DE69002481T2 - Process for coating workpieces with aluminum. - Google Patents

Description

The present invention relates to a method for coating a metal assembly, including at least one aluminum part and steel parts, in particular a method for coating assembled metal parts for automobile bodies.

In coating automobile bodies composed of various metal parts such as steel parts, zinc-plated steel parts, zinc alloy-plated steel parts and aluminum parts, since most of the components are made of steel parts, parts made of only aluminum and occupying small bottles are coated only after punching, degreasing and chemical treatment. The reasons for this are that since chemical coatings for steel parts constituting automobile bodies are formed by zinc phosphate chemicals, aluminum is dissolved away from the aluminum parts by the chemicals and accumulated in the treatment solutions, and that when an amount of aluminum accumulated in the chemical treatment solution exceeds a certain concentration, the formation of chemical coatings on the steel surfaces is inhibited.

It has been proposed by Japanese Patent Publication No. 45-16566 to remove the accumulated aluminum by adding hydrofluoric acid to the chemical coating agents to precipitate the dissolved aluminum. However, it is difficult to to control the exact amount of hydrofluoric acid needed to precipitate the aluminum, and the precipitated aluminum forms sludge such as Kryolite Na₃AlF₆ and Elpasolite NaK₂AlF₆. The sludge should be removed from the chemical treatment bath frequently, or the chemical treatment bath should be renewed frequently.

Jitsumu Myomen Gijutsu (Metal Finishing Practice) Vol. 35, No. 1, 1988, discloses a method for coating assembled components consisting of aluminum parts and steel parts, wherein the aluminum parts and steel parts are treated separately in their own treatment lines and the treated aluminum and steel parts are joined in a final section. However, this method requires separate lines for both the aluminum parts and the steel parts, and the treated aluminum parts and steel parts should be treated very carefully during assembly. In addition, painted parts may have slightly different final colors.

In addition to the above problems, when aluminum parts are subjected to deep drawing by punching, a large amount of a lubricating oil with high viscosity should be used to give the aluminum part good machinability, thereby preventing cracks, scratches and stress. However, it is generally difficult to remove this lubricating oil, which requires a lot of time and effort. Therefore, aluminum parts that can be punched without using lubricating oils are required.

Accordingly, an object of the present invention is to provide a method for coating a metal assembly including at least one aluminum part, wherein the aluminum part and the other parts, namely steel parts, can be treated simultaneously, thereby effectively coating the aluminum component for automobile bodies, etc.

As a result of intensive studies with the above object in mind, the inventors have found that an aluminum member coated with a conductive primer is highly resistant to treatment with chemical solutions necessary for chemically treating steel members, and that the aluminum member coated with a conductive primer can be easily punched, thereby providing deep-drawn products, etc., without using lubricating oils. The present invention is based on these discoveries.

Thus, the invention, as defined in independent claims 1 and 5, comprises a method for coating a metal assembly of Al parts with uncoated and/or zinc or zinc alloy coated steel parts, comprising the following steps:

Degreasing the aluminum part;

Applying a conductive primer to this aluminium part;

Firing the primer to form an organic coating layer;

Combining this aluminum part with the steel part and coating the resulting composite parts with chemical coatings or with chemical and galvanic coatings.

Preferred embodiments of the methods are defined in dependent claims 2 to 4 and as defined in independent claims 1 and 5. According to a further embodiment, the method for coating a metal assembly also comprises the step of performing a formation of an intermediate layer and/or top layer after formation of electrodeposited coatings.

The metal assembly to which the coating process of the present invention can be applied consists of at least one aluminum part and at least one other steel part. A typical combination comprises aluminum parts and steel parts (including zinc-plated steel parts). A combination consists of at least one aluminum part and at least one steel part. Specific combinations of metal parts to be assembled are exemplified as follows:

1. Aluminium parts coated with a conductive primer.

Steel parts (coated with or without a conductive primer).

