EP1181107A2

EP1181107A2 - Weldable, coated metal substrates and methods for preparing and inhibiting corrosion of the same

Info

Publication number: EP1181107A2
Application number: EP00932216A
Authority: EP
Inventors: Ralph C. Gray; Brian T. Hauser; Joseph T. Valko
Original assignee: PPG Industries Ohio Inc
Current assignee: PPG Industries Ohio Inc
Priority date: 1999-05-11
Filing date: 2000-05-10
Publication date: 2002-02-27
Also published as: KR20010113920A; JP2002543998A; CA2372745A1; WO2000068325A2; WO2000068325A3; BR0011519A

Abstract

The present invention provides a weldable, coated metal substrate, the surface of which is pretreated with a Group IIIB or IVB metal compound or mixtures thereof; and a weldable coating containing an electroconductive pigment dispersed throughout a binder deposited upon at least a portion of the pretreatment coating composition.

Description

WELDABLE, COATED METAL SUBSTRATES AND METHODS FOR PREPARING AND INHIBITING CORROSION OF THE SAME

FIELD OF THE INVENTION

This invention relates generally to weldable, corrosion- resistant coated metal substrates and, more particularly, to metal substrates having chrome-free coatings thereon which inhibit corrosion and facilitate forming and welding of the metal substrate.

BACKGROUND OF THE INVENTION

Weldable coatings containing an electrically conductive material, such as zinc, are often used to provide an electroconductive layer on metal substrates. Zinc-rich weldable coatings can be applied directly to ferrous metal surfaces or over ferrous metal which has been treated with a chromium-containing solution. For example, U. S. Patent No. 4,346,143 discloses a process for providing corrosion protection to ferrous metal substrates comprising etching the surface of the substrate with nitric acid followed by applying a zinc-rich coating including a binder material to the etched surface . u. S. Patent Nos. 4,157,924 and 4,186,036 disclose a weldable coating for metallic substrates which contains an epoxy or phenoxy resin, electroconductive pigment such as zinc and a diluent such as glycol ether. As discussed at column 7, lines 37-42, the substrate can be pretreated with a pulverulent metal-free composition containing chromate and/or phosphate ions.

Similarly, European Patent Application No. 0157392 discloses an anti-corrosive primer for metal phosphate- or chromate-treated steel which consists of a mixture of 70 to 95 weight percent zinc, aluminum, a gliding agent and a binding agent .

U. S. Patent No. 3,687,739 discloses a weldable composite coating comprising (1) an undercoating of pulverulent metal and a hexavalent chromium-containing liquid composition and (2) a top coat comprising a particulate, electrically conductive pigment and a binder material.

Although chromium-containing coatings provide excellent corrosion protection, particularly under zinc-rich coatings, they are toxic and present waste disposal problems. Therefore, there is a need for chromium-free treatment solutions for treating metal substrates prior to the application of a weldable coating. The treatment solution should provide corrosion resistance and maintain substrate electroconductivity for welding.

SUMMARY OF THE INVENTION

One aspect of the present invention is a weldable, coated metal substrate comprising: (a) a metal substrate; (b) a pretreatment composition comprising a Group IIIB or IVB metal compound or mixtures thereof deposited upon at least a portion of a surface of the metal substrate; and (c) a weldable coating comprising an electroconductive pigment dispersed throughout a binder deposited upon at least a portion of the pretreatment composition.

Another aspect of the present invention is a method for preparing a weldable, coated metal substrate, comprising the steps of: (a) treating a surface of a metal substrate with a pretreatment composition comprising a Group IIIB or IVB metal compound or mixtures thereof to form a substrate having a pretreated surface; and (b) applying a weldable coating composition to the pretreated surface to form a weldable, coated metal substrate, the weldable coating composition comprising an electroconductive pigment and a binder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used in the specification and claims are to be understood as modified in all instances by the term "about".

