US1965682A

US1965682A - Coating aluminum

Info

Publication number: US1965682A
Application number: US589056A
Authority: US
Inventors: Harold K Work
Original assignee: ALUMINUM COLORS Inc
Current assignee: ALUMINUM COLORS Inc
Priority date: 1932-01-26
Filing date: 1932-01-26
Publication date: 1934-07-10
Anticipated expiration: 1951-07-10
Also published as: DE616812C; DE629629C; GB409679A; FR761876A

Description

Patented July 10, 1934 PATENT OFFICE COATING ALUMINUM Harold K. Work, Oakmont, Pa., assignor, by

mesne assignments, to Aluminum Colors, Incorporated, Indianapolis, Ind., a corporation of Delaware No Drawing. Application January 26, 1932,

' Serial No. 589,056

8 Claims.

' This invention relates to oxide-coated aluminum by which term is'herein defined aluminum and aluminum base alloys coated with a hard adherent coating of appreciable thickness composed in substantial part of aluminum oxide.

The so-called processes of oxide-coating aluminum, which consist essentially in providing aluminum with a hard adherent coating composed in substantial part of aluminum oxide, have become a matter of considerable commercial import. Among such processes, the one finding widest application is based upon the use of sulfuric acid solutions as the coat-forming electrolyte. The process comprises making the aluminum an anode in an electrolytic cell the electrolyte of which is a solution containing up to about per cent of sulfuric acid by weight. About 5 to 40 volts are impressed upon the cell at temperatures usually below about 40 centigrade. Under the combined action of the acid and the current, an oxide coating, the specific characteristics of which depend in substantial part on the concentration of the electrolyte, is formed on the aluminum. While such processes have been considered to be generally satisfactory, they have certain inherent disadvantages which are not easily overcome.

The general object of this invention is the improvement of the above methods by the provision of a modified sulfuric acid solution electrolyte which is capable of producing coatings of improved properties and which is, moreover, excellently adapted to commercial operation. Further and similar objects will appear in the following description of my invention.

I have discovered that the action of sulfuric acid solution electrolytes in the building up of oxide coatings on aluminum may be considerably modified and improved in operation when there is mixed with such solutions a dib'asic organic acid. When an electrolyte containing a mixture of sulfuric acid and dibasic organic ac d is used in connection with the processes of oxide coating above described, the coatings obtained havethe favorable properties of coatings produced in sulfuric acid solution electrolytes but many of the disadvantageous properties of such coatings are rectified. Moreover, the electrolyte is more amenable to commercial operation than is sulfuric acid electrolyte. An example of a property of the oxide coating which is favorably influenced by the use of a sulfuric acid-dibasic organic acid electrolyte in the formation of the coating is the abrasion resistance. One of the common reasons for oxide-coating aluminum is to produce a product having a considerably greater resistance to abrasion than do plain aluminum surfaces. In operation with sulfuric acid electrolytes, it is requisite that the solutions be maintained at temperatures below about 30 centigrade lest the coatings tend to become soft or powdery and the abrasion resistance fall sharply. Therefore, when coating large surfaces, where the power input is substantial, artificial cooling of the electrolyte is necessary if the electrolyte temperature is to be kept low enough to give coatings of satisfactorily high abrasion resistance. Where cold tap water is not available, artificial refrigeration may have to be installed and the attendant expense adds substantially to the cost of operation. The matter of refrigeration has been a considerable commercial drawback in the coating of aluminum in sulfuric acid electrolytes. However, when, in accordance with my invention, an electrolyte of a mixture of sulfuric and dibasic organic acids is used, the necessity for the use of refrigeration is substantially eliminated because not only do the sulfuric acid-dibasic' organic acid electrolytes produce oxide coatings of higher abrasion resistance than does the sulfuric acid electrolyte, but my new electrolyte, likewise, will produce coatings of high abrasion resistance at comparatively high temperatures.

A standard test for the abrasion resistance of such coatings is carried out by rotating a plate of the oxide-coated aluminum against an abrasive wheel under a constant pressure. The number of revolutions of the plate required to penetrate the coating is taken as a measure of the abrasion resistance. Applyingthis test to samples of oxide-coated aluminum which have been prepared bymaking the aluminum an anode in an electrolytic cell the electrolyte of which was composed of a solution containing 25 per cent by weight of sulfuric acid, and to samples prepared by making the aluminum the anode in an electrolytic cell the electrolyte of which was composed of a solution containing3 per cent by weight of sulfuric acid and 3 per cent by weight of oxalic acid, the following results were obtained. Operating the cells at a temperature of about 25 centigrade, the abrasion resistance of the oxidecoated aluminum which was prepared in the sulfuric acid electrolyte was 340 while the abrasion resistance of the oxide-coated aluminum prepared in the sulfuric acid-dibasic organic acid electrolyte was 490. Operating the cell at a temperature of about 37 centigrade, the abrasion resistance of the oxide-coated aluminum produced in the sulfuric acid electrolye was only 50 while the abrasion resistance of the oxidecoated aluminum produced in the sulfuric acid-. dibasic organic acid electrolyte was 580. These 1 figures are but exemplary of the effect produced by the use of electrolytes of various sulfuric acid concentrations and by the use of electrolytes containing various amounts of sulfuric acid and dibasic organic acid.

