US3637470A

US3637470A - Phosphoric acid pretreatment of electrodepositable welded metal seams

Info

Publication number: US3637470A
Application number: US22042A
Authority: US
Inventors: Charles M Hansen; Joseph A Muir; Jeffrey R Leyh
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Inc
Priority date: 1970-03-23
Filing date: 1970-03-23
Publication date: 1972-01-25
Anticipated expiration: 1989-01-25

Abstract

This invention relates to a method of eliminating or reducing the number of microbubbles produced when an ionically solubilized electrodepositable composition is electrocoated upon a welded metal seam. The process of the invention comprises treating the welded seam prior to electrocoating with a solution of phosphoric acid.

Description

nited States Patent Hansen et a].

[ 51 Jan. 25, 1972 [54] PHOSPHORIC ACID PRETREATMENT OF ELECTRODEPOSITABLE WELDED METAL SEAMS [72] Inventors: Charles M. Hansen, Monroeville; Joseph A. Muir, Natrona Heights; Jeffrey R. Leyh, Monroeville, all of Pa.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: Mar. 23, 1970 [21] Appl. No.: 22,042

[52] US. Cl ..204/29, 29/460, 204/181 [51] Int. Cl. ..C23b 5/62 [58] Field of Search ..204/29, 34,56 R, 181, 15;

[56] References Cited UNITED STATES PATENTS 2,847,376 8/1958 Schofield ..204/l5 3,175,964 3/1965 Watanabe et al. 204/56 R Primary ExaminerJohn l-l. Mack Assistant Examiner-W. 1. Solomon Attorney-Chisholm and Spencer [5 7] ABSTRACT 23 Claims, No Drawings PHOSPIIORIC ACID PRETREATMENT OF ELECTRODEPOSITABLE WELDED METAL SEAMS STATE OF THE ART Electrodeposition has become a widely commercially accepted industrial coating technique. The coatings achieved have excellent properties for many applications and electrodeposition results in a coating which does not run off or wash off during baking. Virtually any conductive substrate may be coated by electrodeposition. Those normally employed are metal substrates, including metals such as iron, steel, copper, zinc, brass, tin, nickel, chromium, and aluminum, as well as other metals and pretreated metals. Impregnated paper and other substrates renderedv conductive under the conditions of the coating process may also beemployed as substrates.

In the electrodeposition process, the articles to be electrocoated are immersed in an aqueous dispersion of a solubilized, ionized, film-forming material such as a synthetic organic vehicle resin. An electric current is passed between the article to be coated, serving as an electrode, and a counterelectrode to cause deposition of a coating of the vehicle resin on the article. The article is then withdrawn from the bath, usually rinsed and the coating either air dried or baked in the manner of a conventional finish.

It has been observed that ionically solubilized, electrodepositable compositions, if electrocoated over a welded seam, produce at or near the seam minute bubbles which are entrapped in the electrocoat upon curing, thereby producing potential or actual film weakening and a diminishing of the protective quality of the film.

DESCRIPTION OF THE INVENTION It has now been found that the presence of minute bubbles, hereinafter called microbubbles, in an electrodeposited film at a welded metal seam may be reduced or diminished by treating the seam area with a phosphoric acid solution prior to electrocoating. The welded seam treated by the process of the invention may be essentially any welded ferrous metal seam. The process herein described is particularly effective in treating welded lap seams, that is, where the two pieces of metal being welded are welded with their edges in a lapped relationship. The process of the invention is especially useful in treating a lapped seam wherein the weld is formed by a combination of temperature and pressure rather than a fusion weld, for example, a welded lap seam such as described in U.S. Pat. No. 3,468,453, the disclosure of which is hereby incorporated by reference.

The metal being welded may be virtually any ferrous metal, preferably a ferrous metal which is a low carbon steel havinga surface coated with a protective coating. Preferably this coating will be a passive coating such as chromate phosphate or a similar tin-free steel. For example, see U.S. Pat. No. 3,1 13,845; and Journal of Electrochemical Society: Electrochemical Technology ('Sept., 1969), pages 12994305. However, the coating material is not restricted to a passive coating and can be tin or other corrosive resistant metals.

