the antioxidant, 2,6-di-t-butyl-p
-Cresol, butylated hydroxyanisole, 2,6
-Di-t-butyl-4-ethylphenol, acrylic-
β- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, 2,2-methylenebis (4-methyl-6-t-butylphenol), 2,2-methylenebis (4-ethyl-6-t) -Butylphenol) and the like.

Nonionic surfactants are targeted as the surfactant. For example, polyoxyethylene secondary alcohol ether, polyoxyethylene alkylphenyl ether, polyoxyethylene sterol ether, polyoxyethylene polyoxypropylene block polymer,
Examples include propylene glycol fatty acid ester and polyvinyl alcohol. In the (meth) acrylic acid ester monomer represented by CH ₂ ═C (R ₁ ) COOR ₂ (R ₁ is hydrogen or methyl group, R ₂ is alkyl group) copolymerized with the vinyl-based organic copolymer aqueous emulsion, alkyl When the number of carbon atoms in the group is 3 or less, methyl (meth) acrylate,
There are ethyl (meth) acrylate and propyl (meth) acrylate. If the alkyl group has 4 or more carbon atoms, butyl (meth) acrylate, hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, lauryl (meth) acrylate, (meth) Stearyl acrylate and the like. These acrylic acid esters are copolymerized in order to balance mechanical properties such as elongation and hardness of the film.

Vinyl-based organic copolymerization Total concentration of (meth) acrylic acid ester having 3 or less carbon atoms of alkyl group with respect to total concentration of (meth) acrylic acid ester monomer having 4 or more carbon atoms of alkyl group, which is copolymerized with the aqueous emulsion. The ratio is preferably 1 or less. Further, the acrylic acid ester having an alkyl group having 3 or less carbon atoms may not be copolymerized. If this ratio exceeds 1, many acrylates having 3 or less carbon atoms in the alkyl group will be hydrolyzed to generate alcohol, hexavalent chromium will be reduced to trivalent chromium, and the treatment bath will become unusable. Be stable.

Examples of the non-hydrolyzable vinyl-based monomer containing no ester group include styrene, α-methylstyrene, vinyltoluene and acrylonitrile. These monomers are copolymerized for the purpose of increasing the hardness of the film and improving the heat resistance.
The concentration of the non-hydrolyzable vinyl-based monomer copolymerized with the vinyl-based organic copolymer emulsion is preferably 0.5 mol / l or less. When it exceeds 0.5 mol / l, this type of monomer has a glass transition temperature higher than room temperature, so that the toughness when formed into a film is low and the corrosion resistance of the processed part is lowered.

The carboxyl group-containing vinyl monomer copolymerized with the vinyl organic copolymer emulsion is 0.01
It is preferably not less than mol / l and not more than 1.0 mol / l.
If it is less than 0.01 mol / l, the dispersion stability is lowered and the emulsion aggregates. When it exceeds 1.0 mol / l, the dispersion stability is improved, but the secondary adhesion of the coating film is deteriorated, and when alkali degreasing is performed, the film is dissolved. The carboxyl group-containing vinyl monomer includes acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, fumaric acid and the like. These monomers are copolymerized for the purpose of improving dispersion stability of the vinyl-based organic copolymer aqueous emulsion and improving adhesiveness during film formation.

Inorganic oxide sols such as alumina, zirconia, silica and titania may be added to the treatment bath in order to improve corrosion resistance and film hardness. Hydrophilic colloidal silica having excellent corrosion resistance is preferable. Further, hydrophobic silica may be added to the hydrophilic colloidal silica. The addition amount is preferably 30 g / l or less. If it exceeds 30 g, the compatibility between the resin and colloidal silica decreases,
The film becomes brittle.

Further, the present inventors have also found that the hydroxyl groups are more concentrated than the carboxyl groups on the surface of the coating film obtained by applying and drying the above treatment liquid. The hydroxyl group and the carboxyl group correspond to the hydroxyl group-containing vinyl monomer and the carboxyl group-containing vinyl monomer which are copolymerized with the vinyl organic copolymerizable emulsion, respectively. Since the hydroxyl group has low hydrophilicity to the carboxyl group, it is considered that the hydroxyl group-containing component is phase-separated on the surface. The hydroxyl group / carboxyl group ratio on the surface of the chromate film formed from the treatment bath in the present invention is preferably 2.5 or less.
When it exceeds 2.5, the primary adhesion is improved, but the secondary adhesion is decreased. The thickness of the chromate film is preferably 3 μm or less. If it exceeds 3 μm, the electric resistance between the films increases, so that spot welding of this chromate-treated metal plate is uneconomical because the electrodes of the welding machine are likely to wear. In addition, the finish of the welded surface also deteriorates.

