US3519494A - Method for coating ferrous metal surfaces - Google Patents

Description

United States Patent 3,519,494 METHOD FOR COATING FERROUS METAL SURFACES Rudolf Engesser, Frankfurt am Main, Richard Tuch, Habsburger Allee, Werner Rausch, Stierstadt, Taunus, and Winfried Menzer, Sprendlingen-Hirschsprung, Germany, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed June 21, 1967, Ser. No. 647,595 Claims priority, application Germany, July 12, 1966,

1,521,879 Int. Cl. C23f 7/10 U.S. Cl. 148-615 Claims ABSTRACT OF THE DISCLOSURE A process for forming a protective and/or paint-base coating on ferrous metal surfaces wherein the surface to be treated is contacted with an aqueous acidic zinc phosphate solution containing fluoride, at least one oxidizing agent accelerator, boric acid, and at least three milligrams :per liter of an activating acting titanium phosphate. Desirably, the phosphatizing solution has a total P 0 content within the range of about 2 to 12 grams per liter and the solutions desirably also contain a nonionic wetting agent. Nitrate and nitrite ions are the preferred oxidizing agent accelerators in the solution.

This invention relates to an improved process for coating ferrous metal surfaces and more particularly relates to an improved method for forming thin, hard, tightly adherent zinc phosphate coatings on ferrous metal surfaces, which coatings have excellent corrosion resistant and paintbase properties.

Heretofore, zinc phosphate coating solutions have been widely used in the chemical surface treatment of ferrous metals, such as iron and steel, as a preparation for the subsequent application of paint or lacquer. The zinc phosphate layers produced by these processes are known to increase the corrosion resistance and improve the adhesion of paint or lacquer films which are applied to the treated metal surfaces. Where thin, hard zinc phosphate layers are formed, these have been found to give excellent adhesion values even where the painted part is subjected to bending stresses.

Various modifications of a typical phosphatizing solutions have been proposed in order to obtain this desired, thin phosphate layer. Thus, it is known that the addition of small amounts of a polyphosphate to a nitrate accelerated zinc phosphate bath will result in appreciable reductions in the coating weight of the phosphate layer which is produced. It is also known that similar results can be obtained by the addition of large amounts of calcium to zinc phosphate baths which contain oxidizing agents. In both of these processes, however, difficulties are encountered in the analyses of the processing solutions which are necessary in order to maintain proper solution component concentrations and ratios. Thus, with both of these methods, the replenishing of the operating solution is frequently effected largely on the basis of estimation, which frequently results in a buildup of the polyphosphate or calcium in the solution, both of which are detrimental to the coatings produced.

It is also known, particularly with respect to coating aluminum and aluminum alloys, that thin, hard coating layers may be obtained by the use of a hot aqueous coating solution containing monophosphate, an oxidizing agent, fluoroborate and an excess of boric acid. Frequently, however, the thin, hard phosphate layer produced by this method is not completely homogeneous,

particularly in areas where there are traces of dust, fingerprints, or abraded areas on the metal surface. At these points, there is often produced a lighter colored coating having a different crystal structure then that of the main coating and this has an unfavorable effect on a subsequently applied paint or lacquer, particularly on that applied electrophoretically.

It is, therefore, an object of the present invention to provide an improved process for forming thin, hard zinc phosphate coatings on ferrous surfaces.

A further object of the present invention is to provide an improved process for forming zinc phosphate coatings on ferrous surfaces, which coatings are substantially homogeneous and have excellent corrosion resisting and paint-base properties.

These and other objects 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 producing a thin, hard, sustantially homogeneous phosphate coating on ferrous metal surfaces which comprises contacting the ferrous metal surface to be treated with a coating composition comprising an aqueous acidic zinc phosphate solution, which solution contains at least one oxidizing agent accelerator, boric acid, fluoride, and an activating-acting titanium phosphate in an amount of at least three milligrams per liter, and maintaining the coating composition in contact with the metal surface to be treated for a period suflicient to form the desired coating thereon. In this manner, there is produced a very thin, hard zinc phosphate coating which is substantially homogeneous, and which has a very fine crystalline structure.

More specifically, in the practice of the method of the present invention, the ferrous metal surface to be treated, e.g., a surface of iron or steel, is contacted with an aqueous acidic phosphatizing solution containing zinc ions, phosphate ions, fluoride ions, an oxidizing agent accelerator, boric acid, and at least three miligrams per liter of an activating-acting titanium phosphate. Desirably, the total P 0 content of the solution is within the range of two to twelve grams per liter. As is known in the art, phosphatizing solutions of this general type contain the zinc ions in the form of zinc dihydrogen phosphate and, desirably, have a pH Within the range of about 2.0 to 3.5.

