JPH08109493A - Production of highly corrosion resistant galvannealed steel sheet - Google Patents

Abstract

PURPOSE: To provide a production method of a galvannealed steel sheet for corrosion prevention excellent in corrosion resistance and that after coating. CONSTITUTION: In this production method of the galvannealed steel sheet containing chromium by immersing the steel sheet in an acidic bath containing Zn<2+> ion and Cr<3+> ion, an org. additive having conjugated double bonds is added to the plating bath by more than 0.1 times that of the Cr<3+> ion concn. The bath is excellent in plating processability and corrosion resistance, a deposited chromium amount after long term power supply is hardly changed, the stability of the bath is improved and the management of the bath becomes easy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anti-corrosion galvanized steel sheet having excellent corrosion resistance and corrosion resistance after painting, which is used for automobiles, home appliances, building materials, etc., and more particularly to a high corrosion resistance zinc electroplated steel sheet containing chromium. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Conventionally, electrogalvanized steel sheets have been widely used mainly for automobiles, home appliances, building materials and the like because of their excellent corrosion resistance. Recently, the demand for higher corrosion resistance has increased, and zinc-based alloy plating containing chromium has been proposed to meet this demand.

An example of a technique for depositing chromium together with zinc is disclosed in, for example, JP-A-61-270398 and JP-A-4-36495.
However, these techniques have a small chromium content and have not reached the point of superior corrosion resistance to Zn-Ni alloy plating or Zn-Fe alloy plating.

The reason why it is difficult to contain chromium as described above is that Cr ³⁺ ions form a very stable hydrated complex in an aqueous solution. It is said to be difficult.

The Cr ³⁺ ion is a transition metal which is in various equilibrium states with mononuclear, binuclear and polynuclear in an acidic aqueous solution, and depending on conditions such as temperature, concentration and the presence of other metal ions, Cr ³⁺ is present. It is known that ³⁺ state change is promoted. These phenomena make electrodeposition of zinc alloys containing chromium difficult and hinder industrial production.

With respect to these problems, JP-A-3-1203
In the technique disclosed in Japanese Patent No. 93, tartaric acid is added to deposit chromium. Further, in order to obtain sufficient bath stability, an addition amount of Cr ³⁺ ion concentration × 0.5 (mol / liter) or more is required, and especially Cr ³⁺ ion concentration × 1.1
It is disclosed that the addition of (mol / liter) or more is preferable in operation.

However, with the above-mentioned addition amount, the viscosity of the bath is increased and the conductivity is lowered, so that the current efficiency is lowered and the productivity is unavoidable. Further, since the additive is present in a large amount, it is expected that the additive will be deteriorated in a considerable amount, and the chromium deposition amount may be reduced. Conventionally, no measures have been taken against these problems.

[0008]

SUMMARY OF THE INVENTION The above-mentioned problems can be solved by adding a large amount of additives to a mononuclear compound capable of reducing a polynuclear complex of chromium or a hydrated complex of chromium, which has been impossible to reduce up to now. This is due to the point that it is transformed into a non-hydrated complex of.

In view of the above problems, the present invention can be prepared by adding a small amount of an additive regardless of the state of a chromium complex such as mononuclear or polynuclear, and is excellent in bath stability and high in corrosion resistance. A method for manufacturing a zinc electroplated steel sheet is provided.

[0010]

[Means for Solving the Problems]

The present invention relates to a method for producing an electrogalvanized steel sheet containing chromium by immersing the steel sheet in an acidic bath containing Zn ²⁺ ions and Cr ³⁺ ions. A method for producing a high corrosion resistant zinc electroplated steel sheet, characterized in that an additive is added in an amount of 0.1 times or more the Cr ³⁺ ion concentration during plating.

Further, in the above method for producing a plated steel sheet,
This is a method for producing a highly corrosion-resistant zinc electroplated steel sheet in which the organic additive having a conjugated double bond is any one of maleic acid, a maleate salt, and a maleic acid derivative.

[0013]

The present invention will be described in detail below.

First, the additives to be added to the plating bath in the present invention will be described. In the present invention, an additive having a conjugated double bond is applied. Examples of the additive include maleic acid, maleic acid salts, maleic acid derivatives and the like. When applying these, you may add 1 type (s) or 2 or more types.

The present inventors not only have an effect of co-depositing chromium with maleic acid, a maleic acid salt or a maleic acid derivative as one of the organic additives having a conjugated double bond, The added amount of the Cr ³⁺ ion concentration is 0.
It has been found that an amount of 1 times or more is effective and has no influence on the plating quality such as workability. The reason for this is presumed as follows.

Maleic acid, a maleic acid salt or a maleic acid derivative has a double bond in the ethylene group, and this double bond can resonate with the carbon-oxygen double bond of the carboxyl group. This is generally called a conjugated double bond.

This maleic acid or maleic acid salt and Cr
^{When the 3+} ion forms a complex, the π electron of maleic acid flows into the coordination center Cr ³⁺ ion through the conjugated double bond formed by the ethylene double bond and the carbon-oxygen double bond. , It is possible to stabilize the complex.

On the other hand, with other ligands such as tartaric acid which do not have a conjugated double bond, this stabilizing effect cannot be expected.
It is presumed that the above-described stabilizing action enables precipitation even in a polynuclear chromium complex, which has been difficult to be electrodeposited up to now, and thereby enables chromium precipitation even with a small amount of addition.