As a chromate treatment method for a metal plate, coating with a roll coater, coating with a ringer roll,
Immersion and coating with an air knife squeeze can be used. In addition, as a metal plate to which the present invention is applicable, a galvanized steel plate, a zinc / nickel plated steel plate, a zinc / iron plated steel plate, a zinc / chromium plated steel plate, a zinc / titanium plated steel plate, a zinc / magnesium steel plate, a zinc / manganese plated plate Steel sheets, zinc-based electroplated and hot-dip galvanized steel sheets such as zinc-aluminum-plated steel sheets, aluminum-plated steel sheets, and a small amount of dissimilar metal elements or impurities in these plated layers, for example, cobalt, molybdenum, tungsten, nickel, titanium, chromium. ,aluminum,
Manganese, iron, magnesium, lead, antimony, tin,
Copper, cadmium, plated steel sheet containing arsenic, etc., further plated steel sheet in which inorganic substances such as silica and alumina are dispersed in these plated layers, or aluminum sheet and silicon, copper, magnesium, iron as alloying elements or impurities therein It is an aluminum alloy plate added with manganese, chromium, titanium, zinc or the like, or a cold rolled steel plate.

[0026]

【Example】

Example 1 Various chromate baths were evaluated for bath stability as follows. Stability of treatment bath (1) Vinyl organic copolymer aqueous emulsion resin: An acrylic soap-free emulsion resin was used. The amount added was calculated by converting the amount of the monomer charged during the synthesis of the organic copolymer aqueous emulsion into the concentration in the treatment bath. The copolymerized monomers are as follows. A: OH-containing acrylic acid-based monomer as hydroxyl group-containing vinyl monomer B; Acrylic acid-based monomer as carboxyl group-containing vinyl-based monomer C; Acrylic ester having an alkyl group having 3 or less carbon atoms as hydrolyzable acrylic acid ester monomer An acrylic acid ester having an alkyl group with 4 or more carbon atoms as the hardly hydrolyzable acrylic acid ester monomer E: an aromatic vinyl monomer was used as the non-hydrolyzable vinyl monomer.

The amount of residual monomer in the synthesized emulsion was measured by a gas chromatograph. (2) Kind of chromic acid: Chromic anhydride partially reduced with starch was used. (3) Additive: F; Butanol (BuOH) was used as a hexavalent chromium alcohol-based reducing additive. (4) Inorganic acid: using phosphoric acid (5) Inorganic sol: using hydrophilic colloidal silica

(6) Bath stability evaluation method: The prepared chromate-treated solution was placed in a glass bottle and allowed to stand in a constant temperature bath whose temperature was adjusted to 50 ° C., and visually observed every other day to reach a precipitate or gel state. Confirmed the number of days until. ◎: Stable for 30 days or more ◯: Gelation or aggregation for 20 days or more and less than 30 days Δ: Gelation or aggregation for 10 days or more and less than 20 days ×: Gelation or aggregation for 10 days or less

The results are shown in Table 1. In the examples of the present invention, hexavalent chromium reducing substance concentration, total Cr concentration, Cr
When the (III) / Cr (VI) ratio, the inorganic acid concentration, and the pH are within the appropriate ranges, the stability is high without gelation or aggregation even after 20 days of treatment. In order to produce a chromate-treated metal in a large amount, it is necessary that the treatment bath is stable for at least 20 days.

