US3448055A - Aluminum alloy deoxidizing-desmutting composition and method - Google Patents

Description

3,448,055 ALUMINUM ALLOY DEOXIDIZING-DESMUTTING COMPOSITION AND METHOD Floyd L. Mickelson, Chicago, and Arthur Grwilde, In, Waukegan, Ill., assignors to The Diversey Corporation, Chicago, 11]., a corporation of Illinois No Drawing. Filed Mar. 31, 1965, Ser. No. 444,404 Int. Cl. C23g 1/12 US. Cl. 252-79.3 22 Claims ABSTRACT OF THE DISCLOSURE Disclosed are compositions for, and methods of, deoxidizing and desmutting aluminum based alloys. The compositions contain nitric acid, sulfuric acid, hydrofluoric acid and a fluorinated hydrophobic surfactant.

SC84A), and No: 13 (ASTM-SIZA) which'have the:

following compositions:

CHEMICAL COMPOSITION .sL'UMINUM ALLOY Percent By Weight No. 360 No. 380 No. 13

er Remainder One of the ways of preparing the surface of aluminum castings for further processing is to etch the surface in an alkali solution. This method is not entirely satisfactory because after etching there remains on the surface of the casting a fine pa.rticulate material. This material is referred to as smut. The material is dust-like and loosely held on the surface and often can b ea'sily rubbd'otf. The composition ofthe smut varies with the alloy but generally is composed of the oxides of the alloying metals, including silicon oxide, as well as aluminum oxide and those metallic compounds that do not dissolv'eduring the etching. The smut dulls the metal surface and, if not removed, subsequent deposits of che-mical conversion coatings, such as chromate coatings or paint applications, will not be uniform and will be loosely held where the smut is not removed.

Treatment of aluminum alloy surfaces with conventional acid deoxidizers fails to satisfactorily remove the high temperature oxides formed during the casting operation. A solution of nitric acid and hydrofluoric acid is commonly used for this purpose. Such a solution is highly reactive and within 30 seconds it can etch the casting surface to such a degree as to render it unfit for further processing. Even when carried out carefully, subsequent United States l atent O ice chemical coatings are irregular and far from satisfactory.

Chromic acid type deoxidizers have also been tried to remove smut but they leave behind coii'siderable smut which prevents formation of an acceptable-chemical conversion coating on the surface, such as. 'chromateiconversion coatings which are often applied to aluminum alloy casting surfaces.

According to the present invention, there are provided novel compositions which can be used to treat the sui'faees of aluminum based alloy castings, particularly such castings of aluminum containing a high silicon content, to produce deoxidized, smut free surfacesl simultaneously. The resultfig surfaces are particularly-suitable for the application thereto of chemical conversio'n coatings, such as of the chromate type, for anodizing. the surface of the aluminum casting or for subsequent electroplating processes.

It has been found that the surfaces of aluminum castings can be deoxidized an'd desmuttedby bringing the surface of such an object into contact with a solution of the following composition:

Percent by weight Nitric acid 7-60 Sulfuric acid 10,- Hydrofluoric acid at led 0.5 Phosphoric acid a 4 Surfactant containing a fiuorinated hydrophobic group -'L..- 0-3 Water 5-50 with the percentages of acid being on ananhydrous basis, but advisably of the following compositions:

Percent by weight with the percentage of acids being on an anhydrous basis. Solutions of such compositions will remove the high temperature oxides on aluminum based castings quickly. Also, such solutions will remove smut from aluminum based alloy castings after an alkali treatment or an acid etch cleaning. In doing so, su c h solutions will not significantly etch thebase metal. In additionto removing oxides'and smut, such solutions leave the aluminum surface highly receptive to further treatments, such as the application of conversion coatings, particularly those of the chromate type or paint applications, as well as for anodization of the surface or for subsequent electro plating processes. Conversion coatings applied to the surfaces of such aluminum based alloy materials, after being treated with such solutions, go on uniformly and provide excellent corrosion resistance and good paint adherence.

The novel solutions provided by this invention dissolve oxides'and surface impurities that constitute smut without detrimentally attacking the aluminum base alloy metal. Previously, all solutions used that were active enough to deoxidize and prepare the surface for subsequent treatment detrirnentally attacked the base metal.

The solutions provided herewith remove dried-on smut substantially as readily as freshly-formed smut. Prior desrnutters used in the art removed dried-on smut with much greater difl'iculty than freshly-formed smut but in so doing excessively etched the aluminum casting.

Solutions that are particularly useful, according to the subject invention, but which need not contain phosphoric acid, can have the following composition:

Percent by weight with the percentage of acids being on an anhydrous basis.

