US1869042A - Process of coating aluminum - Google Patents

Description

Patented July 26 y 932 HELMER BENGS'I'ON, OF INDIANAPOLIS, INDIANA, ASSIGNOR, BY MFSNE ASSIGNMENTS,

TO ALUMINUM COLORS INCORPORATED, OE INDIANAPOLIS,

TION 10F INDIANA INDIANA, A CORPORA- PROCESS COATINQ'ALUHINUM NoDrawlng. Application filed June 23, 1930, Serial No. 463,851. Renewed May 10, 1932.

This invention relates toprocesses of coating aluminum; and 1t comprises a method of providing aluminum or aluminum alloys with a. decorative, hard, corrosion-resisting coating layer containing A1 S0 and H 0, said coating being adapted to be dyed, lacquered and otherwise treated, wherein an article of aluminum or aluminum alloy is temporarily made'an anode in rather strong tween 1.5 to 1.7 and the acid advantageously containing a considerable proportion of glycerin or another inhibitor, all as more fully hereinafter set forth and as claimed.

As is well known, ordinary aluminum articles are covered with a hard coherent coating of aluminum oxid formed by the action of air. This coating is self-renewing in the sense that Where it is removed, cracked 01- broken it is, practically at once, renewed by the action of the air. While this coati'n is excellent protection against corrosion an is not unsightly, its existence precludes the ordinary decorative treatments which can be :1 applied to other metals. It is, for example, dilficult to make different finishes by different ways of buffing or polishing; and the coating does not give good anchorage to lacquers and varnishes. For this reason, aluminum articles are seldom decorated in any way. Plating is difficult. v

It is an object of this invention to obviate difiiculties in decorating articles made of aluminum and aluminum alloys by substituting for the ordinary oxid film a special coating produced by anodic treatment of the aluminum in a special way. It has been found that if an article ofaluminum, or of aluminum alloy, is thoroughly cleaned and buffed and is then made an anode in strong sulfuric acid of 1.5 to 1.7 specific gravity there develops a hard coating of substantial thickness. The coating is insoluble in water, contains sulfuric" acid in combination with the alumina in an insoluble form and has an empirical composition corresponding to that of a highly basic hydrated sulfate of alumina. It is minutely pervious but the porosity is sub-microscopic in character and the pores are readily sealed to give a completely prosulfuric acid, the specific gravity being be-.

tective weather-proof layer. This coating is colorless but, as I have found, thecontained material'is in such a reactive form as to take up and readily hold those dyes which are customarily used with mordants. In other words, the alumina compound can be converted into a lake of any desired color or hue.

Decorative on the coating produced as described'may be achieved in a number of other ways. Its thickness is sufficient to permit bulfing and mechanical finishing. And it is found that in spite of the minute character of the pores, it will readily take up oils'and oily materials. This readypermeability by oils is possibly connected with the adsorbent nature of=thematerial of the coating; a view which is supported by the fact that oils once taken up are removed with difliculty by gaso--- line and other oil solvents. Colored oils give various decorative efl'ects.

The strength of the sulfuric acid used in the electrolytic treatment is of great importance. The best results are given by a sulfuric acid of a specific gravity of 1.5 to 1.7; that is an acid containing from 60 to 77 per cent of H SO A 64-65 per centacid is generally useful. Solutions somewhat stronger and weaker than those in this range will operate for my purposes but are open to objection. With higher specific gravities, the conductivity is lessened to an extent making it difiicult to secure efficient current densities while certain lower specific gravities, those between 35 and 60 per cent acid, give too much conductivity and also cause excessive etching and attack on the aluminum. No etching or attack on the aluminum surface is here desired other than that necessary in forming the coating. It has been found highly advantageous, even in using the optimum concentration of sulfuric acid, to add some glycerin to the solution. This decreases etching and attack still further. I use ordinarily about 1 part by volume of, glycerin-to 15 parts by volume of the 1.5-1.7 acid. In the presence of the glycerin the specific gravity can be lowered a little below the range mentioned without too much attack and with the possibility of using higher they cause surface roughening and attack without production of the type of coating here desired. The sulfuric acid should be free of HCl or chlorids, as these cause too much solution loss.

In operation, the apparatus in which the anodic coating is effected is most conveniently made of stoneware or other ceramic or vitreous material. However, containers of wood or metal, lined with materials resistably at 25 ant to sulfuric acid, such as pitch, rubber or sheet lead, may be used. The cathodes may be of any convenient material, but I find cathodes of ordinary chemical lead best adapted. The size and shape of the oathodes, of course, vary according to the work to be done; but for many purposes they may be located on the side walls of the bath and cover about per cent of the side wall area.

