US2375394A - Method of brightening surfaces of aluminum-silicon alloys - Google Patents

Description

Patented May 8, 1945 METHOD OF BRIGHTENING SURFACES OF ALUMINUM-SILICON ALLOYS Martin Tosterud, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa, a corporation oi Pennsylvania N Drawing. Application December 8, 1942,

Serial No. 468,219

1 Claim.

Thi invention relates to an anodic method of brightening aluminum surfaces. This latter term, as used herein and in the appended claim, includes surfaces of pure or commercial grade aluminum and of aluminum base alloys, Methods of brightening aluminum surfaces to prepare them for various commercial purposes, including the decorative arts and for the reflection of light and heat, usually vary in ultimate eifect, depending upon the composition of the treated surface. While most of such methods, including the method of the present invention, will brighten an aluminum surface for use for some commercial purposees, a certain aluminum alloy surface may respond better than another to a given method and the optimum brightening effect may not be secured by the use of another method. The present invention, which has as a general object the provision of a brightening method which is useful for the treatment of any aluminum surface, provides a method which is particularly suited to the brightening of aluminum surfaces which contain high amounts of silicon, such as 2 to 20 per cent by-weight.

The invention broadly comprises making the aluminum surface anode in an electrolytic cell the electrolyte of which is composed of hydrofluoric acid, water and a substance selected from the class composed of glycerine and sugars, and thereafter passing a current through the cell to effect the brightening action. Previous uses of aqueous solutions of hydrofluoric acid, to which glycerine was added, for the treatment of aluminum surfaces (such as the use indicated in U. S. Patent No. 1,011,203 to Jahn) have not had as an object, or a result, the brightening of the treated surface and have often served to darken said surface. It is the anodic treatment of aluminum surfaces in such solutions which achieves the objects, and results, of the present invention.

In the practice of the invention a conventional electrolytic cell, such as those used'inthe anodic oxidation of aluminum surfaces, will be found suitable if it is lined so as to prevent substantial attack on its parts by the hydrofluoric acid of the electrolyte. The electrolyte to be used should preferably contain 5 to 25 per cent by weight of hydrofluoric acid (HF), but greater and lesser amounts are permissible, such as concentrations of 2 to 50 per cent by weight. A limiting factor to the use of small amounts of the acid is the consequent reduction in the activity of the solution. Limiting factors to the use of large concentrations of the acid are the necessity of higher current densities as the concentration increases and the necessity of maintaining low temperatures to avoid severe etching of the treated surface during that interval between the shutting off of the current and the removal of the treated surface from contact with the electrolyte. A non-functional limitation exists where the cell is not provided with a hood or other exhaust means for eliminating injurious fumes. able tc not use a concentration of the acid greatly exceeding 10 per cent by weight of the electrolyte. The glycerine or sugar content of the electrolyte should be about 5 to about '75 per cent by Weight. Concentrations of 10 to 50 per cent by weight of the electrolyte are adequate and are preferred. The total hydrofluoric acid and glycerine or sugar content of the electrolyte should not exceed about per cent by weight since the electrolyte should contain at least 10 per cent by weight of water. The use of glycerine is preferred since it seems to give somewhat better results.

Prior to treatment the aluminum surface is preferably cleaned, either mechanically or with a solvent or both, to remove therefrom superficial impurities such as dirt, oil or grease. The surface is then made anode in the electrolyte and the current is started and maintained until the brightening is effected. The current density employed may vary. Normally higher densities are necessary with higher concentrations of hydrofluoric acid in the electrolyte. Densities in the range of 50 to 450 amperes per'square foot are preferred. The best results on cast aluminum surfaces will be obtained in the upper portion of this range, while when the surface being treated is wrought, the lower portion of the range is preferred. Generally temperatures will vary directly with the current density employed, but it is best to maintain temperatures below about 90 R, if possible, to avoid severe etching of the treated surface in the interval between shutting off the current and removing the surface from contact with the electrolyte. Generally the temperature should be maintained as low as is consistent with the desired speed of the brightening process. After treatment by the method of this invention the aluminum surface may be found to have thereon a smudge or soft superficial fllllll which it is desirable to remove. Such removal may be accomplished, if desired, by the practices described in U. S. Patent No. 2,153,060 to Guthrie.

As a specific example of the practice of the invention, may be cited the treatment of an aluminum casting containing 5 per cent of silicon. The casting was made anode in an electrolyte .consisting of 13 per cent by weight of hydrofluoric In such a case it is advis- 2 assesses acid and 52 per cent by weight of giycerine, the balance being water. A current of 1.5 amperes per square inch was impressed on the cell tor ten minutes, the bath meanwhile being stirred mechanically. The temperature of the operation was 77 F. At the end ofthe operation the casting was quickly removed from electrolyte and its surface washed with water to remove all residual electrolyte therefrom. The brightened surface was thereafter immersed in a hot potassium dichromate solution for about one minute to remove smudge" therefrom.

By the practice of this invention the brighten- 1 ing of any aluminum surface may be achieved.

The eifect obtained is uniform over the surface of the sample being treated and severe etching of the surface is avoided.

amperes per square foot to about 450 amperes.

per square foot until the desired brightening is attained. v

MARTIN TOSIERUD.