Corrosion-resistant trivalent-chromium chemical conversion coating and solution for trivalent-chromium chemical treatment

Abstract

A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm.sup.2 in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm.sup.2.

Claims

1. A method for reducing an amount of hexavalent chromium eluted from a trivalent chromium chemical conversion coating film left for 30 days in a constant temperature and humidity chamber at a temperature of 80° C. and at a humidity of 95% (an amount eluted by immersion of the coating film into hot water at a temperature of 100° C. for 10 minutes) to less than 0.05 μg/cm.sup.2, the method comprising the steps of: preparing chemical conversion treatment liquid for forming a trivalent chromium chemical conversion coating film, and immersing a zinc or zinc alloy plated substrate into the chemical conversion treatment liquid, wherein a content of trivalent chromium ions in the chemical conversion treatment liquid is 0.002 to 0.5 mol/1, a concentration of hexavalent chromium ions in the chemical conversion treatment liquid is 1 ppm or less, a content of cobalt ions in the chemical conversion treatment liquid is 0.1 mol/1 or less, the chemical conversion treatment liquid further contains a hexavalent chromium generation suppressing agent for suppressing generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film, wherein the hexavalent chromium generation suppressing agent is selected from the group consisting of ascorbic acid and salts thereof, tannic acid and salts thereof, and gallic acid and salts thereof, a content of the hexavalent chromium generation suppressing agent in the chemical conversion treatment liquid is 0.1 to 5 g/l, a pH of the chemical conversion treatment liquid is 0.5 to 5, and a concentration of hexavalent chromium in terms of metal atoms in a chemical conversion coating film formed by using the same chemical conversion treatment liquid as the above chemical conversion treatment liquid excluding the hexavalent chromium generation suppressing agent is less than 0.01 μg/cm.sup.2.

2. The method according to claim 1, wherein a concentration of trivalent chromium in terms of metal atoms in the trivalent chromium chemical conversion coating film is 2 to 20 μg/cm.sup.2.

3. The method according to claim 1, wherein a concentration of cobalt in the trivalent chromium chemical conversion coating film is 0.2 to 3.5 μg/cm.sup.2.

4. The method according to claim 1, wherein a concentration of cobalt in the trivalent chromium chemical conversion coating film is less than 0.2 μg/cm.sup.2.

5. The method according to claim 1, wherein the hexavalent chromium generation suppressing agent is a reducing compound.

6. The method according to claim 1, wherein the chemical conversion treatment liquid further comprises a chelating agent in the range of 0.2 to 2 mole per mole of Cr.sup.3+.

7. The method according to claim 1, wherein a content of nitrogen in the chemical conversion treatment liquid is 500 ppm or less in terms of nitrogen atoms.

8. The method according to claim 1, wherein the chemical conversion treatment liquid further comprises a quinoline-based compound or a derivative thereof.

9. A chemical conversion treatment liquid for forming a trivalent chromium chemical conversion coating film on zinc or zinc alloy plating, wherein a content of trivalent chromium ions in the chemical conversion treatment liquid is 0.002 to 0.5 mol/1, a concentration of hexavalent chromium ions in the chemical conversion treatment liquid is 1 ppm or less, a content of cobalt ions in the chemical conversion treatment liquid is 0.1 mol/1 or less, the chemical conversion treatment liquid further contains a hexavalent chromium generation suppressing agent for suppressing generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film, wherein the hexavalent chromium generation suppressing agent is selected from the group consisting of ascorbic acid and salts thereof, tannic acid and salts thereof, and gallic acid and salts thereof, a content of the hexavalent chromium generation suppressing agent in the chemical conversion treatment liquid is 0.1 to 5 g/l, a pH of the chemical conversion treatment liquid is 0.5 to 5, and a concentration of hexavalent chromium in terms of metal atoms in a chemical conversion coating film formed by using the same chemical conversion treatment liquid as the above chemical conversion treatment liquid excluding the hexavalent chromium generation suppressing agent is less than 0.01 μg/cm.sup.2.

10. The chemical conversion treatment liquid according to claim 9, wherein the hexavalent chromium generation suppressing agent is a reducing compound.

11. The chemical conversion treatment liquid according to claim 9, further comprising a chelating agent in the range of 0.2 to 2 mole per mole of Cr.sup.3+.

12. The chemical conversion treatment liquid according to claim 9, wherein a content of nitrogen in the chemical conversion treatment liquid is 500 ppm or less in terms of nitrogen atoms.

13. The chemical conversion treatment liquid according to claim 9, further comprising a quinoline-based compound or a derivative thereof.

Description

BEST MODE FOR CARRYING OUT THE INVENTION

(1) The substrate used in the present invention may be made of any of the following materials: various metals such as iron, nickel and copper; alloys thereof; and metals and alloys such as aluminum, which have been subjected to zincate conversion treatment, and may have any of various shapes such as plate-like, rectangular, column-like, cylindrical and spherical shapes.

