JPH0663113B2 - Method for producing Zn-based electroplated steel sheet having excellent chemical conversion treatability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a Zn-based electroplated steel sheet, and more particularly to a post-treatment method for a Zn-based electroplated steel sheet having excellent chemical conversion treatability on the plated surface. Is.

<Prior art and its problems> Zn-based electroplated steel sheets can be used in a wide range of fields where cold-rolled steel sheets have been used, such as automobiles, home appliances, and building materials, because they can achieve high corrosion resistance without degrading the material quality. Is being expanded.

In particular, in recent years, Zn-Ni, Zn-Fe, and other galvanized steel sheets have been developed for the purpose of improving corrosion resistance, and the application of galvanized steel sheets for rust prevention to automobile bodies has been actively expanded, resulting in a significant increase in production. is doing.

However, at present, the phosphate crystals generated during the phosphate chemical conversion treatment on the Zn-based electroplated steel sheet are hopeite (Zn ₃ (P
In _{O 4) 2 · 4H 2 O} ), the hopeite is produced on cold rolled steel.

Since the adhesiveness of the paint is inferior to that of phosphophyllite (Zn ₂ · Fe (PO ₄ ) ₂ / 4H ₂ O), there is a problem in using it mainly on the inner surface of the vehicle body and on the outside of the vehicle body. .

In order to solve this problem, the present inventors have previously filed a patent application
In No. 61-089747, a method of anodizing the plated surface was proposed.

This technology is a method of removing Zn (OH) ₂ film, which is an insulating material formed on the plating surface by anodically electrolytic treatment when performing Zn-based electroplating, and improving the chemical conversion treatment property of the plating surface. is there.

However, this method has the following problems.

(1) By subjecting the plated surface to an anodic electrolytic treatment, the Zn (OH) ₂ film formed on the plated surface can be removed, and the phosphate conversion treatment is improved. However, for example, Zn or Zn−
When a large amount of Ni-alloy-plated steel sheet is continuously processed on an industrial scale, Zn, Ni, etc. separated from the plated surface are dissolved and accumulated in the electrolytic solution.

When anodically electrolytic treatment of the plating surface is continued using an electrolytic solution in which such metal ions, especially Ni ions are dissolved, and the concentration of these metal ions in the solution becomes 0.2 g / or more, the plating surface is over-etched and Zn In the system alloy plating, a black dot-like film defect occurs, and in Zn plating, a significant film loss occurs, the plating appearance deteriorates, and a phosphate chemical conversion film is not formed in those places, resulting in a film defect.

(2) Therefore, the conventional method is economically disadvantageous because the electrolytic solution must be discarded after a certain amount of treatment. Further, it is necessary to replace the electrolytic solution, which causes downtime of the line. As a result, the line operation is reduced, which hinders productivity and increases costs.

<Purpose of the Invention> The present invention has been made in view of the above circumstances, and when anodizing the plated surface of a Zn-based electroplated steel sheet,
Even if metal ions such as Zn ions and Ni ions are present in the electrolytic solution, it is possible to prevent the occurrence of over-etching of the plated surface without replacing the electrolytic solution, and without degrading the appearance of the plating, and phosphating. An object of the present invention is to provide a method for producing a Zn-based electroplated steel sheet that can easily obtain a plated surface with excellent processability.

<Structure of the Invention> The inventors of the present invention, when anodically electrolytically treating the plated surface of a Zn-based electroplated steel sheet, overetch the plated surface even if metal ions such as Zn ions and Ni ions increase in the electrolytic solution. As a result of various studies on a method for preventing the occurrence of rusting and without impairing the plating appearance and obtaining a plated surface having excellent phosphate conversion treatment properties, oxyacids and / or oxyacid salts or amines were found in the electrolytic solution. Ni-ion, such as
The inventors have found that it is extremely effective to add a substance that forms a chelate compound with Fe ions, and have completed the present invention.