The process of this invention comprises treating the welded area with a solution comprising phosphoric acid. Generally between about 2 percent and about 7 percent phosphoric'acid solution in water is utilized; however, the concentration of the phosphoric acid is not critical and amounts as low as l or even 0.5 percent show some improvement. More concentrated phosphoric acid solutions may be employed; however, solutions substantially in excess of about percent phosphoric acid are less economical since they do not produce substantially accelerated results. In the process of the invention, the welded area is brought in contact with the phosphoric acid solution in any desired manner. Typically the seam is dipped into the solution for a time sufficient to accomplish subsequent microbubble reduction.

The microbubble reduction-is substantially time and temperature dependent in the manner of a typical chemical reaction, thus, while the process may be conducted at room temperature, it is preferred to conduct the treatment at elevated temperatures and preferably between about and 150 F and most preferably at about F.

The welded seam is kept in contact with the phosphoric acid solution for a time sufficient to substantially reduce the microbubble formation in subsequent electrocoating. The optimum time is at least in part dependent on the type of metal, the type of weld, the concentration of phosphoric acid solution and the temperature at which the treatment is carried out; however, the conditions for achievingmeasurable results are not critical and the optimum conditions for each system may be readily determined. In the case of tin-free steel chromium type welded by the process of U.S. Pat. No. 3,469,453, op-

timum results are obtained with a 5 percent phosphoric acid solution at 140 F. in 30 seconds. Longer times do not substantially improve microbubble reduction.

After the welded seam has been treated, preferably the article is then dried, preferably without heating. Air or gas jets or circulating air or gases may be utilized to accelerate the drying. The drying step is primarily conducted to remove excess phosphoric acid solution from the article and also to prevent contamination of the electrodeposition bath; however, if one is not concerned with the long-termstability of the electrodeposition bath, or if the electrodeposition bath employed is not sensitive to phosphoric acid, the drying step may be omitted.

After the article has been treated by the phosphoric acid solution, it is preferably substantially immediately electrocoated. As utilized herein, the terms substantially and immediately"mean within a matter of minutes, preferably within 5 minutes, and at least within about 10 to l5 minutes.

It has been found that the number of microbubbles in the electrodeposited coating tends to increase in relationship to the length of time the treated weld is exposed to an oxygencontaining atmosphere after being treated by the phosphoric acid solution and dried.

The phosphoric-acid treated, welded article may then be electrocoated with an electrodepositable composition in any manner employed by the art.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of this invention. Virtually any water-soluble, water-dispersible, or water-emulsifiable polyacid or polybasic resinous material can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. Any such electrodepositable composition is included among those which can be employed in the present invention, even though the coating obtained might not be entirely satisfactory for certain specialized uses.

A preferred type of electrodepositable coating composition which gives desirable results is the water-dispersible coating compositions comprising at least partially neutralized interpolymers of hydroxyalkyl esters of unsaturated dicarboxylic acid, unsaturated carboxylic acids and at least one other ethylenically unsaturated monomer. These are employed in the composition along with an amine-aldehyde condensation product, with the interpolymer usually making from about 50 percent to about 95 percent by weight of the resinous composition.

The acid monomer of the interpolymer is usually acrylic acid ormethacrylic acid, but other ethylenically unsaturated monocarboxylic and dicarboxylic acids of up to about six carbon atoms can also be employed. The hydroxyalkyl ester is usually hydroxyethyl or hydroxypropyl acrylate or methacrylate, but also desirable are the various hydroxyalkyl esters of the above acids having, for example, up to about five carbon atoms in the hydroxyalkyl radical. Monoor diesters of the dicarboxylic acids mentioned are included. Ordinarily the acid and ester each comprise between about 1 percent and about 20 percent by weight of the interpolymer, with the remainder being'made'up of one or more other copolymerizable ethylenically unsaturated monomers. The most often used are the alkyl acrylates, such as ethyl acrylate; the alkyl methacrylates, such as methyl methacrylate; and the vinyl aromatic hydrocarbons, such as styrene, but others can be utilized.

