US3404046A - Chromating of zinc and aluminum and composition therefor - Google Patents

Description

United States Patent 3,404,046 CHROMATING 0F ZINC AND ALUMINUM AND COMPOSITION THEREFOR William S. Russell, Warren, and Samuel Farina, Mount Clemens, Mich, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y. No Drawing. Filed Sept. 25, 1964, Ser. No. 399,365 11 Claims. (Cl. 148-611) ABSTRACT OF THE DISCLOSURE A process for treating Zinc and aluminum surfaces wherein the surface is first contacted with an alkaline solution having a pH of at least 11, the surface being maintained in contact with the alkaline solution for a period sufficient to effect activation of the surface. The thus-activated surface is then coated with an acidic chr0- mate conversion coating solution, having a pH within the range of about 0.62 to 3.5, and the coated surface is then rinsed with a hexavalent chromium containing rinse solution. A preferred chromate conversion coating solution for use in this method is one which contains at least 0.5 gram per liter of zinc ions, from about 0.5 to 4.5 grams per liter of hexavalent chromium ions, calculated as CrO from about 0.5 to grams per liter of sulfate ions and at least 0.15 gram per liter of a fluoride radical. The solution may also contain at least 0.25 gram per liter of trivalent chromium ions.

This invention relates to improved compositions and processes for providing a protective coating on metal surfaces and more particularly it relates to such improved compositions and processes for providing surfaces which are predominately of zinc or aluminum with a protective chromium-containing coating of the chemical conversion type.

In recent years, there has been an increased demand for a chromate conversion coating for zinciferous surfaces. By zinciferous surfaces it is intended to refer to alloys wherein zinc is the principal ingredient, as well as to hot-dipped galvanized surfaces, electro-deposited zinc coatings and to essentially pure zinc surfaces. To be acceptable to the trade, however, the chromate conversion coatings for such zinciferous surfaces must provide protection for the surface coated against corrosion and also must have good paint bonding qualities, so as to serve as a paint base material.

In the past, the chromium conversion coatings which have been available have, for the most part, not been accepted in the trade because of their inability to satisfactorily meet the requirements for corrosion protection and paint bonding qualities. Although attempts have been made to improve the quality of the chromate chemical conversion coating materials used, in many instances, these improvements have been more than offset by the technological advances in the art of applying coating materials to the substrate to be protected. For example, in strip line applications, wherein the major amount of a satisfactory chromate conversion coating would be used, it is now possible to operate such lines at speeds in excess of 500 feet per minute. At such high speeds, even the improve-d chromate conversion coatings have not been capable of forming an adequate substrate on the zinc surfaces coated for the application of subsequent paint or similar protective films. Additionally, the corrosion protection provided by the coatings applied at such high speeds frequently falls below that which is required in the industry. Accordingly, up to the present time; there has not been available a chromate conversion coating material for providing on aluminum or zinciferous sur- 3,404,046 Patented Oct. 1, 1968 "ice faces a coating of consistently high quality, in terms of the corrosion protection and paint bonding qualities, particularly where such coatings are applied on a high speed strip line which is now commonly used.

It is, therefore, an object of the present invention to provide an improved coating material of the chromate conversion type, which coating material may be applied to aluminum or zinciferous surfaces even on high speed strip lines.

Another object of the present invention is to provide an improved chromate conversion coating for application to aluminum or zinciferous surfaces, which coating has consistently good corrosion resistance and paint bonding qualities.

A further object of the present invention is to provide an improved process for treating surfaces which are predominantly of Zinc or aluminum so as to provide thereon a chromate conversion coating having good corrosion resistance and paint bonding qualities.

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, it has now been found that the formation of a satisfactory chromate conversion coating on surfaces of aluminum or zinc depends to a large extent on the treatments which precede and follow the application of the chromate conversion coating material. Accordingly, the present invention includes a process for treating surfaces which are predominantly of zinc or aluminum, which process comprises contacting the surface to be treated with an alkaline solution having a pH of at least about 11, maintaining the alkaline solution in contact with the surface for a period of time sufiicient to activate the surface, contacting the thus-activated surface with a chromate conversion coating solution, maintaining said coating solution in contact with the metal surface for a period of time sufficient to effect formation of the desired chromate conversion coating thereon and, thereafter, contacting the thus-coated surface with an aqueous solution containing hexavalent chromium, calculated as CrO in an amount of at least 0.025 percent by weight of the solution.

