US2209712A - Method of treating aluminum - Google Patents

Description

Patented July 30, 1940 UNITED STATES] PATENT OFFICE N Drawing.

Claims.

- This invention relates to methods of treatingaluminum and particularly to methods of preparing aluminum for use in electrolytic devices such as electrolytic condensers, rectifiers, lightning arresters and the like. This application is a continuation in part of the copending applications of Joseph B. Brennan Serial No. 707,549, filed...

January 20, 1934, and of Joseph B. Brennan and Leona E. Marsh, Serial No. 118,839, filed January 2, 1937.

In the copending applications referred to above there are disclosed methods of preparing aluminum particularly for use in electrolytic condensers in which the surface of the aluminum is electrolytically treated in order to clean and/or etch the surface of the aluminum. The present application relates especially to the cleaning and/or etching of aluminum in a solution of aluminum chloride (A1013). It is among the objects of my invention to provide a method of cleaning and/ or etching aluminum which can be accurately and easily controlled to produce uniform results. Another object is to provide a method of cleaning and etching aluminum which can be carried out i at extremely low cost. A further object is to provide a method of treating aluminum whereby the surface may be thoroughly cleaned and etched or roughened in such a manner asto produce a large increase in the superficial area thereof. According to my invention the surface ofaluminum is cleaned and etched by subjecting it to electrolysis in an electrolyte consisting principally of a solution of aluminum chloride (AlCla). The 1 without any preliminary cleaning being required,

is immersed in an aqueous solution of aluminum 5 chloride and subjected to electrolytic action as an anode. The cathode preferably is composed of an inert material such as carbon, and the method may .be carried out in a simple apparatus, as will be readily appreciated by those skilled in the art. 0 The aluminum anode and the inert cathode are provided with terminals or leads, which may be connected to positive and negative terminals, respectively, of any convenient source of direct cur rent. The flow of current results in a thorough 5 cleaning and etching of the surface of the alumi- Application May 6, 1937, Serial No; 141,144

.num which appears to have the effect of greatly increasing the superficial area thereof.

In carrying out the electrolysis, I find that neither the strength of the solution, the voltage applied, nor the time of treatment need be held 5 within close ranges. Thus I preferably employ a 3to 5% by weight aqueous solution of alumi num chloride, voltages may vary from 3 to 20 volts, and the time of treatment vary from 5 to 15 minutes. I have found that very satisfactory resultscan be obtained by employing a 5% solution of aluminum chloride, a voltage of about 6 to 10 volts, and a current of about 7 amperes in the case of a piece of aluminum foil measuring 3 in. x 3 in. The current is caused to flow for about 15 minutes, and I find that when a piece of aluminum foil'so treated is pr vided with a dielectric film and incorporated in an electrolytic condenser, the capacity is increased from 4 to 5 times ascompared with an untreated piece-of similar foil incorporated in a similar condenser. This indicates that probably the superficial area of the aluminum is increased from 4 to 5 times by the etching operation. l After subjecting the aluminum foil to electro-- lysis as described, the aluminum is removed from the aluminum chloride solution and the electro-' lyte removed therefrom by washing either in water according to conventional methods or preferably by exposure to steam as disclosed in the copending application of Brennan and Marsh referred to above.

After Washing, the aluminum [is ready for the film forming operation if intended for use as-a condenser anode. The dielectric film may be formed according to any well known method. For example, aluminumnmy be anodized in abath of boric acid or boric acid andammonium borate, depending upon the service for which the condenser is intended. The forming voltages may be controlled in the usual manner, and in the case of high voltage condensers the operation is carried on until the leakage in the forming bath at 550 volts and a temperature of 90 .C. is, about 0.1 to 1 milliampere per microfarad of capacity. This leakage is further reduced when the anode is assembled in a condenser at room temperature. The forming voltages and the method used in forming the dielectric film may be varied depending upon the service for which the condenser is intended, as will be evident to those skilled in the art.

The etching electrolyte may be prepared either by dissolving aluminumchloride directly in water which produces a slightly acid electrolyte beu I cause of the hydrolysis-of the aluminum chloride, or if desired the electrolysis can be started with a 1% to 5% solution of hydrochloric acid. In the latter case, the solution soon has the same characteristics as the solution made by dissolving A1013 in water for the acidity of the solution rapidly decreases as the reactions proceed and aluminum chloride is formed as the result of the action of the hydrochloric acid on the aluminum.

The solution remains slightly acid because of hydrolysis which acidity remains substantially For example, the etching-method may be applied to the production of cathodes for condensers or to etching or cleaning aluminum for decorative purposes.

Various other treatments can be given the aluminum both before and after the electrolytic cleaning and etching step and various other modifications and changes in the invention, all within the spirit and scope thereof, will be evident to those skilled in the art. Therefore it/is to be understood that my patent is not limited to the specific disclosure contained herein or in any manner other than by the scopeof the appended claims when given the range of equivalents to which my patent may be entitled.

I claim:

1. The method of etching aluminum 1 which comprises connecting the aluminum to an external source of electromotive force and subjecting the aluminum to electrolysis as an anode in an electrolyte consisting principally of an aqueous solution of aluminum chloride.

2. The method of preparing aluminum for use as an electrode in an' electrolytic cell which includes the steps of connecting the aluminum to an external source of electromotive force and subjecting the aluminum to electrolysis as an anode in an electrolyte consisting principally of an aqueous solution of aluminum chloride until the surface presents an etched or pitted appearance.

3. A method of preparing aluminum for the reception of a dielectric film including the steps of immersing the aluminum in an electrolyte consisting principally of an aqueous solution of aluminum chloride, connecting the aluminum to an external electric circuit, causing an electric current to flow through the electrolyte, the aluminum being the anode, and thereby causing the surface of the aluminum to be etched, and controlling the etching reaction by controlling the flow of current.

4. A method of etching aluminum which comprises connecting the aluminum to an external source of electromotive force and subjecting the of electromotive force and subjectingaluminunr to electrolysis as an anode in an electrolyte consisting principally of an aqueous solution of aluminum chloride until the efieotive surface area.

of the aluminum is increased from 4 to 5 times.

JOSEPH B. BRENNAN.