JP2001011649A - Galvanized steel sheet excellent in corrosion resistance and its surface treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treated galvanized steel sheet having excellent corrosion resistance equal to that by chromate treatment. SOLUTION: On the surface of a galvanized layer or a galvannealed layer, a film of a molybdenum compd. of 10 to 120 mg/m2 coating weight expressed in terms of Mo, a phosphoric compd. of 0.2 to 2 as the weight ratio expressed in terms of P/Mo and an organic compd. having an oxycarboxylic acid group by the coating weight of 5 to 300 mg/m2 is formed. The Mo-contg. film is formed by coating a galvanized steel sheet or a galvannealed steel sheet with a water base treating soln. contg. molybdate ions of 1 to 100 g/l expressed in terms of Mo, phosphate ions of 0.2 to 2 by the weight ratio expressed in terms of P/Mo and oxycarboxylic acid of 1 to 300 g/l and contg., and need, oxalic acid of 1 to 300 g/l in total with the content of oxycarboxylic acid by 10 to 120 mg/m2 coating weight expressed in terms of Mo, and, without executing water washing, it is dried under heating at 50 to 200 deg.C sheet temp.

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 surface containing a Mo-containing film having excellent corrosion resistance, 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. In the chromate treatment, a large burden is imposed on the treatment of the waste liquid containing Cr ions. Therefore, titanium series,
A chromium-free surface treatment method using a zirconium-based, molybdenum-based, phosphate-based chemical or the like has been studied. In particular, molybdenum-based surface treatments have been reported for a long time.

For example, in Japanese Patent Publication No. 51-2419, a galvanized steel material is immersed in an aqueous solution containing a magnesium or calcium salt of molybdic acid to form a rust-preventive film on the surface of the steel material. According to this method, a film in which tetravalent and hexavalent molybdenum coexist is formed. 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, excellent corrosion resistance and paintability comparable to a chromate treatment agent are imparted.

[0004]

A method of immersing a galvanized steel material in an aqueous solution containing molybdate (Japanese Patent Publication No. 51-1979)
No. 2419), Mo reacts with the surface of the steel material.
A film containing ₅ O ₅ (OH) ₁₀ , MoO _{3 and the} like is formed.
However, since molybdenum is not controlled in valence, the film is hardly densified, and the surface tends to be poor in 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.

[0005]

SUMMARY OF THE INVENTION The present invention has been devised to solve such a problem, and has a weak reducing power.
In addition, by adding oxycarboxylic acid, which has the effect of suppressing the reduction reaction of the reaction product, to the treatment liquid, the amount of soluble hexavalent molybdenum contained in the film can be appropriately controlled to obtain a steel sheet having excellent corrosion resistance. Aim. In order to achieve the object, the zinc-based plated steel sheet of the present invention has a molybdenum compound having a conversion equivalent of Mo of 10 to 120 mg / m ² , P / Mo on the surface of the zinc plating layer or the zinc alloy plating layer.
The weight ratio of the phosphoric acid compound and the organic compound having an oxycarboxylic acid group of 0.2 to 2 in terms of reduced weight ratio is 5 to 300 mg.
/ M ² is formed. The coating may further contain Mg and / or Al in a weight ratio of (Mg + Al) / Mo reduced to 0.6 or less.

[0006] The treatment liquid for forming a film is 1 to 1 in terms of Mo.
Molybdenum oxygenate ion of 00 g / l, phosphate ion having a P / Mo conversion weight ratio of 0.2 to 2, 1 to 300 g / l
Of oxalic acid in a total amount of 1 to 300 g / l with oxycarboxylic acid as needed and oxycarboxylic acid as required. Mg and / or Al with a weight ratio of (Mg + Al) / Mo equivalent of 0.6 or less may be further added to the treatment liquid. The Mo-containing film having excellent corrosion resistance and coating properties is obtained by applying an aqueous treatment solution to a galvanized steel sheet or a zinc alloy-coated steel sheet at a Mo equivalent adhesion amount of 10 to 120 mg / m ² , and at a sheet temperature of 50 to 200 ° C. without washing with water. It is formed by heating and drying.

[0007]

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 surface of the steel material, 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.

[0010] Tartaric acid acts as a reducing agent but 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. The processing solution according to the present invention contains molybdenum oxyacid ion, phosphate ion, tartrate ion and oxalate ion.

