Method for pretreatment of a motor vehicle body prior to painting and motor vehicle body

Abstract

The present invention provides a pretreatment method for coating of an automobile body including a high-tensile steel sheet, which enables satisfactory corrosion resistance after coating, and an automobile body. The solution of the present invention is a pretreatment method for coating of an automobile body, which sequentially performs an alkali degreasing step, a first water washing step, a chemical conversion treatment step, a second water washing step, and a cation electrodeposition coating step, and a chemical conversion treatment agent used in the chemical conversion treatment step contains zirconium (A), free fluoride ions (B), an allylamine-diallylamine copolymer (C), aluminum ions (D), and nitrate ions (E) each at a prescribed concentration, the allylamine-diallylamine copolymer (C) forms an acid addition salt having an anionic counterion, the acid has a pKa in the range of -3.7 to 4.8, and the duallylamine contains 80 mol% or more and 98 mol% or less.

Description

Technical Field

[0001] This invention relates to a pre-painting treatment method for automobile bodies and automobile bodies. Background Technology

[0002] Previously, when applying cationic electrodeposition coatings or powder coatings to the metal substrates of automobile bodies, a chemical conversion treatment was performed on the surface of the metal substrate beforehand to improve corrosion resistance and coating adhesion. In recent years, chemical conversion treatment using chromium-free zinc phosphate has been widely adopted.

[0003] Chemical conversion treatment using zinc phosphate is difficult to perform due to the high reactivity of the treatment agent, resulting in the generation of sludge and a large environmental impact. Therefore, a chemical conversion treatment agent composed of at least one selected from zirconium, titanium, and hafnium, fluorine, and a water-soluble resin has been proposed (see, for example, Patent Document 1).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2004-218074 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] The technology disclosed in Patent Document 1 can effectively perform chemical conversion treatment on metals such as iron, zinc, and aluminum. On the other hand, while high-tensile steel sheets used in automobile bodies are lightweight and possess excellent strength, they are mostly difficult to chemically convert. This is because high-tensile steel sheets not only have a thick oxide film, but also exhibit reduced reactivity to chemical conversion agents due to the alloying elements such as C, Si, and Mn they contain. Therefore, this negatively impacts the formation of the cationic electrodeposited coating after chemical conversion treatment, and sufficient corrosion resistance after cationic electrodeposited coating is desirable.

[0009] The present invention was made in view of the above circumstances, and its object is to provide a pre-treatment method for a car body comprising high-tensile steel sheet that can achieve satisfactory corrosion resistance after coating.

[0010] Methods for solving problems

[0011] (1) This invention relates to a pre-coating treatment method for an automobile body made of raw materials including high-tensile steel sheets. The method involves sequentially performing an alkaline degreasing process, a first water washing process, a chemical conversion treatment process, a second water washing process, and a cationic electrodeposition coating process. The chemical conversion treatment agent used in the aforementioned chemical conversion treatment process comprises zirconium (A), free fluoride ions (B), allylamine-diallylamine copolymer (C), aluminum ions (D), and nitrate ions (E). The concentration of zirconium (A) relative to the total mass of the chemical conversion treatment agent is 50 to 500 ppm by mass (metal element conversion). The concentration of free fluoride ions (B) relative to the total mass of the chemical conversion treatment agent is 5 to 30 ppm by mass. The allylamine-diallylamine copolymer (C) contains diallylamine-derived ions... The content of diallylamine segments is 80 mol% to 98 mol% relative to the total of allylamine segments derived from allylamine and the diallylamine segments described above. The weight average molecular weight of the allylamine-diallylamine copolymer (C) is 5,000 to 100,000, and its concentration is 100 to 350 ppm by mass relative to the total mass of the chemical conversion agent, based on the resin solids concentration. The allylamine-diallylamine copolymer (C) is an acid addition salt with an anionic counterion, and the pKa of the acid forming the acid addition salt is in the range of -3.7 to 4.8. The concentration of aluminum ions (D) is 90 to 500 ppm by mass relative to the total mass of the chemical conversion agent, and the concentration of nitrate ions (E) is 2,000 to 13,000 ppm by mass relative to the total mass of the chemical conversion agent.

