US3502511A - Electrophoretic coating process - Google Patents

Description

United States Patent 3,502,511 ELECTROPHORETIC COATING PROCESS John W. Forsberg, Mentor-on-the-Lake, Ohio, assignor to The Lubrizol Corporation, Wicklilfe, Ohio, at corporation of Ohio No Drawing. Original application Jan. 15, 1965, Ser. No. 425,949, now Patent No. 3,364,081, dated Jan. 16, 1968. Divided and this application Aug. 30, 1967, Ser. No.

Int. Cl. C23f 7/10; C23c 1/10 U.S. Cl. 148-6.15 3 Claims ABSTRACT OF THE DISCLOSURE This application is a division of application Ser. No. 425,949, now US. Patent No. 3,364,081 filed Jan. 15, 1965.

This invention relates to novel aqueous phosphating solutions. In a more particular sense, it relates to novel aqueous phosphating solutions containing lead ions; to the method of phosphating metal articles with these solutions; and to the metal articles which have been phosphated by these solutions.

It is well-known in the metal finishing art that metal surfaces such as aluminum, ferrous, and zinc surfaces may be provided with an inorganic phosphate coating by contacting them with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of siccative organic coating compositions such as paint, laquer, varnish, primers, synthetic resins, enamel, and the like.

Such inorganic phosphate coatings are generally formed on a metal surface by means of aqueous solutions which contain the phosphate ion and, optionally, certain auxiliary ions including metallic ions such as sodium, manganese, zinc, cadmium, copper, and antimony ions. These aqueous solutions may also contain non-metallic ions such as ammonium, chloride, bromide, fluoride, nitrate, sulfate, and borate ions. These auxiliary ions influence the reaction with the metal surface, modify the character of the phosphate coating, and adapt it for a wide variety of applications. Ordinarily, an oxidizing agent such as sodium chlorate, potassium perborate, sodium nitrate, ammonium nitrate, sodium chlorite, potassium perchlorate, or hydrogen peroxide is included in the phosphating solution to depolarize the metal surface being treated and thereby increase the rate at which the phosphate coating is formed on the metal surface. Other auxiliary agents such as anti-sludging agents, coloring agents, and metal cleaning agents may also be incorporated in the phosphating solution. One common type of commercial phosphating solution which contains zinc ion, phosphate ion, and a depolarizing agent is made by dissolving small amounts of zinc dihydrogen phosphate, sodium nitrate, and phosphoric acid in water.

Such phosphating solutions, and others known in the metal finishing art, have been useful in providing an adherent, integral phosphate coating on metal articles, particularly ferrous metal articles and galvanized ferrous metal articles, thereby improving the adhesion thereto of a film of a subsequently applied siccative organic coating composition. However, the use of the prior art phosphating process has been seriously curtailed in some applications for one or more of the following reasons: (1) the phosphating processes required that the phosphating solution be maintained at a temperature of at least about F.; (2) metal surfaces which had been phosphated could not be welded satisfactorily; and (3) the electrodeposition of paint over phosphated metal articles required higher wattages than for plain steel. Conventional commercial phosphate coatings appear to interfere with the passage of the welding current and result in poor welds and/ or premature destruction of the electrodes by excessive arcing.

Any phosphating process adds to the cost of finishing a metal article and, in many instances, this added cost becomes prohibitive when a metal article must be formed and spot-welded before it is phosphated. The formed and spot-welded metal article may of such dimensions and shape that it cannot be phosphated conveniently over its entire surface by ordinary commercial phosphating procedures. Although it has been possible to phosphate unformed steel, particularly rolls of strip steel stock, by high speed, low cost, dip-phosphating or spray-phosphating processes in a continuous manner, the spot-welding of such phosphating steel has been unsuccessful for the reasons indicated above. Furthermore, not all phosphate coatings are able to withstand the high pressures and temperatures required in the forming of the metal. Thus, a definite need has existed for a phosphate coating which can be easily applied to metal surfaces and can be easily Welded and formed.

Accordingly, it is an object of this invention to provide novel aqueous phosphating solutions.

Another object is to provide novel aqueous phosphating solutions which are adapted for phosphating metal articles.

Another object is to provide novel aqueous phosphating solutions which are adapted for phosphating ferrous, zinc, and aluminum surfaces.

Another object is to provide a method for forming an adherent phosphate coating on metal articles.

Another object is to provide metal surfaces which have been provided with an adherent prosphate coating.

Another object is to provide metal surfaces which have been provided with an adherent phosphate coating which is effective to inhibit the corrosion of the metal.

Another object is to provide metal surfaces which have been provided with an adherent phosphate coating which is effective to improve the drawing properties of the metals.

Another object is to provide metal surfaces which have been provided with an adherent phosphate coating which does not interfere with the weldability of the metal.

Still another object is to provide metal surfaces which have been provided with an adherent phosphate coating which is effective to improve the electrophoretic deposition of paint films on the metal surfaces.

Still another object is to provide metal surfaces which have been provided with an adherent phosphate coating which is effective to improve the drawing properties of the metal and which coating does not diminish the weldability of the metal.

A further object is to provide metal surfaces which have been provided with an adherent phosphate coating which effectively prevents corrosion, improves the drawing properties of the metal, does not interfere with weldability of the metal and when free of drawing lubricants and other industrial soils nerves as an excellent base for paints.

These and other objects of the invention are achieved by means of an aqueous phosphating solution consisting essentially of phosphate ion, nitrate ion, and lead ion.