The above interpolymer is at least partially neutralized by reaction with a base as described above; at least about percent and preferably 50 percent or more of the acidic groups are neutralized, and this can be carried out either before or after the incorporation of the interpolymer in the coating composition.

The amine-aldehyde condensation products included in these compositions are, for example, condensation products of melamine, benzoguanamine, or urea with formaldehyde, although other amine-containing amines and amides, including triazines, diazines, triazoles, guanadines, guanamines and alkyl and aryl-substituted derivatives of such compounds can be employed, as can other aldehydes, such as acetaldehyde. The alkylol groups of the products can be etherified by reaction with an alcohol, and the products utilized can be watersoluble or organic solvent-soluble.

Electrodeposition compositions comprising the above interpolymers and an amine-aldehyde resin are more fully described in U.S. Pat. No. 3,403,088, which is hereby incorporated by reference.

Other elcctrodeposition vehicle resins include synthetic polycarboxylic acid resinous materials. Numerous such resins are described in U.S. Pat. Nos. 3,441,489; 3,422,044; 3,403,088; 3,369,983 and 3,366,563, which are incorporated by reference.

These resins include a reaction product or adduct of the drying oil or semidrying oil fatty acid ester with a dicarboxylic acid or anhydride. By drying oil or semidrying oil fatty acid esters are meant esters of fatty acids which are or can be derived from drying oils or semidrying oils, or from such sources as tall oil. Such fatty acids are characterized by containing at least a portion of polyunsaturated fatty acids. Preferably, the drying oil or semidrying oil per se is employed.

Also included among such esters are those in which the esters themselves are modified with other acids, including saturated, unsaturated or aromatic acids or an anhydride thereof. The acid-modified esters are made by transesterification of the ester, as by forming a dior mono-glyceride by alcoholysis, followed by esterification with the acid; they may also be obtained by reacting oil acids with a polyol and reacting the acid with the partial ester. In addition to glycerol, alcoholysis can be carried out using other polyols such as trimethylolpropane, pentaerythritol, sorbitol and the like. If desired, the esters can also be modified with monomers such as cyclopentadiene or styrene and the modified esters produced thereby can be utilized herein. Similarly, other esters of unsaturated fatty acids, for example, those prepared by the esterification of tall oil fatty acids with polyols, are also useful.

Also included within the term drying oil fatty acid esters as set forth herein are alkyd resins prepared utilizing semidrying or drying oils; esters of epoxides with such fatty acids, including esters of diglycidyl ethers of polyhydric compounds as well as other mono-, diand polyepoxides, semidrying or drying oil fatty acid esters of polyols, such as butanediol, trimethylolethane, trimethylolpropane, trimethylolhexane, pentaerythritol, and the like; and semidrying or drying fatty acid esters of resinous polyols such as homopolymers or copolymers of unsaturated aliphatic alcohols, e.g., allyl alcohol or methallyl alcohol, including copolymers of such alcohols with styrene or other ethylenically unsaturated monomers or with nonoil modified alkyd resins containing free hydroxyl groups.

Virtually any alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride can be employed to produce the reaction products described herein. Preferably maleic anhydride is employed.

While the reaction products can be comprised solely of adducts of the fatty acid and the dicarboxylic acid or anhydride,

in many instances it is desirable to incorporate into the reaction product another ethylenically unsaturated monomer. The use of such monomer often produces films and coatings which are harder and more resistant to abrasion and which may have other similar desirable characteristics.

As shown in the art, it is preferred that in certain instances the neutralization reaction can be carried out in such a manner that amido groups are attached to part of the carbonyl carbon atoms derived from the dicarboxylic acid or anhydride.

Compositions within this general class are described in U.S. Pat. Nos. 3,366,563 and 3,369,983.

Another vehicle comprises the fatty acid ester, unsaturated acid or anhydride reaction products and any additional unsaturated modifying materials which are further reacted with the polyol.