The metal substrates used in the practice of the present invention include ferrous metals, non-ferrous metals and combinations thereof. Suitable ferrous metals include iron, steel, and alloys thereof. Non-limiting examples of useful steel materials include cold rolled steel, galvanized (zinc coated) steel, electrogalvanized steel, stainless steel, pickled steel, zinc-iron alloy such as GALVANNEAL, and combinations thereof. Useful non-ferrous metals include aluminum, zinc, magnesium and alloys thereof. Combinations or composites of ferrous and non-ferrous metals can also be used such as GALVALUME and GALFAN zinc-aluminum alloys.

The shape of the metal substrate can be in the form of a sheet, plate, bar, rod or any shape desired. Preferably, the shape of the metal substrate is an elongated strip wound about a spool in the form of a coil. The thickness of the strip preferably ranges from about 0.254 to about 3.18 millimeters (mm) (about 10 to about 125 mils) , and more preferably about 0.3 mm, although the thickness can be greater or less, as desired. The width of the strip generally ranges from about 30.5 to about 183 centimeters (about 12 to about 72 inches), although the width can vary depending upon its intended use. Before depositing the coatings upon the surface of the metal substrate, it is preferred to remove foreign matter from the metal surface by thoroughly cleaning and degreasing the surface. The surface of the metal substrate can be cleaned by physical or chemical means, such as mechanically abrading the surface or cleaning/degreasing with commercially available alkaline or acidic cleaning agents which are well know to those skilled in the art, such as sodium metasilicate and sodium hydroxide. A non-limiting example of a preferred cleaning agent is CHEMKLEEN 1633, an alkaline-based cleaner commercially available from PPG Pretreatment and Specialty Products of Troy, Michigan.

Following the cleaning step, the metal substrate is usually rinsed with water in order to remove any residue. The metal substrate can be air dried using an air knife, by flashing off the water by brief exposure of the substrate to a high temperature or by passing the substrate between squeegee rolls .

In the present invention, a pretreatment coating composition is deposited upon at least a portion of the outer surface of the metal substrate. Preferably, the entire outer surface of the metal substrate is treated with the pretreatment composition.

The pretreatment composition facilitates adhesion of the subsequently applied weldable coating composition to the metal substrate. The pretreatment should be sufficiently thin and/or deformable to permit the heat and force applied to the weldable coating by the welding tool to drive at least a portion of the electroconductive pigment therein through the pretreatment coating to contact or essentially contact the metal substrate and provide an electrically conductive path to permit welding of the coated substrate. As used herein, essentially contact" means that the electrical resistance provided by the pretreatment coating is less than about 1 ohm. The thickness of the pretreatment film can vary, but is generally less than about 1 micrometer, preferably ranges from about 1 to about 500 nanometers, and more preferably is about 10 to about 300 nanometers.

The pretreatment composition comprises one or more Group IIIB or IVB element-containing compounds or mixtures thereof solubulized or dispersed in a carrier medium typically an aqueous medium. The Group IIIB and IVB elements are defined by the CAS Periodic Table of the Elements as shown, for example, in the Handbook of Chemistry and Physics, (60th Ed. 1980) . Transition metal compounds and rare earth metal compounds are compounds of zirconium, titanium, hafnium, yttrium and cerium and mixtures thereof. Typical zirconium compounds may be selected from hexafluorozirconic acid, alkali metal and ammonium salts thereof, ammonium zirconium carbonate, zirconyl nitrate, zirconium carboxylates and zirconium hydroxy carboxylates such as hydrofluorozirconic acid, zirconium acetate, zirconium oxalate, ammonium zirconium glycolate, ammonium zirconium lactate, ammonium zirconium citrate, and mixtures thereof. Hexafluorozirconic acid is preferred. An example of the yttrium compound is yttrium nitrate. An example of the titanium compound is fluorotitanic acid and its salts. An example of the hafnium compound is hafnium nitrate. An example of the cerium compound is cerous nitrate. Preferably, the Group IIIB or IVB metal compounds are in the form of metal salts or acids which are water soluble . The Group IIIB or IVB metal compound is typically present in the carrier medium in an amount of 10 to 5000 ppm metal, preferably 100 to 300 ppm metal.

