JP3111342B2 - Method for producing zinc-based alloy electroplated steel sheet with excellent corrosion resistance - Google Patents

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 Cr-containing zinc-based alloy electroplated steel sheet having excellent corrosion resistance and used as a material for automobiles and home electric appliances.

[0002]

2. Description of the Related Art Zinc-based electroplated steel sheets are widely used as materials for automobiles, home appliances, and building materials. Of these, in automobile applications, higher corrosion resistance is required in order to prevent vehicle body corrosion due to scattered rock salt for preventing road freezing in winter in cold regions. In addition, in automobiles and home appliances, demands for not only corrosion resistance but also appearance quality are increasing, and corrosion-resistant steel sheets having better appearance such as uniformity and glossiness are required.

[0003] Corrosion resistance of galvanized steel sheets can be improved by increasing the amount of zinc plating (adhesion), 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
Zn-based alloy plating containing e, Ni, and Co as alloy components is recognized for its good bare corrosion resistance and corrosion resistance after painting, and has been put to practical use. Recently, Japanese Patent Laid-Open No.
As disclosed in JP-A No. 0393 and JP-A-5-9779, attempts have been made to include Cr, which is a corrosion-resistant metal, in Zn plating. JP-A-3-12039
No. 3 discloses that electroplating is performed by adding tartaric acid to an acidic sulfate bath containing Zn ²⁺ ions and Cr ³⁺ ions.
JP-A-5-9779 discloses a method using a pulse current or a direct current superimposed with an alternating current without using an additive.
It is disclosed to produce n-Cr electroplated steel sheets. According to these, a large amount of Cr can be contained in the Zn plating, but the effect of the inclusion of Cr is not maximized with respect to corrosion resistance, and the appearance is uniform and glossy. This is not a satisfactory level in comparison with conventional alloy plating.

[0004]

Attempts to precipitate Cr together with Zn have conventionally been made using Cr ³⁺ or Cr ⁶⁺ ions, but only a small amount of Cr precipitates from a simple bath. As described in JP-A-5-9779, even when a large amount of Cr is precipitated using a pulse current, not only good appearance is not obtained, but also workability and current efficiency are significantly reduced, and Was not advantageous.

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,
Electroplated steel sheet using a plating bath containing 0.1 to 10 g / l of polysaccharides as iron group metal divalent ion and Cr precipitant, zinc-based alloy electroplated steel sheet excellent in corrosion resistance Manufacturing method.

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,
A method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, further comprising 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]

[Action] Zn ²⁺ ion, iron group metal divalent ion, and Cr
By adding a polysaccharide as a Cr precipitation accelerator to a plating bath containing ³⁺ ions, the above problem can be solved, and a highly corrosion-resistant plating containing a large amount of Cr can be obtained. This is presumed to be because the polysaccharide is adsorbed on the surface of the steel plate serving as a cathode, the steric effect on the structure suppresses the discharge of Zn ²⁺ ions, and the precipitation of Cr ³⁺ ions is performed efficiently. At this time, it is conceivable that Cr ³⁺ ions are preferentially coordinated to 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 less than 0.1 g / l, there is no Cr precipitation effect and 10 g
If it exceeds / l, it is not practical because it becomes difficult to dissolve in the plating bath. A more preferred range is from 0.5 to 5 g / l.
Here, the polysaccharide is a compound having a structure of polyhydroxyaldehyde or polyhydroxyketone, or a substance which gives these compounds by hydrolysis. Among the most effective polysaccharides are agar, potato dextrose agar, nutrient agar, ascorbic acid,
Potato starch, gellite, sorbitol, mannitol, chitin, chitosan, dextran, pullulan, cyclodextrin.

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

As the cationic polymer, a polymer of a quaternary amine salt represented by Chemical Formula 1 to Chemical Formula 5 is preferable. Particularly, a polyamine of Chemical Formula 2 or Chemical Formula 3 (abbreviated as PA) and a polyamine sulfone of Chemical Formula 4 or Chemical Formula 5 (abbreviated as PAS) are preferred. ) Is effective. This is because, Cr ³⁺ to the adsorption action and a sulfonic group into the cathode surface by an amine group
It is presumed that coordination bonds of ions contribute. The average molecular weight is preferably 10 ² to 10 ^6. In addition,
Polymers of primary, secondary and tertiary amines are effective, although they are inferior to the above-mentioned quaternary amine polymers.

[0013]

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[0014]

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[0015]

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[0016]

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[0017]

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In each of the above chemical formulas, R₁, R_Two
Represents a lower alkyl group, and X represents Cl ^-, HSO_Four ^-, H
_TwoPO_Four ^-, R-SO_Three ^-(R is C₁~ C_FourThe alkyl of
Group), NO_Three ^-Represents an anion of

