US2472864A - Composition for and method of chemically coating aluminum - Google Patents

Description

Patented June 14, 1 949 COMPOSITION FOR AND METHOD OF CHEMICALLY COATING ALUMINUM Frank Palin Spruance, Jr.,

Ambler, and James H.

Thirsk, Wyncotc, Pa., assignors to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application February 7, 1947, Serial No. 727,270

12 Claims. (Cl. 148-616) This invention relates to the art of coating aluminum and alloys thereof in which aluminum is the principal ingredient. For example, the invention has been successfully applied not only to pure aluminum but also to a variety of commonly used alloys consisting largely of aluminum and is especially useful with copper bearing aluminum alloys. Alloys typical of those with which the invention is especially useful are the following, the designations given being those employed in the 1946 edition of the Aluminum Company of Americas publication on Aluminum and Its Alloys which gives the composition of each:

The principal objects of the invention are to increase the durability of aluminum or aluminum alloy objects or surfaces; to produce protective o coatings on aluminiferous surfaces which are extremely flexible and which will remain substantially integrated with the surface even though enough tensile stress is applied to the object to cause it to fail; to provide a coating which affords an unusually high degree of resistance to corrosion especially under the severe conditions created by humid or salt-laden atmospheres such as are encountered in the tropics or near or over the sea; to provide a novel solution and a simple process for producing acoating on aluminiferous metals; to improve the degree of durability of the protection afforded by paints, lacquers, etc. which are applied to thecoating of the present invention; and to attain the foregoing objectives by means which are more simple and more economical than any which have been employed hitherto.

Before proceeding with a detailed description of the invention we wish to refer to the fact that, generally speaking, coatings which contain metallic phosphate have not heretofore been entirely satisfactory in preventing corrosion and acting as a base for the application of the final organic finishing coat, under conditions where the surfaces to which the coatings have been applied have been subjected to abnormal strains and stresses. It is well known, for example, that metallic phosphate coatings are somewhat brittle in character and tend to chip or break oil when Iii subjected to distortion. The present invention is particularly useful in overcoming the difficulty just referred to.

The invention is based primarily upon the discovery that the treatment of aluminum or alloys thereof in which aluminum is the principal ingredient by means of aqueous acid solutions containing phosphoric acid, chromic acid, a water soluble compound of fluorine and a water soluble compound of arsenic in a certain well-defined region as to the proportions of the ingredients, leads to the formation of an integral, strongly adherent, smooth and, apparently, amorphous coating which is extremely flexible and has outstanding merit in inhibiting corrosion and improving paint durabilty even under abnormally severe conditions such, as are encountered in humid or salt-laden atmospheres and where great stress or strain are imposed on the metal.

The form in which the active ions are introduced seems to make little or no difference as long as these remain in the solution in the correct proportions and the solution has the proper acidity. For instance, the phosphate ion may be introduced as phosphoric acid or as a salt of phosphoric acid such as monosodium, monopotassium or mono-ammonium phosphate; the fluoride ion may be introduced as a solution of hydrofluoric acid, as sodium fluoride, or as potassium acid fluoride; the dichromate ion may be introduced as chromic acid (ClOa), as potassium, or as sodium chromate or dichromate, and the arsenate ion may be introduced as arsenic pentoxide or as sodium arsenate or other compounds of arsenic soluble in the solution. Naturally the amount of acid which is to be added will depend upon the form in which the essential ions,are added to the solution, as will be further explained below,

As indicated above, the solutions used in carrying out our improved coating process are characterized by a content of acid, fluoride ion, phosphate ion, dichromate ion, and arsenate ion which lie within a certain well-defined area. Generally stated, the region of concentrations within which our solutions must be maintained for satisfactory coating is deflnedas follows:

1. The phosphate ion plus the arsenate ion must be at least 2 grams per liter, and, preferably, at least 6 grams per liter, calculated as equivalent 3 P04. At concentrations below 6.0 grams per liter, the operative concentrations of dichromate ion and fluoride ion become extremely critical; below 2.0 grams per liter, it is hardly possible to maintain the dichromate and fluoride concentrations with suflicient accuracy to operate the process even upon a small area of metal per unit volume of bath. At 6.0 grams per liter of P04 (including arsenate calculated as P04), the range of permissible fluoride and dichromate is large enough to permit practical operation on a succession of metal parts. However, even at this concentration rather careful control and frequent restoration of phosphate, arsenate, fluoride, acid and chromate are necessary to maintain the solution in optimum working condition. A good working concentration of' phosphate calculated as P04 content is between 20 and 100 grams per liter. A practical maximum is about 285 grams per liter.

