JP2001158972A - Galvanized steel sheet having excellent blackening resistance and corrosion resistance, and its surface treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treated and galvanized steel sheet which has excellent corrosion resistance equivalent to that of a chromate treatment and is improved in blackening resistance as well. SOLUTION: A film containing a molybdenum compound of 10 to 120 mg/m2 as a coating weight in terms of Mo, a phosphoric acid compound of 0.2 to 2 as a mass ratio in terms of P/Mo, a cobalt compound of 0.03 to 0.3 as a mass ratio in terms of Co/Mo and an organic compound having an oxycarboxylic acid group of 5 to 300 mg/m2 in coating weight is formed on the surface of a galvanizing layer or zinc alloy coating layer. The film containing Mo and Co is formed by applying an aqueous treating liquid containing molybdic oxygen acid ions of 1 to 100 g/l in terms of Mo, phosphoric acid ions of 0.2 to 2 as the mass ratio in terms of P/Mo, cobalt ions of 0.03 to 0.3 as the mass ratio in terms of Co/Mo. oxycarboxylic acid of 1 to 300 g/l and oxalic acid of 1 to 300 g/l in total amount with the oxycarboxylic acid at need on the galvanized steel sheet or the zinc alloy coated steel sheet at 10 to 120 mg/m2 of a coating weight in terms of Mo, then heating and drying the coating at a sheet temperature of 50 to 200 deg.C without water washing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized steel sheet having a Mo-containing film having excellent blackening resistance and corrosion resistance formed on its surface, an aqueous treatment solution for forming the film, and a surface treatment method.

[0002]

2. Description of the Related Art Galvanized steel sheets are widely used as steel materials having good corrosion resistance. However, when exposed to a humid atmosphere, an exhaust gas atmosphere, or an atmosphere near the coast for a long period of time, white rust is generated on the steel sheet surface, and the appearance of the steel sheet is reduced. Deteriorates. In order to prevent white rust, a method of chromate-treating a galvanized steel sheet is usually adopted. Further, in order to prevent black discoloration of the galvanized steel sheet subjected to the chromate treatment, prior to the chromate treatment, displacement plating is performed with an aqueous solution containing Ni ions and Co ions to precipitate Ni and Co (Japanese Patent Laid-Open No. 59-1984). 1
No. 77381) or a method of treating with an aqueous solution containing a cobalt salt and thermally decomposing the attached cobalt salt to deposit cobalt oxide on the surface of the plating layer (Japanese Patent Publication No. 7-381).
-96699) and the like. However, since a separate treatment is required before the chromate treatment, there is a disadvantage that the manufacturing process is complicated.

[0003] In the chromate treatment, waste containing Cr ions is used.
A large burden is required for processing the liquid. Therefore, titanium-based
Use chemicals such as ruconium, molybdenum, and phosphate
A chromium-free surface treatment method used has been studied.
However, a method of depositing Co and Ni (Japanese Unexamined Patent Publication No.
No. 77381) discloses that Co, N
Steel deposition due to low adhesion of deposited metal to deposit i
Easily detached from the steel sheet surface at the time of steel sheet processing. Deposited metal
Black discoloration is likely to occur at the places where. Koval
Treated with an aqueous solution containing sodium salt
Deposit cobalt oxide on plating layer surface by solution
In the method (Japanese Patent Publication No. 7-96699), the uncoagulated state
To spray an aqueous solution containing cobalt salt on the plating layer
Low Baltic deposition efficiency and N which is harmful to the environment
O _TwoSuch gases tend to be generated during pyrolysis.