In most instances, the aqueous phosphating solution will have a total acidity within the range of from about 5 to 850 points and will consist essentially of from about 0.10% to about 40% of phosphate ion, from about 0.20% to about 55% of nitrate ion, and from about 0.20% to about of lead ion.

The addition of a small amount of a halide ion such as chloride, bromide, fluoride, and iodide ions is found to be beneficial in that it tends generally to provide a coating having fine crystals. As little as 0.001% or as much as 0.5% or more of a halogen ion may be incorporated into the phosphating solution, generally in the form of their salts such as sodium fluoride, sodium chloride, sodium bromide, sodium iodide, potassium chloride, ammonium chloride, etc.

In order to provide commercially satisfactory coating weights and coating speeds, the aqueous phosphating solutions should generally have a total acidity within the range of from about 50 to about 400 points, a phosphate ion concentration of from about 0.5% to about a nitrate ion concentration of from about 2.0% to about 33%, and from about 1.0% to about 25% of lead ion. The use of aqueous phosphating solutions having a total acidity higher than 400 points, e.g., 700 points, makes it possible to form satisfactory phosphate coatings on ferrous, zinc, and aluminum surfaces in as short a time as one second. The commercial applications of such rapid phosphating processes are manifold. For example, it is well-adapted for the continuous phosphating of coldrolled strip steel and zinc surfaces at speeds consonant with those employed in modern, high production rolling mills. The term points total acidity as employed in the phosphating art and this specification represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of a phosphating solution in the presence of phenolphthalein 38 an indicator.

In view of the extensive commercial development of :he phosphating art and the many journal publications and patents describing the application of phosphating solutions, it is believed unnecessary to lengthen this spezification unduly by a detailed recitation of the many ways in which the phosphating step may be accomplished. Suffice it to say that any of the commonly used phos- :hating techniques such as spraying, brushing, dipping, foller-coating, and flow-coating can be employed. The

:emperature of the aqueous phosphating solution may vary Within wide limits, e.g., from about room temperiture to about 240 F. In general, the best results are )btained when the aqueous phosphating solution is used it a temperature of from about room temperature to lbOUt 150 F. A particular feature of the aqueous phos- Jhating solutions of this invention is that a satisfactory phosphate coating can be deposited in a relatively short aeriod of time at room temperature. If desired, however, :he aqueous phosphating solution may be used at higher :emperatures, e.g., 225 F., 250 F., or even 300 F, by :mploying superatmospheric pressures.

In the accepted practice of phosphating metal articles, 1 surface is usually cleaned by a physical and/or chemi- :al means to remove any grease, dirt, or oxides. The :leaned article is then ordinarily rinsed with water before Jeing subjected to the phosphating treatment. The phos- Jhating operation is usually carried out until the weight )f the phosphate coating formed on the metal surface is at least about 25 milligrams per square foot of surface area and is preferably within the range of from about 0 about 1000 milligrams per square foot. The time reuired to form the phosphate coating will vary accordng to the temperature, the concentration of the phoshate solution employed, the particular technique of applying the phosphating solution, and the coating weight desired. In most instances, however, the time required to produce a phosphate coating of a weight suitable for the purposes of this invention will be within the range of from about 5 seconds to about 5 to 10 minutes. When phosphating metal surfaces with high total acid solution, the immersion technique is preferred. This can be a simple dipping technique or a continuously moving kind of immersion exemplified by the immersion of moving steel strip or zinc coated stock in the phosphating solution by means of submerged rollers or other devices which serve the same purpose. Similarly, plates of heavy gauge steel may be conveyed continuously through the phosphating solution. The temperature of the phosphating solution should be preferably between 200 F. for rapid phosphating action and the total immersion time being from about 1 to 20 seconds. A more complete discussion of the utility of high total acid aqueous phosphating solutions is set forth in US. Patent No. 3,144,360. As mentioned previously, one of the novel features of the phosphating solutions of this invention is that the phosphating process can be carried out at lower than normal temperatures.

Upon completion of the phosphating operation, the phosphated metal article is generally rinsed with water and/ or a hot dilute aqueous solution of chromic acid containing from about 0.01 to about 0.2% of CrO The chromic acid rinse appears to seal the phosphate coating and improve its utility as a base for the application of the siccative organic coating. Dilute aqueous solutions of metal chromates, metal dichromates, chromic acid-phosphoric acid mixtures, and chromic acid-metal dichromate mixtures may also be used in place of the dilute aqueous chromic acid.

The phosphating solutions of this invention can be prepared by dissolving suflicient phosphoric acid, nitric acid, and lead oxide and an alkali metal halide (if desired) in Water to yield the desired weight percentages of phosphate, nitrate, lead, and halide ions. The points total acid may be adjusted by additional phosphoric and/or nitric acid. Other salts, bases, and mineral acids such as ammonium dihydrogen phosphate, sodium dihydrogen phosphate, potassium dihydrogen phosphate, potassium nitrate, sodium nitrate, ammonium nitrate, lead nit-rate, ammonium chloride, sodium chloride, sodium bromide, sodium fluoride, and nickel nitrate, may be substituted in the preparation of the phosphating solutions of this invention. Thus, it is apparent that the ions of the phosphating solution of this invention may be derived from a variety of compounds and it appears to be of little consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts selected to provide the required ions, the resulting solution is effective to serve the purposes of this invention. It is necessary only that these salts and acids provide the required characterizing ions.

The phosphating solution may be prepared as a concentrate or as a dilute solution ready for use. When preparing concentrates, it is generally desirable to incorporate an alkali metal nitrate into the concentrate to help maintain all of the components in solution.