2. The ratio of fluoride ion to dichromate ion, by weight, (calculated as chromic acid, CrOa) must be between 0.135 and 0.405, and, preferably, between 0.18 and 0.36.

Too high a ratio'of fluoride ion to dicromate leads to the formation of loose coatings or the absence of coatings and the production of a surface which is merely etched, in contrast with the development of tight, adherent, continuous coatings resistant to abrasion and bending, which are formed under optimum conditions.

A ratio of fluoride ion to dichromate ion which is somewhat too low leads to the development of very thin coatings; at even lower ratios no visible coating action takes place, and the metal remains smooth and bright.

3. The total acidity of the solution must not exceed that corresponding to 3.0 normal acid. For the purposes of this calculation all polybasic acids whose second ionization constants are less than may be considered to be monobasic. For such acids the second and subsequent ionizable hydrogen atomsmake less than a 1.0% contribution to the hydrogen ion content of their aqueous solutions at pH=2.0 or below. The solutions with which we are dealing have pH's near or below 2.0. Examples of such acid are phosphoric and arsenic acids.

4. The arsenic present in the solution should be in amounts of 0.20 to 3.0 grams per liter calculated as AszOs.

It must be explained that the absolute quantities of acid and of anions which may be present under operative conditions are not independent. The actual hydrogen ion concentration of the solution is a function of the dissociation constants of the acids corresponding to the anions present.

Therefore, in the absence of anions of acids weaker than phosphoric, hydrofluoric or chromic acid, the hydrogen ion content of the solution falls with an increase in the quantity of phosphate, fluoride, arsenate and dichromate. It has been found necessary to have in the solution a greater amount of acid, the greater the quantities of these ions present.

It is desirable to express the acidity of the operative solutions in terms of pH. Unfortunately no accurate means of measuring the pH of these solutions has been found. The use of indicators is unreliable because the indicators are oxidized by the dichromate ion present. The electrical pH meter using the glass electrode is unreliable because of the effect of the fluoride ion upon the glass. The hydrogen and quinhydrone 4 electrodes are likewise the oxidizing effect of the dichromate.

The glass electrode, however, seems to give readings which, while they often exhibit a curious excursion with time from a low value to a value as much as a pH unit higher, and then back to a value even lower than at first, seem to have some significance, even though they are not unequivocally interpretable.- Since a recheck in a stand ard buffer solution ofthe glass electrode, after a measurement of one of our coating solutions, shows very little change, it may be assumed that the electrode is not permanently damaged by use for the measurement of our coating solutions.

With these limitations in mindjthe final, nearly steady reading of pH, by means of a commercial glass electrode pH meter, in our improved solutions in correct operating condition, falls in the range 1.6 to 2.2, and for optimum coating conditions, in the range 1.7 to 1.9.

To prepare a solution for operating our improved coating process there mayv be added to water:

1. Sufficient phosphate ion plus arsenate ion in the form of phosphoric acid or any phosphate and arsenic acid, or any arsenate soluble at a pH of about 2, to give a phosphate-arsenate content (as PO4) of at least'2 grams per liter, and better, at least 6.0 grams per liter.

2. Materials containing fluoride and hexavalent chromiumin a quantity sufficient to give a ratio of dissolved fluorine to dissolved hexavalent chromium (calculated as FICI'Os) of between 0.135 and 0.405, or, preferably, between 0.18 and 0.36. The optimum generally is near 0.27.

3. Arsenic in the form of a compound soluble in the solution to give a content of arsenic between 0.2 and 3.0 grams per liter,.calculated as AS205. For this purpose, arsenic pentoxide itself, an arsenate soluble at a DH of about 2, or other compound which will yield dissolved arsenic in the solution, may be employed.

4. An acid, preferably one at least as strong as HF, to give an apparent pH as measured by a commercial glass-electrode pH meter of 1.6 to 2.2 or, preferably, from 1.7 to 1.9 as measured by the lowest value indicated within the first 10 minutes of immersion of the glass electrode in the solution.