Additionally, the pretreatment composition carrier may contain a film forming resin. Suitable resins include reaction products of one or more alkanolamines and an epoxy- functional material containing at least two epoxy groups, such as those disclosed in U. S. Patent No. 5,653,823. Preferably, such resins contain beta hydroxy ester, imide, or sulfide functionality, incorporated by using dimethylolpropionic acid, phthalimide, or mercaptoglycerine as an additional reactant in the preparation of the resin. Alternatively, the reaction product is that of the diglycidyl ether of Bisphenol A (commercially available from Shell Chemical Company as EPON 880), dimethylol propionic acid, and diethanolamine in a 0.6 to 5.0:0.05 to 5.5:1 mole ratio. Other suitable resins include water soluble and water dispersible polyacrylic acids as disclosed in U. S. Patent Nos. 3,912,548 and 5,328,525; phenol-formaldehyde resins as described in U. S. Patent No. 5,662,746, incorporated herein by reference; water soluble polyamides such as those disclosed in WO 95/33869; copolymers of maleic or acrylic acid with allyl ether as described in Canadian Patent Application No. 2,087,352; and water soluble and dispersible resins including epoxy resins, aminoplasts, phenol-formaldehyde resins, tannins, and polyvinyl phenols as discussed in U. S. Patent No. 5,449,415.

In this embodiment of the invention, the film forming resin is present in the pretreatment coating composition in an amount of 0.005% to 30% based on the total weight of the pretreatment composition, and the Group IIIB or IVB metal compound is present in an amount of 10 to 5000, preferably 100 to 1000, ppm metal based on total weight of the pretreatment composition. The weight ratio of the resin to Group IIIB or IVB metal or metal compound is from 2.0 to 10.0, preferably 3.0 to 5.0, based on metal.

The pretreatment coating composition can further comprise surfactants that function as aids to improve wetting of the substrate. Generally, the surfactant materials are present in an amount of less than about 2 weight percent on a basis of total weight of the pretreatment coating composition. Other optional materials in the carrier medium include surfactants that function as defoamers or substrate wetting agents. Preferably, the pretreatment coating composition is essentially free of chromium-containing materials, i.e., contains less than about 2 weight percent of chromium- containing materials (expressed as Cr0₃) , and more preferably less than about 0.05 weight percent of chromium-containing materials. Examples of such chromium-containing materials include chromic acid, chromium trioxide, chromic acid anhydride, dichromate salts such as ammonium dichromate, sodium dichromate, potassium dichromate, and calcium, barium, magnesium, zinc, cadmium and strontium dichromate. Most preferably, the pretreatment coating composition is free of chromium-containing materials.

The pretreatment coating composition is applied to the surface of the metal substrate by any conventional application technique, such as spraying, immersion or roll coating in a batch or continuous process. The temperature of the pretreatment coating composition at application is typically about 10°C to about 85°C, and preferably about 15°C to about 60°C. The pH of the preferred pretreatment coating composition at application generally ranges from about 2.0 to about 5.5, and is preferably about 3.5 to about 5.5. The pH of the medium may be adjusted using mineral acids such as hydrofluoric acid, fluoroboric acid, phosphoric acid, and the like, including mixtures thereof; organic acids such as lactic acid, acetic acid, citric acid, sulfamic acid, or mixtures thereof; and water soluble or water dispersible bases such as sodium hydroxide, ammonium hydroxide, ammonia, or amines such as triethylamine, methylethyl amine, or mixtures thereof.

