JPH03166396A - Highly corrosion resistant composite electroplated steel sheet excellent in chemical conversion treating property and its production - Google Patents

Abstract

PURPOSE:To produce a highly corrosion resistant composite electroplated steel sheet excellent in chemical conversion treating property by making the composition of a lower layer differ from that of an upper layer by using a plating bath containing Zn<2>+ ions, Cr<3+> ions, and organic high polymer and also controlling current density. CONSTITUTION:A steel sheet is plated in a plating bath containing Zn<2+> ions, Cr<3+> ions, and organic high polymer as a Cr precipitation accelerator. At this time, by controlling current density, a lower plating layer of high Cr content is formed and then an upper plating layer of low Cr content is formed to a thin thickness by using the same plating bath. It is preferable to form a Zn-type plating layer containing, by weight, 5-30% Cr and 0.001-5% organic high polymer as the above lower plating layer and also to form a Zn-type plating layer which contains 0.01-<5% Cr and <=1% organic high polymer and in which coating weight is regulated to >=0.5g/m<2> as the above upper plating layer. Further, cation polymer is suitably used as the above organic high polymer. By this method, the highly corrosion resistant composite electroplated steel sheet of high Cr content excellent in chemical conversion treating property can be obtained easily and practically.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties and a method for producing the same, which is used for automobiles, home appliances, building materials, etc.

(Prior Art) It is well known that electrogalvanized steel sheets are already widely used as surface-treated steel sheets that can be mass-produced without impairing the corrosion resistance of cold-rolled steel sheets, the corrosion resistance after painting, and workability.

However, in recent years, attempts have been made to use galvanized steel sheets as anti-rust steel sheets for automobiles in response to the salt sprayed on roads in winter to prevent roads from freezing in cold regions, and there is an increasing demand for corrosion resistance in harsh corrosive environments.

In response to these demands for improving the corrosion resistance of galvanized steel sheets, it is known that the corrosion resistance can be improved by increasing the amount of zinc plating (adhesion amount). Many have been proposed.

Many of these contain iron group elements such as Fe, Ni, and Co as alloy components.

These zinc monoiron group electroplated steel sheets are characterized by excellent corrosion resistance either unpainted or after painting, as seen in Japanese Patent Publication No. 50-29821 and Japanese Patent Publication No. 57-61831, and are not suitable for industrial production. , has been put into practice,
It is strongly desired to further improve corrosion resistance.

As electroplated steel sheets containing Cr in Zn or Zn-based alloy plating, for example, Japanese Patent Publication No. 59-38313 and No. 59-40234, Japanese Patent Application Laid-Open No. 61-130498,
No. 61-270398 and No. 62-54099 are disclosed.

All of these have a trace amount of Cr content of 5% or less, and the effect of Cr on corrosion resistance can only be incidental. Therefore, for the purpose of improving corrosion resistance, it is strongly desired to eutectoid a higher content of Cr. However, conventional Zn-C that can increase the Cr content
rThere was no electroplating technology. That is, even if the concentration of trivalent Cr ions in the plating is simply increased, a plating with good workability cannot be obtained, and there are obstacles such as a sudden decrease in current efficiency, and it is difficult to obtain electroplated steel sheets with a high Cr content industrially. was extremely difficult.

Therefore, the inventors of the present application first applied
In step 8, a water-soluble cationic polymer is introduced into the plating bath containing Zn''+ and Cr3+, and this action promotes Cr precipitation, resulting in a highly corrosion-resistant composite with an unprecedentedly high Cr content of 5% by weight or more. It has been disclosed that an electroplated steel sheet can be obtained and workability can also be ensured by eutectoiding a small amount of cationic polymer.

However, in plating containing a large amount of Cr. Since so-called chemical conversion treatments such as phosphate treatment and chromate treatment are insufficient, as a countermeasure to this problem, it has been necessary to further apply Zn plating or Zn-based alloy plating such as Zn-Ni or Zn-Fe to the upper layer.

(Problem to be solved by the invention) On Zn plating containing a large amount of Cr. If upper layer plating is done with Zn or Zn-based alloy plating, conventional Z
Although good chemical conversion treatment properties similar to n- or Zn-based alloy plating can be obtained, the adhesion of the coating after painting is likely to be influenced by these upper layer platings.

