JP2010215955A - Method for forming multilayer coating film - Google Patents

Abstract

The present invention eliminates the conventional complicated and high-cost rust-proofing treatment process, for example, zinc phosphate treatment process, and, as an alternative, has an excellent rust-proofing treatment process equivalent to or higher than the conventional rust-proofing chemical treatment process. Providing a method for forming a multi-layer coating film that can provide rusting properties and that is suitable for an actual corrosive environment and is excellent in economic efficiency and environmental conservation. ) A non-treated aqueous solution comprising a rare earth metal (A) nitrate having a lower precipitation pH than zinc, a zinc (B) salt, and a rare earth metal (C) nitrate having a higher precipitation pH than zinc; A step of immersing a metal substrate and forming an electrolytic film containing rare earth metal (A), zinc (B) and rare earth metal (C) on the metal substrate by cathodic electrolysis; and (b) step (a) ) Soak the metal substrate on which the electrolytic membrane has been formed in the electrodeposition paint It is a multilayer coating film forming method including a step of forming an electrodeposition coating film by dipping and performing electrodeposition coating,
The mass of the electrolytic membrane after step (b) is 30 to 200 mg / m ² , and the electrolytic membrane is based on a total of 100 parts by mass of the rare earth metal (A), zinc (B), and rare earth metal (C). 20 to 65 parts by mass of rare earth metal (A), 20 to 60 parts by mass of zinc (B), 15 to 50 parts by mass of rare earth metal (C),
A method for forming a multilayer coating film.
[Selection figure] None

Description

  The present invention forms on a raw metal substrate a three-metal component type electrolytic membrane containing a rare earth metal having a lower precipitation pH than zinc, zinc, and a rare earth metal having a higher precipitation pH than zinc. The present invention relates to a multilayer coating film forming method including a process and an electrodeposition coating process, and a multilayer coating film obtained by the multilayer coating film forming method.

  Auto bodies have been commercialized by painting metal bases such as cold-rolled steel sheets and galvanized steel sheets and assembling them. In order to provide such adhesion to the base electrodeposition coating film, such a metal molded article is a pre-treatment stage of the coating process, a degreasing process, a surface adjustment process, a zinc phosphate treatment process, etc. A chemical conversion treatment process was performed.

  In the conventional rust prevention chemical conversion treatment such as zinc phosphate treatment, it is necessary to increase the amount of precipitation per unit area in order to obtain a sufficient base rust prevention effect, which is uneconomical from the viewpoint of material and energy. . Furthermore, since a lot of sludge is deposited, there are practical problems such as adverse effects on environmental conservation.

Therefore, an alternative method in which conventional rust prevention chemical conversion treatment is omitted has been proposed as an economical base rust prevention system in consideration of environmental conservation. For example, Japanese Patent Laid-Open No. 2000-64090 (Patent Document 1) has been proposed. Discloses a metal surface treatment method in which an aqueous solution is electrolyzed using a metal to be treated in an aqueous solution, and the surface treatment method includes yttrium (Y) ions, neodymium (Nd) ions, and praseodymium. (Pr) at least one rare earth metal ion selected from the group consisting of ions, sulfate ions, and zinc ions, each containing 0.05 g / L or more, and the aqueous solution has a pH of 2 to 7, and the resulting film The mass is about 1 g / m ² (see Examples). The ions used in combination with the rare earth metal ions are limited to sulfate ions and zinc ions. In this surface treatment method, rare earth metal ions, sulfate ions and zinc ions are essential components. However, this method achieves base adhesion that is equivalent to or better than that of conventional anti-rust chemical conversion treatments, and also provides practical anti-rust and corrosion resistance after electrodeposition coating, in particular, sufficient anti-rust performance for automotive applications. It has not reached the point where it is achieved. Moreover, when nitrate ion is used instead of sulfate ion, it is known that the rust prevention property of the obtained film is inferior. In particular, sulfate ion is an essential component.

  Japanese Patent Application Laid-Open No. 2000-226690 (Patent Document 2) discloses one or more binders made of inorganic compounds, and metal ion supply substances (aluminum, chromium, molybdenum, tungsten, titanium, zirconium, vanadium, cerium, The metal plate is subjected to cathodic treatment or anodizing treatment in an aqueous solution having a pH of 2 to 11 consisting of one or more of zinc, tin, nickel, cobalt, and copper. A sol-like hydrated oxide, or a coating base, wherein a film made of a sol-like hydrated oxide and a metal is formed on a metal plate, and then dried as it is without being washed with water. A method for producing a surface-treated metal sheet is disclosed. However, Patent Document 2 uses cerium and zinc as one of the metal ion supply substances, but the rare earth element is only cerium, and the combined use of zinc salt and two or more rare earth metals is not disclosed. .

Japanese Patent Laid-Open No. 2003-082496 (Patent Document 3) discloses a method for producing a surface-treated steel sheet in which an electrolytic treatment is performed using a steel sheet as a cathode in an electrolytic solution, from among Al, Mn, Mg, Zn, Ca, and Ce. 0.001~3mol / L 1 kind or two or more metal cations in total selected, 0.001~3mol / L oxygen acid ions including P in terms _{of P} 2 _{O 5,} the oxidizer component 0.001 The ratio [A] / [B] of P ₂ O ₅ converted molar concentration [A] of the oxygen acid ion containing P to ₂ mol / L and the molar concentration [B] of the metal cation is 0.5 to 10 A method for producing a surface-treated steel sheet excellent in corrosion resistance, characterized in that an aqueous solution is used as the electrolytic solution is disclosed. As the oxidizing agent component used in this method, one or two of nitrate ion, nitrite ion, chlorate ion, perchlorate ion, permanganate ion, bromate ion, vanadate ion, and molybdate ion are used. More than seeds are selected. However, in Patent Document 3, the metal cations used are limited to Al, Mn, Mg, Zn, Ca, and Ce.