The presence of the lead ion in the aqueous phosphating solutions of this invention is essential if the novel properties of the phosphate coatings deposited by these aqueous phosphating solutions are to be obtained. The nitrate ion serves as an oxidizing agent to depolarize the metal surface and increase the coating speed of the phosphating solution. The incorporation of halogen ions such as the fluoride and chloride ions is not essential but appears to improve the crystallinity and uniformity of the deposited coating.

Often, the coating action of the phosphating solutions of this invention can be improved by the addition of certain surface-active agents. These agents also serve as dispersants in the phosphating solution and help to maintain the ingredients in solution. Examples of such agents which have been found to be particularly effective are the ethylene oxide condensates, particularly those containing from about 3 to about 25 polyoxythylene groups such as polyoxyelthylene derivatives of oleic acid and polyoxyethylene alkyl phenol derivatives. Also useful as surface-active agents in the coating compositions of this invention are sodium alkyl sulfates, and sulfonated hydrocarbons such as alkyl naphthalene sulfonic acid. It will be appreciated that the surface-active agent selected must be compatible with the acidic phosphate solutions of this invention. Another important requirement of the surface-active agent in that the agent, when added to the phosphating solution, should improve the performance of the solution without afiecting or inhibiting the essential ingredients of the aqueous phosphating solutions of this invention.

The nature of the coating compositions deposited by the aqueous phosphating solutions of this invention is not known with certainty but preliminary investigation (X-ray analysis) indicates that the coating is primarily a lead phosphate of apatite structure (Pb (OH) (PO When a halogen such as chlorine is incorporated into the phosphating solution, the structure of the coating composition appears to be mostly 'Pb (Cl (P00 or a mixture of these two lead phosphates.

Specific examples of the phosphating solutions of this invention are shown in Table I. Except for the points total acid, the values given in the table indicate the percentages by weight of the various ions in the phosphating solution.

The phosphating solutions described in Table I are prepared in the following manner. All parts are by Weight unless otherwise specified.

SOLUTION A To 640 parts of water there is added 219 parts of commercial 70% nitric acid and 24.6 parts of commercial 85% phosphoric acid. The mixture is stirred and 80.4 parts of lead oxide are added. The stirring is continued until all of the lead oxide is in solution whereupon 1.5 parts of sodium chloride in 1 part of iso-octyl phenyl polyethoxy ethanol (wetting agent) are added. The working solution is prepared by adding 1,425 parts of water to the above solution.

6 SOLUTION F SOLUTION G A concentrate is prepared by adding 2,170 parts of nitric acid, 600 parts of potassium nitrate, and 246 parts of commercial 85% phosphoric acid to 4,850 parts of water. The mixture is stirred until all of the potassium nitrate is dissolved whereupon 805 parts of lead oxide is added. After all of the lead oxide is dissolved, 15 parts of sodium chloride and 3 parts of the wetting agent 01 Solution A are added. The working solution having a total acidity of 106 is prepared by adding 1,615 parts of the above concentrate to 2,885 parts of water.

SOLUTION H The phosphate solution having a total acidity of 254 points is prepared by adding 230 parts of the concentrate of Solution B to 84 parts of water.

SOLUTION I To 230 parts of the concentrate of Solution B there is added 84 parts of water and 83 parts of commercial 85 phosphoric acid. The resulting solution has a total acidity of 708 points.

SOLUTION J A phosphating solution having a total acidity of 608 points is prepared by dissolving 230 parts of the concentrate of Solution B in 84 parts of water and 70 parts of commercial 85% phosphoric acid.

TABLE I.PHOSPHATING SOLUTIONS Ions A B o D E r G H I J K L M N 0 P04 0.851 2. 20 0.14 0.55 1.13 2.10 0.840 4.52 10.50 11.12 2.27 2.27 2.27 0.920 1.00 N05 0. as s. 09 0. 5s 2. 25 4. s. 10 7. 07 1s. 00 29. 93 25. 20 8.97 8.97 8. 07 0. 80 s. 52 Pb 2. 02 4. 11 0. 20 1. 03 2. 0s 4. 14 2. es s. 22 4. 02 5. 09 4. 12 4. 12 4. 12 1. 72 3. 41 01-. 0. 05s 0. 002 0.004 0.015 0.030 0.047 0.057 0.124 0.075 .0.077 00123 0.003 F 0.0070 Br 0.0034

Points total acid 110 100 9.0 20.5 70.5 1.47 100 254 70s 008 150 157 157 110 159 SOLUTION B SOLUTION K A concentrate is prepared by adding 204 parts of commercial 70% nitric acid, 54.4 parts of potassium nitrate, and 53.3 parts of commercial 85% phosphoric acid to 540 parts of water. The mixture is stirred and 86 parts of lead oxide is added followed by the addition of 2 parts of sodium chloride. A working solution having a total acidity of 160 points is prepared by adding 223 parts of the above concentrate to 239 parts of water.

SOLUTION C A phosphating solution having a total acidity of 9 points is prepared by dissolving 230 parts of the concentrate prepared for Solution B in 7,457 parts of water.

SOLUTION D A phosphating solution having a total acidity of 36.5 points is prepared by dissolving 230 parts of the concentrate of Solution B in 1,680 parts of water.

SOLUTION E A phosphating solution having a total acidity of 76.5 points is prepared by dissolving 230 parts of the concentrate of Solution B in 727 parts of water.

To 223 parts of the concentrate of Solution K, there is added 239 parts of water and 0.35 part of sodium fluoride.

SOLUTION M To 223 parts of the concentrate of Solution K there is I added 239 parts of water and 0.84 part of sodium bromide.

