JPH0421789A - Formation of composite film on steel sheet or galvanized steel sheet - Google Patents

Abstract

PURPOSE:To enhance corrosion resistance and coating properties of the surface to be treated by depositing the film of specified metal and metal-lic oxide on the surface of steel sheet by cathodic electrolysis and forming a chromate film and a high polymer resin film on one side thereof. CONSTITUTION:A film of metal and metallic oxide having 3-150mg/m<2> is deposited by cathodically electrolizing steel sheet or galvanized steel sheet at 0.3-30.0A/m<2> and 0.3-10.0C/dm<2> amount of electricity in an alkaline metallic aq. aoln. contg. an alkali component such as alkali metallic hydroxide, 0.001-0.6 mol/[ metallic ion such as Fe and Ni and ammonium hydroxide wherein total of ammonium group, etc., is regulated to mol concn. of six times for metallic ion. After this film is washed by water, a chromate film layer of 10-150mg/m<2> expressed in terms of metallic chromium is formed on one side. The film layer of organic high polymer resin contg. 10-120 parts silica sol for 100 parts solid is formed at 0.1-5.0g/m<2> thereon. One side with a double layer film formed thereon is excellent in water resistant secondary adhesive property. The other side is excellent in workability and weldability.

Description

[Detailed description of the invention] [Industrial application field]

The present invention provides a method for applying coatings on the front and back surfaces of a steel plate, an electro-galvanized steel plate, an electro-zinc alloy plated steel plate, a hot-dip galvanized steel plate or an alloyed galvanized steel plate (these plated steel plates are hereinafter collectively referred to as zinc-based plated steel plates). This is a new film formation method that provides different functions, with corrosion resistance and corrosion resistance on one side.
Forming a multilayer film with excellent paint film adhesion and post-painting corrosion resistance, especially water resistant secondary adhesion, and forming a film made of metals and metal oxides with excellent phosphate treatment properties on the other surface, The present invention also relates to a method for forming a composite film with excellent workability and weldability.

[Prior art]

In recent years, steel sheets based on zinc-plated steel sheets with a chromate film and an organic composite silicate film on the top layer have been used as rust-proof steel sheets for automobiles. -The manufacturing process for car bodies involves a body assembly process such as press molding and spot welding, followed by a pre-painting process (degreasing, washing with water, surface conditioning, zinc phosphate treatment, washing with water), and then electrodeposition coating. It is being done. Since rust-proof steel sheets for automobiles are naturally combined with other steel sheets and used through the above manufacturing process, the qualities that the rust-proof steel sheets should have include press formability, spot weldability, resistance to chromium elution in the pre-painting process, Since electrodeposition coating properties are required, various methods have been proposed. For example, this type of improved technology is disclosed in Japanese Patent Application Laid-open No. 82-739383, Japanese Patent Application Laid-Open No. Ei2-289274, Japanese Patent Application Laid-Open No.
The techniques disclosed in Japanese Patent Application Laid-open No. 3-7938, Japanese Patent Application Laid-Open No. 13-22637, and Japanese Patent Application Laid-Open No. 83-143211i5 are well known as conventional examples.