A metal composite of these plates is provided with chemical coatings and electroplating coatings, optionally followed by the formation of an intermediate layer and/or a top layer.

2. Aluminium parts coated with a conductive primer.

Steel parts that can be chemically coated. After the aluminum parts have been punched out, they are assembled with the steel parts and then provided with an intermediate layer and/or a top layer.

3. Aluminium parts coated with a conductive primer.

steel parts.

After the aluminum parts are punched out, they are assembled with steel parts and then chemically coated.

These assembled parts are used for example for automobile bodies or electrical devices. In some applications, plastic parts can be joined with aluminum parts and steel parts.

The conductive primer which can be used in the present invention comprises:

(A) a bisphenol type epoxy resin having a number average molecular weight of 300 to 100,000 in an amount of 30% by weight or more on a solid basis;

(B) at least one hardener selected from a polyisocyanate and a blocked polyisocyanate in an amount of one tenth to twice the weight of the above epoxy resin;

(C) quartz powder having an average particle size of 0.1 to 100 millimeters, in an amount of 5 to 50% by weight on a solids basis; and

(D) a ketone solvent in an amount of 40% or more by weight based on the total weight of the primer.

The solid content of conductive primer is 10 to 50 wt%, which makes it possible to form a thin, uniform, conductive primer layer with excellent corrosion resistance, high conductivity in electroplating, improved formability and excellent adhesion to an electroplated coating layer.

Usable as the binder resin (A) is a bisphenol type epoxy resin which is soluble in an organic solvent and has a number average molecular weight of 300 to 100,000, is excellent in water resistance and alkali resistance and shows good adhesion to a substrate as well as to a topcoat. Examples are the epoxy resins represented by the formula:

wherein R is H or CH₃; A is C(CH₃)₂, -CH₂-, -O-,

or an -S group.

Particularly preferred are the epoxy resins wherein A is C(CH3)2 and which have a number average molecular weight of 1000 to 5000, because of their extremely improved anti-corrosion properties.

If the number average molecular weight of the epoxy resin is less than 300, it is impossible to obtain a high molecular weight polymer when reacted with a polyisocyanate, making the resulting conductive primer layer disadvantageous in anticorrosive properties. On the other hand, if it is more than 100,000, the degree of crosslinking decreases, making the resulting conductive primer layer poor in anticorrosive properties.

In the conductive primer layer used in the present invention, the above bisphenol type epoxy resin (A) should be 30 wt% or more on a solid basis of the conductive primer layer, otherwise the resulting conductive primer layer easily becomes brittle and has poor adhesion after punching.

The hardener (B) used in the present invention is selected from the group consisting of polyisocyanates and blocked polyisocyanates.

Examples of polyisocyanates are aliphatic or alicyclic diisocyanates including hexamethylene diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate and the like; aromatic diisocyanates including toluene diisocyanate, diphenylmethane-4,4'-diisocyanate and the like; triisocyanates as adducts of 1 mole of trimethylolpropane and 3 moles of any of the above-mentioned diisocyanates; trimers of hexamethylene diisocyanate, trimers of toluene diisocyanate and the like.

Any of the above-mentioned polyisocyanates blocked with blocking agents can be used as blocked polyisocyanates. Such blocking agents must be compounds capable of reacting with an isocyanate group and forming addition products which are stable at room temperature but dissociate at high temperatures, such as those encountered during firing, thereby releasing the isocyanate group. Examples of the blocking agents are Lactam blocking agents such as ε-caprolactam and γ-butyrolactam; oxime blocking agents such as methyl ethyl ketoxime and cyclohexanone oxime; alcoholic blocking agents such as methanol, ethanol and isobutyl alcohol; phenolic blocking agents such as phenol, pt-butylphenol and cresol; and beta-diketone blocking agents such as ethyl acetoacetate, methyl acetoacetate, acetylacetoacetate, dimethylmalonate and diethylmalonate.

From the point of view of dissociation at a relatively low temperature, methyl ethyl ketoxime and beta-diketones are particularly preferred.

