US3519495A - Process for coating metal surfaces - Google Patents

Description

United States Patent 3,519,495 PROCESS FOR COATING METAL SURFACES Elmer H. Plaxton, Bloomfield Hills, Mich., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Continuation-impart of applications Ser. No. 378,946 and Ser. No. 378,982, both June 29, 1964. This application Dec. 31, 1968, Ser. No. 789,009

Int. Cl. C23f 7/10 U.S. Cl. 148-6.16 7 Claims ABSTRACT OF THE DISCLOSURE A metal coating process which comprises applying a phosphate or oxalate conversion coating composition to a zinc, aluminum or ferrous metal surface and forming thereon a substantially dry, uniform phosphate or oxalate conversion coating, a portion of which coating is water soluble. Thereafter, a resinous fixing or immobilizing coating composition is applied which reacts out the watersoluble portion of the conversion coating, producing Water-solube complex organic resinous products and forming a dry, uniform composite coating on the metal. A stabilizing or passivating composition containing hexavalent chromium is also applied, which composition may be incorporated in the fixing composition or applied sequentially following the application of the fixing composition. Preferably, all of the coating materials are applied by mist-on type spray application.

This application is a continuation-in-part of my copending applications Ser. No. 378,946 and Ser. No. 378,- 982, filed June 29, 1964, now abandoned.

This invention relates to an improved process for coating metal surfaces and more particularly relates to improvements in the process for applying a protective coating to metallic surfaces such as zinc, iron, and aluminum.

The desirability of providing metal surfaces, and particularly metal surfaces which are predominantly zinc, iron or aluminum, with a protective coating, such as a phosphate coating, has long been recognized. Such coatings are particularly desirable for preventing staining or corrosion of the metal surfaces, or to provide a base for paint or similar protective coatings.

In the past, particularly with surfaces which are predominantly zinc, difiiculties have sometimes been encountered in obtaining a sufficiently heavy or uniform coating to provide the desired protection for the surface. In other instances, the phosphate coating obtained has been sufficiently loose or non-adherent that it has been an unsatisfactory base for paint or other protective coatings. Often, these difficulties have increased or occurred more frequently when the coating has been applied to the surface of a moving strip or sheet of metal. In such instances, because of the speed at which the material is moving, the application of the coating material and the formation of the desired protective coating should be accomplished in a relatively short period of time. Moreover, it sometimes happens that in installations which treat predominantly zinc surfaces, such as those resulting from electroplating zinc or hot-dip galvanizing, it is also desirable to treat surfaces which are predominantly iron or aluminum. Heretofore, it has not generally been possible to treat such a variety of surfaces to provide a satisfactory protective or paint-base coating using the compositions and application techniques which have been available.

Additionally, with processes and compositions which have been available for such applications, close controls were used and frequently one or more rinses of the coated 3,519,495 Patented July 7, 1970 "ice surfaces were employed to obtain acceptable results. Such operations have added to the complexity and costs of prior art processes.

It is, therefore, an object to the present invention to provide novel coating compositions of the reactive phosphate type which may be applied to metal surfaces which are predominantly zinc, iron, or aluminum, to provide an effective protective or paint-base coating.

Another object of the present invention is to provide a novel method whereby the coating compositions of the present invention may be easily and efficiently applied to metal surfaces of zinc, iron, or aluminum, which surfaces are in the form of sheets and/ or strips and/ or other shapes which may be coated using various spraying techniques.

A further object of the present invention is to provide novel compositions and methods for applying such compositions which are particularly adapted for forming paintbase coatings on zinc surfaces produced by hot-dip galvanizing.

Another object of the present invention is to provide a novel process for coating metallic surfaces and particularly, zinc surfaces, which process does not require close controls and rinses.