The molybdenum oxygenate ion is MoO ₃ , H
₂ MoO ₄ , K ₂ MoO ₄ , MgMoO _{4 and the} like are added. The concentration of molybdenum oxyacid ion is M
From the viewpoint of securing a predetermined amount of o, 1 to 100 g in terms of Mo
/ L 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%.
If the concentration in terms of Mo exceeds g / l, sludge is likely 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.

As described above, tartaric acid acts as a reducing agent, but has a low reducing power, and oxalic acid, a reaction product, suppresses the reduction reaction, thereby allowing a large amount of hexavalent molybdenum to be present 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 becomes a large amount in excess of m ² , which is not only 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 plating layer surface. 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. Phosphate ion sources include phosphoric acid and Al
PO 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 weight ratio in terms of P / Mo is 0.2 to 2. The effect of improving the corrosion resistance is exhibited at a P / Mo-converted weight ratio of 0.2 or more, but it is not economical to saturate at a P / Mo-converted weight ratio of 2 and further increase the phosphate ion concentration.

[0016] The reducing ability of the processing solution 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.

Examples of the zinc-based plated steel sheet include a Zn-plated steel sheet and a Zn alloy-plated steel sheet such as Zn-Al, Zn-Mg. Corrosion resistance is improved even when a normal Zn-plated steel sheet is treated, but a Zn alloy-plated steel sheet containing Al, Mg, etc. has a greater corrosion resistance improvement effect. Therefore, Zn
Analysis of the treatment film formed on the surface of the alloy-plated steel sheet revealed that compounds such as Al and Mg were contained in the treatment film. Compounds such as Al and Mg indicate that the plating components were incorporated into the treated film by the reaction between the treatment solution and the plated layer.

Effects of compounds such as Al and Mg on the improvement of corrosion resistance
Al ions and Mg ions
Zn-plated steel sheet using a treatment solution containing
Is also possible. In this case, it is formed on the surface of the Zn plating layer.
Analysis of processed coatings reveals compounds such as Al and Mg
Is done. Compounds such as Al and Mg contained in the film are corrosion resistant
Properties are further improved. For improvement of corrosion resistance, Al and Mg
Reacts with components in the film to form compounds with barrier properties
It is presumed to be derived from that. Al ion is A
lPO_Four , Metal Al, Al (OH)_Three , Al (C_Two O
_Four )_ThreeFor example, Mg ion is MgMoO_Four , Metal M
g, Mg (OH)_Two , MgHPO _Four , MgCO_Three , Mg
(C_Two O_Four ) And so on. Al ion and Mg ion
The effect of improving corrosion resistance by turning on is (Mg + Al) / Mo conversion
(Mg + Al)
If the / Mo equivalent weight ratio exceeds 0.6, the stability of the processing solution may be reduced.
become worse.

[0020]

EXAMPLES Molybdate, phosphate, tartaric acid and oxalic acid were mixed in various ratios to prepare treatment solutions having the compositions shown in Table 1. For comparison, a treatment solution 6 containing no tartaric acid and a treatment solution 7 containing no phosphoric acid were also prepared.

[0021]

As the material to be treated, an electric Zn-plated steel sheet and a hot-dip Zn-Al-Mg alloy-plated steel sheet were used. Electric Zn
The plated steel sheet was prepared by subjecting a weakly deoxidized steel sheet having a thickness of 0.5 mm as a plating base sheet to electroplating with a coating weight of 20 g / m ^{2 in} a usual manner. Fused Zn-Al-Mg
For alloy-plated steel sheet, the same plating base sheet is N ₂ -50% by volume
After reducing and heating in a H ₂ gas atmosphere to activate the surface,
It was introduced into the alloy plating bath Zn-6 wt% Al-3 wt% Mg, and the gas wiping a lifted steel strip from the plating bath to adjust the coating weight to 50 g / m ^2, prepared by naturally cooling did. Apply a treatment solution to each plated steel sheet, place it in an electric oven without washing with water,
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. 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 spraying was continued for a predetermined time, the area ratio of white rust generated on the test piece surface was measured.
10-30% △, 30-50% ▲, 50% or more ×
Was evaluated. As can be seen from the inspection results in Table 3, Test Nos. 1 to 7 in which a film having the composition specified in the present invention was formed.
The zinc-based plated steel sheet of No. showed no generation of white rust even after continuous salt spraying for 72 hours, and was excellent in corrosion resistance. Test No. 11 using a treatment solution to which Al and Mg were added
No. 13 to 13, further excellent corrosion resistance was shown.