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

Next, the effect of the above precipitant 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 the cationic polymer in a complex manner. And the relationship with the appearance of the plating film obtained. Conditions other than the precipitating agent were constant, and Zn ²⁺ ion 30 g / l, Ni ²⁺ ion 25 g /
1, Cr ³⁺ ion 20 g / l, pH 1.5, bath temperature 50
° C, liquid flow rate 60m / min, current density 100A / d
m ² , and a coating weight of 20 g / m ² . Polysaccharide 0.1
When the amount is less than 0.1 g / l, Cr hardly precipitates.
/ L or more, Cr precipitation becomes remarkable, and 0.5 g / l
As described above, a zinc-based alloy electroplated steel sheet having a Cr content of 5% or more can be obtained. Further, when 1 g / l of PAS is added as a cationic polymer, the Cr content tends to increase as compared with the case of using only the polysaccharide, and a glossy uniform appearance can be obtained. The Ni content is in a relative relationship with the Cr content. When the amount of the precipitant is small and the Cr content is low, the Ni content is high, and when the amount of the precipitant is large and the Cr content is high, the Ni content is Dropped. A similar tendency is observed for polysaccharides other than the nutrient agar. Cr is precipitated 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 the plating bath is not particularly limited, but a sulfate bath, a chloride bath, or a mixed bath thereof is desirable. 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 tends to be insufficient at a high current density, and as a result, burning of the plating tends to occur.
If it exceeds 150 g / l, waste of ions due to drag-out cannot be ignored, and crystallization may occur when the liquid temperature is lowered. Although other plating conditions are not particularly limited, pH 0.5 to 3,
The preferred ranges are 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, and optimal conditions can be adopted in consideration of a desired plating composition and current efficiency. In the plating bath, conductivity aids such as Na ⁺ and NH ₄ ⁺ are used, and SiO for further improving corrosion resistance is used.
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 problem in the production method of the present invention.

According to the production method of the present invention, although the corrosion resistance is improved in comparison with Zn plating by including a small amount of Cr in the plating layer, it is compared with a conventional zinc-based alloy plated steel sheet containing no Cr. Has a Cr content of 5
30% is more preferred. That is, when it is 5% or more, not only zinc plating but also higher corrosion resistance than conventional alloy plating such as Zn-Ni and Zn-Fe is exhibited. For example, even if a salt spray test is performed for 500 hours or more, red rust does not easily occur. The reason is that Cr does not passivate in the coexistence with Zn and exhibits sacrificial corrosion protection together with Zn, but the corrosion products formed on the plating surface function as a hardly soluble protective film, and It is presumed to suppress the invasion of corrosion factors. The content of the iron group metal is effective for corrosion resistance and weldability, and is preferably 0.1% or more. Iron group metal is C
If the content exceeds the r content, the property of the iron group metal becomes strong. Therefore, the upper limit of the iron group metal content is less than the Cr content, and more preferably １ ／ or less of the Cr content. Further, when the total of Cr and the iron group metal exceeds 30%, there is a concern that the workability is deteriorated. Here, the iron group metal is Fe,
Co and Ni may be used singly or in combination, but Ni is most preferable in terms of corrosion resistance. A trace amount of a polysaccharide, a cationic polymer or a polyoxyalkylene derivative added as a Cr precipitant, including its decomposition product, may be co-deposited in the plating layer. In this case, if the eutectoid rate is 3% or less in terms of the amount of carbon, 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 an alkali, pickled with a 5% aqueous sulfuric acid solution, and then electroplated under the conditions shown in Tables 1 and 2. Of the Cr precipitants, PA having an average molecular weight of 10,000, PAS having an average molecular weight of 3500 as a cationic polymer, and an average molecular weight of 1500 as a polyoxyalkylene derivative
PEG was used. As a plating bath, use a sulfate bath,
The coating weight was 20 g / m ² . Table 2 also shows the evaluation of the obtained plating composition, corrosion resistance, and appearance. The corrosion resistance was determined by a salt spray test (based on JIS Z 2371) of 600.
Performed for a time, ◎: Red rust generation 1% or less, ○: Red rust generation 10% or less, Δ: Red rust generation 50% or less, ×: Red rust generation more than 50%. The coating appearance, gloss of the plated surface ^{(JIS Z 8741 Gs (60 +} ), black glass plate reference value 93) was measured, ◎: glossiness of 30 or more, ○: Gloss 20 or more, △: Gloss 10 X: The gloss 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 precipitation amount of Cr was small and the corrosion resistance was poor. In Comparative Example 2, 5% of Cr was precipitated by increasing the Cr ³⁺ ion concentration and the current density in the plating bath, but the appearance was poor. Comparative Example 3 is a 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 with a Cr content of 5% or more was obtained, and both corrosion resistance and appearance were good. Examples 14 to 26 of the present invention are examples in which a polysaccharide is used in combination with a cationic polymer or a polyoxyalkylene derivative. The result that the content rate also became high was obtained.

[0027]

According to the present invention, it is possible to eutect Cr into zinc-based electroplating, which has been difficult in the past, and to produce a zinc-based alloy electroplated steel sheet having extremely excellent corrosion resistance and appearance. Therefore, it is extremely effective in producing a zinc-based alloy electroplated steel sheet requiring high corrosion resistance and appearance, mainly for automobiles and home appliances.

[Brief description of the drawings]

FIG. 1 shows the amounts of Cr precipitants, ie, nutrient agar and PAS, added to a plating bath and C in a plating layer.
It is a figure which shows the relationship of r content.

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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-24295 (JP, A) JP-A-1-215998 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25D 1/00-7/12 C25D 15/02

Claims (2)

(57) [Claims] When producing a zinc-based alloy electroplated steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions, and 0.1% of a polysaccharide as a Cr precipitant are used.
A method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, comprising 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⁺ ion, Cr ³⁺ ion, iron group metal divalent ion, and polysaccharide 0.1 to 10 g / l, and cation are further used. A method for producing a zinc-based alloy electroplated steel sheet having excellent corrosion resistance, wherein a steel sheet is electroplated using a plating bath containing 0.1 to 5 g / l of a polymer and / or a polyoxyalkylene derivative.