  The present inventors have already made an untreated metal in an aqueous solution containing a rare earth metal nitrate and zinc salt for the purpose of omitting the conventional complicated and expensive rust prevention chemical conversion treatment step such as a zinc phosphate treatment step. A multilayer coating film forming method including a step of immersing a base material and forming an electrolytic film containing a rare earth metal and zinc on a metal base material by cathodic electrolysis was provided (Japanese Patent Application No. 2007-280464). In the technology of this patent application, a rare earth metal having a lower precipitation pH than zinc is used as the rare earth metal, and a two-metal component-based electrolytic membrane of rare earth metal and zinc can be provided on the metal substrate. The component-based electrolytic membrane has a concentration gradient in which the concentration of the rare earth metal is the highest on the surface of the metal substrate, and the concentration of the rare earth metal decreases with distance from the metal substrate. By the technology of this patent application, conventional rust prevention chemical conversion treatment steps such as a zinc phosphate treatment step can be omitted, and rust prevention properties comparable to these can be provided. However, with this technique, as a rust prevention property, good salt spray test (SST) and salt warm water immersion test (SDT) results can be obtained. However, when an actual corrosive environment is considered, for example, a cycle corrosion test ( It was found that excellent corrosion resistance could not be obtained with the cycle corrosion test (CCT). Therefore, it has been desired to study an electrolytic membrane that can improve the technology of the above-mentioned application and can provide excellent rust prevention properties in accordance with an actual corrosive environment and a multilayer coating film including the electrolytic membrane.

  Further, in recent years, interest in environmental problems has been increasing, and in general, reduction of energy used and environmental conservation are desired.

JP 2000-64090 A JP 2000-226690 A Japanese Patent Laid-Open No. 2003-082496

  The present invention omits the conventional complicated and high-cost rust-proofing treatment process, such as a zinc phosphate treatment process, and, as an alternative, is equivalent to or superior to the conventional rust-proofing treatment process. It is an object of the present invention to provide a method for forming a multilayer coating film that can provide rust prevention and is excellent in economic efficiency and environmental conservation, in which the rust prevention is suitable for an actual corrosive environment. To do.

  As a result of intensive studies, the present inventors have found that a rare earth metal (A) nitrate having a lower precipitation pH than zinc, a zinc (B) salt, and a rare earth metal (C) nitrate having a higher precipitation pH than zinc are included. By immersing an untreated metal substrate in a component aqueous solution and forming an electrolytic film containing rare earth metal (A), zinc (B) and rare earth metal (C) on the metal substrate by cathodic electrolysis The present inventors have found that the metal substrate can be provided with excellent rust prevention properties, and that the electrolytic membrane is excellent in adhesion to the metal substrate and the electrodeposition paint and is suitable for further electrodeposition coating. Accordingly, the present invention provides the following.

(A) a rare earth metal (A) nitrate having a lower precipitation pH than zinc; a salt of zinc (B); and a rare earth metal (C) nitrate having a higher precipitation pH than zinc; A step of immersing the treated metal substrate and forming an electrolytic film containing rare earth metal (A), zinc (B) and rare earth metal (C) on the metal substrate by cathodic electrolysis, and (b) step A method for forming a multilayer coating film comprising a step of forming an electrodeposition coating film by immersing the metal substrate on which the electrolytic film is formed in (a) in an electrodeposition coating and performing electrodeposition coating,
The mass of the electrolytic membrane after step (b) is 30 to 200 mg / m ² , and the electrolytic membrane is based on a total of 100 parts by mass of the rare earth metal (A), zinc (B), and rare earth metal (C). 20 to 65 parts by mass of rare earth metal (A), 20 to 60 parts by mass of zinc (B), 15 to 50 parts by mass of rare earth metal (C),
A method for forming a multilayer coating film.

  In the method for forming a multilayer coating film of the present invention, it is preferable that the electrodeposition paint further contains a salt of a rare earth metal (D).

In the multilayer coating film forming method of the present invention,
The rare earth metal (A) is selected from the group consisting of yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and mixtures thereof. Selected
The rare earth metal (C) is selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and mixtures thereof.
It is preferable.

  The present invention also relates to a multilayer coating film comprising an electrolytic film and an electrodeposition coating film formed on a metal substrate obtained by the above-described multilayer coating film forming method.

  In the multilayer coating film forming method of the present invention, a rare earth metal (A) nitrate having a lower precipitation pH than zinc, a zinc (B) salt, and a rare earth metal (C) nitrate having a higher precipitation pH than zinc are combined. And forming a three-metal component type electrolytic membrane of rare earth metal (A) -zinc (B) -rare earth metal (C) on an untreated metal substrate, and thus, a conventional process such as a zinc phosphate treatment step. It is possible to omit the complicated and high-cost rust prevention chemical conversion treatment process, and to provide an untreated metal substrate with excellent rust prevention and adhesion equivalent to or better than the conventional rust prevention chemical conversion treatment process. can do. In addition, according to the present invention, a conventional complicated and high-cost rust prevention chemical conversion treatment step and the like can be omitted, and thus a multilayer coating film forming method and a multilayer coating film excellent in economic efficiency and environmental conservation are provided. be able to. In addition, since the present invention does not include sulfate ions, even if nitrate ions, which are components of the treatment liquid in the step (a) in the present invention, are mixed into the electrodeposition paint in the next step (b), the electrodeposition paint There is also an advantage that it does not adversely affect. Furthermore, since the step (a) (pretreatment step) included in the method for forming a multilayer coating film of the present invention can provide excellent adhesion to an untreated metal substrate and electrodeposition paint, The invention can provide a base antirust process suitable for electrodeposition coating.

The present invention
Step (a): nitrate of rare earth metal (A) [which may be referred to as (rare earth) metal (A) in this specification) having a lower precipitation pH than zinc and zinc (B) [in this specification, metal (B)) and a nitrate of a rare earth metal (C) having a higher precipitation pH than zinc (sometimes referred to herein as (rare earth) metal (C)). An electrolytic membrane containing rare earth metal (A), zinc (B) and rare earth metal (C) on the metal substrate by cathodic electrolysis by immersing an untreated metal substrate (AB in this specification) A step of forming a -C three-metal component-based electrolyte membrane or simply a three-metal component-based electrolyte membrane), and step (b): the metal group on which the electrolyte membrane was formed in step (a) A multilayer coating that includes the steps of forming an electrodeposition coating by immersing the material in the electrodeposition coating and performing the electrodeposition coating. Hereinafter, each process will be described in detail with respect to the film forming method.

Step (a)
Examples of the metal substrate used in the step (a) of the present invention include cold-rolled steel sheet, high-strength steel, high-tensile steel, cast iron, zinc and galvanized steel, aluminum and aluminum alloy. In the present invention, the above-mentioned metal base material after the general degreasing step is used without being subjected to pretreatment such as untreated, that is, the conventional general surface conditioning step and rust prevention chemical conversion treatment step.

  In the step (a) of the present invention, an aqueous solution containing a rare earth metal (A) nitrate, a zinc (B) salt and a rare earth metal (C) nitrate (hereinafter sometimes referred to as “pretreatment aqueous solution”). Use.

