JPH07300694A - Production of zinc alloy electroplated steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce an electrogalvanized steel sheet contg. Cr and excellent in corrosion resistance. CONSTITUTION:A steel sheet is applied with electroplating by using a plating bath contg. Zn<2+> ions, Cr<3+> ions and 0.1 to 10g/l polysaccharide as a Cr precipitating agent. In the case 0.1 to 5g/l cation polymers and/or polyoxyalkylene derivative is furthermore incorporated into the plating bath, it is more advantageous to the precipitation of Cr and the appearance of the plating.

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 zinc-based electroplated steel sheet containing Cr, which is used as a material for automobiles and home electric appliances, and which has excellent corrosion resistance.

[0002]

2. Description of the Related Art Zinc-based electroplated steel sheets are widely used as materials for automobiles, home appliances, and building materials. Among them, in automotive applications, higher corrosion resistance is required in order to prevent car body corrosion due to sprayed rock salt for road freezing prevention in winter in cold regions. In addition, in automobiles and home appliances, not only corrosion resistance but also quality requirements for appearance are increasing, and corrosion-resistant steel sheets that are more attractive in terms of uniformity and gloss are required.

The corrosion resistance of galvanized steel sheets can be improved by increasing the zinc plating amount (adhesion amount), but this has many problems in terms of weldability and workability. Therefore, many alloy platings have been proposed as a method of suppressing the dissolution of zinc itself and extending the life of zinc plating. Above all, Fe, Ni, Co
Zn-based alloy platings containing as an alloy component have been put into practical use because of their good bare corrosion resistance and post-coating corrosion resistance. Recently, as disclosed in JP-A-3-120393 and JP-A-5-9779, attempts have been made to incorporate Cr, which is a corrosion resistant metal, into Zn plating. Japanese Patent Laid-Open No. 3-120393 discloses Z
It is possible to add tartaric acid to an acidic sulfate bath containing n ²⁺ ions and Cr ³⁺ ions for electroplating.
No. 9 discloses that a Zn-Cr electroplated steel sheet is produced by using a pulse current or a direct current in which an alternating current is superimposed without using an additive. According to these, a large amount of Cr can be contained in the Zn plating, but the effect of the Cr content cannot be maximized with respect to corrosion resistance, and the appearance is uniform and glossy. In terms of points, it is not at a satisfactory level compared to conventional alloy plating.

[0004]

Attempts to precipitate Cr with Zn have been conventionally made using Cr ³⁺ or Cr ⁶⁺ ions, but only a trace amount of Cr is precipitated from a simple bath, As described in JP-A-5-9779, even if a large amount of Cr is deposited using a pulse current, not only a good appearance is not obtained, but also workability and current efficiency are remarkably reduced, which is an industrial problem. Was not in favor of.

The present invention solves the above problems and provides a method for producing a zinc-based electroplated steel sheet containing Cr and having excellent corrosion resistance.

[0006]

The gist of the present invention is as follows.

In producing a zinc-based electroplated steel sheet containing Cr, a steel sheet is prepared using a plating bath containing Zn ²⁺ ions, Cr ³⁺ ions, and 0.1 to 10 g / l of a polysaccharide as a Cr depositing agent. A method for producing a zinc-based electroplated steel sheet having excellent corrosion resistance, which comprises performing electroplating on a steel sheet.

In producing a zinc-based electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, a polysaccharide of 0.1 to 10 g / l, a cationic polymer and //
Alternatively, a method for producing a zinc-based electroplated steel sheet having excellent corrosion resistance is characterized in that the steel sheet is electroplated using a plating bath containing 0.1 to 5 g / l of a polyoxyalkylene derivative.

[0009]

[Function] The above problem can be solved by adding a polysaccharide as a Cr precipitation promoter to a plating bath containing Zn ²⁺ ions and Cr ³⁺ ions, and a plating containing a large amount of Cr and having high corrosion resistance can be obtained. You will get it. The reason for this is
It is presumed that the polysaccharide is adsorbed on the surface of the steel plate serving as the cathode, the discharge of Zn ²⁺ ions is suppressed by the steric effect on the structure, and the precipitation of Cr ³⁺ ions is efficiently performed. At this time, it is also considered that Cr ³⁺ ions are preferentially coordinated with the polysaccharide and the precipitation of Cr ³⁺ ions is further promoted.