Continuous processes are typically used in the coil coating industry and also for mill application. The pretreatment coating composition can be applied by any of these conventional processes. For example, in the coil industry, the substrate is cleaned and rinsed and then usually contacted with the pretreatment coating composition by roll coating with a chemical coater. The treated strip is then dried by heating and painted and baked by conventional coil coating processes. Mill application of the pretreatment composition can be by immersion, spray or roll coating applied to the freshly manufactured metal strip. Excess pretreatment composition is typically removed by wringer rolls. After the pretreatment composition has been applied to the metal surface, the metal can be rinsed with deionized water and dried at room temperature or at elevated temperatures to remove excess moisture from the treated substrate surface and cure any curable coating components to form the pretreatment coating. Alternately, the treated substrate can be heated at about 65°C to about 125°C for about 2 to about 30 seconds to produce a coated substrate having a dried residue of the pretreatment coating composition thereon. If the substrate is already heated from the hot melt production process, no post application heating of the treated substrate is required to facilitate drying. The temperature and time for drying the coating will depend upon such variables as the percentage of solids in the coating, components of the coating composition and type of substrate.

The film coverage of the residue of the pretreatment composition generally ranges from about 1 to about 1000 milligrams per square meter (mg/m²) , and is preferably about 10 to about 400 mg/m².

In the present invention, a weldable coating composition is deposited upon at least a portion of the pretreatment coating. The weldable coating composition comprises one or more electroconductive pigments dispersed throughout a binder which provide electroconductivity and cathodic protection to the weldable coating and one or more binders which adhere the electroconductive pigment to the pretreatment coating.

Non-limiting examples of suitable electroconductive pigments include zinc (preferred) , aluminum, iron, graphite, diiron phosphide and mixtures thereof. Preferred zinc particles are commercially available from ZINCOLI GmbH as ZINCOLI S 620 or 520. The average particle size (equivalent spherical diameter) of the electroconductive pigment particles generally is less than about 10 micrometers, preferably ranges from about 1 to about 5 micrometers, and more preferably about 3 micrometers.

Since the metal substrates are to be subsequently welded, the weldable coating composition must comprise a substantial amount of electroconductive pigment, generally greater than about 10 volume percent and preferably about 30 to about 60 volume percent on a basis of total volume of electroconductive pigment and binder.

The binder is present to secure the electroconductive pigment to the pretreatment coating. Preferably, the binder forms a generally continuous film when applied to the surface of the pretreatment coating. Generally, the amount of binder can range from about 5 to about 50 weight percent of the weldable coating composition on a total solids basis, preferably about 10 to about 30 weight percent and more preferably about 10 to about 20 weight percent. The binder can comprise oligomeric binders, polymeric binders and mixtures thereof. The binder is preferably a resinous polymeric binder material selected from thermosetting binders, thermoplastic binders or mixtures thereof. Non- limiting examples of suitable thermosetting materials include polyesters, epoxy-containing materials, phenoxy-containing materials, polyurethanes, and mixtures thereof, in combination with crosslinkers such as aminoplasts or isocyanates which are discussed below. Non-limiting examples of suitable thermoplastic binders include high molecular weight epoxy resins, defunctionalized epoxy resins, vinyl polymers, polyesters, polyolefins, polyamides, polyurethanes, acrylic polymers and mixtures thereof. Examples of useful binder materials include phenoxy polyether polyols and inorganic silicates.

Particularly preferred binder materials are polyglycidyl ethers of polyhydric phenols having a weight average molecular weight of at least about 2000 and preferably ranging from about 5000 to about 100,000. These materials can be epoxy functional or defunctionalized by reacting the epoxy groups with phenolic materials. Such binders can have epoxy equivalent weights of about 2000 to about one million. Non- limiting examples of useful epoxy resins are commercially available from Shell Chemical Company as EPON® epoxy resins. Preferred EPON® epoxy resins include EPON® 1009, which has an epoxy equivalent weight of about 2300-3800. Useful epoxy defunctionalized resins include phenoxy resins such as EPONOL resin 55-BK-30 which is commercially available from Shell.

Suitable crosslinkers or curing agents are described in U. S. Patent No. 4,346,143 at column 5, lines 45-62 and include blocked or unblocked di- or polyisocyanates such as DESMODUR® BL 1265 toluene diisocyanate blocked with caprolactam, which is commercially available from Bayer, and aminoplasts such as etherified derivatives of urea-melamine- and benzoguanamine-formaldehyde condensates which are commercially available from CYTEC Industries under the trademark CYMEL® and from Solutia under the trademark RESIMENE®.