[0012] (2) The pretreatment method for painting the automobile body according to (1), wherein the pH of the chemical conversion agent is 3.5 to 5.5.

[0013] (3) An automobile body made of a raw material comprising a high-tensile steel sheet with a coating, wherein the concentration of zirconium (A) in the coating on the surface of the high-tensile steel sheet formed by the pre-painting treatment method of the automobile body described in (1) or (2) is 20 to 200 mg / m³ in terms of metal element conversion. 2 .

[0014] Invention Effects

[0015] According to the present invention, a pre-treatment method for painting automobile bodies comprising high-tensile steel sheets can be provided, which can achieve satisfactory corrosion resistance after painting. Detailed Implementation

[0016] Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the embodiments described below.

[0017] <Pre-treatment methods for automotive body painting>

[0018] The pre-treatment method for painting an automobile body according to this embodiment sequentially performs an alkaline degreasing process, a first water washing process, a chemical conversion treatment process, a second water washing process, and a cationic electrodeposition coating process on an automobile body made of raw materials including high-tensile steel sheets.

[0019] (Alkali degreasing process)

[0020] The alkaline degreasing process involves immersing the high-tensile steel sheet of the automobile body, which is to be treated, in a degreasing agent such as a phosphorus-free / nitrogen-free degreasing cleaning solution at a temperature of, for example, 30–55°C for several minutes. Pre-degreasing treatment can also be performed before the alkaline degreasing process.

[0021] (First washing process)

[0022] The first water washing process is a process of washing the degreasing agent after the alkali degreasing process with water, which is carried out by spraying a large amount of water once or multiple times.

[0023] (Chemical conversion treatment process)

[0024] The chemical conversion treatment process is a process of forming a chemical conversion coating on the surface of high-tensile steel sheets used in automobile bodies to produce surface-treated steel sheets. The chemical conversion coating formation process is carried out by bringing a chemical conversion agent into contact with the surface of the high-tensile steel sheet. The method of contact is not particularly limited; examples include immersion coating, spraying, and roller coating. The treatment temperature in the chemical conversion treatment process can be in the range of 20–70°C, preferably in the range of 30–50°C. The treatment time in the chemical conversion treatment process can be in the range of 5–1200 seconds, preferably in the range of 30–120 seconds. The composition of the chemical conversion agent used in the chemical conversion treatment process will be detailed later.

[0025] (Second washing process)

[0026] The second water washing process involves one or more spray treatments or immersion washes without affecting the adhesion and corrosion resistance of the coated surface. The final water washing treatment is preferably performed using ion-exchanged water or pure water. Following the second water washing process, a drying process for the surface-treated steel sheet can be added as needed.

[0027] (Cat electrodeposition coating process)

[0028] The cationic electrodeposition coating process involves applying a cationic electrodeposition coating to the surface-treated steel sheet produced by the aforementioned chemical conversion treatment process to form an electrodeposited coating film on the surface. There are no particular limitations on the cationic electrodeposition coating material used; conventionally known cationic electrodeposition coating materials, including aminated epoxy resins, aminated acrylic resins, and sulfonated epoxy resins, can be used. There are no particular limitations on the cationic electrodeposition coating method using the aforementioned cationic electrodeposition coating material; any known cationic electrodeposition coating method can be applied.

[0029] <Chemical Conversion Treatment Agent>

[0030] The chemical conversion treatment agent of this embodiment can form a chemical conversion film on the surface of the high-tensile steel sheet constituting the automobile body, which can achieve satisfactory corrosion resistance after cationic electrodeposition coating.

[0031] The chemical conversion agent in this embodiment includes zirconium (A), free fluoride ions (B), allylamine-diallylamine copolymer (C), aluminum ions (D), and nitrate ions (E).

[0032] (Zirconium(A))

[0033] Zirconium (A) is a component in chemical conversion coatings. By forming a chemical conversion coating containing zirconium (A) on the surface of high-strength steel plates, the corrosion resistance and wear resistance of the high-tensile steel plates can be improved, as well as the adhesion to cationic electrodeposited coatings can be enhanced.