SOLUTION N To 936 parts of water there is added 108 parts of commercial 70% nitric acid, 20 parts of lead oxide, 12

parts of ammonium dihydrogen phosphate and 0.4 part of sodium chloride.

SOLUTION O A concentrate is prepared from 1,315 parts of commercial 70% nitric acid, 148 parts of commercial 85% phosphoric acid, 360 parts of potassium nitrate, 482 parts of lead oxide, 9 parts of sodium chloride and 2 parts of the wetting agent of Solution A. The solution is prepared by dissolving 4,995 parts of the above concentrate in 7,605 parts of Water.

One feature of the combination of ions which characterizes the phosphating solutions of this invention is that such solutions may be utilized to deposit uniform coatings on either ferrous metal articles, galvanized ferrous metal articles, aluminum articles, and mixtures of ferrous and galvanized ferrous metal articles. A particular feature of the phosphating solutions of this invention is the unique properties of the coatings which are deposited by these solutions on metal articles. For example, metal surfaces which have been coated in accordance with the process of this invention remain weldable. Furthermore, the coating deposited by the solutions of this invention improves the drawing properties of the metals thus coated, and facilitates cold drawing in manufacturing operations. The smooth, uniform coating also provides for protection against corrosion, and is an excellent base for siccative organic coating compositions.

For optimum efiiciency, the amounts of the various ions in the aqueous phosphating solutions of this invention nay be varied depending upon the type of metal article being treated and the particular technique being employed. For example, when the immersion technique and a high aoint bath are utilized, the phosphate ion, nitrate ion, and .ead ion concentration may be as high as 40%, 55% and 30% respectively. More dilute concentrations are preferred where rapid phosphating is not essential. Thus, metal articles are easily phosphated by immersion for 1 period of from seconds to 1 to 2 minutes in a phosphating solution having a total acidity of about 50 :0 about 400 points.

The following examples are submitted to illustrate the itility and the properties of the coatings deposited by he aqueous phosphating solutions of this invention.

Example 1 The ability of the phosphate coatings deposited by the ;olutions of this invention to resist corrosion is shown by ;he results of the following outdoor humidity test. In his test 4" x 8" ZO-gauge SAE 1020 cold-rolled, phosihated steel panels (no top coat) were placed in a horizontal position approximately two feet above a pan of vater in an enclosed outdoor shed. After being exposed n this manner for seven weeks, the panels were removed and inspected for rust. The results of this outdoor humidity est, as shown in Table II, indicate that the phosphate :oatings deposited by the aqueous phosphating solutions )f this invention are effective to reduce the corrosion of ;teel surfaces.

TABLE II.SEVEN WEEK OUTDOOR HUMIDITY TEST After a metal article has been phosphated in accordtnce with the present invention, it is often desirable to .pply a decorative and protective top-coat of a siccative vrganic coating composition such as paint, lacquer, varllSh, synthetic resins, enamel, and the like. Examples of ynthetic resins which may be used are the acrylic, alkyd, :poxy, phenolic, and polyvinyl alcohol resins.

The following examples illustrate the siccative organic coating compositions which may be applied to metal articles which have been phosphated in accordance with this invention.

Coating composition A Short soya alkyd (50% solution in xylene of alkyd resin prepared from 41.6 parts of phthalic anhydride, 18.4 parts of glycerol and 40 parts soya bean acid) 45 Xylene 14.8 Cobalt naphthenate (6% Co) 0.1 Anti-skinning agent 0.1

Coating composition B (red oxide resin modified alkyd baking primer):

Red iron oxide Fe O 20.9 Barium sulfate 8.2 Magnesium silicate 8.2 Resin modified alkyd (70% xylene) 28.6 Naphtha 6.6 Xylene 21.8 Mineral spirits 5.5 Cobalt naphthenate (6% Co) 0.2 Coating composition C (White alkyd baking topcoat):

Titanium dioxide 29.2 Medium castor alkyd (50% solution in xylene of an alkyd resin prepared from 38 parts of phthalic anhydride, 14 parts of glycerol, and

48 parts of castor oil) 48.6 Mineral spirits 19.9 Xylene 2.2 Cobalt naphthenate (6% Co) 0.1

Coating composition D (white acrylic baking topcoat):

Titanium dioxide 25.0 Thermosetting acrylic resin (50% in xylene) 60.0 Xylene 12.2 Cellosolve acetate 2.7 Anti-skinning agent 0.1 Coating composition E (white alkyd baking top coat):

Titanium dioxide 28.4 Medium cottonseed alkyd resin (21 60% solution of an alkyd prepared from 40 parts of phthalic anhydride, 25 parts of glycerol, and

35 parts of cottonseed oil in xylene.) 48.8 High flash naphtha 1.5 Xylene 21.2 Cobalt naphthenate (6% Co) 0.1

Coating composition F (white vinyl baking topcoat):

Titanium dioxide 20.0 Resin stabilizer 0.7 Vinyl chloride-vinyl acetate copolymer (85:15)w 12.5 Toluene 29.5 Methyl isobutyl ketone Epichlorohydrin Methanol 0.1 Tricresyl phosphate 7.2 Coating Composition G (white modified alkyd baking topcoat):

Titanium dioxide 26.8 Zinc oxide 1.4

Medium coconut alkyd resin (60% solution in xylene of an alkyd resin prepared from 40 parts of phthalic anhydride, 25 parts of glycerol and 35 parts of coconut oil) 36.4

Urea (50% xylene solution) 9.4 Melamine (50% xylene solution) 9.4 Xylene 16.6

Petroleum spirits Methylene bis-2,6-di-t-butylphenol Cobalt naphthenate (6% Co) Coating composition I (white alkyd baking topcoat):