More specifically, in the practice of the process of the present invention, a zinciferous surface is activated by contacting it with an alkaline solution having a pH within the range of about 12 to about 13.5, and preferably having an alkali concentration within the range of about 3 to about 12 grams per liter. Various alkaline solutions, which will provide the desired pH as has been indicated hereinabove, may be used. Typically, these solutions are aqueous solutions containing alkali metal hydroxides, alkali metal carbonates, alkali metal phosphates, alkali metal silicates, and the like. Exemplary of the alkali metal phosphates which may be included in the composition are the trialkali metal phosphates, the tetra-alkali metal pyrophosphates, the alkali metal tripolyphosphates, and the like. It will be appreciated that as used in the specification and claims, the term alkali metal compound is intended to refer to the compounds of lithium, sodium, potassium, cesium, and rubidium. Of these, the preferred alkali metal compounds are generally those of sodium and, accordingly, specific reference will be made hereinafter to such compounds. This is not, however, to be taken as a limitation of the alkali metal compounds which may be used as other alkali metal compounds, and particularly potassium compounds have been found to be suitable for use in the process of the present invention. Of the compounds which hav been indicated hereinabove as being suitable for inclusion in the present alkaline treating solution, the preferred compounds for formulating the treating composition are the alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, trisodium or tripotassium phosphate, and the alkali metal orthosilicates, such as sodium orthosilicate.

The aqueous alkaline solution may be applied to the zinciferous surface to be treated using various techniques, such as immersion, flooding, spraying and the like, with spraying techniques generally being preferred. Desirably, the alkaline solution at the time of application to the zinciferous surface is at an elevated temperature, with temperatures within the range of about 60 to about 90 degrees centigrade being typical and temperatures within the range of 65 to about 80 degrees centigrade being preferred. The hot alkaline solution is maintained in contact with the surface to be treated for a period of time sufficient to effect the desired activation of the zinciferous surface. Generally, contact times of up to about 2 minutes are typical, with contact times of about seconds to about 1 minute being preferred. By this use of an alkaline treating solution, and particularly where passivated or chemically treated hot dip galvanized stock is being treated, it is found that there is an appreciabl upgrading in the adhesion rating for paint applied to such stock after it has been coated With a chromate chemical conversion coating.

After the zinciferous surface has been treated with the alkaline solution, it is brought into contact with a chromate conversion coating material for a period of time suflicient to effect the formation of the desired chromate conversion coating on the zinc surface. As will be pointed out in more detail hereinafter, the chromate conversion coating materials used may be of various types, including one of the numerous conventional chromate conversion coating materials which are known in the art. Generally, it is preferable if the zinc surface is subjected to a water rinse both before and after the application of the chromate conversion coating material. In this manner, any alkaline solution remaining on the zinc surface is removed before the surface is contacted with the chromate conversion coating material, and, similarly, any non-reacted conversion coating material is removed from the surface prior to subsequent treating steps. The rinsing operation may be carried out in any convenient manner, as for example, by immersing, spraying, or flooding the treated surface with water.

After the treatment of the zinciferous surface with a chromate conversion coating material, the thus-coated surface is then contacted with an aqueous solution containing hexavalent chromium, calculated as CrO in an amount within the range of about 0.025 to about 0.35 percent by weight of the aqueous solution. This solution, as with the alkaline solution, may be applied to the surface to be treated using various means, including roller, immersion, flooding, or spraying techniques, and the like, with spraying techniques being preferred. Additionally, if desired, after the application of the hexavalent chromium solution to the surface the excess of this solution may be removed from the surface, preferably by wiping or squeegeeing. With respect to such treatment, it has been found that where a squeegeeing or similar treatment of the surface is used, the concentration of the hexavalent chromium solution is desirably within the range of about 0.06 to about 0.35 percent by weight of the solution and preferably within the range of about 0.07 to about 0.15 percent by weight of the solution. Where no squeegee or similar treatment for removal of the excess solution is used, however, it has been found that the concentration of the hexavalent chromium, calculated as CrO is desirably within the range of about 0.025 to about 0.2 percent by weight of the solution and preferably within the range of about 0.05 to about 0.1 percent by weight of the solution.