[0004] In this regard, molybdenum-based surface treatments have been practiced since ancient times because they have no problems in terms of adhesion, environmental aspects, and the like. For example, in Japanese Patent Publication No. 51-2419, a galvanized steel material is immersed in an aqueous solution containing a magnesium salt or a calcium salt of molybdic acid to form a rust preventive film on the surface of the steel material. in this way,
It is said that the film becomes a film in which tetravalent and hexavalent molybdenum coexist. Japanese Patent Application Laid-Open No. 6-146003 introduces a method in which a hexavalent molybdenum compound is partially reduced, and a treatment liquid in which the ratio of hexavalent molybdenum / total molybdenum is adjusted to 0.2 to 0.8 is applied to a steel material surface. Have been. It is stated that since the formed film contains an appropriate amount of hexavalent molybdenum, the steel material has excellent corrosion resistance and paintability comparable to chromate treatment.

[0005]

A method for forming a rust-proof coating by immersing a galvanized steel material in an aqueous solution containing a magnesium or calcium salt of molybdic acid (Japanese Patent Publication No. 51-2)
No. 419), Mo ₅ reacts with the surface of the steel material.
A film containing O ₅ (OH) ₁₀ , MoO _{3 and the} like is generated. However, since molybdenum in the film is not valence controlled,
The film is difficult to densify and tends to have a surface with poor corrosion resistance and paintability. On the other hand, in the method (JP-A-6-146003) using a treatment solution in which the ratio of hexavalent molybdenum / total molybdenum is adjusted to 0.2 to 0.8 by partial reduction, the valence of molybdenum in the film is not controlled. Although it is easy, many compounds of hexavalent molybdenum which are said to have self-healing properties are hardly soluble in water, and are self-healing properties, that is, soluble compounds that elute from the film and repair the damaged part of the film. It is difficult to control the amount of hexavalent molybdenum. For these reasons, the conventional molybdenum-based treating agents have not been able to provide a sufficient rust-preventive effect. In addition, there is room for improvement in suppression of black discoloration.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been devised in order to solve such a problem, and it is possible to omit a step by adding cobalt ions, to have a low reducing power, and to reduce a reaction product. By adding an oxycarboxylic acid having an action of suppressing the reduction reaction to the treatment liquid, the amount of soluble hexavalent molybdenum contained in the film is appropriately controlled, and a zinc-based plated steel sheet having excellent blackening resistance and corrosion resistance is obtained. The purpose is to: In order to achieve the object, the zinc-based plated steel sheet of the present invention has a Mo equivalent adhesion amount of 10 to 1 on the surface of a zinc-based plating layer such as a zinc plating layer or a zinc alloy plating layer.
20 mg / m ² molybdenum compound, 0.2 to 2 phosphoric acid compound as P / Mo converted mass ratio, 0.03 to 0.3 Co compound as Co / Mo converted mass ratio, and adhesion amount 5
A film containing an oxycarboxylic acid group of up to 300 mg / m ² is formed.

[0007] The treatment liquid for film formation is 1 to 1 in terms of Mo.
Molybdenum oxygenate ion of 00 g / l, phosphate ion of 0.2 to 2 as P / Mo converted mass ratio, Co compound of 0.03 to 0.3 as Co / Mo converted mass ratio, and 1 to 3
It has a composition containing oxalic acid of 1 to 300 g / l in a total amount of 00 g / l oxycarboxylic acid and, if necessary, oxycarboxylic acid. A Mo-containing film having excellent corrosion resistance is obtained by applying an aqueous treatment solution to a zinc-based plated steel sheet with a Mo equivalent adhesion amount of 10 to 12%.
Apply at 0mg / m ² , plate temperature 50-2 without washing with water
It is formed by heating and drying at 00 ° C. In the present specification, a steel sheet on which a zinc plating layer or a zinc alloy plating layer is formed by hot-dip plating, electroplating, vapor deposition plating, or the like is generally referred to as a zinc-based plated steel sheet. For zinc alloy plating,
There are Zn-Al, Zn-Mg, Zn-Al-Mg and the like.
Above all, a Zn-Al-Mg-plated steel sheet has a great effect of improving corrosion resistance.