  The rare earth metal (A) used in the present invention is a rare earth metal having a precipitation pH lower than that of zinc. Since the precipitation pH of zinc is around 7 in the case of zinc acetate, lower rare earth metals such as yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and a mixture thereof may be mentioned, dysprosium (Dy), ytterbium (Yb) and a mixture thereof are preferable, and ytterbium (Yb) is more preferable.

  In general, the precipitation pH of rare earth metals (as hydroxides) is known to decrease with an increase in atomic number in lanthanoid series metals (reference: Y. Suzuki, T. Nagayama, M. Sekine). A. Mizuno, K. Yamaguchi, J. Less-Common Met 126, 351 (1986)).

  Specifically, the pH changes when a 0.1 N KOH solution is dropped into a rare earth metal nitrate solution. At that time, a titration curve is created, a tangent line is drawn on the curve portion where the pH changes, and the pH at the point where the tangent line contacts the titration curve is defined as the precipitation pH.

  The nitrate of the rare earth metal (A) is 0.005 to 10%, preferably 0.01 to 5%, more preferably 0.05 to 1% (w / w) with respect to the aqueous solution of the pretreatment agent. If it is less than 0.005%, the deposition efficiency of the electrolytic membrane is lowered, so there is a fear that a satisfactory electrolytic membrane cannot be obtained, and if it exceeds 10%, it is not economically preferable.

  Examples of the metal (B) contained in the aqueous solution of the pretreatment agent, that is, zinc salt include zinc acetate, zinc nitrate, zinc sulfamate, zinc formate, zinc lactate, zinc hypophosphite and the like. It is not limited to.

  The zinc salt is 0.005 to 10%, preferably 0.01 to 5%, more preferably 0.05 to 1% (w / w), and less than 0.005% with respect to the pretreatment agent aqueous solution. Then, since the deposition efficiency of the electrolytic membrane is lowered, there is a possibility that a satisfactory electrolytic membrane cannot be obtained. If it exceeds 10%, it is not economically preferable.

  The rare earth metal (C) used in the present invention is a rare earth metal having a precipitation pH higher than that of zinc. Since the precipitation pH of zinc is around 7 in the case of zinc acetate, higher rare earth metals such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm) and those And the like.

  The nitrate of the rare earth metal (C) is 0.005 to 10%, preferably 0.01 to 5%, more preferably 0.05 to 1% (w / w) with respect to the aqueous solution of the pretreatment agent. If it is less than 0.005%, the deposition efficiency of the electrolytic membrane is lowered, so there is a fear that a satisfactory electrolytic membrane cannot be obtained, and if it exceeds 10%, it is not economically preferable.

The contents of the rare earth metal (A) nitrate, zinc (B) salt and rare earth metal (C) nitrate in the aqueous pretreatment solution are, on a mass basis, rare earth metal (A) nitrate and zinc (B), respectively. For a total of 100 parts by mass of the salt and nitrate of the rare earth metal (C),
10 to 40 parts by weight, preferably 10 to 35 parts by weight, more preferably 20 to 30 parts by weight of the nitrate of the rare earth metal (A);
The salt of zinc (B) is 10-40 parts by weight, preferably 18-33 parts by weight, more preferably 25-30 parts by weight;
The nitrate of rare earth metal (C) is 40 to 70 parts by mass, preferably 45 to 65 parts by mass, more preferably 50 to 60 parts by mass (however, rare earth metal (A) nitrate, zinc (B) salt And the total nitrate of the rare earth metal (C) is 100 parts by mass).
If it deviates from the above range, there is a risk of problems such as a decrease in uniformity of the electrolytic membrane and a failure to obtain desired performance.

  Examples of water used in the pretreatment aqueous solution include tap water, pure water, distilled water, deionized water, and ion exchange water, but are not particularly limited thereto.

  A pretreatment agent aqueous solution can be prepared by dissolving, suspending and / or dispersing the nitrate of rare earth metal (A), the salt of zinc (B) and the nitrate of rare earth metal (C) in water. In particular, the order of addition is not limited.

  The pH of the aqueous pretreatment agent solution is 2 to 7, preferably 4 to 7, and more preferably 5 to 6. If the pH is less than 2, the generation of hydrogen gas during electrolysis becomes intense, so that the deposition efficiency of the electrolytic film is drastically decreased and the denseness of the deposited coating film may be decreased. If the pH exceeds 7, the stability of metal ions is lost, which may cause problems such as precipitation of metal hydroxide in the treatment liquid.

  When the pH of the aqueous solution of the pretreatment agent is higher than the desired pH, chemicals used for adjusting the pH include inorganic acids such as nitric acid, or organic acids such as formic acid and acetic acid, and when the pH is lower than the desired pH, Examples include organic bases such as amines, or inorganic bases such as ammonia and sodium hydroxide, but are not limited thereto.

  In the step (a) of the present invention, an untreated metal base material is immersed as a cathode in the aqueous solution of the pretreatment agent, and cathodic electrolysis (electrodeposition) is performed by applying a voltage and a current. A three-metal component type electrolytic membrane containing metal (A), zinc (B) and rare earth metal (C) is formed.

On the surface of the metal substrate that is the cathode, chemical species such as dissolved oxygen, hydrogen ions, and water are reduced to generate hydroxide ions (OH ⁻ ), which are rare earth metals (A ) Ions, zinc (B) ions, and rare earth metal (C) ions to form a complex of rare earth metal (A), zinc (B), and rare earth metal (C) hydroxides. An electrolytic membrane is deposited on the surface.

  The method for applying the voltage and current is not particularly limited, and examples thereof include a direct current method.

  The applied voltage is 1 to 20V, preferably 2 to 10V, more preferably 3 to 7V. If the applied voltage is less than 1V, there is a risk of insufficient deposition of the metal hydroxide. If it exceeds 20V, the electrolysis of water rather than the deposition of the metal hydroxide. There is a risk of problems such as the generation of hydrogen gas due to.

  The energization time (processing time) is not particularly limited, and is 5 to 300 seconds, preferably 10 to 180 seconds, and more preferably 30 to 120 seconds.

  When the treatment time is shorter than 5 seconds, the electrolytic film is not formed or the corrosion resistance is inferior because the thickness is insufficient even if formed. When the processing time is longer than 300 seconds, an appearance defect called matte burn or burnt may occur. Further, excessive processing time is not preferable because productivity is extremely lowered.

  After electrolysis, the metal substrate may be washed with water as necessary.

