US2438877A - Composition for and method of coating aluminum - Google Patents

Description

March 30, 1948. p, s u cE, JR 2,438,877

comrosxnon FOR-ANQMETHOD 0F comma ALUMINUM Filed s t. 6, 1945 OPERATING RANGE C I'IART USlNG All ALKALI FLUORIDE, CHRONIC ACID.AND PHOSPHORIC AID RATIO mono,

0 so we 150 zoo zoo j 300 350 H, PO GRAMS PER LITER mvszvrok.

Patented Mar. 30, 1948 COMPOSITION FOR AND METHOD OF COATING ALUMINUM Frank Palin Spruance, Jn, Ambler,

Pa., asslgnor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware Application September 6, 1945, Serial No. 614,795

I 12 Claims. This invention relates to the art of coating aluminum and alloys thereof in which aluminum is the principal ingredient. It is particularly directed to the provision of a solution and a simple process for producing a coating on aluminiferous metals, which coating is strongly protective against corrosion and is capable of serving as an excellent base for paint or other similar organic finish.

The principal objects of the invention are to increase, under normal service conditions, the durability of aluminum or aluminum alloy objects or surfaces; to improve the resistance of aluminiferous metals to humid or salt-laden atmospheres such as are encountered in the tropics or near or over the sea; to improve the degree of durability of the protection afforded by paints, lacquers, etc. on surfaces of aluminum or aluminum alloys; to provide a coating for aluminiferous metals which by itself is greatly protective and decorative; and to attain the foregoing objectives by means which are more simple and more economical than any which have been employed hitherto.

The foregoing, together with such other objects and advantages as may appear hereinafter or are incident to my invention, are attained in accordance with the following disclosure which describes preferred materials and conditions useful in carrying out my invention, reference being had to the accompanying drawing in which the single figure is a graph illustratin the permissible variations in the quantities of ingredients entering into the compositions employed with my invention.

This invention is based upon the discovery that the treatment of aluminum or its alloys with aqueous acid solutions containing phosphoric acid, chromic acid, and a water soluble compound of fluorine in a certain well defined region as to the proportions of the ingredients, leads to the formation of an adherent, smooth but not usually shiny, greenish grey to pale green coating which has decorative value as well as outstanding merit in inhibiting corrosion and improving paint durability.

Generally speaking, my novel coating process makes use of an aqueous acid solution containing phosphate ions, fluoride ions, and dichromate ions. The forminwhich the 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 (0103), as potassium, or as sodium chromate or dichromate. 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.

The kind and quantity of cations which may be present are notin themselves important except insofar as their salts act as buffers to regulate the effective acidity of the solution, or as they may cause the loss of active anions by precipitation of salts whose solubility products may be exceeded. Among the cations which may be present in reasonable quantity without doing harm are those of aluminum, trivalent chromium, zinc, copper, manganese, iron, nickel, cobalt, calcium, barium, strontium, tin, and others. Excessive amounts of aluminum, calcium, etc., may tend to cause a loss of fluoride ion by precipitation, but do no other noticeable harm. Fluoride so lost may be replaced and the solution thus restored to its optimum condition.

Foreign anions may also be present in moderate quantities without causing any difllculty. For example, relatively large amounts of sulphate, nitrate, acetate and chloride ion are tolerated without difliculty. Reducing agents are harmful because they cause a loss of dichromate ion by a reduction of hexavalent to trivalent chromium.