(2) The above substrate is plated with zinc or a zinc alloy by the usual method. The zinc plating may be deposited on the substrate using either of the following baths: an acidic/neutral bath such as a sulfuric acid bath, a borofluoride bath, a potassium chloride bath, a sodium chloride bath or an ammonium chloride-potassium chloride bath; or an alkaline bath such as a cyanide bath, a zincate bath or a pyrophoric acid bath, but particularly, a cyanide bath is preferable. The zinc alloy plating may be performed using either an ammonium chloride bath or an alkaline bath such as an organic chelate bath.

(3) In addition, the zinc alloy plating may be zinc-iron alloy plating, zinc-nickel alloy plating, zinc-cobalt alloy plating or tin-zinc alloy plating. Zinc-iron alloy plating is preferable. The zinc or zinc alloy plating may be deposited on a substrate in any thickness, but preferably in a thickness of 1 μm or more, and more preferably in a thickness of 5 to 25 μm.

(4) In the present invention, after the zinc or zinc alloy plating is deposited on a substrate according to the above method, the plated substrate is appropriately pretreated by, for example, being washed with water and optionally activated by a nitric acid, as needed. Thereafter, the zinc or zinc alloy plating is subjected to chemical conversion treatment by a dipping treatment, or the like using a chemical conversion treatment liquid for forming the trivalent chromium chemical conversion coating film according to the present invention.

(5) The chemical conversion treatment liquid of the first aspect of the present invention contains trivalent chromium ions, cobalt ions and hexavalent chromium generation suppressing agent that can suppress generation of hexavalent chromium which is otherwise generated in the trivalent chromium chemical conversion coating film.

(6) In the chemical conversion treatment liquid, any chromium compound containing trivalent chromium ions may be used as a source of trivalent chromium ions. For example, the sources of trivalent chromium salts such as chromium chloride, chromium sulfate, chromium nitrate, chromium phosphate or chromium acetate can be used, or, alternatively, trivalent chromium ions can be obtained by the reduction of hexavalent chromium ions of chromic acid, a dichromate, or the like with a reducing agent. However, the sources are not limited to these examples. One of the above sources of trivalent chromium ions or any combination of at least two of them may be used. A content of trivalent chromium ions in the chemical conversion treatment liquid should preferably be 0.002 to 0.5 mol/l, and should more preferably be 0.02 to 0.1 mol/l. Meanwhile, a concentration of hexavalent chromium ions in the chemical conversion treatment liquid should preferably be 1 ppm or less, and should more preferably be 0.5 ppm or less.

(7) In the chemical conversion treatment liquid, any metal compound containing cobalt may be used as a source of cobalt ions. Examples of such metal compounds include cobalt nitrate, cobalt sulfate, cobalt chloride, cobalt carbonate and cobalt hydroxide. However, the metal compounds are not limited to these examples. One of the above metal compounds or any combination of at least two of them may be used. A content of cobalt ions in the chemical conversion treatment liquid should preferably be 0.1 mol/l or less, should more preferably be 0.001 to 0.06 mol/l, and should still more preferably be 0.005 to 0.04 mol/l.

(8) In the chemical conversion treatment liquid, any additives can be used as the hexavalent chromium generation suppressing agent as long as the additives can suppress generation of hexavalent chromium which is otherwise generated in the trivalent chromium chemical conversion coating film. In order to find out additives that can suppress generation of hexavalent chromium, various additives are added to chemical conversion treatment liquids for forming the trivalent chromate coating film and effect of the additives is examined by experiments. As a result, organic reducing compounds such as ascorbate ions, citrate ions, tannate ions, gallate ions, tartrate ions, hydroxy(iso)quinolines, phenols and thiourea, and inorganic or metal reducing compounds such as phosphate ions, chromium phosphate ions, vanadium ions and titanium ions show the effect. Therefore, preferable hexavalent chromium generation suppressing agents include ascorbic acid, salts thereof, citric acid, salts thereof, tannic acid, salts thereof, gallic acid, salts thereof, tartaric acid, salts thereof, thiourea, phosphoric acid, salts thereof, vanadium compounds, titanium compounds, and the like. A content of the hexavalent chromium generation suppressing agent in the chemical conversion treatment liquid should preferably be 0.1 to 5 g/l, should more preferably be 0.2 to 3 g/l, and should still more preferably be 0.3 to 2 g/l.

(9) A pH of the chemical conversion treatment liquid should preferably be 0.5 to 5, and should more preferably be 2 to 3. The pH can be adjusted to this range by using the inorganic acid ions as described below, and also by using an alkaline agent such as an alkaline hydroxide, ammonia water, or the like.

(10) The chemical conversion treatment liquid may contain one or more kinds selected from inorganic acids, alkaline salts thereof, and the like. Examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, and the like. However, the inorganic acids are not limited to these examples. When one or more kinds selected from inorganic acids, alkaline salts thereof, and the like are contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 1 to 50 g/L, and should more preferably be 4 to 20 g/L.