[0034] There are no particular limitations on the sources of zirconium (A) mentioned above. For example, alkali metal fluorozirconates such as K2ZrF6, zirconium hydrofluoric acid (H2ZrF6), ammonium hexafluorozirconate ((NH4)2ZrF6), ammonium zirconium carbonate ((NH4)2ZrO(CO3)2), tetraalkylammonium modified zirconium, zirconium fluoride, zirconium oxide, etc.

[0035] The concentration of zirconium (A) relative to the total mass of the chemical conversion agent is 50 to 500 ppm by mass (calculated as metal element). When the concentration of zirconium (A) is below 50 ppm, the resulting chemical conversion film cannot achieve sufficient performance. When the concentration of zirconium (A) exceeds 500 ppm by mass, the above-mentioned effects cannot be obtained, which is economically disadvantageous. From the above perspective, the concentration of zirconium (A) is preferably 100 to 500 ppm by mass (calculated as metal element).

[0036] (Free fluoride ions (B))

[0037] Free fluoride ions (B) have the function of etching the surface of a metal substrate. There are no particular limitations on the source of free fluoride ions (B); examples include fluorides such as hydrofluoric acid, ammonium fluoride, boric acid fluoride, ammonium hydrogen fluoride, sodium fluoride, and sodium hydrogen fluoride. Additionally, as complex fluorides, examples include hexafluorosilicates; specific examples include fluorosilicic acid, zinc fluorosilicate, manganese fluorosilicate, magnesium fluorosilicate, nickel fluorosilicate, iron fluorosilicate, and calcium fluorosilicate. Furthermore, fluorine-containing compounds such as alkali metal fluorozirconates, exemplified as sources of zirconium (A), are sources of zirconium (A) and can also serve as sources of free fluoride ions (B).

[0038] The concentration of free fluoride ions (B) relative to the total mass of the chemical conversion agent, expressed as fluorine element, ranges from 5 to 30 ppm by mass. When the concentration of free fluoride ions (B) is below 5 ppm by mass, etching is insufficient, resulting in a poor chemical conversion film. When it exceeds 30 ppm by mass, etching is excessive, preventing the formation of a sufficient chemical conversion film. The concentration of free fluoride ions (B) can be measured, for example, using a commercially available fluoride ion meter (e.g., Toa DKK Corporation IM-32P).

[0039] (Allylamine-Dialylamine copolymer (C))

[0040] The allylamine-diallylamine copolymer (C) has at least two structural units: a segment derived from allylamine and a segment derived from diallylamine (hereinafter sometimes referred to as "allylamine segment" or "diallylamine segment"). Each of these segments can independently be in the state of a quaternary compound. In addition, each of these segments can independently have counterions.

[0041] In this embodiment, the diallylamine segment content in the allylamine-diallylamine copolymer (C) is 80 mol% or more and 98 mol% or less. The diallylamine segment content is defined as the mol% of the diallylamine segment in the allylamine-diallylamine copolymer (C) relative to the total of allylamine and diallylamine segments. When the diallylamine segment content is less than 80 mol%, sufficient corrosion resistance after coating cannot be obtained. When the diallylamine segment content exceeds 98 mol%, the adhesion of the chemical conversion film to the coating film decreases. Furthermore, from the above viewpoint, the diallylamine segment content is preferably 90 mol% or more and 98 mol% or less. For example, heterocyclic structures represented by the following general formulas (1a) and (1b) can be used as the diallylamine segment. The heterocyclic structure can be a saturated heterocyclic structure.

[0042] [Chemistry 1]

[0043]

[0044] (In the above formula, R)1 This indicates a hydrogen atom, alkyl group, or aralkyl group.

[0045] The allylamine segment in the allylamine-diallylamine copolymer (C) is, for example, represented by the following general formula (2).

[0046] [Chemistry 2]

[0047]

[0048] Allylamine-diallylamine copolymer (C) is an acid addition salt with an anionic counterion relative to an ammonium cation. The dissociation constant pKa of the acid forming the above acid addition salt is in the range of -3.7 to 4.8. It should be noted that, in this specification, the dissociation constant pKa of the above acid refers to the value at a temperature of 25°C with water as the solvent. The diallylamine segment constituting the allylamine-diallylamine copolymer (C) as the above acid addition salt is, for example, represented by the following general formulas (1c) and (1d).