These and other objects of the invention will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for coating a metal surface which process comprises applying to the metal surface to be coated a phosphate or oxalate conversion coating composition, effecting formation of a substantially dry, uniform phosphate of oxalate conversion coating on the metal surface a portion of which coating is water-soluble, and applying onto the thus-formed coating a resinous fixing or immobilizing coating composition, reacting out the watersoluble portion of the conversion coating to produce water-insoluble complex organic resinous products and form a substantially dry, uniform composite coating on the metal surface, and applying a hexavalent chromium containing stabilizing or passivating material to the surface to be protected.

Preferably in the present process, the conversion coating is applied using spray techniques and the metal surface to which it is applied is desirably at a temperature of at least about degrees Fahrenheit. Moreover, in effecting the formation of substantially dry, uniform coatings, this may be done by a separate step, e.g., heating, after the application of the coating materials, or this may take place substantially simultaneously with the application of the coating material where the metal surface is sufficiently hot, e.g., at a temperature in excess of about 175 degrees Fahrenheit. Accordingly, both preheating and/or post heating of the metal surface may be used to effect formation of the substantially dry, uniform coatmg.

More specifically, the processing of the present invention, the preferred conversion coating composition is of the phosphate type in which the principal source of the phosphate ion is a metal phosphate wherein the metal has a valence of at least two. Typically, such conversion coating compositions are aqueous acidic solutions of a metal phosphate, such as zinc phosphate, ferrous phosphate, ferris phosphate, nickel phosphate, manganese phosphate, calcium phosphate, chromium phosphate, cobalt phosphate, aluminum phosphate, cadmium phosphate, and the like. Additionally, mixed metallic phosphates such as zinc nickel phosphate, zinc iron phosphate, zinc calcium phosphate, iron manganese phosphate, and the like may also be used. Typically, such compositions contain from about 0.1 to about 10.0 percent 'by weight phosphate, amounts within the range of about 0.3 to about 6.0 percent phosphate being preferred. In some instances, often depending upon'the mode of application, amounts both less than 0.1 percent and greater than percent may be used to obtain good results. As is known in the art, these and other phosphates provide the protective phosphate conversion coating on the metal surfaces being treated.

If desired, the conversion coating compositions may contain phosphoric acid, rather than metal phosphates, as the principal source of phosphate ions. In such compositions, as well as those of metal phosphate type which contain iron, it is generally desirable also to include another, auxiliary, acid, either organic or inorganic, other than phosphoric acid. Exemplary of such auxiliary acids are nitric acid, acrylic acid, methacrylic acid, polyacrylic acid, adipic acid, acetic acid, hydroxyacetic acid, gluconic acid, fumaric acid, malic acid, maleic acid, malonic acid, lactic acid, itaconic acid, tartaric acid, ethylenediarnine tetraaetic acid, citric acid, hydroxymethyl phosphonic acid, oxalic acid, and the like. Typically, these auxiliary acids are present in amounts to provide a ratio of phosphate to the auxiliary acid in the range of about 100:1 to about 2:1, with amounts sutficient to provide a ratio of from about 10:1 to about 4:1 being preferred. It will be appreciated that in many instances, often depending upon the mode of application of the conversion coating composition, the compositions may contain the phosphoric acid and/or the auxiliary acid in amounts both less than and greater than those which have been indicated hereinabove, to obtain comparable results. Generally, however, superior results have been obtained when using conversion coating compositions based on the metal phosphates, rather than phosphoric acid and, hence, these are preferred.

In addition to these materials, others may also be included in the conversion coating composition, e.g., materials which act as accelerators to reduce the time required to obtain a uniform, substantially continuous coating. Such materials include nitrates, nitrites, halides, e.g., bromides, chlorates, bromates, sulfites, perchlorates, iodates, peroxides, periodates, permanganates, organic nitro compounds such as m-nitrobenzene sulfonate, nitro guanidine, nitro methane, nitro ethane, l-nitro propane, nitrobenzene, o-nitro benzaldehyde, p-nitrophenol, p-nitro anilin, p-nitrochloro benzene, picric acid and the like. Other additives which may be used in the composition are those which increase the etching tendency of the solution, such as the so-called complex fluorides, including fiuosilicates, fluoborates, fiuotitanates, fluostannates, fluozirconates, fiuo vanadates, and the like.