[Problems to be Solved by the Invention] The technology disclosed in JP-A-62-73938 incorporates chromium pigments, lubricating components,
In order to improve spot weldability, a conductive component is added. However, since the rust-preventing steel sheet coated with such a coating agent contains chromium pigment, the resistance to chromium elution during the pre-painting treatment process is insufficient. Said JP-A-Sho [i
In the technique disclosed in Japanese Patent Publication No. 2-289274, processability is good due to the use of urethane resin, but corrosion resistance is insufficient. In the techniques of JP-A No. 63-7938 and JP-A-63-22Ei37, chromium pigment is added to each resin in order to increase the rust prevention ability, so the chromium elution resistance in the pre-painting treatment process is low. is insufficient. Furthermore, the technique disclosed in JP-A-63-143265 reduces the chromium elution resistance of the chromate film by adding a reducing agent to the chromate film treatment agent, but this has the problem of poor corrosion resistance. As mentioned above, all of the conventional technologies treat a chromate film on the zinc-based plating surface and then apply an organic composite phosphor film on the top layer. It is difficult to simultaneously satisfy conflicting performance qualities such as (paint film adhesion and post-painting corrosion resistance), processability, and weldability at a high level, and this is a problem. [Means for Solving the Problems] As a concrete means for solving the problems of the conventional example, the present invention provides a steel plate or a zinc-plated steel plate made of alkali metal hydroxide, alkali metal phosphate, or alkali metal carbonate. One or more selected alkali components and ■Fe
, at least one kind of metal ion selected from Ni and Co o, ooi ~ 0.6 mol/IC
(if the total is 0.001 to 0.6 mol/l), and (i) the ammonium group, amino group, or imino group alone or the sum of two or more of these groups is relative to the metal ion. In an aqueous alkaline metal salt solution containing one or more compounds selected from ammonium hydroxide, ammonium salts, aliphatic amino acids, and aliphatic amines, the current density is 0.
3-30.0A/dm2, current flow 0.3-10.0C
/da2, and the surface of the steel plate is subjected to cathodic electrolysis at 3~150μ/da2.
After depositing a film consisting of 1/2 metal and metal oxide and washing it with water, a coated chromate treatment aqueous solution containing hexavalent chromium ions is applied to one side of the film, and then dried to give an Iθ ~ 150 cm in terms of metallic chromium. A chromate film layer of /1/ is formed, and silica sol is added thereon in an amount of 10 to 12 parts by weight per 100 parts (parts by weight, same hereinafter) of the solid content of the organic polymer resin.
Apply an organic polymer resin liquid containing 0 parts and dry it to give 0.1 parts.
The present invention provides a method for forming a composite film on a steel sheet or a zinc-plated steel sheet, characterized in that a film layer of ~5.0 g// is formed. More specifically, before performing the chromate treatment, the alkaline metal salt aqueous solution is first maintained at room temperature to 90"C, and the current density is 0.3 to 30.OA/d2 and the amount of current is 0.3 to I.
By cathodic electrolyzing within the range of O, OC/d■2, a film consisting of metals and metal oxides of 3 to 150■/I is precipitated on the surface of a steel plate or zinc-plated steel plate in a short time, and then washed with water. A surface that is suitable for coating-type chromate film treatment and that is not subjected to coating-type chromate treatment is a surface that is excellent in pre-painting treatment properties in the automobile body manufacturing process. After applying the chromate treatment liquid, drying forms a chromate film layer of lθ ~ 150 mg / wF in terms of metallic chromium, and further, on the chromate film layer, 7Licasol is applied with a solid content of 100% organic polymer resin. 1 for part
Applying an organic polymer resin composition containing 0 to 120 parts,
When dried, the multilayer coating forms a coating layer of 0.1 to 5.0 g/m2, resulting in high corrosion resistance on one side and excellent phosphate treatment properties on the other side, where a coating consisting of metal and metal oxide is formed. It becomes something.

[Explanation of action and composition]