[0049] [Chemistry 3]

[0050]

[0051] (where R is in the formula) 2 and R 3 D represents a hydrogen atom, alkyl group, or aralkyl group. - This indicates a monovalent anion.

[0052] There are no particular limitations on the anionic counterions mentioned above. For example, examples of monovalent anions include carboxylate ions such as formate ions, acetate ions, and benzoate ions, as well as chloride ions, sulfate ions, and nitrate ions. Examples of acids that form acid addition salts include organic acids such as formic acid, acetic acid, and benzoic acid, and inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid.

[0053] The allylamine-diallylamine copolymer (C) may, as needed, have segments derived from allylamine or segments other than diallylamine segments. Examples include segments derived from N,N-dialkylaminoalkyl esters of (meth)acrylate and their salts or quaternary compounds, N,N-dialkylaminoalkyl (meth)acrylamide and their salts or quaternary compounds, vinylimidazole and its salts or quaternary compounds, vinylpyridine and its salts or quaternary compounds, N-alkylallylamine and its salts, N,N-dialkylallylamine and its salts, N-alkyldiallylamine and its salts or quaternary compounds, etc.

[0054] The allylamine-diallylamine copolymer (C) may further contain segments other than those described above, as needed. For example, it may contain segments derived from sulfur dioxide, unsaturated compounds with hydroxyl groups such as 2-hydroxyethyl (meth)acrylate, alkyl (meth)acrylates such as methyl (meth)acrylate and ethyl (meth)acrylate, vinyl acetate, vinyl propionate, unsaturated acids, and (meth)acrylamide.

[0055] The content of segments other than allylamine and diallylamine segments in the allylamine-diallylamine copolymer (C) is preferably 20% or less, more preferably 10% or less, and most preferably 0%. The content of segments not derived from either allylamine or diallylamine segments is defined as the molar percentage of segments in the allylamine-diallylamine copolymer (C) that are not derived from either allylamine or diallylamine segments relative to the total number of segments.

[0056] The concentration of allylamine-diallylamine copolymer (C) relative to the total mass of the chemical conversion treatment agent, expressed as resin solids concentration, is 100–350 ppm by mass. At concentrations below 100 ppm by mass, sufficient adhesion of the chemical conversion film cannot be obtained. At concentrations above 350 ppm by mass, the formation of the chemical conversion film may be hindered. From this perspective, the concentration of allylamine-diallylamine copolymer (C), expressed as resin solids concentration, is preferably 125–300 ppm by mass.

[0057] The weight-average molecular weight of the allylamine-diallylamine copolymer (C) is 5,000 to 100,000. When the weight-average molecular weight is less than 5,000, sufficient adhesion of the chemical conversion film cannot be obtained. When the weight-average molecular weight exceeds 100,000, it may hinder the formation of the chemical conversion film. Based on the above considerations, the weight-average molecular weight of the allylamine-diallylamine copolymer (C) is preferably 20,000 to 100,000.

[0058] The weight-average molecular weight of allylamine-diallylamine copolymer (C) can be determined, for example, by gel permeation chromatography (GPC). As the measuring instrument, a Hitachi L-6000 high-performance liquid chromatograph can be used, along with a Hitachi L-6000 eluent flow path pump, a Shodex RI SE-61 differential refractive index detector, and a column consisting of a dual-linked Asahipck aqueous gel filter type GS-220HQ (exclusion limit molecular weight 3,000) and GS-620HQ (exclusion limit molecular weight 2 million). An example of a GPC determination method is shown below. The sample is adjusted to a concentration of 0.5 g / 100 ml using 20 μl of eluent. The eluent is a 0.4 mol / L aqueous sodium chloride solution. The assay is performed at a column temperature of 30 °C and a flow rate of 1.0 ml / min. As standard samples, calibration curves were obtained using polyethylene glycols with molecular weights of 106, 194, 440, 600, 1470, 4100, 7100, 10300, 12600, and 23000. The weight-average molecular weight (Mw) of the copolymers was then determined based on these calibration curves.

