US3018211A

US3018211A - Composition and process for brightening aluminum and its alloys

Info

Publication number: US3018211A
Application number: US788876A
Authority: US
Inventors: Jr Edward M Duke
Original assignee: Purex Corp Ltd
Current assignee: Purex Corp Ltd
Priority date: 1959-01-26
Filing date: 1959-01-26
Publication date: 1962-01-23
Anticipated expiration: 1979-01-23

Description

Jan. 23, 1962 Filed Jan. 26, 1959 E. M. DUKE, JR COMPOSITION AND PROC ESS FOR BRIGHTENING ALUMINUM AND ITS ALLOYS 2 sheets-sheet 1 O//VU 9/V//7lg/7Qj-t' Q/V//V/BS m k l Q53 Q Nk w kl g n MN u lllgxg @www u Q Q w n Q j Q QSQ @w k @u l w L/ I@ QINNN @DUNES /l s Q Q Q m l l\ Q Q k w w LL fg 'j H s INVENTOR.

Dweo M. oaef, JQ

Jan. 23, 1962 E, M. DUKE, JR 3,018,211

COMPOSITION AND PROCESS FOR BRIGHTENING ALUMINUM AND ITS ALLOYS Filed Jan. 2e, 1959 2 Sheets-Sheet 2 cally mentioned. One or more than one of these comF plex fluoride ions can be present. The source materials for these complex iiuorides can be, for example, the alkali metal or ammonium salts of the complex uorides or any sufliciently soluble complex fluoride salts of a metal which does not form insoluble material with any of the other anions present in the solution, such as the hexavalent chromium-containing anion.

Specic illustrative examples of such complex uorides are sodium, potassium or ammonium uoborates and silicouorides, potassium titanium fluoride, sodium titanium uoride, potassium zirconium fluoride, ammonium beryllium fluoride, ammonium cadmium iiuoride, ammonium aluminum uoride, ammonium chromium uoride, potassium molybdenum uoride, potassium rhenium fluoride, ammonium zirconium uoride, potassium zirconium pentauor-ide, sodium zirconium pentaiiuoride, potassium fenic uoride, potassium zinc fluoride, ammonium titanium fluoride, potassium hafnium uoride, potassium columbium fluoride, and potassium tantalum uoride.

in the case of the AlF4 complex anion, this can be formed either from the double salt, eg., alkali metal or ammonium aluminum fluoride, or from the simple aluminum uoride which in the nitric acid solution of the invention forms the complex AlF4 ion. However, where AlF3 is used as the source of both aluminum ions and complex iiuoride ions, because of the tendency of some aluminum fluoride to form the aforementioned A1114 ions, thus reducing the concentration of free of disassociated aluminum ions, it is preferred to employ alu-minum uoride in conjunction with other aluminum salts, e.g., aluminum sulfate, -to supply sufncient concentration of aluminum ions. The concentration of complex fluoride ions in the solution is preferably about 0.01 to about 0.10 mol per liter of solution. It has been found particularly advantageous to employ a Weight ratio of free aluminum ions to complex uoride ions of about 2 to 1.

The pH of the brightening solutions of the invention having the aforementioned ingredients, is usually about 0.5 or less, and in most cases about 0.4 or less. The temperature of the solutions is preferably maintained between 180 and about 210 F., although lower temperatures can be employed. The employment of such elevated temperatures not only serves to increase the eiiiciency of operation and reduce time of treatment, but lalso aids in conjunction with the complex fluoride ion, and the aforementioned concentration of nitric acid, to maintain in solution the relatively high concentration of aluminum ion required for proper functioning of the bath. Time of treatment in my brightening bath is usually from about 5 to 20 minutes, depending on temperature of the bath, the concentration of the specific components thereof, and the age of the bath. 1I-t will be understood that the temperature and time of treatment are not critical and can vary from the aforementioned ranges.

