US3046165A - Method of and solution for forming phosphate coatings on zinc surfaces - Google Patents

Description

United States Patent Ofifice 3,646,165 Patented July 24, 1962 signors to Parker Rust Proof C m a corporation of Michigan 0 pany, Detroit, Mich.

No Drawing. Filed Dec 14 1960 Ser No 75 660 14 Claims. c1.14s-6.1s

This invention relates to th coatings on zinc surfaces an said surfaces such as ai s amels, lacquers, etc. In the commercial manu fadiiiie iif 21116 and zlnc alloy sheets, strip or the like, it has become customary to subject the sheet or strip to a treatment usually at the mill, to protect the surface and prevent the formation of white corrosion products in the interim between production of the sheet and the ultimate finishing of its surface in its intended end use. A variety of procditferent compositions have been proposed as satisfactory for use in such processes. In most instances the mill treatment process involves the use of an aqueous solution contalning chromic acid in combination with any of a number of other activator ingredients, such as chlorides, sulfates, fluorides, phosphates, complex fluorides borates, etc., to form a protective coating on the surfacei Some of these processes produce colorless coatings, While others produce brown-colored coatings but all are considered to contain a metallic chromate as one of the coatmg components. When an attempt is made to form a phosphate coating on a zinc surface which has been subjected to one of these mill treatments in its original manufacture, it has been found that the use of conventional cleaning procedures prior to ordinary zinc phosphate coatmg compositions are ineffective to form an adherent and corrosion resisting phosphate coating.

It .18 therefore the primary object of this invention to provide an Improved process for forming adherent cor- IOSlOIl resistant phosphate coatings on zinc and zinc alloy surfaces.

Another object of this invention is to provide an improved process for conditioning the surfaces of zinc, electroplated zinc, galvanized zinc, hot dip zinc, electro-galvanlzed zinc and zinc alloy surfaces, to receive a phos phate coating.

A further object of this invention is to provide solutrons for and a process of conditioning zinc and zinc alloy surfaces which have been coated with a mill treat- 7 ment coating so that the surface is rendered receptive to an adherent phosphate coating, and forming phosphate coatings on such pre-conditioned surfaces.

Other objects and advantages will become apparent as the following description is considered in its entirety.

In accordance with this invention, it has been found that any zinc or zinc alloy surface is rendered more receptive to the formation of an adherent phosphate coating thereon if it is preliminarily contacted with an aqueous alkaline solution modified to contain an effective amount of an oxidizing agent.

The process of this invention is beneficial in pre-conditioning the surfaces of pure zinc, electroplated zinc, galvanized substrates such as iron, steel, etc., hot dip zinc surfaces including hot dip zinc containing small quantities of alloying ingredients such as aluminum, etc., and zinc alloys per se or electrodeposited, galvanized or hot dip zinc alloy surfaces. While benefit is obtained on any such zinc-containing surface irrespective of the presence thereon of a hexavalent chromium-containing coating, the greatest benefit Which results from the use of the process of this invention is obtained when the surface has been treated with a corrosion-inhibiting solution, usually containing hexavalent chromium.

The aqueous alkaline solutions which are effective for the purpose of this invention are effective because of the presence therein of the oxidizing agent. Numerous tests have demonstrated that unmodified aqueous alkaline solutions, regardless of degree of alkalinity and method of use including application at boiling temperature, are incapable of conditioning azinc surface to receive a phosphate coating Where that zinc surface is one which has been given a metallic chromate-containing corrosioninhibiting coating at the mill. It has been. found that because of the varied nature of the coatings applied at the mill the surfaces vary in degree of difficulty of conversion to a phosphate coating receptive condition, and the process should be varied accordingly in order to obtain the best results. In all cases, however, the aqueous alkaline solution must contain a certain minimum content of oxidizing agent. The minimum content which is effective is an amount of oxidizing agent having the oxidizing ability of about 0.05 gram per liter of the permanganate ion in an aqueous solution having a pH numerically above 7.

A number of oxidizing agents have been found to be satisfactory for this purpose, including both organic and inorganic types. Somewhat better results: have been obtained from strong, powerful inorganic oxidizing agents and they are preferred. Good results have been obtained from the use of one or more ions selected from the group consisting of the permanganates, perborates, peroxides,

chlorates, bromates, iodates, perchlorates, nitrites and hypochlorites and of this group permanganate is preferred. These ions are conveniently introduced into the aqueous alkaline solution in the form of a metallic salt, and preferably as the alkali metal salt. While the peroxides are also effective, they are somewhat less desirable because they are difiicult to control at the desired concentration in continuous operations. Typically suitable organic oxidizing agents include picric acid, sodium metanitro-benzene sulfonate, o-nitro-chlorobenzene -p-sulfonic acid and m-nitrobenzoic acid, with picric acid giving the best results in a short processing time.

The alkalinity of the solution may be derived from the use of any of the common alkaline salts such as the alkali metal hydroxides, carbonates, phosphates, borates, silicates, polyphosphates, pyrophosphates, mixtures thereof and the like. Weak alkaline solutions, that is, solutions having a pH in the range of 7 to 9 give satisfactory results but require more time at a given temperature than stronger alkaline solutions and solutions having a pH numerically above 9 are preferred. Even when the pH of the solution is above 9, the use of the weaker alkaline salts such as the borates, phosphates or silicates requires higher concentrations, with a given oxidizing agent, than is required when the alkalinity is derived from a hydroxide or carbonate. For high speed production operations, e.g., continuous strip line treatment, a solution containing both a strong alkaline material and a strong oxidizing agent gives the best conditioning action in the shortest time, and the best results have been obtained from the use of a solution having a pH of 9-13 and containing alkali metal hydroxide and alkali metal permanganate.

When permanganate is used, it has been observed that the effectiveness of the solution increases as the concentration of alkaline material increases and the pH increases from 9 toward 14, that is to say, the oxidizing ability of the solution for a given quantity of permanganate increases as the alkalinity increases. At a high pH, in the range of about 11-14, good results have been obtained with extremely small quantities of the permanganate ion in short periods of time. For example, hot dip zinc provided with a metallic chromate-containing mill treatment coating was conditioned to receive an adherent zinc phosphate coating by contacting the surface for two minutes with a solution at about 170 F. containing 7.5 grams per liter of sodium hydroxide and 0.06 gram per liter potassium permanganate. In the usual case, an amount of permanganate ion in the range of about 0.2 g./l. to about 30 g./l. is adequate and provides the best overall results. Similar results are obtained by increasing the concentration of oxidizing agent when the concentration of the alkaline material is decreased. For example, a solution containing 0.1 gram per liter sodium hydroxide was effective to condition mill treated zinc strip when the concentration of potassium permanganate was increased to 10 grams per liter and the solution was applied at 170 F. for two minutes.

As above stated, when weaker alkaline materials are used the concentration should be increased for any given concentration of oxidizing agent, and the general relationship will be apparent from the following comparisons with the above described solution containing 0.1 gram per liter NaOH and 10 grams per liter potassium permanganate. When the sodium hydroxide was replaced with disodium monohydrogen phosphate, it was found that 25 grams per liter of disodi-um phosphate was required to give equivalent results. A similar substitution with sodium meta silicate required 20 grams per liter of sodium meta silicate. In substituting for the permang'anate ion the following general guide is given to enable formulation of solutions having approximately equivalent oxidizing ability. In an aqueous solution containing 7-10 grams per liter of sodium hydroxide mill treated zinc is conditioned to receive an adherent zinc phosphate coating when the solution contains 0.12 gram per liter of potassium permanganate and is applied for one minute at 170 F. Substantially equivalent oxidizing ability is obtained when the permanganate is replaced by 7.5 grams per liter of sodium bromate, grams per liter of picric acid, and 2.5 grams per liter of sodium meta nitrobenzene sulfonate. A solution containing 50 grams per liter sodium hydroxide and grams per liter of sodium chlorate or 10 grams per liter of sodium nitrite also had approximately equivalent oxidizing ability. Hydrogen peroxide is more effective than the chlorates or nitrites and can be used in a quantity of 2 to 3 times the permanganate concentrations. The iodates, perborates, perchlorates and hypochlorites are slightly stronger than the nitrites but are weaker than the bromates and are satisfactory at concentrations intermediate those given for bromates and nitrites. The upper limit of oxidizing agent is not critical and no harm apparently results from the use of concentrations up to the solubility limit although no improvement is obtained from the excess over that required to effect conditioning, as above explained. Concentrations of up to 50-70 grams per liter of permanganate ion, or its equivalent, can be used with the weaker alkaline materials to advantage, although 30-50 grams per liter of permanganate ion is ordinarily sufiicient with solutions having a pH above 9 and a substantial concentration of alkaline material, as above explained.

The oxidizing agent-containing cleaning solutions of this invention may also include conventional surface active agents of the non ionic, anionic or cationic types which are stable in the solutions, if desired. The presence of such a surface active agent has been found to be advantageous, particularly in cases in which the surface to be coated is slightly soiled with finger-prints, oils, greases or the like.

The complete process of this invention comprises the steps of applying the above defined solutions of this invention to the surface to be conditioned by spraying,

brushing or immersion with the solution at a temperature in the range of about F. to boiling and preferably at a temperature of about F. to 180 F. As above indicated, the contact time may vary from about /2 minute to 2 minutes in the ordinary case, but longer contact time is not detrimental to the improved receptivity for phosphate coatings which is imparted to the surface and contact times up to 30 minutes may be employed. The surface to be treated, if dirty, or otherwise contaminated, is preferably preliminarily cleaned with conventional cleaners to remove grease, finger-prints and the like before the application of the conditioning solutions of this invention. After removal of the surface from the conditioning solution, it is rinsed and thereafter may be immediately contacted with an aqueous acidic phosphate coating solution. For this purpose, any of the phosphate coating solutions conventionally employed to form phosphate coatings on zinc surfaces may be used, such as for example, those disclosed in U.S. Patents 2,813,812 and 2,835,617. Alternatively,'the preconditioned surface may be subjected to another conventional conditioning step such as that described in U.S. Patents 2,310,239 or 2,874,081. This treatment comprises contacting the surface with an aqueous solution of disodium phosphate, sodium tri-polyphosphate or tetrasodium pyrophosphate and a titanium containing compound such as titanyl sulfate. After Withdrawal from this solution, it is ready .for contact with the phosphate coating solution. After the phosphate coating has been formed, the surface may be subjected to a conventional aqueous dilute chromic acid rinse, if desired. The following examples are intended to illustrate this invention in greater detail but it is to be understood that the specific materials used therein, the conditions of application and the like are intended to be illustrative only and not to set forth the definitive limits of this invention which have been given hereinabove.

Example I An aqueous solution was prepared containing 11.2 grams per liter of sodium metasilicate, 11.2 grams per liter of tetra sodium pyrophosphate and 10.1 grams per liter of potassium permanganate. The solution was placed in a conventional machine for spray-washing of metal parts and the temperature of the solution was raised to and maintained between about 160 F. and F.

A plurality of hot dipped zinc coated steel panels sold commercially under the trademark Zincgrip and reported to be manufactured by the process of U.S. Patent 2,197,622, were sprayed with the hot alkaline solution to produce a contact time of about 1 minute with the solution, the panels withdrawn and rinsed in Warm water containing a small amount of titanium phosphate made by the procedures of U.S. Patent 2,874,081. Some of the panels were preliminarily treated at the mill with a corrosion inhibiting coating containing hexavalent chromium, while others of the panels were free of any mill treatment coating.

An aqueous acidic zinc phosphate coating solution was prepared and upon analysis found to contain 0.9% P0 0.25% zinc, 0.2% nickel, 0.2% N0 and 0.2% fluoride, added as fiuosilicate. The temperature of the solution was raised to the range of about 150 F. and the rinsed panels from the preceding alkaline solution were contacted with this solution for a contact time of about 1 minute. The panels were then Withdrawn and given a 30-second cold water rinse. Thereafter, the panels were immersed in a dilute aqueous chromic acid solution containing about 0.05% CrO having a free acid of about 0.1 at room temperature for 30 seconds and withdrawn. The panels were then dried in an oven for 3 minutes at 375 F. A number of control samples of both mill-treated and non mill-treated stock were vapor degreased and used as standards in evaluating the efiect of the alkaline-oxidizing solution treatment. Some of the panels were then painted with a commercial two-coated single bake paint system, Du Pont 707-6741, and some of the panels were tested for adhesion, resistance to bending and impact.

The adhesion test comprised manually drawing a knife blade across the surface of the coating and comparing the resistance to dislodgment of the coating with no dislodgment being rated and the difficulty of dislodgment being evaluated numerically from 1-5, with being extremely easy dislodgment or peeling.

The bend test was conducted in accordance with the standard method of ASTM D-522-41, which generally stated, involves bending the panel for 180 around a conical mandrel having a small diameter of /s and a length of 4-8 inches. The results of the bending test are reported in terms of flaking or fracture of the coating and the amount of flaking or fracture is reported as the maximum extent thereof in eighths of an inch measured from the A5 diameter end of the mandrel. Flaking indicates separation of the coating from the panel surface while fracture indicates a crack in the coating without loss of adhesion.

The impact test involves forming about a A deep depression in the coated surface by pressure from a /2 diameter ball, with the pressure being supplied by dropping a weight or hammer on the ball positioned on the surface. The height of the weight drop and the weight itself which is dropped are varied to produce the approximate A depression in the particular gauge metal being used. With 24 gauge metal, for example, a 3 /2 lb. weight dropped 12" forms the desired impression. The internal surface of the concavity and the convex surface of the impression are inspected for coating condition and the condition is reported as flaking or fracture measured in eighths of inches from the crown of the impression.

The mill treated hexavalent chromium-containing coated panels had a, rating of 1 on the adhesion test. The panels which were free of mill treatment coating also rated 1 on the adhesion test. The control samples were compared and found to have an adhesion rating of 5, both for the mill treated panel and the panel which was untreated at the mill.

The bend test showed that the mill treated panel was flaked for A3" and fractured for Ms from the small end of the mandrel, while the untreated panel was flaked for /8 and fractured for 7s. The non mill-treated control sample was flaked for a length of 27s, while the milltreated panel coating was peeled from the surface for a length of 4".

An inspection of the impact test panels showed that the untreated and the treated panels were flaked on the external convex surface in a diameter circle. In comparison of the same surfaces, the mill coated control sample was flaked in a diameter circle and the non millcoated panel was flaked in a diameter circle.

The painted panels were diagonally scratched from corner to corner and subjected to a 5% salt spray test and inspected after 476 hours. The salt spray results are reported in terms of 16ths of an inch creepage of the corrosion from the scratch mark as a range extending from the minimum creepage to the maximum creepage. The non mill-treated panels had a rating of 0-2, while the milltreated panels had a rating of 0-1.5. The non mill-treated control sample was 40% peeled after 72 hours and 100% peeled after 140 hours, at which time it was withdrawn from the test.

Other painted panels were subjected to the standard humidity test and after 504 hours the mill-treated panels showed some general blistering and some rinse blistering at the lower edge of the panel. The non mill-treated panels showed slightly less general blistering over the entire surface of the panel and slightly more lower edge blistering than the mill-treated sample. Non mill-treated control panels were so badly blistered at the end of 72 hours that they were withdrawn from the test.

A number of mill-treated panels were cleaned with conventional cleaners and contacted with the same phosphate coating solution, using the above procedures, for comparative purposes. The cleaner contained tetrasodium pyrophosphate, a wetting agent and titanium phosphate made by the procedures of US. Patent 2,874,081, and was used in the conventional manner. After the phosphate coating step, the panels were inspected and found to be essentially devoid of adherent phosphate coating.

These tests demonstrate that the preliminary treatment with an alkaline solution containing an oxidizing agent renders the mill-treated zinc surfaces at least as receptive to the phasphate coating from a conventional zinc phosphate coating solution as a corresponding zinc surface which was free of mill treatment coating. The tests further demonstrate that a mill-treated zinc surface which is not subjected to the alkaline solution treatment of this invention is non-receptive to an adherent corrosion resisting phosphate coating by use of conventional cleaning procedures and a conventional zinc phosphate coating solution.

in a commercial installation using the permanganatecontaining cleaner and the phosphate coating solution of this example of non-mill-treated continuous hot dip galvanized zinc stock, adherent phosphate coatings were formed on a continuous basis. These phosphate coated surfaces were painted with a vinyl type paint and found to have superior resistance to flaking and fracture during conventional roll forming operations to surfaces otherwise similar except prepared by using conventional cleaning and phosphate coating solutions.

The following solutions are given to further illustrate the relationship between concentration of oxidizing agent, pH, and alkaline material concentration, as well as a large number of specifically satisfactory oxidizing agents.

Each of the examples employs the zinc phosphate coating solution specified in Example I, as well as the dilute aqueous chromic acid rinse solution defined therein and the processing procedures and conditions specified in Example I were followed, except where exceptions are specifically noted.

Example II An aqueous solution was prepared containing 0.1 gram per liter of sodium hydroxide and 10 grams per liter of potassium permanganate. The solution had a pH of approximately 13. With the solution at a temperature of 168 F.-l70 F., mill-treated hexavalent chromiumcontaining coated zinc panels, Armco Zincgrip, were immersed in the bath for 1 minute, rinsed, phosphate coated, water rinsed and dried. The phosphate coating was found to be insufficiently adherent. Additional panels were processed in the same solution, in the same manner for two minutes and the resulting phosphate coating was found to be satisfactorily adherent.

Example III An aqueous solution was prepared containing 1 gram per liter of sodium hydroxide and 10 grams per liter of potassium permanganate. This solution had a pH of approximately 13. Mill-treated hexavalent chromiumcontaining coated zinc panels were immersed in the solution at 168 F.-l70 F. for 1 minute, withdrawn, rinsed, phosphate-coated, water rinsed and dried. The phosphate coating was found to be tight and satisfactorily adherent.

Example IV An aqueous solution was prepared containing 2.5 grams per liter of sodium hydroxide and 0.25 gram per liter of potassium permanganate. The zinc panels were treated in this solution for /2 minute, in accordance with the procedure of the above examples, and the coating was insufficiently adherent, but when the treating time was increased to 1 minute an adherent phosphate coating was obtained.

Another solution was prepared containing 2.5 grams per liter of sodium hydroxide and 0.12 gram per liter of potassium permanganate. Zinc panels of the type above identified processed in this solution in accordance with the above specified steps were coated with phosphate coatings having conventional appearance and moderate adherence, but they were less adherent than the coatings obtained in 1 minute in the solution containing 0.25 gram per liter of potassium permanganate.

Another solution was prepared to contain 7.5 grams per liter of sodium hydroxide and 0.06 gram per liter of potassium permanganate. Zinc panels immersed in this solution at 168 F.17.0 F. for 2 minutes, in accordance with the above specified steps formed satisfactorily adherent phosphate coatings, but the use of shorter time failed to produce satisfactorily adherent coatings.

Example V An aqueous solution was prepared containing 25 grams per liter of disodiurn monohydrogen phosphate and grams per liter of potassium permanganate. This solution had a pH of 8.6. Zinc panels of the above specified type, which were immersed in this solution for /2 minute with subsequent treatment in accordance with the above specified steps, produced phosphate coated panels having only moderate adherence, whereas, panels immersed in this solution for 1 minute produced tightly adherent phosphate coatings.

Separate baths of this type containing 10 grams per liter and 20 grams per liter, respectively, of disodium phosphate and each containing 10 grams per liter 'of potassium permanganate, did not produce on the surface of zinc panels, of the above specified type, an adherent phosphate coating after a '1-minute immersion treatment.

Example VI A series of aqueous solutions were prepared containing 2.5 grams per liter of sodium hydroxide and sodium nitrite in concentrations of 10 grams per liter, grams per liter, grams per liter, grams per liter and 60 grams per liter. None of these solutions satisfactorily conditioned mill-treated hexavalent chromium containing coated zinc panels to a phosphate coating receptive condition.

The sodium hydroxide concentration of the solution containing 10 grams per liter of sodium nitrite was increased to 50 grams per liter and immersion of milltreated zinc panels in the solution for 1 minute conditioned the surface to receive an adherent phosphate coating,

7 and such-a coating was formed by using this solution and the successive steps specified above in Example I.

Example VII A series of aqueous solutions were prepared containing 2.5 grams per liter of sodium hydroxide and sodium chlorate concentrations of 2.5 grams per liter, 12.5 grams per liter, 22.5 grams per liter, and 60 grams per liter. Using the processing conditions specified in Example I, and a contact time of 1 minute, no satisfactory adherent phosphate coatings were produced. When the sodium hydroxide concentration was increased to 50 grams per liter in the solution containing 10 grams per liter of sodium chlorate, satisfactory adherent phosphate coatings were produced on surfaces immersed in the solution for 1 minute.

Example VIII An aqueous solution was prepared containing 2.5 grams per liter sodium hydroxide and 2.5 grams per liter sodium bromate. Another solution wa prepared containing 7.5 sodium hydroxide and 7 .5 sodium bromate. Zinc panels of the above specified type immersed for 1 minute in the solution containing 2.5 grams per liter of sodium bromate failed to produce satisfactory adherent phosphate coatings, whereas, adherent phosphate coatings were produced after the use of 7.5 grams per liter of sodium bromate for 1' minute prior to phosphate coating.

7 8 Example IX An aqueous solution was prepared containing 10 grams per liter of caustic soda and 2.5 grams per liter of sodiurn meta nitrobenzene sulfonate. Zinc panels immersed for 1 minute in this solution at 168 F.170 F. Were coated with an adherent phosphate coating when subjected to the phosphate coating and chromic acid rinsing steps as specified above in Example I.

Example An aqueous solution was prepared containing 10 grains per liter of sodium hydroxide and 3.75 grams per liter of picric acid. Zinc panels immersed for 5 minutes in this solution at 168 F.-170 F. were coated with an adherent phosphate coating When subjected to the phosphate coating and chromic acid rinsing steps as specified above in Example I.

What is claimed is:

1. A method for conditioning zinc and zinc alloy surfaces to receive a phosphate coating which comprises the from the presence therein of about 0.2 gram per liter to about 3 0 grams per liter of the permanganate ion.

3. A method for conditioning zinc and zinc alloy surfaces to receive a phosphate coating which comprises the steps of applying to said surface an aqueous alkaline solution having a pH of about 9 to about 14 and containing an alkalinity equivalent to that produced by the presence therein of at least about 0.1 gram per liter of sodium hydroxide, said solution having dissolved therein an oxidizing agent, said solution having at least the oxidizing ability equivalent to that obtained from the presence therein of about 0.05 gram per liter of permanganate ion.

4. A method in accordance with claim 3 wherein said oxidizing agent is selected from the group consisting of the permanganates, perborates, chlorates, perchlorates, peroxides, bromates, iodates, nitrites and hypochlorites.

S. A method for conditioning zinc and zinc alloys having on the surface thereof a corrosion inhibiting coating, said process comprising the steps of applying to said surface an aqueous alkaline solution having at least the alkalinity equivalent to that resulting from the presence therein of about 0.1 gram per liter of an alkali metal hydroxide, and containing an oxidizing agent in an amount to give to said solution at least the oxidizing ability equivalent to that derived from the presence therein of about 0.2 gram per liter to about 30 grams per liter of the permanganate ion.

6. A method in accordance with claim 5 wherein said corrosion inhibiting coating is one which contains hexa-.

valent chromium.

7. A method in accordance with claim 5 wherein the said oxidizing agent is permanganate ion.

8. A method in accordance with claim 5 wherein the said oxidizing agent is meta nitrobenzene sulfonate.

9. A method in accordance with claim 5 wherein said oxidizing agent is picric acid. 10. A method for forming an adherent phosphate coatmg on zinc and zinc alloy surfaces which comprises the steps of applying to said surfaces an aqueous alkaline solution containing an oxidizing agent, said solution having at least the oxidizing ability equivalent to that of a solution having dissolved therein at least about 0.05 gram per liter of a permanganate ion, and thereafter Contacting the said surface with an aqueous acidic phosphate coating solution so as to form thereon an adherent phosphate coating.

11. A method for forming an adherent phosphate coating on zinc and zinc alloy surfaces, which surfaces are coated with a corrosion inhibiting hexavalent chromiumcontaining coating which comprises the steps of applying to said surface an aqueous alkaline solution having at least the alkalinity equivalent to that resulting from the presence therein of about 0.1 gram per liter of an alkali metal hydroxide and containing dissolved therein an oxidizing agent in an amount sutficient to produce at least the oxidizing ability equivalent to that of a solution having dissolved therein about 0.2 gram per liter to about 30 grams per liter of the permanganate ion, and thereafter contacting said surface with an aqueous acidic phosphate coating solution so as to form thereon an adherent phosphate coating. 1

12. A method in accordance with claim 11 wherein said oxidizing agent is the permanganate ion.

13. A method in accordance with claim 11 wherein said oxidizing agent is meta nitrobenzene sulfonate.

14. A method in accordance with claim 11 wherein said oxidizing agent is picric acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,335,868 Lodeesen Dec. 7, 1943 2,490,062 Jernstedt Dec. 6, 1949 2,859,146 Prust Nov. 4, 1958 2,884,351 Cavanagh et a1 Apr. 28, 1959

Claims (1)

1. A METHOD FOR CONDITIONING ZINC AND ZINC ALLOY SURFACES TO RECEIVE A PHOSPHATE COATING WHICH COMPRISES THE STEPS OF APPLYING TO SAID SURFACE AN AQUEOUS ALKALINE SOLUTION CONTAINING AN OXIDIZING AGENT, SAID SOLUTION HAVING AT LEAST THE OXIDIZING ABILITY EQUIVALENT TO THAT OBTAINED FROM THE PRESENCE THEREIN OF ABOUT 0.05 GRAM PER LITER OF PERMANGANATE ION.