Various oxidizing agent accelerators may be utilized in the present processing solution, such as nitrates, nitrites, combinations of nitrates and nitrites, chlorates, bromates, and the like, as are known to those in the art. Typically, these oxidizing agent accelerators are present in amounts up to about 3% by weight of the phosphatizing solution, with amounts within the range of about 0.01 to 1% by weight being preferred. In many instances, the preferred oxidizing agent accelerator is a combined nitrate-nitrite accelerator and reference will be made hereinafter to phosphatizing solutions containing such accelerators. In such solutions, the nitrate ions are typically present in amounts within the range of about 0.05 to 2% by weight of the phosphatizing solution while the nitrite ions are present in amounts within the range of about 0.001 to 0.05% by weight of the solution. It will be appreciated, however, that as is known to those in the art, amounts of these oxidizing agent accelerators outside of these typical ranges may also be used.

The boric acid component of the subject phosphatizing solution is desirably present in an amount of at least 0.5 gram per liter of H BO with greater amounts, up to the maximum solubility of the boric acid in the solution, being suitable. The boric acid may be introduced into the treating solution as such, i.e., as H BO or it may be added as various boron containing compounds which will form boric acid in the aqueous acidic phosphatizing solution. Exemplary of such boron containing compounds which may be used are alkali metal borates, such as Na B O 1OH O and NaBO -SH O. The boric acid or boric acid forming compounds may be added directly to the phosphatizing bath or they may be added to the concentrate compositions which are used in making up and replenishing the phosphatizing bath and, thus, added to the bath in this manner.

The activating-acting titanium phosphates, which are present in the treating solution in an amount of at least 3 milligrams per liter, are customarily used in aqueous solution as a pretreatment for metal surfaces before the application of a phosphate coating. Typically, these materials are compositions prepared from disodium orthophosphate and a soluble titanium compound such as titanyl sulfate. The processes for the preparation of these activating-titanium phosphates are set forth in German Pats. 1,144,565 and 885,638. Although the activating-titanium phosphate is present in the treating solution in an amount of at least 3 milligrams per liter, greater amounts, up to about 1 gram per liter are often typical. It is to be appreciated, of course, that in some instances amounts of the activating-acting titanium phosphate in excess of these typical amounts may also be used with satisfactory results.

It has been found that by incorporating the titanium phosphate materials in the phosphatizing bath, the rate at which the coating layer is formed is increased. Moreover, it is found that an improvement in the coating itself is obtained which is in excess of that obtained by the individual components of the phosphatizing bath, independent of their concentrations in the solution. Finally, it is found that a high degree of uniformity of the phosphate coating is obtained when the activating-acting titanium phosphates are included in the composition, the phosphate coating being formed uniformly on the metal surface even in those areas which contain traces of dirt, fingerprints, or surface abrasions. It is to be noted, however, that inasmuch as the titanium phosphates are colloidally suspended in the phosphatizing solution, it is desirable that replenishment of the activating-acting titanium phosphate in the bath be carried out frequently or even substantially continuously.

The metal treating solutions of the present invention also contain fluoride, desirably in amounts within the range of 0.05 to 3% by weight of the treating solution. The fluoride may be present in the solution as simple fluorides, which are added as HF, alkali metal fluorides, or the like, or they may be in the form of the more complex fluorides, such as the fluoborates. As such, the fluorides may be added as fiuoroboric acid, or the alkali metal fluoroborates, or the like. It is to be appreciated, of course, that as is known to tthose in the art, the fluorides may be incorporated in the present processing solutions in amounts which are outside of the preferred ranges indicated above, as well as in other forms which will not be detrimental to the processing solution.

In many instances, it has also been found to be desirable to include a non-ionic wetting agent in the phosphatizing solution. In this manner, a degreasing and dirt removing action is imparted to the phosphatizing solution, particularly when the solution is applied by spraying. Desirably, the wetting agent is present in the solution in an amount within the range of about 0.05 to 1 gram per liter of the phosphatizing solution. Although various non-ionic wetting agents may be used, the preferable wetting agents have been found to be the hydroxyethylated alkyl phenols and straight chain fatty alcohols, particularly those having an etherified terminal OH group in the ethylene oxide chain, which latter wetting agents are characterized by their low foaming properties.