The above-mentioned hardener (B) is contained in the conductive primer in an amount of 1/10 to twice the weight of the epoxy resin (A) on a solid basis.

The epoxy resin (A) and the hardener (B) react with each other at a relatively low firing temperature in a short time, for example at a maximum temperature of 100ºC or more, preferably 120ºC to 200ºC in 5 to 60 seconds.

If the mixing ratio of (B)/(A) is less than 1/10, the crosslinking reaction is insufficient, resulting in poor corrosion resistance of the conductive primer layer. On the other hand, if the ratio of (B)/(A) is more than 20/10, the conductive primer layer will be provided with poor water resistance, alkali resistance and adhesion to an overlying layer. Particularly preferred mixing ratio of the hardener (B) to the epoxy resin (A) are such that the NCO equivalent of the hardener (B)/OH equivalent of the epoxy resin (A) is in a range of 0.1/1 to 1.0/1. Under these conditions, excellent anti-corrosion properties can be obtained.

The above-mentioned hardener (B) may be used together with a resol type phenolic resin if desired. This is particularly effective for accelerating the film-forming reaction at a relatively low firing temperature, for example at a maximum temperature of 100ºC to 130ºC.

Particularly preferred resol-type phenolic resins are the compounds of the formula:

wherein n is an integer from 0 to 4; W is -CH₂- or -CH₂-O-CH₂-; R is CH₃, H, or

Such a resol type phenolic resin is preferably added in an amount of up to ten times the weight of the hardener (B) on a solid basis. If the weight ratio of the phenolic resin/hardener (B) is more than 10/1, the alkali resistance of the resulting conductive primer layer is likely to be lowered.

In the present invention, silica fume (C) having an average particle size of 0.1 to 100 mils serves to impart electrical conductivity to the primer layer because ions can pass through the pores formed by the silica fume in the primer layer in the chemical coating and electroplating processes. If the average particle size of the silica is less than 0.1 mils, the conductive primer layer is insufficient not only in electrical conductivity but also in adhesion to an overlying layer and in alkali resistance. On the other hand, if the average particle size of the silica fume exceeds 100 mils, the corrosion resistance of the resulting conductive primer layer is lowered.

The silica dust (C) amounts to 5 to 50 wt%, preferably 15 to 30 wt% of the conductive primer layer on a solid basis. To date, such a large amount of silica dust has not been used because it makes the conductive primer layer highly viscous. However, this problem has been solved by using a special ketone solvent (D).

If the silica fume (C) accounts for less than 5 wt% of the primer layer on a solid basis, the corrosion resistance of the resulting conductive primer layer becomes poor. On the other hand, if the proportion exceeds 50 wt%, the adhesion properties of the conductive primer layer after punching tend to to be demeaned in an undesirable manner.

The epoxy resin (A), the hardener (B) and the quartz powder (C) are dissolved or dispersed in a ketone solvent. The ketone solvent (D) should be 40 wt% or more to adjust the solid content of the conductive primer layer to 10 to 50 wt%.

If the solid content of the conductive primer layer is less than 10 wt%, it is uneconomical because too much solvent is consumed. On the other hand, if it exceeds 50 wt%, it is hardly possible to form a thin, uniform conductive primer layer.

Preferred examples of the ketone solvents (D) are methyl isobutyl ketone, acetone, cyclohexanone and isophorone. If desired, additional solvents other than the ketone solvent may be used together therewith. However, solvents that can react with polyisocyanates, such as water and alcohol, should be avoided.

From the viewpoint of forming pre-coated aluminum parts, the conductive primer layer may further comprise 0.1 to 10% by weight, based on the conductive primer layer, of such lubricants as polyolefins, carboxylates, metal carboxylates, polyalkylene glycols, molybdenum disulfide, silicon compounds and fluorinated compounds. Particularly preferred lubricants are polyethylene waxes. with densities of 0.94 or more, molecular weights of 1000 to 10,000 and acid values of 15 KOH mg/g or less.