Although the presence of phosphoric acid in the solutions is not essential, the inclusion of phosphoric acid permits a greater water tolerance in the solutions without etching of the aluminum based alloy surfaces. Since generally the aluminum castings are washed prior to immersion in the desmutting and deoxidizing solution, water is dragged therein from the prior rinse. This causes the water concentration to increase in the desmuttingsolutions. In solutions without phosphoric acid, after the 'water reaches a concentration of 40 to 45%, serious etching may take place. The inclusion of phosphoric acid, however, reduces the tendency of the desmutting solution to etch the aluminum surface and thus more water can be tolerated. In any event, it is generally advisable to employ solutions containing no more than about 46% by weight of. water even when phosphoric acid is included. Without phosphoric acid being present, a water concentration of about 20 to 40% is generally most suitable in the solutions.

Solutions containing phosphoric acid which are particularly useful in this invention can have the following compositions:

Percent by weight Nitric acid 14-34 Sulfuric acid 13-30 Hydrofluoric acid 0.5-5 Phosphoric acid 15-50 Surfactant containing a fluorinated hydrophobic group -3 Water -50 with the percentage of acids being on an anhydrous basis. Such solutions are generally most suitable when they contain to 46% water, and advisably at least water.

The surfactant is optional but advisably included in the solutions. In commercial production, the inclusion of a surfactarft permits a fast rinsing and thus prevents discolorations and streaking which may be encountered in the rinse tank due to momentary dilution and possible etching by dragged-out desmutting solutions.

Any suitable surfactant which is stable in the desmutting solution can be used. A particularly suitable surfactant is the anionic surfactant having a completely fluorinated hydrophobic tail group and solubilizing group known as Fluorochemical FQ-95 which is commercially available. United States Patent No. 2,732,398 discloses such fluorocarbon surfactants. Fluorochemical FC-95 has the following solubility in grams per 1000 grams of solvent at C.:

. Grams Acetone 100 Benzene 0.8 Carbon tetrachloride 0.04 Ethyl alcohol 2 Heptane 0.03 Isopropyl alcohol 3 Isopropyl ether 0.1 Methyl'falcohol -1..- 60 Perchloroethylene 0.07 Toluene 1 Water 1 2 It is anionic, white, a free flowing powder with heat stability to 390 C.

Solutions which can be used to desmut and deoxidize and providean aluminum surface highly receptive for receiving a chromate conversion coating or for anodization generally should have a ratio, on an anhydrous acid basis, of about 1 part by weight of sulfuric acid to about 0.1 to 6 partsby weight of nitric acid, and advisably about 1 part by weight of sulfuric acid to about 0.3 to 3.0 parts by weight of nitric acid. As a general rule, as the ratio of nitric acid to sulfuric acid increases, desmutting is achieved but the surface of the aluminum alloy becomes less receptive to conversion coatings. On the other hand, as the ratio of sulfuric acid to nitric acid increases, the desmuttingability of the solutions decreases and the solutions tend to attack the base metal thus depositing more smut.

.More particularly, the ratio should be 1 part of sulfuric acid to about 0.5 to 2.0 parts by weight of nitric acid. The best results, however, are generally obtained when the amount by weight of nitric acid is greater than the weight of sulfuric acid. It is furthermore considered that the optimum ratio is about 1 part by weight of sulfuric acid on an anhydrous basis to 1.10 parts by weight of nitric acid on an anhydrous basis.

Good desmutting and deoxidizing is obtained with the solutions provided herewith when they contain a minimum of 0.5% by weight of hydrofluoric acid on an anhydrous basis. If the level of hydrofluoric acid is lower than 0.5%, the desmutting activity of the solutions falls off and the results are not nearly as satisfactory. Furthermore, there is rarely any need for the hydrofluoric acid concentration to be greater than 5% by weight of the solution although it can be higher, and generally it is adequate to have present no more than 2% by weight of hydrofluoric acid.

The fluoride ion can be supplied in the solutions by use of hydrofluoric acid or a suitable fluoride salt. One of the most suitable salts which can be used is ammonium biflgoride. Other salts which can be used are sodium biflrioride, fiuoro boric acid and sodium silicofluoride. For the purpose of this application, the term hydrofluoric acid is considered to encompass the resulting acids acids formed by the utilization of salts in the solutions to provide the needed hydrofluoric acid.

Some specific solutions which are highly useful in this invention are the following:

, Percent by weight Nitric acid 41.00

Sulfuric acid 37.00

Hydrofluoric acid 1.30 Surfactant containing a fluorinated hydrophobic group 0-1 Water, balance to with the percentage of acids being on an anhydrous basis.

