KR950000312B1 - Method for chromate treatment of galvanized steel sheet - Google Patents

Abstract

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Description

[Name of invention]

Chromate treatment method of galvanized steel sheet

Detailed description of the invention

[Technical Field]

The present invention relates to a chromate treatment method for imparting excellent electrodeposition coating and workability to a metal surface. In more detail, the present invention relates to a chromate treatment method that gives excellent electrodeposition coating properties and workability by treating the surface of the zinc-based plated steel sheet by the chromate treatment method.

[Background]

Galvanized and zinc alloy coated steel sheets are generally excellent in corrosion resistance, but many problems have been pointed out.

As one of these problems, the surface of the zinc-based plated steel sheet, in particular, the surface of the alloyed hot-dip galvanized steel sheet subjected to the alloying process is not uniform in the deposition of the plated metal, so that the surface shape of the steel sheet is uneven and the electrical conductivity of the surface is uneven. Therefore, there existed a problem that craters generate | occur | produce easily at the time of electrodeposition coating by these nonuniformity. In addition, these zinc-based plated steel sheets cause powdering during processing such as press molding, and thus have problems in processing efficiency.

The prior art has dealt with the following problems associated with zinc (alloy) plating.

Japanese Unexamined Patent Application Publication No. 57-67195 discloses that at least one side of a double-sided galvanized steel sheet is subjected to railway gold having a predetermined thickness, thereby improving phosphate chemical treatment properties and coating film adhesion.

Japanese Unexamined Patent Publication No. 60-37880 discloses a surface-treated steel sheet excellent in secondary adhesion of a cationic electrodeposition coating film by subjecting it to a zinc, zinc-based composite or zinc alloy plated steel sheet, followed by a thin chromate treatment. Suggest a method.

Japanese Patent Application Laid-Open No. 59-0171645 proposes to improve the powdering property by forming a zinc-rich coating film containing a specific ratio of zinc powder and zinc-magnesium alloy powder on a specific galvanized steel sheet via a chromate coating. .

In addition, the method disclosed in Japanese Patent Laid-Open No. 60-105535 is a method of laminating a specific amount of chromate film and a zinc thick film layer having a specific thickness on a Zn-Ni alloy plated steel sheet, and the powdering by the zinc rich film layer is less. Being done is written.

As disclosed in Japanese Patent Application Laid-Open No. 56-36868, there is also a method of forming a specific plating amount of Ni plating layer on the surface of the galvanized steel and forming a chromate coating having a specific deposition amount.

The method disclosed in Japanese Patent Application Laid-Open No. 60-18751 improves coating adhesion by applying an aqueous solution containing chromic anhydride, colloidal silicate, and pyrophosphoric acid to the surface of a galvanized steel sheet, and then drying it without washing with water. Suggest to do.

The method disclosed in Japanese Patent Laid-Open No. 61-73900 proposes to prevent cratering in cationic electrodeposition by forming a specific amount of pure zinc plated coating on the plating surface of the zinc alloy plated steel sheet.

The above-mentioned prior art made for the purpose of improving workability such as electrodeposition coating or press working on the surface of steel sheet is coated with coating film, chromate coating, zinc rich coating, resin coating layer, etc. Although it was a requirement, neither of these methods was able to exhibit the outstanding performance of both electrodeposition coating property and press workability. In addition, there is a problem that the treatment process becomes complicated and management becomes difficult.

In addition, the inventors of the present invention improve the conventional chromate treatment + zinc (alloy) plating treatment (JP-A 60-37880, JP-A 60-105535, JP-A 59-171645). The method employed has been found to be a method in which a special plating is added in addition to chromate treatment, but the plating treatment itself is a conventional method, and thus there is a limit in improving electrodeposition coating and press workability.

[Initiation of invention]

The present invention first forms a substituted plating film layer from the additive metal ions in the chromate treatment liquid on the surface of the steel sheet by applying a specific chromate treatment liquid on the surface of the zinc-based plated steel sheet, and then dry without washing with water. It is made to form a chromate coating layer, making it a main point. By using this novel chromate treatment method, a plating layer is formed on a zinc-based plated steel sheet by an electroless method only by applying a treatment liquid, and a chromate film is formed by a simple treatment step of only drying after coating. It has been found that the problems of the prior art are solved and the corrosion resistance and the coating adhesion are also improved.

