US1947981A - Plating aluminum - Google Patents

Description

Patented Feb. 20, 1934 UNITED STATES PATENT OFFICE many, assignor to gesellschaft, Siemensstadt,

Siemens & Halske Aktiennear Berlin, Ger-' many, a corporation of Germany Nov Drawing. Application October 31, 1931,

Serial No. 572,440, and in Germany November 4 Claims. (Cl. 204-3) This invention relates to plating aluminum; and it comprises a method of electrically plating articles of aluminum and aluminum alloys with other metals wherein such an article is electro- 5 lytically treated until an oxidized film develops,

is thereafter made a cathode in an alkaline solution containing a depositable metal until the oxid film is partially removed and a very thin platingof said metal-is formed and is thereafter 'electrically coated with metal deposited on said very thin plating; said very thin plating being usually either copper or brass; all as more-fully hereinafter set forth and as claimed.

' As is well known, itis impracticable to produce good electric platings of other metals on articles made of aluminum or aluminum alloy, by known methods. This is for the reason that all such articles on exposure to the air become covered.

In the present invention I use an expedient which consists in first producing an oxidized coating of different nature and-afterwards simultaneously partially removing it and depositing a film coating of another metal on, the aluminum as the coating is partially removed. The metal of the surface.

As is well known, aluminum when made an anode in many solutions becomes covered with a protective coating of oxidized nature having a 'uniateral conductivity; a phenomenon which permits the use of aluminum in rectifiers and wet electrical condensers. This coating is different from the air-film always occurring on aluminum and enables the production of a me tallic coating on the aluminum if the: coated articles are subsequently treated by the processhereinafter described.

I have found that by providing aluminum with such an anodic coating, the coating can be,par-,

5 tially removed when the article is made a cathode in an alkaline bath and if the bath contains a metal which can be deposited, simultaneously with the partial removal of the oxid'coating I can deposit this metal in the form of an adherent 5 film coating. The metal placed in solution in the film coating produced in this way sticks to the bath to form the film coating can be any of thoseusually deposited as foundations for galvanic coatings. In practice, we use a bath depositing either copper or brass; that is, for example, a bath containing double cyanids of copper and alkali, or of copper and alkali and of zinc and alkali, as the case may be. Cadmium plating baths may be used.

Thefilm coati'ng deposited from an alkaline bath, although very thin, adheres well to the underlying layer. The film coating can be afterwards plated in the regular way in any convenient bath with nomore difllculty than an article of the same kind made of copper, or of brass, as the case may be.

In practice, the metal for the film coating is best copper or brass, although cadmium, silver or gold can be used if desired. The metal for the main coating may be any of the usual metals deposited by electroplating that is, copper, brass, gold, silver, nickel, chromium, .cadmium, etc. The present process is applicable to the coating of aluminum with any metal which may be desired for decoration or protection.

Where alternating current'is used for the primary oxidation the first and second step of the process may be conducted in a single bath; the

article being first provided with an electrochemically produced oxid coating by connection with an alternating current. Then the alternating current is replaced by a direct current, the article being made a cathode.

In providing the oxid coating, which is the first operation in our process, either alternating or direct current may be used; the article being made an anode in the latter event.

The article is made a cathode and voltage may beapplied as is necessary, that usually used being ordinarily over 5 volts. After formation of the primary metallic or film coating, the main coating is applied.

I In a practical embodiment of the described invention, an article of aluminum is made an anode for 15 to 30 minutes in a chromic acid bath of the kind described. This converts the usual 110 .the deposition of brass.

air film on the aluminum into a diflerent type of coating, also of oxidized nature. After forming the oxid coating the article is removed from the chromic acid bath, washed and made a cathode at about 6 volts in an alkaline bath adapted for After two to three minutes passage of the current enough metal is deposited on the aluminum in adherent relationship therewith to serve as a basis for subsequent plating. After washing, the article may be directly plated with any desirable metal among those named hereinbefore.

In another practical embodiment of our invention an aluminum article is placed in a bath containing double copper-alkali cyanids and zincalkali cyanids, together with a counter electrode of sheet iron and an alternating current is imposed. This current may be at 30 to 40 volts and the current density may be 800 amperes per square meter. After two minutes exposure to alternating current, direct current is substituted, making the aluminum a cathode without changing the treating bath. With 5 volts and a current density of 300 amperes per square meter, in about five minutes it is found that the oxid coating is partially removed and an extremely thin skin of brass is deposited on the article. The brassed article may be removed from the bath and plated with gold, copper, brass, tin, silver, nickel, chromium, cadmium, etc., in any suitable bath.

Plated aluminum articles produced under the present invention may be buffed, polished and otherwise finished in the usual ways.-

- What is claimed is: g

1. In brass coating articles made of aluminum and aluminum alloys, the process which comprises imposing alternating current on such an article in a brassing bath containing double cyanids of copper and alkali and of zinc and alkali, continuing such current until an oxidized coating is produced, substituting a low potential direct current with the article a cathode for the alternating current and then finish plating the article in the usual manner.

2. In the plating of articles of aluminum and of aluminum alloys with firmly coherent, substantial metal coatings, the process which comprises passing an electric current between such articles through a bath containing electrolytes of such nature as to produce an electrolytic coating of oxidized nature, flash coating the so-treated articles electrolytically in an alkaline bath with a metal selected from a group consisting of copper, brass, cadmium, silver and gold, and finishcoating said flash-coated articles electrolytically.