Preferably, the weldable coating composition comprises one or more diluents for adjusting the viscosity of the composition so that it can be applied to the metal substrate by conventional coating techniques. The diluent should be selected so as not to detrimentally affect the adhesion of the weldable coating to the pretreatment coating upon the metal substrate. Suitable diluents include ketones such as cyclohexanone (preferred) , acetone, methyl ethyl ketone, methyl isobutyl ketone and isophorone; esters and ethers such as 2-ethoxyethyl acetate, propylene glycol monomethyl ethers such as DOWANOL PM, dipropylene glycol monomethyl ethers such as DOWANOL DPM or propylene glycol methyl ether acetates such as PM ACETATE which is commercially available from Dow Chemical; and aromatic solvents such as toluene, xylene, aromatic solvent blends derived from petroleum such as SOLVESSO®. The amount of diluent can vary depending upon the method of coating, the binder components and the pigment-to- binder ratio, but generally ranges from about 10 to about 50 weight percent on a basis of total weight of the weldable coating composition.

The weldable coating composition can further comprise optional ingredients such as phosphorus-containing materials, including metal phosphates or the organophosphates discussed in detail above; inorganic lubricants such as GLEITMO 1000S molybdenum disulfide particles which are commercially available from Fuchs of Germany; extender pigments such as iron oxides and iron phosphides; flow control agents; thixotropic agents such as silica, montmorillonite clay and hydrogenated castor oil; anti-settling agents such as aluminum stearate and polyethylene powder; dehydrating agents which inhibit gas formation such as silica, lime or sodium aluminum silicate; and wetting agents including salts of sulfated castor oil derivatives such as DEHYSOL R. Other pigments such as carbon black, iron oxide, magnesium silicate (talc) , zinc oxide and corrosion inhibiting pigments including zinc phosphate and molybdates such as calcium molybdate, zinc molybdate, barium molybdate and strontium molybdate and mixtures thereof can be included in the weldable coating composition. Generally, these optional ingredients comprise less than about 20 weight percent of the weldable coating composition on a total solids basis, and usually about 5 to about 15 weight percent. Preferably, the weldable coating composition is essentially free of chromium- containing materials, i.e., comprises less than about 2 weight percent of chromium-containing materials and more preferably is free of chromium-containing materials.

The preferred weldable coating composition includes EPON® 1009 epoxy-functional resin, zinc dust, salt of a sulfated castor oil derivative, silica, molybdenum disulfide, red iron oxide, toluene diisocyanate blocked with caprolactam, melamine resin, dipropylene glycol methyl ether, propylene glycol methyl ether acetate and cyclohexanone . The weldable coating composition can be applied to the surface of the pretreatment coating by any conventional method well known to those skilled in the art, such as dip coating, direct roll coating, reverse roll coating, curtain coating, air and airless spraying, electrostatic spraying, pressure spraying, brushing such as rotary brush coating or a combination of any of the techniques discussed above.

The thickness of the weldable coating can vary depending upon the use to which the coated metal substrate will be subjected. Generally, to achieve sufficient corrosion resistance for coil metal for automotive use, the applied weldable coating should have a film thickness of at least about 1 micrometer (about 0.5 mils), preferably about 1 to about 20 micrometers and more preferably about 2 to about 5 micrometers. For other substrates and other applications, thinner or thicker coatings can be used.

After application, the weldable coating is preferably dried and/or any curable components thereof are cured to form a dried residue of the weldable coating upon the substrate. The dried residue can be formed at an elevated temperature ranging up to about 300°C peak metal temperature. Many of the binders such as those prepared from epoxy-containing materials require curing at an elevated temperature for a period of time sufficient to vaporize any diluents in the coating and to cure or set the binder. In general, baking temperatures will be dependent upon film thickness and the components of the binder. For preferred binders prepared from epoxy-containing materials, peak metal temperatures of about 150°C to about 300°C are preferred.