That is, according to the present invention, after subjecting a steel sheet to Zn-based electroplating, one or more of Na, K, Li, Mg and Al phosphates or sulfates, and further Ni ions and Fe. PH containing ions and substances that form chelate compounds
Manufacture of Zn-based electroplated steel sheet with excellent chemical conversion treatability, which is characterized by performing anodic electrolytic treatment at a current density of 5 A / dm ² or more and an electric quantity of 10 coulomb / dm ² or more in an electrolyte solution of 4 or more. It provides a method.

Hereinafter, the anodic electrolytic treatment method of the Zn-based electroplated steel sheet having excellent chemical conversion treatability of 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-based alloy plating, as well as P, Co, Cr, Sn, S
One or two of b, V, Fe, Ti, Ni, Mn, As, Bi, etc.
Intentionally added or inevitably mixed more than one kind,
Further, any Zn-based alloy or composite plating film such as those in which particles of alumina, silica, titania, chromate, etc. are dispersed is plated with an acidic liquid such as a sulfuric acid bath, a chloride bath or a mixed bath thereof. It is a thing. The electrolytic solution contains, as a main component, at least one kind of phosphate or sulfate such as Na, K, Li, Mg and Al. Since the deposition potential of the metal ion is lower than the hydrogen reduction potential, the deposition of the metal does not occur in the electrolysis of the aqueous solution. Therefore, the maintenance of the metal ion concentration in the bath and the maintenance of the cathode are easy, and the acid produced on the anode and the alkali produced on the cathode are balanced in an equivalence relation, which is also suitable for maintaining the bath pH. Because there is.

Further, the sulfate or phosphate is selected because no harmful gas is generated in the electrolysis of these aqueous solutions.

Then, as a feature of the present invention, it is added to the electrolytic solution
Examples of the substance that forms a chelate compound with Ni ions and Fe ions include oxyacids and / or oxyacid salts and amines.

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

When an oxyacid and / or an alkali salt of oxyacid is added to the electrolytic solution, a plating surface having excellent chemical conversion treatability can be obtained without deteriorating the appearance of the plating surface and chemical conversion treatability. However, it is considered that various metals are chelated with oxygen of hydroxyl group and carboxyl group to form a water-soluble complex salt and act as a masking agent.

The addition amount of the oxyacid and / or oxyacid salt is preferably 5 g / or more as oxyacid ion. This is because if the amount of oxyacid ions is less than 5 g /, it will be difficult to clean the plated surface and the effect of preventing black dot-like over-etching generated on the plated surface will be small. Further, the upper limit is preferably not more than the solubility of oxyacid and / or oxyacid salt.

Further, in place of the oxyacid, amines added to the electrolytic solution include methylamine, ethylamine, propylamine, isopropylamine, butylamine, amylamine, hexylamine, heptylamine, octylamine, nonylamine, decylamine, undecylamine. ,
Aliphatic primary amines such as dodecylamine, tridecylamine, tetradecylamine, pentadecylamine, cetylamine, dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, diamylamine and other aliphatic secondary amines, and trimethylamine, Examples thereof include 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 plating surface having excellent chemical conversion treatability can be obtained without deteriorating the appearance and chemical conversion treatability of the plated surface because the amine forms a complex salt with a metal ion to form a metal ion. It is believed that this is because it chelates and acts as a masking agent for metal ions.

The addition amount of the amine is preferably 2 g / or more. This is because when the amount of amine is less than 2 g /, it becomes difficult to clean the plated surface, and the effect of preventing black dot-like overetching that occurs on the plated surface is small. Further, the upper limit is preferably the solubility of amine or less.

When the pH of the electrolytic solution is 4 or more, it is effective in improving the chemical conversion treatability. This is because if the pH is less than 4, the chemical dissolution of the plating layer remarkably increases and plating cracks occur, resulting in a decrease in corrosion resistance.

Further, when the current density is 5 A / dm ² or more, the chemical conversion treatability is improved, but when the current density is less than 5 A / dm ² , a sufficient effect cannot be obtained.
In addition, the process requires a long time, which is not economical.

The upper limit of the current density is preferably 150 A / dm ² or less in consideration of power loss due to increase in electrolysis voltage.