That is, the present invention consists of 4.0 to 51.0 g / l of hexavalent chromium, 6.0 to 38.0 g / l of trivalent chromium, and 0.5 to 99.0 g / l of phosphate ions, and 0.2 to 1.4 weight ratio of trivalent / hexavalent chromium. Aqueous chromium which added 1 type (s) or 2 or more types chosen from a sulfate ion, a nitrate ion, and a fluoride ion in a phosphorus aqueous solution, 1 type (s) or 2 or more types selected from Co, Ni, Sn, Cu, Fe, and Pb as a metal ion. The acid treatment solution is applied to the surface of a zinc-based plated steel sheet and subsequently dried to form a chromate coating having a chromium deposition amount of 20 to 160 mg / m 2 on the surface thereof. will be.

First, the composition of the aqueous chromate solution used for the method of this invention is demonstrated. This chromate solution contains water as a solvent and contains 4.0 to 51.0 g / l of hexavalent chromium and 6.0 to 38.0 g / l of trivalent chromium as basic components. At hexavalent chromium concentrations below 4.0 g / l and trivalent chromium concentrations below 6.0 g / l, it is difficult to form a chromate coating showing satisfactory corrosion resistance, on the contrary, hexavalent chromium concentrations exceeding 51.0 g / l and 38.0 g / l. At an excess of trivalent chromium concentration, the viscosity of the chromate solution increases and the stability of the chromate solution deteriorates, making it difficult to control the amount of chromium deposition. Also important in the amount of chromium is the trivalent and hexavalent chromium content ratio, and the trivalent / hexavalent chromium weight ratio needs to be in the range of 0.2 to 1.4.

The chromium weight ratio is controlled by adding known reducing agents such as ethanol, methanol, oxalic acid, starch, sugar and the like as necessary. When the chromium weight ratio is less than 0.2, the hexavalent chromium concentration in the chromate solution increases as much. Therefore, when acidic acid and metal ions other than phosphoric acid are added to this liquid together with one or two or more kinds, the action of acid ions other than phosphoric acid As a result, the reduction reaction of hexavalent chromium in the chromate solution easily occurs, resulting in the degradation of the chromate solution. On the contrary, when the chromium weight ratio exceeds 1.4, the chromate solution gels easily and the corrosion resistance of the chromate film to be formed decreases.

As another component, the chromate solution of the present invention contains 0.5 to 99.0 g / l of phosphate ions. Phosphate ions are preferably added in the form of orthophosphoric acid (H ₃ PO ₄ ). If the amount of phosphate ions is less than 0.5 g / l, the corrosion resistance and alkali resistance of the chromate film decreases. On the contrary, if the amount of the phosphate ion exceeds 97.0 g / l, the formation of the substituted plating film layer by the chromate solution is insufficient. It is thought that this is mainly due to the action of hexavalent chromium passivating the surface of the workpiece.

Further, one or two or more sulfate ions, nitrate ions, and hydrofluoric acid ions as acid ions added to the chromate treatment liquid are etched from the surface when the chromate treatment liquid is applied to the workpiece, and Zn is used. The said metal ion is substituted, and the plating layer of the ion of the said metal is formed. After that, when the object to be treated with the chromate solution is dried without washing with water, a normal non-rinse type chromate coating layer is formed. In addition, the acid ion and the metal ion need not be added separately as long as it is an acid salt such as copper nitrate, copper sulfate, nickel sulfate, or the like.

If the concentration of one or two or more acid ions selected from sulfate, nitrate and fluoride ions in the chromate treatment liquid is less than 0.01 mol / l, the amount of etching by the acid ions on the surface of the workpiece is insufficient. Substituted plating layers of metal ions are less likely to be formed. Moreover, the etching amount by the said acid ion on the surface of a to-be-processed surface exceeding 2.9 mol / l becomes excess, and the corrosion prevention performance by the zinc-based plating of a to-be-processed material, ie, a zinc-based plated steel sheet, is lost. Therefore, the concentration of these acid ions is preferably 0.01 to 2.9 mol / l.