After the weldable coating has been dried and/or cured, the metal substrate can be stored or forwarded to other operations, such as forming, shaping, cutting and/or welding operations to form the substrate into parts such as fenders or doors and/or to a subsequent electrocoat or top coating operations. While the metal is being stored, transported or subjected to subsequent operations, the coatings protect the metal surface from corrosion, such as white and red rust, due to exposure to atmospheric conditions.

Since the coated metal substrate prepared according to the present invention is electroconductive, top coat of the coated substrate by electrodeposition is of particular interest. Compositions and methods for electrodepositing coatings are well known to those skilled in the art and a detailed discussion thereof is not believed to be necessary. Useful compositions and methods are discussed in U. S. Patent No. 5,530,043 (relating to anionic electrodeposition) and U. S. Patent Nos. 5,760,107, 5,820,987 and 4,933,056 (relating to cationic electrodeposition) .

The weldable coated metal substrate optionally can be coated with a metal phosphate coating, such as zinc phosphate, which is deposited upon at least a portion of the weldable coating. Methods of application and compositions for such metal phosphate coatings are disclosed in U. S. Patent Nos. 4,941,930 and 5,238,506. The present invention will now be illustrated by the following specific, non-limiting examples. All parts and percentages are by weight unless otherwise indicated.

EXAMPLES

The following examples show the use of surface treatments comprising Group IVB or IIIB elements applied to steel substrates which are subsequently coated with weldable coatings according to the present invention.

Preparation and Coating of Substrates

Electrogalvanized and hot-dipped galvannealled steel substrates used for coating and testing were provided by USX. Each panel was about 10.16 centimeters (cm) (4 inches) wide, about 30.48 cm (12 inches) long and about 0.76 to 0.79 mm (0.030 to 0.031 inches) thick. The steel panels were subjected to an alkaline cleaning process by spraying with a 2% by volume bath of CHEMKLEEN 163 which is available from PPG Industries, Inc. at a temperature of 60°C (140°F) for 60 seconds. The panels were removed from the alkaline cleaning bath, rinsed with room temperature water (about 21°C (70°F) ) for 15 seconds and dried with warm air blowers.

The panels were treated with one of the following pretreatment coatings: a solution of fluorozirconic acid containing 100 ppm Zr at pH 3.75 (Example 1) or a of fluorozirconic acid containing 1000 ppm Zr at pH 3.79 (Example 2). All pretreatment solutions were applied via roll coat application at 3.4 x 10⁵ Pa (50 psi) and a rate of 56.4 meters/min (185 ft/min) . Panels were immediately baked for 15 seconds to a peak metal temperature of 110°C + 6°C (230°F+ 10°F) .

After drying, all panels were coated with BONAZINC 3001 zinc-rich, epoxy resin-containing weldable coating, which is commercially available from PPG Industries, Inc., on both sides of the panel with a #12 drawbar (resulting in a dried film thickness of between 3.0 microns and 4.0 microns) and baked at 390°C (735°F) for about 40-50 seconds until a peak metal temperature of 254°C (490°F) was achieved. The panels were then cooled at ambient temperature.

Corrosion Testing

Corrosion performance was determined by putting the above-coated panels in the General Motors 9511P cyclic corrosion test. Relative ratings according to the percentage of red rust which formed over the entire tested surface of the panel, are shown in Table 1.

TABLE 1

Corrosion results from GM 9511P Test (8 cycles;

Values based on the average of two panels.

The pretreatment coating and weldable coating provide the metal substrate of the present invention with improved adhesion and flexibility and resistance to humidity, salt spray corrosion and components of subsequently applied coatings. In addition, the disposal and use problems associated with chromium can be reduced or eliminated.