My process has been successfully applied to a variety of commonly used alloys consisting largely of aluminum as well as pure aluminum. A few of the well known alloys to which the process has been successfully applied, are the follow ing:

Percent alloying element Name On Mn Mg Si Cr Al 99.24- 98. Balance Balance Balance The solutions used in performing my improved coating process are characterized by a content of acid, fluoride ion, phosphate ion, and dichromate ion which are within a certain well-defined curacy to operate the process even upon a small area of metal per unit volume of bath. At 6.0 grams per liter of P04 the range of permissible fluoride and dichromate is large enough to permit practical operation on a succession of metal parts. rather careful control and frequent restoration of phosphate, fluoride, acid and chromate are necessary to maintain the solution in optimum working condition. A'good working concentration of P04 content is between 20 and 100 grams per liter. A practical maximum is about'285 grams per liter. Y r 2. The ratio of fluoride ion to dichromate ion, by weight (calculated as chromic acid, 010:) must be between 0.135 and 0.405, and, preferably, between 0.18 and 0.36.

Too high a ratio of fluoride ion to dichromat 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 atoms make 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 'aIrsenic acids.

Somewhat too high an acidity results in a powdery, non-adherent coating; still higher acidity results in a strong etch and no attached coating.

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. In the absence of anions of acids weaker than phosphoric acid, hydrofluoric acid, or chromic acid, the hydrogen ion content of the solution falls with increase in the quantity of phosphate, fluoride, 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 However, even at this concentration 4 indicators is unreliable because the indicators are oxidized by the dichromate ion present. The electrical pH meter, using the glass electrode is unreliable becauses of the efiect oi the fluoride ion upon the glass. The hydrogen and quinhydrone electrodes are likewise inapplicable because of the oxidizing eflect of the dichromate.

The glass electrode, however, seems to give readings which, whilethey 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 standard buffer solution of the glass electrode, after a measurement of one of my coating solutions, shows very little change, it may be assumed that the electrode is not permanently damaged by use for the measurement of my coating solutions.

steady reading of pH, by means of a commercial glass electrode pH meter, in my 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 my improved coating process there may be added to water:

1. Sufficient phosphate ion, in.the form of phosphoric acid or any phosphate soluble at a pH of about 2, to give a phosphate content (as P04) of at least 2 grams per liter, and better, 6.0 grams per liter.

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

3. 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, and fluoride are present in the proper quantities, the exact quantity of acid to be used may be checked by the appearance of the coating produced duringactual processing of'aluminum surfaces. As stated above, too low an acidity leads to no coating or a very thin coat. 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 ofi on removal of the treated part from the bath or on rinsing it with water.

In carrying out my improved process the surfaces to be coated should be 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 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 flhn of solution is allowed to act for some time, it is desirable to use a solution considerably more concentrated than that of Formula No. 1.

The action of the solution may be accelerated by heat. The 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 may be reduced from about to minutes to 1 to 2 minutes, or

even less by such a rise in temperature.

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

Formula No. 2

HaPO; rams" 12 NaF do 3.1 CrOa do 3.6 Water, to make liters 1 Formula No. 3

HaPO4 g ams-.. 24 NaF do 5.0 CrO: do 6.8 Water, to make ..liters 1 Formula No. 4

NaH2PO4.H2O grams. 31.8 NaF do 5.0 KnCrzOr d 10.6 HCl do 4.8 Water, to make liter 1 Formula N0. 5

NaHzPOaHzO grams 66.5 NaI-lFz do 4.2 KzCrzOv do 14.7 H2804 do 4.8 Water, to make 1'H'ors 1 Formula No. 6

NamPotr'no grams 31.8 AlFs do 5.0 KzCrzOq do 10.6 HCl do 4.6 Water, to make liters 1 Of these examples of coating solutions Formu las Nos. 1, 3 and 4 produced normal weight coatings, and Formulas Nos. 5 and 6 produced slightly lighter coatings,

Maintenance of my 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.

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

8. Dichromate is consumed by reduction to trivalent chromium, some of which is included in v the coating, some of which remains dissolved in thesolution.

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 sufflcient degree as the fluoride and/or phosphate.

These losses, as well as gross loss of solution due to drag-ou 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 difliculty by decantationor filtration.

After the treatment with my improved solution, as described, the 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 where 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 fllrn, especially if the surface is to be subjected or exposed to humid conditions.

