JPH11106984A - Method for producing Zn-Ni-based electroplated steel sheet excellent in phosphatability and water resistance - Google Patents

Abstract

(57) [Summary] [PROBLEMS] When a Zn-Ni plated steel sheet is subjected to a phosphate treatment, even if the chemical conversion treatment conditions are deteriorated, the chemical conversion defects such as non-uniform chemical formation and coarse formation of the chemical conversion crystal, poor water adhesion resistance, etc. Provided is a method for producing a Zn—Ni-based electroplated steel sheet in which performance degradation does not occur. SOLUTION: After subjecting a steel plate to Zn-Ni-based electrogalvanizing with a Ni content of 8 to 16 wt% and an adhesion amount of 10 to 60 g / m ² , one or both of a bilophosphate and a tripolyphosphate are removed. A total of 5 to 50 g / L of a weak alkaline solution having a pH of 8 or more and less than 11 is brought into contact with a solution in which Ni ions are added in an amount of 0.1 g / L or more and 10 g / L or less. Precipitate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Zn-Ni-based electroplated steel sheet having excellent phosphatability and water resistance.

[0002]

2. Description of the Related Art Zn-Ni electroplated steel sheets have corrosion resistance,
Because it satisfies the properties such as workability and weldability at a high level,
It is used in large quantities as rust-proof steel sheets for automobile bodies. However, the surface of the Zn-Ni-based electroplating is more inert than pure zinc plating, and the formation of a non-uniform chemical conversion film (chemical conversion) that is visually identified in a phosphate treatment usually performed as a pretreatment for painting a car body. Irregularities) or coarse phosphate crystal formation identified by a microscope may be observed.

[0003] In particular, when the chemical conversion treatment solution has deteriorated with time or when the pretreatment is insufficiently degreased, such a chemical conversion failure frequently occurs. If the chemical conversion is poor, the adhesion after coating, especially the secondary adhesion to water, may deteriorate, which is a major problem in quality.

[0004] Therefore, there has been a demand for a Zn-Ni-based electroplated steel sheet which does not cause chemical conversion failure even if the chemical conversion treatment conditions are degraded, and many studies as described below have been made to meet such a demand.

JP-A-58-45382 discloses Zn
A technique is disclosed in which a Ni-based alloy is electroplated and then brought into contact with an acidic solution having a pH of 2.5 or less for 5 seconds or more. According to this technique, the surface is etched by the acid solution to release the internal stress of the plating layer, and fine cracks are generated, thereby facilitating dissolution of Zn in the chemical conversion treatment and forming dense phosphate crystals. It is said that you can.

Japanese Unexamined Patent Publication No. 58-210194 discloses that a surface oxide of a Zn—Ni-based electroplating is removed by anodic electrolysis, cathodic electrolysis, a combination thereof, immersion, or the like in an acidic, neutral, or alkaline solution. Discloses a technique for improving the chemical conversion property and the water resistance.

JP-A-63-93879 discloses Zn
-Ni-based electroplated surface layer of the Zn-rich layer Zn-Ni plating solution, dilute hydrochloric acid, by 0.01 to 1 g / m ² dissolved in dilute sulfuric acid, a technique for improving chemical convertibility, the water adhesion is disclosed .

Japanese Patent Application Laid-Open No. 61-91391 discloses a lower Zn-Ni plating (Ni 8 to 16%, 5 to 50 g /
m ² ), an upper layer Zn-Ni plating (Ni 16 to 80%, 0.5 to 5 g / m ² ) with an increased Ni content is performed to dissolve Ni during the chemical conversion treatment to improve water adhesion. Techniques are disclosed.

JP-A-2-70088 discloses that Zn-
0.1 current density 1 to 10 A / dm ² on Ni plating
A technique for plating at g / m ² is disclosed. According to this technique, it is said that the conversion film becomes uniform because the amount of zinc hydroxide remaining on the surface is small and uniform in plating at a low current density.

[0010]

However, the above-mentioned prior art is not necessarily a reasonable technology for preventing formation failure, such as dissolving a plated film or performing plating at a low current density with low productivity. The effect is not sufficient.

The present invention has been made in view of such circumstances, and when a Zn—Ni-plated steel sheet is subjected to a phosphate treatment, even if the chemical conversion treatment conditions are deteriorated, the chemical conversion unevenness and the formation of the chemical conversion crystal are increased. It is an object of the present invention to provide a method for producing a Zn—Ni-based electroplated steel sheet in which performance deterioration such as poor chemical conversion and poor water adhesion does not occur.