Inasmuch as the conversion coating composition is desirably in the form of an aqueous solution, the above and other additives are preferably added to the composition in a form which is readily dispersible in the aqueous solution. Many Water dispersible forms of these compounds may be used provided they have no adverse effect on the coating composition, the metal substrate, or the coatings subsequently produced or applied. Typically, such additives may be present in the conversion coating composition in amounts within the range of about 0.01 to about 6.0 percent by weight, and desirably in amounts within the range of 0.02 to about 4.0 percent by weight, although in many instances both lesser and greater amounts may be used.

In many instances, the oxalate radical may be substituted for the phosphate in these compositions to obtain comparable results. Examplary of this is a conversion coating composition solution based on ferric oxalate, rather than the phosphate. Additionally, adjuvants other than those which have been indicated hereinabove may also be included in the compositions where particular properties and/or characteristics of the coating are desired.

Typically, these conversion coating compositions are utilized at a pH within the range of about 0.7 to about 3.5 and the compositions are applied in such a manner so as to obtain a coating weight of phosphate ions of from 4 about 10 to about 250 milligrams per square foot, of metal surface being treated. Desirably, the coatings obtained contain phosphate in the range of about 20 to about milligrams per square foot. Here again both lesser and greater amounts may be used.

The conversion coating compositions are desirably applied to metal surfaces, which are preferably at a temperature of degrees Fahrenheit, or higher, immediately before impingement of the phosphate composition. Typically, the temperatures are within a range of about 200 to about 600 degrees Fahrenheit, with temperatures in the range of about 275 to about 425 degrees Fahrenheit being most preferred. Although various metal surfaces may be treated in this manner, e.g., surfaces which are predominantly zinc, iron, or aluminum, excellent results have been obtained when coating zinc surfaces and, in particular, zinc surfaces obtained by hot-dip galvanizing. In such preferred applications, the process of the present invention may be carried out immediately following the hot-dip galvanizing operation, while the surfaces are still hot from the galvanizing bath. In this manner, the retained heat of the metal from the hot-dip galvanizing operation is utilized so that the application of additional heat to the surfaces may not be necessary. It will, of course, be appreciated that where the metal surface to be treated is not already hot from some previous processing step, preheating of the surface prior to the application of the conversion coating composition, may be carried out in many convenient ways to obtain the desired temperatures that have been indicated hereinabove.

The phosphate conversion coating compositions of the present invention may be applied to the metal surfaces by various means, for example, by spraying, immersion, flooding, roll-on and the like. Of these, various spraying techniques are generally utilized with the preferred method of application being by misting. In this latter technique, the coating solution to be applied is subjected to atomization so as to obtain many very finely divided liquid particles which are deposited on the metal surface in such a manner that there is substantially no liquid run-off from the surface and a dry, substantially uniform coating is very quickly obtained, as the liquid components of the composition are evaporated by the heat of the metal surface. Typically, this will be effected by steam or other gas atomization. In such misting processes, gasatomized particles having a size in the range of about 15 to about 350 microns of the aqueous coating solution are directed toward a heated metal surface so as to form a coating on the surface by inter-reaction with the heated surface. The size of the particles, and the quantity and number of successive applications of these particles to the surface, are all controlled so that the particles of coating composition deposited on the surface remain in substantially the loci of their original impacts, and the surface is uniformly coated. Generally, it has been found that by utilizing such misting techniques for applying the coat ing compositions of the present invention, the time required to obtain a substantially dry, uniform coating on the surface being treated is greatly reduced and more efficient use of the coating composition is made with substantially no waste or run-off from the surface. Accordingly, hereinafter in referring to the application of the conversion coating compositions to the surfaces to be treated, primary reference will be made to the use of misting techniques to accomplish this application. This is not, however, to be taken as a limitation of the application techniques which may be used, as other and perhaps more conventional methods have been used in the process of the present invention to obtain excellent results.