The compounds serving as the source for the essential ions of the bright dip composition of the invention, can be added separately lto a water solution in amounts sulficient to form the acid bath having the preferred aforementioned ranges of said ions, or alternatively, a solid composition composed of a mixture of two or more of said compounds in -proper proportions can be added to an aqueous nitric acid solution in proper concentration to form the bright dip bath.

When processing aluminum parts to be brightened according to the invention, it is first preferred to remove sols from the surface of the parts by treatment in a precleaning composition, such as, for example, a solution formed by 'dissolving in Water a composition composed of 95% tetrasodium pyrophosphate, 3% sodium chromate and 2% sodium dodecyl benzene sulfonate, in a concentration of 6 ounces of the com-position per gallon of solution. The parts should then be rinsed thoroughly in cold or warm running water to remove any residue from 4 the parts. lf desired, the parts can then be pre-etched in a caustic alkali solution, eg., 10% NaOH solution, for a few minutes. When such pre-etching is employed, this tends to increase the tank `life of the bright dip bath of the invention.

The aluminum parts can then be ideoxidized by treatment, e.g., for 2 to 5 minutes, in a deoxidizer of the type disclosed in U.S. application Serial No. 710,403 of Isidore Pollack, tiled January 22, 1958. An illustrative bath of this nature is composed of 91% sodium bisulfate, 8% chromic acid (CrO3) and 1% potassium titanium fluoride dissolved in water in an amount of 8 ounces of said composition per gallon of solution. The parts are then rinsed thoroughly in cold running water.

The parts are then treated in the bright dip bath of the invention, preferably under the pH conditions, temperature and time period noted above. Gentle mechanical agitation o fthe Work pieces in the bath, or agitation of the bath itself, enhances the specular brightness. However, excessive agitation is to be avoided, especially in aged baths, as it tends to accelerate graining therein. To prolong the bath lige, it is recommended that about 1.0% of the bath be discarded for each 10 sq. ft. of aluminum processed per working gallon of solution.

After the aluminum parts have been brightened according to the invention, the parts are rinsed thoroughly in cold, agitated, overowing water. The Work surface can then be passivated in a chromic acid passivating solution, eg., 0.5% chromic acid (Cr03) solution, adjusted to pH of 2.0 with nitric acid, for a short period, e.g., 10 to 20 seconds, to form a thin inert layer of oxide on the brightened aluminum surface without adversely affecting the specular brightness thereof.

It will be understood that treatment in any one or more of the aforementioned auxiliary treating baths, that is,

pre-cleaner, pre-etching, deoxidizer or passivating baths,

is optional, and can be omitted if desired.

It has been found that treatment in the bright dip bath of the invention produces a highly polished mirror-like surface on commercial aluminum, e.g., 1100 and 3003 aluminum, and on alloys such as 2024 and 6061, a bright smooth surface of high specular reflectance is obtained, which is not quite as reflective as the surface formed on 1100 or 3003 aluminum. However, the bright dip bath hereof is effective on various aluminum alloys in addition to those specifically mentioned above.

The following examples illustrate practice of the invention and the advantages thereof:

EXAMPLE l Two solutions A and B are made up as follows:

Percent by Weight Aluminum nitrate (Al(NO3):.0H2O). Water 89. 8

Solution A contains 1.27 mol hydrogen ion, 1.27 mol nitrate ion, .15 mol hexavalent chromium and .039 mol TiPS ion, based on a liter of solution. Solution B contains 1.27 mol hydrogen ion, 1.39 mol nitrate ion, .l5 mol lhexavalent chromium, .039 mol TiFG ion and .14 mol aluminum ion, based on a liter of solution.

A number of panels of 3003 aluminum were treated in solution A at 200 F. and a number of other panels of the same alloy were treated in solution B maintained at the same temperature as solution A.