If the amount of electricity is 10 coulombs / dm ² or more, the chemical conversion treatability can be improved, and if it is less than 10 coulombs / dm ² , a sufficient effect cannot be obtained. There is no upper limit on the amount of electricity, but if the amount of electricity increases,
The amount of coating weight decreases due to metal dissolution, which is not economical and is preferably 500 coulomb / dm ² or less.

When electrolyzing a plated steel sheet, the polarity is positive (plus) to improve the chemical conversion processability, and if the polarity is negative (minus), the purpose cannot be achieved. As a proof of this, the reason why the phosphate treatment property in the present invention is improved is not clear, but it is considered as follows.

For example, the electrodeposition mechanism of Zn-Ni plating is known to be an abnormal eutectoid type.

Anomalous eutectoid is a phenomenon in which Zn ^{2 +} ions, which are hard to deposit, are preferentially incorporated into the electrodeposited layer over Ni ⁺ ions, which are likely to deposit. This is because the pH of the steel surface rises when electric current starts and a Zn (OH) ₂ film is formed first, and electrodeposition occurs through this Zn (OH) ₂ film. Therefore, Ni electrodeposition is hindered,
The molar ratio of Zn and Ni in the plating bath is Zn / Ni = 1/2, whereas the molar ratio of Zn and Ni in the plating layer is Zn / Ni = 6 to 7
Because the ratio is reversed.

The formation reaction of the chemical conversion coating on the Zn-based electroplating surface is as follows: Zn + 2H ₃ PO ₄ → Zn (H ₂ PO ₄ ) ₂ + H ₂ ↑ (initiation) (1) 3Zn (H ₂ PO ₄ ) ₂ + 4H ₂ O → Zn ₃ (PO ₄ ) ₂ 4H ₂ O + 4H ₃ PO ₄ (2) (Chemical conversion film). At the end of plating, the Zn (OH) ₂ coating covers the plating surface. Since the Zn (OH) ₂ coating does not have electron conductivity, it inhibits the metal dissolution in the above reaction (1) and lowers the chemical conversion treatability.

The Zn (OH) ₂ film is very thin, and when the plate immediately after plating is wet, it is gel-like and only covers the plating surface. Therefore, if washing water is strongly applied to the plating surface, a part of it is removed.

It can be inferred that some of the Zn-Ni platings with good chemical conversion treatability happened to be the portions with a small amount of Zn (OH) ₂ coating. However, it is extremely difficult to maintain and manage the equipment and it is impossible to obtain stable characteristics if the washing water is evenly applied to the entire plated steel sheet.

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

_{H 2 O → 1/2 O} 2 ↑ + 2H + + 2e - ...... (3) Zn → Zn 2 + + 2e - ...... (4) Accordingly, the Zn _{(OH) 2} which is an insulating material, from the oxygen gas and the lower layer It is removed from the plating surface at the same time as the dissolution of Zn.

The above reaction occurs on the entire surface of the steel sheet, and a Zn-Ni plated steel sheet from which Zn (OH) _{2 on} the surface is uniformly removed is obtained. Zn
_On the surface with a small amount of (OH) _2, the initial reaction of chemical conversion treatment of metal dissolution occurs from the entire steel sheet, and a large number of initial crystal nuclei are generated. Although the initial crystals grow, they are many in number, so that growth is mutually inhibited and a dense chemical crystal is obtained.

That is, by using the steel sheet side as the anode, it is possible to obtain a dense chemical conversion crystal.

When the steel plate side is made the cathode (minus), hydrogen gas is generated as a cathode reaction and metal ions as impurities are precipitated if they exist, so the Zn (OH) ₂ film is not sufficiently removed, and rather the electrodeposited metal is formed. It causes unevenness and the chemical conversion processability is not improved.

The steel material used in the present invention is not limited to its type and size, and may be subjected to pretreatment such as degreasing, pickling, and water washing according to a conventional method.

In addition, after these pretreatments, Zn-based electroplating is performed, and then anodic electrolytic treatment may be performed based on the method of the present invention.