It will be appreciated by those in the art that the application of the phosphate conversion coatings by misting to the heated zinc surfaces may be accomplished with many different types of equipment, depending upon the particular circumstances involved. For example, where the metal being treated is in the form of strips or large sheets, these may be passed through one or more stationary spraying or misting zones, wherein the size and number of the spraying zones, the concentration of the phosphate conversion coating solution and the particle size of the atomized solution may all be varied so as to obtain the desired coating weight, within the ranges indicated hereinabove. Alternatively, and particularly in treating smaller work pieces, hand spraying may be employed wherein multiple passes of the spray over the surface to be treated, for varying periods of time, may also be utilized to control and obtain the desired coating weights. These and other application methods are apparent to those in the art and the particular techniques utilized will, of course, depend upon the specific factors involved. In general, however, it has been found that the coating weights applied can be controlled over the greatest range by variations in the concentration of the coating solution.

After the formation of a substantially uniform phosphate conversion coating, 9. portion of which is watersoluble, on the metal surface has been effected, a resinous fixing or immobilizing composition is then applied to the thus-formed coating. As has been indicated hereinabove, due to the heat of the metal surface in the preferred method, the formation of a substantially dry, uniform phosphate conversion coating takes place very quickly and in many instances, substantially simultaneously With the application of the phosphate conversion coating material. Thus, the application of the fixing coating composition can be effected substantially immediately after the application of the phosphate conversion coating material, with no rinsing or curing of the phosphate coating.

The fixing coating composition contains resinous materials which are reactive with the water-soluble portion of the previously formed phosphate conversion coating, which materials are preferably dissolved or dispersed in an aqueous media. In referring to fixing or immobilizing the phosphate conversion coating, it is meant that the second coating material applied reacts out the Watersoluble portion of the previously applied phosphate coating to produce water-insoluble complex organic residues and form a composite coating which is more adherent to a subsequently applied paint or similar protective coating and which provides an improved barrier between the metal substrate and materials with which it may come in contact which are likely to be corrosive or to lift the paint from the surface, such as water, thereby immobilizing or fixing the conversion coating on the metal surface.

The reaction(s) by which this fixing is accomplished include the formation of complex organic resinous products by chemical bonding (as opposed to physical adsorption) to enhance the adherence of the initial conversion coating and at the same time to promote adherence of a subsequently applied paint film. It is known, for example, that acidic materials can change the physical and chemical properties of many resinous materials. Small amounts of acids can act as catalysts for increasing the molecular weight of some resins, can enter into complexes with reactive groups of resins, etc. In looking at these phenomena from another viewpoint these resinous materials have the net effect of acting as scavengers for the acidic water-soluble residues from the initial conversion coating. While the mode of action is not entirely clear, the effects are definite when viewed as paint base coatings.

Resinous materials that may be used in this fixing composition include dimethyl hydantoin-formaldehyde (a low molecular weight resin like material), water-soluble acrylic, melamine, or urea-formaldehyde resins, emulsified acrylics, alkyds, vinyls, polyvinyl acetate, polyvinyl butyrals, polybltadiene-styrenes, polyvinyl pyrrolidones, polyesters, and epoxies. These materials may be used singly or in combination to obtain the desired effects.

As with the phosphate conversion coating material, the concentration of the fixing components of the composition may be varied, typical concentrations being within the range of about 0.1 to about 6 percent by weight of the total composition, with amounts within the rang of about 0.3 to about 1 percent being preferred. Similarly, the amount of the immobilizing component deposited on the surface being treated may also be varied, amounts within the range of about 1 to about milligrams per square foot being typical with amounts within the range of about 15 to about 50 milligrams per square foot being preferred. The above concentrations and coating weights are exemplary of those which may be used, as both greater and lesser amounts give good results in many instances.