(11) In addition, the chemical conversion treatment liquid may contain one or more kinds of hydroxycarboxylic acids, monocarboxylic acids, polyvalent carboxylic acids, aminocarboxylic acids, alkaline salts thereof, and the like as chelating agents for trivalent chromium ions. Examples of hydroxycarboxylic acids include malic acid, and the like. However, the hydroxycarboxylic acids are not limited to these examples. Examples of monocarboxylic acids include formic acid, acetic acid, and the like. However, the monocarboxylic acids are not limited to these examples. Examples of polyvalent carboxylic acids include: dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, adipic acid and diglycolic acid; tricarboxylic acids such as propanetricarboxylic acid, and the like. However, the polyvalent carboxylic acids are not limited to these examples. Examples of aminocarboxylic acids include glycine, aspartic acid, and the like. However, the aminocarboxylic acids are not limited to these examples. Among these, polyvalent carboxylic acids are preferable, and oxalic acid, malonic acid and succinic acid are more preferable. When the chelating agent for trivalent chromium ions is contained, a concentration thereof in the chemical conversion treatment liquid should preferably be in the range of 0.2 to 2 mole, should more preferably be in the range of 0.3 to 2 mole, should still more preferably be in the range of 0.5 to 2 mole, and should still further more preferably be in the range of 0.7 to 1.8 mole per mole of trivalent chromium ions.

(12) In addition, the chemical conversion treatment liquid may contain one or more kinds of silicon compounds. Examples of silicon compounds include colloidal silica, sodium silicate, potassium silicate, lithium silicate, and the like. However, the silicon compounds are not limited to these examples. When the silicon compound is contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 1 to 20 g/l, and should more preferably be 2 to 10 g/l in terms of Si. Colloidal silica is particularly preferable. A concentration thereof should preferably be 1 to 100 ml/1 as a 20% SiO2 aqueous solution. By adding colloidal silica, a coating film with a bilayer structure of a Si—O layer and a Cr—O layer can be formed, whereby the corrosion resistance can be further improved. In addition, the chemical conversion treatment liquid may contain one or more kinds of agents for reducing a coating film overall friction coefficient. Examples of such agents for reducing a coating film overall friction coefficient include quinoline-based compounds such as quinoline sulfonic acid, quinaldic acid, quinophthalone and derivatives thereof described in Japanese Patent Application Publication No. 2005-248233. When the agent for reducing a coating film overall friction coefficient is contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 0.1 to 25 g/l, and should more preferably be 0.2 to 15 g/l. The trivalent chromium chemical conversion coating film according to the present invention formed by treatment with the chemical conversion treatment liquid containing such a agent for reducing a coating film overall friction coefficient is a coating film that has a reduced coating film overall friction coefficient.

(13) The rest of the chemical conversion treatment liquid other than the above essential components is water.

(14) Usually, a nitrogen-containing compound, mainly nitrate ions, is used in large amount in a trivalent chromium chemical conversion treatment liquid, for improving the corrosion resistance of a trivalent chromium chemical conversion coating film. Consequently, the nitrogen atom concentration in the treatment liquid is high, for example 3 to 9 g/l, and there is a problem in terms of the wastewater treatment. In the chemical conversion treatment liquid according to the present invention, nitrate ions may be used in an amount similar to a conventional treatment liquid. However, even when nitrate ions are considerably decreased, and a nitrogen atom concentration, in the chemical conversion treatment liquid, mainly derived from nitrate ions is considerably reduced to 500 ppm/1 or less, the trivalent chromium chemical conversion coating film that has an excellent corrosion resistance and from which elution of hexavalent chromium is suppressed when the coating film is left can be obtained from the chemical conversion treatment liquid. In the above treatment liquid, a content is specifically 500 ppm or less in terms of nitrogen atoms, should preferably be in the range of 30 to 400 ppm and should more preferably be in the range of 50 to 300 ppm, for example. A metal reducing compound is preferable as the hexavalent chromium generation suppressing agent. Particularly, vanadium compounds, titanium compounds, magnesium compounds and combination thereof are preferable.

(15) In addition, cobalt ions may be contained but are not necessarily contained. However, cobalt ions should be contained preferably in the range of 0.001 to 0.06 mol/l, and more preferably in the range of 0.005 to 0.04 mol/l, because corrosion resistance of the chemical conversion coating film under heating is further improved.

(16) A method for forming a trivalent chromium chemical conversion coating film on zinc or zinc alloy plating by using the chemical conversion treatment liquid is commonly to immerse a zinc or zinc alloy plated substrate into the chemical conversion treatment liquid. A temperature of the chemical conversion treatment liquid at immersion is, for example, 10 to 70° C. The temperature should preferably be 30 to 50° C. An immersion time should preferably be 5 to 600 seconds, and should more preferably be 15 to 120 seconds. Meanwhile, immersion into a diluted nitric acid solution, a diluted sulfuric acid solution, a diluted hydrochloric acid solution, a diluted hydrofluoric acid solution, or the like may be performed before trivalent chromium chemical conversion treatment, for activating the surface of the zinc or zinc alloy plating. The conditions and treatment operations other than those described above may follow the conventional hexavalent chromate treatment method.

