US3347713A

US3347713A - Process for forming chromate coatings on aluminum

Info

Publication number: US3347713A
Application number: US342275A
Authority: US
Inventors: Herman J Lodeesen; William S Russell
Original assignee: Hooker Chemical Corp
Current assignee: Henkel Corp
Priority date: 1964-02-03
Filing date: 1964-02-03
Publication date: 1967-10-17
Anticipated expiration: 1984-10-17
Also published as: SE323263B; DK120370B; BR6566830D0; AT250121B; NL6501351A; CH440909A; DE1248427B; GB1049131A; BE656609A

Description

United States Patent 3,347,713 PROCESS FOR FORMING CHROMATE (IOATINGS 0N ALUMINUM Herman J. Lodeesen, Clawson, and Wiiliarn S. Russell,

Warren, Mich, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Feb. 3, 1964, Ser. No. 342,275

16 Claims. (Cl. 148-62) ABSTRACT OF THE DECLOSURE An aqueous acidic solution useful for forming a protective coating on aluminum or aluminum alloy surfaces, which contains, in percent W./v. 0.05 to 1% CrO 0.16 to 2.7% fluoride ions, 0.01 to.0.4% tungsten and 0.22 to 3.2% Al(F) complex. Preferably, this solution has a pH Within the range of about 1.1 to 2.3 and the protective coating is formed on the aluminum and aluminum alloy surfaces by contacting the surface with this solution.

This invention relates to procedures and solutions for forming corrosion resistant and paint bonding coatings on the surfaces of aluminum and aluminum alloys. More particularly, this invention concerns aqueous acidic solutions of the general type which contain the hexavalent chromium ion and fluoride ions and which are improved by the inclusion therein of a modifying constituent which improves the speed, efliciency and ease of operation of the solution to produce adherent, corrosion resisting chromate coatings on aluminum or aluminum alloy surfaces.

The present invention relates to solutions for and methods of treating aluminum-containing surfaces which are in the same general category as those described in such typical United States patents as 2,276,353; 2,471,909; 2,472,864; 2,796,370; 2,796,371; 2,507,956; 2,843,513; 2,859,144; 2,868,679; 2,839,439; and 3,009,842. In these disclosures and other efforts to provide an effective process, it is apparent that an attempt has been made to concurrently provide a process useful for a broad range of aluminum and aluminum alloy surfaces and yet one which is simple to prepare initially and maintain in elfective coating forming condition during use. As this art has developed, however, the trend has been to add to operative solutions additional anions or cations to attain some specific objective, and by such modifications the control of the solutions during continuous use has been complicated and rendered more difficult. This invention deviates from the heretofore employed approach in providing a simplified composition which employs a substitute for the single modifying anion ferricyanide that is disclosed and claimed in United States Patent 2,796,370.

In the commercial operation of chromate-fiuoride ferricyanide activated solutions for coating aluminum a number of operational difficulties have been encountered. Such solutions must constantly contain a certain minimum quantity of the ferricyanide ion in order to preserve the coating rate and coating efliciency at levels which are commercially acceptable from the combined standpoints of quality and economy. It has been observed that the ferricyanide ion is sensitive to temperature degradation at temperatures of above about 120125 F., and unless care is constantly exercised to avoid exceeding this temperature, even at localized areas of an operating tank installation, the ferricyanide becomes degraded and incapable of maintaining the solution in acceptanble coating-forming condition. It has also been found that such solutions are relatively sensitive to acidity and require attentive maintenance of the necessary acidity in a fairly narrow range of acidity. Moreover, chromate-fluoride ferricyanide solutions lose their coating ability upon mere standing at room temperature without use and replenishment of them is complicated because the ferricyanide and chromate-fluoride components must be added separately and maintained in separate containers to avoid reaction between chromate and ferricyanide.

It is, therefore, the primary object of this invention to provide an improved chromate-fluoride solution which operates at high speed and high efficiency to form a corrosion-resistant, paint receptive coating on aluminum which is capable of operating at high coating rate and high efficiency over a wide range of pH and temperature and which is suitable for use at temperatures up to those approaching the boiling point of the solution.

Another object of this invention is to provide an improved chromate-fluoride aqueous acidic coating solution for aluminum which is stable upon standing at room temperature without use.