The mass [the metal conversion mass (total precipitation amount)] of the electrolytic membrane is 30 to 200 mg / m ² , preferably 80 to 150 mg / m ² . If the mass of the electrolytic membrane is less than 30 mg / m ² , the corrosion resistance value of the electrolytic membrane will decrease, which may lead to problems such as reduced corrosion resistance after the electrolytic membrane and electrodeposition coating, If it exceeds 200 mg / m ² , the film formability of the electrolytic membrane is lowered, which may cause problems such as the occurrence of defects.

The amount and ratio of the rare earth metal (A), zinc (B) and rare earth metal (C) in the electrolytic membrane is 100 parts by mass in total of the rare earth metal (A), zinc (B) and rare earth metal (C) on a mass basis. Against
20 to 65 parts by mass of the rare earth metal (A), preferably 30 to 65 parts by mass, more preferably 35 to 63 parts by mass;
Zinc (B) is 20-60 parts by mass, preferably 20-50 parts by mass, more preferably 21-40 parts by mass;
Rare earth metal (C) is 15 to 50 parts by mass, preferably 17 to 40 parts by mass; more preferably 18 to 30 parts by mass (however, rare earth metal (A), zinc (B) and rare earth metal (C) Is 100 parts by mass).
If it deviates from the above range, there is a risk of problems such as deterioration in the uniformity and corrosivity of the electrolytic membrane.

  The mass of the electrolytic membrane and the contents and proportions of the rare earth metal (A), zinc (B) and rare earth metal (C) are determined after removing the uncured electrodeposition coating after the step (b) described later. (For example, an uncured electrodeposition coating film is washed as it is and dissolved and removed using acetone). Then, in the electrolytic membrane remaining, a conventionally known fluorescent X-ray measurement is performed, and the mass of the electrolytic membrane (metal equivalent mass) is calculated. It can be determined by quantification. The rare earth metal (A), zinc (B) and rare earth metal (C) are based on the determined mass (metal equivalent mass) of the rare earth metal (A), zinc (B) and rare earth metal (C). The content and ratio of each can be calculated.

  The electrolytic membrane in the step (a) can be used as an alternative to the conventional complicated and high-cost rust prevention chemical conversion treatment step, for example, zinc phosphate treatment step, etc., and is excellent in economic efficiency and environmental conservation. An untreated metal substrate can be provided with excellent rust prevention property equal to or higher than that of the process, in particular, rust prevention property and adhesion in accordance with the actual corrosive environment.

Step (b)
The electrodeposition paint used in the step (b) is not particularly limited, and an electrodeposition paint known to those skilled in the art can be used. For example, the electrodeposition paint is a base resin having a cationic group, a curing agent, and a necessity. Accordingly, electrodeposition paints containing pigments are preferred.

  The base resin having a cationic group contained in the electrodeposition coating is a cation-modified epoxy resin obtained by modifying an oxirane ring in the resin skeleton with an organic amine compound. In general, a cation-modified epoxy resin is produced by opening a ring of an oxirane ring in a starting material resin molecule with a primary amine, a secondary amine, a tertiary amine, and / or an amine such as an acid salt thereof. A typical example of the starting material resin is a polyphenol polyglycidyl ether type epoxy resin which is a reaction product of a polycyclic phenol compound such as bisphenol A, bisphenol F, bisphenol S, phenol novolak, cresol novolak, and epichlorohydrin. Examples of other starting material resins include oxazolidone ring-containing epoxy resins described in JP-A-5-306327. These epoxy resins are obtained by a reaction between a diisocyanate compound or a bisurethane compound obtained by blocking an NCO group of a diisocyanate compound with a lower alcohol such as methanol or ethanol, and epichlorohydrin.

  The starting material resin can be used by extending the chain with a bifunctional polyester polyol, polyether polyol, bisphenol, dibasic carboxylic acid or the like before the oxirane ring-opening reaction with amines.

  Similarly, before the oxirane ring-opening reaction with amines, 2-ethylhexanol, nonylphenol, ethylene glycol for some oxirane rings for the purpose of adjusting molecular weight or amine equivalent, improving heat flow, etc. Monohydroxy compounds such as mono-2-ethylhexyl ether, ethylene glycol mono n-butyl ether, and propylene glycol mono-2-ethylhexyl ether can also be added and used.

  Examples of amines that can be used for opening an oxirane ring and introducing an amino group include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine , N, N-dimethylbenzylamine, primary amines such as N, N-dimethylethanolamine, secondary amines or tertiary amines and / or their acid salts. Further, ketimine block primary amino group-containing secondary amine such as aminoethylethanolamine methyl isobutyl ketimine, diethylenetriamine diketimine can also be used. These amines must be reacted with at least an equivalent amount relative to the oxirane ring in order to open all the oxirane rings.

  The number average molecular weight of the cation-modified epoxy resin is in the range of 1,500 to 5,000, preferably 1,600 to 3,000. When the number average molecular weight is less than 1,500, physical properties such as solvent resistance and corrosion resistance of the cured coating film may be inferior. On the other hand, when it exceeds 5,000, it is difficult to control the viscosity of the resin solution and it is difficult to synthesize the resin solution, and it may be difficult to handle in operation such as emulsification dispersion of the obtained resin. Furthermore, because of the high viscosity, the flowability during heating and curing is poor, and the appearance of the coating film may be significantly impaired.

  The cation-modified epoxy resin is preferably molecularly designed so that the hydroxyl value is in the range of 50 to 250. If the hydroxyl value is less than 50, curing of the coating is inferior. On the other hand, if it exceeds 250, excessive hydroxyl groups remain in the coating film after curing, resulting in a decrease in water resistance.

  The cation-modified epoxy resin is preferably molecularly designed so that the amine value is in the range of 40 to 150. If the amine value is less than 40, the emulsification dispersion failure in the aqueous medium due to acid neutralization, which will be described in detail below, is caused. On the other hand, if it exceeds 150, excess amino groups remain in the coating film after curing, resulting in water resistance. May decrease.

  The curing agent contained in the electrodeposition paint may be of any type as long as each resin component can be cured at the time of heating, and among them, a block polymer suitable as a curing agent for the electrodeposition paint application. Isocyanates are recommended. Examples of the polyisocyanate that is a raw material of the block polyisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate (including trimer), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate, isophorone diisocyanate, 4,4′- Examples include alicyclic polyisocyanates such as methylene bis (cyclohexyl isocyanate), and aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate. The blocked polyisocyanate can be obtained by blocking these with an appropriate sealant.