In addition, in particular, with Zn-based alloy plating, there is a problem that the potential difference with the underlying composite plating layer containing a large amount of Cr becomes large, and corrosion of the plating layer tends to progress due to the formation of local batteries between the upper and lower plating layers. There is.

In other words, in order to ensure chemical conversion treatment properties, Zn or Z
When n-based alloy plating is applied as the upper layer plating, the lower layer Cr
This will reduce the good corrosion resistance and coating adhesion that a composite plating layer that contains a large amount of .

Therefore, it is necessary to impart good chemical conversion treatment properties without impairing these properties. Furthermore, as a matter of course, equipment dedicated to upper layer plating is required, and if control and management of upper layer plating is also included, a large amount of cost is required.

In view of the above problems, the present invention provides a two-layer type highly corrosion-resistant composite electroplated steel sheet with high Cr content and excellent chemical conversion treatment property by using the same plating bath and controlling the plating composition of the lower and upper layers. and its manufacturing method.

(Means for Solving the Problems) The present invention introduces an organic polymer such as a water-soluble cationic polymer into a plating bath containing Zn''+ and Cr'+, and promotes Cr precipitation by this action. , First, a composite electroplated layer with a high Cr content was obtained as the lower layer plating, and then,
By applying a thin composite electroplated layer with a low Cr content as the upper layer plating in the same plating bath, we succeeded in obtaining a highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties and in its manufacturing method.

The gist of the present invention is (1) Cr5 to 3 as the lower plating layer on the surface of the m plate.
A Zn-based composite plating layer containing 0% by weight of organic polymer and 0.00 1 to 5% by weight of organic polymer is formed, and an upper plating layer is formed on the Zn-based composite plating layer containing 0.01 to 5% by weight of Cr and 1 to 5% by weight of organic polymer. A highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties, characterized by having a ZN-based composite electroplated layer containing less than or equal to % by weight, A highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties as claimed in claim (1). (3) Claim in which the organic polymer is a cationic polymer (
Controlling the current density using the highly corrosion-resistant composite electroplated steel sheet with excellent chemical treatment properties described in item 1), and (4) a plating bath containing Zn2+ ions, Cr'+ ions, and organic polymers. This is a method for producing a highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties, which is characterized by changing the plating composition of the lower layer plating and the upper layer plating.

(Function) The corrosion resistance of the composite electroplated steel sheet of the present invention is mainly due to Cr.
This is the effect of First, the Cr content of the lower layer plating is 5 to 5.
30% by weight is preferred. If it is less than 5% by weight, although a slight corrosion resistance improvement effect is recognized, the tendency for red rust to occur remains, and the corrosion resistance is not sufficient. When the amount is 5% by weight or more, the occurrence of red rust is suppressed in, for example, a salt spray test, and an epoch-making effect appears.

Such high corrosion resistance cannot be achieved by conventionally known Zn plating or alloy plating such as Zn-Fe or Zn-Ni.

When Cr coexists with Zn, it does not become passivated, and together with Zn it participates in the sacrificial anticorrosion effect, and the corrosion products of Cr deposit a poorly soluble protective film on the corroded areas, thereby suppressing corrosion. This is thought to be the reason why it exhibits high corrosion resistance.

When the Cr content exceeds 30% by weight, corrosion resistance is good, but even with the eutectoid effect of organic polymers such as cationic polymers, which will be described later, if the plating layer peels off during processing such as press working, so-called powdering property may occur. It is difficult to apply in practice.

The organic polymer is a precipitation accelerator for Cr, and improves powdering resistance during processing by eutectoiding with a small amount of Cr in the plating layer. It is presumed that this mutual effect of organic polymers is due to the fact that Cr ions inhibit the uniform deposition of Zn and prevent a plating structure lacking in uniformity and smoothness. be done. That is, the content of the organic polymer is 0.00, which is thought to form a w1@licked layer in which Zn and Cr are uniformly mixed or alloyed through the eutectoid organic polymer.
1 to 5% by weight is preferred. If the content is less than 0.001% by weight, it will have little effect on powdering resistance during processing, and if the content exceeds 5% by weight, it will be difficult to obtain even if the concentration of the organic polymer in the plating bath is increased. First, if a large amount is eutectoid, it will cause a decrease in plating adhesion. From the viewpoint of processability, it is sufficient if the organic polymer is eutectoid at a content of 1/1000 or more of the Cr content.