Another object of this invention is to provide a process for coating aluminum which employs a chromate-fluoride aqueous acidic solution that is capable of being maintained in optimum coating condition 'by replenishing with a single replenishing material; a further object is to provide replenishing materials for use in the improved process of this invention.

A further important object of this invention is to provide a process for coating aluminum which employs a chromate-fluoride aqueous acidic solution which is economical to use, easy to control and maintain in optimum coating-forming condition and capable of providing adherent, corrosion-resistant, colored coatings having predetermined shades, as desired.

The present invention is based on the discovery that aqueous acidic solutions containing the hexavalent chromium ion and the fluoride ion can be converted into highspeed coating-forming solutions by incorporating therein the tungsten anion so long as certain quantities and relative proportions of tne hexavalent chromium and tungsten anions are maintained in the solution and the solution contains a controlled proportion of a complex aluminum fluoride ion.

It has been found that relatively minor proportions of the tungsten anion are capable of changing CrOg-fluoride ion aqueous acidic solution from a slow, low-efliciency coating solution into a high speed, high efficiency solution which is easy to control and maintain in'peak coating condition during continuous use. The solutions are predominantly chromate solutions and may contain from about 0.5 to about 10 grams/liter CrO In these solutions which also contain a sufficient quantity of the fluoride ion to cause attack of the aluminum surface and resulting coating formation, it has been found that at least a minimum of about 0.1 grams/liter of tungsten is necessary in the solution in order to obtain a significant improvement in the rate and efficiency of coating formation. This minimum quantity of tungsten is most effective to form high speed, adherent, corrosion resistant coatings on the surfaces of aluminum and aluminum alloys when there is also present in the solution a quantity of an aluminum fluoride complex ion equivalent to about 2.2 to about 32 grams/liter of Al(F) In the absence of such a controlled quantity of Al(F) it has been observed that the highest speed of coating and highest efiiciency are not attained, although the tungsten anion alone, produces a markedimprovernent in the coating ability of a CrO fluoride ion activated aqueous acidic solution. The aluminum fluoride complex ion is expressed as Al(F) because it is present in the operating solution as an equilibrium of AIF ions which may contain from 1 to 6 fluoride atoms per aluminum atom and in the solutions of this invention this equilibrium averages out to be approximately equivalent to the AlF ion. The expression AMP) is intended to represent any aluminum fluoride ion and the quantities thereof refer to an amount of such ion equivalent to the AIR, ion.

Coating efliciency as used herein and in the appended claims refers to the relative quantity of coating formed to the amount of metal dissolved from the surface of the metal being coated and is specifically the ratio obtained by dividing the metal loss in milligrams per square ft. of surface treated by the coating weight on the same area in milligrams per square ft. Thus, as the numerical ratio decreases, the coating efficiency increases and the lowest numbers represent the highest efliciency of coating formation. For example, a chromic acid solution containing the fluoride ion as its sole activator ion dissolves between about 40 and about 200 milligrams per square ft. of aluminum metal in order to form on that surface a coating having a weight of 100 milligrams per square ft.; such coating formation is relatively slow and the coating efliciency is between 0.4 and 2.0, which is a relatively low coating efficiency. In comparison, the use of a typical coating solution of this invention forms 100 milligrams per square ft. of coating while dissolving only between about to about 33 milligrams of aluminum metal per square ft.; such coatings are formed at high speed, measured in seconds, and the coating efficiency varies between 0.15 and 0.33, which is a relatively high efiiciency range. The improved compositions of this invention include, the hexavalent chromium ion, the fluoride ion, the tungsten ion and an aluminum fluoride ion and these ions may be present in the relative proportions, expressed in percent, weight/volume, as set forth in the Formulations I and II which follow:

Formulation I Concentration,

Solution component: percent w./v. CrO 0.05-1 Fluoride ion 0.16-2.7 Tungsten anion as tungsten 0.01-0.4 Al(F) complex 0.22-3.2'

A preferred formulation for the purposes of this invention is set forth below as Formulation II.

Formulation II Concentration,

Solution component: percent w./v. CrO 0.20.5 Fluoride ion 0.16-1.6 Tungsten anion as tungsten 0.030.1 Al(F) complex 022-19 The heaxavalent chromium ion may be supplied as chromic acid or one or more of the water soluble salts thereof, including the sodium, potassium or ammonium chromates or dichromates or admixtures thereof with chromic acid and its salts.