This step is, of course, subject to the limitations stated above. In any case, provided the phosphate, dichromate, fluoride and arsenic are present in the proper quantities, the exact quantity of acid to be used may be checked by the appearance of the coating produced during the actual processing of aluminum surfaces. Too low an acidity leads to no coating or a very thin coating. Too high an acidity leads to loose powdery coatings; still higher acidity to a strong etch, sometimes preceded by the formation of visible coatings which wash off on removal of the treated part from the bath or on rinsing it with water.

In carrying out our improved process the surfaces to be coated should be moderately clean. The cleaning, which forms no part of the present invention,'may be carried out by conventional methods. For example, grease and dirt may be removed by a mild silicate alkali spray or by the use of an emulsion of a grease solvent. Heavy oxide films may b removed by acid or caustic soda treatments. The cleaned work, which may be wet or dry, istreated with a solution of proper composition, of which one example is the followinapplicable because of Foam No. I

Phosphoric acid, 75% grams 61 Sodium fluoride -do 5 Chromlc acid (CrOs) do-; Arsenic acid do 2 Water to make liter 1 The treatment may be performed by immersing the surfaces to be coated in the solution, by flowing or spraying the solution upon the work, or by other convenient techniques in which the solution is allowed to act upon the work; If the solution is merely applied to the work momentarily after which the adhering film of solution is allowed to act for some time, it is desirable to use a solution considerably more concentrated than that of Formula No. I.

The action of the solution may be accelerated by heat. Th solution may be kept at any temperature from ordinary room temperature to 180 F. or more. Similar coatings appear to be formed independent of temperature but the time for complete coating formation at room temperature may be reduced from about 5 to 10 minutes to 1 to 2 minutes, or even less by such a rise in temperature.

Two typical alternative bath formulas are given below by way of illustration, illustrating a few of the many variations in composition which our improved compositions may take within their operating range.

FORMULA No. II

Maintenance of our solutions in operating condition during the processing of a succession of surfaces requires merely that the proportion of dissolved ions and acidity be kept within the prescribed limits by suitable additions of chemicals. It is to be noted that the coating operation consumes chemicals as follows:

1. Acid is consumed by attack on the metal. This is accompanied by an evolution of hydrogen during the coating operation.

2. Phosphate, arsenate and fluoride are included in the coating, as evidenced by its analysis.

3. Dichromate is consumed by reduction to trivalent chromium, some of which is included in the coating. some of which remains dissolved in the solution. a

4. The accumulation of dissolved aluminum in the solution leads, ultimately, to a loss of fluoride as a precipitate of aluminum fluoride.

5. Some trivalent chromium may be precipitated when this accumulates to a suflicient degree as the fluoride and/or phosphate.

These losses, as well as gross loss of solution due to drag-out on the surface of the work, must be replaced to maintain the bath within its operating limits. The precipitates referred to are apparently without effect except as they are mechanically objectionable. In any case, they may be removed without difflculty by decantation or filtration.

After the treatment with our improved solution, as described, the coated surfaces can either be rinsed with water and then dried, or be dried" first, followed by a water rinse and a second drying. In the second instance the adhering treating solution dries upon the coated surface, and when it is not desired to paint the surface it may be left in an unrinsed condition after it has been dried. However, if paint or other organic finish is to be applied to the coated and dried surface, it should then be thoroughly rinsed with pure water to remove all soluble salts because such salts are likely to cause blistering of the paint or other organic film, especially if the surface is to be subjected or exposed to humid conditions.

We have found that the corrosion resistance imparted to the surface is distinctly improved by permitting the adhering treating solution to dry upon the coated surface before any rinsing takes place and that such dried and unrinsed surfaces are satisfactory for many purposes where a subsequent organic finish is not desired but, as stated, where paint is to be applied rinsing is necessary to remove any residue of soluble salts which may be present.

The drying of the coated surfaces with their adhering solution may be accomplished at ordinary room temperature or at elevated temperatures where expedition of the drying process is desirable. If the nitric acid-insoluble type of coating described below is not desired, the time and temperature of exposure to the drying oven should be limited to that required to remove physical moisture only. Longer heating, especially at temperatures above the boiling point of water, will drive off chemically bound water and insolubilize the coatings as described below.