In order to increase the electrical conductivity of the conductive primer layer, powders of conductive material such as carbon black, graphite powder, zinc oxide powder, titanium oxide powder, aluminum powder, copper powder, etc. can be added in appropriate proportions.

If desired, the conductive primer layer may further contain known anticorrosive pigments (e.g. chromate pigments, in particular zinc chromate, lead chromate and barium chromate, phosphate pigments and plumbate pigments), extender pigments (e.g. carbonate pigments and silicate pigments), color pigments (e.g. titanium oxide and carbon black), anticorrosive agents (e.g. amine compounds and phenolic carboxylic acids) and dispersion stabilizers.

The primer layer used in the present invention forms an organic coating layer which is electrically conductive for the purpose of the present invention. The term "conductive" used herein means that the aluminum parts coated with the conductive primer layer can be provided with chemical coatings and electroplating, namely, that electrical conduction can be achieved through the conductive primer in the process of electroplating, etc. The reasons for this can be considered that since the conductive primer layer is thin and porous, an electroplating composition of the Aluminum parts can be attracted via the conductive primer.

Furthermore, in a case where punching such as deep drawing is carried out, the conductive primer provides aluminum parts with excellent machinability, making it unnecessary to use lubricating oils. This is an outstanding advantage because the process of removing lubricating oils from the punched aluminum parts can be omitted. Furthermore, when the aluminum parts are combined with the steel parts, it is preferable to use a conductive primer to give the same coating appearance to the aluminum parts and the steel parts.

In order to sufficiently achieve the object of the present invention, the aluminum parts are coated with the conductive primer on one or both sides thereof and then transported to an automobile body manufacturing line where the aluminum parts are punched and combined with the steel parts for subsequent treatments. However, according to an alternative method, after assembling the aluminum parts with the steel parts, the conductive primer may be applied to the assembled parts by spraying and then baked.

The conductive primer is usually applied to the aluminum parts in a dry film thickness of 10 µm or less. If the dry film thickness exceeds 10 µm, the conductive primer shows high insulation, which makes it difficult to to provide them with chemical and electroplating coatings. Accordingly, the resulting coated aluminum parts have poor corrosion resistance, impact resistance and workability. The preferred dry film thickness of the conductive primer layer is 0.1 to 5 µm. If it is less than 0.1 µm, the conductive primer-coated aluminum parts are susceptible to erosion in a zinc phosphate treatment bath and allow a large amount of aluminum to be dissolved in the zinc phosphate treatment bath. In addition, in the case of stamping, the aluminum parts tend to show cracks and strains. A particularly preferred dry film thickness of the conductive primer layer is 0.5 to 3 µm.

The chemical coating (coating) can be carried out using known coating chemicals. The preferred coating chemicals for the purpose of the present invention are zinc phosphate chemicals containing zinc ions, phosphate ions, hydrogen chloride ions, nitrite ions and, if necessary, nickel ions, nitrate ions and chlorine ions in suitable proportions. A typical example of the coating chemicals consists essentially of about 0.4 to about 1 g/l of zinc, about 5 to about 40 g/l of phosphate, about 0.01 to about 0.2 g/l of nitrite and about 2 to about 5 g/l of chlorate. Such coating chemicals suitable as primers for forming electroplated coatings are exemplified in Japanese Patent Laid-Open No. 55-145180.

The coating chemicals may be applied to the aluminum and steel parts separately or in combination. It should be noted that the coating chemicals are not necessarily indispensable to the aluminum parts, but the coating chemicals can provide the aluminum parts with better corrosion resistance. When a composition of aluminum parts coated with the conductive primer and steel parts for forming automobile bodies is immersed in a chemical treatment bath of zinc phosphate chemicals, the pre-coated aluminum parts are not eroded by the chemical treatment bath, so that substantially no aluminum sludge is formed in the bath.

Next, the assembled parts are provided with electroplating coatings. The electroplating coating composition can be of both cationic and anionic types as known in the art. A uniform electroplating coating layer with good appearance can be formed on the pre-coated aluminum parts and the chemically treated steel parts.

If necessary, an intermediate layer and/or a top layer may be formed on the assembled parts thus treated. As an alternative method, without subjecting the aluminum parts coated with the conductive primer to chemical coating with zinc phosphate chemicals, electroplating may be carried out, optionally followed by the formation of an intermediate layer and/or a top layer.

For example, if oils, dusts, stresses, etc. adhere to the conductive primer layers, it is preferable that they be removed by solvents, etc. before subsequent treatments.

Preferred examples of the coating method according to the present invention are as follows:

(1) (a-1) Aluminium parts are degreased using a known method.

(a-2) Conductive primer is applied with a roller and fired at a maximum temperature of 100-160ºC for 20 s.

(b-1) assembling the aluminum parts with steel parts to form automobile bodies;

(b-2) formation of chemical coatings;

(b-3) formation of electroplated coatings;

(b-4) Formation of an intermediate layer and/or a top layer.

(2) (a-1) Aluminium parts are degreased using a known method.

(a-2) A conductive primer is applied with a roller and fired at a maximum temperature of 100-160ºC for 20 s.

(b-1) punching;

(b-2) Assembling the aluminum parts with the steel parts to form automobile bodies, etc.;

(b-3) formation of electroplated coatings;

(b-4) Formation of an intermediate layer/a top layer.

(3) (a-1) Aluminium parts are degreased using a known process.

(a-2) A conductive primer is applied with a roller and fired at a maximum temperature of 100-160ºC for 20 s;

(b-1) punching;

(b-2) assembling the aluminum parts with the steel parts to form automobile bodies;

(b-3) forming an undercoat layer by spraying;

(b-4) Forming an intermediate layer and/or a top layer.

In the above methods, steps (a-1), (a-2) may be carried out in aluminum processing lines, and steps (b-1), (b-2) ... may be carried out in automobile body assembly lines.

The present invention is described in more detail by means of the following examples.

example 1

The following coating process is carried out:

(a-1) Manufacturing of aluminum parts (degreased by a known method).

(a-2) Applying an epoxy resin type conductive primer (NIPPE METALCOAT Pl9 CLEAR, manufactured by Nippon Paint Co., Ltd.) to the aluminum parts in a dry film thickness of 0.5 µm.

(a-3) Punching of the pre-coated aluminum parts (a-2).

(b-1) Assembling the stamped aluminum parts (a-3) and a stamped steel plate (cold-rolled steel parts) to form an automobile body.

(b-2) Formation of chemical coatings.

(i)Degrease with alkali by immersing the automobile body in a 2% RIDOLINE SD 400 (manufactured by Nippon Paint Co., Ltd.) solution at 60ºC for 3 min.

(ii) Rinse with water.

(iii) Surface adjustment by immersing the automobile body in a 0.1% FIXODINE 5N-10 (manufactured by Nippon Paint Co., Ltd.) solution at 25ºC for 30 min.

(iv) Formation of coating chemicals by immersing the automobile body in a solution containing zinc phosphate chemicals (GRANODINE DP 3000) manufactured by Nippon Paint Co., Ltd.) having a concentration:

Concentration = total acid/free acid = 18/0.9 = 20

at 50ºC for 2 min.

(v) Rinse with water.

(vi) Final rinsing (if necessary) by spraying a 0.1% DEOXYLITE 41 (manufactured by Nippon Paint Co., Ltd.) solution at 20ºC for 1 s.

(vii) Rinse with pure water by spraying at 20ºC for 1 s.

(viii) Drying at 150ºC for 2 min.

(b-3) Formation of electroplating coatings. Cationic electroplating coating paint: POWERTOP U600, manufactured by Nippon Paint Co., Ltd.;

Applied voltage: 200 V.

Application time: 3 min.

Bath temperature: 30ºC.

Firing: 185ºC, 20 min.

Thickness of dry film: 25 um

(b-4) Formation of an intermediate layer with ORGA P-41 GRAY, manufactured by Nippon Paint Co., Ltd.

Firing: 140ºC, 30 min.

Thickness of dry film: 35 um

(b-5) Formation of an upper class with:

(1) SUPERLAC M-80 METALLIC, manufactured by Nippon Paint Co., Ltd. and

(2) SUPERLAC 0-50 CLEAR, manufactured by Nippon Paint Co., Ltd.

in a wet-on-wet coating process.

Firing: 140ºC, 30 min.

Thickness of dry film: 20/35 um

Various properties are evaluated and the results are shown in Table 1.

Example 2

The same coating procedure as in Example 1 is carried out except for changing the dry film thickness of NIPPE METALCOAT P19 CLEAR to 5 µm. Various properties are evaluated and the results are shown in Table 1.

Example 3

The following coating process is carried out:

(a-1) Manufacturing of aluminum parts (degreased by a known method).

(b-l) Applying a conductive primer with an epoxy resin type (NIPPE METALCOAT Pl9 CLEAR, manufactured by Nippon Paint Co., Ltd.) to the aluminum parts in a dry film thickness of 1 µm.

(b-2) Punching of the pre-coated aluminum parts.

(b-3) Assembling the aluminum stamped parts and a steel stamped part (cold rolled steel parts) to form an automobile body.

The formation of coating chemicals, electroplating coatings, an intermediate layer and a top layer are carried out according to the same procedures as in Example 1. Various properties are evaluated and the results are shown in Table 1.

Comparative examples 1 to 4

A combination of aluminum parts and steel parts is coated as follows:

[1] Treatment of aluminium parts

(1) Coating with different amounts of lubricating oil.

(2) Punching the aluminum parts with different punching degrees.

(3) Degreasing with alkalis by immersing the stamped aluminum parts in a 1% RIDOLINE 53 solution (manufactured by Nippon Paint Co., Ltd.) for 1 min.

(4) Rinse with water.

(5) Formation of chemical coatings by immersion in a 2% chrome match chemical solution (ALODINE 1200, manufactured by Nippon Paint Co., Ltd.) at 40ºC for 30 s.

(6) Rinse with water.

(7) Drying.

(8) Formation of electroplated coatings according to the same procedure as in Example 1.

[2] Treatment of steel parts

(1) Chemical coating

(i)Degrease with alkalis by immersing the steel parts in a 2% RIDOLINE SD400 solution (manufactured by Nippon Paint Co., Ltd.) at 60ºC for 3 min.

(ii) Rinse with water.

(iii) Surface adjustment by immersing the steel parts in a 0.1% FTXODINE 5N-10 solution (manufactured by Nippon Paint Co., Ltd.) at 25ºC for 30 min.

(iv) Formation of coating chemicals by immersing the steel parts in zinc phosphate chemicals (GRANODINE DP 3000, manufactured by Nippon Paint Co., Ltd.) with the concentration:

Concentration = total acid/free acid = 18/0.9 = 20

at 50ºC for 2 min.

(v) Rinse with water.

(vi) Final rinsing (if necessary) by spraying a 0.1% solution of DEOXYLITE 41 (manufactured by Nippon Paint Co., Ltd.) at 20ºC for 1 s.

(vii) Rinse with pure water by spraying at 20ºC for 1 s.

(viii) Drying at 150ºC for 2 min.

(2) Formation of electroplated coatings according to the same procedure as in Example 1.

[3] Assembling the aluminum parts and the steel parts.

[4] Forming an intermediate layer/top layer on the assembled plates.

Various properties are evaluated by the same method as in Example 1, and the results are shown in Table 2. Table 1 Example No. Punching degree (1) Manufacturing cost (2) Productivity (3) Coating Thickness (µm) of conductive primer layer Appearance (4) Hardness (5) Impact resistance (6) Adhesion (7) Machinability (8)

Annotation:

(1) A: Punching without deep drawing.