The phosphatizing solutions of the present invention may be formulated using any convenient source of materials which provide the desired ccmponents in the solution. Examplary of such materials are zinc oxide, phosphoric acid, zinc dihydrogen phosphate, zinc nitrate, and the like. Additionally, the nitrite ions, which are accelerators in the bath, may conveniently be added as the alkali metal nitrite, such as sodium nitrite. Moreover, as has been indicated hereinabove, boric acid may be added either as such or as a boron containing compound such as an alkali metal borate, which will form boric acid in the solution and the fluoride ions may be added as the simple or complex fluorides, either as the. acid or in the form of fluoride salts. While other materials may be used to formulate the phosphatizing solutions, as are known to those in the art, it is to be appreciated that in choosing these materials, the materials used are desirably those which will not introduce extraneous ions into the treating solution or, that at least will not introduce ions into the solution which are detrimental either to the solution itself or to the coating which is produced.

Desirably, the phosphatizing solutions are at a pH within the range of about 2.0 to 3.5 and are used at a temperature within the range of about 40 to degrees centigrade, although operations outside of these ranges may also be carried out in many instances. The phosphatizing solution may be applied using any suitable application technique, including immersion, flooding, spraying, and the like, although the advantages of the present process are particularly apparent when using flooding and spraying methods.

The phosphatizing solutions as described above, are brought into contact with the ferrous metal surfaces to be treated, using any suitable application techniques, as have been indicated. The metal surfaces are maintained in contact with the phosphatizing solution for a period suflicient to effect the formation of the desired phosphate coating on the metal. Typical cotnact times which may be used are within the range of about 1 to 4 minutes, although contact times outside of this typical range will be used in many instances, depending upon the particular application techniques which are employed. After the desired phosphate coating has been formed on the metal surface, the surface may then be given a final rinse with a trivalent or hexavalent chromium-containing solution, aqueous solutions containing from about 0.01 to 1% by weight of CrO either alone or in admixture with other acids such as phosphoric acid, being typical of the rinse solutions which may be used. The thus-treated metal surface may then be given a protective coating of a paint or lacquer, which paint or lacquer coating may be applied by conventional dip, spray or flooding techniques, or by electrophoretic means, the conditions of such application techniques being known to those in the art. The zinc phosphate coatings which are thus formed on the ferrous metal surfaces treated are found to have a very fine-grain crystalline structure and are dark in color. Typically, the weights of these coatings are within the range of about 1 to 2 grams per square meter.

It is found that with the phosphatizing solutions described above, which contain the non-ionic wetting agent, the present process may often be carried out using fewer treating stages, because of the combined degreasing and dirt removing action with the phosphatizing action of the solution. Thus, in the first treating zone, the metal surfaces are both degreased and phosphatized, water rinsed in the second zone, and then given a final rinse or treatment with the aqueous acid solution of hexavalent or trivalent chromium, in the third zone. In many instances, because of the excellent corrosion protection provided b the coatings which are produced in accordance with the present method, particularly when these coatings are used in conjunction with paint or lacquer film, it is possible to eliminate the final after rinse with hexavalent or trivalent chromium, without any appreciable reduction in the corrosion protection which is obtained.

It has also been found in using the phosphatizing solutions which have been described, substantially all of the grease and dirt removed from the metal surfaces is retained in the phosphate sludge which forms in the coating solution. Thus, the solution containers and the walls in the phosphatizing zone remain substantially free of greasecontaining deposits and the grease-containing posphate sludge is easily removed from the coating bath by filtration. This combined degreasing and phosphatizing also results in far less incrustration of spray nozzles in the phosphatizing bath, as well as of the heat recorder, bath and tunnel walls, as compared to that obtained in a sequential degreasing and phosphatizing process. Additionally, with this combined phosphatizing and degreasing action, any prepassivation of the metal surface is minimized. In contrast, this prepassivation is frequently encountered in a sequential degreasing-phosphatizing process, as a result of the initial reaction of the phosphatizing solution mist with the clean work surface as it is introduced into the phosphatizing zone. This reaction results in the formation of thin, tarnish layers on the metal surface which hinder the formation of a homogenous zinc phosphate coating during the phosphatizing step.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percent are by weight. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

EXAMPLE 1 An aqueous phosphatizing solution was prepared which contained the following components in the amounts indicated:

Components: Grams per liter Zinc 3.15 P 6.75 N0 2.55 NaNO 0.15

This solution was then modified by the addition of the additive as shown in the following table, and the ratio of free P 0 to total P 0 was adjusted by the addition of sodium hydroxide to the solution to the values shown in the table. Steel plates degreased in an alkaline spray cleaner were rinsed with water and then sprayed for three minutes with these solutions which were at a temperature of about 60 degrees centigrade. The coating weights obtained in each instance and the uniformity of the coating are given in the table. It is to be noted that the activatingacting titanium phosphate, which is added to some of the solutions, was a mixture of 90% of Na HPO and of titanium phosphate.

6 EXAMPLE 2 The steel plates which had been treated with solutions a through k of Example 1 were then coated electrophoretically with a lacquer and with a monolayer acrylate lacquer. The thus-coated plates were then tested by bending over a conical mandrel and were also subjected to the salt spray test using the procedure ASTM Bl17-54-T. It 7 was found that the lacquer adhesion of the plates which had been treated with solutions a, c, d, e, f and g, was poor, in that the lacquer was loosened when using a bend mandrel diameter of from 10 to 30 millimeters. Additionally, after 48 hours in the salt spray test, the electrophoretically applied lacquer on the plates was loosened at distances of from 2 to 3 millimeters from the scratch site while the acrylate resin lacquer, after 96 hours in the salt spray showed loosening of from 1 to 1.5 millimeters at the scratch site. The panels which had been treated in solutions b, h and i of Example 1 although having acceptable lacquer adhesion and corrosion resistance, evidenced nonuniformity in the lacquer coatings, due to the poor uniformity of the phosphate coating which had previously been applied. In contrast, the plates treated with solution been applied. In contrast, the plates treated with solutions i and k of Example 1 were found to have excellent lacquer adhesion and corrosion resistance and, additionally, had a substantially completely uniform lacquer coating.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is intended that each element recited in any of the following claims is 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 ma be utilized.

What is claimed is:

1. A process for forming a thin, hard, substantially uniform phosphate coating on ferrous metal surfaces which comprises contacting the ferrous metal surface to be treated with a coating composition consisting essentially of an aqueous acidic zinc phosphate solution, having a total P 0 content of from about 2 to 12 grams per liter, which solution contains at least one oxidizing agent accelerator in an amount up to about 3% by weight of the solution, boric acid in an amount of at least about 0.5 gram per liter, fluoride ions in an amount within the range of about 0.05 to 3% by weight of the solution, and at least 3 milligrams per liter of an activating titanium phosphate Coating weight Uniformity Free P205, in grams] of Additive and amount Total P205 sq. meter coating None 0. 12 2. 2-2.4 Poor.

4 grams/liter NazB 0 .10H20 0. 12 1. 3-1. 5 Do. one 0. 08 2. 6-2. 8 Do. 4 grams/liter NazB O1.10HzO 0. 08 2. 4-2. 6 D0. e 0.2 gram/liter activating-acting titanium phosphate..- 0. 12 2. 22. 3 Average f 0.6 gram/liter NaBF4 0. 12 2. 0-2. 2 Poor. g do 0. 08 2. 3-2. 5 D0; h. 0 6 gram/liter NaBF4, 4 grams/liter NazB40 0. 08 1. 3 Do. ido 0.12 1.3 Do 3. 0.6 gram/liter NaBF4, 4 grams/liter N82B4O1.l0H2O, 0. 08 1. 3 Good.

0.2 gram/liter activating-acting titanium phosphate. k do 0. 12 1. 3 Do.

From the above results, it is seen that only with solutions j and k, which solutions contain fluoride, boric acid, and the activating-acting titanium phosphate, in accordance with the method of the present invention, are the desired thin, hard zinc phosphate coatings obtained which are substantially uniform in nature.

and maintaining the ferrous metal surface to be treated in contact with the solution for a period sufiicient to form the desired coatin.

2.. The method as claimed in claim 1 wherein the oxidizing agent accelerator in a combination of nitrate and 7 nitrite ions.

7 8 3. The method as claimed in claim 2 wherein the phos- References Cited phatizing solution has a pH Within the range of about 210 UNITED STATES PATENTS to 2,322,349 6/1943 Iernstedt 148-6.15 4. The method as claimed 1n claim 3 wherein the con- 2,479,564 8/1949 Gilbert 4g 5 tact of the treating solution with ferrous metal surface 5 3,090,709 5/1953 H ick 148-6.15 is effected by spraying the solution on the metal surface. 3,420,715 1/ 1969 Ayres 1486.1S X

5. The method as claimed in claim 4 wherein the oxidiz- FOREIGN PATENTS ing agent is present in an amount within the range of about 0.01 to 1% by weight, and the titanium phosphate is 10 present in an amount up to about 1 gram per liter. R ALPH KENDALL, p i Examimr 655,079 7/ 1951 Great Britain.