[0008]

In order to obtain corrosion resistance comparable to a chromate film, it is necessary to form a film having strong barrier properties and a self-healing effect. The barrier properties are improved by including a polyionizable compound in the film to make the film insoluble or hardly soluble. On the other hand, the self-healing action is an action obtained by contacting a defective portion with a compound or metal ion once eluted to form an insoluble or hardly soluble compound and covering the defective portion. Need to include. In this regard, molybdate compounds can form polymolybdates, and tetravalent molybdates are often insoluble or hardly soluble, and there are also soluble hexavalent molybdates. Acid-based coatings are preferred.

The present inventors have conducted various investigations and examinations of molybdenum-based treating agents from the viewpoint of components and compositions from such a viewpoint. As a result, it was found that when a treatment solution containing phosphoric acid, oxycarboxylic acid, and hexavalent molybdenum was brought into contact with a zinc-based plated steel sheet, a treatment film having a barrier effect and a self-healing action was formed. When a treatment solution containing phosphate ions and oxalate ions is applied to a galvanized steel sheet, a film containing an insoluble or sparingly soluble molybdenum compound mainly composed of hexavalent is formed without undergoing any reduction reaction. It is supposed that the suppression of the reduction reaction is due to the action of oxalate ions contained in the processing solution and the formation of insoluble compounds of phosphoric acid, oxalic acid and hexavalent molybdenum in the film, although the detailed reason is unknown. .

In a system containing oxycarboxylate ions instead of oxalate ions, a film containing tetravalent and hexavalent molybdenum compounds is formed on the steel material surface, and the hexavalent molybdenum compounds become soluble. In this case, it is presumed that hexavalent molybdenum is reduced by the reduction reaction due to contact with the steel material surface and the reducing action of oxycarboxylic acid, and a tetravalent molybdenum compound is generated. Examples of the oxycarboxylic acid include tartaric acid, citric acid, lactic acid, and butyric acid. Since all of them have the same action and effect, tartaric acid is represented in the following description.

Although tartaric acid acts as a reducing agent, it has a very low reducing ability and is oxidized to oxalic acid which inhibits the reduction reaction. Therefore, a large amount of hexavalent molybdenum is present in the film and reacts with phosphoric acid, tartaric acid, oxalic acid, and the like to form a complex compound that is partially soluble. Of the soluble components, phosphoric acid,
Once the component containing tartaric acid and hexavalent molybdenum elutes and flows to the defect, it reacts with the zinc plating layer at the defect. The reaction product is an insoluble molybdenum compound, which covers the defect and suppresses corrosion.

[0012] The treatment liquid according to the present invention contains molybdenum oxygenate ions, phosphate ions, cobalt ions and tartrate ions. Molybdenum oxygenate ions are MoO ₃ , H ₂
It is added as MoO ₄ , K ₂ MoO ₄ , MgMoO ₄ or the like. The concentration of molybdenum oxyacid ion is 1 to 100 g / l in terms of Mo in order to secure a predetermined amount of Mo in the film.
Adjust to the range. If the Mo conversion concentration is less than 1 g / l, the amount of Mo in the film tends to be insufficient, and conversely 100 g / l.
If the Mo-converted concentration exceeds 1, the sludge tends to be generated during continuous production. Hexavalent molybdenum contained in the treatment liquid may be partially reduced. However, in a system containing tartaric acid having a low reducing power, the amount of reduced hexavalent molybdenum is very small.

The amount of Mo in the coating is preferably in the range of 10 to 120 g / m ^{2 in} order to obtain sufficient corrosion resistance. The effect of improving corrosion resistance by Mo is exhibited when the amount of Mo is 10 g / m ² or more, but is saturated at 120 g / m ² , and increasing it more is not economical. Further, even if a part is reduced to tetravalent molybdenum, the corrosion resistance is not adversely affected, but if a large amount of hexavalent molybdenum is reduced, the self-healing action of the treated film is deteriorated. However, in the treatment solution according to the present invention, since tartaric acid having a low reducing power is added and oxalic acid, which is a reaction product of tartaric acid, suppresses the reduction reaction, a large amount of hexavalent molybdenum is reduced. Absent.