As with the phosphate conversion coating, the fixing composition may be applied to the surface to be treated without heating the surface, as for example, the surface temperatures being substantially at room temperature, e.g., 65-75 degrees Fahrenheit. Preferably, however, the surface temperatures are at least about degrees Fahrenheit, and are typically within the range of about 200 to about 600 degrees Fahrenheit with emperaures below about 350 degrees Fahrenheit being most preferred. The fixing coating composition is applied to the surfaces using various convenient techniques, such as by roller coating, by immersing, by flooding, by spraying, utilizing various suitable spraying techniques, with the misting techniques of application being preferred. When the composition is applied to a surface at an elevated temperature, particularly when using misting techniques, the immobilizing or fixing composition reacts very quickly, and generally substantially simultaneously with its application, with the previously formed phosphate conversion coating to form a composite, substantially dry and uniform coating on the metal surface.

In addition to the fixing coating composition, there is also applied to the surface being treated, either after the application of the immobilizing composition or in conjunction therewith, a hexavalent chromium containing stabilizing or passivating composition. Generally, it is preferred that the stabilizing composition be incorporated in the fixing coating composition. Thus, components which are incorporated in the resinous fixing compositions to effect this stabilization or passivation are hexavalent chromium bearing materials of soluble or insoluble types, with compatibility and competitive factors important in the selection thereof. Exemplary of these are ammonium dichromate, zinc or chromium dichromate, zinc tetroxy chromate zinc yellow, or other metallic chromates and including chromic acid. Still other components may also be included to assist the physical properties of the fixing composition, including wetting agents, protective colloids, inerts such as silica, and the like.

While the preferred method of this invention discloses the use of a two stage process involving (a) an initial conversion coating followed by (b) a 'fixing agent and passivator combination, it is to be appreciated that the fixing composition and the passivator may be applied in sequence rather than together if desired. It is important to note, however, that no rinses are required between stages nor after the final stage.

In applying the hexavalent chromium containing stabilizing composition, preferably as a part of the fixing composition, or as a separate step, the composition of the stabilizing solution may be varied. Typically, the composition, based on CrO content is within the range of about 0.01 to about 2 percent by weight, with amounts within the range of about 0.1 to about 1 percent by weight being preferred. Similarly, the coating weight of the composition on the surface treated may also be varied, with coatings containing CrO within the range of about 0.1 to about 20 milligrams per square foot being typical and coating Weights containing CrO in the range of about 1 to about 8 milligrams per square foot being preferred. It will be appreciated that in many instances the concentrations used and the coating weights obtained may be higher or lower than the above values and still obtain good results.

The stabilizing composition, when not used as a part of the fixing composition, may be applied to the articles using many convenient means, including immersion and roller application, although spray techniques, and particularly misting techniques are preferred. Additionally, with regard to the temperature of the surface to which the stabilizing coating is applied, as with the application of the conversion coating and the fixing coating, it is not essential that the surface be heated. Accordingly, temperatures from room temperature, e.g., about 65-75 degrees F. up to temperatures of about 400 Fahrenheit, may be used. Typically, and where hot-dip galvanized surfaces are being treated, the surface temperature will be within the range of about 200 to about 350 degrees Fahrenheit during the application of the stabilizing coating. It will, of course, be appreciated that where it is desired to obtain the stabilizing coating in a substantially dry condition in a relatively short period of time, surface temperatures and application techniques will be utilized which will provide this result. For example, surface temperatures above about 200 degrees Fahrenheit and the misting application techniques are used.

In carrying out the preferred method of the present invention, hot-dip galvanized surfaces, generally in the form of sheet or coil stock, are passed through a series of spray or misting zones wherein the compositions of the present invention are applied in either two or three stages, preferable two, by including the hexavalent chromium stabilizer in the immobilizing composition. It has been found that the applications of these compositions to metal surfaces may be effected over a wide range of speeds for the work traveling through the misting application zone. For example, excellent results are obtained when applying the compositions to metal surfaces traveling at speeds from about to over 500 feet per minute. By this method, there is obtained a high speed process for providing conversion coatings on metal surfaces, which process eliminates the need of rinsing the conversion coating after its application. The coatings produced'by the method of the present invention are found to provide excellent corrosion protection on the metal surfaces to which they are applied and are also found to be excellent paint base coatings. Additionally, they also prevent white corrosion on storage of hot dip galvanized metal.

