US2833679A - Method of coating process aluminum and its alloys - Google Patents

Description

METHOD OF COATING PROCESS ALUMINUM. AND ITS ALLOYS Morris A. Steinberg, University Heights, Ohio, assignor to Horizons Incorporated, Princeton, N. J., a corporation, of New Jersey No Drawing. Application April 15, 1955 Serial No. 501,709

3 Claims. (Cl. 148-6.11)

This invention relates to the coating of aluminum and alloys which are composed principally of aluminum. More particularly, it relates to a process in which the aluminum is coated with metals which are below aluminum in the electromotive series.

In accordance with my invention, objects formed of aluminum or aluminum alloys are coated either by simple immersion in a reactive composition or by such immersion followed by dipping into a molten bath of the metal or alloy which is to serve as the coating.

The production of adherent metallic coatings on aluminum and alloys of aluminum has been extensively investigated. Workers in this art have long recognized that the problem is complicated by the superficial oxide film that invariably is formed on any freshly exposed aluminum surface by reaction with the ordinary atmosphere. Failure to effect the removal of all of this oxide film leads to coatings which flake off because of poor adherence to the oxide. Such coatings, therefore, fail to provide the desired benefit for which they were applied, for instance in soldering or joining the aluminum. I

It has been generally proposed to remove the superficial oxide film either by mechanical means or by chemical action as a condition precedent to the application of any coating metal. One commonly adopted expedient has been to fioat a molten flux layer upon a molten bath of a coating metal and to dip the aluminum or aluminum alloy object into the flux and thence, after the surface has been chemically cleaned, into the molten metal bath.

I have now discovered that the above procedure may be greatly simplified and that satisfactory, tightly adherent coatings may be produced on the aluminum objects without the necessity of a molten metal coating bath. In

accordance with one procedure, I provide a molten flux bath in which one of the essential constituents comprises a combination of chlorides which will dissolve the aluminum oxide film, and one or more metal chlorides of a metal with which the freshly exposed aluminum base metal will react, and by simple displacement will decompose the metal chloride and cause the metal to deposit thereon as a metallic coating.

In another embodiment of my invention I provide flux consisting of the above ingredients and disposed over a molten bath of the metal to be deposited as a coating on the aluminum or aluminum alloy surface. In this modification of my invention I have found it preferable to include in the flux the chloride of the metal or metals forming the molten bath upon which my reaction flux floats.

One object of my invention is to provide a composition which will effectively remove the oxide layer from aluminum objects at temperatures below 300 C. and at the same time will effectively deposit a-metal on the freshly cleaned aluminum or aluminum alloy surface.

Another object of my invention is to providea simple and inexpensive method for coating aluminum with a uniform, tightly adherent metallic coating. A further United States PatentO 2.53am Patented May 6, 1958 2 object of my invention is to provide a composition and method which are particularly effective for removing the oxide film from the surface of aluminum or aluminum below 300 C. and a second component which comprises the chloride of at least one metal below aluminum in the electromotive series is prepared in the following manner.

The first component is prepared by mixing aluminum I chloride and one or more alkali metal chlorides of the group consisting of lithium chloride, sodium chloride, and potassium chloride and heating the mixture gently to a temperature below 300 C. at which the constituents form a clear melt. The melt may be solidified and comminuted and stored for future use or it may have added to it the second essential constituent of my. composition.

Chlorides which have been found suitable as the second constituent are the following:

AgCl HgCl AuCl InCl AuCl MnCl CdCl NiCl CoCl v PbCl 8%? fi s CuCl SHCIZ FeCl TlCl GaCl ZHCI When it is desired to coat an aluminum object by the first procedure, simple immersion in the reaction flux, a mixture of the aluminum chloride-alkali metal chloride component with the coating metal chloride or chlorides is melted in a suitable vessel and stirred to obtain a homogeneous mixture. Since many of these mixtures will melt at temperatures as low as 200 C., inexpensive glass vessels or glass-lined vessels may be employedwith substantial economic advantages. Once the composition has been melted and homogenized,it is merely necessary to immerse the aluminum or aluminum alloy object into the flux and to move it gently back and forth therein'in order to insure an intimatecontact between the'fiux and all surface portions of the object to be coated.

The relative proportions of aluminum chloride, alkali metal chloride and coating metal chlorideor'chlorides are selected so that the homogenized composition will have a relatively low melting point preferably below about 200 C. before it is used as well as after it is used. To produce a composition with the desired low melting point, I have found that the proportion of aluminum chloride in the aluminum chloride-alkali metal chloride component of the mixture should be between 52 mol percent and 82 mol percent and the proportion of alkali metal chloride in the aluminum chloride-alkali metal chloride component should be between 48 mol percent and 18 mol percent; and the amount of coating metal chloride should constitute less than about 50% by weight of the mixture and should constitute at least a sufficientamount to de-.

ride-alkali metal chloride component that the'composition tends to lose to a significant extent the ability to dissolve the aluminum oxide film and thatwith suchamounts of aluminum chloride the composition tends to become unstable and to lose aluminumchloride by volatil-ization Once the oxide film has been removed from the aluminum or aluminum alloy object to be coated, the exposed metal reacts with the coating metal chloride or chlorides present in the molten flux as follows: i

where M 01, is a metal chloride chosen from the above mentioned metal chlorides and x and y are integers.