Water-soluble cationic polymers are effective as organic polymers used in the present invention, and quaternary amine polymers are particularly effective polymers. The molecular weight is in this case 103
~10' is desirable.

Specifically, among the following amine polymers, polyamine sulfone (categorized as PAS) and polyamine (PA) are Cr.
Most effective as a precipitation promoter.

It is thought that the adsorption effect by the amine group and the bond between the sulfone group and the metal ion or metal contribute. Basically, it is composed of the following quaternary amine salts (ammonium salts) or cobolymers.

Some compounds will be specifically listed below.

Examples include polymers obtained from diallylamine. Rel R2 represents a lower alkyl group, and is abbreviated as CH2/\-[-CHCH-CH2Cl{2CH2\/N...X haR1R, hPA.

or, ? E- CH2- CH- CH- CH211 CH ■　CH, C R1R2 CH2 or CH, CH2 \/ N...X X is CQ HSO. 82PO4 (R is C-C4 ? rukyl group). Indicates the anion of No3-.

Another example is a polymer synthesized from vinylbenzyl. R■l R21 R3 represents hydrocarbon, 1,000H
2- CH± CH2 N...X/\R1R2R3? is C flood, HSO, -, H PO. -, R-S
Indicates anion of O, -, No, \.

Alternatively, allylamine polymer may be mentioned.

? It is abbreviated as CH2-CIli -+ PAR.

l N...X /1\ R■R2R3 Rt, R2, R3 represent hydrocarbon, X is CQ-,
IISO4-,1{2PO,-, R-So. -, N
Indicates the anion of o3-.

In addition, polymers of primary, secondary, and tertiary amines are also effective as Cr precipitation promoters, although they are not as effective as the above-mentioned quaternary amine polymers. In addition to these thione polymers, polyoxyalkylene derivatives, especially polyethylene glycol (P
(abbreviated as EG) is valid. Plating amount is 10-50
g/rrf can ensure sufficient corrosion resistance. Also, Zn,
Cr. In addition to organic polymers, Fe, Ni, Co,
Pb, Sn, Ag, In, Bi, Cu, S
Even if small amounts of B, As, AQ, Ti, Na, P, S, etc. inevitably eutectoid, the effect of the lower layer plating does not essentially change.

Next, the Cr content of the upper layer plating is preferably less than 0.01 to 5% by weight. When combined with a lower layer plating with a high Cr content in this range, chemical conversion treatment properties can be improved without impairing the high corrosion resistance of the lower layer plating. 5th view%
If the content is above 10% by weight, phosphate crystals tend to become coarse even if phosphate treatment is performed, and if the content is 10% by weight or more, hardly any phosphate crystals will precipitate. In addition, in the case of glomate treatment, particularly in the reaction type (etching type) and electrolytic type, the production efficiency of chromate 1 film decreases. 0.01%
If it is less than that, the corrosion resistance after painting will be reduced although the chemical treatment properties will be good. In the present invention, since the upper layer plating uses a plating bath containing the same organic polymer as the lower layer plating, the organic polymer is inevitably eutectoid during the upper layer plating as well. The content of organic polymers has a positive correlation with the Cr content, so if the Cr content in the upper layer plating is as low as 0.01 to less than 5% by weight, only a very small amount will precipitate, but chemical conversion treatment In order not to impair the properties, the amount must be 1% by weight or less.

The adhesion amount of the upper layer plating is 0. 5g/n? That's all.

If it is less than 0.5 g/rrl', the coverage of the upper layer plating with respect to the lower layer plating will be insufficient, and chemical conversion treatment properties will not improve.

There is no particular upper limit, but from the point of view of cost and productivity 5 g
/rr? is preferred.

Next, regarding the method for manufacturing a composite electroplated steel sheet of the present invention, the plating bath includes Zn"+ ions, Cr3+ ions, and water-soluble thionic polymers such as polymers of quaternary amines such as PAS, PEG, etc. Contains 0.01-20gIQ of organic polymer as a Cr precipitation accelerator, p+I0.5-3
An acidic plating bath with a bath temperature of 40 to 70°C is used. The plating bath may be a sulfuric acid bath, a chloride bath, or a mixed bath thereof.

Sin2, Tie.
, AQ 20, etc. may be added.

Furthermore, Na", K", NH. Adding salts such as + ions is effective in increasing the conductivity of the bath.

In the manufacturing method of the present invention, using the above plating bath,
Composite electroplating with high Cr content on the lower layer and low Cr on the upper layer
It is characterized in that it forms a composite electroplating with a different content.

This classification can be done by changing the current density, bath temperature, and liquid flow rate, but the classification based on current density is the simplest and most effective.

That is, as shown in the drawing, by changing only the current density under constant conditions of plating bath, bath temperature, and liquid flow rate,
The r content varies. In FIG. 1, the plating bath is Zn
"45 g/A, Cr"20 gIQ, P
AS1 gIQ, Na”1 6gIQ, ptl
2. A sulfuric acid bath with a bath temperature of 40°C and a liquid flow rate of 60 m/w.
It is in. As is clear from FIG. 1, the Cr content increases with the current density.

By utilizing this phenomenon, it is possible to form plating layers with different Cr contents in the upper and lower layers by controlling only the current density in the same plating bath. The relationship between current density and Cr content varies depending on the plating bath composition, P11, and bath temperature, so based on the current density of around 5% by weight of Cr, the lower layer plating should be at a higher current density, and the upper layer plating should be at a higher current density than this. It is sufficient to use a low current density. In this way, in a continuous plating line having a large number of plating cells, lower layer plating and upper layer plating with a predetermined Cr content can be continuously applied using the same plating bath and the same plating cell.

In addition, the amount of adhesion can be controlled by current density. Depending on line speed,
This is possible by allocating the number of cells.

The structure of the present invention does not necessarily have to be applied to both sides of the steel plate, but can be applied to only one side depending on the application, and the other side can be left as the steel plate surface, or coated with another plating layer or a plating layer covered with an organic film. You can also use it as

The base steel plate to which the present invention is applied is usually a mild steel plate that has been dull finish rolled, but it may also be a mild steel plate that has been bright finish rolled, a high tensile strength steel plate that contains a large amount of Mn, S, P, etc. as steel, Cr, Cu, It can also be applied to highly corrosion-resistant steel plates that contain a large amount of Ni, P, etc. and have a low corrosion rate. (Example) After degreasing a cold rolled steel plate with alkali and pickling with 5% sulfuric acid,
After washing with water, electroplating was performed under the following conditions.

The plating bath consisted of ln2+, Cr3+, organic polymers (PA with an average molecular weight of 10,000, PAS with an average molecular weight of 3500).
, PEG with an average molecular weight of 1500) and Na”30g#l
This is a sulfuric acid acidic bath containing 100% sulfuric acid, and the concentration, pH, and bath temperature were varied. The liquid flow rate was 60 m/min. Using the above plating bath, lower layer plating and upper layer plating having different plating compositions and coating amounts were successively applied by changing the current density and the amount of current applied. Some of the samples were subjected to Zn plating or Zn-based alloy plating as the upper layer plating for comparison.

The manufacturing conditions, plating composition, and performance evaluation results of the composite electroplated steel sheet manufactured in this way are shown in Table 1 and
Shown in Tables 2 and 3.

Note that Zn and Cr in the plating layer were analyzed by atomic absorption spectrometry, and organic polymers were analyzed for C by combustion method and converted into organic polymer weight. The performance evaluation method is as follows.

(1) Chemical conversion treatment ■ Immersion type phosphate treatment (P manufactured by Nippon Parriki Rising Co., Ltd.)
R3020) under standard conditions (immersion for 2 minutes) to evaluate phosphate crystals.

0: Dense, average grain size, 15 μl or less △: Coarse, average grain size, over 15 μl
H2 5040.25gIQ, 40°C) at a current density of 10A/d and current flow rate of 10C/drn to evaluate the film amount.

O: Total CrJL20iIg/rrr or more △: Total Cr amount 1
0mg/r&more than 20I! Less than Ig/rrr×: Total Cr
Amount 10mg/rr? Less than (2) Corrosion resistance after painting Immersion phosphate treatment (same as above) and cationic electrodeposition coating (Power Top U-600 manufactured by Nippon Paint Co., Ltd.) 30μ
Give Peng. A test piece with a crosscut reaching the base metal was subjected to 30 cycles of CCT as described below, and evaluated by the amount of reduction in plate thickness at the crosscut portion.

0.1m or less: O O. More than 1 nn to less than 0.2 mm: △ More than 0.2 mm: × (3) Water resistant adhesion The above test piece was further coated with an intermediate coat and a top coat to give a total film thickness of 10
After immersing the sample in distilled water at 40°C for 500 hours, a 2m grid test was performed and the number of pieces peeled off was evaluated.

O pieces 201 to 5 pieces: Δ 6 pieces or more: X (4) Workability After performing a cylindrical press of 50φ x 25H, tape peeling was performed on the processed surface, and the weight loss was evaluated.

5 l Tlg or less: 0 More than 5 ■ to 10 [ng or less: △ More than 10 ■: × First, Comparative Example 1 does not have an upper layer plating, and Comparative Example 2 has a high Cr content in the upper layer plating. Sample No. 4 has a high organic polymer content in the upper layer plating, and Comparative Example 5 has a low adhesion amount of the upper layer plating, so both have poor chemical conversion treatment properties and water resistant adhesion. Comparative Example 3 has poor water resistant adhesion because the Cr content of the upper layer plating is low.

Comparative Example 6 has poor corrosion resistance after coating because the Cr content of the lower layer plating is low. In Comparative Example 4, the lower layer plating has a high Cr content, so the workability is also poor.

In Comparative Examples 7 and 8, the upper layer plating was conventional Zn plating and Zn-Ni alloy plating, so the performance after painting was insufficient.

On the other hand, Examples 1 to 20 of the present invention have good chemical conversion treatment properties, post-painting performance, and workability.

Table 3 (Effects of the Invention) As described above, according to the present invention, a highly corrosion-resistant composite electroplated steel sheet with a high Cr content and excellent chemical conversion treatability can be obtained. Moreover, the manufacturing method thereof is substantial and simple, and is suitable for use as a rust-proof steel plate for automobiles, home appliances, building materials, etc., and as a manufacturing method thereof.

[Brief explanation of the drawing]

The upper figure is a diagram showing the relationship between current density and Cr content in the plating layer. Figure 1 Current density (A/drr+2) Procedural amendment 1. Display of the incident 1999 Patent Application No. 303368 2. 3. Name of the invention Relationship with the case concerning a person who amends a highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties and its manufacturing method Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (6
6 5) Yutaka Saito, Representative of Nippon Steel Corporation 4. Agent: 105 ThL (503) 4877 Address: 8th floor, Daiichi Mori Building, 1-12-1 Nishi-Shinbashi, Minato-ku, Tokyo 6. Column 7 for detailed description of the invention in the specification to be amended. Contents of correction

Claims (4)

[Claims] (1) Zn containing 5 to 30% by weight of Cr and 0.001 to 5% by weight of organic polymer as a lower plating layer on the steel sheet surface.
A composite electroplating layer is formed thereon, and an upper plating layer containing 0.01 to less than 5% by weight of Cr and 1% by weight of an organic polymer.
A highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties, characterized by forming a Zn-based composite electroplated layer containing the following: (2) A highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatability according to claim (1), wherein the coating weight of the upper layer plating is 0.5 g/m^2 or more. (3) Claim in which the organic polymer is a cationic polymer (
A highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties as described in item 1). (4) The plating composition of the lower layer plating and the upper layer plating is changed by controlling the current density using a plating bath containing Zn^2^+ ions, Cr^3^+ ions, and organic polymers. A method for manufacturing a highly corrosion-resistant composite electroplated steel sheet with excellent chemical conversion treatment properties.