Although tartaric acid acts as a reducing agent as described above, it has a low reducing power, and oxalic acid, a reaction product, suppresses the reduction reaction, so that a large amount of hexavalent molybdenum is allowed to exist in the film. . Further, it remains as a tartrate in the film and becomes a part of a soluble component effective for a self-healing action. In this regard, when alcohols, carboxylic acids, saccharides and the like disclosed in JP-A-6-146003 are used as the organic reducing agent, the reduction of hexavalent molybdenum proceeds by the amount of the reducing agent. Therefore, the organic reducing agent is decomposed,
The added reducing agent does not work as a corrosion resistance improving component such as tartaric acid. In addition, some of the non-reduced hexavalent molybdenum exists as a sparingly soluble hexavalent molybdenum salt, which makes it difficult to control the soluble components in the film.

The concentration of tartaric acid is from 1 to 300 g in order to secure a predetermined amount of hexavalent molybdenum in the film and to improve the corrosion resistance.
/ L. When the tartaric acid concentration is less than 1 g / l, the amount of tartaric acid contained in the film falls below 5 mg / m ² ,
Sufficient corrosion resistance cannot be obtained. Conversely, at a tartaric acid concentration exceeding 300 g / l, the amount of tartaric acid contained in the film is 300 g / l.
It is a large amount exceeding m ² , which is not economical but also increases the amount of soluble Mo. As a result, the amount of insoluble molybdenum in the film is reduced, a sufficient barrier film is not formed, and the corrosion resistance tends to decrease.

The phosphate ion enhances the reactivity between the treatment solution and the surface of the steel material, and dramatically improves the adhesion strength of the treatment film formed on the surface of the zinc-based plating layer. It is inferred that the strength of the treated film is also improved by the binding of the molybdate ions contained in the treatment solution and the zinc ions eluted from the plating layer with the phosphate ions. Phosphoric acid, AlPO ₄ , KH ₂ PO ₄ , K ₂ HPO ₄ , (NH ₄ ) H ₂
There are various phosphates such as PO ₄ , MgHPO ₄ and Mg ₃ (PO ₄ ) ₂ . The phosphate ion concentration is adjusted so that the mass ratio in terms of P / Mo is 0.2 to 2. The effect of improving corrosion resistance is manifested at a P / Mo converted mass ratio of 0.2 or more.
It is not economical to saturate at a / Mo conversion mass ratio of 2 and further increase the phosphate ion concentration.

[0017] Cobalt ions are added to prevent black discoloration of the galvanized steel sheet. It is presumed that the cause of the black discoloration is that a light-absorbing oxide film containing Zn as a main component, which is caused by a change in the state of the elements on the surface of the zinc-based plating layer or a change in the crystal orientation, is grayish black. Is done. On the other hand, if cobalt atoms are present on the surface of the zinc-based plating layer, the cathode reaction on the zinc-based plating layer surface is suppressed by a small amount of cobalt ions flowing out in a corrosive environment, and the corrosion (oxidation) rate of zinc decreases. It is considered that the appearance of the black discoloration phenomenon appears to be delayed. Cobalt ion sources effective for suppressing black discoloration include Co
(OH) ₂ , Co (NO ₃ ) ₂ , CoSO ₄ , Co (COO
H) There are ₂ etc. Cobalt ions are added to the aqueous treatment liquid such that the cobalt compound in the film has a Co / Mo equivalent mass ratio of 0.03 to 0.3. The effect of improving the blackening resistance by the cobalt compound is Co / Mo of 0.03 or more.
It becomes remarkable in the reduced mass ratio. However, Co exceeding 0.3
/ Mo equivalent mass ratio is not only economically disadvantageous,
There is a tendency that the stability of the aqueous treatment liquid and the corrosion resistance of the film are reduced.

