JPH06212483A - Method for manufacturing high corrosion resistant electrogalvanized steel sheet - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing an electrogalvanized steel sheet containing Cr or Cr and an iron group metal, which has excellent corrosion resistance. [Composition] When manufacturing an electrogalvanized steel sheet containing Cr or Cr and an iron group metal, Zn ²⁺ ions, C
r ³⁺ ion, divalent iron group metal ion and 0.1 to 10 g /
A method for producing a highly corrosion-resistant Zn-based electroplated steel sheet, which comprises electroplating using a plating bath containing 1 of polyvinyl alcohol or a derivative thereof. The plating bath may further contain 0.01 to 20 g / l of a cationic polymer or a polyoxyalkylene derivative.

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 galvanized steel sheet having high corrosion resistance containing Cr or Cr and an iron group metal.

[0002]

2. Description of the Related Art Electrogalvanized steel sheets are used for automobiles, home appliances,
Widely used mainly for 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. Although the corrosion resistance of galvanized steel sheets can be improved by increasing the zinc plating amount (adhesion amount),
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, F
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. Also, JP-A-61-27039
No. 8, JP-A-62-54099, and JP-A-3-
As disclosed in Japanese Patent No. 120393, etc., Zn
Although attempts have been made to add Cr to Zn-based alloy plating, the corrosion resistance has not reached that of Zn-Ni alloy plating or Zn-Fe alloy plating.

[0003]

Attempts to precipitate Cr with zinc have been conventionally made using Cr ³⁺ and Cr ⁶⁺ ions, but only a trace amount of Cr is precipitated from a simple bath, Several% of C due to increase of Cr concentration and current density
Even if r is deposited, not only a good appearance is not obtained, but also a workability and a current efficiency are significantly lowered, and there is no industrially advantageous method. The present invention solves the above problems and provides C
It is intended to provide a method for producing a high corrosion resistant galvanized steel sheet containing r or Cr and an iron group metal.

[0004]

Means for Solving the Problems The gist of the present invention is as follows. (1) In producing an electrogalvanized steel sheet containing Cr, Zn ²⁺ ions, Cr ³⁺ ions and 0.1 to 10 g are prepared.
A method for producing a highly corrosion-resistant galvanized steel sheet, which comprises performing electroplating using a plating bath containing polyvinyl alcohol of 1 / l. (2) When manufacturing an electrogalvanized steel sheet containing Cr and an iron group metal, Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions and 0.1 to 10 g / l of polyvinyl alcohol are contained. A method for producing a high corrosion resistant electrogalvanized steel sheet, which comprises performing electroplating using a plating bath. (3) High corrosion resistance electricity according to the above (1) or (2), characterized in that the plating bath further contains 0.01 to 20 g / l of a cationic polymer and / or a polyoxyalkylene derivative for electroplating. It is a manufacturing method of a galvanized steel sheet.

[0005]

[Effect] Even if a conventional simple bath composed of Zn ²⁺ ions and Cr ³⁺ or Cr ⁶⁺ ions is used, only a small amount of Cr is deposited.
Even if r was deposited, a plating that could withstand practical use was not obtained in terms of appearance and workability. However, this problem is solved by adding polyvinyl alcohol as a Cr precipitation promoter in a zinc-based plating bath containing Cr ³⁺ ions,
It is possible to obtain a plating that has excellent appearance and workability and that has a high degree of corrosion resistance and contains a large amount of Cr. The reason for this is presumed that polyvinyl alcohol forms a complex with Cr ³⁺ ions and facilitates the precipitation of Cr ³⁺ ions while suppressing the precipitation of Zn ²⁺ ions.

The addition amount of polyvinyl alcohol is 0.1
10 g / l. If it is less than 0.1 g / l, there is no Cr precipitation effect, and if it exceeds 10 g / l, it is difficult to dissolve it in the plating bath, which is not practical. More preferable range is 0.1 to 2
g / l. Here, polyvinyl alcohol is
An organic compound represented by (—CH ₂ —CH (OH) —) _n having a molecular weight of 10 ² to 10 ⁶ may be used, and particularly a molecular weight of 10 ³ to 10 ⁵ may be used for Cr precipitation. Is effective. In the plating bath, in addition to polyvinyl alcohol, a cationic polymer or a polyoxyalkylene derivative may be added in an amount of 0.01 to 20 g / l, whereby the appearance quality can be improved and the Cr precipitation can be prevented. However, it is more advantageous. Either one or both of the cationic polymer and the polyoxyalkylene derivative may be used.