Where paint or other organic finish is to be applied to the coated surface it may be unnecessary in many instances to go to the trouble of an initial drying of the adhering residues of treating solution, in which event the treated surfaces may be immediately rinsed with pure water to remove the soluble salts and then dried. Such treatment yields excellent results although, as indicated above, the corrosion resistance of the coating is not quite equal to that of coatings produced by permitting the adhering treating solution to dry before any rinsing takes place.

If any small amount of soluble salts should be left on the finally dried surface they may be rendered much less harmful lithe surface is treated. after coating and drying, with a dilute solution containing free chromic acid. Thus, if there is any possibility that the water used for rinsing is too high in dissolved salts for absolute safety, it has been found desirable finally to rinse the coated and dried surfaces with a dilute chromic acid solution containing from /2 to 8 ounces of chromic acid per gallons of water, after which the surfaces are again dried. This treatment cannot be harmful and may, therefore, be applied as a matter of routine whether or not the water supply is known to be too high in soluble salts.

We should like to note that, if desired, rinsing of the dried residues of treating solution may in effect be combined with the final chromic acid rinse although to prevent undue contamination of the chromic acid solution we prefer to rinse first with plain water and then with the dilute chromic acid solution.

The coatings produced by our improved process fluorine, phosphorus, chromium, arsenic, oxygen and hydrogen as their principal constituents. On heating they lose up to 40% of their weight. Physically such heating seems to produce no obvious change other than a possible darkening of the color of the coating. Chemically, however, the coatinngs become much more resistant after dehydration by heating. For example, as produced by treatment with our improved process, but before heating the coatings are fairly readily soluble in 70% nitric acid. After heating they dissolve only with great difllculty and on long boiling in this acid. The inertness of the heated coatings is of considerable advantage in the protection of aluminum which is exposed to corrosive materials and environment.

Although, as previously stated, our coating solutions can be prepared from a variety of starting substances, possibly the simplest, cheapest, and most easily available combination of chemicals from which to prepare them is an alkali fluoride. phosphoric acid, chromic acid, and arsenic pentoxide.

Although exact maximum and minimum amounts of fluoride and dichromate to be used in our improved solutions are diflicult to specify. aside from the FZC1O3 ratio, it has been found, generally, that:

1. The fluoride ion content should lie between 0.9 and 12.5 grams per liter, and preferably between 2.0 and 6.0 grams per liter.

2. The dichromate ion content should correspond to a total CrOa content of between 3.75 and 60 grams per liter, and preferably between 6.0 and 20 grams per liter.

A good balance between economy in draggedout chemical, ease of control, and good results in coating is obtained in the preferred range specifled.

Since the essential ingredients of our coating solution are fluoride ion, phosphate ion, dichromate ion, arsenate ion and hydrogen ion, it has been found desirable in making up and replenishing the solution to use concentrated admixtures which need only to be added to water or to acidifled water to produce operative solutions of the proper composition. Such admixtures have very obvious advantages.

For making a fresh solution, the concentrated admixture may contain, for example, compounds of fluorine, of phosphorus (as orthophosphate), of arsenic (as pentavalent arsenic) and of hexavalent chromium, all in a form soluble in water at pH about 2. The composition should contain the constituents in the following proportions:

' Bat Fluorine, by weight i"'1 Chromium calculated as ClOs 2.47 to 7.40 Phosphate calculated as P04 1.8 to 67.5 Arsenic as AS205 .2 to 3 The above admixtures may or may not be compounded to include free acid. The inclusion of acid is desirable from the standpoint of ease in preparing the actual coating solutions, since nothing but water and the concentrated admixture is necessary. However, strong acid solutions containing fluoride and chromate are corrosive and, therefore, dangerous to handle. For this reason, the acid is often omitted from the concentrated make-up composition.

Concentrated compositions may be made up as solutions, slurries or solids. For ease in shipping and handling, solid admixtures are particularly 8 desirable. To get usable coating solutions, these need only to be added to water, acidulated to the proper degree.

A preferred embodiment of our invention as regards make-up material for our improved coating solution, embodying only easily obtainable chemicals is as follows:

Foams No. IV

When the total material of Formula No. IV is dissolved in to 300 gallons of water, from 3.0 to 4.3 gallons of 20 B. hydrochloric acid will be required to yield a solution in optimum 0perative condition. The actual amount of hydrochloric acid will depend on the amount of water used.

No purpose would be served in multiplying the number of suchformulas given.

It must be noted that the consumption of the various ingredients during the coating of a succession of surfaces is not in the same proportion in which these constituents exist in the solution. In general, the relative rates of consumption of the ingradients are generally about as follows, by weight:

Fluorine, grams 1.0 Chromium. as ClOa 0.7 to 1.4 Acid, (gram equivalents of replaceable hydrogen) 0.06 to 0.14 Phosphate, as P04, grams 0.5 to 1.0 Arsenate. as A5205, grams 0.006 to 0.012

Since the tolerance of our improved coating solution for variations from the optimum ratio of ingredients is reasonably large, it is possible for limited periods to effect replenishment of the solution with any of the concentrated make-up materials previously described. However, it has been found that if the same solution is to be used for coating a large area of metal, the relative rate of consumption of the ingredients is sufllciently different from their initial relative concentrations so that concentrated admixtures designed for making up the original solution are not capable of indefinitely maintaining the proper ratios of constituents in the working bath. Therefore, we have found it desirable to prepare admixtures having a ratio of ingredients more nearly like that in which they are consumed. For this purpose, compounds of fluorine, phosphorus (as orthophosphate), of arsenic (as arsenic pentoxide) and of hexavalent chromium, all soluble in water at about pH 2, are admixed in the ratio shown above.

If the replenishing material is also to contain acid, which otherwise must be added separately, the ratio grams fluorine: gram equivalents of acid, (replaceable hydrogen) should be from 1:0.05 to 1:010.

Examples of solid and liquid replenishing admixtures are as follows:

FORMULA No. V

FORMULA N0. VI

(Liquid) Pounds HF 52.8 CrOa 28.9 H3PO4 (75%) 17.9 A5205 0.

Replenishing the bath with the liquid replenishing material alone usually results in maintaining it in good operating condition for a long time. However, it is not wise to prepare the liquid replenishing material too far in advance as it is not entirely stable, although its rate of decomposition is quite low and for this reason the solid replenishing material is usually preferred.

It will be seen from the foregoing that in the concentrated admixtures, for each part by weight of fluorine. the quantity of chromium, calculated as CrOa, will range from a minimum of 0.7 parts to a maximum of 7.40 parts, that the quantity of phosphorus calculated as P04. will range from a minimum of 0.5 to a maximum of 67.5 parts for each part by weight of fluorine and finally that the quantity of pentavalent arsenic as AS205 will range from a minimum of 0.006 parts to a maximum of 3.0 parts for each part by weight of fluorine. It will also be seen that when the admixture is to be used for preparing a fresh solution, the minimum amount of chromium should be 2.47 parts by weight for each part of fluorine. that the minimum amount of phosphorus should be 1.8 parts for each part of fluorine, and that the minimum amount of arsenic should be 0.2 part for each part of fluorine. Still further it will be seen that when the concentrated admixture is to be employed in replenishing a used solution the maximum amount of chromium for each part of fluorine should be 1.4 parts, the maximum amount of phosphorus for each part of fluorine should be 1.0 part and,

the maximum amount of arsenic should be 0.012 part for each part by weight of fluorine.

Attention is called to the following copending applications of one of the present applicants; namely, Spruance, Jr.;

S 3242: l 11mg Date 614. 795 Sent. 6,1045

Grams per liter Fluorine 0.9 to 12.5

Chromic acid (CrOa) 3.75 to 60 Phosphate plus arsenate (calculated as P04) 6.0 to 258.0 Arsenate (AS205) 0.2 t 3.0

the ratio of fluoride ion to dichromate, expressed as FzCrOa, being between 0.135 and 0.405; and and the pH of the solution being between about 1.6 and 2.2, as measured by the lowest value indicated by a glass electrode pH meter within the first ten minutes of immersion of the electrode in the solution.