(17) Meanwhile, the second aspect of the present invention is washing water or a finishing liquid for a trivalent chromium chemical conversion coating film formed on zinc or zinc alloy plating. The washing water or the finishing liquid is used for suppressing hexavalent chromium which is otherwise generated in the trivalent chromium chemical conversion coating film. The washing water or the finishing liquid contains a hexavalent chromium generation suppressing agent that can suppress generation of hexavalent chromium which is otherwise generated in the trivalent chromium chemical conversion coating film. A method for forming a trivalent chromium chemical conversion coating film to which the washing water or the finishing liquid is applied is not particularly limited, and may be any known method. The washing water or the finishing liquid is particularly effective in the following cases: the trivalent chromium chemical conversion coating film contains Co.sup.2+ and Co.sup.3+; the trivalent chromium chemical conversion coating film is a coating film formed from a chemical conversion treatment liquid having a stronger oxidizing effect caused by a combination of chloric acid-nitric acid, or the like; the trivalent chromium chemical conversion coating film contains a manganese compound such as manganese dioxide; and the trivalent chromium chemical conversion coating film contains, in the coating film, a large amount of ions of an element, other than Co, whose valence can vary. Meanwhile, the hexavalent chromium generation suppressing agent is already described above.

(18) A content of the hexavalent chromium generation suppressing agent in the washing water or the finishing liquid should preferably be 0.1 to 10 g/l, should more preferably be 0.2 to 5 g/l, and should still more preferably be 0.3 to 3 g/l. Examples of preferable hexavalent chromium generation suppressing agents include ascorbic acid, salts thereof, citric acid, salts thereof, tannic acid, salts thereof, gallic acid, salts thereof, tartaric acid, salts thereof, thiourea, phosphoric acid, salts thereof, and the like.

(19) A pH of the washing water or the finishing liquid should preferably be 2 to 10, and should more preferably be 3 to 6. The pH may be adjusted to this range by using the inorganic acid ions as described below, or by using an alkaline agent such as an alkaline hydroxide, ammonia water, or the like.

(20) A method for treating a trivalent chromium chemical conversion coating film using the washing water or the finishing liquid is not particularly limited, and conventional and known methods such as immersing, applying, spraying, and the like may be used. However, to immerse a trivalent chromium chemical conversion coating film into the washing water or the finishing liquid is commonly employed. A temperature of the washing water or the finishing liquid at immersion is, for example, 10 to 70° C. The temperature should preferably be 20 to 50° C. An immersion time should preferably be 5 to 120 seconds, and should more preferably be 5 to 15 seconds.

(21) Meanwhile, a chemical conversion treatment liquid of the third aspect of the present invention contains trivalent chromium ions, cobalt ions and a sulfur compound.

(22) In the chemical conversion treatment liquid, any chromium compound containing trivalent chromium ions may be used as a source of trivalent chromium ions. For example, the source should be trivalent chromium salt such as chromium chloride, chromium sulfate, chromium nitrate, chromium phosphate or chromium acetate, or, alternatively, trivalent chromium ions can be obtained by the reduction of hexavalent chromium ions of chromic acid, a dichromate, or the like with a reducing agent. However, the source is not limited to these examples. One of the above sources of trivalent chromium ions or any combination of at least two of them may be used. A content of trivalent chromium ions in the chemical conversion treatment liquid should preferably be 0.002 to 0.5 mol/l, and should more preferably be 0.02 to 0.1 mol/l.

(23) Meanwhile, a concentration of hexavalent chromium ions in the chemical conversion treatment liquid should preferably be 1 ppm or less, and should more preferably be 0.5 ppm or less.

(24) A content of cobalt ions in the chemical conversion treatment liquid is 250 ppm or less. The chemical conversion treatment liquid does not necessarily contain cobalt ions. Since the sulfur compound is contained, the formed trivalent chromium chemical conversion coating film has a sufficient corrosion resistance even when cobalt ions are not contained. A content of cobalt ions in the chemical conversion treatment liquid should preferably be 100 to 250 ppm, and should more preferably be 150 to 200 ppm, when a higher corrosion resistance is required. When the chemical conversion treatment liquid contains cobalt ions, any metal compound containing cobalt can be used as a source of cobalt ions. Examples of such metal compounds include cobalt nitrate, cobalt sulfate, cobalt chloride, cobalt carbonate and cobalt hydroxide. However, the metal compounds are not limited to these examples. One of the above metal compounds or any combination of at least two of them may be used.

(25) In the chemical conversion treatment liquid, an organic sulfur compound is preferable as the sulfur compound. Specific examples of organic sulfur compounds include thiourea, thioglycerin, thioacetic acid, potassium thioacetate, thiodiacetic acid, 3,3-thiodipropionic acid, thiosemicarbazide, thioglycolic acid, thiodiglycolic acid, thiomaleic acid, thioacetamide, dithioglycolic acid, dithiodiglycolic acid, alkaline salts thereof, and the like. In addition, one of the above sulfur compounds or a mixture of two or more of them can be used. A content of the sulfur compound in the chemical conversion treatment liquid should preferably be 100 to 1500 ppm, should more preferably be 300 to 1000 ppm, and should still more preferably be 400 to 800 ppm in terms of sulfur atoms. By adding the sulfur compound, the formed trivalent chromium chemical conversion coating film has a sufficient corrosion resistance, even when a concentration of cobalt ions in the coating film is 0.2 μg/cm.sup.2 or less, and preferably 0.17 μg/cm.sup.2 or less. In addition, in the trivalent chromium chemical conversion coating film formed from the chemical conversion treatment liquid, hexavalent chromium which is otherwise generated in the trivalent chromium chemical conversion coating film can be suppressed because of low concentration of cobalt ions in the coating film.