Titanium dioxide 29.1 Medium castor alkyd (50% solution in xylene of an alkyd resin prepared from 37 parts of phthalic acid, 15 parts of glycerline, and 48v parts of castor oil) 48.7 Mineral Spirits 22.1 Cobalt Naphthenate (6% Co) 0.1

Application of the organic coating compositions can be effected by any of the ordinary techniques such as brushing, spraying, dipping, roller-coating, flow-coating, etc. The topcoated phosphated article is dried in the manner best suited for the particular siccative organic coating composition employed such as air-drying at ambient temperature, drying in a current of hot air, baking in an oven, or baking under a battery of infra-red lamps. In most instances, the thickness of the dried film of the siccative organic coating composition will be within the range of from about 0.1 to about 10 mils, more often from about 0.3 to about mils.

The following test is used to demonstrate the efiicacy of the aqueous phosphating solutions of this invention in improving the adhesion of films of siccative organic coating compositions to metal articles such as aluminum, steel, and galvanized steel. The results of such test indicate clearly the utility of the phosphating solutions of this invention.

Salt fag test.A number of 4" x 8" panels of 20- gauge SAE 1020 cold-rolled steel Were cleaned, phosphated with various phosphating solutions according to the sechedule detailed in Table III, rinsed with cold water, and finally spray rinsed for 10 seconds at 75- 80 F. with a dilute aqueous zinc dichromate solution (1 gram CrO /liter). The phosphated panels were then sprayed with a one-coat white, amine modified alkyd based appliance paint which was then baked for 30 minutes at 300 F. The average combined coating thickness was 1 mi]. The paint film on each panel was ruptured down to the bare metal by scoring a 6-inch line on the surface of each panel. The scored panels were then subjected to the salt-fog test described in ASTM Procedure B117-62. On this test the panels are placed in a cabinet containing a 5% aqueous sodium chloride solution at 95 F. Air is bubbled through the solution to produce a corrosive salt atmosphere which acts on the surface of the test panels, suspended above the level of the salt solution.

The panels are allowed to remain in this atmosphere for 168 hours whereupon they are removed, washed with water, and dried with a cloth. A pressure sensitive tape is then applied to the panel and removed suddenly. This procedure is repeated until no more paint can be removed in this manner. The panels are then inspected to determine the amount (percent) of paint still adhering to the metal substrate. The loss of adhesion caused by corrosion from the scribed lines is measured in thirtyseconds of an inch. This corrosion along the scribed lines is called creep. The results of this salt-fog corrosion test, shown in Table III, indicate that the aqueous phosphating solutions of this invention substantially improve the rust preventive properties of panel coatings as demonstrated by the improved resistance to undercutting by corrosion from the scribed line and by the overall increased adhesion of the paint to the substrate.

TABLE III.SALT FOG TEST Paint base Test results (168 hrs.)

Immersion Soln Coating Paint; time temp. wt. Creep adhesion Solution (590.) F.) (mg/ft (N (percent) Working solution contained 2.8 g./liter of Fe++.

Example 3 As mentioned previously, the coatings deposited by the aqueous phosphating solutions of this invention are particularly useful in the preparation of metals for drawing and forming operations. Before a metal is subjected to a drawing operation, it is generally covered with a lubricant. Metal drawing lubricants are generally classified into two groups commonly referred to as wet lubricants and dry lubricants. Both types are applied as liquids, the wet lubricant remaining liquid, but the dry lubricant being dried to form a solid film. The wet lubricants are more easily applied and require a minimum amount of floor space and equipment. These lubricants are usually applied by spraying, swabbing, or with a roller. Spraying is generally unsatisfactory because of the waste due to over-spraying.

The application of a dry lubricant to a metal surface requires, in addition to the coating step, a drying step. Since the effectiveness of a dried lubricant is dependent upon the degree of dehydration of the film, the lubricity of the film increasing as the amount of residual water decreases, it is necessary to remove substantially all of the water from the film.

Examples of lubricants which have been found useful as aids in the drawing of metal include petroleum oils, e.g., 5,000 S.S.U. at F.; chlorinated wax; soaps prepared by neutralizing mixed fatty acids with a mixture of amines and caustic soda: dispersions of such soaps; beeswax; dry soap type films; heavy or light duty pigmented emulsions; heavy duty non-pigmented emulsions; etc. The following lubricants are examples of coating compositions which will produce films which have satisfactory drawing properties.

Drawing lubricant A: Percent Tetraethylene pentamine 3.2 Sodium hydroxide 3.5 Oleic acid 22.8

Water 71.5

Drawing lubricant B:

Triethanolamine 5.0 Sodium hydroxide 1.8 Tall oil 23.2

Water 70.0

Drawing lubricant C:

Monoethanolamine 2.8 Potassium hydroxide 2.5 Coconut fatty acids 24.7 Water 70.0

Drawing lubricant D:

Monoisopropanolamine 3.0 Potassium hydroxide 1.0 Soya bean fatty acids 23.5 Polyoxyethylene trioleate 8.0 Water 64.5

Drawing lubricant E:

Sodium tallow soap 16.0 Potassium carbonate 20.0 Boric acid 20.0 Borax 38.0

Polyethylene glycol (molecular weight 6000) 6.0

1 1 Drawing lubricant F: Percent Sodium tallow soap c- 15.0 Borax 85.0 Drawing lubricant G:

Sodium stearate 45 Sodium sulfite 4 Borax 38 Water 5 Drawing lubricant H:

Chlorinated wax (50% chlorine) SAE mineral lubrication oil The solid lubricants such as those illustrated by lubricants E and F may be applied to the metal surface in paste form by adding a small amount of water, or the metal surface may be coated by immersion in a boiling solution containing about 1 pound of the lubricant per gallon of water.