[0012]

In order to solve the above-mentioned problems, the present invention provides a Zn-Ni-based electrogalvanized steel sheet having a Ni content of 8 to 16 wt% and an adhesion amount of 10 to 60 g / m ² on a steel sheet. After adding Ni ion to a weak alkaline solution having a pH of 8 or more and less than 11 containing Ni-phosphate and / or tripolyphosphate in an amount of 5 to 50 g / L, Ni ions are added in an amount of 0.1 g / L to 10 g / L. A method for producing a Zn-Ni-based electroplated steel sheet having excellent phosphatability and water-resistant adhesion, characterized in that metallic Ni is deposited on the outermost surface of a plating layer by contacting the solution with a immersion liquid. .

[0013] In the case where non-uniform formation or poor water-resistant adhesion is caused by Zn-Ni electrogalvanizing, the formed crystals become coarse. This coarsening occurs when the surface reactivity in the chemical conversion treatment is low and the density of the chemical conversion crystal nuclei is low. Therefore, it is considered that if factors that inhibit the surface reactivity are removed and a substance that becomes a chemical conversion nucleus is added to the surface, the chemical conversion crystal becomes finer as a whole, and it is possible to prevent chemical conversion unevenness and poor water adhesion.

The present inventors have repeatedly studied such a technique for modifying the surface of Zn-Ni plating, and as a result, have found that Zn-N
After i-plating, it is immersed for a short time in a solution obtained by dissolving a small amount of Ni in a weak alkaline solution containing either pyrophosphate salt such as sodium pyrophosphate and / or tripolyphosphate such as sodium tripolyphosphate as a main component. It has been found that the formation of fine particles of the chemical conversion does not cause the occurrence of non-uniform chemical conversion and the deterioration of the water-resistant adhesion.

[0015] A weak alkaline solution containing one or both of bilophosphate and tripolyphosphate as a main component is made of Zn.
-When Ni plating is immersed, Zn forms a complex with these pyrophosphoric acid and tripolyphosphoric acid and dissolves. However, since the dissolving action is milder than that of an acidic solution or a strong alkaline solution, Zn is mainly oxidized on the surface. It is believed that the membrane is dissolved. N dissolved at the same time as this dissolution reaction (oxidation reaction)
The i-ions are reduced and deposited in a trace amount uniformly on the Zn-Ni plating surface. The precipitated Ni provides chemical conversion crystal nucleus generation sites during the chemical conversion treatment, and generates fine chemical conversion crystals.
Since such fine chemical conversion crystals are uniformly generated, it is considered that non-uniform chemical conversion and poor water-resistant adhesion do not occur.

This treatment can be carried out in a very short time, and does not use an acidic solution or an alkaline solution as in the prior art.
In addition, there is no need for electrolytic treatment required in a part of the prior art, and the amount of Ni to be deposited is very small, so that there is an advantage that the treatment cost is extremely low.

[0017]

Embodiments of the present invention will be described below. In the present invention, first, N
A Zn—Ni-based electrogalvanizing with an i content of 8 to 16 wt% and an adhesion amount of 10 to 60 g / m ² is performed. Here, the reason why the Ni content of the plating layer is set to 8 to 16 wt% is that the corrosion resistance becomes the best in the Ni content region. Further, the reason why the adhesion amount is set to 10 to 60 g / m ² is that 10 g / m ²
If it is less than m ² , the corrosion resistance for automotive body applications will not be sufficiently satisfied, and if it exceeds 60 g / m ² , the production cost will be high, and workability, welding accuracy, etc. will be deteriorated. Further, the type of the base steel sheet is not particularly limited, and generally, all steel sheets to which Zn-Ni-based electrogalvanizing is applied can be used.

Next, the thus formed Zn-N
Ni deposition treatment is performed on the i-based electrogalvanized layer. In this treatment, the plating layer surface is coated with 5 to 50 g / L of either or both of bilophosphate and tripolyphosphate.
Containing a weak alkaline solution having a pH of 8 or more and less than 11
This is performed by bringing the liquid into contact with a liquid to which i-ion is added in an amount of 0.1 g / L or more and 10 g / L or less. Here, a sodium salt, a potassium salt, and an ammonium salt can be used as the pyrophosphate and the tripolyphosphate. That is, sodium pyrophosphate, potassium pyrophosphate, ammonium pyrophosphate, sodium tripolyphosphate, potassium tripolyphosphate, and ammonium tripolyphosphate can be used.

Here, either or both of the bilophosphate and the tripolyphosphate in the solution were used mainly because of the mild dissolution of the solution, the Zn on the surface of the plating layer was mainly used.
This is because the oxide film is dissolved. In addition, these concentrations are 5
The reason for setting to 50 g / L is that if it is less than 5 g / L, the effect of dissolving the zinc oxide film on the plating surface decreases,
This is because the effect of dissolving i becomes weaker, while if it exceeds 50 g / L, the above-mentioned effect is saturated, which is not preferable in terms of economy and detergency.