In order that those skilled in the art may even more readily understand the present invention and the manner in which it may be practiced, the following specific examples are given. It is to be understood, of course, that these examples are illustrative of the present invention and are not intended to be taken as limitations thereof.

In this application, unless otherwise indicated, the temperatures are expressed in degrees Fahrenheit, the parts and percentages are by weight, and the coating weights are in milligrams per square foot. Additionally, unless otherwise indicated, the coating compositions were applied to high-speed continuous hot-dip galvanized surfaces, known commercially as Zincgrip and said to be obtained by the continuous hot-dip zinc coating process disclosed in US. Pat. 2,197,622. After these surfaces were coated, they were painted with an alkyd-melamine baking enamel and then subjected to 5 percent salt spray, humidity, and physical tests. The salt spray test is the American Society for Testing and Materials (ASTM) test B117-61 with painted panels scribed as given in ASTM test D-l65461. This uses a 5 percent sodium chloride fog. The ratings given depend on the creepage from the scratch, given in of an inch. Ratings given as spot (S) indicate no creepage except in a small area. In the humidity test, panels were exposed in a walk-in room at 100 percent relative humidity at 100 degrees Fahrenheit, for the designated period of time. The blistering was rated according to ASTM designation D7l4-56 and is reported as follows. D-dense; MDmedium dense; Mmedium; FMfew medium; F-few; and VF-very few. In both the salt spray and humidity tests, unless otherwise indicated, the exposure time was 504 hours. In the physical test, adhesion is determined by knife blade and the results are reported on the scale of 0 to 10, where 10 is excellent, 8 is good, 6 is fair, 4 is poor, 2 is very poor, 0 is complete loss of adhesion. In the forming test, painted panels were subjected to a severe deformation producing parallel short radius right angle bends and paired three dimensional short radius shoulders in one operation. Failures or degrees thereof are shown by percentage peeling of the paint.

In the following examples, the Zincgrip panels were solvent cleaned and then heated to a temperature within the range of about 275 to about 325 degrees Fahrenheit, generally about 300 degrees Fahrenheit. Thereafter, the various coatings were applied to the heated surface by misting techniques, the conversion coating solution generally having a P0 content of about 1 percent and being applied at a P0 coating weight of about 40 to milligrams per square foot. The fixing coating composition generally had a concentration of about 0.5-30 percent and was applied at a coating rate of about 15 to 35 milligrams per square foot while the stabilizing composition generally had a CrO content of about 0.2 percent and was applied to give a CrO coating weight of about 3 milligrams per square foot. Using the procedures as have been described hereinabove, the following examples were carried out and the indicated results were obtained. In these examples are given data on those experiments in which significant improved results in properties of embodiments of this invention were found.

Conversion Fixing Stabilizing Example Coating Coating Coating 1 Aqueous solution containing 0.1% Zn, 0.15% None Aqueous solution containing Fe, 1.0% P04 and 0.05% HNOa. 0.2% 010 2 Aql( )7llSHS(%tiOn containing 1% HaPO and ..do D

0. 5 0 3. 3 Same as 1 except 0.25% HNOa Aqueous solution containing 1.5% 01 dimethylhydantoin- Do.

formaldehyde resin. 4 Same as 2 Same as 3 except 3.0% of resin Do. 5 Commercial zinc phosphate solution as dis- Aqueous solution containing 0.8% ofmelamine-iormalde- Do.

closed in U.S. 2,835,617. hyde resin.

6 Aqueous solution containing 1.0% P0 0.15% Fe and 0.25% N03.

Same as 6 except with copolymer of styrene and polyvinyl 12 Same as 2 except 0.25% HNO3 Aqueous dispersion containing 0.5% polyvinyl butyral and 0.5% zine tetroxy chromate. Same as 6 except with copolyrner of butadiene and styrene pyrollidone. Aqueous dispersion containing 0.5% dimethylolurea and 0.2% CrOa as CI2(C!2O7)a. Aqueous dispersion containing 0.5% melamineformaldehyde resin and 0.5% zinc yellow (K20. 4Zn0. 401-03. 3H3O) Aqueous dispersion containing 1.5% dimethylbydantoinformaldehyde resin and 0.5% zinc yellow. Aqueous dispersion containing 0.8% polyvinyl butynol an? 0.5% zinc yellow.

l 48 hours.