The oxide-coatings produced by the use of sulfuric acid-dibasic organic acid electrolytes in accordance with my invention retain the desirable characteristics of the oxide coatings produced in sulfuric acid electrolyte. white; they are hard; they are composed in substantial part of aluminum oxide;

providing it with a protective coating of great practical value and of considerable beauty.

It is of course necessary, in making the sulfuric acid-dibasic acid electrolytes, to use a di basic organic acid which, under the conditions named, is stable and is soluble. Among such acids '1 have found that certain are to be preferred and that, although the advantages of my invention may be realized with the other dibasic organic acids, and often to a very useful end, oxalic acid (112C204), malic acid (HsC4O5), and

' malonic acid (C3H4O4), when mixed with sulfuric acid to form the electrolyte, produce coatings in which appear to be realized the greatest benefits of my invention. Among other dibasic acids which are useful are maleic acid (114C404), succinic acid (C4H6O4), and so forth. The dibasic organic acid may be mixed with sulfuric acid in varying proportions and excellent oxide coatings produced, by the use thereof, on aluminum. Concentrations of sulfuric acid as high as 70 per cent by weight may be used, and'in the practiceof my invention I have found that commercial electrolytes containing from about 0.5 to 15 per cent by weight of sulfuric acid and about 0.5 per cent by weight up to about the saturation limit (usually about 9 per cent by weight) of oxalic acid are particularly advantageous in large scale operation. For instance, an electrolyte containing 0.5 per cent by weight of sulfuric acid and 3 per cent by weight of oxalic acid has given very good results as has likewise an electrolyte containing 9 per cent by weight of sulfuric acid and 5 per cent by weight of oxalic acid. Also an electrolye containing 3 per cent by weight of sulfuric acid and 3 per cent by weight of malic acid has been found very suitable for operation, as

.has also a solution containing 3 per cent by weight of sulfuric acid and 3 per cent by weight of malonic acid.

In the practice of my invention the sulfuric acid-dibasic organic acid electrolyte is placed in an electrolytic cell the cathode of which may be any suitable material such as aluminum or lead and the anode of which is the aluminum or aluminum alloy article which is to be coated. A voltage, which is usually about 10 to 30 volts but which may be as high as 50 volts, is impressed upon the cell and the operation is continued until a coating of the desired thickness is formed.

desirable and since the thickness is a direct function of the time of treatment, it is a desirable The coatings are and they strongly adhere to the surface of the aluminum,-.

property of my electrolyte that thick coatings,

hard of surface, may be built up on aluminum.

Although I have referred to electrolytes made of solutions of sulfuric and dibasic organic acids,

'I have intended by this designation to include acid solutions of the salts of sulfuric acid and dibasic organic acid when those salts are such that their combined presence in solution does not precipitate one or another of the acid radicals and when the metals from which such salts are formed do not tend to plate out of solution upon the cathode being treated. Such, for instance, are the salts of the alkali metals, including ammonium, and acid solutions of these salts are within the purview of my invention.

' When, herein and in the appended claims, it is stated that the aluminumis made the anode, it is to be understood that the use of either direct or alternating current is contemplated and that solong as the aluminum is the anode with direct current or an electrode with alternating current, the anodic oxide coating will be produced.

I claim:

l. The process of producing oxide-coated aluminum which comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains in solution 0.5 to '70 per cent sulfuric acid and a dibasic organic acid of the group consisting of oxalic, malic, ma-

lonic,'maleie and succinic acids in the range of -0.5 per cent to the saturation limit.

2.'The process of producing oxide-coated aluminum which'comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains in solution 0.5 to 15 per cent sulfuric acid and 0.5 to 9.0 per cent oxalic acid.

5. A process of producing oxide-coated alumi- 1 num articles which comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains in solution about 0.5 per cent sulfuric acid and about 3 per cent oxalic acid.

6. A process of producing oxide-coated aluminum which comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains in solution about 3 per cent of sulfuric acid and about 3 per cent of malonic acid.

7. The process of producing oxide-coated aluminum which comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains in solution about 3 per cent sulfuric acid and about 3 per cent malic acid.

8. The process of producingoxide-coated aluminum which comprises making the article to be coated an anode in an electrolytic cell, the electrolyte of which contains insolution 0.5 to

70 per cent sulfuric acid and oxalic acid in the rarge of 0.5 per cent to the saturation limit.

HAROLD K. WORK.

- -.CERTIFIGATE 0F GORREG.TION.

Patent No. l,965, 682. July. lfl,1.}1934.

' HAROLD K. ,iwonlt.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction is follows: Page 1, for the patent number "I, 695, 682" in the upper right-hand corner read 1,965,682: and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

e. Signed and sealedthis 25th day or September, A. I); 1934,

Leslie Frazer (Seal) Acting Commissioner of Patents-