Essentially any polyol can be employed, but diols are preferred. When higher polyols, such as trimethylolpropane, glycerol, pentaerythritol and the like are utilized, they are employed in small amounts, or in conjunction with the diol, or in the presence of a monohydric alcohol, and are used with adducts having a relatively low proportion of acidic component. Water-insoluble diols are often preferable.

The polyol reaction products and reaction conditions are more fully described in application Ser. No. 450,205, filed Apr. 22, 1965, now U.S. Pat. No. 3,565,781 as well as in the art cited above.

Still another electrodepositable composition of desirable properties comprises an alkyd-amine vehicle, that is, a vehicle containing an alkyd resin and an amine-aldehyde resin. A number of these are known in the art and may be employed. Preferred are water-dispersible alkyds such as those in which a conventional alkyd (such as a glyceryl phthalate resin) which may be modified with drying oil fatty acids, is made with a high acid number (e.g., 50 to 70) and solubilized with ammonia or an amine, or those in which a surface-active agent, which as a polyalkylene glycol (e.g., Carbowax") is incorporated. High acid number alkyds are also made by employing a tricarboxylic acid, such as trimellitic acid or anhydride, along with a polyol in making the alkyd.

The above alkyds are combined with an amine-aldehyde resin, such as those described hereinabove. Preferred are water-soluble condensation products of melamine or a similar triazine with formaldehyde with subsequent reaction with an alkanol. An example of such a product is hexakis(methoxymethyl)melamine.

The alkyd-amine compositions are dispersed in water and they ordinarily contain from about 10 percent to about 50 percent by weight of amine resin based on the total resinous components.

Yet another electrodepositable composition of desirable properties comprises mixed esters of a resinous polyol. These resin esters comprise mixed esters of an unsaturated fatty acid adduct. Generally the polyols which are utilized with these resins are essentially any polyol having a molecular weight between about 500 and 5,000. Such resinous polyols include those resinous materials containing oxirane rings which can be opened in, prior to, or during the esterification reaction to provide an apparent hydroxy site. The vehicle resins are formed by reacting a portion of the hydroxyl groups of the polyol with the fatty acid, the ratio of the reactions being such that at least an average of one hydroxyl group per molecule of the polyol remains unreacted. The remaining functionality is then reacted with the unsaturated fatty acid adduct of an olefinically unsaturated dicarboxylic anhydride, such as maleic anhydride, this second esterification reaction being conducted under conditions so that esterification occurs through the anhydride ring, thereby introducing free acid groups into the molecule. Mixed acids of the class described are disclosed in Belgian Pat. No. 641,642, as well as in copending application Ser. No. 568,144, filed July 27, 1966 now abandoned.

In order to produce an electrodepositable composition, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used. There may likewise be used ammonia or organic bases, especially watersoluble amines, such as, for example, the mono-, diand trilower alkylamines such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine,

dipropylamine, dibutylamine, and m-methyl-butylamine, triethylamine, tributylamine, methyldiethylamine, dimethylbutylamine, and the like; cyclic amines such as morpholine, pyrrolidine, piperidine; diamines such as hydrazine, methylhydrazine, 2,3-toluene diamine, ethyl diamine and piperizine and substituted amines such as hydroxylamine, ethanolamine, diethanolamine, butanolamine, hexanolamine and methyl diethanolamine, octanolamine, diglycolamine, and other polyglycolamines, triethanolamine, and methylethanolamine, n-amino-ethanolamine and methyldiethanolamine and polyamines such as diethylene triamine.

There may be present in the electrodepositable composition any of the conventional types of pigments employed in the art. There is often incorporated into the pigment composition a dispersing or surface-active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the bath in order to aid dispersibility, viscosity and/or film quality, a nonionic modifier or solvent. Examples of such materials are aliphatic, naphthenic and aromatic hydrocarbons or mixtures of the same; monoand dialkyl ethers of glycols, pine oil and other solvents compatible with the resin system. The presently preferred modifier is 4-methoxy-4-methyl-pentanone-2 (Pent- Oxone). 7

There may also be included in the coating composition, if desired, additives such as antioxidants, for example, orthoamylphenol or cresol. It is especially advantageous to include such antioxidants in coating compositions which are used in baths which may be exposed to atmospheric oxygen at elevated temperatures and with agitation over extended periods of time.