From the above speculation, as long as it is an additive having a conjugated double bond and capable of easily sending π electrons to the coordination center, it is the same as maleic acid and maleic acid salts and maleic acid derivatives in the present invention. The effect of can be expected.

Examples thereof include phthalate and acetylsalicylate. As the counterion of maleate,
Na ⁺ , K ⁺ , NH ₄ ^{+ and the} like are preferable. As the maleic acid derivative, ethoxymaleic acid, maleic acid amide and the like are preferable. The repeating unit of the ethoxy group of ethoxymaleic acid is preferably n = 1 to 20.

Next, the organic additive having a conjugated double bond represented by maleic acid, maleic acid salt or maleic acid derivative has a small chromium precipitation effect when the amount is less than 0.1 times the Cr ³⁺ ion concentration. Therefore, it is necessary to add 0.1 times or more.

On the other hand, although the upper limit is not particularly defined, it is desirable that it is not more than 1 time the concentration of Cr ³⁺ ions present in the aqueous solution. It is preferably 0.3 to 0.9 times the Cr ³⁺ ion concentration. Excessive addition is not preferable because it may deteriorate the physical properties of plating such as plating adhesion and appearance quality, and may deteriorate productivity due to deterioration of additives, increase of viscosity of plating solution, and decrease of conductivity.

In the present invention, in addition to the above maleic acid, maleic acid salt or maleic acid derivative, a second additive such as polyoxyalkylene, polyvinyl ether or polyethyleneoxyphenol derivative may be added.

In the present invention, if necessary, Ni
By adding a small amount (0.1 mol / liter or less) of metal ions such as ²⁺ , Co ²⁺ , Fe ³⁺ , Mn ²⁺ , Pb ^2+, etc., it is possible to deposit as a third component in the plating film. it can.

The high corrosion resistance zinc electroplating containing chromium obtained by the present invention can be changed by changing the plating bath composition and the current density.
The plating bath may be either a sulfuric acid acidic bath or a hydrochloric acid acidic bath. Current density is usually 50 A / dm ^{2 to} 250 A /
It is appropriately determined within the range of dm ² .

The chromium content in the plating film is preferably 5% by weight or more in consideration of corrosion resistance, while when it is 40% by weight or more, the plating becomes powdery during processing, so-called powdering property is deteriorated. Therefore, less than that is desirable. Further, the base steel sheet for plating is not specified, and the components and the method of manufacturing the steel sheet are not limited.

[0027]

Embodiments of the present invention will be described below.

A cold-rolled steel sheet having a plate thickness of 0.8 mm was used as a base steel sheet, and Zn ²⁺ ions = 0.5 to 0.6 mo as a plating bath.
1 / liter, Cr ³⁺ ion = 0.4 to 0.5 mol /
An acidic bath having a pH of 1.5 and containing liters (Zn ²⁺ Cr ³⁺ = 1.0 mol / liter) was used.

The additives shown in Table 1 were added to this acidic bath, and plating was performed at a bath temperature of 50 ° C. and a current density of 50 to 200 A / dm ² . The amount of adhesion 20 g / m ² thus obtained
For the plated steel sheet, the weight of chromium in the plating film was measured, and the workability and corrosion resistance of the plating were evaluated. The evaluation is as follows.

The workability of the plating was evaluated based on the plating peeling condition of the contact bending portion after the contact bending, and ◎ (no plating peeling), ○ (small plating peeling), × (large plating peeling)
It went in three steps.

For corrosion resistance, a salt water spray test according to JIS Z 2371 was carried out for 672 hours, and ◎ (red rust 1% or less), ○ (red rust less than 10%), × (red rust more than 10%) were evaluated.
It was done in stages.

The bath stability is 100,000 C /
The chromium deposition amount after dm ² energization was measured and compared with the initial chromium deposition amount.
Those with 3% or less were marked with x.

The following evaluation results are shown in Tables 1 to 4.

In the evaluation results, the results of the embodiment (N
o. 1-43), the chromium weight in the plating film is 5%
An electrogalvanized steel sheet containing -18% of chromium was obtained.
The plating processability and plating corrosion resistance were excellent. Also, the amount of chromium deposited after long-term energization is almost unchanged,
The bath stability was excellent.

On the other hand, as shown in Comparative Examples (Nos. 1 to 8), when the addition amount was too small, almost no chromium was deposited in the plating film, and the effect on the corrosion resistance was not exhibited.
Further, even if the added amount was too large, peeling of the plating was observed. Further, when tartaric acid was used as an additive having no conjugated double bond (No. 9), the amount of chromium deposited was decreased after long-term energization, and the bath stability was poor.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

As described above, according to the method for producing a zinc electroplated steel sheet of the present invention, an organic additive having a conjugated double bond is not affected by the complex state of Cr ³⁺ ion concentration during plating. It is possible to produce a high corrosion resistant zinc electroplated steel sheet that contains chromium and has excellent plating workability and plating corrosion resistance by simply adding 0.1 times or more. The bath stability is also improved, and bath management becomes easier.

Claims (2)

[Claims] 1. A method for producing an electrogalvanized steel sheet containing chromium by immersing the steel sheet in an acidic bath containing Zn ²⁺ ions and Cr ³⁺ ions, wherein an organic additive having a conjugated double bond is used. ^Is added in an amount of 0.1 times or more of the Cr ³⁺ ion concentration during plating. 2. The method for producing a galvanized steel sheet with high corrosion resistance according to claim 1, wherein the organic additive having a conjugated double bond is any one of maleic acid, maleate and maleic acid derivative.