EXAMPLE 14 The procedures of Examples 6, 7 and 8 were repeated with the exception that the coating compositions were applied to panels of cold rolled steel, rather than zinc panels. Additionally, in the case of Example 6, the quantities of the components of the immobilizing coating composition were quadrupled and in the case of Examples 7 and 8, the amounts of these components were doubled. In each instance, the knife adhesion and forming test results were 10 and 0, respectively.

EXAMPLE 15 A series of runs was made wherein the conversion coating was an aqueous solution containing from 0.1 to 0.5 percent acetic acid and oxalic acid in the amounts of 0.1, 0.2, 0.5, and 1 percent. In the four runs made, the temperatures of the Zincgrip panels onto which the coating was misted were 200 F., 300 F., and 375 F. In each instance, the application of the conversion coating material was followed by a mist-on application of a 0.2 percent aqueous solution of CrO The typical results obtained on testing the painted panels showed from to 3 creepage after 504 hours in the salt spray, blister failure only after 504 hours in the humidity test and fair to good knife adhesion and forming tests.

EXAMPLE 16 The procedure of Example 15 was repeated with the exception that the conversion coating composition was a ferric oxalate solution containing 0.5 percent iron which was applied to panels which were at a temperature of 375 degrees Fahrenheit. The test results on the painted panels showed good humidity and physical test ratings.

EXAMPLE 17 Salt Humid- Adhe- Spray ity sion Forming Zine dichromate control N 1 VF, F 3 80% peel. DMHF Zinc dichromate... N, N VF, F 0

1 Failed in 48 hrs. (major or peeling). N orE.-N=Zero creepage.

EXAMPLE 18 A ferrous phosphate solution containing 1.8% P0 025% N0 0.15% Fe was misted on 300 degrees Fahrenheit Zincgrip panels to give 40-50 milligrams P0 per square foot and was followed by an aqueous resin soluble chromate composition, misted on to provide 3 milligrams CrO per square foot. This latter composition contained 1.5 percent dimethylhydantoin-formaldehyde resin and sufficient zinc dichromate solution to give 0.2 percent CrO On panels painted with a high temperature baking vinyl, this treatment improved the knife adhesion from 7 to 1-0, when compared to the use of zinc dichromate without the resin present.

EXAMPLE 19 A series of zinc and nickel phosphate solutions were misted onto commercial Zincgrip panels at 300 degrees Fahrenheit in an amount to obtain a coating weight of 4050 milligrams P0 per square foot.

The metal content of the phosphate solutions was .02 molar with approximately 0.1 molar P0 and 0.04 molar N0 The Zn/Ni ratio was varied as follows: 1:0, 2:1, 1:1, 1:2, and 0:1. A further variant was introduced by adding H SiF at 0.0 to 0.2 percent.

The phosphate coating was followed quickly with a fixer prepared from: zinc yellow pigment 0.5%; SiO 0.05%, dimethylhydantoin-formaldehyde resin 1.5%.

This composition was misted toprovide approximately 3 milligrams CrO per square foot.

These coatings were painted with a high temperature vinyl composition. All variants showed excellent performance in accelerated and physical tests.

A similar series was made with the metal level raised to 0.05 molar also with excellent results.