Other additives which may be included in coating compositions, if desired, include, for example, wetting agents such as petroleum sulfonates, sulfated fatty amines, or their amides, esters of sodium isothionates, alkyl phenoxypolyethylene alkanols, or phosphate esters including ethoxylated alkylphenol phosphates. Other additives which may be employed include antifoaming agents, suspending agents, bactericides, and the like.

In formulating the coating composition, ordinary tap water may be employed. However, such water may contain a relatively high level of metals and cations which, while not rendering the process inoperative, may result in variations of properties of the baths when used in electrodeposition. Thus, in common practice, deionized water, i.e., water from which free ions have been removed by the passage through ion exchange resins, is invariably used to make up coating compositions of the instant invention.

in addition to the electrodepositable vehicle resins described above, there may be present in the electrodepositable composition other resinous materials which are noncarboxylic acid materials. For example, as shown above, there may be added up to about 50 percent by weight of an aminealdehyde condensation product.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in U.S. Pat. No. 3,392,165, which is incorporated herein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is equally applicable to cationic type vehicle resins, that is, polybases solubilized by means of an acid, for example, an amine-terminated polyamide or an acrylic polymer solubilized with acetic acid.

Another case of such cationic polymers is described in copending application Ser. No. 772,366, filed Oct. 28, I968.

In a manner similar to the anionic resins described above, the cationic resins may be formulated with adjuvants, such as pigments, solvents, surfactants, cross-linking agents, and the like.

The polyacids are anionic in nature and are dispersed or dissolved in water with alkaline materials such as amines or alkaline metal hydroxides and, when subjected to an electric current, they migrate to the anode. The polybasic resins. solubilized by acids, are cationic in character and when these resins are water-dispersed or solubilized with an acid such as acetic acid, the material deposits on the cathode under an electric current.

The invention is further described in conjunction with the following example which is to be considered illustrative rather than limiting. All parts and percentages in the example and throughout this specification are by weight unless otherwise stated.

EXAMPLE The articles coated in this example were cans of tin free, steel-chromium type can stock welded by the method of U.S. Pat. No. 3,468,453.

The pretreatment bath comprised a 5 percent phosphoric acid solution at 140 F.

The electrodeposited bath had the following composition:

Parts by Weight Acrylic resin 223.0

(74% solids) 55 percent buryl acrylate 25 percent styrene 5 percent hydroxypropyl methacyrlate l5 percent methacrylic acid (26% solvent) percent methyl cellosolve 20 percent isopropyl alcohol Viscosity 860,000 centipoises; acid value 7| Melamine resin (XM H6) 42.0 Diisopropanolamine 23.4 Dcionized water 3.5 I l .6

The welded cans were treated in the acid bath, water rinsed and air dried by a compressed air stream, and then electrocoated at 600 volts for 4 seconds at a bath temperature of F. and the coating baked for 4 minutes at 385 F. Untreated cans had an MBI of 10-15; cans treated in the acid bath over the range of 30 seconds to 2 minutes showed an MB! of 0-1. (MBI being Microbubble lndex, representing the percent volume of bubbles in the seam area.)

Likewise, at a pretreatment temperature of 120 F. with the remaining conditions constant, MBI ranged from 0 to 4.

In a similar manner, varying metals and welding by varied techniques, as well as treatment baths of varying temperatures and concentrations may be employed to obtain results within the scope of this invention. Likewise, the electrodepositable resin may be replaced with any of the resins hereinabove disclosed.

According to the provisions of the Patent statutes, there are described above the invention and what are now considered its best embodiments; however, within the scope of the appended claims, it is to be understood that the invention can be practiced otherwise than as specifically described.

