US2431595A - Method for cleaning aluminum - Google Patents

Description

Patented Nov. 25, .1947

METHOD For: CLEANING ALUMINUM Wayne E. White, Plum-Township, Allegheny County, Pa., assignor to Aluminum Company of America, Pittsburgh, 2a., a corporation of Pennsylvania UNITED. STATE No Drawing. Application August ii, 1942, Serial N0. 453,875 7 3 Claims. (c1. 134-2) This invention relates to the cleaning of aluminum surfaces and is more particularly concerned with the provision of a suitable material, as well as a novel method, for accomplishing this purpose. The term aluminum as herein employed, is intended to include both aluminum as well as aluminum base alloys.

A variety of impurities and filmsmay collect on aluminum surfaces and must often be removed therefrom. For example, aluminum sheet during fabrication may acquire oil stains and other films. Another coating which must be removed upon occasion from aluminum surfaces is one consisting substantially of aluminum oxide. A thin film of this substance always forms on aluminum surfaces when they are exposed to the atmosphere, while a much heavier coating of oxide may be produced artificially on aluminum surfaces by various'well known methods. Such oxide films, which are removed with dimculty, often serve to bind oils and other staining materials with which the aluminum surfaces may have come in contact, thereby necessitating the removal of the oxide coating in order to accomplish efiective cleaning of the surface. In many cases, even when the article is not stained or discolored in this fashion, it becomes necessary for one reason or another to remove the heavy, artificially produced, oxide coatings. vWhenever possible, the cleaning of the. surface should be accomplished without etching or otherwise attacking the underlying aluminum surface to any appreciable extent.

It is an object of this invention to provide a method of cleaning and brightening aluminum, which is of such a nature as to remove surface films and impurities readily Without materially attackin the aluminum and to provide a material for use in cleaning and brightening aluminum. A more particular object of the invention is to provide an improved method of cleaning aluminum whereby this result may be obtained gy merely immersing the article in a suitable ath.

It is my discovery that molten ammonium fluoride (NH4F) will remove the various surface coatings and films, including the heaviest of oxide coatings, from aluminum surfaces without material attack upon the underlying aluminum surface, even when the aluminum article is kept in contact with the molten ammonium fluoride for long periods of time. In many cases this cleaning action will further result in a decided brightening'of the aluminum surface in consequence of the removal of these various coatings and allied impurities. The cleaning process may be carried out by merely immersing the aluminum surface to be cleaned in a bath of molten ammonium fluoride for a suitable time. Ammonium fluoride becomes molten at a. temperature of about 246" F., and while the molten salt may be successfully employed as an aluminum cleaning agent at any temperature above its melting point, I prefer to maintain the'bath within a temperature range of about.300 to 375 F. When the temperature of the bath falls below this range, the cleaning action becomes somewhat slower than is otherwise desirable, whereas at temperatures above 375 F. the bath will fume unduly.

The time necessary to leave the aluminum article or materials in the molten cleaning bath will vary considerably, depending on the nature of the material and of the surface film or 'lmpurity present thereon. Usually an immersion period of about one-half minute'will suffice, though this period may be as short as 5 seconds, as when cleaning and brightening aluminum already having a relatively clean surface. On the other hand, it may be necessary in some instances to employ a much longer immersion period. The aluminum material being cleaned may be kept in the bath for comparatively long periods of time without harmful result, however, because of the relative lack of interaction between the molten salt and the aluminum.

Instead of employing a bath consisting of ammonium fluoride, there may be employed a substance which yields molten ammonium fluoride which is available for cleaning the aluminum under the conditions existing, and in the appended claims any reference to available ammonium fluoride is intended to include substances of this type as well as ammonium fluoride. For example, the material commonly referred to as ammonium acid fluoride (NH4F.IIF) falls within this group since upon being heated it furnishes ammonium fluoride which serves to clean aluminum immersed, therein. However, when'using ammonium acid fluoride incarrying out this invention, the bath should not be entirely composed of this substance, for when the concentration thereof of the bath exceeds about '75 per cent by weight, the molten salt will unduly attack and etch the aluminum surface. For this reason, when employing ammonium acid fluoride, it is necessary to incorporate in the bath at least 25 per cent by weight of a suitable diluent. Ammonium fluoride and ammonium carbonate are suitable diluents. In case ammonium carbonate is used its concentration should preferably not be over about per cent by weight, since, as distin- .guished from ammonium fluoride, ammonium fluoride which serves as a cleaning agent for the aluminum, any reference in the appended claims to a bath consisting of ammonium fluoride is also intended to embrace bath containing ammonium acid fluoride.

