JP2000192297A - High corrosion resistance surface treated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel sheet excellent in electric conductivity by providing the surface of a galvanizing film contg. specified ratios of CO and high polymer organic matter with a film of a water soluble resin or the like with a specified thickness contg. oxide fine grains having specified average primary grain size and content. SOLUTION: On at least one side of a base material composed of low carbon steel, a plating film contg., by weight, 0.5 to 5.0% Co and 0.5 to 5.0% high polymer organic matter, and the balance substantial zinc is formed by the coating weight of about 10 to 100 g/m2. Moreover, as the high polymer organic matter, dextrin, dextran or the like is used. The surface of this plating film is provided with a protecting film of 10 to 500 nm average thickness contg. 0 to 40 wt.% oxide fine grains with <=20 nm average primary grain size, and the balance substantial water soluble resin or water dispersible resin. Furthermore, as the oxide fine grains, silica, alumina or the like is used, and, as the water soluble resin or water dispersible resin, an epoxy resin, a polyurethane resin, an acrylic resin or the like is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a non-painted part having excellent corrosion resistance, corrosion resistance after painting, workability and spot weldability, which is suitable as a material for home appliances and building materials, and a conductivity not containing a hexavalent chromium compound. It relates to a surface-treated steel sheet having excellent high corrosion resistance.

[0002]

2. Description of the Related Art Various kinds of surface-treated steel sheets are used in various fields such as home electric appliances and building materials. In response to this, an organic composite coated steel sheet or a coated steel sheet which is excellent in corrosion resistance and is provided with other various performances in a well-balanced manner is often used. For example, JP-A-6-15
No. 5658 discloses a rust-prevention-treated steel sheet mainly including a chromate treatment layer and an organic resin thin film having a thickness of about 1 μm on a Zn—Ni alloy plating layer.

[0003] Chromate treatment is widely used as a post-treatment of galvanized steel sheets to improve the corrosion resistance in the unpainted state. This treatment is performed by applying a treatment solution containing hexavalent chromium to a galvanized steel sheet, and the resulting chromate film contains hexavalent chromium. The excellent corrosion resistance is exhibited despite the fact that the chromate film is a thin film, because it contains hexavalent chromium.

[0004] In recent years, chromium compounds, particularly hexavalent chromium compounds, have attracted attention as environmental pollutants, and there is a demand to avoid using these substances. In response to such a demand, a rust-preventive treatment method that does not contain hexavalent chromium, a so-called non-chromate treatment method or a chromate-free treatment method (hereinafter, simply referred to as “non-chromate treatment”) has been disclosed. Chemicals used in these treatments are roughly classified into organic and inorganic.

[0005] Japanese Patent Application Laid-Open No. 1-151977 discloses a surface treatment of a zinc-based and zinc-based alloy-plated steel sheet, which comprises treating with an aqueous solution containing a tannic acid resin and a silicate colloid as a treatment method using an organic chemical. A method is disclosed.

[0006] Japanese Patent Application Laid-Open No. 4-48081 discloses a non-chromate treatment method using an inorganic chemical, in which a steel material is immersed in an aqueous solution of ammonium molybdate and then dried to plate a molybdenum film on the surface of the steel material. A method for forming a molybdenum film is disclosed.

[0007]

However, when the above non-chromate treatment is applied to a conventional hot-dip galvanized steel sheet or electrogalvanized steel sheet, the corrosion resistance obtained is 6%.
There has been a problem that the corrosion resistance of a zinc-based plated steel sheet subjected to chromate treatment containing valent chromium does not reach.

[0008] If static electricity is charged during use of home electric appliances or building materials, problems such as electric shock and dust adhesion at the time of use occur, and comfort is lost. To avoid such charging phenomena,
It is considered that a material having excellent conductivity, which is discharged only when the materials come into contact with each other, is desirable as a material to be assembled and used as a chassis material or the like. When a thick protective film is provided on the plating layer, the conductivity is often hindered. Therefore, a plated steel sheet having a thick film is not preferable for home appliances and building materials. However, it is common practice to provide a thick protective film on the plating layer in order to improve the corrosion resistance.

An object of the present invention is to solve these problems,
An object of the present invention is to provide a surface-treated steel sheet which has no or no corrosion resistance after coating and has a corrosion resistance equal to or higher than that of a conventional chromate-treated product and has excellent conductivity.