[0018] The reducing ability of the treatment liquid can also be adjusted by including oxalate ions. The oxalate ions are supplied from reaction products of oxalate and tartaric acid added to the processing solution.
Oxalic acid exhibits an action of suppressing the reduction reaction of hexavalent molybdenum, and is effective in securing a predetermined amount of hexavalent molybdenum in the film. As the number of oxalate ions increases, the amount of oxalate that suppresses the reduction reaction of hexavalent molybdenum is contained in the film in a larger amount. However, when a large amount of oxalate ions exceeding 300 g / l is contained, the amount of insoluble Mo increases, and the self-healing action of the film tends to decrease. In order to secure a predetermined amount of oxalate in the film, the total amount of
It is preferable to adjust the oxalate ion concentration so as to be 0 g / l or less.

The treatment liquid according to the present invention is applied to a galvanized steel sheet by a roll coating method, a spin coating method, a spray method or the like. After the application, the coating is dried without being washed with water, whereby a film having predetermined performance is formed on the surface of the plating layer. Drying of the film is possible at room temperature, but it is preferable to keep the temperature at 50 ° C. or higher to shorten the drying time in consideration of continuous operation. However, if the drying temperature exceeds 200 ° C., the carboxylic acid contained in the film may be decomposed by heating, and the corrosion resistance of the film may be reduced. Next, the effects of the present invention will be specifically shown by examples. In the following examples, hot-dip galvanized steel sheets were used. However, similar effects of improving corrosion resistance and blackening resistance were observed in hot-dip galvanized steel sheets, electrogalvanized steel sheets, and vapor-deposited zinc-based steel sheets.

[0020]

EXAMPLE A molybdate, a phosphate, a cobalt salt, tartaric acid and oxalic acid were mixed in various ratios to prepare a treating solution having the composition shown in Table 1. For comparison, a treatment liquid 6 containing no cobalt salt, a treatment liquid 7 containing no tartaric acid and a treatment liquid 8 containing no phosphoric acid, and a treatment liquid 9 containing no cobalt salt and tartaric acid were also prepared.

[0021]

A hot-dip Zn-Al-Mg alloy plated steel sheet was used as the material to be treated. The hot-dip Zn-Al-Mg alloy plated steel sheet is made of a weakly deoxidized steel sheet with a thickness of 0.5 mm as the plating base sheet.
After surface activation by reduction heating in a gas atmosphere of N ₂ -50% H ₂ , the steel strip was introduced into an alloy plating bath of Zn-6 mass% Al-3 mass% Mg, and the steel strip pulled up from the plating bath was removed. It was manufactured by adjusting the coating weight to 50 g / m ² by gas wiping and cooling naturally. Each treatment liquid shown in Table 1 was applied to a hot-dip Zn-Al-Mg alloy-plated steel sheet, and charged into an electric oven without washing with water, and the sheet temperature was set to 50.
It heat-dried at -200 degreeC. Table 2 shows the results of analyzing the film formed on the plating layer surface.

[0023]

A test piece was cut out from each of the plated steel sheets after the treatment, and subjected to a corrosion resistance test and a blackening resistance test. In the corrosion resistance test, the end face of the test piece was sealed, and a 5% NaCl aqueous solution at 35 ° C. was sprayed according to JIS Z2371. After the salt spray was continued for a predetermined time, the area ratio of white rust generated on the test piece surface was measured.
〜１０ for 10%, △ for 10 to 30%, ▲ for 30 to 50%
50% or more was evaluated as x. In the blackening resistance test, the test piece was allowed to stand for 10 days in an atmosphere at a temperature of 50 ° C. and a relative humidity of 98%, and the lightness difference ΔL before and after the test was determined. 5 to △, 5 to 10 ▲, 10
The above was evaluated as x, and the blackening resistance was evaluated. As can be seen from the investigation results in Table 3, the zinc-coated steel sheets of Test Nos. 1 to 7 in which the films having the compositions specified in the present invention were formed, the generation of white rust was detected even after the salt spraying was continued for 72 hours. No change in brightness was observed.