As the cationic polymer, the following quaternary
Polymers of amine salts are preferred, especially polyamine sulfone.
(PAS) and polyamines (PA) are effective
is there. The reason for this is that absorption by the amine group on the cathode surface
Adhesion and Cr to sulfone group ³⁺Contribution of coordination bond of ions
It is estimated that 10 as the average molecular weight²~ 1
0⁶Is preferred.

[0008]

[Chemical 1]

[0009]

[Chemical 2]

[0010]

[Chemical 3]

R ₁ and R ₂ represent a lower alkyl group, X represents Cl ⁻ , HSO ₄ ⁻ , H ₂ PO ₄ ⁻ , R—SO ₃ ^— (R represents C ₁ to
C ₄ alkyl group), and NO ₃ ⁻ anion. In addition, polymers of primary, secondary, and tertiary amines are effective even though they do not reach the above-mentioned quaternary amine polymers.

The polyoxyalkylene derivative is R ₂ --O.
An organic compound represented by — (R ₁ —O) _n —H. The average molecular weight is preferably 10 ² to 10 ⁶ . Among them, especially polyethylene glycol (abbreviated as PEG) and HO
- is _{(CH 2 -CH 2 -O) n} -H is effective. FIG. 1 shows the relationship between the amount of the Cr deposition accelerator added and the Cr content in the plating layer, and FIG. 2 shows the relationship between the amount of the Cr deposition accelerator added and the appearance of the obtained plating film. Conditions other than the addition amount of the Cr precipitation accelerator, that is, the plating bath composition and the plating conditions are constant. When the content of polyvinyl alcohol is less than 0.1 g / l, the Cr content is about 1%, but when it is 0.1 g / l or more, the Cr content becomes 5% or more. Also, as an example of a cationic polymer, when PAS is added at 1 g / l, Cr
The content tends to increase as compared with the case of using polyvinyl alcohol alone, and a glossy and uniform appearance can be obtained.

As the plating bath, a sulfate bath, a chloride bath,
Any of these mixing baths can be applied. The metal ion concentration is preferably 30 to 200 g / l in total of Zn ²⁺ ions, Cr ³⁺ ions, and iron group metal divalent ions. 30 g /
If it is less than 1, the current supply tends to be insufficient at a high current density, resulting in plating burn. When it exceeds 200 g / l, the waste of ions due to dragout cannot be ignored, and there is a concern of crystallization when the liquid temperature becomes low. Other plating conditions are not particularly limited, but pH 0.5 to 3, bath temperature 40 to 7
0 ° C, liquid flow rate 1m / min or more, current density 20A / dm
² or more is a preferred range, the desired plating composition,
Optimal conditions can be adopted in consideration of current efficiency. In the plating bath, an auxiliary conductivity agent such as Na ⁺ , NH ₄ ⁺ , SiO ₂ or Ti for further improving corrosion resistance
Oxide particles such as O ₂ and Al ₂ 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.

Thus, an electrogalvanized steel sheet containing Cr and an iron group metal can be manufactured, but the Cr content in the plating layer is preferably 5% or more. Even if it is less than 5%, an improvement in corrosion resistance is recognized as compared with galvanizing, but if it is 5% or more, not only zinc plating but also Zn-N
It exhibits high corrosion resistance superior to alloy plating of i, Zn-Fe and the like, 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. When 0.1% or more of iron group metal is contained in addition to Cr, it is effective for corrosion resistance and weldability. Regarding the iron group metal content, if it is higher than the Cr content, the properties of the iron group metal become stronger,
It is desirable that the Cr content is less than, and more preferably 1/2 or less of the Cr content.