The fluoride ion may be supplied by any fluorine-containing compound which is capable of ionizing in the aqueous acidic solutions of this invention to provide the fluoride ion, such as hydrofluoric acid, fluosilicic acid, or fluoboric acid and the sodium, potassium and ammonium salts thereof. The aluminum fluoride ion may be supplied as such or may be formed in the bath as a complex from free aluminum and fluoride ion. For this purpose the fluoride may be present as HF, HBF or H SiF The tungsten ion may be supplied in the solution of this invention by adding thereto any ionizable tungsten compound such as tungstic acid, or its sodium or potassium or ammonium salts, which will give the tungsten anion when oxidized by chromic acid or the salts thereof.

The solutions of this invention may be used to form coatings on the surfaces of aluminum or aluminum alloys by dipping, brushing, or spraying the solution on the sur- 4 face after conventional cleaning procedures have been employed to free the surface of oil, grease, oxideor the like. The solutions may alsobe applied to form coatings by atomizing the solution on the surface in a heated condition. Generally stated, this atomization application procedure comprises the steps of preliminarily heating the aluminum or aluminum alloy surface to be coatedto a temperature above about F. and atomizing on that heated surface a quantity of the coating solution sufficient to form the desired coating but insufficient to cause the droplets of atomized coating solution to coalesce or puddle on the surface, the coating resulting from the substantially instantaneous flashing of the liquid from the solution and each individual atomizedparticle droplet remaining substantially' in the locus of its original contact withthe aluminum or aluminum alloy surface. The coatings of this invention are slightly colored and vary in appearance from iridescent to light-gold to yellow to brown, generally as the coating weight increases.

The highspeed, high efficiency coating method of this invention broadly comprises the above enumerated steps of using the selected form of application of the coating solution to the aluminum or aluminum alloy surface to pending uponthe acidity and the concentration of the.

tungsten and aluminum fluoride complex ion in the solution, it is possible to obtain an increase in the coating rate of from 2-5 times that obtainedat room temperature from a solution containing a constant quantity of CrO and the fluoride ion. The coating rate-has been found to increase much more slowly between 120 F. and F., and for practical purposes the coating rate is substantially uniform throughout this range. It is, therefore, preferred to operate the solutions of this invention at a temperature in the range of about 120-160 F. Higher temperatures than 160 F. may be employed, for example, F. or even up to the boiling point of the solution, but no particular advantage is obtained by operating at temperatures higher than about 160 F.

The pH of the operating solution significantly affects the coating rate and coating efliciency of the solution being applied to the aluminum or aluminumalloy surface but excellent results are obtained over a fairly wide range of pH variations. The solutions may be operated satisfactorily in the pH range of about 1.1 to about 2.3 and preferably in the range of about 1.6 to about 2.1. The pH range refers to measurements taken by using an electrical pH meter employing a glass electrode and a calomel electrode by immersing the electrodes in freshportions of the solution and observing the indicated value. The glass electrode is maintained in optimum condition by taking care to rinse the electrode, immediately upon its removal from the solution being tested, in a solution of five normal hydrochloric acid by inserting in that solution for two to three minutes and thereafter rinsing the electrode in pure water. Between measurements, the electrode is kept immersed in pure water and prior to each use is checked against a standard buffer.

The tungsten anion containing chromate-fluoride solutions of this invention are basically different from any of the heretofore known chromate solutions for coating aluminum, and although the entire reason for this fact is not known with certainty, the evidence is clear that the concurrent chemical reactions occur, ionic balances are changing and resultant concentrations and relative proportions of active ingredients migrate toward a condition of inbalance and loss of coating-forming ability. In the use of the solutions of this invention, for example, at least the following changes are simultaneously occurring: hexavalent chromium, fluoride and tungsten anions are used in metal attack and coating formation, aluminum is released during metal attack, remains in the solution and forms varying aluminum fluoride complexes depending upon pH and temperature, some hexavalent chromium is reduced to trivalent chromium which remains in the solution and at least partially forms complexes with fluoride, acid is consumed in coating formation and the pH tends to rise, some of the tungsten anion and aluminum fluoride complexes apparently form additional stabilizing complexes and as an overall result the relative proportions of hexavalent chromium, fluoride and tungsten tend to change.