The following examples will further illustrate the preferred manner of practicing this aspect of my invention:

Eitample I Example II A clear melt was prepared by heating a charge comprising 60 parts by weight of aluminum chloride covered by 20 parts by weight of sodium chloride and 20 parts by weight of stannous chloride to a temperature of about 300 C. A piece of aluminum was immersed in the melt for 2 minutes after which itvwas withdrawn. A bright,

, uniform tin platewas obtained on thealuminum free from spots or streaksr I The second aspect of my invention involves the use of a molten metal bath disposed beneath a supernatant molten flux layer. The procedure is substantially similar to that previously outlined for preparing the flux and the fol lowing examples will serve to illustrate the preferred practice of this aspect'of my invention:

1 Eatample III A flux composition was prepared by melting 60 parts by weight of aluminum chloride, 20 parts by weight of sodium chloride and 20 parts by weight of stannous chloride in a glass beaker covered with a watch glass by heating toabout 300 6;, at whichtemperaturea clear melt formed -Into this melt there was added about 2000 grams of metallietin. whichfon continued heating, was readily melted and form'e'dga pool beneath the molten flux. A piece of aluminum was immersed in the flux and then droppedintothe moltentin, Upon withdrawal from the bath it was found to be covered completely with a uniform, tightlyadherent layer of tin, free from streaks or spots. 1 t t Example I V t A flux composition comprised of 60 parts by weight of aluminum chloride, 20, parts by weight of sodium chloride, 10 parts byiweight ofc'admium chloride and 1 0 parts by weight of zinc'chloridelwas charged into a glass beaker and melted as in Example, IIIL Into. this melt there was added about 2500 grams of a cadmiumzinc; alloy which was readily meltedtherein. ,A piece of aluminum was immersed first in the flux and thenin the molten metal and when withdrawn was found to be covered with a layer of a cadmium-zinc alloy This combination of baseand coating is particularly advantageous since the. cadmium zinc does not form an electrolytic couple with the alumi num base and henceis particularly resistant; tocertain types of corrosion. i i Y discovered am thod rd po ing metall c "coatings on i s o a umi m or al oys. of a .uminu,ni e the y From the'above examples it will be apparent-that I have, i

simple immersion in a reactive flux containing one or more chlorides which react with the aluminum and a combination of ingredients which dissolve the aluminum oxide film from the object to be coated or by immersion in such a flux followed by immersion in a molten metal or metal alloy bath.

While there are enumerated a large number of chlo-' rides which have been found to be satisfactory for the production of coatings, it will be understood that for reasons of economy, the chlorides of lead, tin, cadmium and zinc, and combinations of these chlorides are preferred in the practice of my invention.

Furthermore, although the flux compositions described will generally be molten at temperatures below about 300 0., they may be employed in cooperation with molten metal or alloy compositions which melt at appreciably higher temperatures. Thus, the flux bath of Example 1 has been employed as the supernatant flux on a pool of molten zinc (M. P. 420 C.) without excessive fuming or breakdown of the flux.

Another factor which it is necessary to consider in the practice of my method is the choice of appropriate reactive chlorides to be added to the aluminum chloride-alkali metalchloride basic flux composition. Obviously, chlorides shouldbe chosen which do not react with the underlying pool of molten metal, since this would lead to waste of both chloride and molten bath material. For this reason,

. it has been found particularly useful to add to the flux,

the chloride of the coating metal over which the fiux is floated. Of course when the flux is employed without any molten pool, any of the chlorides above listed may be used.

By filming first a coating of a metal on the aluminum substrate by decomposition of a chloride constituent of the fiuxand then dipping an article into a diiierent metal, it is possible to provide coatings of improved adherence to the base metal, particularly where the intermediate metal is one which bonds more readily to the substrate than the outermost metal.

The term aluminum in the following claims is intended to cover the metal itself as well as alloys which are composed principally of aluminum, particularly alloys with upwards of of aluminum.

I claim:

1. The method of coating aluminum and aluminumbase alloys with a coating metal which is more electronegative than aluminum, which comprises establishing a molten body of the coating metal, forming on at least a portion of the surface of said molten body of coating metal a fused, homogeneous salt composition comprising a major amount of an aluminum chloride-alkali metal chloride constituent consisting essentially of between about 52 mol percent and 82 mol percent of aluminum chloride and between about 18 mol percent and 48 mol percent of at least one alkali metal chloride from the group consisting of sodium chloride, potassium chloride and lithium chloride, and between about 1 percent and 25 percent by weight of the aluminum chloride-alkali metal chloride constituent of a chloride ofat least one metal from the group consisting of zinc chloride, stannous chloride, lead chloride and cadmium chloride, passing the article to be coated downwardly through the supernatant fused salt composition and into the underlying molten body of coating metal, whereby the aluminum oxide film on the article is removed and a coating of the metal of the metal chloride of said group of metal chlorides is formed on the article, as the article passes through the supernatant fused salt composition and before it is immersed into the molten body of the coating metal, and thereafter withdrawing the article from the molten body of said coating metal with a coating-of the coating metal thereon.