The lubricant which is chosen for this purpose must have the ability to allow the metal to flow properly and to minimize galling and scoring of the tools and/or the fabricated parts. Thus, an important feature of any lubricant system is the ability of the lubricant to reduce or control friction. The advantage of depositing the phosphate coating of this invention on metal surfaces prior to lubrication and drawing, is demonstrated by the following laboratory test which provides a method of measuring the coefiicient of friction of treated and untreated steel panels.

Friction test.In this test, a lubricated test strip (2 x 24'') is placed between a pair of flat polished dies upon which is placed a load by means of a calibrated torque wrench acting on a screw. The dies may be heated to increase the severity of the test condition. This assembly is mounted in a tensile testing machine and the metal strip is moved through the dies. The force (measured in pounds) required to move the strip at a rate of 4 inches per minute through the dies at a given temperature and iaw load is observed and recorded. The results of the test ire reported in terms of dynamic coeflicient of friction which is defined by the following equation:

Dynamic coefficient of friction force (lbs.) required to move the strip at rate of 4 inches/ minutes through the dies total jaw load (lbs.) The more desirable metal treatments, therefore, are charicterized by a low dynamic coeflicient of friction. This riction test is described in detail by W. J. Wojtowicz in Lubricating Engineering, vol. 11, pages 1747 (1955). The results of this simple sliding friction test on lubricated :old-rolled steel panels, summarized in Table IV, indicate :learly the utility of the coating compositions deposited by he aqueous phosphating solutions of this invention in relucing the coefiicient of friction of coated steel.

TABLE IV.COEFFICIENT OF FRICTION TEST! Phosphate Treatment Dwaamic Immersion Soln temp. Coating wt. coefi. of

lolution time (see) (F.) (mg/ft?) friction 2 lone 0. 102 75.80 393 0 062 A commercial drawing lubricant is applied over the panels before asting.

2 Jaw load, 20,000 lbs; die temperature, 180F.

Example 4 rent, welding time, and electrode pressure may be made in the manner known to those versed in the art of welding.

It has been found that the strength of a spotwelded phosphated metal article prepared in accordance with the process of this invention is equivalent to that of a conventional, welded uncoated metal article. Thus, the advantages of a metal article of this invention with respect to corrosion resistance and paint adhesion are achieved with no sacrifice of weld strength. Furthermore, tests indicate that the phosphate coating of this invention substantially extends the life of spot-welding electrodes by minimizing arcing and metal spitting. The following example illustrates how the life of spot-welding electrodes is extended.

Welding test.A large number of clean, degreased, 4" x 12" panels of SAE 1020 ZO-gauge cold-rolled steel were phosphated by immersion in solution F for 20 seconds at a temperature of about F. Thereafter, the panels were water-rinsed, and immersed for 10 seconds in an aqueous solution of chromic acid at room temperature and blown dry with warm air. The phosphate coating weight on the panels was found to be 300i mg. per square foot.

Using 1" x 3 coupons, 2 Welds were made 1 /2 inches apart having 0.25" to 0.26" diameter buttons. Then, using the phosphated 4" x 8" cold rolled steel panels, 2000 spot-welds were made with a commercial spot-welding machine equipped with electrodes having a 0.25" diameter. The force of the electrodes against the metal was 550 pounds and the welding current was maintained at 12,500 amperes. The welding rate was 15 spot-Welds per minute. Coupons (1" x 3") were welded and checked for button diameter every 250 welds by placing one coupon in a vice and removing the outer coupon with a pair of pliers. If the button measures less than 0.22" in diameter, the test was terminated as a failure.

Also, the test is terminated if there is constant or violent metal expulsion, or excessive smoke or fumes which are irritating to the operator.

After 2000 welds, the button diameter was observed to be substantially the same as the first button, and the electrodes were still in good condition and did not require dressing.

According to well-recognized industry standards, uncoated steel panels should give satisfactory welds, and the attainment of 1000 welds without electrode dressing is considered to be excellent. Thus, the consistency of the button diameter and the extended life of the spot-welding electrodes indicate that steel surfaces with have been coated in accordance with the process of this invention are easily welded.

Example 5 The durability and versatility of the phosphate coatings deposited by the solutions of this invention are shown by the following procedure which compares the corrosion resistance of phosphated and non-phosphated steel panels before and after being subjected to the sliding friction test of Example 3.

Salt-fog test.Two 2" x 5" strips were cut from the bottom of each panel which has been subjected to the friction test. One of the strips was placed over the other with an overlap of about 1 inch and they were spot-welded together. The lubricant used in the friction test was then removed by passing the panels through a four-stage spray line as follows:

Stage: Description 1 Treat the panel for 30' seconds at 170 F.

with an aqueous alkaline cleaner compounded from one ounce of a commercial alkali base cle a n in g composition and one gallon of water.

2 Spray rinse the panel for 60 seconds with water at a temperature of 100- 3 Spray rinse the panel for 60 seconds with cold water.

4 Force dry the panel.

The cleaned, welded panels were then spray-painted with a one-coat white, amine modified alkyd-based appliance paint which was then baked for 30 minutes at 300 F. The average combined coating thickness was approximately 1 mil. The paint film on each panel was ruptured by scoring a 6-inch line to the bare metal of each panel, and the panels were subjected to the salt-fog test described in Example 2 for a period of 72 hours. After being exposed to the salt fog, the panels were removed, washed with water, and dried with a cloth. The amount of paint still adhering to the metal substrate was determined according to the procedure described in Example 2. The results summarized in Table V, indicate that the coatings deposited by the aqueous phosphating solutions of this invention maintain their corrosion resisting properties even after the drawing operation.