On the other hand, Test No. 8,
16. Test Nos. 9.1 and 1 using a treatment solution containing no tartaric acid
7. Test Nos. 10 and 1 using a processing solution containing no phosphoric acid
In No. 8, white rust was generated by spraying salt water for 24 hours. Also,
In Test Nos. 14 and 15 using the original plate containing Al and Mg and the processing solution, no white rust was detected after 24 hours of salt spray, but white rust was generated when salt spray was continued for 72 hours. I was As is clear from this comparison, the galvanized steel sheet treated according to the present invention exhibited excellent corrosion resistance. The corrosion resistance was also good.

[0026]

[0027]

As described above, the zinc-based galvanized steel sheet of the present invention has a low reducing power, and the reaction product has a function of suppressing the reduction of hexavalent molybdenum. A Mo-containing film is formed on the surface of a galvanized layer or a zinc alloy plated layer by treating a galvanized steel sheet or a zinc alloy plated steel sheet with a treatment liquid containing the same. Zinc-based coated steel sheets with Mo-containing coatings have excellent corrosion resistance and paintability comparable to chromate coatings, and the burden on waste liquid treatment is reduced, so chrome-free surface treatment that does not adversely affect the environment Used in a wide range of fields as steel sheets.

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 AA22 BA01 BA03 BB08 CA13 CA16 CA18 CA23 CA26 CA29 CA37 CA38 DA02 DA11 DA15

Claims (8)

[Claims] 1. A molybdenum compound having a Mo equivalent adhesion of 10 to 120 mg / m ² , a phosphoric acid compound having a P / Mo equivalent weight ratio of 0.2 to 2 on a surface of a zinc plating layer or a zinc alloy plating layer. A galvanized steel sheet having excellent corrosion resistance, wherein a film having an amount of an organic compound having a carboxylic acid group attached of 5 to 300 mg / m ² is formed. 2. The coating according to claim 1, further comprising Mg and / or Al in a weight ratio of 0.6 or less in terms of (Mg + Al) / Mo.
A galvanized steel sheet with excellent corrosion resistance as described. 3. A zinc-based plating containing molybdenum oxygenate ion of 1-100 g / l in terms of Mo, phosphate ion having a P / Mo equivalent weight ratio of 0.2-2, and oxycarboxylic acid of 1-300 g / l. Aqueous treatment liquid for steel sheet. 4. A total amount of 1 to 4 with an oxycarboxylic acid.
The aqueous treatment liquid for a galvanized steel sheet according to claim 3, which contains 300 g / l of oxalic acid. 5. The composition according to claim 3, further comprising Mg and / or Al having a weight ratio of (Mg + Al) / Mo reduced to 0.6 or less.
The aqueous treatment liquid for a zinc-based plated steel sheet according to the above. 6. An aqueous treatment liquid containing 1-100 g / l of molybdenum oxyacid ion in terms of Mo, a phosphate ion having a P / Mo equivalent weight ratio of 0.2-2, and oxycarboxylic acid in a range of 1-300 g / l. Is applied to a galvanized steel sheet or a zinc alloy-coated steel sheet at a Mo equivalent adhesion amount of 10 to 120 mg / m ² , and is heated and dried at a sheet temperature of 50 to 200 ° C. without washing with water. Steel sheet surface treatment method. 7. Molybdenum oxygenate ion of 1 to 100 g / l in terms of Mo, phosphate ion having a P / Mo equivalent weight ratio of 0.2 to 2, oxycarboxylic acid of 1 to 300 g / l,
An aqueous treatment liquid containing 0.6 or less of (Mg + Al) / Mo-equivalent weight ratio containing Mg and / or Al is applied to a galvanized steel sheet or a zinc alloy-coated steel sheet with a Mo-equivalent adhesion amount of 10 to 120.
mg / m ² , plate temperature 50-20 without washing with water
A surface treatment method for a galvanized steel sheet having excellent corrosion resistance, characterized by heating and drying at 0 ° C. 8. The composition of claim 1, further comprising a total amount of 1 to
The method according to claim 6 or 7, wherein a treatment solution containing 300 g / l of oxalic acid is used.