[0059] The allylamine-diallylamine copolymer (C) can be modified within the scope of the present invention without prejudice to its purpose. For example, a portion of the amino group of the allylamine-diallylamine copolymer (C) can be modified by methods such as acetylation, or it can be crosslinked by a crosslinking agent to a degree that does not affect solubility.

[0060] The method for preparing the allylamine-diallylamine copolymer (C) is not particularly limited. For example, a method can be described by carrying out free radical polymerization of a mixture of allylamine, diallylamine, and other monomers as needed in the presence of a free radical polymerization initiator in a suitable solvent. Regarding the polymerization conditions, conditions known to those skilled in the art can be appropriately selected.

[0061] (Other polymers)

[0062] The chemical conversion treatment agent of this embodiment may contain polymers other than allylamine-diallylamine copolymer (C). Examples of polymers other than allylamine-diallylamine copolymer (C) include polyallylamine resin, polyethyleneamine resin, polydiallylamine resin, urethane resin, acrylic resin, polyester resin, and natural polymer derivatives such as chitosan-chitosan derivatives and cellulose derivatives. When the chemical conversion treatment agent of this embodiment contains polymers other than allylamine-diallylamine copolymer (C), the mass of the solid component of allylamine-diallylamine copolymer (C) relative to the total mass of the solid components of all polymers is preferably 80% by mass or more, more preferably 90% by mass or more, and most preferably 95% by mass or more.

[0063] (Aluminum ion(D))

[0064] By including aluminum ions (D) in the chemical conversion treatment agent, the corrosion resistance after cationic electrodeposition coating can be further improved. There are no particular limitations on the source of aluminum ions (D), and examples include aluminum oxides, hydroxides, fluorides, chlorides, sulfates, nitrates, borates, carbonates, and organic acid salts. The concentration of aluminum ions (D) relative to the total mass of the chemical conversion treatment agent is 90–500 ppm by mass, preferably 90–350 ppm by mass.

[0065] (nitrate ion (E))

[0066] Nitrate ions (E) function as an oxidant to promote the chemical conversion coating formation reaction. In addition to the aforementioned aluminum nitrates and nitrate ions serving as an anionic counterion of the aforementioned allylamine-diallylamine copolymer (C), sources of nitrate ions (E) include nitric acid, sodium nitrate, potassium nitrate, and ammonium nitrate. The concentration of nitrate ions (E) relative to the total mass of the chemical conversion treatment agent is 2000–13000 ppm by mass, preferably 3000–12000 ppm by mass.

[0067] (Other ingredients)

[0068] The chemical conversion treatment agent of this embodiment preferably further contains a silane coupling agent. By containing a silane coupling agent in the chemical conversion treatment agent, the coating adhesion of the chemical conversion film can be further improved. There are no particular limitations on the silane coupling agent; for example, one or more silane coupling agents selected from the group consisting of amino-containing silane coupling agents, epoxy-containing silane coupling agents, hydrolysates of amino-containing silane coupling agents, hydrolysates of epoxy-containing silane coupling agents, polymers of amino-containing silane coupling agents, and polymers of epoxy-containing silane coupling agents are preferred.

[0069] There are no particular limitations on the amino-containing silane coupling agents mentioned above. For example, known silane coupling agents such as N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N,N-bis[3-(trimethoxysilyl)propyl]ethylenediamine can be cited. Commercially available amino-containing silane coupling agents such as KBM-602, KBM-603, KBE-603, KBM-903, KBE-9103, KBM-573 (all manufactured by Shin-Etsu Chemical Industry Co., Ltd.), and XS1003 (manufactured by Chisso Co., Ltd.) can also be used.

[0070] There are no particular limitations on the aforementioned epoxy-containing silane coupling agents. Examples include 3-epoxypropoxypropyltrimethoxysilane, 3-epoxypropoxypropyltriethoxysilane, 3-epoxypropoxypropylmethyldimethoxysilane, 3-epoxypropoxypropylmethyldiethoxysilane, 3-epoxypropoxypropyldiethylethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, and 5,6-epoxyhexyltriethoxysilane. Commercially available "KBM-403", "KBE-403", "KBE-402", and "KBM-303" (all manufactured by Shin-Etsu Chemical Co., Ltd.) can also be used.