When the one or two or more concentrations selected from Co, Ni, Sn, Cu, Fe, and Pb as the metal ions in the chromate treatment solution are less than 0.003 mol / l, the amount of the coating film is small, and if the concentration exceeds 0.85 mol / l, The amount of the plating film is excessively high, and the adhesion to the object to be treated of the chromate coating layer is lowered. On the other hand, when the total amount of metal ions added to the chromate treatment liquid is less than the acid ions required to deposit both Zn and the surface of the object as a substitution, the acid ions in the chromate treatment liquid are insufficient. If the metal ions in the treatment liquid exceed 0.85 mol / l, the metal ions, which were not precipitated as the substitution plating, remain in the chromate coating and change to metal oxides and hydroxides over time, resulting in lowering the corrosion resistance of the chromates.

The chromate solution used in the method of the present invention is applied to the surface of the zinc-based plated steel sheet by, for example, a roll coater or the like and subsequently dried. In the present invention, the drying conditions are not specified, but preferably the plate temperature of the steel sheet to be dried is 3 to 60 seconds at 60 to 260 ° C. Moreover, chromium adhesion amount is the range of 20-160 mg / m <2>. If the amount of chromium is less than 20 g / m 2, the corrosion resistance of the chromate film and the corrosion resistance after coating are insufficient, and if it exceeds 160 mg / m 2, it is difficult to control the amount of chromium coating of the chromate film, and the effect of improving the corrosion resistance is saturated, and further effects. In addition, since part of the chromate film is easily removed by external force, it is a cause of deterioration of the coating film adhesion property and a decrease in weldability.

Since the chromate coating formed on the workpiece by the method of the present invention has a substituted metal plating layer, it is possible to eliminate the unevenness of the surface shape of the workpiece and the nonuniformity of the surface electrical conductivity, and to make it difficult to generate craters during electrodeposition coating. It works. In addition, since the lubricity is imparted to the surface of the zinc-based plated steel sheet with the substitution plating as described above, the tool easily slips against the workpiece during press molding, and the powdering phenomenon accompanying the peeling of the zinc-based plated layer is prevented. It has the effect of preventing and improving a processing capability.

Best Mode for Carrying Out the Invention

Next, the Example and comparative example of this invention are shown. Zn-Ni-plated steel sheet and alloyed hot-dip galvanized steel sheet obtained by diluting the chromate agent of the compositions of Examples 1 to 6 and Comparative Examples 1 to 6 of the composition shown in Table 1 with water and trichloroethylene degreased appropriately. Each raw material was applied with a roll coater and subsequently dried at 180 ° C without washing with water.

In addition, the amount of chromium deposited on the chromate coating layer was about 70 mg / m 2 as measured by a fluorescent X-ray apparatus. Moreover, it confirmed that the substitution plating was affixed by copper fluorescence X-ray. The electrodeposited coating made by Japan Kansai Paint Co., Ltd. (Electron-9400) was set to 270V of electrodeposition voltage and 27 ° C of bath temperature on the angular gold plate after the chromate treatment. After baking for 20 minutes in oven, the coating film adhesion test was performed.

The performance evaluation of the test material produced by the above method was performed about each following item.

(1) Primary adhesion test

① Cross-cut adhesion test: 100 squares of 1 mm square were cut out with a cutter so as to reach the base, a cellophane tape was attached thereto, and then peeled off, and the remaining oil of the coating film was examined.

② DuPont impact test: A weight of 12.7 mm (1/2 inch) in diameter and 500 gr in weight was dropped on the painted surface at a height of 50 cm, and the degree of abnormality of the painted surface was visually observed.

③Erichsen test: Erichsen test extruded the coating surface by 6mm by using the Erikxen extruder, and observed the degree of abnormality such as cracking and peeling of the coating surface.

Coating film adhesiveness of the said item was evaluated by dividing into four steps according to the peeling degree of a coating film.