[0010]

Means for Solving the Problems The present inventors diligently studied a surface-treated steel sheet having good corrosion resistance without painting, corrosion resistance after painting, conductivity, and spot weldability without using a hexavalent chromium compound. The following findings were obtained.

In order to obtain excellent corrosion resistance, it is necessary to provide a protective film suitable for the plating layer in addition to increasing the corrosion resistance of the plating layer itself. It is important that the protective film is not merely a thick film, but a protective film that can sufficiently exhibit the corrosion resistance mechanism of the plating layer.

As for the corrosion resistance of the plating layer, it is desirable that both the corrosion resistance without painting and the corrosion resistance after painting are excellent. It is known that corrosion resistance without painting is improved by alloying a plating film to make it electrically noble. On the other hand, since the corrosion resistance after painting is basically ensured by the sacrificial anticorrosion ability of the plating film, a plating layer made of a metal, such as zinc, having a potential lower than that of iron is preferable.

[0013] By adding a relatively small amount of Co and a high-molecular organic substance, preferably a high-molecular-weight organic substance having water solubility or water dispersibility, to the zinc plating film, the corrosion resistance after painting is improved while the corrosion resistance after painting is improved. It can be equal to or more than pure zinc. Although the effects of these Co and high molecular organic substances have not been elucidated in detail, the following performance expression mechanism is presumed.

The high molecular organic substance added to the plating bath is taken into the plating film when forming the plating film, and has an effect of making the zinc crystals finer. These finely divided crystals produce fine corrosion products in a corrosive environment. Since the fine corrosion products have a greater environmental barrier effect than the coarse corrosion products, the corrosion resistance in an unpainted state is improved. Further, it is expected that the high-molecular organic matter taken into the plating film is eluted in the corrosive environment, and the corrosion product is miniaturized.

On the other hand, the effect of Co is to make the plating film a little noble as an alloy element, to improve bare corrosion resistance,
In collaboration with high molecular organic substances, the above corrosion products can be refined
It is thought to contribute to stabilization. Further, since the hardness of the plating film is slightly increased, the sliding characteristics at the time of press working and the continuous spot welding property of spot welding are improved. In addition, it is considered that the addition of Co improves the reactivity of the chemical conversion treatment and improves the adhesion between the chemical conversion treatment film and the plating film, so that the corrosion resistance after painting can be improved.

As described above, even if the plating film alone is used, it is possible to secure a considerable level of corrosion resistance in a non-painted state. However, in order to ensure corrosion resistance in a non-painted portion equal to or higher than that of a conventional surface-treated steel sheet having a chromate film. It is preferable to provide a protective film on the plating film. In particular, in the case of home appliance materials, it is necessary to suppress the appearance of white rust in appearance,
Preferably, a protective coating is provided.

In order to secure the conductivity required for home appliances and building materials, it is preferable to make the protective film thin. In order to secure a desired corrosion resistance with a thin protective film, it is necessary to provide a film that exhibits corrosion resistance by a synergistic effect with a corrosion product of plating. This can be achieved by using a suitably permeable coating.

The protective film controls the permeability of corrosion factors such as water and salts to the plating layer, and
It is preferable that the formation of corrosion products of plating is not completely suppressed. Specifically, a resin-based film and an inorganic film can be considered.

The resin used as the main agent in the resin-based protective film is preferably a resin which is considered to be fast and has excellent workability.

When a protective film is provided on the plating film,
The reactivity and adhesion between the two determine the corrosion resistance. In the plated steel sheet of the present invention, the plating layer preferably contains a water-soluble or water-dispersible high molecular organic substance. Therefore, if the protective film thereon is formed of a water-soluble or water-dispersible organic or inorganic chemical, an effect of improving the adhesion between them can be obtained. Rather than using a general solvent-based paint as a protective film agent, a film formed from a water-based paint has better adhesion and also has an advantageous effect on corrosion resistance.

In a resin-based protective coating,
To moderately suppress the permeability of corrosion factors such as water and salts, and to stabilize corrosion products of plating,
It is more preferable to mix oxide fine particles in the protective film.

The substance forming the protective film is made of an inorganic substance such as a metal oxide or hydroxide such as Mo, a phosphoric acid compound, a silicate compound or the like instead of the above-mentioned organic resin. The same function can be achieved even with a film having the same structure.

The present invention has been completed based on the new findings obtained by the present inventors as described above,
The gist lies in the highly weather-resistant surface-treated steel sheet described in the following (1) and (2) and the manufacturing method described in the following (3) and (4).