[0071] The chemical conversion treatment agent of this embodiment may contain components other than those described above. For example, zinc is preferably further included as a chemical conversion coating forming component. This further improves the corrosion resistance of the metal substrate to which the chemical conversion coating is formed. In addition to the above, the chemical conversion coating forming component may also include at least one metal component selected from the group consisting of magnesium, calcium, gallium, indium, and copper. Furthermore, it may further include at least one metal component selected from the group consisting of manganese, iron, cobalt, nickel, and chromium. The supply source of the above coating forming component is not particularly limited, and examples include oxides, hydroxides, fluorides, chlorides, sulfates, nitrates, borates, carbonates, and organic acid salts of various metals. The above supply source may also be included in the chemical conversion treatment agent as a leaching component of the metal substrate undergoing chemical conversion treatment in the chemical conversion bath.

[0072] The chemical conversion treatment agent of this embodiment may contain an oxidizing agent other than nitrate ions (E). This promotes the formation of a chemical conversion coating and further improves the corrosion resistance of the metal substrate. Examples of oxidizing agents include inorganic acids or their salts. Examples of inorganic acids include hydrochloric acid, bromic acid, chloric acid, hydrogen peroxide, HMnO4, and HVO3. It should be noted that the chemical conversion treatment agent may contain compounds containing sulfonic acid groups or their salts as oxidizing agents.

[0073] The chemical conversion treatment agent of this embodiment preferably does not contain phosphate ions. In this specification, "not containing phosphate ions" means that phosphate ions are present to the extent that they do not function as a component of the chemical conversion treatment agent. Since the chemical conversion treatment agent of this embodiment does not contain phosphate ions, phosphorus, which contributes to environmental pollution, is not substantially used. Furthermore, the generation of sludge such as ferric phosphate and zinc phosphate, which occurs when using zinc phosphate treatment agents, can be suppressed.

[0074] (pH)

[0075] The pH of the aforementioned chemical conversion treatment agent is preferably 3.5 to 5.5. When the pH is below 3.5, etching is excessive, and a sufficient chemical conversion film cannot be formed. When the pH exceeds 5.5, etching is insufficient, and a good chemical conversion film cannot be obtained. From this perspective, a pH of 4.0 to 4.5 is more preferable. To adjust the pH of the aforementioned chemical conversion treatment agent, acidic compounds such as nitric acid and sulfuric acid, as well as alkaline compounds such as sodium hydroxide, potassium hydroxide, and ammonia, can be used.

[0076] <Car body made of raw materials including high-tensile steel sheets>

[0077] The chemical conversion treatment agent of this embodiment forms a chemical conversion coating on the surface of an automobile body made of raw materials including high-tensile steel sheets. The chemical conversion treatment agent of this embodiment can impart sufficient corrosion resistance even to high-tensile steel sheets for which conventional chemical conversion treatment agents struggle to provide adequate corrosion resistance. High-tensile steel sheets refer to steel sheets with a certain or higher tensile strength. Examples of high-tensile steel sheets include, for example, high-tensile hot-rolled steel sheets, high-tensile cold-rolled steel sheets, and high-tensile galvanized steel sheets.

[0078] At least a portion of the vehicle body, which is the object to be coated by the pre-coating treatment method of the vehicle body according to this embodiment, is made of high-strength steel sheet. The vehicle body may be entirely made of high-tensile steel sheet, or it may have portions made of high-tensile steel sheet and portions made of steel sheet other than high-tensile steel sheet. Examples of steel sheets other than high-tensile steel sheet that can constitute the object to be coated include, for example, cold-rolled steel sheet, hot-rolled steel sheet, stainless steel, galvanized or zinc-based alloy steel sheet. Examples of galvanized or zinc-based alloy steel sheets include, for example, galvanized steel sheet, galvanized-nickel steel sheet, galvanized-iron steel sheet, galvanized-chromium steel sheet, galvanized-aluminum steel sheet, galvanized-titanium steel sheet, galvanized-magnesium steel sheet, galvanized-manganese steel sheet, and other galvanized electroplated, melt-plated, or vapor-plated steel sheets.

