JPS63274797A - Production of zn or zn alloy electroplated steel sheet having superior chemical treatability - Google Patents

Abstract

PURPOSE:To improve the chemical treatability of the plated surface of a Zn or Zn alloy electroplated steel sheet by anodizing the plated surface in an electrolytic soln. having a specified compsn. under specified electrical conditions. CONSTITUTION:A Zn or Zn alloy plated steel sheet is immersed in an electrolytic soln. of >=4pH contg. at least one kind of phosphate or sulfate of a metal such as Na, K, Li, Mg or Al and a substance forming a chelate compd. with Ni and Fe ions, e.g., an oxyacid such as citric acid, tartaric acid, lactic acid, malic acid or hydroacrylic acid, a salt thereof or amine, and the plated surface is anodized with >=10C/dm<2> electricity at >=5A/dm<2> current density. Even when Fe and Ni ions increase in the electrolytic soln., the suitability of the Zn or Zn alloy plated surface to phosphating can be improved without requiring the exchange of the electrolytic soln. because the ions form a chelate compd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing Zn-based electroplated steel sheets, and in particular to a post-treatment method for Zn-based electroplated steel sheets with excellent chemical conversion treatment properties on the plated surface. be.

<Conventional technology and its problems> Zn-based electroplated steel sheets can achieve high corrosion resistance without deteriorating the material quality, so they are used in a wide range of fields where cold-rolled steel sheets have traditionally been used, such as automobiles, home appliances, and building materials. is being expanded.

In particular, in recent years, Zn-Ni, Zn
Electric alloy plated steel sheets such as -Fe have been developed, and their application to rust-proof plated steel sheets for automobile bodies is being actively expanded, and production is increasing significantly.

However, currently, the phosphate crystals generated during phosphate chemical treatment on Zn-based electroplated steel sheets are called hopeite (Zn).
n3 (F'04) 2 ・4) 120), this hopeite is the phosphor generated on the cold-rolled steel plate.
phyllite (Zn2 ・Fa (PO4) 2
・Since the adhesion of the paint is inferior to that of 4H20), it is mainly used on the inside of the car body, and there are problems with using it on the outside of the car body.

In order to solve this problem, the present inventors previously proposed a method of subjecting the plated surface to anodic electrolytic treatment in Japanese Patent Application No. 61-089747.

This technology is a method that improves the chemical conversion treatment properties of the plated surface by removing the Zn(OH)2 film, which is an insulating material that is formed on the plated surface, by anode electrolysis treatment when Zn-based electroplating is performed. be.

However, this method has the following problems.

(1) By subjecting the plated surface to anodic electrolytic treatment, the Zn(OH)2 film formed on the plated surface can be removed, and the phosphate chemical conversion treatment properties are improved. However, for example, when Zn or Zn-Ni alloy plated steel sheets are continuously processed in large quantities on an industrial scale, Zn, Ni, etc. peeled off from the plated surface are dissolved in the electrolyte and accumulated.

The anodic electrolytic treatment of the plating surface is continued using an electrolytic solution in which such metal ions, especially Ni ions, are dissolved, until the concentration of these metal ions in the solution is 0.2 g/1. If this occurs, the plated surface will be over-etched, resulting in black dot-like film defects in Zn-based alloy plating and significant film loss in Zn plating, deteriorating the appearance of the plating. is not formed, resulting in film defects.

(2) Therefore, in the conventional method, the electrolyte must be discarded after a certain amount of treatment, which is economically disadvantageous. Additionally, the electrolyte needs to be replaced, resulting in line downtime. This reduces line operation and
This hinders productivity and increases costs.

<Object of the invention> The present invention has been made in view of the above circumstances.
When performing anodic electrolytic treatment on the plated surface of electroplated steel sheets, even if metal ions such as Zn ions and Ni ions are present in the electrolyte, over-etching of the plated surface can occur without replacing the electrolyte. It is an object of the present invention to provide a method for manufacturing a Zn-based electroplated steel sheet that can easily obtain a plated surface with excellent phosphate chemical conversion treatment properties without impairing the plating appearance.

<Structure of the Invention> The present inventors have discovered that when performing anodic electrolytic treatment on the plated surface of a Zn-based electroplated steel sheet, even if metal ions such as Zn ions and Ni ions increase in the electrolyte, over-etching of the plated surface does not occur. As a result of various studies on how to obtain a plated surface with excellent phosphate chemical conversion treatment properties without impairing the appearance of the plating, we found that oxyacids and/or oxyalts or amines in the electrolyte Ni such as
It was discovered that it is extremely effective to add a substance that forms a chelate compound with Fe ions and Fe ions, and this led to the completion of the present invention.