EXAMPLE 20 An aqueous fixing-stabilizing composition was formulated containing the following components in the amounts indicated:

Components: Percent by Weight (net solids) Thermoplastic cross-linkable acrylic nonionic emulsion having a pH of 6.1-6.7 0.08 Acrylic copolymer emulsion having a pH of 3 0.32 Low molecular weight anionic copolymer of acylamide and acrylic acid 0.12 Alkyd resin emulsion having a pH of 6-9 0.60 CrO (added as a solution of zinc dichromate in hydroxyethyl cellulose, neutralized to a pH of 7.0-7.2. with NH OH) 0.20 Cobalt-manganeses naphthenate driers 0.018

Commercial Zincgrip panels were heated to provide a 300 F. surface for impingement of a phosphatizing composition containing ferric nitrate and phosphoric acid followed immediately with the resin-chromate composition above. The resulting coating contained 40-50 mg. PO ft. and 3 mg. CrO /ft. Panels were painted with a Dulux baking enamel and tested for 504 hours in 5% salt fog: creepage from scratch was 0-1 "s). A simi- 'larly treated panel painted with a more water sensitive b aking enamel and tested by water immersion at F. for 504 hours showed no blistering and retained knife adhesion of 9' while wet.

Another similarly treated panel, painted with 1.0 mil of a post-forming vinyl paint was tested in comporison with painted untreated control and a painted panel treated with a conventional commercial phosphate and the following results were obtained:

Misted Conventional Coating Untreated Phosphate Salt Spray creepage: (504 hr.) Nil Nil Pittsburgh Lockseam (6" length "s of peel) 10 96 64 Adhesion (knive) 10 4 10 1 Failed 48 hr.

1 1 recited in any of the following claims is intended to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for coating aluminum, zinc and ferrous metal surfaces which comprises (a) applynig to the metal surface to be coated a phosphate or oxalate conversion coating composition;

(b) forming on the metal surface a substantially dry, uniform conversion coating, a portion of which is water-soluble;

,(c) applying an aqeuous resinous fixing composition to deposit at least about 1 milligram of resin per square foot, the resinous component of said resinous fixing compositions selected from dimethyl hydantionforrnaldehyde, Water-soluble acrylic, melamine, and urea-formaldehyde resins, emulsified acrylics, alkyds, alkyds, vinyls, polyvinyl acetate, polyvinyl butyrals, polybutadiene-styrenes, polyvinyl pyrrolidones, polyesters, epoxies and mixtures thereof;

((1) reacting out the water-soluble portion of said conversion coating to produce water-insoluble organic resinous products and form a substantially dry, uniform composite coating; and

(e) applying a hexavalent chromium-containing stabilizing composition.

2. The method as claimed in claim 1 wherein the stabilizing composition is applied concurrently with the fixing composition by including the hexavalent chromium containing composition in the resinous fixing composition.

3. The method as claimed in claim 2 wherein the conversion coating composition is an aqueous phosphate composition in which the predominate source of phosphate ions are phosphates of at least one of the metals zinc, ferric iron, ferrous iron, nickel chromium, cobalt, cadmium, calcium, and manganese.

4. The method as claimed in claim 3 wherein the conversion coating composition is sprayed onto the'metal surface and the metal surface is at a temperature of at least about 175 F.

5. The method as claimed in claim 4 wherein the phosphate concentration of the conversion coating is from about 0.1 to 10% by Weight, which coating is applied so as to provide a phosphate coating weight of from about 10 to 250 milligrams per square foot and the fixing coating composition contains the resinous fixing material in an amount of from about 0.1 to 6.0% by weight and the hexavalent chromium containing material in an amount of from about 0.01 to 2% by weight hexavalent chromium, the fixing coating composition being applied so as to pro vide a coating weight of. from about 1 to 100 milligrams per square foot and a coating weight of CrO of from about 0.1 to 20 milligrams per square foot.

6. The method as claimedvin claim 5 wherein the temperature of the metal surface to which the conversion coating is applied is from about 175 to 600 F.

7. A metal surface having a protective coating thereon produced by the method of claim 1.

References Cited UNITED STATES PATENTS 2,562,117 7/1951 Osdal l48-6.27 X 2,813,813 11/1957 Ley et al. 1486.15 3,053,691 9/1962 Hartman et al 1486.2 X 3,281,284 10/1966 Dwors 1486.l6

OTHER REFERENCES German Auslegeschri'ft 1,147,820, April 1963.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 1486.2