Paint Retention Paint Base (percent, 72 hrs.)

Immer- Soln Coating After Before sion temp. wt. friction friction Solution time F.) (mg/ 13. test test None 57 65 60 75-80 393 83 87 30 75-80 135 77 72 Example 6 The unique properties of the coatings deposited by the aqueous phosphating solutions of this invention are further demonstrated by electrocoating steel panels (some containing a phosphate coating) with water-soluble paints.

Present methods of applying a paint film to metal articles (sprays, brushing, etc.) generally result in the loss of large amounts of paint due to over-spraying or to the accumulation of excessive paint material in certain areas of the metal article such as the edges. These techniques also suffer the disadvantage of often depositing uneven coatings, especially in hard-to-reach areas, which result in poor surface finishes. These difiiculties are, however, not encountered when the electrophoretic process of painting is utilized.

In the electrophoretic process of painting metal surfaces, the metal article to be coated is placed in an electrolytic solution which contains emulsified colloidal paint particles. An electric charge is routed through both the metal surface and the Water-based primer by placing a positive charge on the metal surface (acting as an electrode) and a negative charge on a second electrode, generally'the container. In this electric field, the colloidal particles of the primer which are in suspension move either toward the negative or positive electrode depending on the charge carried by the dispersed particles. In the instant situation, namely, the metal surface having a positive charge, negative paint particles are attracted to the metal surface. Upon contact with the metal surface, the colloidal particles lose their electrical charge, thereby breaking the emulsion and depositing as a coating on the electrode. The metal article is then removed from the solution, rinsed, and baked in an oven to cure the electrophoretically deposited coating.

The electrical potential applied in the process of electrophoresis will be determined by the thickness of the desired coating, the conductivity and composition of the coating bath, and the time alloted for the formation of the coating. Voltages of from 50 to about 1000 volts have proven satisfactory at a current density of from 0.1 to about amperes per square foot.

The electrolytic solutions which are utilized in the electrophoretic coating processes generally comprise emulsified paint particles in a colloidal state dispersed in a conducting liquid medium. A red primer which may be electrophoretically deposited is prepared as follows. A mixture of 63 parts (all parts are by weight unless otherwise specified) of a-film-forming material consisting of a styrene-allyl alcohol copolymer is mixed with 37 parts of linseed fatty acid, and the mixture is esterified at 260 C. to an acid number of 5 and a maximum viscosity of about 2.5 poises measured when the copolymer is reduced to a 60% solution in xylene. Fifty-one parts of the above prepared mixture is blended in a roller mill with 45 parts of red oxide pigment and 4 parts of linseed fatty acids. This blended mixture (38 parts) is further blended with 50 parts of the above film forming mixture, 11.8 parts of melamine formaldehyde, and 0.2 part. of cobalt naphthenate.

An emulsifying agent is prepared consisting of 3.5 parts of commercial 28% ammonium hydroxide and 96.5 parts of demineralized water. The emulsifying agent is added slowly to the paint mixture until the so-called inversion point is reached. The balance of the diluted ammonium hydroxide is then added, and the resulting emulsion is further refined in a colloid mill to produce a more stable emulsion.

The above described paint composition is useful as an automobile primer paint and may be electrophoretically deposited on automobile parts from an electrolyte prepared by mixing 2 parts of the above paint composition with 6 parts of water and /s part of concentrated ammonia. The resulting electrolyte is a colloidal dispersion.

In order to improve the adhesion of the paint to the metal surface, the metal surface is generally pretreated with, for example, a phosphate solution. Ordinarily the phosphate coatings deposited by the known phosphating solutions such as those which deposits zinc phosphate, manganese phosphate, one or more alkaline earth metal phosphates, or zinc or manganese phosphate modified with an alkaline earth metal phosphate, increase the electrical resistance of the metal surface. In such cases, greater power (wattage) is required to electrophoretically deposit a satisfactory coating of paint on the metal surface. However, metal surfaces which are coated with the phosphate coatings of this invention can be electrophoretically coated with paint at a power input which is less than that required for plain steel. This unusual effect is demonstrated as follows.

Paint electrodeposition test-In this test, an aqueous paint solution is prepared by dissolving one part of a commercial water-soluble red oxide primer in 3 parts of distilled water, placed in a stainless steel container, and maintained at a temperature of about 73 -76 F. A magnetic stirrer is used to keep the solution circulating. A 5 ampere power supply (0 to 500 volts DC output) is used to supply the current. The steel panels (4" x 8" 20 gauge strip steel) are pretreated and electrocoated one at a time 'by applying a positive, charge on the test panel and a negative charge on the steel container. The power supply is adjusted to give 1.1 amperes constant current in 2-5 seconds, and this current is maintained at this level while the voltage is increased to a level which will result in 45 seconds, in a deposition of a cured coating of about 1 mil thickness. This voltage is recorded as the Constant Peak Voltage. The power is turned off and the coated panel is removed from the electrolyte, rinsed with cold water, and baked in an oven at 350 ilO" F. for 20 minutes.