On the other hand, Test No. 8,
Test No. 10 using a treatment solution containing no tartaric acid
12. In Test No. 11 using a treatment solution containing no phosphoric acid, white rust was generated by spraying salt water for 24 hours. In Test Nos. 9 and 12 using a treatment solution containing no cobalt salt and Test No. 10 in which the amount of deposited cobalt was insufficient, the test piece surface after the blackening resistance test was gray-black. As is clear from this comparison, the galvanized steel sheet treated according to the present invention exhibited excellent corrosion resistance and blackening resistance.

[0026]

[0027]

As described above, the galvanized steel sheet according to the present invention comprises a cobalt salt having a function of delaying blackening of the surface of a plating layer, a weak reducing power, and a reaction product of hexavalent molybdenum. By treating a galvanized steel sheet or a zinc alloy-plated steel sheet with a molybdenum oxyacid-containing treatment liquid to which an oxycarboxylic acid exhibiting the action of suppressing reduction is added, Mo,
A Co-containing film is formed. The galvanized steel sheet with the Mo, Co-containing film formed has excellent corrosion resistance comparable to that of the chromate film, has improved blackening resistance, and is chromium-free, reducing the burden on waste liquid treatment and reducing environmental impact. It is used in a wide range of fields as a chromium-free surface-treated steel sheet that has no adverse effect.

Continued on the front page (72) Inventor Yasumi Ariyoshi 5 Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Co., Ltd. Inside the Technical Research Laboratory (72) Inventor Minoru Saito 5 Ishizu Nishimachi, Sakai City, Osaka Nisshin Steel Co., Ltd. Technical Research In-house F-term (reference) 4K026 AA02 AA07 AA12 AA13 AA22 BA01 BA03 BB08 BB10 CA16 CA18 CA23 CA26 CA29 CA36 CA38 CA40 DA11 DA15 DA16

Claims (5)

[Claims] 1. A molybdenum compound having a Mo equivalent adhesion amount of 10 to 120 mg / m ^{2 on} the surface of a zinc-based plating layer,
A phosphate compound having a P / Mo converted mass ratio of 0.2 to 2,
A black film characterized in that a film containing a cobalt compound having a Co / Mo equivalent mass ratio of 0.03 to 0.3 and an organic compound having an oxycarboxylic acid group having an adhesion amount of 5 to 300 mg / m ² is formed. Galvanized steel sheet with excellent modification and corrosion resistance. 2. A molybdenum oxyacid ion of 1 to 100 g / l in terms of Mo, and a mass ratio of 0.2 to 2 in terms of P / Mo.
Aqueous treatment solution for zinc-based plated steel sheet, containing phosphate ion of 1 to 300 g / l and oxycarboxylic acid of 1 to 300 g / l and cobalt ion of 0.03 to 0.3 in terms of Co / Mo conversion mass ratio. 3. A total amount of 1 to 3 with oxycarboxylic acid.
The aqueous treatment solution for a zinc-based plated steel sheet according to claim 2, comprising 300 g / l of oxalic acid. 4. Molybdenum oxygenate ion of 1 to 100 g / l in terms of Mo, and a mass ratio of P / Mo of 0.2 to 2 g / l.
Ion as a Co / Mo equivalent mass ratio of 0.03
Aqueous treatment solution containing 0.3 to 0.3 g of cobalt ion and 1 to 300 g / l of oxycarboxylic acid
o A surface treatment method for a galvanized steel sheet excellent in blackening resistance and corrosion resistance, which is applied at a conversion amount of 10 to 120 mg / m ² and heated and dried at a sheet temperature of 50 to 200 ° C without washing with water. 5. A total amount of 1 to 5 with an oxycarboxylic acid.
The method according to claim 4, wherein an aqueous treatment solution containing 300 g / l of oxalic acid is used.