The aqueous hexavalent chromium solution used is a water solution containing a source of hexavalent chromium, in the amounts as have been indicated hereinabove. Various water-soluble or water dispcrsible sources of hexavalent chromium may be used in formulating this solution, provided the anions or cations introduced with the hexavalent chromium do not have a detrimental effect on either the solution itself or on the coated zinc surfaces which are treated. Exemplary of hexavalent chromium materials which may be used are chromic acid (CrO the alkali metal chromates, such as sodium chromate and potassium chromate, the alkali metal dichromates, such as sodium dichromate and potassium dichromate, and the like. Preferably, the aqueous hexavalent chromium containing solution is maintained at an elevated temperature while it is in contact with the surface to be treated, temperatures within the range of about 35 to about 60 degrees centigrade and contact times of up to about 60 seconds being typical. By this use of the hexavalent chromium containing solution, in the manner as has been described hereinabove, following the application of the chromate conversion coating, it is found that there is obtained a marked improvement in the salt spray protection provided for the zinc surfaces which have been treated, particularly when these surfaces are commercial hot dip galvanized surfaces.

As has been indicated hereinabove, numerous chromate conversion coating materials may be used with the process of the present invention. Exemplary, but by no means all inclusive of the chromate conversion coating composition which may be used are those described in U.S. Patents 2,035,380, 2,296,884, 2,393,640, 2,393,633, 2,393,664, 2,393,665, 2,477,310, 2,483,510, 2,494,908, 2,497,905, 2,499,231, 2,504,434, 2,524,577, 2,796,372, 2,799,601, 2,843,513, 2,844,496 3,090,710, 3,130,085 and 3,130,086.

In addition to the above compositions, it has been found that particularly good results are obtained when the chromate conversion coating composition used in the process of the present invention is an aqueous solution which comprises at least 0.5 gram per liter of zinc ions, from 0.5 to about 4.5 grams per liter of hexavalent chromium ions, calculated as CrO from about 0.5 to about 40 grams per liter of sulfate ions and at least 0.15 gram per liter of a fluoride radical. In many instances, it has also been found desirable to incorporate in the above composition trivalent chromium ions in an amount of at least 0.25 gram per liter. This composition, when applied to surfaces which are predominantly of zinc or aluminum, produces a chromate conversion coating which is highly corrosion resistant and has excellent paint bonding characteristics. Accordingly, this composition is the preferred chromate conversion coating material for use in the process of the present invention.

More specifically, in the preferred composition for use in the process of the present invention, the zinc ions, which are calculated as Zn, are present in an amount of at least 0.5 gram per liter and preferably in an amount within the range of about 1 to about 15 grams per liter. The zinc ions may be added to the composition of the present invention in any convenient form which is water dispersible and preferably water soluble, provided that when the zinc is added as a zinc salt, the anion portion of the salt does not have a detrimental effect on either the solution itself or on the coating which is ultimately produced. Accordingly, the Zinc may be added to the present composition as zinc oxide, zinc sulfate, zinc dichromate, zinc fluosilicate, and the like, or, in some instances, may even be added as zinc metal. Typically, the zinc is added to the composition as the salt of one of the other anions which are part of the composition, and preferably as the zinc sulfate.

With regard to the hexavalent chromium ion, this is calculated as CrO and is present in the composition in an amount within the range of about 0.5 to about 4.5 grams per liter, and preferably in an amount within the range of about 1 to about 3 grams per liter. As with the zinc ions, the hexavalent chromium ions may be introduced into the present composition in any suitable water dispersible form in which the anions or cations introduced with the hexavalent chromium are not detrimental to the composition or the subsequently produced coating. In many instances, it has been found to be preferable to add the hexavalent chromium ions to the composition in the form of chromic acid, CrO or, as it is sometimes referred to, chromic trioxide. Additionally, however, the hexavalent chromium may be introduced into the composition as a hexavalent chromium salt of zinc, such as zinc chromate or zinc dichromate.

The sulfate ions in the present composition are present in an amount within the range of about 0.5 to about 40 grams per liter and preferably in an amount within the range of about 2 to about 35 grams per liter. Generally, it is preferred to introduce the sulfate ions into the solution of the present invention as the salt of one of the metal substituents of the composition such as in the form of zinc sulfate, chromic sulfate, and the like. In some instances, as for example when it is desired to adjust the pH of the solution, the sulfate ions may be added as sulfuric acid.

As has been indicated hereinabove, the preferred aqueous acidic composition for use in the process of the present invention also contains a fluoride radical in an amount of at least 0.15 gram per liter, and preferably in an amount within the range of about 1 to about grams per liter. Typical of the fluoride radicals which may be utilized in the present composition are complex fluoride radicals such as fluotitanate (TiF fluoborate (BF fluosilicate (SiF and fluoaluminate (AlF as well as simple fluoride radical, i.e., (F)-. These, of course, may be introduced into the composition in any substantially water dispersible and preferably water soluble form, such as in the form of the corresponding zinc salt. In some instances, and in particular, when a complex fluoride radical is used, such as the fluosilicate radical, it may be introduced into the composition as the acid, i.e., fluosilicic acid or as HF.

In addition to the above components, it has also been found to be desirable to incorporate into the preferred chromate conversion composition of the present invention, trivalent chromium ions, generally in an amount of at least about 0.25 gram per liter, and preferably in an amount within the range of about 3 to about 9 grams per liter. As has been indicated hereinabove, the trivalent chromium is desirably introduced into the composition as chromic sulfate although other trivalent chromium salts may also be used, provided that the anions of these salts have no detrimental effect on either the coating solution or on the coatings produced on the surfaces treated.

In addition to the trivalent chromium ions, in many instances, it has also been found to be desirable to incorporate in the composition of the present invention nitrate ions in an amount of at least 0.05 percent by weight of the composition. Generally, it is preferred that where nitrate ions are included in the composition, they are added as nitric acid although water dispersible nitrate salts may also be used. Where the nitrate ions are added in the preferred manner, i.e., as nitric acid, it is generally desirable that the amount of nitric acid added is not sufficient to lower the pH of the solution below that at which the efficient coating of the zinc or aluminum surfaces is obtained. In this latter regard it has been found to be desirable that the pH of the coating solution of the present invention be maintained within the range of about 0.62 to about 3.5 with a pH value within the range of about 1.6 and about 2.5 being preferred. Accordingly, with regard to the addition of nitric acid to the present composition, it is desirable that the quantity of the nitric acid added is not sufficient to lower the pH of the solution below about 0.62. It will be appreciated by those in the art, that the desired pH of the present coating solution may be maintained by adjusting the amounts of the various components of the composition, within the ranges as has been indicated hereinabove. Generally, and perhaps most typically, the pH of the solution will be adjusted by the addition of sulfuric acid thereto.

Typically, and in a preferred embodiment of the present invention, the subject coating composition will be in aqueous acid solution having a pH within the range of about 1.6 to about 2.5, which contains the following components in amounts indicated: Zinc, calculated as Zn, from about 1 to 15 grams per liter, hexavalent chromium, calculated as CrO from about 1 to about 3 grams per liter, sulfate ions, calculated as 80,, from about 2 to about 35 grams per liter, complex fluorides, such as SiF in an amount of at least about 0.15 gram per liter. Desirably, the composition will also contain trivalent chromium in an amount of at least about 0.25 gram per liter.

As with the alkaline solution and the hexavalent chromium rinse solution, the chromate conversion coating solution, including the preferred solution, as has been described above, may be applied to the zinciferous surfaces to be treated using various coating techniques. For example, the coating may be applied by immersing the surfaces to be coated in the coating solution, or by applying the solution to the surfaces by means of coating rolls, flooding, spraying, and the like. For many applications, and in particular, when applying the composition to a zinc surface in the form of a continuous strip, spraying techniques are preferred for applying the chromium conversion coating compositions. Generally, the temperatures at which the chromate conversion coating compositions are applied have not been found to be critical, the temperature of the compositions being variable over a relatively wide range. For example, application temperatures within the range of about room temperature, i.e. about 20 degrees centigrade, up to 70 degrees centigrade are typical with temperatures within the range of about 25 to about 45 degrees centigrade being preferred. In many instances, even lower temperatures such as temperatures as low as 12 degrees centigrade as Well as higher temperatures, such as to degrees centigrade have also been used to obtain excellent chromate conversion coatings on the surfaces treated.

In processing zinc surfaces in accordance with the method of the present invention, the following processing cycle has been found to be desirable. Initially, the zinc surfaces to be coated are treated with an alkaline solution, as has been heretofore described, which solution is preferably at a temperature of about 65 to 80 degrees centigrade and may be applied to the surface being treated using the various application techniques as indicated hereinabove, with the spraying techniques being preferable. After the application of the akaline solution to the surface to be treated, the surface is desirably rinsed in water, and preferably in a hot water spray using water at a temperature within the range of about 60-85 degrees centigrade. Thereafter, a chromate conversion coating solution and preferably the preferred composition described above, is applied to the zinc surface, desirably by spraying. After the desired chromate conversion coating has been formed on the zinc surface, the thus formed coating is preferably given a cold water rinse, again preferably using a Water spray, and is then contacted with the above described aqueous hexavalent chromium rinse solution, preferably by spraying the hexavalent chromium solution at a temperature within the range of about 35 to about 55 degrees centigrade. Thereafter, the excess moisture is preferably removed from the treated zinc surface, as by means of a squeegee, and the surface is then dried, temperatures within the range of about 100 to degrees centigrade and times up to about 5 minutes being typical.

In order that those in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. It is to be appreciated, however, that these examples are merely exemplary of the preferred embodiments of the present invention and are not intended to be taken as a limitation of the invention.

In this application, unless otherwise indicated, the temperatures are expressed in degrees centigrade and the parts and percentages are by weight. Additionally, unless otherwise indicated, the coating compositions are applied to high-speed continuous ht-dip galvanized surfaces, known commercially as zincgrip and said to be obtained by the continous hot-dip zinc coating process disclosed in US. Patent 2,197,622. After these surfaces were coated, they were painted with a vinyl paint and then subjected to a 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-1654-61. 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 D71456 and is reported as follows. D--dense; MD medium dense; Mmedium; FMfew medium; F-few; and VF-very few. In both the salt spray and humidity tests, 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 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 coating the surfaces which were tested, various aqueous chromate conversion coating solutions were formulated by admixing with water, in the amounts indicated, the components as given hereinafter in Examples l-20. In these examples, the fluoride ion was added as the fiuosilicate radical.

Components in grams/liter Example pH CrO Cr Zn+ SO F- N03 Alkaline treating solutions were also formulated by admixing in Water, in the amounts indicated, the components as set forth in the following table:

were then rinsed with hot water by spray for about 15 seconds. The thus-rinsed panels were then' sprayed for from 5 to 15 seconds with one of the chromate conversion coating solutions given in Examples 1-20, the solution being at a temperature of about 40 degrees centigrade. The thus-coated panels were then rinsed by spraying with cold water for about 15 seconds and were then sprayed for about 15 seconds with one of the following hexavalent chromium rinse solutions, which solutions were also at a temperature of about 40 degrees centigrade. These solutions were formulated by admixing in Water in the amounts indicated, the components as set forth in the following table:

The thus-rinsed panels were squeegeed and then oven dried for about 1 minute at 120 degrees Centigrade.

The thus-dried panels were then painted with the vinyl paint, cured for 10 minutes at about 150 degrees centigrade and then subjected to the 5 percent salt spray test as described above. In each instance, after 504 hours in the salt spray test, the panels were found to exhibit only trac amounts of corrosion, substantially all the panels having ratings within the range of 0-2. The panels were also subjected to the humidity, adhesion and physical tests, as described above. In all instances, after the humidity test, the panels tested had ratings of F and VF. The ratings after the adhesion test on substantially all panels was 810, with most of the panels having a rating of 10. In the physical testing, substantially no peeling was encountered and in all cases, the ratings were less than about 8 percent.