  Examples of the sealant include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol; ethylene glycol Cellosolves such as monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type double-ended diols such as polyethylene glycol, polypropylene glycol and polytetramethylene ether glycol phenol; ethylene glycol, propylene glycol and 1,4-butanediol Polyester-type double-ended polyols obtained from diols such as oxalic acid, succinic acid, adipic acid, suberic acid, sebacic acid and the like; para-t-butylphenol And phenols such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam are preferably used. .

  It is desirable that the block polyisocyanate is blocked beforehand by using a sealing agent alone or by using a plurality of types. The blocking rate is preferably set to 100% in order to ensure the storage stability of the paint unless there is a purpose of denaturing reaction with the resin components.

  The blending ratio of the block polyisocyanate to the base resin having the cationic group varies depending on the degree of cross-linking required for the purpose of using the cured coating film, but it is solid in consideration of coating film properties and intermediate coating compatibility. As a quantity, the range of 15-40 mass% is preferable. If the blending ratio is less than 15% by mass, the coating film may be poorly cured. As a result, the physical properties of the coating film such as mechanical strength may be lowered, and the coating film may be affected by paint thinner during intermediate coating. May invite. On the other hand, if it exceeds 40% by mass, it may be excessively cured, resulting in poor coating properties such as impact resistance. In addition, block polyisocyanate may be used in combination of two or more kinds for convenience of coating film physical properties, degree of curing and adjustment of curing temperature.

  In the base resin having a cationic group, an amino group in the resin is converted to an appropriate amount of inorganic acid such as hydrochloric acid, nitric acid, hypophosphorous acid, formic acid, acetic acid (including acetic anhydride and glacial acetic acid), lactic acid, sulfamic acid, It is prepared by neutralizing with an organic acid such as acetylglycine acid and emulsifying and dispersing in water as a cationized emulsion. Further, when emulsifying and dispersing, usually, emulsion particles containing a curing agent as a core and a base resin as a shell (soot) are formed.

  The electrodeposition paint used in the method of the present invention may further contain a pigment. Any pigment that can be used in paints can be used without particular limitation. Examples include color pigments such as carbon black, titanium dioxide, graphite, and extender pigments such as kaolin, aluminum silicate (clay), talc, calcium carbonate, silica, and inorganic colloids (silica sol, alumina sol, titanium sol, zirconia sol, etc.) And lead-free rust preventive pigments such as phosphoric acid pigments (aluminum phosphomolybdate, (poly) zinc phosphate, calcium phosphate, etc.) and molybdic acid pigments (aluminum phosphomolybdate, zinc phosphomolybdate, etc.).

  Among these, titanium dioxide, carbon black, aluminum silicate (clay), silica, aluminum phosphomolybdate, and (poly) zinc phosphate are particularly important as pigments contained in the electrodeposition paint. In particular, titanium dioxide and carbon black are most suitable for electrodeposition coatings because they have high concealability as color pigments and are inexpensive.

  In addition, although the said pigment can also be used independently, it is common to use multiple types according to the objective.

  The method for introducing the pigment into the electrodeposition coating is not particularly limited, and examples thereof include pigment dispersion resins (for example, epoxy sulfonium salt type resins, epoxy type quaternary ammonium salt type resins, epoxy type tertiary amine type resins). A pigment dispersion paste is prepared in advance by dispersing a pigment in the base resin having a cationic group, which may be an acrylic quaternary ammonium salt type resin or the like. Can be blended.

  Mass ratio of pigment (P) to total mass (P + V) of pigment (P) and resin solid content (V) contained in the electrodeposition paint {P / (P + V)} × 100% (hereinafter referred to as PWC) However, it is preferable that it exists in the range of 5-30 mass%. If the mass ratio is less than 5% by mass, the barrier of the corrosion factor such as water and oxygen to the coating film is excessively lowered due to insufficient pigment, and corrosion resistance at a practical level may not be exhibited. However, if such inconvenience does not occur, the pigment concentration may be set to zero as much as possible, and a clear or nearly clear electrodeposition paint may be formed and supplied to the present invention. Further, if the above mass ratio exceeds 30% by mass, an excess of pigment causes an increase in viscosity at the time of curing, and the flow property is lowered and the appearance of the coating film may be remarkably deteriorated. However, the resin solid content (V) is the total solid of all the resin binders constituting the electrodeposition coating film including the base resin, which is the main resin of the electrodeposition paint, and the curing agent as well as the pigment dispersion resin. Indicates the amount.

  Furthermore, the electrodeposition paint used in step (b) may further contain a salt of rare earth metal (D).

  The salt of rare earth metal (D) produces rare earth metal (D) ions during the electrodeposition process, which rarely precipitates more than the base resin, curing agent and any pigments with cationic groups. Therefore, it is preferentially deposited on the electrolyte membrane formed in the step (a), filling the gap between the electrolyte membranes formed in the step (a), and further forming a dense continuous film that is combined. Can be formed.

  As the salt of rare earth metal (D), cerium (Ce), neodymium (Nd), samarium (Sm), praseodymium (Pr), yttrium (Y), dysprosium (Dy), holmium (Ho), erbium ( An organic acid salt and an inorganic acid salt of at least one rare earth metal (D) selected from the group consisting of Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and mixtures thereof, for example Selected from rare earth metals (D) and organic acids (eg acetic acid, formic acid, lactic acid, sulfamic acid etc., preferably acetic acid, formic acid and sulfamic acid) or inorganic acids (eg hypophosphorous acid, nitric acid, sulfuric acid etc.) And at least one acid salt.

  The content of rare earth metal (D) salt in the electrodeposition coating is 0.005 to 2.5%, preferably 0.01 to 1%, based on the total solid content of the electrodeposition coating. More preferably, it is 0.05 to 1% (w / w). If the content is less than 0.005%, corrosion resistance based on sufficient adhesion to the base may not be obtained, and if it exceeds 2.5%, the dispersion stability of the electrodeposition paint component and the electrodeposition coating film The smoothness and water resistance of the resin may decrease.

  The method for introducing the salt of the rare earth metal (D) into the electrodeposition coating is not particularly limited, and can be carried out in the same manner as a normal pigment dispersion method. For example, a salt of rare earth metal (D) can be dispersed in a dispersion resin in advance to prepare a dispersion paste, which can be blended with the electrodeposition paint. Or it can disperse | distribute or melt | dissolve and mix | blend as it is after preparation of the resin emulsion for coating materials, or after preparation of a coating material. In addition, as a resin for dispersion, general materials for cationic electrodeposition paints (epoxy sulfonium salt type resin, epoxy type quaternary ammonium salt type resin, epoxy type tertiary amine type resin, acrylic type quaternary ammonium salt type) Resin).