(26) In the chemical conversion treatment liquid, a high corrosion resistance can be maintained even when nitrogen content in the treatment liquid is considerably reduced. A preferable nitrogen content is 500 ppm or less, and preferably 200 ppm or less in terms of nitrogen atoms. The content should more preferably be 40 to 200 ppm, and should still more preferably be 60 to 130 ppm.

(27) In addition, the chemical conversion treatment liquid may contain one or more kinds of silicon compounds. Examples of silicon compounds include colloidal silica, sodium silicate, potassium silicate, lithium silicate, and the like. However, the silicon compounds are not limited to these examples. When the silicon compound is contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 1 to 20 g/l, and should more preferably be 2 to 10 g/l in terms of Si. Colloidal silica is particularly preferable. A concentration thereof should preferably be 1 to 100 ml/1 as a 20% SiO.sub.2 aqueous solution. By adding colloidal silica, a coating film with a bilayer structure of a Si—O layer and a Cr—O layer can be formed, whereby corrosion resistance can be further improved.

(28) In addition, the chemical conversion treatment liquid may contain one or more kinds of agents for reducing a coating film overall friction coefficient. Examples of such agents for reducing a coating film overall friction coefficient include quinoline-based compounds such as quinoline sulfonic acid, quinaldic acid, quinophthalone and derivatives thereof described in Japanese Patent Application Publication No. 2005-248233. When the agent for reducing a coating film overall friction coefficient is contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 0.1 to 25 g/l, and should more preferably be 0.2 to 15 g/l. The trivalent chromium chemical conversion coating film according to the present invention formed by treatment with the chemical conversion treatment liquid containing such a agent for reducing a coating film overall friction coefficient is a coating film that has a reduced coating film overall friction coefficient.

(29) In addition, the chemical conversion treatment liquid may contain one or more kinds selected from inorganic acids, alkaline salts thereof, and the like. Examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, and the like. However, the inorganic acids are not limited to these examples. When one or more kinds selected from inorganic acids, the alkaline salts thereof, and the like are contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 0.01 to 50 g/L, and should more preferably be 0.05 to 20 g/L.

(30) In addition, the chemical conversion treatment liquid may contain one or more kinds selected from phosphorus oxoacids such as hypophosphorous acid, phosphoric acid, alkaline salts thereof, and the like. When one or more kinds selected from phosphorus oxoacids such as hypophosphorous acid, phosphoric acid, alkaline salts thereof, and the like are contained, the concentration thereof in the chemical conversion treatment liquid should preferably be 0.1 to 50 g/L, and should more preferably be 4 to 25 g/L.

(31) Furthermore, the chemical conversion treatment liquid may contain one or more kinds of hydroxycarboxylic acids, monocarboxylic acids, polyvalent carboxylic acids, aminocarboxylic acids, alkaline salts thereof, and the like as chelating agents for trivalent chromium ions. Examples of hydroxycarboxylic acids include malic acid, and the like. However, the hydroxycarboxylic acids are not limited to these examples. Examples of monocarboxylic acids include formic acid, acetic acid, and the like. However, the monocarboxylic acids are not limited to these examples. Examples of polyvalent carboxylic acids include: dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, adipic acid and diglycolic acid; tricarboxylic acids such as propanetricarboxylic acid, and the like. However, the polyvalent carboxylic acids are not limited to these examples. Examples of aminocarboxylic acids include glycine, aspartic acid, and the like. However, the aminocarboxylic acids are not limited to these examples. Among these, polyvalent carboxylic acids are preferable, and oxalic acid, malonic acid and succinic acid are more preferable. When the above carboxylic acids and alkaline salts thereof are contained, a concentration thereof in the chemical conversion treatment liquid should preferably be in the range of 0.2 to 2 mole, should more preferably be in the range of 0.3 to 2 mole, should still more preferably be in the range of 0.5 to 2 mole, and should still further more preferably be in the range of 0.7 to 1.8 mole per mole of trivalent chromium ions.

(32) In addition, the chemical conversion treatment liquid may contain one or more kinds of ions of metal selected from Mg, Al, Mn, Ti, W, V, Mo, Ni, Fe, Zn, Zr, Ca, Nb, Ta, Sn and Ce. When the metal ions are contained, a concentration thereof in the chemical conversion treatment liquid should preferably be 1 to 10 g/l, and should more preferably be 2 to 8 g/l.

(33) A pH of the chemical conversion treatment liquid should preferably be 0.5 to 5, and should more preferably be 2 to 3. The pH may be adjusted to this range by using the inorganic acid ions as described below, or by using an alkaline agent such as an alkaline hydroxide, ammonia water, or the like.

(34) The rest of the chemical conversion treatment liquid other than the above essential components is water.