When the aluminum articles are removed from the ,molten salt bath, a certain amount of salt adheres to the surface and solidifies there. Water may be employed to remove it, but because of the corroslveness to aluminum of aqueous ammonium fluoride solutions,'itis important that the removal of the salt be rapid,'else the metal surface will be marred by the etching action of the aqueous solution. If crevices are present which will tend to entrap the salt and prevent its rapid removal by water, or if for any other reason a quick water rinse is not satisfactory, a bath of hot glycerol or but will not promote the etching reaction as does water.

After the treatment with glycerol or ethylene glycol the cleaned material may be rinsed with water.

When in use, the bath which is here proposed will always fume to a greater or lesser extent. For this reason, the molten salt should be kept in a vessel which may be covered except when placing articles to be cleaned in the bath or when removing them therefrom. If this should not prove to be convenient, a suitable paraffin 'or ceresin wax may be floated on the bath. The oil layer so produced effectively prevents all fuming and consequent loss of the bath constituents through evaporation. When using an oil layer of this nature, it has been found advisable to employ a small well which is not covered by the oil and through which the material to be cleaned may be introduced or removed from the bath, thereby avoiding contamination of the surface of the material by the oil. Nickel is not appreciably attacked by ammonium fluoride or ammonium acid fluoride, and for that reason it has been found most satisfactory to employ vessels composed of nickel to hold the molten salt. Iron may be employed for this purpose, but it is somewhat susceptible to attack by ammonium fluoride, and for this reason nickel containers are preferred. To illustrate the manner in which this invention flnds application, the following examples will be given:

Example 1 A group of aluminum sand castings bearing on their surface a thick gray film comprising aluthe castings were allowed to remain in the bath.

for periods as long as 60 minutes. All of the castings were thoroughly washed with water to remove all traces of ammonium fluoride as soon as they were removed from the bath. Whereas before the immersion treatment the surface of the castings had been dull and discolored, after the immersion the castings presented a bright appearance and were free of any but a thin film of aluminum oxide such as is formed upon aluminum when exposed to air. The castings were weighed both before and after the immersion in the salt bath, and it was observed that even in the case of those pieces which had been left in the bath for 60 minutes, the loss in weight due to the attack of the molten ammonium fluoride upon the aluminum was negligible. The castings cleaned in this fashion were readily joined by spot welding, whereas the nature of the surface of the castings prior to cleaning in the bath precluded joining by this method.

Example 2 Bright, rolled aluminum foil having the reflectivity of about 81 per cent was cleaned and brightened by immersion for a period of about 10 seconds in a bath consisting of 50 per cent by weight ammonium fluoride and 50 per cent by weight ammonium acid fluoride. This bath was maintained at a temperature of about 325 F. Upon subsequent withdrawal from the bath and washing with water, the foil was found to have been cleaned and brightened so that its reflectivity was approximately per cent. There was no visible evidence of any interaction between the foil and the molten salt. The reflectivity of the aluminum surface was measured with the Taylor-Baumgartner Reflectometer, described in the November, 1937 issue of the General Electric Review, at pages 525-527 inclusive.

I claim:

1. A method of cleaning aluminum surfaces,

comprising immersing the article to be cleaned in a bath containing a predominant quantity of available molten ammonium fluoride for a period of at least 5 seconds, said bath being maintained at a temperature of at least 240 F., and thereafter removing the article from the bath.

2. A method of cleaning aluminum surfaces, comprising immersing the article to be cleaned in a bath consisting of molten ammonium fluoride for a period of at least 5 seconds, said bath being maintained at a, temperature of at least 240 F., and thereafter withdrawing the article from the bath.

3. A method of cleaning aluminum surfaces, comprising immersing the article to be cleaned in a bath consisting of molten ammonium acid fluoride and a molten diluent therefor for a period of at least 5 seconds, said bath being maintained at a temperature of at least 240 F., and thereafter withdrawing the article from the bath.

WAYNE E. WHITE.

REFERENCES CITED The following references are of record in the OTHER REFERENCES Handbook of Chemistry and Physics, 26th ed. 1942-1943, Chemical Rubber Publishing 00., Cleveland, pages 334 to 337. (Copy in Div, 3.)

Mellor, J. W., A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 11, pages 519-521, 784, 785, Longman, London, 1922. (Copy in Div. 59.)