2. The method of coating aluminum and aluminumbase alloys with a coating metal which is more electro-- negative than aluminum, which comprises establishing a molten body of the coating metal, forming on at least a portion of the surface of said molten body of coating metal a fused homogeneous salt composition consisting essentially of between about 52 mol percent and 82 mol percent of aluminum chloride and between about 18 mol percent and 48 mol percent of at least one alkali metal chloride from the group consisting of sodium chloride, potassium chloride and lithium chloride, and between about 1 percent and 25 percent by weight of a chloride of at least one metal more electIonegative than aluminum, passing the article to be coated downwardly through the supernatant fused salt composition and into the underlying molten body of coating metal, whereby the aluminum oxide film on the article is removed therefrom and a coating of the metal of the metal chloride more electronegative than aluminum is formed on the article as the article passes through the supernatant fused salt composition and before it is immersed into the molten body of the coating metal, and thereafter withdrawing the article from the molten body of the coating metal with a coating of the coating metal thereon.

3. The method of coating aluminum and aluminumbase alloys with a coating metal which is more electronegative than aluminum, which comprises establishing a molten body of a fused, homogeneous salt composition, consisting essentially of a major amount of aluminum chloride and a minor amount of at least one alkali metal said coating metal, immersing the aluminum article in the fused salt bath, whereby the oxide film on the articleis removed by the molten salt and a coating of the metal of the metal chloride more electronegative than aluminum is formed on the article, thereafter further coating the freshly coated article by immersing it into the body of molten metal, and withdrawing the article from the body of molten metal with a final coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,293,823 Luckey Feb. 11, 1919 1,534,105 Gann Apr. 21, 1925 1,550,280 Post .Aug. 18, 1925 2,299,166 Miller Oct. 20, 1942 2,330,904 Miller Oct. 5, 1943 2,443,169 Shanklin June 8, 1948 2,480,498 Miller Aug. 30, 1949 2,481,053 Wassermann Sept. 6, 1949 2,596,466 Bowden May 13, 1952 2,674,790 Edson et a1. Apr. 13, 1954 2,723,929

Wagner et a1 Nov. 15, 1955

Claims (1)

1. THE METHOD OF COATING ALUMINUM AND ALUMINUMBASE, ALLOYS WITH A COATING METAL WHICH IS MORE ELECTRONEGATIVE THAN ALUMINUM, WHICH COMPRISES ESTABLISHING A MOLTEN BODY OF THE COATING METAL, FORMING ON AT LEAST A PORTION OF THE SURFACE OF SAID MOLTEN BODY OF COATING METAL A FUSED, HOMOGENEOUS SALT COMPOSITION COMPRISING A MAJOR AMOUNT OF AN ALUMINUM CHLORIDE-ALKALI METAL CHLORIDE CONSTITUENT CONSISTING ESSENTIALLY OF BETWEEN ABOUT 52 MOL PERCENT AND 82 MOL PERCENT OF ALUMINUM CHLORIDE AND BETWEEN ABOUT 18 MOL PERCENT AND 48 MOL PERCENT OF AT LEAST ONE ALKALI METAL CHLORIDE FROM THE GROUP CONSISTING OF SODIUM CHLORIDE, POTASSIUM CHLORIDE AND LITHIUM CHLORIDE, AND BETWEEN ABOUT 1 PERCENT AND 25 PERCENT BY WEIGHT OF THE ALUMINUM CHLORIDE-ALKALI METAL CHLORIDE CONSTITUENT OF A CHLORIDE OF AT LEAST ONE METAL FROM THE GROUP CONSISTING OF ZINC CHLORIDE, STANNOUS CHLORIDE, LEAD CHLORIDE AND CADMIUM CHLORIDE, PASSING THE ARTICLE TO BE COATED DOWNWARDLY THROUGH THE SUPERNATANT FUSED SALT COMPOSITION AND INTO THE UNDERLYING MOLTEN BODY OF COATING METAL, WHEREBY THE ALUMINUM OXIDE FILM ON THE ARTICLE IS REMOVED AND A COATING OF THE METAL OF THE METAL CHLORIDE OF SAID GROUP OF METAL CHLORIDES IS FORMED ON THE ARTICLE, AS THE ARTICLE PASSES THROUGH THE SUPERNATANT FUSED SALT COMPOSITION AND BEFORE IT IS IMMERSED INTO THE MOLTEN BODY OF THE COATING METAL, AND THEREAFTER WITHDRAWING THE ARTICLE FROM THE MOLTEN BODY OF SAID COATING METAL WITH A COATING OF THE COATING METAL THEREON.