The present invention uses an ammonium salt, an aliphatic amino acid, and an aliphatic amine as a complexing agent in an alkaline metal salt aqueous solution before forming a chromate film on the surface of a steel plate or a zinc-based alloy plated copper plate. When the above-mentioned complexing agent is contained in a molar concentration that is six times or more than the molar concentration of Ni or CO metal ions, metals consisting of metals and metal oxides can be melted into steel sheets or zinc in short-time cathodic electrolytic treatment of 1 second or less. A uniform and dense film made of metal containing 3 to 150 mg/m2 of metal oxide is formed on the surface of the plated steel sheet. In the subsequent process, one side is washed with water and coated with chromate treatment, and the weight of the chromate film obtained by the coated chromate treatment is lθ ~ 150 g/+/ in terms of metallic chromium. The coating weight is 0.1 to 5.0 g/v! The organic polymer resin composition contains 10 to 120 parts per 100 parts of the solid content of the polymer resin! This gives different functions to the two surfaces of the copper plate or zinc-plated steel plate, and the function of one of the surfaces is to provide excellent corrosion resistance, coating adhesion, and It has excellent corrosion resistance after painting, especially excellent water resistant secondary adhesion, and other surface functions are excellent in phosphate treatment, and
A composite film with excellent workability and weldability can be formed. That is, in the present invention, the alkaline metal salt aqueous solution contains one or more alkali components selected from alkali metal hydroxides, alkali metal phosphates, and alkali metal carbonates and F.
At least one metal ion selected from e + Ni+ or Co 0.001 to 0.6 mol/l (
If there are two or more types of metal ions, their total is 0.
．． 001 to 0.6 mol/1) and one or more ammonium groups, ammonium groups, or imino groups in one or more compounds selected from ammonium hydroxide, ammonium salts, aliphatic amino acids, and aliphatic amines, or 2 of the base of
An alkaline aqueous solution containing at least 6 times the molar concentration of the metal in total at room temperature to 90°C.
The current density was maintained at 0.3-30. Cathodic electrolysis is performed within the range of 0.3 to 10.0 C/dm2 of OA/di2 current flow,
A film consisting of 3 to 150 mg/m2 of metal and metal oxide is deposited on the surface of the steel plate in a short time, and washed with water to firmly coat the coated chromate film formed on one side of the steel plate or zinc. The weight of the applied chromate film is lθ~150■/W in terms of metallic chromium! Following this, apply silica sol on top of the chromate film layer in an amount of 1 part per 100 parts of the solid content of the organic polymer resin.
Applying an organic polymer resin composition containing 0 to 120 parts,
By drying and forming a film weight of 0.1 to 5.0 g/rl, it improves chromium elution resistance in the pre-painting process on one side, and improves water resistant secondary adhesion, corrosion resistance,
It has excellent paintability (paint film adhesion and post-painting corrosion resistance), and the other surfaces, that is, the chromate film and the surface where the organic polymer composition film is not applied on the chromate film, have excellent pre-painting properties, processability, This is a method for forming a composite film on steel sheets or zinc-plated steel sheets that simultaneously satisfies various performance qualities such as weldability at a high level. The pH value of the aqueous alkaline metal salt solution used in the present invention is 7.
It can be any value exceeding I4. Electrolysis in an aqueous acidic metal salt solution with a pH value of less than 7 is not preferred because it impairs the pre-painting properties of the surface to which the coated chromate film and the organic polymer resin composition film are not applied. Further, as the alkali agent, one or more selected from alkali metal hydroxides, alkali metal phosphates, and alkali metal carbonates can be selected. Specific examples of the alkali metal salt include lithium, sodium, and potassium, but the salt is not particularly limited. Next, at least one metal ion selected from Fe, Ni, and Co is added in the form of a metal salt of carbonic acid, phosphoric acid, sulfuric acid, nitric acid, or carboxylic acid, but the appropriate amount of addition is 0. Concentration from .001 to 0.6 mol/l, 2
When more than one type of metal ion is added, the total concentration of these metal ions is 0.001 to 0.6 mol/l. If the concentration of metal ions is less than 0.001 mol/l, it will be difficult to form a film of at least 3 mol/l/l on the surface of the copper plate under the electrolytic conditions specified in Section 1 of the present invention.
On the other hand, if the content exceeds 0.6 mol/l, the amount of film deposited cannot be increased in accordance with the increase in content, and the cost of the alkaline metal salt aqueous solution increases, which is not economical. Next, the complexing agents applied to complex the aforementioned metal ions are ammonium hydroxide, ammonium salts are ammonium carbonate, ammonium phosphate, ammonium sulfate, and ammonium nitrate, and aliphatic amino acids are Monoaminocarboxylic acids include glycine and alanine, oxyamino acids include serine, monoaminodicarboxylic acids include aspartic acid and glutamic acid, and aliphatic amines include 01 to C8 aliphatic monoamines, aliphatic diamines, and aliphatic triamines. It is at least one type of compound selected from. Incidentally, aliphatic amino acids and aliphatic amines can be specifically mentioned as mentioned above, but in the present invention, aliphatic amino acids and aliphatic amines other than the above-mentioned examples can be used as long as the purpose of metal complexation is achieved. can also be applied. Next, the amount of the complexing agent added to the alkaline metal salt aqueous solution is determined based on the molar concentration of ammonium groups, amino groups, or imino groups in the complexing agent that is Fe+ N++ contained in the aqueous solution.
At least one type of metal ion selected from Co is contained in a molar concentration (if two or more types of metal ions are included, the total molar concentration thereof) at least 6 times or more. In order to electrolytically treat a large amount of steel sheets or galvanized steel sheets over a long period of time at high current density, the stability of the electrolytic treatment solution is extremely important. As a result of studies by the present inventors, if the amount of complexing agent added to the alkaline metal salt aqueous solution is less than 6 times the molar concentration of metal ions such as Fe, Ni, and CO, precipitation of the metal ions tends to occur. The amount of film formed by metals and metal oxides on the surface of steel sheets or zinc-plated steel sheets fluctuates,
If a large amount of precipitation occurs, the amount of precipitation will decrease in less than 1 second.
It becomes difficult to form a film of 1/2 or more, and it becomes difficult to firmly bond the subsequently formed coated chromate film to the surface of the steel plate or zinc-based plated copper plate. The alkaline metal salt aqueous solution used in the present invention may optionally contain an alkali metal salt of a fatty acid, such as an alkali metal salt of gluconic acid, preferably sodium gluconate.
~507! Can be added. The alkaline metal salt aqueous solution of the present invention can be prepared at room temperature to 90''C.
Current density at a temperature of 0.3 to 30. A steel plate or a zinc-plated steel plate is subjected to cathodic electrolysis for a short time at an OA/dm2 and a current flow rate of 0.3 to 10.0 C/dm2. Here, if the current density is less than 0-3A/d+2, metal precipitation will be less than 3mg/m2 in a short time treatment of 1 second or less, and the amount of metal precipitation will be insufficient; if the current density exceeds 30A/d+a2, metal precipitation will be It exceeds 150+ag//. The above electrolytic conditions are intended to form a uniform and dense film consisting of metal fine particles containing metal oxides of 3 to 150/I in metal terms on the surface of steel sheets or zinc-plated steel sheets. It is set. 3
If n/T is less than 11, the metal precipitation will be insufficient and it will be difficult to firmly form the subsequently formed coated chromate film.On the other hand, if it exceeds 150/2, the subsequently formed coated chromate film will be difficult to form. This impairs the adhesion of the steel plate to the surface of the steel plate. Next, the coating type chromate treatment liquid that can be used in the present invention is not specified, but is a commercially available chromate treatment liquid such as ZM-R1415A, LN-
4513H (manufactured by Nippon Parkerizing Co., Ltd.) or the like can be used, but the important point is that the weight of the chromate film must be 10 to 150 μ/l in terms of metallic chromium. If the weight of the chromate film is less than lθmg/m2 in terms of metallic chromium, the corrosion resistance will be insufficient, and if it is more than 150 mg/m2, chromium elution will increase in the pre-painting treatment process, which will be disadvantageous in terms of pollution prevention and Chromium ions accumulate in the treatment solution, which adversely affects the formation of the zinc phosphate film, and it becomes difficult to form a normal zinc phosphate film on the surface of the steel sheet on which the multilayer film is not formed, resulting in inconvenience. Next, the film weight of the organic polymer resin composition used in the present invention is 0.1 to 5.0 g/n? It is necessary that the following is true. If it is less than 0.1 g/r1, suppression of chromium elution in the painting pretreatment process will be insufficient.5.
If it exceeds 0 g/m2, the surface resistance will increase and the spot weldability and electrodeposition coating performance will deteriorate. In addition, the content of the sori force sol is 10 to 12 parts per 100 parts of the solid content of the organic polymer resin.
It is necessary to make it 0 copies. /Ricasol content is 1
If the content is less than 0 parts, the resin film will not have enough barrier material, resulting in insufficient corrosion resistance, and if the content exceeds 120 parts, the resin film will become brittle and processability will deteriorate. Examples of the organic polymer resin used in the organic polymer resin composition include acrylic acid, acrylic esters, methacrylic acid, and acrylic polymers mainly containing methacrylic esters; Urethane resins, epoxy resins based on epichlorohydrin and bisphenol A, alkyd resins, etc. are dissolved or dispersed in water by the reaction of polyols with hydroxyl groups and polyisocyanates with two incyanate groups in the molecule. A water-based resin liquid or a solvent-based resin liquid dissolved or dispersed in any solvent capable of dissolving or dispersing the above-mentioned resin can be used, but there is no particular limitation. Next, although there are no particular limitations on the Nrikasol, for example, Aerosil #200. #300. #38
0 (manufactured by Nippon Aerosil), Snowtex-0°-N,
-C (manufactured by Nissan Chemical), etc. can be used.

【Example】

EXAMPLES The present invention will be specifically described below with reference to Examples 1 to 6 and Comparative Examples 1 to 4. (1) Composite film formation was performed by the following steps. Steel plate to be treated f! +) → Alkaline degreasing (X2) → Water washing → Electrolytic treatment → Water washing → Roll squeezing → Drying (air drying) → One side chromate application → Drying (bundle 3) → One side resin composition application → Drying (
Bundle 4) (Back 1) The steel plate to be treated is one side electrolytic zinc nickel alloy plated steel plate (plating amount 20g / j N N iv t%
: 12%) and double-sided alloyed galvanized steel sheet (plating amount 20g/m2/20g/ze), the size of which is 2.
An oiling material of 00x300 m++ and plate thickness of 0.8w+i was used. (Y2) Alkaline degreasing was carried out using a 2% aqueous solution of a weakly alkaline degreaser (Bulklean 342, manufactured by Nihon Paru Ising Co., Ltd.) at a temperature of 60° C. for 30 seconds. (X3) Drying was carried out at a plate temperature of 100° C. and a drying time of 7 seconds. (Book 4) Drying was carried out at a board temperature of 150° C. and a drying time of 15 seconds. (2) Table 1 shows the bath composition, bath temperature, and electrolytic conditions of the alkaline metal salt aqueous solution. Table 2 shows the organic polymer resin composition. Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 3. (3) Performance test (a) Chromium elution resistance test A composite film-formed steel plate was pretreated for painting under the following conditions, and the amount of chromium deposited before and after was measured using fluorescent X-rays (mg / r
l) L, chromium elution resistance was expressed by the following formula. That is, the smaller the % value, the better the chromium elution resistance is, and the value 0 indicates that the test was not affected by the pre-painting treatment at all. Chromium elution resistance = Initial chromium adhesion amount (w/J) (b) After extruding a corrosion resistance test piece (size 70 x 150 soda liter) 61 times using an Erichsen tester, salt water spray specified in JIS-Z-2371 was applied. The test was conducted for 1000 hours for the electrolytic zinc-nickel alloy plated steel sheet and for 100 hours for the alloyed galvanized steel sheet, and the corrosion resistance was evaluated from the state of white rust occurrence over the entire area of the test piece. ◎: White rust occurrence area rate 0% ○: Same as above, less than 10% △: Same as above, 10% or more and less than 30% ×:
Same as above 30% or more (c) After pre-painting the painted plate corrosion resistance test piece, apply cationic electrodeposition coating for automobiles, make scratches in the coating film that reach the base metal with a cutter,
Electrolytic zinc nickel alloy plated steel plate 50
The test was carried out for 0 hours, and the alloyed galvanized steel sheet was tested for 1000 hours. After the test, the adhesive tape (cellotape) was peeled off, and the maximum peeling width on one side from the scratch was measured (unit: mu). (d) Water resistant secondary adhesion (burlap test) A test piece painted using the same process as in (c) was immersed in deionized water at 40°C for 240°C.
After soaking for an hour, place 100 grid squares on the paint film.
The pieces were cut with a cutter to reach the base metal, and adhesive tape (Cello tape) was applied to the test surface and rapidly peeled off, and the number of peeled pieces of the paint film was observed. (e) Determine the appearance of the phosphate film after pre-painting treatment on the surface that is not coated with phosphate-treated chromate or resin film. ○: Good △: Some imperfections and unevenness ×: Some imperfections and unevenness Since weldability deteriorates, weldability can be determined based on the degree of deterioration. In other words, for every 100 points, 30X 100 m+a
Welded another test piece, and the tensile strength of that test piece was 400
The number of hits until kg could be maintained was recorded. Welding surface: Coated surface - Uncoated surface Pressure force: 200 kg Current: 8.5 kA Current application time: 10 cycles Electrode: R40 (radius type) material, chromium-copper (g) Painting pre-treatment process: × 3 minutes Conditions, drug type: Degreasing; FC-L4460 (manufactured by Nippon Barcalinong), agent A: 20 g/l, agent B: 12 g/l, spray =
43℃ x 2 minutes. Surface adjustment; PL-ZN (manufactured by Nippon Parkerizing)
:1g/no, spray=30 seconds. Chemical: PB-L3020 (manufactured by Nihon Baru Ising), M agent: 48g#, AD-4813: 5g/l, A
D-4813: 17g/I, immersion, 43°C for 2 minutes. table table table