With lead lined containers the lead lining may very conveniently serve as a. cathode. The articles to be treated may be hung 1n the bath upon the usual conducting rods. Pointed clamps may be used with sufficient passage of current so that the uncoated area in the finished article is merely an imperceptible spot. The direct current voltage used is ordinarily about 12 volts between terminals. With a voltage of this order no rheostats or regulating devices are necessary. The bath is cooled in any of the ordinary ways. With large baths, cooling coils or fcooling jackets are generally desirable. Circulation of the electrolyte, with agitation thereof, in well known ways, is also desirable. Heat is evolved in the operations. It is best to keep the temperature between 20 to 30, prefer- C. The anode current density,

- with the usual spacing between anodes and cathodes, is ordinarily between 6 and 10 amperes per square foot of surface of the article being treated.

The aluminum articles to be coated should be subjected to the usual preliminary cleaning treatments. Where a lustrous coating is wanted the aluminum should be'butfed before dipping. In any event, grease and other foreign matter should be removed.

For most purposes the time required for the anodic treatment to produce the desired coatingis 10 to 30 minutes. However, a useful layer may be formed in 5 minutes or less and of course an extended time of an hour or more does no harm. Flow of current decreases with the increase in thickness of the coating and after a' normal coating has been formed, prolongation of treatment efiects no material change. Coating is uniformat all of dissolved sulfate of alumina and this can be further reduced by the use of inhibitors. For the most part, the aluminum oxidized in the anodic treatment reappears in the coating. Such aluminum as does dissolve forms aluminum sulfate which after a time crystallizes out.

If the work is not completely clean there is apt to be uneven coloring dueto varying thicknesses of coating. In such an event, it

is best to remove the coating and reprocess the article. For removing the coating a 5 to 10 per cent hydrofluoric acid dip is most satisfactory. On immersion the coating rapidly dissolves, solution taking perhaps 15 seconds. There is then a vigorous evolution of gas as the acid reaches the metal. When this occurs, the metal is removed, washed and is ready for processing.

After treatment the article isremoved, rinsed and dried. The coating is insoluble in Water at the ordinary temperature and is not injured by rinsing. Thorough rinsing to remove contained electrolyte is desirable. The wet coating after rinsing may be dyed by treatment with a solution of a dye; the dye being taken up to form what may be termed a lake. The dye is found to penetratethe coating and give uniformity of color at all points. Exhaustion of the dye bath is usually good- The coating may be buffed and mechanically finished, before or after the application of dye. Alternatively, in lieu of dyeing the coating it may be oiled With a suitable oil such as lanolin or lard oil. Neutral petroleum oils and waxes may also be used. Apparently the oil is taken up in the body of the coating and not merely supen ficially. In bufiing, the coating usually takes up oily matter from the bufiing compound. The dyed coating is usually dried and buffed or otherwise finished. It may be oiled.

The anodic treatment described usually gives a coating of about 0.0005 inch thick. With a somewhat greater time of anodic treatment the thickness of coating may be 0.0015 inch or more; These thicknesses are suflicient for all ordinary purposes, and while somewhat thicker coatings may be secured the result is more attack on the underlying aluminum The coating thickness varies directly with the potential of the bath and, for a time, with sulfuric acid diluted to 1.5 to 1.7 specific gravity.

5. In electro-coating articles of aluminum and aluminum alloys, the process which comprises making such an article an anode in sulfuric acid diluted to 1.5 to 1.7 specific gravity, such acid containing a little glycerin.

6. In electro-coating' articles of aluminum and aluminum alloys, the process which comprises passing current between such an article and another electrode in an electrolytic bath of sulfuric acid dilutedto 1.5 to 1.7 specific gravity.

7. In electro-coatingarticlesof aluminum andaluminum alloys,.the process which comprises anodically coating the said articles in a bath of sulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing, and dyeing the said coating.

8. In electro-coating articles of aluminum and aluminum alloys, the. process which comprises anodically coating the said articles in a bath of sulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing the coating and plugging the pores thereof, whereby the article becomes resistant to corrosion.

9. In electro-coating articles of aluminum and aluminum alloys, the process which comprises anodically coating the said articles in a bath of sulfuric acid diluted to 1.5 to 1.7 specific gravity, rinsing the coating, plugging the pores thereof and finally bufling the said coating. 1

10. In electro-coating articles of aluminum and aluminum alloys, the process which com prises anodically coating the said articles in a bath of sulfuric acid diluted to 1.5 to 1.7 specific gravity and thereafter mechanically finishing the article.