As the anodic electrolytic treatment tank, conventionally known methods such as horizontal type, vertical type and radial cell can be applied, and the liquid temperature is from room temperature to
It can be arbitrarily selected between 70 ° C.

After finishing the anodic electrolytic treatment, the plated steel sheet is washed with water and dried to obtain a product.

<Examples> Next, the present invention will be specifically described based on Examples.

In each of the following examples, SPCC with a plate thickness of 0.7 mm is used.
It was plated using a corresponding cold rolled steel sheet.

[Example 1] (1) Zn-Ni bath composition _{NiSO 4 · 6H 2 O 250g /} ZnSO 4 · 7H 2 O 130g / Na 2 SO 4 40g / (2) pH 1.6 (3) bath temperature 60 ° C. (4) current density 50A / dm ^{2 (5)} plating time 15 sec (6) after the electric Zn-Ni alloy plated cold rolled steel sheet by plating conditions of the plating basis weight 20 g / m ^2,
Immediately, the plate surface was washed with water, the plate surface was still wet, and then the anodizing treatment of the plated surface was carried out under the conditions shown in Table 1, followed by washing with water and drying.

The visual evaluation of the plated surface after electrolytic treatment is as follows by visual measurement.

〇… Uniform and no black dot over-etching △… Partly black dot over-etching ×… Overall black dot over-etching After this, a phosphate treatment liquid Granodine SD manufactured by Nippon Paint
Chemical conversion treatment was performed using 2000 (Dip method). The chemical conversion film is evaluated by visual observation and scanning electron microscope observation for uniformity, presence or absence of film defects, and crystal density.
The coating amount was determined by the gravimetric method (using an ammonium chromate solution).

The evaluation criteria are as follows.

◯: Uniform and dense crystal film Δ: Slight dot-like film defects or uneven crystal X: Dotted film defects on the entire surface Table 1 shows the results.

[Example 2] (1) Zn bath composition ZnSO ₄ 7H ₂ O 400 g / Na ₂ SO ₄ 35 g / (2) pH 2 (3) Bath temperature 55 ° C (4) Current density 40 A / dm ² (5) Plating Time 20sec (6) Cold-rolled steel sheet is electro-Zn plated under the plating condition of coating weight of 20g / m ² and immediately rinsed with water, and the anodic electrolysis of the plated surface is continued under the conditions shown in Table 2 while the plate surface remains wet. After treatment, it was washed with water and dried. The electrolytic treatment liquid is the above-mentioned Zn after the Zn-Ni plating treatment.
As the plated product, Ni ions were mixed in the treatment liquid.

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

Evaluation of plating surface appearance after electrolytic treatment: 〇… uniform and no film loss due to overetching △… some lack of film due to overetching,
Or, the crystals are non-uniform ..... The film is missing due to over-etching on almost the entire surface. Chemical film appearance evaluation: ◯ ... Uniform and dense crystalline film ..... Some of the film defects .... The film is missing on almost the entire surface. .

<Effects of the Invention> As described above in detail, according to the present invention, Zn in the electrolytic solution,
Even if metal ions such as Ni and Fe are present, over-etching that occurs on the plated surface is prevented, the appearance after chemical conversion treatment is excellent, and a dense and fine phosphate crystal plated surface is obtained.

Further, according to the present invention, since it is possible to obtain a Zn-based electroplated steel sheet having excellent chemical conversion treatability on the plated surface without exchanging the electrolytic solution, the line operation rate is high and the productivity is high.
There is an effect that a method for manufacturing a Zn-based electroplated steel sheet can be obtained.

Claims (1)

[Claims] 1. A steel sheet is plated with Zn-based electroplating, and then Na,
Current density in an electrolyte solution of pH 4 or more containing one or more of K, Li, Mg and Al phosphates or sulfates, and Ni ion and Fe ion and a substance that forms a chelate compound. Is a 5 A / dm ² or more and an electric quantity of 10 Coulomb / dm ² or more, and anodically electrolytic treatment is performed.