The square feet of aluminum processed in each of `solutions A and B and the corresponding remaining strength or acidity of the bath as percent by weight nitric acid are determined at various intervals. These values are plotted, giving the curves shown in the FIG. l of the accompanying drawing. The horizontal line indicated at X and corresponding to a concentration of about 5.2% by weight nitric acid represents an average concentration noted herein as the transitional concentration, above which polishing and/ or brightening occurs and below which the polishing and brightening action appears to decrease substantially and graining occurs. The greater the distance above the line X, the higher the specular reflectance produced on the part surface.

From the plots shown in the drawing, it is seen that using solution A not containing aluminum nitrate initially, the bath strength, i.e., its acidity in terms of nitric acid concentration, decreases rapidly and the bath reaches the transitional concentration when less than 8 square feet of aluminum per gallon of solution is processed. When using solution B of the invention, containing the required concentration of aluminum ions, together with the required concentration of nitrate and hexavalent chromium, about l5 square feet of aluminum surface per gallon of solution are processed before the transitional concentration is reached, showing the eiectiveness of the presence of such aluminum ions in increasing the life and elliciency of the bright dip. Moreover, solution B can be revivied by addition of nitric acid to process say 30 square feet or more of aluminum per gallon of solution before the solution is exhausted, whereas with addition of the same amount of nitric acid to the solution A the bath is exhausted when only 16 to 20 square -feet of aluminum per gallon of solution are processed.

.-Further, at points above the transitional concentration indicated by line X on each of curves A and B, for any given value of square feet of aluminum processed per gallon of solution, the specular reflectance of the aluminum treated with solution B containing the requisite concentration of aluminum ions, is superior to that for solution A. For example, at the value on the abscissa corresponding to processing of six square feet of aluminum in each of the baths, the specular reflectance of parts processed in solution B is materially greater than that for solution A. As the number of square feet of aluminum processed per gallon of solution increases, the greater is the difference between the high specular reilectivity produced in solution B as compared to the inferior brightening action of solution A. Also, processing of aluminum parts in solution B in which more than 8 and less than 15 square feet of aluminum per gallon of solution have previously been processed continues to give a high polished surface, whereas after processing about 8 square feet of aluminum per gallon of solution using solution A, further operation proceeds in the region below the transitional concentration, giving inferior results, including graining.

During the processing of the aluminum panels in each of solutions A and B, as the nitric acid is consumed, aluminum from the panels is dissolved in the solution, increasing the aluminum ion concentration. However, in the case of solution A the nitric acid is consumed much faster than in solution Bof the invention, placing substantially more aluminum into solution than solution B. When between 6 and 8 square feet of aluminum have been processed in solution A, there is a large amount of aluminum ion present substantially in excess of the 0.20 mol per liter upper limit for the concentration of aluminum cation according to the invention, and such excess aluminum ion functions as a contaminant in the bath. About 13 to 14 square feet of aluminum can be processed in solution B before it becomes contaminated with an excess of aluminum ion comparable to the excess aluminum ion present when only 6 to 8 square feet of aluminum have been processed in solution A. Hence it is apparent that bath A becomes quickly contaminated by excess aluminum ion due to rapid consumption of nitric acid, whereas bath B of the invention containing a predetermined concentration of aluminum ion initially in solution reduces the consumption of nitric acid and can be used for processing many more square feet of aluminum before it becomes contaminated to an equal degree with excess aluminum ion, as compared to bath A.

FIGURE 2 of the drawing is a plot of square feet of aluminum processed per gallon of solution against percentage of contaminants in the form of excess aluminum, precipitated and reduced chromates and other contaminants including iluoride precipitates, formed during the reaction.

Curve A represents the contamination level of bath A and curve B the contamination level of bath B. Thus, for example, when 8 square feet of aluminum has been processed using solution A, the contamination level is about 4.2%, and according to FIGURE 1, at this point bath A is no longer effective as a brightening bath. On the other hand, at 8 square feet of aluminum processed in solution B, the contamination level for bath B containing aluminum ion initially is only about 2.7%, and at this level of contamination, bath B is still effective as a brightener. Bath B does not reach the 4.2% contamination level until about 13 square feet of aluminum has been processed.

EXAMPLE 2 The following solutions C to K are prepared, a practical range of proportions of the components which can be used also being given for each of the specific compositions:

Table I Percent by Weight C Range 3. 60 to 12.0 s Iso to 4I 0o 8 26 to 1 30 Remainder emainder 8 3 3 8 26 Remainder 16 to .8O .26 to 1. 3 8O to 6 3.6 to 12.6 Remainder 1100, 3003, 2024 and 6061 aluminum alloys are treated in each of the compositions C to K noted above for a period of about minutes, the temperature ofreach of the solutions being maintained at about 200 F.

In each case, the respective parts treated in the above solutions develop a smooth, polished bright surface of high specular reilectance, with absence of pitting and graining, and with substantially no loss of base metal. However, best results are noted with compositions containing titanium, zirconium and aluminum complexes with iluorine, that is, compositions C to G and K. Particularly with the 1100 and 3003 aluminum samples, the compositions of Table I produce mirror-like surfaces. Of the compositions E and F, wherein the uorine complex is the AlF4 ion, composition F containing additional free aluminum ion furnished by the aluminum sulfate produces better results. In composition G, the NaA1O2 in nitric acid solution breaks down to give aluminum nitrate.

From the foregoing, it is seen that the invention provides improved novel compositions and procedure for eiiicient brightening and polishing of aluminum, Without accentuating the formation of smut or grain, and with substantially no base metal loss.

The term consisting essentially of as used in the deiinition of the ingredients present in the composition claimed is intended `to exclude the presence of other materials in such amounts as to interfere substantially with the properties and characteristics possessed by the composition set forth but to permit the presence of other materials in such amounts as not substantially to atiect said properties and characteristics adversely.

The term aluminum employed in the claims is intended to denote commercially pure aluminum and aluminum alloys.

While I have described particular embodiments of my invention for purposes of illustration, it should be understood that various modications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A chemical brightener bath for aluminum, which comprises an acid aqueous solution consisting essentially of about .60 to about 2.00 mols per liter of hydrogen ions, about :60 to about 3.00 mols per liter of nitrate ions, a concentration of hexavalent chromium-containing ion equivalent to about .05 to about 0.40 mol per liter of hexavalent chromium, about .01 to about .10 mol per liter of complex fluoride ions selected from the group consisting of the SiFG, TiF6, ZrF, BF., and AlF4 ions, and about .02 to about .20 mol per liter of aluminum cations, the pH of said solution being not greater than about 0.5.

2. A chemical brightener bath as defined in claim 1, wherein said complex fluoride ions are TiFG ions.

3. A chemical brightener bath for aluminum, which comprises an acid aqueous solution consisting essentially of nitric acid, aluminum nitrate, chromic acid and potassium titanium fluoride, the amounts of said ingredients present being such as to provide about .60 to about 2.00 mols per liter of hydrogen ions, about .60 to about 3.00 mols per liter of nitrate ions, chromate ions in a concentration equiv-alent to about .05 to about 0.40 mol per liter of hexavalent chromium, about .01 to about .10 mol per liter of uotitanate ions, and about .02 to about .20 mol per liter of aluminum cations, the pH of said solution being not greater than about 0.5.

4. A chemical brightener bath for aluminum, which comprises an acid aqueous solution consisting essentially of nitric acid, aluminum uoride and chromic acid, the amounts of said ingredients present being such as to provide about .60 to about 2.00 mols per liter of hydrogen ions, about .60 to about 3.00 rnols per liter of nitrate ions, chromate ionsin a concentration equivalent to about .05 to about 0.40 mol per liter of heXav-alent chromium, about .01 to about .10 mol per liter of AIF., ions, and about .02 to about .20 mol per liter of aluminum cations, the pH of said solution being not greater than about 0.5.

5. A process for chemically brightening aluminum, which comprises treating said aluminum in an aqueous acid solution consisting essentially of about .60 to about 2.00 mols per liter of hydrogen ions, about .60 to about 3.00 mols per liter of nitrate ions, la concentration of hexavalent chromium-containing ion equivalent to about .05 to about 0.40 mol per liter of hex-avalent chromium, about .01 to about .10 mol per liter of complex fluoride ions selected from the group consisting of the SiF, TiFe, ZrFG, BF., and AlF4 ions, and about .02 to about .20 mol per liter of aluminum cations, the pH of said solution being not greater than about 0.5.

` 6. A process as dened in claim 5, wherein the temperature of said solution being maintained between about and about 210 F.

7. A composition which consists essentially by weight of .50 to 6.0 parts of a soluble hexavalent chromium-containing compound, 0.1 to 2.0 parts of a soluble complex uoride of the group consisting of soluble silicoliuondes, titanium liuorides, zirconium uorides, uoborates and aluminum uorides; and .16 to 3.8 parts of a soluble aluminum salt, said composition when added to an aqueous nitric acid solution having a nitric acid concentration furnishing about .60 to about 3.00 mols per liter of nitrate ions, forming =a brightening bath for aluminum, comprisf ing hexavalent chromium-containing ion equivalent to about .05 to about 0.40 mol per liter of hexavalent chromium, about .01 to about .10 mol per liter of complex fluoride ions selected from the group consisting of the Sil-76, TiFs, ZrF, BF., and AIF., ions, and about .02 to about .20 mol per liter of aluminum cations, and about .60 to about 2.00 mols per liter of hydrogen ions, said bath having a pH not greater than about 0.5.

References Cited in the rile of this patent UNITED STATES PATENTS

Claims

1. A CHEMICAL BRIGHTENER BATH FOR ALUMINUM, WHICH COMPRISES AN ACID AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF ABOUT .60 TO ABOUT 2.00 MOLS PER LITER OF HYDROGEN IONS, ABOUT .60 TO ABOUT 3.00 MOLS PER LITER OF NITRATE IONS, A CONCENTRATION OF HEXAVALENT CHROMIUM-CONTAINING ION EQUIVALENT TO ABOUT .05 TO ABOUT 0.40 MOL PER LITER OF HEXAVALENT CHROMIUM, ABOUT .01 TO ABOUT .10 MOL PER LITER OF COMPLEX FLUORIDE IONS SELECTED FROM THE GROUP CONSISTING OF THE SIF6, TIF6, ZRF6, BF4 AND ALF4 IONS, AND ABOUT .02 TO ABOUT .20 MOL PER LITER OF ALUMINUM CATIONS, THE PH OF SAID SOLUTION BEING NOT GREATER THAN ABOUT 0.5.

5. A PROCESS FOR CHEMICALLY BRIGHTENING ALUMINUM. WHICH COMPRISES TREATING SAID ALUMINUM IN AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF ABOUT .60 TO ABOUT 2.00 MOLS PER LITER OF HYDROGEN IONS, ABOUT .60 TO ABOUT 3.00 MOLS PER LITER OF NITRATE IONS, A CONCENTRATION OF HEXAVALENT CHROMIUM-CONTAINING ION EQUIVALENT TO ABOUT .05 TO ABOUT 0.40 MOL PER LITER OF HEXAVALENT CHROMIUM, ABOUT .01 TO ABOUT .10 MOL PER LITER OF COMPLEX FLUORIDE IONS SELECTED FROM THE GROUP CONSISTING OF THE SIF6, TIF6, ZRF6, BF4 AND ALF4 IONS, AND ABOUT .02 TO ABOUT .20 MOL PER LITER OF ALUMINUM CATIONS, THE PH OF SAID SOLUTION BEING NOT GREATER THAN ABOUT 0.5.