(35) A method for forming a trivalent chromium chemical conversion coating film on zinc or zinc alloy plating by using the chemical conversion treatment liquid is commonly to immerse a zinc or zinc alloy plated substrate into the chemical conversion treatment liquid. A temperature of the chemical conversion treatment liquid at immersion is, for example, 10 to 70° C. The temperature should preferably be 25 to 35° C. An immersion time should preferably be 5 to 600 seconds, and should more preferably be 15 to 120 seconds. Meanwhile, immersion into a diluted nitric acid solution, a diluted sulfuric acid solution, a diluted hydrochloric acid solution, a diluted hydrofluoric acid solution, or the like may be performed before trivalent chromium chemical conversion treatment, for activating the surface of the zinc or zinc alloy plating. The conditions and treatment operations other than those described above may follow the conventional hexavalent chromate treatment method.

(36) Overcoating the trivalent chromium chemical conversion coating film formed by using the chemical conversion treatment liquid according to the present invention can improve the corrosion resistance thereof, and thus is a highly effective means for achieving longer-lasting corrosion resistance. For example, the trivalent chromium chemical conversion coating film is firstly formed on the zinc or zinc alloy plating using the chemical conversion treatment liquid according to the present invention, then washed with water, then immersed into an overcoating solution or subjected to an electrolytic treatment therein, and thereafter dried. Alternatively, the trivalent chromium chemical conversion coating film may be dried after formation thereof, and thereafter further immersed into an overcoating solution or subjected to an electrolytic treatment therein, and then dried. Here, as the overcoating, as well as an inorganic coating film made of silicates, phosphates, or the like, an organic coating film made of polyethylene, polyvinyl chloride, polystyrene, polypropylene, methacrylate resin, polycarbonate, polyamide, polyacetal, fluorine resin, urea resin, phenolic resin, unsaturated polyester resin, polyurethane, alkyd resin, epoxy resin, melamine resin, or the like may be effectively used.

(37) As the overcoating solution for overcoating such a film, DIPCOAT W or CC445 available from Dipsol Chemicals Co., Ltd. or the like may be used. The thickness of the overcoating may be any value, but should preferably be 0.1 to 30 μm.

(38) Next, the present invention is described by referring to Examples and Comparative Examples.

EXAMPLES

Examples 1 to 8

(39) An M6 bolt (material: iron), which had been plated with zinc using a zincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in Examples 1 to 2 and 6 to 8 or acidic zinc (EZ-960 available from. Dipsol Chemicals Co., Ltd.) in Examples 3 to 5 in a thickness of 8 μm, was immersed in a chemical conversion treatment liquid shown in Table 1 under conditions shown in Table 1. In addition, in Examples 6 to 8, the immersed bolt was immersed in a finishing liquid shown in Table 1 under conditions shown in Table 1. After immersion, the coating film was dried under conditions at 80° C. for 10 minutes.

(40) TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 7 8 Composition of trivalent chromium chemical conversion treatment liquid Cr.sup.3− (mol/l) 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 Cr.sup.6+ 0 0 0 0 0 0 0 0 Oxalic acid (g/l) 12 12 12 12 12 12 12 12 (mol/mol of Cr.sup.3+) (1.7) (1.7) (1.7) (1.7) (1.7) (1.7) (1.7) (1.7) Co.sup.2+ (mol/l) 0.034 0.034 0.034 0.034 0.034 0.034 0.034 0.034 hexavalent chromium generation Tannic Gallic Thiourea Vanadium ion phosphate ion — — — suppressing agent (0.5 g/l) acid acid (vanadium (sodium chloride) dihydrogen phosphate) Nitrogen content in terms of 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 nitrogen atoms (g/l) pH 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 Treatment temperature (° C.) 30 30 30 30 30 30 30 30 Treatment time (seconds) 40 40 40 40 40 40 40 40 Composition of finishing liquid Ascorbic acid (g/l) 2 Tannic acid (g/l) 1 Chromium phosphate (g/l) + 2 + 3 citric acid (g/l) pH of finishing liquid 4 4 6 Finishing treatment 25 40 25 temperature (° C.) Finishing treatment 15 5 15 time (seconds)

(41) A 40% chromium nitrate aqueous solution was employed as a source of Cr.sup.3+, and cobalt nitrate was employed as a source of Co.sup.2+. The rest of the solution was water.

Comparative Example 1

(42) An M6 bolt (material: iron), which had been plated with zinc using a zincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in a thickness of 8 μm, was subjected to a hexavalent chromate treatment. As the hexavalent chromate treatment liquid, Z-493 (10 ml/l) available from Dipsol Chemicals Co., Ltd. was used, and the bolt was immersed at 25° C. for 20 seconds. After immersion, the coating film was dried under conditions at 60° C. for 10 minutes.

Comparative Example 2

(43) A trivalent chromium chemical conversion coating film was formed on an M6 bolt (material: iron), which had been plated with zinc using a zincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in a thickness of 8 um. As the chemical conversion treatment liquid, a chemical conversion treatment liquid having the following composition was used, and the bolt was immersed at 30° C. for 40 seconds. After immersion, the coating film was dried under conditions at 80° C. for 10 minutes.