◎: 0% of peeling off

○: less than 10% of peeling

△: more than 10% of peeling off film

×: 30% or more peeling of coating

(2) salt spray test

Based on JISZ2371, the crosscut was cut out with a cutter until it reached the base from a coating film, and it carried out for 1000 hours. Corrosion resistance was evaluated in the rust state with respect to the whole area of the test piece.

◎: Rust generation area 0%

○: less than 10% of rust area

△: Rust generation area of 10% or more but less than 30%

×: rust area more than 30%

(3) 2nd adhesion test

After the salt spray test was finished for 1000 hours, a checkerboard test was performed on the painted surface in the same manner as the first adhesion test. The evaluation method is the same as the first close contact test.

(4) electrodeposition coating

After chromate treatment in the test material manufacturing method, the electrodeposition paint (EL-9400) manufactured by Japan and Kansai Paint Co., Ltd. was set at an electrodeposition voltage of 350 V and a coating temperature of 240 ° C., washed with water, and then washed in a 165 ° C. oven. Baking was performed for 20 minutes.

Determination of electrodeposition coating property was evaluated by measuring the number of craters produced on the coating surface, and dividing it into the following four steps by the number per 1m <2>.

◎: Less than 20 craters

○: More than 20 craters and less than 40 craters

(Triangle | delta): More than 40 craters and less than 60 craters

×: number of craters 60 or more

(5) processability

In order to evaluate the workability, in particular the amount of powdering, of the chromate treated steel sheet in the test material fabrication method, the treated steel sheet having a thickness of 1.4 mm was subjected to bending radius of 1 mm 180 °, tapered after bending tape, peeled off and powdered. The results were evaluated by dividing into four steps.

◎: powdered radish

○: Small amount of powdering

△: powdering weight

×: Large amount of powdering

Tables 2 and 3 show the results of the performance evaluation tests based on the test items of Examples 1 to 6 and Comparative Examples 1 to 6. In addition, Table 2 is a Zn-Ni plated steel sheet, Table 3 is an evaluation result of the embodiment of the alloyed hot-dip galvanized steel sheet.

TABLE 1

Cr ⁶⁺ is used for chromic anhydride Cr ³⁺ is used for reducing chromic anhydride with methanol. Preparation of the liquid was carried out in 300 ml of water, and after preparation, it was made into an appropriate aqueous solution concentration.

TABLE 2

Target plate: Zn-Ni plated steel sheet

TABLE 3

Treated plate: Alloyed hot dip galvanized steel sheet

Industrial availability

As can be seen from Table 2 and Table 3, the material to be treated by the chromate treatment method of the present invention is excellent in electrodeposition coating property, workability, corrosion resistance, coating film adhesion, and the like. In particular, it was O that the electrodeposition coating property and workability of a comparative example were the best, and it was confirmed that the performance of this invention is excellent. Therefore, the method concerning this invention is suitably applied to the process of the galvanized steel plate shape | molded and electrodeposition-coated, and the performance can be improved further.

Claims (2)

Sulfuric acid in an aqueous solution of 4.0 to 51.0 g / l of trivalent chromium, 6.0 to 38.0 g / l of trivalent chromium, and 0.5 to 99.0 g / l of phosphate ions, and 0.2 to 1.4 of trivalent / hexavalent chromium weight ratio. An aqueous chromate treatment solution containing one or two or more selected from ions, nitrates and fluorides, and one or two or more selected from Co, Ni, Sn, Cu, Fe, and Pb as a metal ion A method of treating chromate in a zinc-based plated steel sheet, which is coated on the surface of the zinc-based plated steel sheet and subsequently dried without washing with water, thereby forming a chromium film having a chromium deposition amount of 20 to 160 mg / m 2 on the surface thereof. The concentration of one or two or more acid ions selected from sulfate, nitrate and fluoride ions in the chromate treatment solution is in the range of 0.01 to 2.90 mol / l, and Co, Ni as a metal ion. A method for treating chromate of a zinc-based plated steel sheet, characterized in that the concentration of one or two or more selected from among Sn, Cu, Fe, and Pb is in the range of 0.003 to 0.85 mol / l.