In testimony whereof, I have hereunto affixed my signature. 1 HELMER BENGSTON.

the duration of treatment. Thickness varies somewhat with the strength of acid. In the solution in the acid asaluminum sulfate and after/a time the acid becomes saturated and deposits crystals. To dispose of this crystallized sulfate and keep down the temperature of the bath, it is usually expedient to keep a portion of the bath in cyclic circulation to and through a filter and cooling means and back to the bath. A leadlined pump may be used.

While I have described making the aluminum an anode in a sulfuric acid bath with employment of direct current of the usual type used in plating, an alternating current may be used in lieu of direct current. The rectifier action of aluminum makes this feasible. I have secured good coatings with low voltage alternating current. Even 8 volts have sufiiced. In some instances this applicability of alternating current is advantageous since a small step-down transformer can be used in lieu of the more complex equipment necessary in securing low voltage direct current from ordinary'60 cycle 110 volt alternating current.

Some properties of a coating produced by the anodic treatment are as follows: it is a non-conductor of electricity when dry and particularly when oiled, evincing a good resistance against spark puncture; it is readily reactive with, adsorbs and permanently holds organic dyes; it is resistant to water and wear; it stands bufing and takes a high polish; it withstands heating without lifting or breaking away'from the metal; it withstands punching, stamping and the like to a reasonable degree; it binds paints, oils, enamels, lacquers and the like; it shows no definite crystalline structure under X-ray examination; and it is a coating of reasonable thickness rather than a film. While the coating is, as stated, amorphous or non-crystalline under the X-ray, it has a laminar structure. Next to the metal is an extremely thin, extremely hard, glaze-like layer while there is a much thicker outer layer of considerably less hardness integrally attached thereto. The hardness of this outer layer however is sufficient to permit bufling and polishing etc., as stated. The sub-microscopic porosity extends through both layers and is of a type like that with silica gels, the amount of internal surface being very great. This, together with the transparent nature of the coating, is an explanation of the bright and true colors which can be obtained in dyeing. In one particular coating about 0.00066 inch thick, the porosity represented about 40 per cent of the total film. The coating is resistant to heat, withstanding temperatures up to, say, 300 C.,

without shelling or cracking and without much lossof water. With well rinsed material, the amount of hygroscopic moisture given up at 100 C., is very little. The restof the H 0 is more permanently held. The coating is'tolerably resistant to acids but loses sulfuric acid in alkaline liquids. It is insoluble in cold water but gives up sulfate to boiling water. Two hours boiling with distilled water are however required for completion of the action. .Rinsed, dry material does not corrode in the air and with the pores plugged or filled, corrosion by sea water does not occur. As illustrating the adsorptive power of the coating material, it may be mentioned that on application of lacquers c0ntaining dyes, some of the dye sometimes migrates into the underlying coating.

. Aside from its advantageous mechanical properties, the present coating owes most of its value to its highly developed adsorptive power; a power which enables it .to take up i and hold oils, coloring matters, etc. against withdrawal. I attribute this high adsorptive power in part to the large amount of fixedly contained SO or basic aluminum sulfate as the case may be. This basic sulfate is in a form resisting action by cold water and it is extremely active as an adsorbent. In the article of the present invention, the fixedly held S0 may range between, say, 16 percent and 23-24; per cent of the coating. Using weaker sulfuric acid than 1.5-1.7 specific gravity, it is not possible to fix as much S0 and the adsorptive properties are not as good.

The present application contains certain matter in common with prior and copending application 368,742 filed J une 5, 1929, and is filed as a substitute for and continuationin part'of said application. Aluminum coated with the described coating produced by the present process is not specifically claimed herein, it forming the subject matter of another'and copending a lication, Serial No. 460,543, filed June 11, 1930.

What I claim is;

1. In electro-coating articles of aluminu and aluminum alloys, the process which comprises making such an article an anode in sulfuric acid diluted to about 64-65 per cent 2. In electro-coating articles of aluminum and aluminum alloys, the process which comprises making such an article an anode in sulfuric acid diluted to about 64-65 per cent H 80 and containing a little glycerin.

3. In electro-coating articles of aluminumand aluminum alloys, the process which com-- prises making such an article an anode in sulfuric acid diluted to about 64-65 per cent H 80, and containing an inhibitor.

4. In electro-coating articles of aluminum and aluminum alloys, the process which comprises making such an article an anode in'