In solutions otherwise similar except that they contain no tungsten anion, some of the above enumerated changes occur, but in each case to a substantially different degree and with a significantly different final effect. For example, when using a simple CrO -fluoride ion activated aqueous acidic solution within the herein disclosed pH range, a small portion of the hexavalent ion decreases by becoming a part of the coating and a large part of the hexavalent chromium is reduced to trivalent chromium and remains in the solution, some fluoride ion complexes with the trivalent chromium thus released in the solution, some fluoride complexes with the aluminum released during metal attack and the acid content decreases; however, the fluoride ion concentration decrease in the simple CrQ -fluoride ion solution is substantially greater than occurs in the solution which is otherwise similar except that it contains the tungsten anion. As a result of the decrease in fluoride ion concentration and build-up of other ions the solution solution soon loses its ability to form a coating. In the simple chromate-fluoride solutions, the chromate-fluoride-ferricyanide solutions, the chromatophosphate-fluoride solutions, one of the heretofore employed procedures has been to add acid to the solution, such as sulfuric, nitric or other mineral acid. The acid addition has restored coating ability and increased the coating rate, and this effect has been explained to be due to a shift in the equilibrium or aluminum fluoride complexes toward those containing less fluoride, thus freeing fluoride for metal attack and coating formation. The addition of such a mineral acid to the tungsten anion containing solutions of this invention does not effect a comparable change in the coating rate; rather the solutions operate over a wide range of pH to provide approximately the same coating rate. In a somewhat similar fashion, it has been observed that the solutions of this invention provide about the same coating rate and coating efliciency over a range of temperature from about 120 'to' about 160 F., whereas the ordinary effect of raising the temperature of an aqueous acidic metal coating solution in this range is to increase the coating weight proportionately. There is a slight advantage in coating rate at about 130 F.140

F. but it is not commercially significantly better than other portions of the range between 120 F. and 160 F.

The presence in the operating solutions of this invention of foreign cations, that is, cations other than the alkali metal ions normally introduced together with the fluoride or tungsten ion, such as divalent or trivalent metal ions, have been found to be undesirable and to detrimentally affect the coating rate and coating efliciencies which may be obtained by using the solutions of this invention in the manner above described. This detrimental eflFect on coating rate and coating efiiciency is particularly noticeable in those solutions of this invention which contain the lower quantities of tungsten and/ or fluoride ions, and although minor quantities of such undesirable cations could be off-set by the addition of anions such as the sulfate or nitrate anion, to thus permit the attainment of the benefits of concurrently using the tungsten, fluoride and aluminum fluoride complex ions, it is nevertheless undesirable to introduce them into or have such other cations present in the solutions. A particularly advantageous procedure for maintaining the solutions of this invention in optimum operating condition is to operate the solutions in conjunction with an ion exchange unit of the type and by the procedures described in Roy A. Halversen United States Patent 2,967,- 791, issued Jan. 10, 1961, and for the majority of installations, this method of operation is preferred. In operating the solutions of this invention with such an ion exchange unit which employs a cation exchange resin, the solutions are maintained free of metallic cations other than aluminum which occurs in the solutions as aluminum flouride complex ions and these complexes are controlled in concentration in the solution by the ion exchange unit.

Using the method of this invention, corrosion resistant, adherent coatings are formed on the aluminum or aluminum alloy surfaces in a matter of a few seconds, for example, about 1 to about 20 seconds and by modifying the compositions as to tungsten anion content, fluoride ion content, aluminum fluoride complex ion content, and as to pH and acidity, it is possible to maintain the coating rate suificiently fast to form a coating weight on continuous strips of aluminum or aluminum alloys having a weight in the range of about 15 to about 40 milligrams per square ft. in 1 to 2 seconds of contact time.

The below given examples set forth in somewhat greater detail typical operating solutions and operating procedures for the purpose of illustrating certain of the above referred to effects of variation in solution composition and operating conditions. It is to be understood, however, that these examples are presented for illustrative purposes only and do not themselves represent the definite compositional limits or limits on the operational steps of this invention which have been defined hereinabove and hereinafter in the appended claims.

EXAMPLE I gold in color and had coating weights varying from about 20 to about 50 milligrams per square ft., average, depending upon the contact time. The coating efilciency calculated as metal loss in milligrams per square ft. divided .by the coating weight in milligrams per square ft. averaged 0.2. This solution was maintained free of trivalent chromium ions and at the above level of aluminum fluoride complex by constantly cycling a small portion of the solution through an ion exchange resin column filled with Dowex 50, 20-100 mesh cation exchange resin, and after many thousands of surface feet of'aluminum had been processed through the solution, the coating obtained was still similar in appearance and the coating weight and the coating efiiciency remained substantially constant.

EXAMPLE II A solution was prepared containing 0.25% CrO 0.02% fluoride, as HF, 0.02% tungsten, added as Na WO and 0.22% AMP) (average AlF The solution had a pH of 1.68, a total fluoride of 0.17% and when applied by spraying at F. for 7-15 seconds contact time to the same aluminum builder stock employed in Example I, produced coatings that were approximately the same weight range, had an average coating efficiency of 0.25.

7. EXAMPLE m A 750 gallon tank for immersion coating was filled with a solution containing 0.5% CrO 0.25% fluoride, as H SiF 0.04% tungsten, added as Na WO 0.23% chromic nitrate and 0.22% AlF(F) (average AlF The solution had a pH of 1.55, a total fluoride of 0.4% and was raised in temperature to about 120 F. 3003 aluminum processed through the solution by immersion for 30 seconds to 1 minute produced adherent, gold-colored coatings having an average weight of 40-60 milligrams per square ft. At this temperature the average coating efliciency was 0.25.

This solution was operated at varying temperatures by spraying portions of the. solution on 3003 cleaned aluminum panels for a -second contact time. At 100 F. the coating weight was 55 milligrams per square ft., at 110 F. it was 62 milligrams per square ft., at 120 F. it was 84 milligrams per square ft., at 135 F. it was 81 milligrams per square ft., at 150. F. it was 67 milligrams per square ft., and at 160 F. it was 50 milligrams per square.

EXAMPLE IV A 2000 gallon solution was prepared containing 0.5 CrO 0.08% tungsten, added as Na WO 1.37% chromic nitrate, 0.02% fluoride as HF, and 1.87% Al(F) (average MP The solution had a pH of 1.71, atotal fluoride of 1.23% and was used in spray coating of 3003 aluminum at 120 F. for 7-15 second contact time and produced adherent light-gold to dark-gold coatings at coating weights averaging between about and about 50 milligrams per square ft., with a coating eificiency average of 0.3.

EXAMPLE V A 2000 gallon solution was prepared containing 1.0% CrO 0.41% fluoride, as HBF 2.28% chromic nitrate, 0.3l%' tungsten, added as Na WO and 3.11% Al(F) average AlF This solution had a pH of 1.96, a total fluoride of 2.51% and when operated in accordance with the procedures and on the materials specified in Example I, for similar coatings, had an average coating efficiency of 0.33.

EXAMPLE VI A solution was prepared containing 0.49% CrO 0.04% fluoride, as HF, 0.09% tungsten, added as Na WO and 0.236% Al(F) (average A11 The solution contained a total fluoride of 0.2 and had a pH of 168. The solution was heated to 120 F. and sprayed on 3003 aluminum panels for a 15-second contact time to produce a uniform adherent coating having an average coating weight of 78 milligrams per square it. at a coating efficiency of 0.19.

The above bath, when operated at 90 F., and under otherwise identical conditions, was found to produce adherent coatings having an average weight of 21 milligrams per square ft.; when operated at 100 F. produced coatings having an average weight of 54 milligrams per square ft.; when operated at 130 F. produced coatings having an average weight of 98 milligrams per square ft.; when operated at 140 F. produced coatings having an average weight of 98 milligrams per square it; when operated at 150 F. produced coatings having an average weight of 86 milligrams per square ft.; and when operated at 160 F. produced coatings having an average weight of 76 milligrams per square ft.

To portions of the above bat-h, sodium hydroxide Was added sufiicient to produce a pH of 1.81, and under the same conditions of application at 120 F. and on similar panels, an average coating weight of 77 milligrams per square ft. was obtained at a coating efliciency, of 0.20. To another portion of the solution, additional sodium hydroxide was added to produce a pH of 1.91 and when operated under similar conditions produced coatings having an average weight of 71 milligrams per square ft. at a coating efficiency of 0.24. To another portion of the above solution, suflicienttsodium hydroxide was added to produce a pH of 2.12 and when operatedunder similar conditions was found. to produce adherent coatings having an average weight of 45 milligrams per square ft. at a coating efli'ciency of 0.27. To another portion of the above solution, sutlieient sodium hydroxide was added to produce a pH of 2.28, and when this solution was oper ated, under similar conditions, adherent coatings were produced having an averageweight of 22 milligrams per square ft. at a coating efliciency of 0.28.

Another solution was prepared containing 0.49% .CrO 0.09% tungsten, as Na WO 0.02% fluoride, as HF and 0.266% Al(F) (average AlF This solution had a pH of 1.70 and a total fluoride content of 0.20%. This solution, operating at F., was sprayedon 3003 aluminum panels for 15 seconds contact time and produced an average coating weight of. 37 milligrams per square ft. at a coating efiiciency of 0.28. 0.48% HNO was added to this solution to convert the solution into one having a pH of 1.21. and when operated under similar conditions produced adherent coatings having an average weight of l 66 milligrams per square it. at a coating efficiency of 0.27. A single package replenishing material is suitable for use in maintaining the operating solutions of this invention in optimum coating-forming condition and a replenishing material for this purpose may satisfactorily contain:

Parts by weight CrO 15-20 HF 7-12 HNO 1.4-7 Na2WO4'2H O Al(F) (average AlF 0.6-1.6

CrO 17-19 HF 7-8 .HNO 1 4-1 6 N32W042H2O 3-5 A1(F) (average A1F 0.6-1.0 H BO 1.2-2.0

An especially satisfactory replenishing material for use with a continuous strip-line operation in which no cation exchange resin bed is employed is the following:

Parts by weight cro 18-20 HNO 5-7 Na2WO42H2O 3-6 A1(F) (average A1F 0.8-1.5. HF 8-11 A replenishing material particularly satisfactory in production operations in which the parts to be coated are moved through a spray installation on a monorail conveyor is the following:

Parts by weight CI'O3 HF 8-11 HNO 4.5-6.5 N21 WO 2H O 3 grams/liter CrO at least 0.1 gram per liter of the tungsten anion, about 2.2 to about 32 grams per liter of an aluminum fluoride complex ion and sufficient fluoride ion to cause said solution to coat said aluminum surface.

2. A method for forming a protective coating on the surfaces of aluminum and aluminum alloys which comprises the step of applying to the surface thereof an aqueous acidic solution consisting essentially of, in percent w./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 022-32 A1(F) complex.

3. A method for forming a protective coating on the surfaces of aluminum and aluminum alloys which comprises the step of applying to the surface thereof an aqueous acidic solution consisting essentially of, in percent W./v., 0.2-0.5 CrO 0.16-1.6 fluoride ion, 0.03-0.1 tungsten and 0.22-1.9 A1(F) complex.

4. A method for forming a protective coating on the surfaces of aluminum and aluminum alloys which comprises treating the surface thereof with an aqueous acidic solution consisting essentially of, in percent w./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 022-32 A1(F) complex, said solution having a pH in the range of about 1.1 to 2.3.

5. A method in accordance with claim 4 wherein said pH is maintained in said range by the periodic addition thereto of an acid in an amount equivalent to about 0.05% to about 3% of nitric acid.

6. A method for forming a protective coating on the surfaces of aluminum and its alloys which comprises the step of applying to said surface an aqueous acidic solution consisting essentially of, in percent W./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 0.22-3.2 A1(F) complex, said solution having a pH in the range of about 1.1 to about 2.3, periodically cycling a portion of said solution through a cation exchange resin bed to maintain said aluminum fluoride complex ion within said range.

7. A method for forming a protective coating on the surfaces of aluminum and its alloys which comprises the step of applying to said surface an aqueous acidic solution consisting essentially of, in percent W./v., 0.2-0.5 CrO O.16-l.6 fluoride ion, 0.03-0.1 tungsten and 0.22-1.9 A1(F) complex, said solution having a pH in the range of about 1.1 to about 2.3, periodically cycling a portion of said solution through a cation exchange resin bed to maintain said aluminum fluoride complex within said range.

8. A method for forming a protective coating on the surfaces of aluminum and its alloys which comprises the step of applying to said surface an aqueous acidic solution consisting essentially of, in percent w./v.,0.05-1 CrO 0.16-1.6 fluoride ion, 0.03-0.1 tungsten and 0.2-3 A1(F) complex ion, said solution having a pH in the range of about 1.6 to about 2.1, periodically cycling a portion of said solution through a cation exchange resin bed to maintain said aluminum fluoride complex within said range.

9. An aqueous acidic solution for forming a protective coating on the surface of aluminum and aluminum alloys which consists essentially of an aqueous acidic solution containing about 0.5 to about 10 grams/liter CrO the tungsten anion, an aluminum fluoride complex and at least about 0.16% fluoride ion.

10. An aqueous acidic solution for forming a protective coating on the surface of aluminum and aluminum alloys which consists essentially of an aqueous acidic solution containing, in percent w./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.0l-0.4 tungsten and 022-32 A1(F) complex.

11. An aqueous acidic solution for forming protective coating on the surface of aluminum and aluminum alloys which consists essentially of an aqueous acidic solution containing, in percent w./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.01-0.4 tungsten and 0.22-3.2 (A1('F) complex, said solution having a pH in the range of 1.1 to 2.3.

12. An aqueous acidic solution for forming a protective coating on the surface of aluminum and aluminum alloys which consists essentially of an aqueous acidic solution containing in percent w./v., 0.05-1 CrO 0.16-2.7 fluoride ion, 0.0l-0.4 tungsten and 0.22-3.2 A1(F) complex, said solution having a pH in the range of 1.6 to about 2.1.

13. A replenishing material for forming solutions for coating aluminum and aluminum alloys, which consists esentially of in parts by weight: CrO 15-20; HF, 7-12; HNO 1.4-7; Na WO .2H O, 3-6.5; A1(F) (average A1F3), 0.6-1.6.

14. A replenishing material for forming solutions for coating aluminum and aluminum alloys, which consists essentially of in parts by weight: CrO 17-19; HF, 7-8; HNO 1.4-1.6; Na WO 2H O, 3-5; A1(F) (average A1F 0.6-1.0; H BO 1.2-2.0.

15. A replenishing material for forming solutions for coating aluminum and aluminum alloys, which consists essentially of in parts by Weight, CrO 18-20; HF, 10-12; HNO 5-7; Na WO 2H O, 3-6; A1(F) (average A1F 0.8-1.5.

16. A replenishing material for forming solutions for coating aluminum and aluminum alloys, which consists essentially of in parts by weight: CrO 15-17; HF, 8-11; I-INO 4.5-6.5; Na WO 2H O, 3.5-6.5; A1(F) (average A1F 1-1.6.

References Cited UNITED STATES PATENTS 2,276,353 3/ 1942 Thompson 1486.27 X 2,868,679 1/ 1959 Pimbley 1486.2 2,967,791 1/ 1961 Halversen 1486.16

ALFRED L. LEAVITT, Primary Examiner. RALPH S. KENDALL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,347,713 October 17, 1967 Herman J. Lodeesen et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 68, "acceptanble" should read acceptable Column 2, line 38, "tne" should read the Column 5, line 38, "solution solution" should read solution line 46, "or aluminum" should read of aluminum Column 8,

line 58, "HF--8-ll" should read HF-10l2 Signed and sealed this 20th day of January 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

Claims

1. A METHOD FOR FORMING A PROTECTIVE COATING ON THE SURFACES OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES TREATING THE SURFACE THEREOF WITH AN AQUEOUS ACIDIC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 0.5 TO ABOUT 10 GRAMS/LITER CRO3, AT LEAST 0.1 GRAM PER LITER OF THE TUNGSTEN ANION, ABOUT 2.2 TO ABOUT 32 GRAMS PER LITER OF AN ALUMINUM FLUORIDE COMPLEX ION AND SUFFICIENT FLUORIDE ION TO CAUSE SAID SOLUTION TO COAT SAID ALUMINUM SURFACE.