(44) TABLE-US-00002 Cr.sup.3+ 4 g/l (40% chromium nitrate was used. 0.077 mol/l in terms of Cr) Oxalic acid 12 g/l Co(NO.sub.3).sub.2 10 g/l (0.034 mol/l in terms of Co) pH 2.3

(45) Table 2 shows concentrations of Cr.sup.3+, concentrations of Cr.sup.6+ and concentrations of Co.sup.2+ in the chemical conversion coating films obtained in Examples 1 to 8 and Comparative Examples 1 and 2, appearances, results of salt spray test (JIS Z-2371) and amounts of hexavalent chromium eluted after the storage test. As shown in Table 3, the coating films of Examples 1 to 8 exhibited corrosion resistance equal to or better than that of the conventional hexavalent chromate chemical conversion coating film of Comparative Example 1. In addition, the amounts of hexavalent chromium eluted after storage test were less than the measurement limit value.

(46) TABLE-US-00003 TABLE 2 Corrosion resistance Amount of Concentration Time required Cr.sup.6+ eluted in coating for the after shelf film (μg/cm.sup.2) Appearance of coating formation of test Cr.sup.3+ Cr.sup.6+ Co.sup.2+ film white rust (Hrs) (μg/cm.sup.2) Example 1 8.5 0 0.7 Pale reddish green 300 <0.05 Example 2 10.5 0 2.5 Pale reddish green 300 <0.05 Example 3 6.7 0 1.6 Pale reddish green 300 <0.05 Example 4 7.0 0 0.6 Pale reddish green 300 <0.05 Example 5 5.4 0 0.4 Pale reddish green 300 <0.05 Example 6 10.0 0 2.3 Pale reddish green 300 <0.05 Example 7 10.8 0 1.0 Pale reddish green 300 <0.05 Example 8 10.7 0 2.9 Pale reddish green 300 <0.05 Comparative 20 6.8 0 Reddish green 240 6.8 Example 1 Comparative 11 0.01 2.8 Pale reddish green 240 0.12 Example 2

Examples 9 to 11

(47) An M6 Bolt (material: iron), which had been plated with zinc using a zincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in a thickness of 8 μm, were immersed into a chemical conversion treatment liquid shown in Table 3 under conditions shown in Table 3. After immersion, the coating film was dried under conditions at 80° C. for 10 minutes.

(48) TABLE-US-00004 TABLE 3 Example 9 10 11 Cr.sup.3+ (mol/l) 0.038 0.038 0.038 Cr.sup.6+ (ppm) 0 0 0 Nitrogen 90 90 90 content in terms of nitrogen atoms (ppm) SO.sub.4.sup.2− (g/L) 0 6.0 6.0 Cl.sup.− (g/L) 4.4 0 0 Thiodiglycolic 2 2 0 acid (g/L) (430) (430) (in terms of sulfur content (ppm)) Thiourea (g/L) 0 0 2 (in terms of (840) sulfur content (ppm)) CO.sup.2+ (ppm) 200 200 200 Si (g/L) 2 2 2 ph of treatment 2.4 2.4 2.4 liquid Treatment 30 30 30 temperature (° C.) Treatment time 60 40 40 (seconds)

(49) A 35% chromium chloride aqueous solution (Example 9) or a 35% chromium sulfate aqueous solution (Examples 10 and 11) was employed as a source of Cr.sup.3+. Cobalt chloride (Example 9) or cobalt sulfate (Examples 10 and 11) was employed as a source of Co.sup.2+. Si was an acidic colloidal silica (SNOWTEX-O available from Nissan Chemical Industries, Ltd.). The rest of the solution was water.

Examples 12 to 14

(50) Overcoating was performed on the trivalent chromium chemical conversion coating film of Example 9. Table 4 shows the overcoating conditions.

(51) TABLE-US-00005 TABLE 4 Example 12 13 14 Type of Chromium Inorganic Methacrylate overcoating phosphate-based silicate-based resin-based inorganic inorganic Si-dispersed-type coating film coating film organic coating film Treatment 150 ml/l 200 ml/l Undiluted liquid concentration was used Treatment 45° C., 10 seconds 25° C., 25° C., conditions 30 seconds 30 seconds Name of agent ZTB-118 CC-445Y DIPCOAT W available from available from available from Dipsol Dipsol Dipsol Chemicals Chemicals Co., Chemicals Co., Co., Ltd. Ltd. Ltd.

(52) Table 5 shows concentrations of Cr.sup.3+, concentrations of Cr.sup.6+ and concentrations of Co.sup.2+ in the chemical conversion coating films obtained in Examples 9 to 14 and Comparative Examples 1 and 2, appearances, results of salt spray test (JIS Z-2371) and amounts of hexavalent chromium eluted after storage test. As shown in Table 5, the coating films of Examples 9 to 14 exhibited corrosion resistance equal to or better than that of the conventional hexavalent chromate chemical conversion coating film of Comparative Example 1. In addition, the overcoated coating films (Examples 12 to 14) exhibited better corrosion resistance than the conventional hexavalent chromate chemical conversion coating film. In addition, the amounts of hexavalent chromium eluted after storage test were less than the measurement limit value.