[0030]

[Table 1]

Example 2 Among the processing baths evaluated in Example 1, various plating metal plates were subjected to chromate treatment using a processing bath which was stable for 10 days or more, and the characteristics thereof were evaluated. (1) Kind of metal plate: The following metal plates were used for the chromate treatment. GI; hot-dip galvanized steel sheet (coating weight 90 g / m
² ) EG; electrogalvanized steel sheet (coating weight: 20 g / m
² ) Zn-Al; hot dip zinc-aluminum plated steel sheet (coating adhesion amount 90 g / m ² Al / Zn = 55/45 (2) Treatment bath: Of the treatment baths constructed in Example 1,
A treatment bath that was stable for 10 days or more was selected and applied 10 days after the construction bath. (3) Coating method: A roll coater was used for coating. The thickness of the chromate film after drying was 0.5 μm.

(4) Performance evaluation method Chromium elution amount: The treated metal plate was immersed in boiling water for 30 minutes, and the total Cr amount after immersion was measured from the total Cr amount before immersion. It was confirmed. In addition, Cr
The amount is a value converted to metallic Cr by fluorescent X-ray. Dissolution rate = (Cr amount before immersion−Cr amount after immersion) / (C before immersion)
r amount) × 100 [%] ◎: less than 5% ◯: 5% or more and less than 15% △: 15% or more and less than 30% x: 30% or more

Corrosion resistance of processed part: After the sample material was Erichsen processed to a height of 7 mm, a salt spray test according to JIS Z2371 was carried out for 72 hours to evaluate the degree of rust generation in the processed part. ⊚: No rust occurred ◯: White rust 5% or less △: White rust 5% or more and less than 15% ×: White rust 15% or more

Primary adhesion: A melamine alkyd paint is applied to the test material in a thickness of 20 μm, dried under predetermined conditions, and then 1 m with a cutter knife according to JIS K5400.
The cross section of m was put on the painted surface, peeled off with a cellophane tape, and the peeled area of the coating film was examined. ◎: No peeling ○: Peeling rate less than 3% △: Peeling rate 3% or more and less than 10% ×: Peeling rate over 10%

Secondary adhesion: A melamine alkyd-based paint of 20 μm is applied to the test material, dried under predetermined conditions, and immersed in warm water at 50 ° C. for 3 hours, and then JISK540.
According to 0, a 1 mm grid was put on the painted surface with a cutter knife, peeled off with a cellophane tape, and the peeled area of the coating film was examined. ◎: No peeling ○: Peeling rate less than 10% △: Peeling rate 10% or more and less than 30% ×: Peeling rate over 30%

(5) Analysis of film surface composition (hydroxyl group / carboxyl group ratio) A narrow scan derived from carbon by XPS was applied to the chromate film, and COO ⁻ and ≡CO− of the spectrum obtained were applied.
Waveform separation, the integrated intensity of the corresponding spectrum was converted to concentration, and the concentration ratio was defined as the hydroxyl group / carboxyl group ratio. The results are shown in Table 2.

[0037]

[Table 2]

Example 3 The following metal plate was used and treated and evaluated in the same manner as in Example 2. In the corrosion resistance evaluation of the processed part, the salt spray test was conducted for 144 hours. (1) Kind of metal plate: An aluminum plated plate specified in JIS 8642H-HDAl was used as a metal plate for chromate treatment. Table 3 shows the results. In the examples of the present invention, hexavalent chromium reducing substance concentration, total Cr
Concentration, Cr (III) / Cr (VI) ratio, inorganic acid concentration, and pH within the appropriate range have high stability of the treatment bath and excellent properties when used as a chromate-treated metal plate. There is. Further, the chromate metal plate produced in the treatment bath having a bath life of 10 days or more and less than 20 days has low corrosion resistance in the processed part. It was assumed that this was because the treatment bath was unstable, so the emulsion particles were not uniformly dispersed and a dense film could not be formed, which caused the film to be unable to follow the processing. It is thought that the damage was severe. It is considered that this insufficient film formation also affects the chromium elution property and the coating film adhesion. Furthermore, when the residual monomer content is 100 ppm or less, the defects such as pinholes derived from the residual monomer are reduced, and the corrosion resistance of the processed portion is also improved. When the ratio of hydroxyl group / carboxyl group on the surface of the chromate film is within the proper range, the primary and secondary adhesiveness is also good. further,
It is also understood that the other performances are good, and that all items are superior to the comparative example.

[0039]

[Table 3]

[0040]

EFFECTS OF THE INVENTION According to the present invention, a surface treatment capable of reducing the amount of chromium eluted during alkaline degreasing while maintaining the corrosion resistance in the processed portion, which is the original function of chromate treatment, without gelling or aggregating for a long period of time. A metal plate can be manufactured stably.

