JP3358479B2 - Method for producing composite zinc-based electroplated metal sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a composite zinc-based electroplated metal sheet having excellent corrosion resistance and workability, which is suitable as a material for automobiles, home electric appliances, building materials and the like. More specifically, the present invention relates to a method for efficiently performing electroplating in which a polymer organic substance is codeposited in a plating film.

[0002]

2. Description of the Related Art Various surface-treated metal sheets, particularly surface-treated steel sheets, have been used in various fields such as automobiles, home appliances, building materials, and the like. The demand for is increasing.

For example, a surface-treated steel sheet used for an exterior of an automobile is required to have a 10-year corrosion resistance against pitting in a cold region in which rock salt for snow melting is sprayed on a road in winter and an external rust resistance for 5 years. Achievable high corrosion resistance is required. Even if an impact load such as a stone jump is applied in a low temperature environment (-50 ° C to 0 ° C), the adhesion of the plating film is not impaired (hereinafter referred to as low temperature impact resistance), and the coating film after coating is applied. It is also necessary to ensure the corrosion resistance (hereinafter referred to as corrosion resistance after painting) of the scratched portion and the end face of the coated steel plate.

[0004] In addition to the corrosion resistance, a rust-preventive steel sheet for an automobile exterior has a plating film that does not stick to a mold when the steel sheet is processed, and that a plating film layer does not powder or fall off ( Hereinafter, the workability of the plating film is described), and it is necessary that the welding workability in the assembling process is not impaired. Furthermore, since these metal plates are used in large quantities, it is also important to establish a manufacturing technology that can be supplied economically and stably.

In order to improve the corrosion resistance of galvanized steel sheets, which are the most common rust-proof metal sheets, Zn, Fe, Ni, C
There is a zinc alloy plated steel sheet plated with an alloy with at least one metal selected from the group consisting of o, Mn, Cr, Al and the like. Also,
In order to improve corrosion resistance, a multi-layer galvanized steel sheet provided with two or more plating films has also been proposed. For example, described in JP-A-60-215789, after galvanized,
A steel plate in which the adhesion of a coating film is improved by further plating a zinc-based alloy layer containing Ni, Co, etc., and JP-B-58-15554
There is a steel sheet described in Japanese Patent Application Publication No. H10-110, in which an Fe-based flash plating is further performed on an alloyed hot-dip galvanized steel sheet to improve the electrodeposition coating property. However, these zinc-based alloy-plated steel sheets or multi-layer plated steel sheets cannot satisfy the requirements for high corrosion resistance, workability of a plating film, or economical and efficient production.

As a method for solving these problems, Japanese Patent Application Laid-Open No. 7-70796 discloses a composite comprising Zn and one or more of Fe, Co and Ni having a C-concentrated layer on the surface of a plating film. A zinc alloy plated metal sheet is disclosed. In addition to corrosion resistance without coating and corrosion resistance after coating, this metal plate has improved workability of plating film, spot weldability and low-temperature impact resistance. It is expected as a suitable surface treatment plate.

Japanese Patent Application Laid-Open No. 8-209382 discloses a method for producing a zinc-based electroplated steel sheet in which C is eutectoid in a plating film. This is because, by electroplating using a zinc alloy plating bath containing dextrin and / or dextran (hereinafter simply referred to as “dextrin or the like”), C is co-deposited in the plating film and the orientation of the η phase of the plating film is obtained. This is a method for producing a steel sheet in which corrosion resistance, workability, and the like are improved by changing the properties.

However, when dextrin or the like, which is a high-molecular organic substance described in this production method, is added to an electroplating bath, the dextrin or the like does not completely dissolve and undissolved residues are formed, which precipitate in the plating tank and cause May float inside.

When electroplating is performed, the presence of lead ions in the plating bath tends to impair the heat resistance of the product and the corrosion resistance without painting. Lead ions elute from the electrode holding material or the electrode itself. Normally, strontium carbonate is added to the plating bath to co-precipitate lead, and this co-precipitate is filtered through a filter provided in the plating bath circulation path, etc., while maintaining the cleanliness of the plating bath. We are promoting. If undissolved residues such as dextrin increase in the plating bath, they remain and accumulate in the filter, shortening the life of the filter. Furthermore, dextrin or the like remaining on the filter when a plated steel sheet of another plating type is produced may be mixed into the plating bath and deteriorate the quality of the product.

Therefore, when plating is performed using a plating bath to which dextrin or the like is added, another plating solution circulation path is provided so that the plating bath can be circulated without passing through a filter, or the frequency of washing the filter is increased. Must be performed. When a plating bath circulation path that does not pass through a filter is provided, the adverse effect of lead ions cannot be removed, so that the product performance is easily impaired, and increasing the frequency of cleaning the filter impairs the workability of plating.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for efficiently producing a composite zinc-based electroplated metal sheet having excellent corrosion resistance and workability.