[0079] Regarding the automobile body of this embodiment, which is made of raw materials including high-tensile steel sheets, the zirconium (A) content in the coating formed on the surface of the high-tensile steel sheet by a chemical conversion treatment agent is preferably 20 to 200 mg / m² (metal element conversion). 2 The content of the aforementioned metal component (A) is less than 20 mg / m³. 2 At this time, a uniform chemical conversion film cannot be obtained. The content of the metal component (A) exceeds 200 mg / m³. 2 If the desired effect is not achieved, it will be economically disadvantageous.

[0080] Example

[0081] The present invention will now be described in more detail based on embodiments. However, the present invention is not limited to the embodiments described below.

[0082] (Example 1)

[0083] Commercially available cold-rolled high-tensile steel sheet (standard test piece made by the company, 7cm×15cm×0.1cm) was used as the substrate, and surface treatment was performed under the following conditions.

[0084] As an alkaline degreasing process, the substrate was immersed in 2% by mass "Surfcleaner EC90" (a degreasing agent manufactured by Nippon Paint Surface Treatment Co., Ltd.) at 40°C for 2 minutes. As a first water wash process, tap water was sprayed for 30 seconds. As a chemical conversion treatment process, a chemical conversion agent was prepared using zirconium hydrofluoric acid, acidic sodium fluoride, allylamine-diallylamine copolymer (allylamine segment: 20 mol%, diallylamine segment: 80 mol%, weight average molecular weight 5000, acetate (pKa 4.8) salt), aluminum nitrate nonahydrate, and sodium nitrate, as shown in Table 1. The concentration of Zr was 50 ppm by mass (metal element conversion), the free fluoride ion concentration was 15 ppm by mass, and the allylamine-diallylamine copolymer concentration was 100 ppm by mass (resin solids concentration). The pH was adjusted to 4.0 using sodium hydroxide. The temperature of the chemical conversion agent was adjusted to 40°C, and the substrate was immersed for 120 seconds.

[0085] As a second water washing process, a 30-second spray treatment was performed using tap water. A further 30-second spray treatment was then performed using ion-exchanged water. Finally, as a drying process, the film was dried in an electric drying oven at 80°C for 5 minutes. The zirconium content (mg / m³) in the chemically converted coating was determined using a ZSX PrimusII (Rigaku Corporation X-ray analyzer). 2 ), as shown in Table 1.

[0086] As a cationic electrodeposition coating formation process, "Powernics 1050" (cationic electrodeposition coating manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was used to perform cationic electrodeposition coating with a dry film thickness of 20 μm. After washing with water, the coating was sintered at 170°C for 20 minutes to produce the test plate of Example 1.

[0087] (Examples 2-11, Comparative Examples 1-13)

[0088] Except that the chemical conversion agent in the chemical conversion treatment process is configured as shown in Table 1, the test plates of the above-described examples and comparative examples were prepared in the same manner as in Example 1. The detailed configurations of the chemical conversion agents of the above-described examples and comparative examples are shown below.

[0089] The allylamine-diallylamine copolymer (C) used in the following examples and comparative examples was a commercially available product as shown below. In Comparative Example 9, a diallylamine copolymer was used instead of allylamine-diallylamine copolymer (C).

[0090] Example 3, Comparative Example 5: "PAA-D19-A", Examples 8, 10 and Comparative Example 1: "PAA-D19-HCl", Comparative Example 7: "PAA-D41-HCl", Comparative Example 8: "PAA-D1-HCl", Comparative Example 9: "PAS-21" (all manufactured by Nittobo Pharmaceutical Co., Ltd.).

[0091] In addition, in the examples and comparative examples, hydrochloric acid was used as an addition acid with a pKa of -3.7.

[0092] [Peeling test after saline-temperature water test (SDT)]

[0093] For the test plates of the examples and comparative examples, after being transversely cut to the substrate, they were immersed in a 5% by mass NaCl aqueous solution at 55°C for 240 hours. Next, they were washed with tap water and further dried at room temperature. Then, a Cellotape (registered trademark) peel test was performed on the transverse section of the electrodeposited coating to determine the maximum peel width on one side from the transverse cut. Evaluation was conducted according to the following criteria, with a score of 2 or higher considered acceptable. The results are shown in Table 1.

[0094] 3: Less than 1.0mm

[0095] 2: 1.0mm or more and less than 2.5mm

[0096] 1: 2.5mm or more

[0097] [Total area of ​​bubbling after SDT (Saline Temperature Test)]

[0098] For the test panels of the examples and comparative examples, the electrodeposited coated panels were immersed in a 5% (w / w) NaCl aqueous solution at 55°C for 240 hours. They were then washed with tap water and further dried at room temperature. Subsequently, the total area ratio of bubbles generated across the entire surface of the electrodeposited coating was measured. Evaluation was performed according to the following criteria, with a value of 3 or higher considered acceptable. The results are shown in Table 1.

[0099] 3:0%

[0100] 2: 0% or more but less than 1.0%

[0101] 1: 1.0% or more

[0102] [Combined Cyclic Corrosion Test (CCT)]

[0103] For the test plates of the examples and comparative examples, after being cut transversely to the substrate, a composite cyclic corrosion test was performed. The test method consisted of 50 cycles, with each cycle defined as a composite cyclic corrosion test under the following conditions.

[0104]

[0105]

[0106] After the CCT test described above, the maximum expansion amplitude on both sides of the cut portion was measured. Evaluation was conducted according to the following criteria, with a value of 3 or higher considered acceptable. The results are shown in Table 1.

[0107] 4: Less than 3.0mm

[0108] 3: 3.0mm or more and less than 3.5mm

[0109] 2: 3.5mm or more and less than 4.0mm

[0110] 1: 4.0mm or more

[0111] [Table 1]

[0112]

[0113] The results in Table 1 confirm that the chemical conversion treatment agents of each embodiment can achieve satisfactory corrosion resistance after cationic electrodeposition coating compared with the chemical conversion treatment agents of the comparative examples.

Claims

1. A pretreatment method for an automobile body before painting, comprising sequentially performing an alkaline degreasing process, a first water washing process, a chemical conversion treatment process, a second water washing process, and a cationic electrodeposition coating process. The vehicle body is constructed from raw materials including high-tensile steel plates. The chemical conversion agent used in the chemical conversion treatment process contains zirconium (A), free fluoride ions (B), allylamine-diallylamine copolymer (C), aluminum ions (D), and nitrate ions (E). The concentration of zirconium (A) relative to the total mass of the chemical conversion agent, calculated in metal elemental form, is 50-500 ppm by mass. The concentration of the free fluoride ions (B) relative to the total mass of the chemical conversion agent is 5-30 ppm by mass. The proportion of diallylamine segments derived from diallylamine in the allylamine-diallylamine copolymer (C) is 80 mol% or more and 98 mol% or less relative to the total of the allylamine segments derived from allylamine and the diallylamine segments. The allylamine-diallylamine copolymer (C) has a weight-average molecular weight of 5,000 to 100,000 and a concentration of 100 to 350 ppm by weight relative to the total mass of the chemical conversion agent, based on the resin solids concentration. The allylamine-diallylamine copolymer (C) is an acid addition salt with anionic counterion properties, and the pKa of the acid forming the acid addition salt is in the range of -3.7 to 4.

8. The concentration of aluminum ions (D) relative to the total mass of the chemical conversion agent is 90-500 ppm by mass. The concentration of nitrate ions (E) is 2000~12000 ppm by mass relative to the total mass of the chemical conversion agent.

2. The pretreatment method for painting an automobile body according to claim 1, wherein the pH of the chemical conversion agent is 3.5 to 5.

5.

3. An automobile body comprising a raw material including a high-tensile steel sheet with a coating, wherein the zirconium (A) content in the coating on the surface of the high-tensile steel sheet formed by the pre-painting treatment method of claim 1 or 2 is 20-200 mg / m² (based on metal element conversion). 2 .

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