US1727331A - Process of coating aluminum electrolytically - Google Patents

Description

sept. 1o, 1929. j C. L. BEM. 1,727,331

PROCESS QF COATING ALUMINUM ELECTROLYTICALLY Filed Jan. 1'?. 1927 .ilwe aqueous aad bath Subsantzay free #om aUfaZL lsalzsuchj as L 5% sulfuric 9 acid SOZzLOn/V//T/ he cathde.

ATTORNEY.

Patented Sept. 10, 1929.

vUNITED STATES PATENT OFFICE.

CARL L. BEAL, F ROCHESTER, NEW YORK, ASSIGNOR T0 EASTMA-N KODAK COMPANY,

OF ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK.

.PROCESS OF' COATING ALUMINUM ELECTROLYTICALLY.

Application led January 17, 1927. Serial No. 161,515.

This invention relates to processes of coating aluminum and alloys containing the same, electrolyt-ically. One object of the invention is to provide a rocess in which an electrolytic coating o adequate density and adherence can be united to an aluminum surface in spite of the tendency of the latter to form a thin resistant film of oxide. Another object of the invention is to provide a process which will accomplish the above result at the minimum of trouble and expense. Other objects will hereinafter appear.

In the accompanying drawing,-

Fig. 1 is a diagrammatic vertical section on a reduced scale of one form of apparatus in which the first be conveniently carried out;

Fig. 2 is a similar view of one form of apparatus in which the second pretreatment step may conveniently be carried out;

Fig. 3 is a similar view of one form of apparatus in which the electrodeposition may conveniently take place.

It is well known that aluminum has been difficult to coat reliably, because its surface, inzcontact with the air, rapidlyv forms a tenacious film, which even when it is thin, resists the adherence of an electrodeposited coating over it. For instance, it has been difficult to )roduce a good reliable zinc coating electro ytically on aluminum.

I have found that this trouble can be avoided by first treating' the surface of. the aluminum in a dilute aqueous alkaline solution, preferably hot, and then making it a cathode in an acid bath, these successive pretreatment steps bringing the surface t0 a condition in which it can bereadily electroplated in a standard commercial bath, such as an acid-sulfate zinc plating bat i The first pretreatment step does not re'- quire any electrolysis, and is very rapid and economical. Various alkaline aqueous so-l lutions maybe employed, such as those of sodium hydroxid, sodium carbonate, or trisodium phosphate. The concentration of the solution may vary considerably. I have found it convenient to work with one having a hydroxyl ion concentration of the same order of magnitude as that of a 5% solupretreatment step may tion of trisodium phosphate. Torking with such a solution at a temperature slightly below boiling, say 190 to 200 F. the surface to be treated requires only about a halfl minute immersion. Where it is undesirable to carry any of this alkaline bath into the bath for the second retreatment step, the aricle can be dipped or a few seconds in an acid bath, say 10% aqueous hydrochloric acid, but this is not indispensable, although desirable.

The surface under treatment, after undergoing the action of the alkaline solution in the first step, is then made a cathode in an aqueous acid solution. I For example, it may be electrolyzed for about 3 to 5 minutes at a current density of 0.4' amperes per square inch with a. voltage of around 2. The acid solution can be, for example, a 5% aqueous sulfuric acid one, but the concentration can obviously be varied considerably, and any other acid can be substituted in an amount which will furnish a correspondingly sufcient hydrogen ion concentration.

It will be noted that the final cleaning, or second pretreatment ste takes place in an electrolyte which contains substantially noy alkali metal salts. This is important because any considerable concentration of the ionsof such metal causes conditions in the cathode region,.which favor further filming of the aluminum 'surface which forms the l bi Icathode. In my process the chief cations during the second pretreatment step are those of hydrogen (H*) and these'tend to dissolve the oxide and hydroxid that were previously formed or loosened during the first or alkaline pretreatment step. AIoreover, the hydrogen polarization of the cathode surface prevents oxidation. IVhile I do not wish to be restricted to a particular theory of operation. since the above described pretreatment steps have been found in actual practice to be unexpectedly effective, nevertheless, I believe that hydrogen, in addition'95 to maintaining a non-oxidizing zone around the cathode, also mechanically eliminates some of the aluminum hydroxid coating, the

bubbles acting on this coating. after the preliminary attack of the alkaline immersion.

The surface of the aluminum having undergone the pretreatment described above, can4 be lated in any well known acid-plating bat i, such as the standard acid-sulfate 5 zinc plating bath operating at a current density of 0.1 to 0.2 upon the treated surface.l As such bathsare well known, the lde-4 tails of them are omitted here.

In the drawing, 1 represents any suitable container in which is the dilute aqueous alkaline solution inl which the article 3, having the surface of the readily oxidizable metal, is suspended in any suitable Way, say by rod 4 and hook. 5. i

1'5 In Fig. 2 any suitable vessel 6 contains the dilute acid solution 7 in which the artifcle to be treated 3 is immersed as the cathjode, say by electroconducting hook 7 from i cathode bar 8. Suitable anodes 9 are connected to anode bars 10. The bars 8 and .10 are connected to the poles of any suitable source of current 11.. The anodes 9 are ...preferably of a material which does not foul the bath, such as carbon.

:l In Fig. 3 any suitable vessel 12 has suspended in the bath, as 'the cathode, the treatedarticle 3 suspended by any suitable electroconducting hook 75 from cathode bar 8. The anodes 13 are connected to anode bars 14. Bars 8 and 14 are connected to any suitable source of current 11. In the vesselis ai standard electro-plating bath 15 of the type customarily used. In fact, Fig. 3

is merely a diagrammatic representation of currentelectroplating practice.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In the process of electroplatingl an aluminum surface, the steps of treating said surface in a dilute aqueous alkaline bath Wit-hout electrolysis, treating said surface as the cathode in an acid dilute aqueous bath under non-oxidizing conditions, and thereafter electrodepositing a coating upon said 'treated surface.

day of January 1927.

CARL L. BEAL.

Q. In the .process of electroplating upon an aluminum surface, the steps of immersing said surface 1n an aqueous alkaline solution to attack the film on said surface,

treating said surface as the cathode in a In-theprocess of electrodepositing al coating upon an aluminum surface, the steps 6.0 of immersng said surface in a hot aqueous f bath having an alkalinity of the orderof magnitude of a 5% solution of trisodium phosphate, dipping the' surface in a dilute Y aqueous acid solution, making said surface u a cathode in an aqueous acid bath, substan-

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