The pH of this solution is substantially determined by the amount of added bilophosphate and tripolyphosphate, and the pH is in the range of 8 to 11 unless an acid or alkali is added. acid·
When an alkali is added, a precipitate that is considered to be a Ni compound is generated when the pH is less than 8, and conversely, when the pH exceeds 12, the effect of Ni is not exhibited. Although the cause is not clear, it is considered that Ni ions are stabilized and do not precipitate on the Zn-Ni surface.

The solution contains Ni ions in an amount of 0.1 g / L or more.
The addition in the range of 0 g / L or less means that Zn-
This is because a small amount of Ni is uniformly deposited on the surface of the Ni plating layer. If the amount of Ni ions is less than 0.1 g / L, the amount of Ni deposited is insufficient, and if it exceeds 10 g / L, the effect is saturated, which is economically undesirable. Ni ion is 5g /
If it exceeds L, a precipitate considered to be a Ni compound is formed, so that the content is preferably 5 g / L or less. In order to form Ni ions, Ni sulfate or the like is added to the Ni treatment liquid. As the salts at this time, any salts that can be dissolved in the Ni treatment liquid can be used.

[0022]

【Example】

Example 1 First, the following Zn-Ni electroplating was applied to a cold-rolled steel sheet under the following plating bath composition and plating conditions.

(1) Plating bath composition Zinc sulfate: 100 to 200 g / L, nickel sulfate: 30
0 to 400 g / L, sodium sulfate: 60 g / L (2) Plating conditions pH: 1.5, temperature: 60 ° C., flow rate: 2 m / s, current density: 50 to 130 A / dm ² , (3) plating film Ni Content: 8 to 16 wt%, plating amount: 20 to 30 g
/ M ² ,

Subsequently, surface treatment and Ni precipitation treatment were performed with the treatment liquids shown in Table 1. At this time, nickel sulfate was added to the treatment liquid as a Ni source (shown as the Ni concentration in Table 1). Then immersion type phosphate treatment (treatment time 2
) And visually evaluate the presence or absence of formation unevenness, and evaluate the crystal morphology and size using a scanning electron microscope. Further, a cationic electrodeposition coating (20 μm) and a melamine alkyd coating as a middle / overcoat are applied at a thickness of 40 μm, respectively. Number evaluation). The above evaluation results are also shown in Table 1. In addition, Table 2 shows the evaluation criteria for non-uniformity of formation, crystal size, and water resistance.

[0025]

[Table 1]

[0026]

[Table 2]

As is apparent from Table 1, Examples 1 to 19
In all of the samples, chemical conversion unevenness did not occur, the phosphate crystals were fine, and the water-resistant adhesion was good. On the other hand, in Comparative Examples 1 to 4, chemical conversion unevenness was observed, the phosphate crystals were coarse, and the water resistance was poor.

Comparative Example 1 is a case of an alkaline treatment liquid having a pH of 12, and it is considered that Ni did not precipitate. In this pH range Ni is HNiO ₂ ^- it is considered to be present stably in the form of, believed substitution plating reaction with zinc did not occur.

Comparative Example 2 was a case of a weakly acidic liquid, and although the chemical conversion unevenness was slightly improved, the phosphate crystals were coarse and the water resistance was not improved. Comparative Examples 3 and 4 are N
In the case where i was not added, non-uniform chemical formation was observed, the phosphate crystal was coarse, and the water adhesion was poor. Also,
Comparative Example 5 is a result in which the amount of Ni added is 0.01 g / L,
Since the amount of Ni precipitation is small, slight formation unevenness is observed,
The phosphate crystals were also slightly coarse, and the water adhesion was insufficient.

[0030]

As described above, according to the present invention,
After performing Zn-Ni plating, a Ni deposition process is performed by a predetermined solution to uniformly deposit a small amount of Ni on the surface of the plating layer.
Even if the chemical conversion treatment conditions are deteriorated, there is no performance deterioration such as poor chemical formation such as non-uniform chemical formation or coarse formation of chemical conversion crystals and poor water adhesion resistance.

Claims (1)

[Claims] After subjecting a steel sheet to Zn-Ni-based electrogalvanizing with a Ni content of 8 to 16 wt% and an adhesion amount of 10 to 60 g / m ² , one or both of a bilophosphate and a tripolyphosphate are applied. Is brought into contact with a solution in which Ni ions are added in an amount of 0.1 g / L or more and 10 g / L or less to a weak alkaline solution having a pH of 8 or more and less than 11 containing 5 to 50 g / L in total, so that metallic Ni is formed on the outermost surface of the plating layer. A method for producing a Zn—Ni-based electroplated steel sheet having excellent phosphatability and water-resistant adhesion, characterized in that Zn is precipitated.