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 further intended that each element 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 process for treating surfaces which are predominantly of zinc or aluminum, which process comprises contacting the surface to be treated with an alkaline solution having a pH of at least about 11, maintaining the alkaline solution in contact with the surface for a Components in grams/liter Alkaline solution N 2.0 H Trisodium N220 O 3 Wetting agent Sequentering Sodium ortho Sodium Petroleum pH phosphate agent sihcate tnpolydlstillate phosphate The zincgrip panels were sprayed with one of the above alkaline solutions for 30 seconds, the solution being period of time suificient to activate the surface, which contact time is at least 15 seconds, contacting the thusat a. temperature of about degrees centigrade, and activated surface with an acidic chromate conversion coating solution having a pH within the range of about 0.62 to 3.5, said chromate conversion coating solution comprising zinc ions in an amount of at least 0.5 gram per liter, hexavalent chromium ions, calculated as CrO in an amount within the range of about 0.5 to 4.5 grams per liter, sulfate ions in an amount within the range of about 0.5 to about 40 grams per liter, and at least 0.15 gram per liter of a fluoride radical, maintaining said coating solution in contact with said surface for a period of time sufficient to effect formation of the desired chromate conversion coating thereon and, thereafter, contacting the thus-coated surface with an aqueous solution containing hexavalent chromium, calculated as CrO in an amount of at least 0.25% by weight of the solution.

2. The method as claimed in claim 1 wherein the chromate conversion coating solution also contains at least about 0.25 gram per liter of trivalent chromium ions.

3. The method as claimed in claim 2 wherein the chromate conversion coating solution is comprised of zinc ions in an amount within the range of about 1 to about grams per liter, hexavalent chromium ions, calculated as CrO in an amount within the range of about 1 to about 3 grams per liter, sulfate ions in an amount within the range of about 2 to about 35 grams per liter, from about 1 to about 15 grams per liter of a complex fluoride radical selected from the group consisting of fluotitanites, fluoborates, fluosilicates, fluoaluminates, and from about 3 to about 9 grams per liter of trivalent chromium ions.

4. The method as claimed in claim 3 wherein the alkalin solution, the chromate conversion coating solution, and the aqueous rinse solution are all applied to the metal surface to be treated by spraying.

5. A chromate conversion coating composition for providing on surfaces which are predominantly of zinc or aluminum, a highly corrosion resistant chromate coating having excellent paint bonding characteristics, when applied in accordance with the method of claim 1 which composition is an aqueous solution comprising zinc ions in an amount of at least 0.5 gram per liter, hexavalent chromium ions, calculated as CrO in an amount within the range of about 0.5 to 4.5 grams per liter, sulfate ions in an amount within the range of about 0.5 to about 40 grams per liter, and at least 0.15 gram per liter of a fluoride radical.

6. The composition as claimed in claim 5 wherein there is also contained at least 0.25 gram per liter of trivalent chromium.

7. The composition as claimed in claim 6 wherein there is also present nitrate ions in an amount at least 0.5 percent by weight of the composition.

8. The composition as claimed in claim 6 wherein the zinc ions are present in an amount within the range of about 1 to 15 grams per liter, the hexavalent chromium ions, calculated as CrO are present in an amount within the range of about 1 to 3 grams per liter, the sulfate ions are present in an amount within the range of about 2 to 35 grams per liter, the fluoride radical is a complex fluoride radical which is present in an amount within the range of about 1 to 15 grams per liter, and the trivalent chrominum ions are present in an amount within the range of about 3 to 9 grams per liter.

9. The composition as claimed in claim 8 wherein the complex fluoride radical in the composition is selected from the group consisting of fluotitanates, fluoborates, fluosilicates, and fluoaluminates.

10. The composition as claimed in claim 8 wherein the pH of the coating solution is within the range of about 0.62 to about 3.5.

11. The composition as claimed in claim 10 wherein the pH of the solution is within the range of about 1.6 to about 2.5.

References Cited UNITED STATES PATENTS 1,946,152 2/1934 Edwards 148-6.2 X 2,213,263 9/1940 Thompson et al. 1486.2 2,276,353 3/1942 Thompson 148--6.2 2,493,934 1/1950 Waring 148-62 2,858,245 10/ 1958 Carroll et al. 148-6.2 3,018,211 1/1962 Duke 1486.2 X 3,130,086 4/1964 Otto 1486.21 2,483,510 10/1949 StareCk 148-621 FOREIGN PATENTS 846,3 63 8/1960 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

R. L. BROWDY, Assistant Examiner.