That is, according to the present invention, after applying Zn-based electroplating to a steel sheet, one or more of phosphates or sulfates of Na, Li, Mg, and A° C.
In addition, pi containing a substance that forms a chelate compound with Ni ions and Fe ions (in an electrolyte of 4 or more, with a current density of 5 A/dm2 or more and an electrical quantity of 10 coulombs/dr)
The present invention provides a method for manufacturing a Zn-based electroplated steel sheet with excellent chemical conversion treatment properties, which is characterized by performing anodic electrolysis treatment at a temperature of n'' or more.

Hereinafter, the method for anodic electrolytic treatment of Zn-based electroplated steel sheets having excellent chemical conversion treatment properties according to the present invention will be described in detail.

The Zn-based electroplated steel sheet in the present invention broadly includes a Zn-plated steel sheet and a Zn-based alloy-plated steel sheet mainly containing Zn. For example, Zn-Ni system, Zn-Mn system, Zn-Cr system, Zn-Fe system alloy plating, and furthermore, P, Co, Cr, Sn, Sb, V, Fe.

Those in which one or more of Ti, Ni, MnAs, Bi, etc. are intentionally added or unavoidably mixed, etc.
Any Zn-based alloy or composite plating film is plated in an acidic solution such as a sulfuric acid bath, a chloride bath, or a mixed bath thereof. The electrolyte contains Na, Li, Mg5
At least 1 f of phosphate or sulfate such as Au
The main component is ffi-. Since the deposition potential of the metal ions is higher than the hydrogen reduction potential, the metals do not precipitate in electrolysis of an aqueous solution. This makes it easy to maintain the metal ion concentration in the bath and maintain the cathode.
This is because the acid produced on the anode and the alkali produced on the cathode are balanced in equivalence, which is suitable for maintaining the bath pH.

In addition, sulfate or phosphate was selected because these aqueous electrolyses do not generate harmful gases.

As a feature of the present invention, the substance that forms a chelate compound with Ni ions and Fe ions added to the electrolytic solution includes oxyacids and/or oxyacid salts, amines, and the like.

The oxyacid and/or oxyacid salt to be added to the electrolytic solution include aliphatic oxyacids such as citric acid, tartaric acid, lactic acid, malic acid, hydroacrylic acid, α-oxybutyric acid, glyceric acid, glycolic acid, and tartronic acid. Examples include aromatic oxyacids such as salicylic acid, mandelic acid, trobic acid, gallic acid, m-oxybenzoic acid, p-oxybenzoic acid, and/or their oxyacid salts.

When an oxyacid and/or an alkali salt of an oxyacid is added to the electrolytic solution, a plated surface with excellent chemical conversion treatment properties can be obtained without deteriorating the appearance or chemical conversion treatment properties of the plated surface. This is thought to be due to the fact that various metals are chelated with the oxygen of hydroxyl groups and carboxyl groups to form water-soluble complex salts, which act as masking agents.

The amount of the oxyacid and/or oxyacid salt added is preferably 5 g/fL or more in terms of oxyacid ions.
This is because if the oxyacid ion content is less than 5 g/It, it becomes difficult to clean the plated surface and there is little effect in preventing overetching in the form of black spots on the plated surface. Further, the upper limit is preferably equal to or lower than the solubility of the oxyacid and/or oxyacid.

In addition, examples of amines added to the electrolytic solution instead of the oxyacid include methylamine, ethylamine, propylamine, isopropylamine, butylamine, amylamine, heptylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine. , aliphatic primary amines such as dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, cetylamine, aliphatic tertiary amines and trimethylamine such as dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, cyamylamine, etc. , aliphatic tertiary amines such as triethylamine, tripropylamine, tributylamine, and triamylamine, aliphatic unsaturated amines, alicyclic amines, and aromatic amines.

When an amine is added to the electrolytic solution, a plated surface with excellent chemical conversion treatment properties can be obtained without deteriorating the appearance or chemical conversion treatment properties of the plated surface.The reason why the amine forms a complex salt with metal ions and the metal ions This is thought to be because it chelates metal ions and acts as a masking agent for metal ions.