[0012]

The gist of the present invention resides in a method for producing a composite zinc-based electroplated metal sheet described in the following (1) and (2).

(1) Using a zinc-based plating bath containing 0.01 to 10% by weight of a high molecular weight organic material having a number average molecular weight of 10 ⁸ or less and a non-reducing terminal group to reducing terminal group ratio of 15 or more. A method for producing a composite zinc-based electroplated metal sheet, comprising electroplating.

(2) One of amylopectin, dextrin and dextran having a number average molecular weight of 10 ⁸ or less and a ratio of non-reducing terminal groups to reducing terminal groups of 15 or more.
A method for producing a composite zinc-based electroplated metal sheet, wherein electroplating is performed using a zinc-based plating bath containing 0.01% to 10% by weight of a total of two or more species.

When electroplating is performed using a plating bath containing a high molecular organic substance in a zinc-based electroplating bath, in order to ensure the quality of the plated metal sheet and the efficiency of the plating operation, the high molecular organic substance must be used. It is necessary to sufficiently dissolve in the plating bath.

Starch is a typical high molecular organic substance, but starch itself is not sufficiently soluble in a plating bath. Ordinary starch has the general formula (C ₆ H ₁₀ O ₅ ) _n · H
It is a polysaccharide represented by ₂ O and is said to be composed of 20 to 25% by weight of amylose and 75 to 80% by weight of amylopectin.

Amylopectin has relatively good solubility. A molecule constituting amylopectin is a high molecular weight organic substance having a highly branched molecular structure in which D glucose is branched during the course of linear polymerization of D glucose. It has a terminal group and a number of non-reducing terminal groups. If there are many non-reducing end groups in one molecule, the molecular form is stabilized, so that self-aggregation is difficult and the solubility is excellent.

Amylose has poor solubility in a solution.
The molecule constituting amylose is a high molecular organic substance having a molecular structure in which D-glucose is not branched and polymerized in a linear manner. Since there is only one non-reducing terminal group in one molecule and the molecular form is unstable, self-aggregation is liable to occur. It seems that the solubility is not good.

As the content of metal salts (for example, metal ions such as Zn, Ni, and Co in a plating bath) increases in the solution, the organic polymer becomes more difficult to dissolve. This is because, when metal ions are added to water, the surrounding water molecules are fixed as water of hydration by the hydration power, so that the amount of water molecules effective as a solvent for the solute is reduced and the solute is precipitated. It is understood to be due to the salting out phenomenon. In order to ensure the solubility of the high molecular organic substance in the plating bath, it is necessary to consider this salting out phenomenon.

As described above, the solubility of high molecular organic substances in the plating bath is considered to be good if the molecular structure of these constituents is rich in non-reducing end groups. The present inventors examined the solubility of the high-molecular organic substance in the plating bath, and found that the number-average molecular weight of the high-molecular organic substance and the ratio of the non-reducing end group to the reducing terminal group contained in the high-molecular organic substance were determined. It has been found that the solubility in the plating bath can be determined by combining the above.

Amylopectin is preferably used as a raw material for supplying a polymer organic substance to the plating film. When ordinary starch is added to a plating bath, a precipitate mainly composed of amylose is liable to be formed, which is not preferable. When starch, which is a mixture of amylose and amylopectin, is hydrolyzed, dextrin or the like, which is a polysaccharide in which decomposition products having various degrees of polymerization are mixed, is obtained. Its molecular weight ranges from a high molecular weight in which starch is slightly hydrolyzed to a low molecular weight in which no iodine starch reaction is exhibited. Decomposition products generated from amylose are likely to be composed of linear molecules containing one non-reducing terminal group in one molecule, and are not easily dissolved in the plating bath. For this reason, when ordinary dextrin or the like is also added to the plating bath, a precipitate is easily formed.

By subjecting such a dextrin or the like to a heat treatment and repolymerization, a dextrin or the like composed of a molecule having a plurality of non-reducing end groups in one molecule can be obtained (hereinafter referred to as " Repolymerized dextrin, etc.)). This repolymerized dextrin and the like can also be used as a source of a high molecular organic substance because it is sufficiently dissolved in the plating bath.

The solubility in the plating bath was determined by adding a powder of a high molecular organic substance to the plating bath under the following conditions, stirring the mixture for 10 minutes, and visually determining the dissolution state. If not, it is considered good.

[0024] Component ZnSO ₄ · 7H ₂ plating baths O: 350 g / liter (35 _{wt%) CoSO 4 · 7H 2 O} : 60g / l (6 _{wt%) Na 2 SO 4: 75g} / l (7.5 wt%) High molecular organic matter: 0.1 to 100 g / liter (0.01 to 10% by weight) pH: 1.8 Temperature of liquid: 40 to 65 ° C

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing a composite zinc-based electroplated metal sheet of the present invention will be described in detail. The percentages shown below for the contents of alloy elements, high molecular weight organic substances, etc., mean weight%.

The base material used for the composite zinc-based electroplated metal plate to which the production method of the present invention is applied is an automobile, a home appliance,
Various cold-rolled steel sheets used as materials such as building materials are suitable. However, depending on the application, hot-rolled steel plate, aluminum plate to reduce the body weight, or
A metal plate or the like plated with Zn or Al may be used as a base material. The base material is subjected to a pretreatment such as degreasing or pickling according to a conventional method, and then electroplated.

The electroplating bath: the number-average molecular weight of ¹⁰⁸ or less, the ratio of the non-reducing end group to the reducing end groups (hereinafter,
A zinc-based plating bath containing 0.01 to 10% by weight of a high-molecular organic material having a “ratio of non-reducing end groups” of 15 or more is used.

Here, the ratio of the non-reducing terminal group is expressed by the ratio (Wnr / Wn) of the amount of glucose having a non-reducing terminal group (Wnr) to the amount of glucose having a reducing terminal group (Wr).
Defined in r). Note that glucose is a unit constituting an organic substance. The amount of glucose having a non-reducing terminal group is determined by completely methylating and hydrolyzing a high-molecular-weight organic substance, and then determining the non-reducing terminal amount by chromatography (Haworth
Method). The amount of glucose having a reducing terminal group can be determined by oxidizing the reducing terminal group with I ₂ and determining the amount of consumed I ₂ by the WS method (Wilstedtard-Schötzel method).

There is no particular limitation on the metal element source contained in the zinc-based plating bath in the present invention. Zn alone,
In order to improve corrosion resistance, etc., Fe, Ni, Co are added to Zn.
, Mn, Cr, Al and the like, may contain one or more alloying element sources. However, in order to maintain good corrosion resistance (perforation resistance) without painting, Fe, Co, N
It is desirable to use a plating bath containing one or more of i in a total of 0.01 to 10%. If the total of these alloy elements exceeds 10%, η phase (zinc phase)
Disappears, the sacrificial corrosion protection function of the plating film is reduced, and the end face corrosion resistance after coating may be impaired. From the viewpoint of both economy and performance, it is more desirable to contain Co in the range of 0.4 to 2%.

As the plating bath, any of an acidic bath (for example, a sulfate bath and a chloride bath) and an alkaline bath (for example, a cyanide bath) can be used. However, it is preferable to use an acidic bath, particularly a sulfate bath, since a metal salt tends to precipitate in an alkaline bath. The metal element source constituting the plating film is preferably added to the plating bath as metal ions such as sulfate, acetate, carbonate, molybdate, hypophosphite, and organic metal salt.

High-molecular-weight organic matter: If the high-molecular-weight organic matter added to the plating bath is not sufficiently dissolved, it adversely affects the plating operation and the quality of the plated plate. For this reason, in the present invention, a high molecular organic substance having a number average molecular weight of 10 ⁸ or less and a ratio of non-reducing end groups of 15 or more is used.

Since the solubility in the plating bath is impaired as the number average molecular weight of the high molecular weight organic substance increases, the upper limit of the number average molecular weight is set at 10 ⁸ . The lower limit of the number average molecular weight is not particularly specified, but is preferably 10 ³ or more. Number average molecular weight is
If less than 10 ^3, there is a case where corrosion resistance in unpainted plating film is impaired. From the viewpoint of solubility in the plating bath and effects on corrosion resistance, those having a number average molecular weight of 10 ⁵ to 10 ⁷ are more preferably used.

Further, in order to ensure the solubility in the plating solution, the ratio of the non-reducing terminal groups needs to be 15 or more. On the other hand, as the ratio of non-reducing end groups increases, the solubility in the plating solution saturates, and it becomes difficult to obtain, resulting in lack of economy. Therefore, the ratio of non-reducing end groups is 1000
It is preferable to use the following.

When the content of the organic polymer in the plating bath is 0.01
%, The amount of high molecular weight organic material to be eutectoid in the plating film is insufficient, and the corrosion resistance and workability of the plating film become insufficient. When the content of the high molecular organic matter in the plating bath exceeds 10%, undissolved residues in the plating bath increase to contaminate the plating bath and increase the viscosity of the plating bath, so that the plating operation becomes difficult. Further, suspended particles in the plating bath adhere to the plating surface and impair the appearance quality of the product. For this reason,
The content of the organic polymer in the plating bath is 0.01 to 10%.

The object of the present invention can be achieved if the high molecular organic substance satisfies the above conditions. Repolymerized dextrin or the like in which the ratio of non-reducing terminal groups is increased by repolymerization may be used. In particular, amylopectin, which is one of the components of starch, is preferable because it is inexpensive and has a high ratio of non-reducing end groups. These high-molecular organic substances may be directly introduced into the plating bath, but it is desirable to add the high-molecular organic substances in a slurry form in order to shorten the dissolution time in the plating bath.

Electroplating method: Electroplating methods include:
There are continuous plating method where plating current is supplied continuously and pulse plating method where plating current is supplied intermittently.
In the present invention, any method may be used. The plating may be applied to the required surface of the base metal plate, may be applied to only one surface, or may be applied to both surfaces. Although there is no particular limitation on the amount of adhesion, if the amount is too small, the corrosion resistance is impaired. If the amount is too large, the effect on the corrosion resistance is saturated, so that the economic efficiency is impaired. For this reason, 5-200 g / m ² is preferable.

As the molecular weight of the high molecular weight organic substance, the number average molecular weight measured by gel permeation chromatography, which is one type of liquid chromatography, which is analyzed by a column filled with an inert crosslinked polymer gel, is used as an index. The content of the high molecular organic substance in the plating bath is measured by the phenol sulfuric acid method. The content of the high molecular organic substance in the plating film is measured by dissolving the plating film with 5% sulfuric acid, decomposing the high molecular organic substance in the solution into glucose, and quantifying the amount of glucose by the phenol sulfuric acid method.

The composition of the plating bath and the plating conditions in a typical continuous electroplating method for a sulfuric acid acid bath are as follows.

[0039]

[0040]

【Example】

(Example 1) Various polymer organic substances were added at various ratios to a plating bath having the following composition, and the resulting mixture was stirred for 10 minutes. The dissolution state was evaluated according to the following criteria, and complete dissolution was judged to be good. .

The plating bath composition _{ZnSO 4 · 7H 2 O: 350g} / l (35 _{wt%) CoSO 4 · 7H 2 O} : 60g / l (6 _{wt%) Na 2 SO 4: 75g} / l (7.5 wt%) solution Temperature: 40, 50, 60 ° C Solubility criteria: ○: Completely dissolved.

△: Suspended particles, small amount of sediment.

×: There is a large amount of precipitate.

The results of these evaluations are shown in Table 1.

[0045]

[Table 1]

As can be seen from Table 1, a high molecular weight organic material satisfying the number average molecular weight and the ratio of non-reducing end groups specified in the present invention was completely dissolved in the plating bath up to 10% by weight. If any of the conditions were not satisfied, undissolved residues were generated.

(Example 2) A low-carbon cold-rolled steel sheet specified in JIS G 3141 having a thickness of 0.7 mm was used as a base material, degreased, and a sulfate bath containing 1% by weight of a high-molecular organic substance was added. Was used to perform electroplating, and the workability of plating was evaluated. The polymer organic material, the molecular weight of 5 × 10 ^4, and amylopectin ratio of non-reducing end group is 204, the molecular weight of 5 × 10 ^4,
Dextrin with a non-reducing end group ratio of 10 was used.

The plating conditions are as follows.

[0049] Plating bath composition _{ZnSO 4 · 7H 2 O: 350g} / l (35 _{wt%) CoSO 4 · 7H 2 O} : 60g / l (6 _{wt%) Na 2 SO 4: 75g} / l (7.5 wt%) pH : 1.8 Bath temperature: 50 ± 2 ℃ Plating current: 50A / dm ² × 24sec. Current flow rate: 1.0m / sec Aiming adhesion amount: 30g / m ² Precipitation when amylopectin is used as high molecular organic matter There was no suspended matter, and the product quality was good with the same workability as in the case where no high-molecular organic substance was added. When dextrin was used, a small amount of precipitate was generated and the filter was contaminated, so that the plating operation had to be interrupted and the filter had to be replaced before changing the plating type.

[0050]

According to the method for producing a composite zinc alloy-plated metal sheet of the present invention, the production efficiency is high and the plating bath is less contaminated, the productivity is high and the product quality stability is excellent as compared with the conventional production method. It is a method and can contribute to the stable supply of a metal plate having excellent corrosion resistance.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C25D 15/02-15/02 C25D 5/26

Claims (2)

(57) [Claims] 1. A number average molecular weight of ¹⁰⁸ or less, electric with zinc-based plating bath the ratio of non-reducing end group to the reducing end group contains from 0.01 to 10% by weight of a polymer organic material is 15 or more A method for producing a composite zinc-based electroplated metal sheet, characterized by plating. 2. Amylopectin, dextrin and dextran having a number average molecular weight of 10 ⁸ or less and a ratio of non-reducing terminal groups to reducing terminal groups of 15 or more in total of 0.01 to 2 A method for producing a composite zinc-based electroplated metal sheet, comprising electroplating using a zinc-based plating bath containing 10% by weight.