  The electrodeposition paint is adjusted so that the total solid content is in the range of 5 to 40% by mass, preferably 10 to 25% by mass. An aqueous medium (water alone or a mixture of water and a hydrophilic organic solvent) is used to adjust the total solid concentration.

  Furthermore, a small amount of additives may be introduced into the electrodeposition paint. Examples of additives include UV absorbers, antioxidants, surfactants, coating surface smoothers, curing catalysts (eg, dibutyltin oxide, dioctyltin dilaurate, dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, dibutyltin diacetate). Organotin compounds such as benzoate and dioctyltin dibenzoate), curing accelerators (for example, zinc acetate), and the like.

  The electrodeposition coating method is not particularly limited, and a method known in the art can be used. For example, cathodic electrodeposition coating using a metal substrate as a cathode is preferable. By maintaining the temperature at 15 to 35 ° C. and setting the applied voltage to 50 to 450 V, preferably 100 to 400 V, mainly precipitates from rare earth metal salts (even if they are not present) Alternatively, the base resin having a cationic group, the curing agent and the pigment, which are electrodeposition paint vehicles, are preferentially deposited. When the applied voltage is less than 50V, the deposition property of the vehicle component of the electrodeposition paint is insufficient, and when the applied voltage exceeds 450V, the vehicle component is deposited in excess of an appropriate amount. This is not preferable because it may have an appearance.

  The energization time (processing time) is 30 to 300 seconds, preferably 30 to 180 seconds. When processing time is too short for 30 seconds, an electrodeposition coating film does not produce | generate, or even if it produces | generates, since thickness is insufficient, corrosion resistance is inferior. Further, an excessive treatment time exceeding 300 seconds is not preferable because productivity is extremely lowered.

  After electrodeposition coating, an electrodeposition-cured coating film having a high degree of crosslinking can be obtained by performing a curing reaction at 120 to 200 ° C., preferably 140 to 180 ° C. However, if it exceeds 200 ° C., the coating film becomes excessively hard and brittle, while if it is less than 120 ° C., curing is not sufficient, and film properties such as solvent resistance and film strength are unfavorable. The curing time is preferably 10 to 30 minutes.

  As described above, in the present invention, in the step (a), a three-metal component-based metal composite layer containing rare earth metal (A), zinc (B), and rare earth metal (C) as an electrolytic film on the metal substrate. The electrodeposition coating film can be formed on the metal composite layer by the step (b), and the multilayer coating film comprising the metal composite layer and the electrodeposition coating film is formed according to the present invention. be able to.

  In the present invention, an anti-rust property is obtained by forming, on an untreated metal substrate, an electrolytic film in which a metal of a three metal component system of rare earth metal (A) -zinc (B) -rare earth metal (C) is combined. In particular, the rust prevention property in accordance with the actual corrosive environment is greatly improved. Examples of the actual corrosive environment include a corrosive environment of a cycle corrosion test (CCT) according to JASO M609-91 “Automobile Material Corrosion Test Method” known in the art, and the metal of the present invention. The multilayer coating film on the metal substrate including the composite layer and the electrodeposition coating film can exhibit excellent rust prevention properties even under such an environment. This is because the chemical stability of the electrolytic membrane is remarkably improved by the composite of the metal.

  Moreover, since the antirust property obtained by this invention is equivalent to or more than conventional antirust chemical conversion treatments such as zinc phosphate treatment, it is very useful as an alternative. Furthermore, the metal composite layer obtained by the present invention can provide excellent adhesion to an electrodeposition coating film, which is very beneficial.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited only to these examples. In the examples, “parts” means “parts by mass” unless otherwise specified.

Production Example 1 (Production of aqueous solution of pretreatment agent)
The salt of rare earth metal (A), the salt of zinc (B) and the salt of rare earth metal (C) were dissolved in ion-exchanged water to prepare 1000 ml of a pretreatment agent aqueous solution (solutions in Tables 2 to 6).

Production Example 2 (Production of a base resin having a cationic group)
In a reaction vessel equipped with a stirrer, decanter, nitrogen inlet tube, thermometer and dropping funnel, 2400 parts of bisphenol A type epoxy resin (trade name DER-331J, manufactured by Dow Chemical Company) with an epoxy equivalent of 188, 141 parts of methanol, methyl After charging 168 parts of isobutyl ketone and 0.5 part of dibutyltin dilaurate and stirring them uniformly at 40 ° C., 30 parts of 2,4- / 2,6-tolylene diisocyanate (80/20 mass ratio mixture) is added to 30 parts. When dripped over a period of time, heat was generated and the temperature rose to 70 ° C. To this was added 5 parts of N, N-dimethylbenzylamine, the temperature in the system was raised to 120 ° C, and the reaction was continued at 120 ° C for 3 hours until the epoxy equivalent reached 500 while distilling off methanol. Further, 644 parts of methyl isobutyl ketone, 341 parts of bisphenol A and 413 parts of 2-ethylhexanoic acid were added, the temperature inside the system was kept at 120 ° C., and the reaction was continued until the epoxy equivalent reached 1070. Cooled to 110 ° C. Subsequently, a mixture of 241 parts of diethylenetriamine diketimine (methyl isobutyl ketone solution having a solid content of 73%) and 192 parts of N-methylethanolamine was added and reacted at 110 ° C. for 1 hour to obtain a cation-modified epoxy resin. The number average molecular weight of this resin was 2,100, the amine value = 74, and the hydroxyl value was 160. Moreover, it was confirmed from the measurement of an infrared absorption spectrum etc. that the resin has an oxazolidone ring (absorption wave number: 1750 cm ⁻¹ ).

Production Example 3 (Production of curing agent for electrodeposition paint)
Put 222 parts of isophorone diisocyanate in a reaction vessel equipped with a stirrer, nitrogen introduction pipe, cooling pipe and thermometer, dilute with 56 parts of methyl isobutyl ketone, add 0.2 part of dibutyltin dilaurate, and heat up to 50 ° C. Later, 17 parts of methyl ethyl ketoxime was added such that the temperature of the contents did not exceed 70 ° C. And it is kept at 70 ° C. for 1 hour until the absorption of the isocyanate residue is substantially disappeared by infrared absorption spectrum, and then diluted with 43 parts of n-butanol to obtain the desired blocked isocyanate curing agent solution (solid content: 70%). Got.