I 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 7 should be limited to that required to remove physical moisture only. Longer heating, especially attemperatures above the boilingpoint 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 if the surface is treated, after coating and drying, with a dilute solution containing free chromic acid. Thus, it 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 100 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.

I 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 I prefer to rinse first with plain water and then with the dilute chromic acid solution.

The coatings produced by my improved process contain aluminum, fluorine, phosphorus, chromium, 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; chemically, however, the coatings become much more resistant after dehydration by heating. For example, as produced by treatment with my improved process but before heatin 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. This inertness of the heated coatings is of considerable advantage in the protection of aluminum which is exposed to certain corrosive chemicals.

Although, as previously stated, my coating solutions can be prepared from 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 and chromic acid (CrOz). The limits of composition within which the coating process is operative have been indicated. A very large number of charts and diagrams can be ,drawn to indicate the operable regions for my solutions and process depending on the starting materials. However, for illustrative purposes, and as a guide to the operation of the process with the particular chemicals just specified, reference is made to the chart shown in the accompanying drawing. In this chart each point represents a group of solutions for which the ratio of fluoride ion: chromic acid used in preparing Hie solutions is plotted as ordinate against the phosphoric acid used in grams per liter, as abscissa. The cross-hatched area marked Operative range, good coating represents the field of compositions within which my novel solutions and process have been found to be operable. The effect of alteration in the proportions of ingredients so that the composition of a solution falls outside the operative range, is indicated by an appropriate description on the chart. Compositions selected as optimum in performance are indicated by the heavy dotted line marked Optimum line.

Although exact maximum and minimum amounts of fluoride and dichromate to be used in my improved solutions are difficult to specify, aside from the FzCrOa 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 corre- 8 spond to a total CrO: content of between 3.75 and grams per liter, and preferably between 6.0 and zogramsperliteni A good balance between economy in drasged-. out chemical. ease oi'controL and good results in coating is obtained in the preferred ranze specified.

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

1. Only one chemical. or at the most two, need to be weighed or measured to make up the solution.

2. Shipping space and weight are saved by the omission oi much of the water from the admixture.

3. Errors in calculating and measuring the proportions of the ingredients are minimized, since the proportions are fixed by the composition of the concentrated admixture which can be prepared and checked once and for all.

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

Parts by weight Fluorine 1 Chromium, calculated as CrOa 2.47 to.7.40 Phosphate, calculated as P04 2 to '70 The best compositions should contain, for each part of fluorine, 2.88 to 5.55 parts chromium, calculated as CrOa and 7 to 50 parts of phosphate, calculated as P04.

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 chro'mate are corrosive and somewhat dangerous to handle. Therefore, acid may be omitted from the composition.

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

Preferred embodiments of my invention as regards make-up material for my improved coating solution, embodying only easily obtainable chemicals are as follows:

Formula No. 7

Lbs. Sodium fluoride 6.0 Chromic acid (CrOa) 12.0 Monosodium dihydrogen phosphate (NaHzPOLHzO) 82.0

When the total material of Formula No. 7 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 operative condition, depending on the volume of water used.

Another formula for a. concentrate, suitable Formula No. 8

This material must be stored and shipped in containers suitably corrosion resistant.

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

For reasons of cheapness and availability I prefer to use an alkali fluoride or acid fluoride, an alkali chromate, an alkali dichromate, or free chromic acid, and an alkali phosphate or free phosphoric acid in making compositions of this type.

It must be noted that the consumption of fluoride, phosphate, dichromate, and acid during the coating of a succession of surfaces is not usually in the same proportion in which these constituents exist in the solution. In general, the consumption of these ingredients depends somewhat upon the kind and surface finish of the metal treated. The relative rates of consumption of the ingredients are generally about as follows, by weight:

Fluorine grams- 1.0 Chromium, as CrOa do 0.7 to 1.4 Acid (gram equivalents of replaceable hydrogen) 0.06 to 0.14 Phosphate, as PO; grams 0.5 to 1.0

Since the tolerance of my improved coating solution for variations from the optimum ratio of ingredients is reasonably large, within the limits previously stated, it is possible for limited periods to effect replenishment of the solution with any of the concentrated make-up materials previously described, as, for example, those of Formulas 7 and 8, without failure of the coating process. However, it has been found that ii the same solution is to be used for coating a large area of metal, the relative rate of consumption of the ingredients is sufficiently diiierent 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.