【Effect of the invention】

As explained above, the method for forming a composite film on a steel plate or a zinc-plated steel plate according to the present invention is based on the method of forming a composite film on a steel plate or a zinc-based plated steel plate by selecting an alkali metal salt of hydroxide, an alkali metal phosphate, or an alkali metal carbonate. one or more alkali components; and ■ at least one metal ion selected from Fe, Ni, and Co from 0.001 to 0.6.
mol/l (if there are two or more types of metal ions, their total is 0.001 to 0.6 mol/l); and (1) an ammonium group, an amine group, or an imino group, or two of these groups; An alkaline metal salt containing one or more compounds selected from ammonium hydroxide, ammonium salts, aliphatic amino acids, and aliphatic amines, the total of which is at least 6 times the molar concentration of the metal ion. In aqueous solution, current density 0.3-30.0A/d
a2, cathodic electrolysis is carried out at a current flow of 0.3 to IO, OC/dm2 to deposit a film consisting of metals and metal oxides of 3 to 150 mg/I on the surface of the steel sheet, and after washing with water, one side of the film is coated with hexavalent A paint-type chromate treatment aqueous solution containing chromium ions is applied and then dried to give
A chromate film layer of 150 mg/I is formed, and an organic polymer resin liquid containing 10 to 120 parts of silica sol per 100 parts of solid content of the polymer resin is applied thereon and dried to form a chromate film layer of 0.1 to 5 .0g/n? By forming a film layer, a multi-layer film with excellent corrosion resistance, paint film adhesion and post-painting corrosion resistance, especially water resistant secondary adhesion, is formed on one side of the steel plate or zinc-plated steel plate. Not only are films made of metals and oxides with excellent phosphate treatment properties formed on other surfaces, but these films also have various properties that cannot be achieved with conventional methods, such as excellent workability and weldability. It has excellent effects.

Claims (4)

[Claims] (1) A steel plate or a zinc-plated steel plate is mixed with one or more alkali components selected from [1] alkali metal hydroxides, alkali metal phosphates, and alkali metal carbonates, and [2] Fe, Ni. and at least one selected from Co
0.001 to 0.6 mol/l of different types of metal ions (if there are two or more types of metal ions, their total is 0.00
1 to 0.6 mol/l) and [3] water in which the molar concentration of an ammonium group, an amino group, or an imino group, or the sum of two or more of these groups, is at least 6 times that of the metal ion. One or two selected from ammonium oxide, ammonium salts, aliphatic amino acids, and aliphatic amines
In an alkaline metal salt aqueous solution containing more than one type of compound, the current density is 0.3 to 30.0 A/dm^2, and the amount of current is 0.
．． Cathodic electrolysis is performed at 3 to 10.0 C/dm^2 to deposit a film consisting of metals and metal oxides at a concentration of 3 to 150 mg/m^2 on the surface of the steel sheet, and after washing with water, hexavalent chromium ions are applied to one side of the steel plate. A coating-type chromate treatment aqueous solution containing
A chromate film layer of g/m^2 is formed, and silica sol is applied thereon at a rate of 10 parts per 100 parts of the solid content of the organic polymer resin.
Formation of a composite film on a steel sheet or zinc-plated steel sheet, characterized in that an organic polymer resin liquid containing ~120 parts is applied and dried to form a film layer of 0.1 to 5.0 g/m^2 Method. (2) The method for forming a composite film on a steel sheet or a zinc-plated steel sheet according to claim (1), wherein the metal ions contained in the alkaline metal salt aqueous solution are metal salts of carbonic acid, phosphoric acid, sulfuric acid, nitric acid, or carboxylic acid. (3) The ammonium salt used in the alkaline metal salt aqueous solution is selected from ammonium carbonate, ammonium phosphate, ammonium sulfate, and ammonium nitrate; the aliphatic amino acid is selected from monoaminocarboxylic acids, oxyamino acids, and monoaminodicarboxylic acids; Group amines are C
The method for forming a composite film on a steel sheet or a zinc-plated steel sheet according to claim (1), wherein the aliphatic monoamine, diamine, and triamine are selected from _1 to C_8. (4) The steel sheet or galvanized steel sheet according to claim (1), (2) or (3), wherein the alkaline metal salt aqueous solution further contains 1 to 50 g/l of an alkali metal salt of a fatty acid. A method for forming a composite film.