Production Example 4 (Production of pigment-dispersed resin)
A reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe, and a thermometer was charged with 710 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 198 (trade name Epon 829, manufactured by Shell Chemical Co., Ltd.) and 289.6 parts of bisphenol A, The mixture was reacted at 150 to 160 ° C. for 1 hour under a nitrogen atmosphere, then cooled to 120 ° C., and 406.4 parts of a methyl isobutyl ketone solution of 2-ethylhexanolated half-blocked tolylene diisocyanate (solid content 95%) was added. After maintaining the reaction mixture at 110-120 ° C. for 1 hour, ethylene glycol mono n-butyl ether 1584.1 was added. And it cooled to 85-95 degreeC and made it uniform.

  In parallel with the production of the reaction product, in another reaction vessel, 384 parts of a methyl isobutyl ketone solution of 2-ethylhexanolated half-blocked tolylene diisocyanate (solid content 95%) in N, N-dimethylethanolamine 104.6. The mixture was stirred at 80 ° C. for 1 hour, and then 141.1 parts of 75% aqueous lactic acid was added. Further, 47.0 parts of ethylene glycol mono-n-butyl ether was mixed and stirred for 30 minutes. (Solid content 85%) was prepared. Then, 620.46 parts of this quaternizing agent was added to the previous reaction product, and the mixture was maintained at 85 to 95 ° C. until an acid value of 1 was reached, and a resin solution of pigment dispersion resin (average molecular weight 2200) (resin solid content 56% )

Production Example 5 (Production of pigment dispersion paste for electrodeposition paint)
Using a sand mill, based on the formulation shown in the following Table 1 including the pigment-dispersed resin obtained in Production Example 4, the obtained mixture was dispersed at 40 ° C. until the particle size became 5 μm or less, and the pigment was prepared. A dispersion paste (solid content 59%) was obtained.

Production Example 6 (Production of electrodeposition paint)
350 g (solid content) of the base resin obtained in Production Example 2 and 150 g (solid content) of the curing agent obtained in Production Example 3 were mixed, and ethylene glycol mono-2-ethylhexyl ether was 3% based on the solid content ( 15 g). Next, neutralize glacial acetic acid to a neutralization rate of 40.5%, add ion-exchanged water to dilute slowly, and then remove methyl isobutyl ketone under reduced pressure to a solid content of 36%. did. To the thus obtained 2000 g of the emulsion, 460.0 g of the pigment dispersion paste obtained in Production Example 5 and 2252 g of ion-exchanged water are added, and the electrodeposition paint having a solid content of 20.0% by mass with respect to the resin solid content. Was prepared.

  A rare earth metal salt was directly added to the electrodeposition coating composition obtained above in the form of an aqueous solution so as to have the concentrations shown in Tables 2 to 6, and the electrodeposition coatings shown in Tables 2 to 6 below were prepared. .

(Examples 1-7 and Comparative Examples 1-8)
Surfactor SC-53 (manufactured by Nippon Paint Co., Ltd.) was used as a cathode for each pretreatment agent aqueous solution shown in Tables 2 to 6 prepared by the method described in Production Example 1 as a cathode. After degreasing and washing with water, electrolytic treatment was performed under the conditions shown in the same table. Thereafter, the electrolytically treated base material was sufficiently washed with pure water, and then each of the electrodeposition paints shown in Tables 2 to 6 prepared in Production Example 6 was subjected to the electrodeposition coating process under the electrodeposition coating conditions in the same table. After the electrodeposition coating was performed so that the dry film thickness of the electrodeposition coating film was 20 μm, it was cured at 170 ° C. for 20 minutes to obtain a coating film. The amount of the electrolytic membrane deposited was determined by dissolving and removing the uncured electrodeposition coating film with acetone after the electrodeposition coating step and quantifying it by fluorescent X-ray measurement. The measurement conditions for the fluorescent X-ray measurement here were as follows.

Fluorescent X-ray Measurement Conditions A predetermined amount of standard solution is dropped on an untreated steel plate using a standard solution of each measurement element (manufactured by Wako Pure Chemical Industries, Ltd.) using a scanning fluorescent X-ray analyzer manufactured by Rigaku Co., Ltd. and dried. A calibration curve was prepared from the prepared samples, and analysis and quantification were performed based on the calibration curve.

(Comparative Example 9)
Electrodeposition paints and electrodeposition coating conditions shown in Table 6 using zinc phosphate-treated steel sheets obtained by treating surface untreated cold-rolled steel sheets (JIS G3141, SPCC-SD) with Surfdyne SD-5000 (manufactured by Nippon Paint Co., Ltd.) Was used for electrodeposition coating to obtain a dry film thickness of 20 μm to obtain an electrodeposition coating film.

  About the obtained multilayer coating film, the corrosion resistance by the cycle corrosion test (Cycle Corrosion Test (CCT)) and the salt warm water immersion test (Salt Dip Test (SDT)), the adhesion by the electrolytic peeling test, and the coating film appearance The results are shown in Tables 2-6 below. Each test method is as follows.

(Test method)
(1) Rust prevention evaluation:
Cycle Corrosion Test (CCT)
Cross-cut scratches were made with a knife so as to reach the substrate in the coating film of the electrodeposition coating plate after curing, and JASO M609-91 “Automobile Material Corrosion Test Method” was performed 100 cycles. Thereafter, the occurrence of rust and blistering from the crosscut portion was observed, and the rust prevention property in accordance with the actual corrosive environment was evaluated. The evaluation criteria are as follows.
Evaluation criteria
A: Maximum width of rust or swelling is less than 4 mm from the cut part (both sides)
○: The maximum width of rust or swelling is 4 mm or more and less than 6 mm from the cut part (both sides)
Δ: The maximum width of rust or blister is 6 mm or more and less than 9 mm (both sides) from the cut part, and blisters may occur on the flat part.
X: The maximum width of rust or blisters is 9 mm or more (both sides) from the cut part, or blisters are generated on the entire surface.

Salt warm water immersion test (Salt Dip Test (SDT))
The cured electrodeposition coated plate was immersed in 5% saline at 50 ° C. for 900 hours, and then the tape was peeled from the entire surface of the electrodeposited coated plate to determine the peeled area ratio.
Evaluation criteria
A: Peel area ratio 10% or less
○: peeling area ratio 30% or less
Δ: peeling area ratio 50% or less
X: Peel area ratio 50% or more

(2) Adhesion Evaluation: Electrolytic Peeling (Cathode Peeling) Test Cut the cured electrodeposition coated plate, electrolyze at a current value of 0.1 mA for 72 hours, and then peel off the tape and peel off both sides. The adhesion was evaluated by the width. The evaluation criteria are as follows.
Evaluation criteria
A: Peel width 3 mm or less
○: peeling width 3 mm to 6 mm
Δ: peeling width 6 mm to 10 mm
X: Peeling width 10 mm or more

(3) Coating film appearance The presence or absence of an abnormality in the coating film appearance was visually determined. The evaluation criteria were as follows.
Evaluation criteria ○: No problem ×: Appearance defects such as rough skin

(Test results)

* The total amount of deposited metal, the zinc content, and the rare earth metal (A) content are the mass (mg) of the electrolyte membrane remaining after removing the uncured electrodeposition coating film with acetone after washing with water after step (b). / m ^2), as well as means each weight percent of zinc and rare earth metal (a) included therein.

  As is clear from the above results, the performance of the multilayer coating film (Examples 1 to 7) by the multilayer coating film forming method of the present invention such as rust prevention, adhesion, and coating film appearance is treated with zinc phosphate. It was equal to or more than the electrodeposition coating film (Comparative Example 9) formed on the steel plate. In particular, according to the present invention, it was possible to obtain rust prevention properties in accordance with the actual corrosive environment, that is, excellent CCT test results.

  In addition, the multilayer coating films of Comparative Examples 1 to 8 are clearly low in rust prevention and adhesion as compared with the multilayer coating films of the present invention (Examples 1 to 7). The rust preventive property did not reach far to the electrodeposition coating film (Comparative Example 9) formed on the zinc phosphate-treated steel sheet.

  In Comparative Example 1, the total precipitation amount of the metal after the step (b) is lower than the specified value of the present invention, the zinc content is higher than the specified value of the present invention, and the rare earth metal (A) content is the present invention. Therefore, the rust prevention property (both CCT and SDT) is remarkably lowered.

  In Comparative Example 2, since the zinc content is lower than the specified value of the present invention and the rare earth metal (A) content is higher than the specified value of the present invention, the antirust property (both CCT and SDT) is remarkably high. descend.

  In Comparative Example 3, the zinc content in the electrolytic membrane after step (b) is higher than the specified value of the present invention, and the rare earth metal (A) content is lower than the specified value of the present invention. Sexuality (both CCT and SDT, especially CCT) and adhesion are significantly reduced.

  In Comparative Example 4, the aqueous solution of the pretreatment agent used in step (a) does not contain any salt of rare earth metal (C) having a precipitation pH higher than that of zinc, and Yb—Zn two-metal component electrolysis. A membrane was obtained. In the Yb—Zn two-metal component system, rust prevention, adhesion and coating appearance by SDT were good to some extent, but rust prevention by CCT, that is, rust prevention in accordance with the actual corrosive environment is extremely low. It was a thing.

  In Comparative Example 5, the aqueous solution of the pretreatment agent used in step (a) was a mixture of zinc acetate and lanthanum nitrate, but did not contain a rare earth metal (A) having a lower precipitation pH than zinc. . As a result, rust prevention (both CCT and SDT) and adhesion were significantly reduced.

  In Comparative Example 6, the aqueous solution of the pretreatment agent used in step (a) is a mixture of zinc acetate and cerium nitrate. Like Comparative Example 5, the rare earth metal (A) having a lower precipitation pH than zinc. Was not included. As a result, rust prevention (both CCT and SDT) and adhesion were significantly reduced.

  In Comparative Example 7, the aqueous solution of the pretreatment agent used in step (a) contains only ytterbium nitrate (Yb) and contains a zinc salt and a salt of rare earth metal (C) having a higher precipitation pH than zinc. It was not. As a result, the total amount of metal deposited after the step (b) was remarkably reduced, and since zinc was not contained, the rust prevention property (particularly SDT) was remarkably reduced.

  In Comparative Example 8, the aqueous solution of the pretreatment agent used in step (a) contains only zinc nitrate, a salt of rare earth metal (A) having a lower precipitation pH than zinc, and a higher precipitation pH than zinc. It did not contain any salt of rare earth metal (C). As a result, the rust prevention property (particularly CCT) was significantly reduced.

  The multi-layer coating film having the three metal component-based electrolytic film and the electrodeposition coating film of the present invention was able to provide excellent rust prevention properties, particularly rust prevention properties suitable for corrosive environments.

  In the present invention, by forming such an electrolytic membrane, the conventional complicated and high-cost rust prevention chemical conversion treatment step such as a zinc phosphate treatment step can be omitted, and the conventional rust prevention chemical conversion treatment step. It is possible to provide excellent rust prevention and adhesion equivalent to or higher than the above. Furthermore, in the method for forming a multilayer coating film of the present invention, by forming the above-mentioned electrolytic film on an untreated metal substrate, the adhesion with the electrodeposition coating film formed thereon is also improved, As a result, an excellent coating film appearance can be provided. In addition, the present invention provides a multilayer coating film forming method and a multilayer coating film excellent in economic efficiency and environmental conservation by reducing sludge and shortening the processing time by omitting the rust prevention chemical conversion treatment step. it can. Furthermore, since the step (a) included in the method for forming a multilayer coating film of the present invention can provide excellent adhesion to an untreated metal base material, the present invention is a base rust preventive suitable for electrodeposition coating. The treatment process can be provided simply, and it is very useful especially in the painting of automobile bodies.

Claims (4)

(A) a rare earth metal (A) nitrate having a lower precipitation pH than zinc; a salt of zinc (B); and a rare earth metal (C) nitrate having a higher precipitation pH than zinc; A step of immersing the treated metal substrate and forming an electrolytic film containing rare earth metal (A), zinc (B) and rare earth metal (C) on the metal substrate by cathodic electrolysis, and (b) step A method for forming a multilayer coating film comprising a step of forming an electrodeposition coating film by immersing the metal substrate on which the electrolytic film is formed in (a) in an electrodeposition coating and performing electrodeposition coating,
The mass of the electrolytic membrane after step (b) is 30 to 200 mg / m ² , and the electrolytic membrane is based on a total of 100 parts by mass of the rare earth metal (A), zinc (B), and rare earth metal (C). 20 to 65 parts by mass of rare earth metal (A), 20 to 60 parts by mass of zinc (B), 15 to 50 parts by mass of rare earth metal (C),
A method for forming a multilayer coating film.   The method for forming a multilayer coating film according to claim 1, wherein the electrodeposition paint further contains a salt of a rare earth metal (D). The rare earth metal (A) is selected from the group consisting of yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and mixtures thereof. Selected
The rare earth metal (C) is selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and mixtures thereof.
The method for forming a multilayer coating film according to claim 1.   The multilayer coating film which consists of an electrolytic membrane and an electrodeposition coating film formed on the metal base material obtained by the multilayer coating-film formation method of any one of Claims 1-3.