It has, therefore, been found desirable to prepare admixtures having a ratio of ingredients more nearly like that in which they are consumed. For this purpose compounds of fluorine, of phosphorus (as orthophosphate), and of hexavalent chromium, all soluble in water at above pH 2, are admixed in the ratio shown above, namely, grams, fluorine: grams, chromium (as CrOz): grams. phosphate (as P04): 1.010.? to 1.4105 to 1.0.

If the replenishing material is also to contain acid, which must otherwise be added separately, the ratio grams fluorine: gram equivalents of acid (replaceable hydrogen), should be from 1 :0.6 to 1:0.14.

An example of a solid replenishing admixture i011 iinggredients of the solution other than the ac d Formula No. 9 Lbs. Potassium fluo 60.6 iChromic acid (CrOa) 21.9

Monosodium dihydrogen phosphate (NaI hPOLHaO) 17.5

Total 100.0

This material is added, as required to maintain along with an acid in sufflcient quantity to maintain proper pH and activity.

If desired, a liquid replenishing material may be formulated out 01' hydrofluoric, chromic' and phosphoric acids within the teachings as to proportions given above but in such case it should be noted that the resulting solutions are not-entirely stable. However, their rate of decomposition is quite low and I have found that they may be used to and will usually maintain a bath in good operating condition for a long time.

For reasons of cheapness and availability I prefer to use an alkali fluoride or acid fluoride, an alkali chromate, an alkali dichromate, or free chromic acid, and an alkali phosphate or free phosphoric acid in making compositions of this type.

The present application is a continuation-inpart of my earlier application Serial No. 602,512, filed June 30, 1945, now abandoned.

I claim:

1. An acid aqueous solution for coating metal from the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient, the essential active coating-producing ingredients of which solution consist of fluoride ion, dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromio acid (CrOa) 3.75to 60.0 Phosphate (P04) 2 to 285 Grams per liter Fluorine 2.0 to 6.0 Chromie acid (CrOa) 6.0 to 20.0 Phosphate (P04) 20.0 to 100.0

the ratio of fluoride to dichromate, expressed as F:Cr03, being between 0.18 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 metal chosen from the group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating, the step which onsists in subjecting the metal to the action of an acid aqueous solution the essential active coating producing ingredients of which are fluoride ion, dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromic acid (CrOa) 3.'75to 60.0 Phosphate (P04) 2 to285 the ratio of fluoride ion to dichromate, expressed as F:Cra, being between 0.135 and 0.405; 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 metal chosen from the group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating consisting in subjecting the metal to the action of an acid aqueous solution the essential active coating-producing ingredients of which are fluoride ion, dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromic acid (CrO a) 3.75 to 60.0 Phosphate (P04) 2 to285 the ratio of fluoride ion to dichromate, expressed as FzCrOa, being between 0.135 and 0.405; 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 flrst ten minutes of immersion of the electrode in the solution: and allowing adhering coating solution to dry upon the metal.

5. A process for coating metal chosen from the group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating consisting in subjecting the metal to the action of an acid aqueous solution the essential active coatingproducing ingredients of which are fluoride ion, dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrlcally equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromi acid (CrOa) 3.75150 60.0 Phosphate (P04) 2 to285 the rat o of fluoride ion to dichromate, expressed as F:Cr0a, being between 0.135 and 0.405: 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; allowing adhering coating solution to dry upon the metal, rinsing the dried metal, and again drying it.