Claims

WE CLAIM :

1. A weldable, coated metal substrate comprising: (a) a metal substrate; (b) a pretreatment composition comprising a Group IIIB or IVB metal compound or mixtures thereof deposited upon at least a portion of a surface of the metal substrate; and

(c) a weldable coating comprising an electroconductive pigment dispersed throughout a binder deposited upon at least a portion of the pretreatment composition.

2. The coated metal substrate according to claim 1, wherein the metal substrate comprises a ferrous metal.

3. The coated metal substrate according to claim 2, wherein the ferrous metal is selected from the group consisting of iron, steel and alloys thereof.

4. The coated metal substrate according to claim 3, wherein the ferrous metal comprises steel selected from the group consisting of cold rolled steel, galvanized steel, electrogalvanized steel, stainless steel and combinations thereof.

5. The coated metal substrate according to claim 1, wherein the metal substrate comprises a non-ferrous metal selected from the group consisting of aluminum, zinc, magnesium and alloys thereof.

6. The coated metal substrate according to claim 1, wherein the Group IIIB or IVB metal compound is a zirconium compound.

7. The coated metal substrate according to claim 6, wherein the zirconium compound is hexafluorozirconic acid.

8. The coated metal substrate according to claim 1, wherein the pretreatment coating composition further comprises a film forming resin.

9. The coated metal substrate according to claim 1, wherein the Group IIIB or IVB metal compound is present in an aqueous medium.

10. The coated metal substrate according to claim 1, wherein the pretreatment composition is essentially free of chromium-containing materials.

11. The coated metal substrate according to claim 1, wherein the electroconductive pigment is selected from the group consisting of zinc, aluminum, iron, graphite, diiron phosphide and mixtures thereof.

12. The coated metal substrate according to claim 1, wherein the binder is selected from the group consisting of oligomeric binders, polymeric binders and mixtures thereof.

13. The coated metal substrate according to claim 1, wherein the binder is selected from the group consisting of thermosetting binders, thermoplastic binders and mixtures thereof.

14. The coated metal substrate according to claim 13, wherein the binder comprises a thermosetting binder selected from the group consisting of polyesters, epoxy-containing materials, phenoxy-containing materials, aminoplasts, polyurethanes and mixtures thereof.

15. The coated metal substrate according to claim 14, wherein the binder comprises an epoxy-containing material which is a polyglycidyl ether of Bisphenol A.

16. The coated metal substrate according to claim 13, wherein the binder comprises a thermoplastic binder selected from the group consisting of vinyl polymers, polyesters, polyolefins, polyamides, polyurethanes, acrylic polymers and mixtures thereof.

17. The coated metal substrate according to claim 14, wherein the weldable coating further comprises a crosslinker for crosslinking the thermosetting binder.

18. The coated metal substrate according to claim 1, wherein the weldable coating further comprises a phosphorus- containing material.

19. The coated metal substrate according to claim 1, wherein the weldable coating further comprises an inorganic lubricant .

20. The coated metal substrate according to claim 19, wherein the inorganic lubricant is molybdenum disulfide.

21. The coated metal substrate according to claim 1, wherein the coated metal substrate further comprises a metal phosphate coating deposited upon at least a portion of the weldable coating composition.

22. The coated metal substrate according to claim 1, wherein the coated metal substrate further comprises an electrodeposited coating deposited upon at least a portion of the weldable coating.

23. A method for preparing a weldable, coated metal substrate, comprising the steps of:

(a) treating a surface of a metal substrate with a pretreatment composition comprising a Group IIIB or IVB metal compound or mixtures thereof to form a substrate having a pretreated surface; and (b) applying a weldable coating composition to the pretreated surface to form the weldable, coated metal substrate, the weldable coating composition comprising an electroconductive pigment dispersed throughout a binder.

24. The method according to claim 23, further comprising an initial step of cleaning the surface of the metal substrate with a cleaning composition prior to treating the surface of the metal substrate with the pretreatment composition.

25. The method according to claim 23, further comprising an additional step of heating the coated metal substrate after applying the weldable composition in step (b) .