The results of this test (summarized in Table VI) indicate that steel panels which are coated in accordance with the process of this invention can be electrophoretically coated with paint at a lower voltage than e ther plain steel or steel coated with one of the ordinary phosphate coatings. The ordinary zinc phosphate coating deposited by the control solution actually acts as an insulator for Cured coating thickness (mils) turfaec treatment Constant peak voltage I one 175 l. Dontrol Soln 1 225 1. 0 iolution F 2 120 1. O

l The cleaned panels are sprayed with an aqueous phosphating solution ontaining 1.89% commercial 85% phosphoric acid, 0.266% commercial 0% nitric acid, 0.441% zinc oxide and 0.113% nickel nitrate hexahydrate or 1 minute at 150 F and rinsed with (1) water and (2) a dilute aqueous inc dichromate solution (1 gram CrO /liter).

2 The clean steel panels were immersed in the phosphating solution or 20 seconds at 126130F. and then rinsed as in (1) above.

Example 7 The utility of the coating compositions obtained by the rocess of this invention as paint bases is further illusrated by subjecting the electrophoretically painted, phos- Jhate coated steel panels to the salt fog corrosion test iescribed in Example 2 with the following changes: The

steel panels which were phosphated according to the process of this invention before the panels were formed and welded into the test shape. The procedure for preparing the zinc phosphate test shapes of (2) differs from the procedure in (3) because the zinc phosphate coated steel panels cannot be welded together, thus, the welding step must be carried out before the panels are phosphated with this solution.

The results which are summarized in Table VIII are, therefore, especially significant since the coating contained on the test shapes, prepared in accordance with the process of this invention, have been subjected to severe forming and welding conditions while the zinc phosphate coating deposited by the control solution was not. The test results show, therefore, that the phosphate coatings deposited by the aqueous phosphating solutions of this invention possess the unique properties of successfully withstanding the severe conditions of forming and welding without any loss of its excellent properties as a paint base.

TABLE VIII.SALT FOG TEST ON FORMED AND WELDED STEEL Areas of corrosion 1 The electrocoat primer described in Example 6. 2 The solution described in footnote 1 of Table VI. 3 The conditions were the same as those described in footnote 2 of Table VI.

TABLE VlI.-SALT FO G TEST ON STEEL PANELS IAlNTED BY ELECTRO PHO RESIS Test Results (240 hours) Creep (A2) .urfaoe treatment Edge Blister corrosion rior to painting (in ch) Jone 10,10 Va )ontrol solution 3, 2 o2, A0 olution F 2, 2 lio, As

1 Same as footnote 1 of Table VI. 2 Same as footnote 2 of Table Vi.

'Example 8 The unique ability of the phosphate coatings deposited iy the aqueous phosphating solutions of this invention to vithstand forming and welding, and to be satisfactory lectrocoated with paint is demonstrated by the followng test which subjects the coating to such procedures beore the steel panels are placed in a salt fog test.

In this test, several 4" X 8 20-gauge steel panels are ormed into a sawtooth shape, the long edge of the tooth eing 4 inches and the vertical edge being 1.75 inches. Each formed panel is then spot welded to a regular flat teel panel. These test pieces are then given the desired urface treatment and painted. Before being exposed to the alt-fog corrosion test, two holes are drilled through he center of the 4-inch section of the tooth, the center of ach hole being one inch from the outer edge of the ection. This procedure increases the severity of the test ince the bare metal is exposed at the edges of the hole.

Table VIII contains a comparison of the salt-fog test esults on the above described test shapes using (1) plain teel, (2) plain steel which was formed and welded beore a zinc phosphate coating was deposited, and (3) Example 9 The ease with which panels, coated in accordance with the process of this invention, can be welded and electrocoated is believed to be a result of the electrical conductivity of the novel phosphate coatings, another unique property. As mentioned previously, the use of the wellknown aqueous phosphating solutions such as those depositing a zinc phosphate type of coating may not be satisfactorily welded, and attempts to electrocoat such coated panels require added power input. In order to demonstrate the relative electrical resistance of various solid powders, including those obtained from the aqueous solutions of this invention and those obtained from aqueous phosphating solutions of the prior art, the following test was devised.

Each powder was packed in a glass tube 6 mm. in diameter and about 20 mm. long. The powder was packed in the tube with the help of a glass rod and cotton was used to plug the ends of the tube. After placing the tubes on their side, the probes of an ohmmeter were placed at each end and the resistance was observed on the ohmmeter. The results of this test which are contained in Table IX demonstrate that the apatitic lead phosphate coating obtained in the process of this invention is conductive while zinc phosphate is not conductive.

TABLE IX.ELECTRICAL RESISTANCE OF SOLIDS 2 The solid precipitated from the solution by slowly adding 360 ml of 2 1 N sodium hydroxide to 1,200 grams of the stirred solution. The precipitate wrs dried overnight at 0., and powdered. A slight yellow color developed. X-ray analysis indicates that the crystalline structure of this solid is apatitie in structure.

What is claimed is:

1. In the process of electrophoretically depositing a siccative organic coating on a metal surface, the improvement which comprises first forming an adherent phosphate coating on the metal surface by contacting said surface with an aqueous phosphating solution having a total acid ity of about 5-850 points and consisting essentially of about 0.10-40% phosphate ion, about 0.2055% nitrate ion, and about 0.2030% lead ion.

2. The method of claim 1 wherein the phosphating solution has a total acidity of about 50-400 points and consists essentially of about 0.535% phosphate ion, about 20-33% nitrate ion, and about 10-25% lead ion.

3. The method of claim 2 wherein the phosphating solution contains about 0.001-0.5% of a halogen ion.

References Cited UNITED STATES PATENTS Lum 1486.15 Ley et a1. 1486.15 Hays 1486.15 Kubie 1486.15 Palm 1486.15 Ehren et al l48--6.15 Gilchrist Q. 204-481 X RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R.