2. The solution of claim 1 in which the ions are present in amounts stoichiometrically equivalent to:

Grams per liter Fluorine 2.0 to 6.0

Chromic acid (CrOa) 6.0 to 20.0 Phosphate plus arsenate (calculated as P04) 20.0 to 100.0

Arsenate (AS205) 0.2 to 3.0

the ratio of fluoride to dichromate, expressed as F: CrOa, being between 0.1.8 and 0.36, and in which the pH of the solution, measured as described, lies between 1.7 and 1.9.

3. In a process for coating surfaces of aluminum and alloys thereof in which aluminum is the principal ingredient, the step which consists in subjecting the surface to the action of an acid aqueous solution the essential active coatingproducing ingredients of which are fluoride ion, dichromate ion, phosphate ion, arsenate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine g 0.9 to 12.5 Chromic acid (C103) 3.75 to 60.0 Phosphate plus arsenate (calculated as P04) 6.0 to 285.0 Arsenate (AS205) 0.2 to 3.0

the ratio of fluoride ion to dichromate, expressed as F:Cr0a, being between 0.135 and 0.405; and the pH of the solution being between about 1.6 and 2.2, as measured by the lowest value indicated by a glass electrode pH meter within the first ten minutes of immersion of the electrode in the solution.

4. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1 and then allowing adhering solution to dry upon the surface.

5. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1, allowing adhering solution to dry upon the surface, rinsing the dried surface, and again drying it.

6. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1, allowing adhering solution to dry upon the surface. rinsing the Surface in a solution of chromic acid containing from A.; to 8 ounces of chromic acid per gallons; and again drying it.

7. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1; rinsing the surface and drying it.

8. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1; rinsing the surface in a solution of chromic acid containing 11 from to 8 ounces of chromic acid per 100 gallons; and drying it.

9. A process for coating aluminum and alloys thereof in which aluminum is the principal ingredient consisting in subjecting the surface to a solution such as defined in claim 1 and then at least partially dehydrating the coating formed by heating it.

10. For use in a process for coating aluminiferous metal in which the metal is treated with an aqueous acid solution as defined in claim 1; a concentrated admixture comprising compounds of fluorine, of phosphorus as orthophosphate, pentavalent arsenic, and of hexavalent chmmium, all in a form soluble in water at pH2, in which admixture, for each part by weight of fluorine there are: (1) a minimum of 0.7 parts and a maximum of 7.40 parts of chromium calculated as C103. (2) a minimum of 0.5 and a maximum of 67.5 parts of phosphorus, calculated as P04, and (3) a minimum of 0.006 parts and a maximum of 3.0 parts of arsenic, calculated as AS205; the minimum amount of chromium being 2.47 parts when the admixture is used for preparing a fresh solution and the maximum amount being 1.4 parts when the admixture is used for replenishing a used solution; the minimum amount of phosphorus being 1.8 parts when the admixture is used for preparing a fresh solution and the maximum amount being 1.0 part when the admixture is used for replenishing a used solution; and the minimum amount of arsenic as AS205 being 0.2 part when the admixture is used for preparing a fresh solution and the maximum amount being 0.012 part when the admixture is used for replenishing a used solution.

11. An admixture for preparing a coating solution for use in coating aluminum and alloys thereof in which aluminum is the principal ingradient, the essential active coating-producing ingredients of which admixture consist of compounds of fluorine yielding fluoride ions in solution, of phosphorus as orthophosphate, of pentavalent arsenic, and of hexavalent chromium, all in forms soluble in water at pH2 and in which for each part by weight of fluorine there are from 2.47 to 7.40 parts of chromium, calculated as CrOa; from 1.8 to 67.5 parts of phosphorus, calculated as P04; and from 0.2 to 3.0 parts of arsenic, calculated as AS205.

12. An admixture solution for use in coating aluminum and alloys gredient, the essential active coating-producing ingredients of which admixture consist of compounds of fluorine yielding fluoride ions in solution, of phosphorus as orthophosphate, of penta valent arsenic, and of hexavalent chromium, all in a form soluble in water at pH2 and in which for each part by weight of fluoride there are from 0.7 to 1.4 parts of chromium, calculated as CrOa; 0.5 to 1.0 part of phosphorus, calculated as P04; and 0.006 to 0.012 AS205.

FRANK PALI N SPRUANCE, JR.

JAMES H. THIRSK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS