JPH07157889A - Production of plated steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a plating solution composition capable of making the upper limit of applied current density higher than that of phenol sulfonate which is conventionally used in electroplating a steel sheet with Sn or the like. CONSTITUTION:An aliphatic sulfonic acid is used as sulfonic acid in the plating solution to be a Hf supply source. Concretely, in the plating solution composition using sulfonic acid as a base material, the plating solution containing 20-100g/l Sn<2+> using an aliphatic sulfonic acid such as methane sulfonic acid as the H<+> supply source, 20-120g/l methane sulfonic acid (100-60g/l expressed in the terms of free sulfuric acid) and 10-40ml/l surface active agent (a solution containing 3-10wt.% polypropylene glycol (500-7000 in molecular weight)) is used and the upper limit of current density is increased up to 300A/dm<2> at room temp to 80 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a plated steel sheet having excellent corrosion resistance, particularly a plated steel sheet having excellent corrosion resistance as a fuel tank for automobiles.

[0002]

2. Description of the Related Art In a fuel tank for automobiles, olefins in gasoline are oxidized due to long-term storage of stored gasoline.
It is corroded by organic acids such as formic acid and acetic acid generated thereby, and cannot be used for a long time.

In order to improve the corrosion resistance of this fuel tank, Japanese Examined Patent Publication (Kokoku) No. 1-36558 discloses a steel sheet on which an alloy underlayer of Sn and Pb is formed in order to provide lubricity at the time of tank molding. It is described that upper layer plating such as Sn, Ni, and Co is applied.

In performing the upper layer plating, Sn
In the case of plating, for example, stannous sulfate 60 g / l (Sn
⁽²⁺ conversion 33 g / l), sulfuric acid conversion phenol sulfonic acid 20 g / l, ENSA 10 g / l as a brightener were mixed to form a plating solution, and the current density was 5 at room temperature to 80 ° C.
Treatment is performed under electrolytic treatment conditions of about 50 A / dm ² .

This phenolsulfonic acid ionizes in the plating solution and becomes a conductive substance, and in addition to that, Sn is used to supply Sn ^{2+ in} a system for electrotin plating using an insoluble anode. H ⁺ required for lysis reaction
Will be the source of supply. Further phenolsulfonic acid ions ionized _{(C 6 H 4 (OH)} SO 3 -) is Sn ²⁺ in the bath
It has a function of forming a complex as shown on the right side of the following formula, and is said to prevent Sn ²⁺ in the bath from being peroxidized by Sn ²⁺ + O ₂ → SnO ₂ and becoming sludge. There is.

[0006]

[Chemical 1] However, when this phenol sulfonic acid is used, the electrolytic conditions are limited in terms of plating performance such as corrosion resistance, plating adhesion, and plating appearance, and the current density is practically 5 to 75 A / dm ² ( Sn ²¹ : 20-50 g /
In the range of (1), below this range, a so-called low current phenomenon occurs in which precipitated crystal grains are coarsened, uniform coverage is impaired, a blackish appearance is exhibited, and corrosion resistance is poor. If the upper limit is exceeded, Sn ²⁺
Supply to the plating interface controls the plating reaction, the hydrogen generation reaction proceeds in parallel with the Sn precipitation, the plated surface becomes dendritic (dendritic), and the powder is deposited, resulting in uniform coverage. As a result, the appearance and corrosion resistance are deteriorated.

Therefore, in the case of such two-layer plating, expanding the allowable current density range of the upper layer plating, in particular, expanding the upper limit of the current density means that the amount of plating adhered per electrode can be increased. Therefore, the line speed can be increased and productivity can be improved in existing equipment with a fixed number of plating cells, or the number of plating cells can be reduced in new equipment. It means to be advantageous.

[0008]

DISCLOSURE OF THE INVENTION The object of the present invention is to increase the upper limit of the current density to be applied when electroplating Sn or the like on a steel sheet in comparison with phenol sulfonic acid which has been conventionally used, and to improve the plating efficiency. It is to provide a method for manufacturing a plated steel sheet that raises the price.

[0009]

The present invention has achieved its object by using an aliphatic sulfonic acid as an H ⁺ source in a plating solution composition based on sulfonic acid.
As the aliphatic sulfonic acid, methanesulfonic acid (CH ₃
SO ₃ H) can be preferably used in terms of economy and easy availability.

In this case, as the plating solution, Sn ²⁺ is ^added to 20
~ 100g / l and 20 ~ 120g of methanesulfonic acid
/ L (10 to 60 g / l in terms of sulfuric acid) and 10 to 40 m of an aqueous solution containing 3 to 10% by weight of polypropylene glycol having a molecular weight of 500 to 7,000 as a surfactant.
Using a plating solution containing 1 / l, room temperature to 80 ° C
And the plating treatment is performed under a current density of 5 to 300 A / dm ² .

Particularly, the plating solution of the present invention is used for Pb-Sn.
By plating an Sn overcoat layer having a thickness of 0.05 to 7 μm on the alloy undercoat layer at room temperature, it can be suitably applied to a plated steel sheet as a fuel tank forming material.

As the Pb-Sn base plating treatment, N
It is also possible to perform a pretreatment by i plating.

Further, the Pb-Sn alloy layer contains Pb.
In order to prevent corrosion from pinholes in the -Sn alloy layer, a phosphate coating layer mainly composed of a compound containing lead and phosphorus can be provided.

[0014]

When an aliphatic sulfonic acid such as methane sulfonic acid is used as the plating solution constituent liquid, the applicable current density range can be expanded as compared with the conventional phenol sulfonic acid. Further, since the upper limit value of the applicable current density increases in proportion to the increase of the Sn ²⁺ concentration, the upper limit value can be expanded by using methanesulfonic acid.

[0015]

EXAMPLE An example in which the present invention is applied to the production of a steel plate for a fuel tank in which two plating layers are formed on a steel plate will be described.

Under the following conditions, both surfaces of a steel sheet that has been subjected to the usual pretreatments required for degreasing and pickling plating and the cleaning and activation treatments normally required for hot dipping and electroplating are performed. Pb that is plated and contains 8% Sn
Was formed to a thickness of 3.5 μm.

First, by electroplating method, 0.05 μm
After applying Ni plating of thickness, Pb-8% S at 350 ° C
The coating layer was dipped in an n-alloy plating bath for 7 seconds to control the plating amount with a high-pressure gas to form a coating layer having a thickness of 3.5 μm.

Further, on this Pb-Sn undercoat plating layer, it was immersed in a 0.2% phosphoric acid aqueous solution at 50 ° C. for 10 seconds, squeezed by a roll, dried, and 5 per surface in terms of P.
A coating layer of mg / m ² was formed.

A coating layer containing a compound containing 5 mg / m ² of lead and phosphorus as main components was provided on one side.

Further, the phosphate-coated underlayer is coated with S by using a plating solution mainly containing methanesulfonic acid.
An n-plating upper layer was formed to a thickness of 3 μm.

The conditions for forming the Sn plating upper layer were as follows.

Plating bath composition: Sn ²⁺ 33 g / l Methanesulfonic acid 40 g / l (20 g / l in terms of sulfuric acid) Polypropylene glycol (surface active) 10 g / l (molecular weight 2000) 5 wt% aqueous solution Plating conditions: Plating Liquid temperature Normal temperature to 80 Under the above processing conditions, Sn is changed to 3 μm by changing the current density.
Plated thick.

Table 1 shows the production conditions (bath composition, bath temperature, current density, etc.), the surface condition of the plating layer and the plating performance evaluation.

[0024]

[Table 1] The evaluation test was carried out by the following method.

Corrosion resistance evaluation by salt spray test on the outer surface Flat plate and after processing (0.8 mm × 500 mm × 500 mm
A blank spraying process from a blank size to a square cylinder having a drawing depth of 100 mm) was performed, and a salt spray test for a predetermined time shown in Table 1 was performed to evaluate the corrosion resistance.

The evaluation criteria are as shown below.

◎ ・ ・ ・ Number of red rust generated 3 / dm ² or less ○ ・ ・ ・ Number of red rust generated 10 / dm ² or less △ ・ ・ ・ Number of red rust generated 20 / dm ² or less × ・ ・ ・ Number of red rust generated 20 More than 100 pieces / dm ² Evaluation for gasoline and deteriorated gasoline Promotion of various fuels shown in Table 1 inside a 0.8 mm x 500 mm x 500 mm blank size to a square tube drawing test piece with a drawing depth of 110 mm The corrosion-promoting fuel solution assumed for the test was filled, the solution was renewed every month, and the test was carried out for one year.

As the deteriorated gasoline, a solution containing 1000 ppm of formic acid and 4000 ppm of acetic acid which had been accelerated and deteriorated was used.

As can be seen from the table, the current density is 5 to 1
Under the condition of 00 A / dm ² , it was possible to obtain a plating layer having excellent plating adhesion and corrosion resistance and an excellent surface appearance. However, when the current density is 5 A / dm ² or less, the current density is low, so the Sn ²⁺ discharge rate in the plating process is low, and the crystal growth rate is higher than the crystal nucleus generation rate. As a result, the generation of crystal nuclei is suppressed, the crystal grows slowly and large, and the crystal grains become coarse,
The crystal surface becomes rough, the base cannot be uniformly coated, the corrosion resistance deteriorates, and the appearance has a blackish appearance.

At 100 A / dm ^{2 and} above, Sn ²⁺
Supply to the plating interface controls the plating reaction, the hydrogen generation reaction proceeds in parallel with the Sn precipitation, the plated surface becomes dendritic (dendritic), and the powder is deposited, resulting in uniform coverage. However, the plating adhesion and corrosion resistance were deteriorated, and the appearance had a blackish appearance (so-called “plating burnt”), and it could not be put to practical use.

For comparison, a phenol sulfonic acid was used instead of methane sulfonic acid, and the treatment was carried out under the same Sn ²⁺ concentration condition. When the current density is in the range of 5 to 50 A / dm ² , the supply of Sn ^{2+ to} the plating interface and the Sn precipitation proceed in a well-balanced manner, and the discharge rate of Sn ²⁺ is also large to some extent, and the generation rate of crystal nuclei (hence, The production amount is large, so a dense and uniform plating layer is formed, and the underlying plating layer can be uniformly coated, so the surface gloss is slightly inferior, but an upper plating layer with excellent plating adhesion and corrosion resistance is obtained. I was able to. However, if it exceeds 50 A / dm ² , the undercoat can be uniformly coated as with the phenomenon above the upper limit of the permissible current density when methanesulfonic acid is used, so both the appearance and corrosion resistance deteriorate and it cannot be put to practical use. It was a thing. As described above, the applied current density of the aliphatic sulfonic acid such as methane in the present invention can be doubled as compared with the conventionally used phenol sulfonic acid. This is related to the mobility of Sn ²⁺ from the offshore (bulk) in the plating bath to the plating interface, and the kinematic viscosity of phenolsulfonic acid and methanesulfonic acid and Sn ^{2+ in the} liquid. Comparing the diffusion coefficients of methane sulfonic acid, methanesulfonic acid has a smaller kinematic viscosity coefficient and a larger diffusion coefficient, so that Sn ²⁺ can be easily supplied to the plating interface, and as a result, the current density upper limit value can be expanded. it is conceivable that.

On the other hand, with respect to the surface gloss, although the reason is not clear, the amount of crystal nuclei generated on the plating surface of the methanesulfonic acid bath is slightly larger than that of the phenolsulfonic acid bath. Since it is dense and uniform, it seems that the surface gloss is slightly better when methanesulfonic acid is used.

[0033]

The present invention has the following effects.

(1) It is possible to apply a higher current density than in the case of using a conventional sulfonic acid, so that the productivity is improved by increasing the line speed.

(2) A higher current density can be applied than in the case of using a conventional sulfonic acid, and the number of plating cells can be reduced in new equipment.

(3) A plated surface having a surface gloss superior to that in the case of using a conventional sulfonic acid can be obtained.

Claims (5)

[Claims] 1. In a method for producing a plated steel sheet in which a steel plate surface is metal-plated with a plating solution composition containing sulfonic acid as a base, an aliphatic sulfonic acid is used as the sulfonic acid in the plating solution composition, and an H ⁺ supply source is used. Method for producing plated steel sheet. 2. The method for producing a plated steel sheet according to claim 1, wherein the aliphatic sulfonic acid is methanesulfonic acid. 3. The plated steel sheet according to claim 1, wherein the steel sheet surface is metal-plated, is a fuel tank-forming steel sheet for plating a Sn overcoat layer on an undercoat layer, and conditions for metal plating are provided. As for the plating solution composition, the plating solution composition based on sulfonic acid has Sn ²⁺ of 20 to 100 g / l and methanesulfonic acid of 20 to 120 g / l (10 to 60 g / l in terms of free sulfuric acid).
1) and 10 to 40 ml / l of an aqueous solution containing 3 to 10% by weight of a surfactant made of polypropylene glycol having a molecular weight of 500 to 7,000, and the electrolytic treatment condition is that the plating solution is at room temperature to 80%. A method for producing a plated steel sheet, which has a current density of 5 to 300 A / dm ² at 0 ° C., and in which the metal plating applied to the steel sheet surface is an Sn plating layer applied to a thickness of 0.05 to 7 μm. 4. The Pb-S according to claim 3, wherein the undercoat layer provided on the steel plate contains Sn in an amount of 3 to 30% by weight.
A method for producing a plated steel sheet which is an alloy layer of n. 5. The Pb-S according to claim 3, wherein the undercoating layer provided on the steel plate contains Sn in an amount of 3 to 30% by weight.
The manufacturing method of the plated steel plate which consists of a phosphate layer formed on the alloy layer of n and the alloy layer of the same Pb-Sn.