Percent by weight Nitric acid 33.60

Sulfuric acid 30.50

Hydrofluoric acid 1.20 Surfactant containing a fluorinated hydrophobic group 0.02

Water 34.68

with the percentage of acids being on an anhydrous basis.

One of the most useful of the solutions, having optimum deoxidizing and desmutting ability with good water tolerance and reduced etching tendency, has the composition:

Percent by weight immersed in a solution as providedby this invention. w

Subsequently, the casting can be treated to apply a chromate conversion coating.

In another preparatory treatment, particularly for castings made of aluminum alloys of high silicon content, the casting can be first cleaned in an alkaline cleaner to remove oils, greases and mold lubricating materials, the casting rinsed with water and the part then given an etch in an alkali etching bath followed -by a rinse in clear water. An etching bath containing 4 ounces of alkali per gallon at a temperature of 140 F. will generally etch adequately in 3 minutes. Following such etching treatment, the casting can be dipped into a desmuttingdeoxidizing solution as provided by this invention, generally for at least 4 minutes, followed by a rinse in clean water. Subsequently, the casting can be placed in a chromate conversion coating solution of conventional composition, ,then removed and rinsed with clear water and the casting dried, advisably in a current of warm air.

Many castings are degreased using per or trichlorethylcne and then given a chromate conversion coating. Use of the present invention as an intermediate step improves the appearance of the conversion coating achieved.

Still another alternative preparation which can be used comprises treating the casting with a phosphoric acid etching and cleaning bath followed by a rinsein clear water, next treating the casting with a caustic etch bath followed by a water rinse and then subjecting the casting to a novel solution as provided herewith. After that, the casting can be rinsed with water, the part given a conventional chromate conversion coating, followed by a water rinse andthen dried with blown air. Drying of the wet casting with a strong current of air reduces the possibility of streaking.

Any suitable, conventional chromate conversion coating can be applied to aluminum based alloy products after being treated with the solutions of this invention. The chromate conversion coatings adhere excellently to the metal after such treatment and provide a superior base upon which to apply paint. Chromate conversion coatings applied to objects treated with these solutions are uniform coatings, golden in color, and do not shift in shade significantly. The quality of a chromate conversion coating can be determined by the salt-spray method, as given in ASTM-B-l17-62. Using this test, very excellent results have been obtained with chromate conversion coatings applied to aluminum allow surfaces treated with the desmutting-deoxidizing solutions of this invention.

The following examples are presented to illustrate the invention. In all of the examples, the chromate coating was applied using a solution consisting of chromic acid (7 g./l.) and fluoride ion (0.4 g./l.).

EXAMPLE 1 A deoxidizing solution was prepared to contain:

Percent by weight A casting of 380 aluminum alloy high in silicon content was cleaned in a 3 oz./gal. concentration non-etch alkaline cleaner at 170 F., and then rinsed. The casting was next immersed in the deoxidizing solution, which was at 750 F., for 4 minutes and then rinsed in clear water. Only a very slight etching was discernible. The casting was then treated with the chromate conversion coating solution, rinsed with water and dried in a stream of warm air. The result was a uniform golden chromate coating with no streaking or shades of other colors. This coating had good corrosion resistance and paint adhesion.

EXAMPLEZ A deoxidizing solution was prepared to contain:

Percent by weight Nitric acid, 70% conc. 37.40 Sulfuric acid, 97% conc. 23.60 Phosphoric acid, 75% conc. 19.60 Water 14.69 Hydrofiuoric acid, 47% conc. 4.70 Fluorochemical surfactant FC- 0.01

Castings of 380 and 360 aluminum alloys high in silicon were first treated with a 15% phosphoric acid clean ing and etching solution and then rinsed in clear water. The next treatment was with a 4 oz./gal. concentration of caustic e'tch solution at F. for 2 minutes followed by a clear water rinse. Then the castings were immersed in the deoxidizing solution of the above composition at L F. for 5 minutes. No etching occurred during the treatment. The stings were rinsed in cleapwater and then dipped into the conventional chromate conversion coating solution at 75 F. for 2 minutes. The parts were rinsed and dried in a current of warm air. An excellent uniform, golden chromate conversion coating was obtained.

EXAMPLE 3 Castings of type 360 and 380 high silicon aluminum alloys were treated with a -15% phosphoric acid cleaning etching solution, rinsed, treated in a 4 oz./gal. caustic etch solution at 140 F., rinsed and treated in a desmutter-deoxidizer solution of the following composition.