(53) TABLE-US-00006 TABLE 5 Concentration Corrosion in resistance Amount of Cr.sup.6+ coating film Time required for eluted after (μg/cm.sup.2) Appearance of the formation of shelf Cr.sup.3+ Cr.sup.6+ Co.sup.2+ coating film white rust (Hrs) test (μg/cm.sup.2) Example 9 7.0 0 0.1 Light blue 240 <0.05 Example 10 5.8 0 0.1 Light blue 240 <0.05 Example 11 6.0 0 0.1 Light blue 240 <0.05 Example 12 7.0 0 0.1 light interference 1000 or more <0.05 color Example 13 7.0 0 0.1 colorless 1000 or more <0.05 Example 14 7.0 0 0.1 colorless 1000 or more <0.05 Comparative 20 6.8 0 Reddish green 240 6.8 Example 1 Comparative 11 0.11 2.8 Pale reddish green 240 0.12 Example 2
(Measurement of Cr.sup.6+ Concentration in Coating Film)

(54) A coating film sample (50 cm.sup.2) was immersed into approximately 50 ml of hot water at a temperature of 100° C. for 10 minutes. An amount of hexavalent chromium eluted from the coating film sample was determined by the absorption spectroscopy using diphenylcarbazide (in accordance with EN-15205).

(55) (Measurement of Cr.sup.3+ Concentration and Co.sup.2+ Concentration in Coating Film)

(56) After measuring the Cr.sup.6+ concentration, the same sample was dissolved into hydrochloric acid, and Cr.sup.3+ and Co.sup.2+ concentrations in the solution was measured by ICP optical emission spectrometry.

(57) (Salt Spray Test)

(58) In a salt spray test, evaluation was made in accordance with JIS-Z-2371.

(59) (Procedure of Storage Test and Measurement of Amount of Hexavalent Chromium Eluted after the Storage Test)

(60) A storage test was performed as an acceleration test by employing a method in which a sample for the elution test was left for 30 days in a constant temperature and humidity chamber maintained at a temperature of 80° C. and a humidity of 95%. Then, the sample after the storage test was immersed into hot water at a temperature of 100° C. for 10 minutes by a method similar to the above-described measuring method of Cr.sup.6+ concentration in a coating film. An amount of hexavalent chromium eluted from the coating film sample was determined by absorption spectroscopy using diphenylcarbazide (in accordance with EN-15205).

Examples 15 to 20

(61) An M6 Bolt (material: iron), which had been plated with zinc using a zincate (NZ-200 available from Dipsol Chemicals Co., Ltd.) in a thickness of 8 μm, was immersed into a chemical conversion treatment liquid shown in Table 6 under conditions shown in Table 6. After immersion, the coating film was dried under conditions at 80° C. for 10 minutes.

(62) TABLE-US-00007 TABLE 6 Example 15 16 17 18 19 20 Cr.sup.3+ (mol/l) 0.038 0.038 0.038 0.038 0.038 0.038 Cr.sup.6+ (ppm) 0 0 0 0 0 0 Nitrogen 135 270 270 90 90 90 content in terms of nitrogen atoms (ppm) Tartaric 0 0 0 2 2 0 acid(g/l) (0.35) (0.35) (mol/mol of Cr.sup.3+) Malic 5 5 0 0 0 5 acid(g/l) (0.97) (0.97) (0.97) (mol/mol of Cr.sup.3+) SO.sub.4.sup.2− (g/l) 0 0 2 2 0 0 Cl.sup.− (g/l) 4 4 4 4 4 4 Co.sup.2+ (mol/l) 0.015 0.008 0 0.008 0.008 0.008 VOSO.sub.4 (g/l) 1 1 0 0 1 0 Ti(SO.sub.4).sub.2 0 0 1 0 0 1 MgSO.sub.4 (g/l) 0 0 0 2 0 0 Si (g/l) 5 2 5 10 5 10 ph of 2.0 2.1 2.0 2.3 2.4 2.5 treatment liquid Treatment 40 40 30 30 30 30 temperature (° C.) Treatment 20 30 60 40 40 60 time (seconds)

(63) A 35% chromium chloride aqueous solution was employed as a source of Cr.sup.3+. Cobalt chloride was employed as a source of Co.sup.2+. Si was an acidic colloidal silica (SNOWTEX-O available from Nissan Chemical Industries, Ltd.). The rest of the solution was water. Note that, the nitrogen content was derived from NO.sub.3.sup.−.

(64) TABLE-US-00008 TABLE 7 Corrosion resistance Time required for the Concentration in formation of Amount of Cr.sup.6+ coating film (μg/cm.sup.2) Appearance of white rust eluted after shelf Cr.sup.3+ Cr.sup.6+ Co.sup.2+ coating film (Hrs) test (μg/cm.sup.2) Example 15 7.2 0 0.15 Light blue 240 <0.05 Example 16 7.0 0 0.1 Light blue 240 <0.05 Example 17 6.2 0 0 Light blue 168 <0.05 Example 18 5.0 0 0.15 Light blue 144 <0.05 Example 19 4.3 0 0.1 Light blue 144 <0.05 Example 20 4.0 0 0.1 Light blue 144 <0.05