Regarding the sum of Cr and the iron group metal,
If this amount increases, the workability decreases, so 30% or less,
It is more preferably 20% or less. Here, the iron group metal refers to Fe, Co, and Ni, and may be used alone or in combination. Ni is the most preferable in terms of contribution to corrosion resistance. Further, polyvinyl alcohol, which is a Cr deposition accelerator, or a cationic polymer or a polyoxyalkylene derivative may be co-deposited in a trace amount in the plating layer.
If the eutectoid ratio is 5% or less, a dense and uniform plating layer is formed, which is advantageous for formability.

[0016]

EXAMPLES Cold-rolled steel sheets were 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 precipitation accelerators, polyvinyl alcohol has A: average molecular weight of 10,000 and B: average molecular weight of 100,000, and cationic polymer has PA of average molecular weight of 10,000, PAS of average molecular weight of 3500, and polyoxyalkylene derivative. PEG with an average molecular weight of 1500
Was used. A sulfate bath was used as the plating bath, and the coating weight was 20 g / m ² . Table 2 shows the plating composition thus obtained, the corrosion resistance, and the evaluation of the appearance. The corrosion resistance was evaluated by performing a salt spray test (according to JIS Z2371) for 500 hours and following the procedure below.

◎: Red rust occurrence is less than 1% ○: Red rust occurrence is 10%
Below △: 50% or less of red rust generation ×: Over 50% of red rust generation For the plating appearance, the gloss of the plating surface (JIS Z 87
41 G _S (60 °) and black glass plate reference value 93) were measured and judged according to the following procedure. ◎: Gloss 30 or more ○: Gloss 20 or more △: Gloss 10 or more ×: Gloss less than 10

In Tables 1 and 2, Comparative Example 1 contains a small amount of polyvinyl alcohol, and therefore C in the plating layer
The r content is low and the corrosion resistance is poor. In Comparative Example 2, although the current density was extremely increased and Cr was deposited by several% with the same plating bath composition as Comparative Example 1, good corrosion resistance could not be obtained, and the appearance was also poor. Comparative Example 3 is an example containing an iron group metal, but since the addition amount of polyvinyl alcohol is small, the Cr content is also low and the corrosion resistance is insufficient. Comparative Example 4 is Zn-Ni alloy plating, and although the appearance is good, the corrosion resistance is lower than that of the example of the present invention. Inventive Examples 1 to 7 are Zn
-Cr and Inventive Examples 8 to 16 are Zn-Cr-iron group metals, but all have a Cr content of 5% or more and are excellent in corrosion resistance and appearance. Inventive Examples 4 to 7 and Inventive Examples 12 to 16 are examples in which polyvinyl alcohol and a cationic polymer are used in combination.
Further, as compared with the examples of using polyvinyl alcohol alone of Examples 8 to 11 of the present invention, the result that the appearance was further improved was obtained, and Cr
The content also tended to increase.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

As described above, the present invention makes it possible to coprecipitate Cr into electrogalvanizing, which has been difficult in the past, by charging a specific additive into the plating bath, and has excellent corrosion resistance. This is to obtain an electrogalvanized steel sheet. Therefore, it is extremely effective for producing a rust-preventing steel plate which requires high corrosion resistance mainly for automobile body applications.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of polyvinyl alcohol and a cationic polymer added to a plating bath and the Cr content in a plating layer.

FIG. 2 is a diagram showing the relationship between the addition amounts of polyvinyl alcohol and a cationic polymer in a plating bath and the appearance of the obtained plating film.

Claims (3)

[Claims] 1. An electrogalvanized steel sheet containing Cr is manufactured.
When making, Zn ²⁺Ion, Cr³⁺Ion and 0.1
Plating containing polyvinyl alcohol of 10 to 10 g / l
High resistance to electroplating using a bath
Method for producing galvanic galvanized steel sheet. 2. In producing an electrogalvanized steel sheet containing Cr and an iron group metal, Zn ²⁺ ions, Cr ³⁺ ions, iron group metal divalent ions and 0.1 to 10 g / l of polyvinyl alcohol are added. A method for producing a highly corrosion-resistant galvanized steel sheet, which comprises performing electroplating using a contained plating bath. 3. Further 0.01 to 20 g / l in the plating bath
The method for producing a highly corrosion-resistant galvanized steel sheet according to claim 1 or 2, wherein the electropolymerization is performed by incorporating the cationic polymer and / or the polyoxyalkylene derivative.