Percent by weight Sulfuric acid 13.50 Nitric acid 15.00 Hydrofluoric acid 0.50 Phosphoric acid 25.00 Water 45.97 Fluorochemical surfactant FC-95 0.03

EXAMPLE 4 The formulations of Table 1 show variations in ratio of nitric acid to sulfuric acid which gave good desmutting and deoxidizing in less than 10 minutes. The peroentages of acids are given on an anhydrous basis. Castings of type 380 high silicon aluminum alloy were used in all the tests and, in addition, alloys 360 and 13 were used in solution formulation 3. The castings were pretreated as in Example 2 before treatment with the desmutting and deoxidizing solutions. The castings were subsequently given a chromate coating and examined.

Compositions in Percent TABLE 1.EFFECT 0F VARIATION IN NITRIC ACID:SULFURIC ACID RATIO Appearance of chromate conversion coating..-

l Excellent desmutting. 1 Golden brawn.

9 EXAMPLE 8 TABLE 5. COMPOSITIONS USING INORGANIC FLUORIDES Composition in percent Sulfuric acid.. Nitric acid.... Phosphoric aci Additive Fluorochemical surfactant FO-95 Water Alloy type.

Time, minutes Desmutting 1 Ammonium bifluorlde. 2 Sodium bifluoride.

I Very good.

4 Desmuts but slight etching.

EXAMPLE 9 To facilitate packaging and shipping, it is convenient to make and ship a dual product composed of two separate mixtures. Mixture A can consist of an aqueous solution of the following composition:

. 9 eiefiassss esssea 1c sesess Percent by weight Nitric acid 14-54 Sulfuric acid 13-59 Phosphoric acid -50 Fluorochemical surfactant FC-95 0-3 Water -46 with the percentages of acids being on an anhydrous basis. Mixture B can consist of a mixture containing a source of active fluoride ion. It can be a liquid or dry mixture. A dry mixture can have the following composition:

Percent by weight Ammonium bifiuoride 0-100 Sodium silicofluoride 0-90 Sodium fluoride 0-100 Fluorochemical surfactant FC-95 0-3 with the sum of the ammonium bifluoride and sodium fluoride being at least 10% by weight of the composition.

A liquid type mixture B can have the composition: h

Percent by weight Hydrofluoric acid 10-99 Phosphoric acid 0-90 Sulfuric acid 0-90 Fluorochemical surfactant FC-95 0-3 Water, balance to 100%.

with the percentages of acids being on an anhydrousbasis. To prepare a deoxidizing-desmutting solution, about 1 to 10 ounces of a Mixture B in dry or liquid form can be added to 1 gallon of Mixture A.

A preferred Mixture A can have the composition:

Percent by weight Sulfuric acid, 66 B. 26 0 Nitric acid, 42 Be. 41.70 Phosphoric acid, 75% cone. 22.00 Fluorochemical surfactant FO-95 0.01 Water 9.89

A preferred dry Mixture B can have the composition:

Percent by weight Ammonium bifluoride 60.00 Sodium silicofluoride 39.995 Fluorochemical surfactant FC-95 0.005

To prepare a deoxidizing-desmutting solution, 5 ounces of the preferred dry Mixture B can be added to 1 gallon of preferred Mixture A. A deoxidizing-desmutting solusion prepared in this way gave good results with No. 380, No. 360, and No. 13 aluminum alloy castings. Desmutting was achieved in 4 to 5 minutes and a chromate conversion coating that was subsequently applied firmly adhered and had a good golden color.

Compositons of the Mixture B type can also be used as make up additives in deoxidizing-desmutting solutions depleted of fluoride ions through use.

Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.

What is claimed is:

1. A solution for deoxidizing and desmutting an aluminum based alloy object consisting essentially of:

Percent by weight Nitric acid 29-54 Sulfuric acid 17-59 Hydrofluoric acid 0.5-5 Water 5-45 with the percentages of acids being on an anhydrous basis.

2. A solution for deoxidizing and desmutting an aluminum based alloy object consisting essentially of:

Percent by weight Nitric acid 14-34 Sulfuric acid 13-30 Hydrofiuoric acid 0.5-5 Phosphoric acid 15-50 Water 5-50 Percent by weight Nitric acid 29-54 Sulfuric acid 17-59 Hydrofluoric acid 0.5-5 Water 5-45 with the percentages of acids being on an anhydrous basis, and with 1 part of sulfuric acid present with from about 0.3 to 3.0 parts of nitric acid.