The amount of the amine added is preferably 2 g/f1 or more. This is because if the amine content is less than 2 g/°C, it becomes difficult to clean the plated surface and there is little effect in preventing overetching in the form of black spots on the plated surface. Further, the upper limit is preferably equal to or lower than the solubility of the amine.

When the pH of the electrolytic solution is 4 or more, it is effective in improving chemical conversion treatment properties. This is because if the pH is less than 4, the chemical dissolution of the plating layer will significantly increase, causing plating cracking and reducing corrosion resistance.

Further, when the current density is 5 A/dm' or more, the chemical conversion treatment property is improved, but when it is less than 5 A/dm2, a sufficient effect cannot be obtained and the process requires a long time, which is not economical.

The upper limit of the current density is preferably 150 A/dm'' or less in consideration of power loss due to increase in electrolytic voltage.

The amount of electricity is 10 kron/drr? The chemical conversion treatment property can be improved with the above, but a sufficient effect cannot be obtained with less than 10 coron/dm'. Although there is no upper limit to the amount of electricity, if the amount of electricity increases, the plating weight will decrease due to metal melting, which is not economical, and it is preferably 500 coron/dm' or less.

When electrolyzing a plated steel sheet, it is possible to improve the chemical conversion properties by setting the polarity to an anode (positive), but if the polarity is set to a cathode (negative), the purpose cannot be achieved. In support of this, the reason why the phosphate treatment properties are improved in the present invention is thought to be as follows, although it is not clear.

For example, it is known that the electrodeposition mechanism of Zn-Ni plating is an anomalous eutectoid type.

The abnormal eutectoid is a phenomenon in which Zn2+ ions, which are difficult to precipitate, are preferentially taken into the deposited layer over Ni+ ions, which are originally easy to precipitate. This is due to the p
As l increases, a Zn(OH)2 film is first formed, and this 2
Electrodeposition occurs through the n(O)I)2 film. Therefore, N
Electrodeposition of t is inhibited, and while the molar ratio of Zn and Ni in the plating bath is Zn/Ni-1/2, the molar ratio of Zn and Ni in the plating layer is Zn/N1=6~ This is because the ratio is reversed.

The formation reaction of the chemical conversion film on the Zn-based electroplated surface is as follows: Zn+ 283 PO4- Zn()12 PO4) 2 + H2↑(init
ation) −・= (1) 3Zn(H2PO4)
2 +482 0 -Zn3 (PO4) 2
4H20+483 PO4"" (2) (chemical conversion film). At the end of the cleaning process, the Zn (OH) 2 film is
Covers the plating surface. Since the Zn (O) I) 2 film does not have electron conductivity, it inhibits metal dissolution in the above reaction +1) and reduces chemical conversion treatment properties.

The Zn(Of()2 film is very thin, and when the plate is wet immediately after plating, it is in a gel state and just covers the plating surface. Therefore, if you apply washing water strongly to the plating surface, it will be removed. section is excluded.

The reason why some parts of the Zn--Ni plating show good chemical conversion treatment properties can be assumed to be that those parts happened to have a small amount of Zn (OH)2 film. However, applying washing water evenly to the entire plated steel sheet makes maintenance and management of the equipment extremely difficult, and it is impossible to obtain stable characteristics.

Here, according to the present invention, by making the steel plate side an anode (positive) in the final section of plating, the following anode reaction mainly occurs.

H20→1/2 02 ↑+2tl” + 2e-・-
(3)”in → Zn2+ + 2e
-... (4) Therefore, Zn (
O)l)2 is removed from the plating surface simultaneously with the dissolution of oxygen gas and Zn from the underlying layer.・ The above reaction occurs on the entire surface of the steel sheet, and the Zn (OH
) A Zn-Ni plated steel sheet from which 2 has been uniformly removed is obtained. On the surface with less Zn (DH)2, the initial chemical conversion reaction of metal dissolution occurs from the entire steel sheet, and a large number of initial crystal nuclei are generated. Initial crystals grow, but because they are numerous,
By mutually inhibiting growth, dense chemical crystals can be obtained.

That is, by using the steel plate side as the anode, a dense chemically formed crystal can be obtained.

When the steel plate side is made the cathode (negative), hydrogen gas is generated as a cathode reaction, and if metal ions exist as impurities, they precipitate, so there is sufficient Zn (OH)2
The film is not removed; rather, the electrodeposited metal causes uneven chemical conversion, and chemical conversion treatment properties are not improved.

The steel used in the present invention is not limited in type or size, and may be pretreated by conventional methods such as degreasing, pickling, and water washing.

Furthermore, after these pretreatments are followed by Zn-based electroplating, an anodic electrolytic treatment may be performed based on the method of the present invention.

Conventionally known methods such as horizontal type, vertical type, and radial cell can be used as the anodic electrolysis treatment tank, and the liquid temperature is between room temperature and
It can be arbitrarily selected between 70°C.

After completing the anodic electrolytic treatment, the plated steel sheet is washed with water, dried, and made into a product.

<Example> Next, the present invention will be specifically explained based on examples.

In each of the following examples, the plate thickness is 0.7 mm.
It was plated using a cold-rolled steel plate equivalent to 5 pcc.

[Example 1 (1) Zn-Ni bath composition NiSO4/6) 120
250 g/ILZnSO4+ 7H20130g/f
lNa2504 40 g/It (2) pH 1,6 (3) Bath temperature 60°C (4)
Current density 50 A/drd (5
) Plating time: 15 seconds (6) Plating size: After electrolytic Zn-Ni alloy plating was performed on a cold-rolled steel sheet under the plating conditions of 20 g/m', immediately rinsed with water, and continued as shown in Table 1 while the sheet surface was still wet. The plated surface was subjected to anodic electrolysis under the following conditions, and then washed with water and dried.

The appearance evaluation of the membrane surface after electrolytic treatment was as follows by visual measurement.

O... Uniform, no black dot-like over-etching △...
・There is over-etching in the form of black dots in some areas.
The chemical conversion treatment was performed using the p method. The chemical conversion coating was evaluated visually and by scanning electron microscopy in terms of uniformity, presence or absence of coating defects, crystal density, etc., and the coating weight was determined by gravimetric method (using ammonium chromate solution).

The evaluation criteria are as follows.

○: Uniform and dense crystal film Δ: Some dot-like film defects, or crystals are uneven ×: There are dot-like film defects on the entire surface The results are shown in Table 1.

[Example 2] (1) Zn bath composition 1nsO4・0120 400
g/ ANa2 S04 35 g
/ fl (2) pH2 (3) Bath temperature 55℃ (4)
Current density 40 A/d m” (5
) Plating time: 20 seconds (6) Plating size: After electroplating a cold rolled steel sheet with Zn under the plating conditions of 20g/lr1'', immediately rinse with water, and continue to coat the plated surface under the conditions shown in Table 2 while the sheet surface is still wet. After that, it was washed with water and dried.The electrolytic treatment solution was obtained by performing the Zn plating after the Zn--Ni plating treatment, and Ni ions were mixed in the treatment solution.

The test methods for the appearance of the plated surface after electrolytic treatment and chemical conversion treatment properties were the same as in Example 1, and the evaluation criteria were as follows.

Evaluation of plating surface appearance after electrolytic treatment: O...uniform, no film missing due to over-etching △...some missing parts of the film due to over-etching, or uneven crystals ×...over-etching over almost the entire surface Appearance evaluation of chemical conversion coating with coating missing: ○... Uniform and dense crystalline coating △... Slightly missing parts of the coating ×... The results are shown in Table 2, with coating missing on almost the entire surface.

<Effects of the Invention> As detailed above, according to the present invention, Z
Even if metal ions such as n, Ni, and Fe are present, over-etching that occurs on the plated surface is prevented, and a plated surface with dense and fine phosphate crystals with excellent appearance after chemical conversion treatment can be obtained.

Furthermore, according to the present invention, a Zn-based electroplated steel sheet with excellent chemical conversion treatment property on the plated surface can be obtained without replacing the electrolyte, so that the Zn-based electroplated steel sheet with high line operation rate and high productivity can be obtained. This has the effect of providing a manufacturing method.

Claims (1)

[Claims] (1) After applying Zn-based electroplating to the steel plate, Na, K,
1 of phosphates or sulfates of Li, Mg and Al
A current density of 5 A/dm^2 or more and a quantity of electricity of 10 coulombs/dm^2 or more in an electrolytic solution with a pH of 4 or more containing the species or two or more species and a substance that forms a chelate compound with Ni ions and Fe ions. 1. A method for manufacturing a Zn-based electroplated steel sheet with excellent chemical conversion treatment properties, which comprises performing an anodic electrolytic treatment.