B: Punching with deep drawing.

(2),(3) Evaluated at step (b-1) and following, expressed by the following relative scores:

: excellent

: good

Δ: mediocre

x: bad

xx: very bad

(4) Evaluations are obtained by the coating appearance evaluation device by "NSIC" manufactured by Suga Shikenki K.K.

: 80 or more

: 79-76

Δ: 75-71

x: 70 or less

(5) Measured according to JIS K 5400 6.14

: F or harder

: B-2B

x: 3B or softer

(6) Measured according to JIS K 5400 6.13 B at 7.27 cm (1/2 inch), 500 g, 30 cm.

: No damage to the coating.

Δ: Slight damage to the coating.

x: Extreme damage to the coating.

(7) Measured according to JIS K 5400 7.2. Each coating is cut to obtain 100 isolated coating units of 2 mm x 2 mm according to JIS K 5400 6.15 and bonded with a cellophane tape. After After immersion in warm water at 50ºC for 400 h, the cellophane tape was removed to evaluate how many isolated coating units remained on the test piece.

: 100/100

: 99/100

Δ: 90-98/100

x: 89 or less/100

(8) After the punching process step, cracks, scratches, stresses, etc. are evaluated:

: No damage observed through a magnifying glass (x 10).

: No damage observed with the naked eye.

Δ: Minor damage (cracks, scratches, stresses) observed with the naked eye.

x : Major damage observed with the naked eye. Table 2 Comparative Example No. Amount of lubricating oil (1) Punching rate (2) Manufacturing cost (3) Productivity (4) Coating Thickness (µm) of conductive primer layer Appearance (5) Hardness (6) Impact resistance (7) Adhesion (8) Removability of lubricating oil (9) Machinability (10)

Annotation:

(1) L: 10-15 g/m².

M: 5-10 g/m².

S: 1-5 g/m².

(2)-(8), (10) the same as in Table 1.

(9) After removing the lubricating oil, the water repellency was evaluated.

: No water repellency (almost defatted).

: Slight water repellency (almost degreased).

Δ : Water repellent (incompletely degreased).

x : Extreme water repellency.

As described above in detail, according to the coating method of the present invention, a metal assembly including at least one aluminum part and at least one steel part can be provided with chemical coatings and electroplating coatings without suffering from erosion of the aluminum parts. In addition, the aluminum parts coated with the conductive primer can be punched into an arbitrary shape, such as a deep-drawn shape, without using a lubricating oil. This is very advantageous in terms of productivity because the process step of removing a lubricating oil from the punched aluminum part usually requires a lot of time and effort.

Claims (6)

1. Method for coating a metal assembly of Al parts with uncoated and/or zinc or zinc alloy coated steel parts, comprising the following steps: Degreasing the aluminum part; Applying a conductive primer to this aluminium part and Firing the primer to form an organic coating layer; Combining the aluminum part with the steel part and applying chemical and electroplating coatings to the resulting assembled parts. 2. The method of claim 1, further comprising the step of forming an intermediate layer and/or top layer after the formation of electroplated coatings. 3. The method of claim 1, further comprising the step of forming chemical coatings on the aluminum part prior to applying the conductive primer layer thereto. 4. The method of claim 1, further comprising the step of punching the aluminum part provided with coated with this conductive primer before being combined with the steel part. 5. Method for coating a metal assembly of aluminum parts with uncoated and/or zinc or zinc alloy coated steel parts, comprising the following steps: Degreasing the aluminum part; Applying a conductive primer to this aluminium part and firing the primer to form an organic coating layer; Punching the aluminum part; Combining the stamped aluminum part with the steel part and then applying chemical coatings to the resulting composite parts. 6. A method according to one or more of the preceding claims, wherein the thickness of the conductive primer, expressed as the thickness of a dry film, is 0.1 to 10 µm.