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[Procedure amendment]

[Submission date] August 18, 1995

[Procedure amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction contents]

[0002]

2. Description of the Related Art Chromate treatment has hitherto been known as rust-preventing treatment for steel sheets, zinc-based plated steel sheets, aluminum-based plated steel sheets, aluminum sheets or aluminum alloy sheets. There are an electrolytic chromate treatment containing trivalent chromium as a main component, a reactive chromate treatment, and a coating chromate treatment containing hexavalent chromium and dried without washing after coating. In recent years, chromate-treated metal plates have been widely used for home appliances, building materials, automobiles and the like, and various performances have been demanded by consumers. Examples include uniformity of appearance, anti-fingerprint property, adhesion to paint, corrosion resistance of flat plate material when used bare, corrosion resistance at processed part and scratched part, and chromium elution resistance at alkali degreasing. With respect to resistance to chromium dissolution of the coating type chromate containing much hexavalent chromium, from the viewpoint of environmental protection for the purpose of environmental abatement believed to be important to aim the improvement of one layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical display location C23C 22/30 C23C 22/30 (72) Inventor Yuji Fujioka 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Shin Nippon Steel Co., Ltd. Advanced Technology Research Laboratories (72) Inventor Yujiro Miyauchi 1 Kimitsu, Kimitsu City, Chiba Shin Nippon Steel Co., Ltd. Kimitsu Steel Co., Ltd.

Claims (7)

[Claims] 1. A hexavalent chromium reducing substance as an essential component is 0.001 mol / l or more and 0.30 mol / l or less,
Total Cr concentration is 0.02 mol / l or more 0.35 mol /
l or less, Cr (III) / Cr (VI) = 25/75 or more 75
/ 25 or less, and the inorganic acid concentration is 0.15 mol / l or more and 0.
Less than 9 mol / l, and the pH of the entire bath containing the vinyl-based organic copolymer aqueous emulsion, thereby obtained
Is a chromate treatment bath having excellent stability. 2. A hexavalent chromium-reducing substance is copolymerized with (a) a vinyl-based organic copolymer aqueous emulsion to form a hydroxyl group-containing vinyl monomer, (b) a hydroxyl group-containing additive,
The chromate treatment bath according to claim 1, which is at least one kind of (c) an ester-based additive which is hydrolyzed at a pH of 2 or less to produce alcohol. 3. At least one kind of (meth) acrylic acid ester monomer represented by CH ₂ ═C (R ₁ ) COOR ₂ (R ₁ is hydrogen or methyl group, R ₂ is alkyl group) is copolymerized. It is a vinyl-based organic copolymer aqueous emulsion, and the total concentration of (meth) acrylic acid ester monomers having 3 or less alkyl group carbon atoms relative to the total concentration of (meth) acrylic acid ester monomer alkyl groups having 4 or more carbon atoms. The chromate treatment bath according to claim 1, wherein the ratio is 1 or less. 4. A (meth) acrylic acid ester monomer represented by CH ₂ ═C (R ₁ ) COOR ₂ (R ₁ is hydrogen or a methyl group, and R ₂ is an alkyl group) is copolymerized with at least one kind. A vinyl-based organic copolymer aqueous emulsion, which is a vinyl-based organic copolymer aqueous emulsion in which a (meth) acrylic acid ester monomer having an alkyl group having 3 or less carbon atoms is not copolymerized. The chromate treatment bath described in 3. 5. The chromate treatment according to claim 1, wherein the non-hydrolyzable vinyl-based monomer not containing an ester group is a vinyl-based organic copolymer aqueous emulsion in which at least one kind is copolymerized. bath. 6. The chromate treatment according to claim 1, wherein at least one kind of monomers copolymerized with the vinyl-based organic copolymer aqueous emulsion is a carboxyl group-containing vinyl-based monomer. bath. 7. A chromate treatment, wherein the chromate-treated metal plate produced by the chromate treatment bath according to any one of claims 1 to 6 has a hydroxyl group / carboxyl group ratio of organic substances on the coating surface of 2.5 or less. Metal plate.