The amount of polysaccharide added is 0.1 to 10 g / l. If it is less than 0.1 g / l, there is no Cr precipitation effect and it is 10 g.
If it exceeds 1 / l, it is difficult to dissolve in the plating bath, which is not practical. A more preferable range is 0.5 to 5 g / l.
Here, the polysaccharide is a compound having a structure of polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds by hydrolysis. Among the polysaccharides that are particularly effective are agar, potato dextrose agar, nutrient agar, ascorbic acid,
Mention may be made of potato starch, gellite, sorbitol, mannitol, chitin, chitosan, dextran, pullulan, cyclodextrin.

When a cationic polymer or a polyoxyalkylene derivative is added to the plating bath in addition to these polysaccharides, the appearance quality is improved and Cr is added.
It is more advantageous in increasing the amount of precipitation. The addition amount of these is 0.1 to 5 g / l. If it is less than 0.1 g / l, the above effect is insufficient, and if it exceeds 5 g / l, the effect of addition is saturated. Either one or both of the cationic polymer and the polyoxyalkylene derivative may be used.

The cationic polymer is preferably a polymer of a quaternary amine salt represented by Chemical Formulas 1 to 5, particularly polyamines represented by Chemical Formulas 2 and 3 (abbreviated as PA) and polyamine sulfones represented by Chemical Formulas 4 and 5 (abbreviated as PAS). ) Is effective. The reason for this is that the amine group adsorbs to the cathode surface and the sulfone group contains Cr ^3+.
It is presumed that the coordination bond of ions contributes. The average molecular weight is preferably 10 ² to 10 ⁶ . Also, besides this,
Polymers of primary, secondary, and tertiary amines are effective even though they do not reach the above-mentioned quaternary amine polymers.

[0013]

[Chemical 1]

[0014]

[Chemical 2]

[0015]

[Chemical 3]

[0016]

[Chemical 4]

[0017]

[Chemical 5]

In each of the above chemical formulas, R₁, R₂
Represents a lower alkyl group, X represents Cl ^-, HSO_Four ^-, H
₂PO_Four ^-, R-SO₃ ^-(R is C₁~ C_FourThe alkyl
Group), NO₃ ^-Represents the anion of.

The polyoxyalkylene derivative is R ₂ --O.
_{- (R 1 -O) n-} H, R 2 -O- (R 1 -O) n-H
Is an organic compound represented by. The average molecular weight is 10 ^2-1
0 ⁶ is preferred. Of these, especially polyethylene glycol (PEG _{substantially) HO- (CH 2 -CH 2 -O} ) n-H
Is effective.

Next, the effect of the above depositing agent will be described with reference to the drawings. FIG. 1 shows the relationship between the amount of addition and the Cr content in the plating layer when potato dextrose agar was used as the polysaccharide and when PAS was additionally added as an example of the cationic polymer. , Showing the relationship with the appearance of the obtained plating film. Conditions other than the precipitant are constant, Zn ²⁺ ions 33 g / l, Cr ³⁺ ions 26 g
/ L, pH 1.5, bath temperature 50 ° C, liquid flow rate 60 m / mi
n, current density 100 A / dm ² , coating weight 20 g /
It was set to m ² . When the polysaccharide content is less than 0.1 g / l, Cr hardly precipitates, but when it is 0.1 g / l or more, Cr precipitation becomes remarkable, and when it is 0.5 g / l or more, a zinc-based electroplated steel sheet having a Cr content of 5% or more is obtained. You will get it. Also,
If 1 g / l of PAS is added as a cationic polymer,
The Cr content tends to increase as compared with the case of using the polysaccharide alone, and a glossy and uniform appearance can be obtained. There is a similar tendency for polysaccharides other than potato dextrose agar, Cr begins to precipitate at 0.1 g / l or more, and 5% or more Cr is present at 0.5 to 1 g / l or more.
The content rate can be obtained.

The type of plating bath is not particularly limited, but a sulfate bath, a chloride bath, or a mixed bath thereof is preferable. The metal ion concentration is preferably 30 to 150 g / l in total of Zn ²⁺ ions and Cr ³⁺ ions. If it is less than 30 g / l, the supply of ions is likely to be insufficient at a high current density, and as a result, plating burn is likely to occur. If it exceeds 150 g / l, the waste of ions due to dragout cannot be ignored, and crystallization may occur when the liquid temperature becomes low. Other plating conditions are not particularly limited, but pH 0.5 to 3, bath temperature 40 to 70 ° C., liquid flow rate 1 m / min or more, and current density 20 A / dm ² or more are preferable ranges, and The optimum conditions can be adopted in consideration of the plating composition and the current efficiency. In the plating bath, an auxiliary conductivity agent such as Na ⁺ and NH ₄ ⁺ , SiO ₂ , TiO ₂ , and Al ₂ for further improving corrosion resistance.
Oxide particles such as O ₃ and chromate particles such as BaCrO ₄ may be added, and there is no essential obstacle to the production method of the present invention.

According to the manufacturing method of the present invention, the corrosion resistance is improved in comparison with Zn plating by including Cr even in a small amount in the plating layer. However, in comparison with the conventional alloy plating, the Cr content is 5 to 5%. 30% is preferable. That is, when it is 5% or more, not only zinc plating but also Zn-N
It exhibits high corrosion resistance superior to that of conventional alloy plating such as i and Zn-Fe, and red rust does not easily occur even if a salt spray test is performed for 500 hours or more. The reason for this is that Cr does not passivate in the presence of Zn and exhibits a sacrificial anticorrosive action together with Zn, but the corrosion product formed on the plating surface functions as a sparingly soluble protective film, and It is presumed that this is to suppress the invasion of the corrosion factors. Cr content is 30%
If it exceeds, workability is deteriorated, which is not preferable. Further, a small amount of a polysaccharide, which is added as a Cr depositing agent, or a cationic polymer or a polyoxyalkylene derivative, may be co-deposited in the plating layer including the decomposition products thereof. In this case, if the eutectoid rate of carbon is 3% or less,
A dense and uniform plating layer is formed, which has an advantageous effect on workability.

[0023]

Example A cold rolled steel sheet was degreased with alkali, pickled with a 5% aqueous solution of sulfuric acid, and then electroplated under the conditions shown in Table 1.
Among Cr depositing agents, PA having an average molecular weight of 10,000, PAS having an average molecular weight of 3500 as a cationic polymer, and PE having an average molecular weight of 1500 as a polyoxyalkylene derivative.
G was used. A sulfate bath was used as the plating bath, and the coating weight was 20 g / m ² . Table 1 also shows the obtained plating composition, corrosion resistance, and appearance evaluation. In addition, corrosion resistance was tested by a salt spray test (according to JIS Z 2371) for 500 hours. It was determined. Regarding the plating appearance, the glossiness of the plating surface (JIS Z 8741 Gs (60 ⁺ ), black glass plate reference value 93) was measured, and ◎: glossiness 30 or more, ◯: glossiness 20 or more, Δ: glossiness 10 Above, x: Glossiness was determined to be less than 10.

[0024]

[Table 1]

In Comparative Example 1, since the amount of polysaccharide in the plating bath was small, the amount of Cr deposited was small and the corrosion resistance was poor. In Comparative Example 2, Cr ³⁺ ion concentration and current density in the plating bath were increased to deposit 6% of Cr, but the appearance was poor. Comparative Example 3 is Zn-Ni alloy plating, and although the appearance is good, the corrosion resistance is lower than that of the present invention. On the other hand, in each of the examples of the present invention, plating having a Cr content of 5% or more is obtained, and both corrosion resistance and appearance are good. In addition, Examples 14 to 18 of the present invention are examples in which a polysaccharide and a cationic polymer or a polyoxyalkylene derivative are used in combination, but the appearance is further improved as compared with the examples of the single use of polysaccharides of Examples 1 to 13 of the present invention. As a result, the content was also increased.

[0026]

INDUSTRIAL APPLICABILITY The present invention enables the co-deposition of Cr into zinc-based electroplating, which has been difficult in the past, and can produce a zinc-based electroplated steel sheet having excellent corrosion resistance and appearance. Therefore, it is extremely effective for producing a zinc-based electroplated steel sheet that requires high corrosion resistance and appearance, mainly for automobiles and home appliances.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the Cr content in a plating layer and the addition amount of a Cr depositing agent, ie, potato dextrose agar and PAS, in a plating bath.

FIG. 2 is a diagram showing the relationship between the amount of Cr depositing agent added to the plating bath and the appearance of the obtained plating film.

Claims (2)

[Claims] 1. When manufacturing a zinc-based electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, and C
A method for producing a zinc-based electroplated steel sheet having excellent corrosion resistance, which comprises electroplating a steel sheet using a plating bath containing 0.1 to 10 g / l of a polysaccharide as a precipitating agent. 2. When producing a zinc-based electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, a polysaccharide of 0.1 to 10 g / l, and further a cationic polymer and / or a polyoxyalkylene derivative 0. A method for producing a zinc-based electroplated steel sheet having excellent corrosion resistance, which comprises electroplating a steel sheet using a plating bath containing 1 to 5 g / l.