Patented Aug. 3, 1948 ELECTRODEPOSITION OF ALUMINUM Thomas P. Wier, Jr., Berkeley, Calif., assignor to The William Marsh Rice Institute for the Advancement of Literature, Science and Art, a corporation of Texas No Drawing. Application February 29, 1944, Serial No. 524,487

1 Claim. 1

This invention relates to a process for the electrodeposition of aluminum on dissimilar metals.

In a co-pending application Serial No. 522,375 filed February 14, 1944, Hurley has disclosed that aluminum can be advantageously deposited from and electrolyte comprising (1) an aluminum halide such as aluminum chloride or aluminum bromide and (2) various lower aliphatic pyridinium halides such as N-ethyl pyridinium chloride at temperatures generally above 100 C. In another co-pending application of Wier and Hurley, Serial No. 524,486, filed February 29, 1944, it has been disclosed that aluminum can also be deposited on dissimilar metals at room temperature "by utilizing a bath including certain aromatic compounds in addition to the aluminum chloride and the mono-pyridinium mono-halides. The plates secured by utilizing these bath-s with direct current are brittle and tend to crack, break or flake away from the base, particularly if the article is repeatedly flexed. When using the above mentioned baths, I have discovered that by superimposing a suitable alternating current on the direct current, adherence of the plate to the base metal is increased with the result that the articles can thereafter be worked and bent to shape. Further, the plate secured is more uniform and, in addition, its thickness can be increased. Also, by superimposing a suitable alternating current on the direct current, at the same direct current density the voltage required is lowered somewhat. In addition, higher cathode direct current densities can be used to obtain plates of the same general type as with direct current only. These advantages are obtained without any decrease in cathode current efliciency.

In the experiments whose results are described herein, the magnitude of the alternating current was an approximately sinusoidal function of time and the frequency was about sixty cycles per second. A current of this character is usually most conveniently obtained. However, higher or lower frequencies or other alternating current and the type of plate desired. When it is desired to employ high direct current densities, the ratio of alternating current to direct current should be increased. Likewise, when thick, non-brittle deposits are to be obtained, the ratio should be increased. The exact value of the ratio is usually not highly critical and can readily be determined for a given set of conditions. The direct current and alternating current voltages employed are usually low (of the order of a few volts, usually less than ten volts) and depend upon the several factors mentioned above.

X-ray examination of plates made in accordance with the present invention shows that when direct current alone is employed, the plates consist of partly oriented, small crystals, whereas when the alternating current is imposed in addition, the plate is composed of larger crystals which are not oriented. It is believed that the improved adherence and other properties of the plates are accounted for in this way.

The following example is set forth by way of further illustration of the practice of the present invention, and not by way of limitation:

A plating mixture was prepared by mixing 2 mols of aluminum chloride (taken as AlCls) and 1 mol of fused ethyl pridinium bromide. After cooling the mixture it was saturated with benzene. About 50 cc. of this solution together with suflicient excess benzene to provide a protective benzene layer about 0.5 cm. in depth was then placed in a cylindrical glass electrolysis cell. This cell was 4.5 cm. in diameter and about 10 cm. high. It was fitted with a cover designed to exclude moisture. The anode was a circular sheet of aluminum fitting against the wall of the cell. The cathodes consisted of suitably cleaned strips of brass 1.34 cm. wide and about 15 cm. long; these were immersed to a depth of 3.55 cm. in the bath. The source of direct current was connected across the electrodes and'the source 01 alternating current was connected in parallel with the direct current. The circuits were so arranged as to permit separate variation of the alternating current and of the direct current. The magnitude of the current passing through the cell was an approximately sinusoidal function of time and was symmetrical about a value represented by the direct current. The frequency of the alternating current was approximately cycles per second.

In a series of plating tests, varying amounts of alternating current were superimposed on a constant amount of direct current. The density of direct current at the cathode was 1.20 amps/dun.

3 The direct current voltage was between 0.7 and 0.8 volt. The time of electrolysis was 10 minutes in each instance. The effect of variation in the root-mean-square value of the alternating current with respect to the direct current on the types of plate is shown in the following table:

5 being about two to three times the value of the direct current.

THOMAS P. WIER, JR.

. I Type assurances omen o U gg y plate with small brown 10 The following references are of record in the 1.0 man till 5 signs a? plate secured. when 519 of his patent: 1.5 r ncizfily shfiy E white. Not UNITED STATES PATENTS muc apparent p ysical change, but no brown coloration. Number Name Date 33%; mwhat 15 1,567,791 Duhme D c, 29, 1925 Uniiorm white deposit, not uc 1,785,389 Piersol Dec, 16, 1930 1,911,122 Keyes et al. May 23, 1933 1,939,397 Keyes et a1. Dec. 12, 1933 Satisfactory plates on other metals such as iron Mathers t all g and copper were also made, using this process.

I claim:

In a. process of electroplating with aluminum, the step of subjecting an article of a metal from the group consisting of brass, copper and iron, serving as a cathode, simultaneously to an alternating current and a. direct current, passed between said cathode and an aluminum anode and through a fused, anhydrous bath consisting of OTHER REFERENCES Journal of Physical Chemistry, vol. 35 (1931), p. 2289.

Chemical Abstracts, vol. 32, No. 18, Sept. 20,

25 1938, Col. 6956.

Elektrochemie Nichtwassriger Losungen by Paul Walden, pp. 194, 200, 239.