7. A solution according to claim 6 containing at least 0.8% of hydrofluoric acid.

8. A solution for deoxidizing and desmutting an aluminum based alloy object consisting essentially of:

Percent by weight Nitric acid 14-54 Sulfuric acid 13-59 Hydrofiuoric acid 0.5-5 Phosphoric acid -0-50 Water 10-46 with the percentages of acids being on an anhydrous basis. 9. A solution according to claim 8 in which 1 part of sulfuric acid is present with from about 0.3 to 3.0 parts of nitric acid.

10. A solution according to claim 8 in which the percent of nitric acid is greater than the percent of sulfuric acid.

1 1 11. A solution for deoxidizing and desmutting an aluminum based alloy object consisting of:

Percent by weight Nitric acid 41.00 Sulfuric acid 37.00 Hydrofiuoric acid 1.30 Water, balance to 100% with the percentages of acids being on an anhydrous basis.

12. A solution for deoxidizing and desmutting an aluminum based alloy object consisting of:

Percent by weight Nitric acid 26.00 Sulfuric acid 24.00 Hydrofiuoric acid 1.50 Phosphoric acid 16.00

Water, balance to 100% with the percentages of acids being on an anhydrous basis.

13. The method of deoxidizing and desmutting the surface of an aluminum based alloy object which comprises immersing the object in a solution consisting essentially of the composition:

Percent by weight Nitric acid 14-54 Sulfuric acid 17-59 Hydrofiuoric acid 0.5-5 Phosphoric acid 0-50 Water -46 'with the percentages of acids being on an anhydrous basis, and subsequently rinsing the object with water.

14. The method of claim 13 in which the solution contains 1 part of sulfuric acid per 0.3 to 3.0 parts of nitric acid.

15. The method of claim 13 in which the solution contains a greater percentage of nitric acid than sulfuric acid.

16. The method of deoxidizing and desmutting the surface of an aluminum based alloy object which comprises immersing the object in a solution consisting essentially of the composition:

Percent by weight Nitric acid 29-54 Sulfuric acid 17-59 Hydrofiuoric acid 0.5-5 Water 5-45 with the percentages of acid being on an anhydrous basis, and subsequently rinsing the object with water.

17. The method of claim 16 in which the solution contains a greater percentage of nitric acid than sulfuric acid.

18. The method of deoxidizing and desmutting the surface of an aluminum based alloy object which comprises immersing the object in a solution consisting essentially of the composition:

Percent by weight Nitric acid 14-34 Sulfuric acid 13-30 Hydrofiuoric acid 0.5-4.4 Phosphoric acid 15-50 Water -46 with the percentages of acids being on an anhydrous basis, and subsequently rinsing the object with water.

19. The method of claim 18 in which the solution contains a greater percentage of nitric acid than sulfuric acid.

20. The method of preparing the surface of an aluminum based alloy object which comprises simultaneously deoxidizing and desmutting the surface by immersing the object in a solution consisting essentially of the composition:

Percent by weight Nitric acid 7-60 Sulfuric acid 10-70 Hydrofiuoric acid at least 0.5 Phosphoric acid 0-50 Water 5-50 with the percentages of acids being on an anhydrous basis, and subsequently rinsing the object with water.

21. A package for shipping for deoxidizing and desmutting an aluminum based alloy object consisting essentially of a first portion having about the following composition:

Percent by weight Sulfuric acid, 66 B 26.40 Nitric acid, 42 B 41.70 Phosphoric acid, conc. 22.00 Surfactant containing a fiuorinated hydrophobic group 0.01 Water 9.89

and a second portion having about the following composition:

Percent by weight Ammonium bifiuoride 60.000 Sodium silicofluoride 39.995

and the two portions are adaptable to being mixed together to form a solution suitable for simultaneously deoxidizing and desmutting the surface of an aluminum based object by immersing said object in said solution.

22. A composition for use in solution as make up additive for deoxidizing and desmutting the surface of an aluminum based alloy object having about the following ingredients Percent by weight Ammonium bifiuoride 60.000 Sodium silicotluoride 39.995

References Cited UNITED STATES PATENTS 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 448 O55 Dated June 3 1969 fls) Floyd L. Mickelson and Arthur G. Wilde. Jr,

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

[- Column 2, line 31, "compositions" should be composition T Column 4, line 12, delete "acids". Column 5, line 53, change "allow" to alloy Column 6, line 2, change "750" to 75 Column 12, lines 26 and 27, delete "Surfactant containing a fluorinated fiydropfiofiic gr0up...0.0l".

SIGNED AND SEALED MAR 3 1970 L) AM Edward M. Member. I!-

. mm 1. W, JR.

Attestmg Officer comissioner of Patents