JPH07300693A - Production of zinc alloy electroplated steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To produce an electrogalvanized steel sheet contg. Cr and iron group metals and excellent in corrosion resistance. CONSTITUTION:A steel sheet is applied with electroplating by using a plating bath contg. Zn<2+> ions, iron-group bivalent 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 alloy 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. Among them, F, which is an iron group metal
The Zn-based alloy plating containing e, Ni, and Co as alloy components has been put into practical use because of its excellent bare corrosion resistance and post-painting corrosion resistance. Recently, Japanese Patent Laid-Open No. 3-12
As disclosed in Japanese Patent Application No. 0393 and Japanese Patent Application Laid-Open No. 5-9779, attempts have been made to incorporate Cr, which is a corrosion resistant metal, into Zn plating. JP-A-3-12039
No. 3, gazette discloses that tartaric acid is added to an acidic sulfate bath containing Zn ²⁺ ions and Cr ³⁺ ions for electroplating.
Japanese Patent Application Laid-Open No. 5-9779 discloses a method in which a pulse current or a direct current in which an alternating current is superposed is used without using an additive.
Manufacturing n-Cr electroplated steel sheets is disclosed. 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 alloy 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 alloy electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions,
Zinc-based alloy electroplated steel sheet with excellent corrosion resistance, characterized in that the steel sheet is electroplated using a plating bath containing 0.1 to 10 g / l of a ferrous group metal divalent ion and a polysaccharide as a Cr depositing agent. Manufacturing method.

In producing a zinc-based alloy electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions,
Iron group metal divalent ion, and polysaccharide 0.1 to 10 g / l,
Further, a method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, which comprises electroplating a steel sheet using a plating bath containing 0.1 to 5 g / l of a cationic polymer and / or a polyoxyalkylene derivative.

[0009]

[Function] Zn ²⁺ ion, iron group metal divalent ion, and Cr
By adding a polysaccharide as a Cr precipitation promoter in a plating bath containing ³⁺ ions, the above problem can be solved and a plating having a high degree of corrosion resistance containing a large amount of Cr can be obtained. It is presumed that the reason is that the polysaccharide is adsorbed on the surface of the steel plate serving as a 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 nutrient agar was used as the polysaccharide and when PAS was added as an example of a cationic polymer. , Showing the relationship with the appearance of the obtained plating film. Conditions other than the precipitant were constant, and Zn ²⁺ ions were 30 g / l and Ni ²⁺ ions were 25 g / l.
1, Cr ³⁺ ion 20 g / l, pH 1.5, bath temperature 50
° C, liquid flow rate 60 m / min, current density 100 A / d
m ² and coating weight 20 g / m ² . Polysaccharide 0.1
If less than g / l, almost no Cr precipitates, but 0.1 g
/ G or more, Cr precipitation becomes remarkable, 0.5 g / l
With the above, a zinc-based alloy electroplated steel sheet having a Cr content of 5% or more can be obtained. When PAS is added as the cationic polymer in an amount of 1 g / l, 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. The Ni content has a relative relationship with the Cr content, and the Ni content is high when the amount of the depositing agent is small and the Cr content is low, and the Ni content is high when the amount of the depositing agent is large and the Cr content is high. Fell. There is a similar tendency for polysaccharides other than nutrient agar, Cr starts to precipitate at 0.1 g / l or more, and a Cr content of 5% or more is obtained at 0.5 to 1 g / l or more. become.

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, iron group metal divalent 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,
A bath temperature of 40 to 70 ° C., a liquid flow rate of 1 m / min or more, and a current density of 20 A / dm ² or more are preferable ranges, and optimum conditions can be adopted in consideration of a target plating composition and current efficiency. In the plating bath, a conductivity aid such as Na ⁺ , NH ₄ ^+, and SiO for further improving corrosion resistance.
₂ , oxide particles such as TiO ₂ and Al ₂ O ₃ , BaCr
Chromate particles such as O 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, but in comparison with the conventional zinc-based alloy plated steel sheet containing no Cr, Is a Cr content of 5 to
30% is more preferable. That is, when it is 5% or more, not only zinc plating but also high corrosion resistance exceeding conventional alloy plating such as Zn-Ni and Zn-Fe is exhibited, 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. The iron group metal content is effective for corrosion resistance and weldability, and is preferably 0.1% or more. Iron group metal is C
Since the properties of the iron group metal become stronger when the r content is exceeded, the upper limit of the iron group metal content is less than the Cr content, and more preferably 1/2 or less of the Cr content. Further, if the total amount of Cr and the iron group metal exceeds 30%, workability may be deteriorated, so that it is preferably 30% or less. Here, the iron group metal is Fe,
It refers to Co and Ni and may be used alone or in combination, but Ni is the most preferable from the viewpoint of corrosion resistance. A small amount of a polysaccharide, which is added as a Cr depositing agent, or a cationic polymer or a polyoxyalkylene derivative, including its decomposition products, may be co-deposited in the plating layer. In this case, if the eutectoid ratio is 3% or less as the carbon amount, 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 Tables 1 and 2. Among the Cr depositing agents, PA having an average molecular weight of 10,000, PAS having an average molecular weight of 3500 as the cation polymer, and 1500 having an average molecular weight of 1500 as the polyoxyalkylene derivative.
PEG was used. A sulfate bath is used as the plating bath,
The coating weight was 20 g / m ² . Table 2 also shows the obtained plating composition, corrosion resistance, and evaluation of appearance. In addition, the corrosion resistance is 600 in salt spray test (based on JIS Z 2371).
Time: ⊚: red rust generation 1% or less, ◯: red rust generation 10% or less, Δ: red rust generation 50% or less, x: red rust generation 50% or more. 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]

[0025]

[Table 2]

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 5% 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. Further, Examples 14 to 26 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, and Cr As a result, the content was also increased.

[0027]

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 alloy electroplated steel sheet having excellent corrosion resistance and appearance. Therefore, it is extremely effective for producing a zinc-based alloy electroplated steel sheet that requires a high degree of corrosion resistance and appearance, mainly for automobiles and home appliances.

[Brief description of drawings]

FIG. 1 is the amount of Cr depositing agent, that is, nutrient agar and PAS, added to the plating bath and C in the plating layer.
It is a figure which shows the relationship of r content rate.

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 alloy electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions, and a polysaccharide as a Cr depositing agent are added in an amount of 0.1.
A method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, which comprises electroplating a steel sheet using a plating bath containing 1 to 10 g / l. 2. When producing a zinc-based alloy electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions, and polysaccharides 0.1 to 10 g / l, and further cations. A method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, which comprises electroplating a steel sheet using a plating bath containing a polymer and / or a polyoxyalkylene derivative of 0.1 to 5 g / l.