We claim:

1. A method of pretreating a welded metal seam prior to electrodeposition of an ionically solubilized organic resin which comprises contacting the welded seam with an aqueous solution of phosphoric acid wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

2. A method as in claim 1 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure. p

3. A method as in claim 2 wherein the metal is a low carbon steel having a passive coating.

4. A method as in claim 1 which comprises A. contacting a welded metal seam with an aqueous solution of phosphoric acid at a temperature above about 100 F. for a time sufficient to reduce microbubble formation during electrodeposition;

B. rinsing said seam;

C. substantially immediately, at least within about 15 minutes, subjecting said seam to electrodeposition wherein an ionically solubilized synthetic resin is coated upon said seam.

5. A method as in claim 4 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

6. A method as in claim 5 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.

7. A method as in claim 6 wherein the metal is a low carbon steel having a passive coating.

8. A method as in claim 4 wherein the ionically solubilized resin is a polycarboxylic acid resin.

9. A method as in claim 8 wherein the resin is an acrylic resin.

10. A method as in claim 9 wherein said acid resin contains hydroxyl groups.

11. A method as in claim 10 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

12. A method as in claim 11 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.

13. A method as in claim 12 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

14. A method as in claim 4 which comprises A. contacting a welded metal seam with an aqueous solution of phosphoric acid at a temperature above about F. for a time sufficient to reduce microbubble formation during electrodeposition;

B. rinsing said seam;

C. subjecting said seam to electrodeposition wherein an ionically solubilized synthetic resin is coated upon said seam within at least 5 minutes.

15. A method as in claim 14 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

16. A method as in claim 15 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.

17. A method as in claim 16 wherein the metal is a low carbon steel having a passive coating.

18. A method as in claim 14 wherein the ionically solubilized resin is a polycarboxylic acid resin.

19. A method as in claim 18 wherein the resin is an acrylic resin.

20. A method as in claim 19 wherein said acid resin contains hydroxyl groups.

21. A method as in claim 20 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

22. A method as in claim 20 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.

23. A method as in claim 22 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.

Claims

2. A method as in claim 1 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.
3. A method as in claim 2 wherein the metal is a low carbon steel having a passive coating.
4. A method as in claim 1 which comprises A. contacting a welded metal seam with an aqueous solution of phosphoric acid at a temperature above about 100* F. for a time sufficient to reduce microbubble formation during electrodeposition; B. rinsing said seam; C. substantially immediately, at least within about 15 minutes, subjecting said seam to electrodeposition wherein an ionically solubilized synthetic resin is coated upon said seam.
5. A method as in claim 4 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.
6. A method as in claim 5 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.
7. A method as in claim 6 wherein the metal is a low carbon steel having a passive coating.
8. A method as in claim 4 wherein the ionically solubilized resin is a polycarboxylic acid resin.
9. A method as in claim 8 wherein the resin is an acrylic resin.
10. A method as in claim 9 wherein said acid resin contains hydroxyl groups.
11. A method as in claim 10 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.
12. A method as in claim 11 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.
13. A method as in claim 12 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.
14. A method as in claim 4 which comprises A. contacting a welded metal seam with an aqueous solution of phosphoric acid at a temperature above about 100* F. for a time sufficient to reduce microbubble formation during electrodeposition; B. rinsing said seam; C. subjecting said seam to electrodeposition wherein an ionically solubilized synthetic resin is coated upon said seam within at least 5 minutes.
15. A method as in claim 14 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.
16. A method as in claim 15 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.
17. A method as in claim 16 wherein the metal is a low carbon steel having a passive coating.
18. A method as in claim 14 wherein the ionically sOlubilized resin is a polycarboxylic acid resin.
19. A method as in claim 18 wherein the resin is an acrylic resin.
20. A method as in claim 19 wherein said acid resin contains hydroxyl groups.
21. A method as in claim 20 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.
22. A method as in claim 20 wherein the seam is a lap seam welded below the fusion temperature of the metal by a combination of heat and pressure.
23. A method as in claim 22 wherein the aqueous solution comprises between about 2 percent and about 7 percent by weight of phosphoric acid.