6. A process for coating metal chosen from the group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating consisting in subjecting the metal to the action of an acid aqueous solution the essential active coatingproducing ingredients of which are fluoride ion,

dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromic acid (CrOa) 3.75 to 60.0 Phosphate (P04) 2 to 285 the ratio of fluoride ion to dichromate, expressed as FtCrOa, being between 0.135 and 0.405; the pH of the solution being between about 1.6 and 2.2, as-measured by the lowest value indicated assaerr 12 1 by a glass-electrode pH meter within the fl'rst'tenminutes of immersion of the electrode'in the 6 solution; allowing adhering coating solution to dry-upon the metal and rinsing the metal in a solution of chromic acid containing from ,9 to 8 ounces of chromic acid per gallons; and again drying it.

7. A process for coating metal chosen from the: group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating consisting in subjecting the metal to the action of an acid aqueous solution the essential active coatln3- producing ingredients of which are fluoride ion. dichromate ion, phosphate ion and hydrogen'ion. the ions being present in amounts stoichiometrically equivalent to V I t Grams per-liter Fluorine 0.9 to 12.5 vChromic acid (CrOa) 3.75 to160.0

Phosphate (P0 2' to .285

theratio offluoride ion to dichromate, expressed as FzCrOa, being between 0.135 and 0.405: the

pH of the solution being between about 1.6 and 2.2, as measured by the lowest value indicated 1 by a glass-electrode pH meter within the first ten minutes 01- immersion of the electrode in the solution; rinsing the metal and drying it.

8. A process for coating metal chosen fromthe group consisting of aluminum and alloys thereof in which aluminum is the principal ingredient with an adherent greenish coating consisting in subjecting the metal to the action of an acid aqueous solution the essential active coating-producing ingredients of which are fluoride ion. dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrlcally equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromic acid (CrOa) 3.75 to 60.0 Phosphate (P04) 2 to 285 the ratio .of fluoride ion to dichromate, expressed as Fzcroa, being between 0.135 and 0.405; the

pH of the solution being between about 1.6 and subjecting the metal to the action of an said aqueous solution the essential active coating-producing ingredients of which are fluoride ion. dichromate ion, phosphate ion and hydrogen ion, the ions being present in amounts stoichiometrically equivalent to Grams per liter Fluorine 0.9 to 12.5 Chromic acid (CrOa) 3.75 to 60.0 Phosphate (P04) 2 to 285 the ratio of fluoride ion to dichromate, expressed as FzCrOa, being between 0.135 and 0.405; 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 flrst ten minutes of immersion of the electrode in the solution, and which includes at least partially dehydrating the coating formed, by heating it.

10. An admixture for preparing a coating solution for use in coating metal from the class consisting oi aluminum and alloys thereof in which aluminum is the principal ingredient, the essential active coating-producing ingredients of which admixture consist of compounds of fluorine yieldin: fluoride ions in solution, of phosphorus as orthophosphate, and of hexavalent chromium, all in forms soluble in water at pH 2, and in which for each part by weight of fluorine there are i from 2.47 to 7.40 parts of chromium, calculated as 010:, and 2 to 70 parts of phosphorus, ca]

7 culated as P04.

11. An admixturevof the type oi! claim 10 in which for each part, by weight, of fluorine there are from 2.88 to 5.55 parts of chromium, calculated as CrOa, and 1 to 50 parts of phosphorus, calculated as P04.

culated as P04.

12. An admixture for replenishing a coating which aluminum is the principal ingredient, the

essential active coating-producing ingredients of 14 which admixture consist 01 compounds of fluorine yielding fluoride ions in solution, of phosphorus as orthophosphate, and of hexavalent chromium, all in forms soluble in water at pH 2, and in which, for each part by weight of fluorine, there are from 0.7 to 1.4 parts ofchromium, calculated as (310:, and from 0.5 to 1.0 part of phosphorus cal- FRANK PALIN SPRUANCE, JR.

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

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