(1) 0.5 to 0.5%
An oxide containing 5.0% by weight of cobalt and 0.5 to 5.0% by weight of a high molecular weight organic material and having an average primary particle diameter of 200 nm or less on a plating film substantially composed of zinc. Fine particles are contained in an amount of 0 to 40% by weight, the balance being substantially composed of a water-soluble resin or a water-dispersible resin, and having an average thickness of 10 to 5%.
A highly corrosion-resistant surface-treated steel sheet comprising a film having a thickness of 00 nm.

(2) On at least one side of the base material, 0.5 to
Mo, W, Ti, V, Cr ³ containing 5.0% by weight of cobalt and 0.5 to 5.0% by weight of a high molecular weight organic material, with the balance being substantially zinc. ⁺ , Nb, Z
an inorganic film having an average thickness of 10 to 500 nm, containing an oxide of r, a hydroxide of these metals, and one or more members selected from the group consisting of phosphoric acid compounds and silicate compounds. Highly corrosion-resistant surface-treated steel sheet.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail. The percentages described below mean weight%.

Base material: Low-carbon steel is suitable for the base material, but it is not limited to this. Ultra-low-carbon steel, low-carbon steel, or Si, Mn, Ti, Nb, V, Mo, B, etc. An appropriate steel sheet can be selected according to the purpose of use and the site of use, such as an alloy steel containing. As the type of the steel sheet, a cold-rolled steel sheet is preferable, but a hot-rolled steel sheet may be used.

Plating film: In order to maintain good corrosion resistance (perforation resistance) without coating, the plating film contains 0.5% or more of Co in addition to zinc in proportion to the weight of the plating film. If the Co content is less than 0.5%, the effect of improving corrosion resistance without coating is insufficient. 0.
When the content is 5% or more, the plating film becomes hard, the sliding resistance is reduced, the formability of the plating film is improved, and the spot weldability is also improved. Preferably, the content is 0.8% or more. If the Co content exceeds 5.0%, the plating film becomes noble, the sacrificial corrosion protection ability is reduced, and the end face corrosion resistance after coating cannot be secured.

The plating film contains 0.5 to 5.0% of a high molecular weight organic substance based on the weight of the plating film. When the content of the organic polymer in the plating film is less than 0.5%, the corrosion resistance without coating is insufficient. If the content of the high-molecular organic substance exceeds 5.0%, the amount of gas generated from the organic substance at the time of welding increases, which is not preferable because the weldability of the plated product is impaired.

If the average molecular weight of the high molecular weight organic substance is less than 10 ³ , the corrosion resistance of the plating film without coating may not be improved. Therefore, the average molecular weight is preferably set to 10 ³ or more. More preferably, it is 10 ⁴ or more. For corrosion resistance, the larger the molecular weight, the better, but the higher the molecular weight, the more difficult it is to dissolve in the plating bath, so that those with an average molecular weight of more than 10 ⁸ are difficult to plate.

As the high molecular organic substance, those having water solubility or water dispersibility are preferable, and among them, dextrin,
Any one of dextran and amylopectin
It is preferred to use a species or a mixture of two or more.

The content of the high molecular weight organic material deposited in the plating film is determined by dissolving the plating film with sulfuric acid or the like, decomposing the high molecular weight organic material in the solution into glucose, and quantifying the amount of glucose by the phenol sulfuric acid method. Can be measured. The molecular weight of the high molecular weight organic substance is determined by gel permeation chromatography (GP
It can be measured by the method C).

It is preferable that the coating weight is in the range of 10 to 100 g / m ² . When the adhesion amount is less than 10 g / m ² , the rust prevention effect becomes insufficient. On the other hand, 100 g / m
^If it exceeds ² , the corrosion resistance will be saturated and the cost will increase. A more preferable adhesion amount range is 20 to 40 g / m ² . The plating may be either single-sided plating or double-sided plating.

Protective film: When the protective film is a resin-based protective film, the type of the resin may be a water-soluble or water-dispersible resin among various resins usually used for an organic coated metal plate. I just need. In particular, epoxy resin, polyurethane resin, acrylic resin, polyolefin resin, phenol resin and the like are suitable, but need not be limited to these,
For example, a material such as a natural resin may be used.

The term "water-dispersible" refers to those resins which can be stably dispersed in water as an emulsion. Specifically, those having a hydrophilic functional group that stabilizes the emulsion and those having a structure in which a hydrophilic polymer is adsorbed after emulsification are preferable.

In the case of forming a resin-based protective film, it is preferable to mix oxide fine particles having an average primary particle size of 200 nm or less in the protective film. When the oxide fine particles are contained, the hydrophilicity of the coating is improved, and the coating property is improved when electrodeposition coating is performed thereon, and coating defects such as pinholes and no coating are less likely to occur. Furthermore, the permeability (porosity) of the resin film is improved, so that corrosion-promoting substances such as moisture from the external environment can easily penetrate the resin film, and such corrosion-promoting substances can easily reach the plating film surface. Become. Therefore, the generation of dense and stable corrosion products at the plating film interface is promoted, and the corrosion resistance is further improved.

As the oxide particles, silica, alumina,
Antimony oxide, titania and the like are preferred. Among them, colloidal silica or fumed silica is preferable as silica.

When the content of the oxide fine particles exceeds 40%, the electric resistance of the resin film becomes too high, and it becomes difficult to secure the ground. Further, when the content of the oxide fine particles is excessive, the non-coating corrosion resistance is not preferable because the cohesive force of the resin film is reduced and the film is easily peeled off. Further, the resin film becomes brittle, and the corrosion resistance of the processed portion may be significantly deteriorated. Therefore, it is preferable that the content be 40% or less even when it is contained.

If the particle size of the oxide particles is too large, the cohesive force becomes insufficient and the film-forming property is deteriorated. Therefore, the average particle size of the oxide particles is 200 nm or less. Preferably 50n
m or less, more preferably 30 nm or less.

In the case of forming a resin-based protective film, a crosslinking agent, a lubricant, a coloring agent, a rust inhibitor and the like may be contained as necessary in addition to the oxide fine particles.

The cross-linking agent is added to densify the resin film and improve the corrosion resistance. For example, when the organic resin is an epoxy resin, a phenol resin, an amino resin, a blocked isocyanate, or the like is used. A general addition amount is about 5 to 20% based on the main resin.

The lubricant improves the lubricity of the film and improves the press workability and the like. Generally, a polyolefin-based synthetic wax such as polyethylene is added. The addition amount is preferably 1 to 10%.

Further, a coloring pigment may be added for discriminating the front and back surfaces and for improving the appearance. Examples are iron oxide,
Examples include inorganic pigments such as titanium oxide and carbon, and organic pigments such as diazo. The addition amount is preferably about 1 to 20%.

When further improvement in corrosion resistance is required,
Rust inhibitors can also be added to the organic coating. However, needless to say, it is necessary to use a rust inhibitor other than the hexavalent chromium compound. Examples of such rust preventives include tannic acid and benzotriazole as organic ones, and phosphoric acid, molybdate, calcium phosphate and the like as inorganic ones. The addition amount is 1 to
10% is preferred.

When the protective film is made of an inorganic film, it is preferable that the protective film has an appropriate barrier property and corrosion factor permeability as in the case of the organic film. It is also important that the film has good workability.

Representative examples of such inorganic coatings include oxides of Mo, W, Ti, V, Cr ³⁺ , Nb, and Zr, hydroxides of these metals, and phosphate compounds and silicate compounds. A film formed by applying, reacting, drying or baking an aqueous solution containing one or more members of the group consisting of is preferred.

The film thickness of such a film is 10 to 50 on average.
It needs to be 0 nm. Here, the average film thickness is an average adhesion amount in an area equivalent to several tens of mm square by a gravimetric method, a fluorescent X-ray method, a chemical analysis of a solution, or the like.

If the film thickness is less than 10 nm, the effect of improving corrosion resistance without coating is not sufficient. Therefore, the thickness of the protective film is 10
nm or more. Preferably it is 50 nm or more. When the thickness of the protective film exceeds 500 nm, the conductivity and spot weldability of the film become insufficient. In addition, the electrodeposition paintability is impaired. Therefore, the thickness of the protective film is set to 500 nm or less. Preferably it is 400 nm or less, more preferably 300 nm or less.

The protective film made of the above-mentioned inorganic oxide / hydroxide may contain oxide fine particles having an average primary particle size of 200 nm or less. The inclusion of oxidized fine particles has the effect of improving the permeability of corrosion factors, stabilizing corrosion products, and improving corrosion resistance. When it is contained, the content is preferably 40% by weight or less based on the weight of the protective film after drying. If it exceeds 40% by weight, the protective film becomes brittle and the workability is lowered, which is not preferable.

Production method: The highly weather-resistant surface-treated steel sheet of the present invention is obtained by electroplating a base steel sheet using a known plating apparatus, and applying a paint capable of forming a coating film on the surface under the above-mentioned conditions by a roll coating method or the like. It can be easily manufactured by coating by a known method such as a spray method, drying according to a known method, and baking. The plating method is electroplating, and may be either single-sided plating or double-sided plating.

The electroplating bath is an acid bath (eg, a sulfate bath,
Any of a chloride bath) and an alkaline bath (eg, a cyanide bath) may be used, but an acidic bath, particularly a sulfate bath is preferred. In the plating bath, a zinc source and a Co source are contained as sulfates, acetates, or carbonates containing these metals in an amount to achieve a target composition.

The electroplating bath further contains a water-soluble or water-dispersible organic polymer. By using a water-soluble or water-dispersible polymer organic material, it has good reactivity with the water-soluble or water-dispersible organic or inorganic protective film formed on the plating film, and has good adhesion. And the corrosion resistance is also improved.

In order to improve the corrosion resistance without coating, it is preferable that the high-molecular-weight organic material contains those having an average molecular weight of 10 ³ or more, more preferably 10 ⁴ or more. However, if the molecular weight is excessively large, the solubility in the plating bath is impaired. Therefore, it is preferable to use one having an average molecular weight of 10 ⁸ or less, more preferably 10 ⁷ or less.

As such a high molecular organic substance, dextrin or dextran (hereinafter simply referred to as “dextrin”) obtained by hydrolyzing starch, or a high molecular organic substance such as amylopectin which is a component of starch is preferable. It is. The amount of the high molecular weight organic material contained in the plating solution is preferably in the range of 0.05 to 10% based on the weight of the plating solution.

The plating may be performed under known conditions except for using the plating bath as described above.

Next, a resin having water solubility or water dispersibility, Mo, W, Ti, V, C
oxides of r ³⁺ , Nb, Zr, hydroxides of these metals,
And a coating composition containing one or more members selected from the group consisting of phosphoric acid compounds and silicate compounds, or a coating composition further containing oxide fine particles, to obtain a desired dry film thickness. What is necessary is just to apply | coat in quantity and dry.

[0057]

EXAMPLE A cold-rolled steel sheet having a thickness of 0.8 mm was used as a base material, and after degreased the surface, the following plating bath was used, plating bath temperature: 40 to 65 ° C., current density: 20 to 150 A.
/ Dm ² and the flow rate of the plating solution: 0.5 to 4 m / sec. As the high molecular weight organic substance, dextrin having a weight average molecular weight of 10,000 (symbol a) or dextran having a weight average molecular weight of 40,000 (symbol b) was used.

[0058] electroplating bath composition; ZnSO ₄ · 7H
₂ O: 20 to 40 _{wt%, CoSO 4 · 7H 2 O} : 10
40 wt%, Na ₂ SO _4: 5 to 10 wt%, Te Bu dextrin or de dextran: 0.001 wt%, pH:. 1 to
4. As the base liquid for the coating for the protective film, those shown in Table 1 were used.

[0059]

[Table 1]

As additives, colloidal silica having a primary average particle size of 20 nm, alumina having a thickness of 20 nm, and phosphoric acid were prepared, and these were mixed in various ratios with a base solution to prepare a paint for a protective film. These paints are applied on the plating film using a bar coater so that a predetermined dry film thickness is obtained, and the maximum temperature (PMT) is reached using an electric oven.
However, for organic base materials (symbols A and B), 130
℃, 10 for inorganic base materials (codes C, D and E)
The sample was baked under the condition of 0 ° C. to prepare a sample provided with a protective film.

The performance of these samples was evaluated by the following method. Unpainted corrosion resistance; a salt spray test was performed on the test piece having a protective film by the following corrosion cycle, and the area ratio was 5%.
The time until the appearance of rust was generated was measured and evaluated according to the following criteria.

Corrosion cycle: [Salt spray (5% -NaC
1, 35 ° C, 7 hours) → dry (50 ° C, 2 hours) → wet (RH 85%, 50 ° C, 15 hours)]. ◎: 72 hours or more, ：: 48 hours or more, less than 72 hours, Δ: 24 hours or more, less than 48 hours, ×: less than 24 hours.

Corrosion resistance after coating: A sample having a protective film was further coated with a melamine alkyd type solvent-based paint using a bar coater to a dry film thickness of 20 μm.
Bake for 15 minutes in an oven set at 50 ° C,
Painted samples were also made.

After the coated surface was scratched with a cutter knife to reach the steel plate base, a salt spray test under the same conditions as above was performed for 120 hours, and the maximum value of the swollen width of the coating film from the scratch was measured. The evaluation was based on the following criteria.

◎: Blister width <0.5 mm, ：: Blister width <1.0 mm, Δ: Blister width <2.0 mm, ×: Blister width <3.0 mm.

Conductivity: Conductivity was measured using an SQ meter. Based on the conductivity of a commercially available electrogalvanized steel sheet subjected to chromate treatment with a chromium deposition amount of 30 mg / m ² , a value equal to or less than that value was regarded as acceptable.

Weldability: Two test pieces having a protective film were stacked, and a pressure of 300 kgf was applied using a dome-shaped electrode.
The spot welding was conducted at a current of 60 Hz and a current of 27,000 A for 6 cycles.
The test was continuously performed under the condition of a pause for 2 seconds. Welding was repeated 20 times at intervals of 1 point / 2 seconds, and the welding cycle of 40 seconds pause was repeated. Three shear test pieces were collected every 100 points, a shear test was performed on the welded portion, and a nugget diameter of the fractured portion was obtained. Was measured. The number of hit points until the nugget diameter began to decrease sharply was obtained, and the evaluation was made based on the following criteria.

◎: 2000 points or more, ： １: 1500 points or more, Δ: 1000 points or more and less than 1500 points ×: Less than 1000 points. Table 2 shows the obtained evaluation results.

[0069]

[Table 2]

As shown in Table 2, the test numbers 1 to 12 satisfying the conditions specified by the present invention showed good performance in all evaluation items. On the other hand, Test No. 1 in which the content of Co or a high molecular organic substance in the plating film was less than the lower limit specified in the present invention.
3, 14, 16, and 22 to 24, the uncoated corrosion resistance was not good (test No. 25 contained no Co?).
Test No. 1 whose Co content exceeded the upper limit specified by the present invention
In No. 5, the sacrificial corrosion resistance was reduced and the end face corrosion resistance after coating was not good. In Test No. 17 in which the content of the high-molecular organic substance exceeded the range specified by the present invention, the spot weldability was not good. In Test Nos. 19 and 20 in which the thickness of the protective film was less than the thickness specified by the present invention, the corrosion resistance without painting was not good. In Test Nos. 18 and 21 in which the thickness of the protective film was larger than the thickness specified by the present invention, the conductivity and the spot weldability were poor.

The plated steel sheet of Test No. 25 is a commercially available zinc-based alloy-coated steel sheet containing 13% of Ni. However, since the Ni content of the plating layer is high and the organic layer does not contain a high molecular weight organic material, it has good corrosion resistance after painting. Did not.

[0072]

The surface-treated steel sheet of the present invention satisfies all the required performances of corrosion resistance without painting, corrosion resistance after painting, conductivity, and weldability, even though the coating does not contain a hexavalent chromium compound. It is useful for automobiles, home appliances, and building materials.

Claims (2)

[Claims] 1. The method according to claim 1, wherein at least one side of the base material has a thickness of 0.5 to 5.
Oxide fine particles having an average primary particle diameter of 200 nm or less are contained on a plating film containing 0% by weight of cobalt and 0.5 to 5.0% by weight of a high molecular weight organic material, and the balance substantially consisting of zinc. 0 to 40% by weight, the balance being substantially composed of a water-soluble resin or a water-dispersible resin, and having an average thickness of 10 to 500%.
A highly corrosion-resistant surface-treated steel sheet provided with a film having a thickness of nm. 2. The method according to claim 1, wherein at least one side of the base material has a thickness of 0.5 to 5.
0% by weight of cobalt and 0.5 to 5.0% by weight of a high molecular weight organic material, the balance being substantially on zinc, Mo, W, Ti, V, Cr ³⁺ , An inorganic film containing an oxide of Nb and Zr, a hydroxide of these metals, and one or more members selected from the group consisting of phosphoric acid compounds and silicate compounds, and having an average thickness of 10 to 500 nm A highly corrosion-resistant surface-treated steel sheet comprising: