US2872346A

US2872346A - Metal plating bath

Info

Publication number: US2872346A
Application number: US585951A
Authority: US
Inventors: Miller Adolph
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-05-21
Filing date: 1956-05-21
Publication date: 1959-02-03
Anticipated expiration: 1976-02-03

Description

United States Patent METAL PLATLNG BATH Adolph Miller, Great Neck, N. Y.

No Drawing. Application May 21, 1956 7 Serial No. 585,951

14 Claims. (Cl. 117-65) The present invention relates broadly to a metal plating bath and more particularly to a method for immersion plating copper on aluminum and aluminum-base alloys having a propensity to polarize under ordinaryplating conditions. Theinvention also relates to a method for preparing the surface of aluminum-base foil, sheet, Wire and other products by immersion plating in order to render the base metal surface wettable by commercial grades of solder thereby improving the bonding between the base metal and the solder applied to it.

While in recent years many uses have been developed for aluminum as a packaging material, as a material of construction, and in the making of components for use in the electrical field, this metal has certain inherent disadvantages which militate against its use in certain applications which call for the use of low melting solder as a bonding agent in producing bonded joints or connections between aluminum and another metal, or between aluminum and aluminum Aluminum oxidizes easily at ordinary room temperatures, forming a thin, tenacious, substantially. invisible oxide coating which is difiicult to remove. The presence of this oxide coating inhibits the solderability of aluminum by preventing the underlying base metal from being wetted by well known commercial solders. Hence other forms of jointing are resorted to, such as crimping in the case of aluminum foil, riveting in the case ofthin sections, etc.

Attempts have been made to prepare aluminum surfaces for soldering by producing thereon a thin metal plate (e. g. nickel, copper, etc.) easily wettable by solder. However, these attempts have not been too successful as it was difficult to obtain a good adherent coating. Generally, the methods tried were electrolytic. Alumi-' num had a tendency to polarize and prevent metals from depositing on it. Even if metal did deposit on the surface, it was generally spongy and not too adherent. This was particularly true when attempts were made to pro duce a lead plate or a tin plate from baths containing plumbates and stannates, respectively.

Thus, while many attempts were made to produce an adherent metal plate on the surface of aluminum, none was too satisfactory when carried into practice of a commercial scale.

I have now discovered. a method whereby aluminum and alloys based thereon can be coated with an adherent plate of metal.

It is the object of the present invention to provide a novel metal plating bath.

Another object is to provide a plating bath capable of producing a substantially adherent metal plate on aluminum and its alloys.

A further object is to provide a method whereby aluminum can be soldered.

Other objects will clearly appear from the following description.

Broadly speaking, the present invention is carried out by using a novel immersion plating bath containing a substantially alkaline, water-soluble organic'amine. It

2,872,345 Patented Feb. 3, 1959 has been discovered that such solutions containing a platable metal in the form of an amine complex produce relatively adherent plates on aluminum without resorting to electrolysis, that is by chemical dipping, particularlyat high pHs, for example at a pH at least about 9 and higher. The baths may be employed electrolyticaly in situations where thick metal plates are desirable. However, metal plates formed by chemical dipping (i. e. immersion), while very thin, have been found quite suitable in conferring solderability to aluminum and its alloys. 7

Metal plating baths containing organic amine complexes have been tried for electroplating copper on iron and zinc articles. The reason for using such baths was that generally acid copper baths had the disadvantage of coating iron and zinc by immersion before electroplating occurred, the coating being substantially spongy and nonadherent, thereby interfering with the obtaining of an adherent electroplate. Amine solutions on the other hand prevented immersion plating from occurring, thus assuring adherent electroplates of copper on iron and ZlIlC.

Contrary to the foregoing, I have discovered that amine plating solutions of pH of at least about 9 and above have particular utility in the immersion plating of such metals as aluminum and its alloys.

Organic amines which have been found particularly satisfactory in producing metal plating baths of the invention include alkylene amines, alkyl amines and alkylol amines such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, monoethanolamine, diethanolamine and triethanolamine as well asn-butylamine. Where copper is employed as the plating metal, it is possible to plate it-on aluminum from the foregoing baths over a broad range of composition. The deposits obtained are bright, smooth, substantially :adherent and wettable by commercial grade solders.

Tests have indicated that metal organic amines behave differently from metal complexes based on ammonia. For example, attempts made to produce a metal plate on aluminum from baths based on ammonia complexes of either nickel, copper, zinc or cadmium were not successful, regardless whether the plating was attempted by chemical dipping or by electrolysis. One explanation is that aluminum has a greater propensity to polarize in the ammonia bath than in a bath comprising an organic amine. I n

By way of example, in formulating a copper plating bath in accordance with the invention, either copper sulphate or copper acetate may be employed as the starting material. Bath temperatures used for plating may depend on the bath composition. ried out chemically at room temperature or at an elevated temperature. Room temperature for the purposes of this invention may be in the neighborhood of about 23 to 27 C. while elevated temperature is construed to be in the neighborhood of about 70 to C. Generally, elevated temperatures have the effect of increasing the speed of deposition.

vAs illustrative of the various compositions which were produced in accordance with the invention, the following examples based on ethylene diamine are given:

The pH was over about 11.

1 70% in water.

Thus, plating may be car-- 3 This solution produced an effective copper plate on aluminum by chemical dipping at room temperature. After annealing in a gas flame, the copper-plated aluminum formeda strong joint with a soft solder.

Example 2 The following bath composition enabled the formation of a copper plate by chemical dipping at both room and elevated temperatures:

Copper sulphate grns 10 Ethylene diamine gms 10 Water grns 90 Percent copper by weight 2.3 Percent ethylene diamine by weight 6.4

The pH was over about 10.5.

1 70% in water.

Example 3 Copper sulpha gms' 10 Ethylene diamine gms 1 5 Water Q ems 90 Percent copper by Weight 2.4 Percent ethylene diamine by weight 3.9

The pH was over about 9.5. (b) Copper sulphate gms-.. 5 Ethylene diamine gms 1 Water ems 85 Percent copper by weight 1.3 Percent ethylene diamine by weight 8.3

The pH was over about 10.5.

Copper sulphate ...---..-..gms..- 5 Ethylene diamine g 1 20 Wat urns" 75 Percent copper by weight. 1.3 Percent ethylene diamine by weight 16.6

The pH was over about 11. (d) Copper sulphate 1 Ethylene diamine grns 1 1 Water arms" 90 Percent copper by weight 0.28 Percent ethylene diamine by Weight 0.9

The pH was over about 9.5.

Copper sulphate gms 2 Ethylene diamine gms 1 1 Sodium hydroxide gms 2 Water gms 90 Percent copper by weight 0.54 Percent ethylene diamine by weight 0.88 Percent sodium hydroxide 0.21

The pH was over about 10.

(f) Cupric acetate grns 1 Ethylene diamine gms 1.2 Water gms 90 Percent copper by weight 0.28 Percent ethylene diamine by Weight 1.1

The pH was over about 9.5.

1 83% in water.

1 10% in water.

All of the above baths yielded good copper deposits by chemical dipping at elevated temperatures. Deposits were also obtained electrolytically. Not much difierence could be discerned between the two types of plates other than the fact that the electrolytic method enabled faster deposition and the formation of thicker plates.

Good plating results were also-obtained with diethylene triarnine solution as a solution base.

4 Example 4 Copper sulphate gms 10 Diethylene triamine "gins-.. 18 Water arm Percent copper by weight 2.5 Percent diethylene triamine by weight 17.5

The pH was over about 11.

Copper sulph 2ms 1 Diethylene triamine ems 1 Water arm 90 Percent copper by weight 0.28 Percent diethylene triamine by weight 1.09

The pH was over about 9.5.

- Copper sulpha ems 1 Diethylene triarnine gms ()5 Water gms 90 Sodium hydroxide gms 1 Percent copper by weight 0.28 Percent diethylene triamine by weight 1.09 Percent sodium hydroxide by weight 0.11

The pH was over about :10.

1 10% in water.

v electroplating.

Similar results were obtained with tetraethylene pentarnine compositions as follows:

Example 5 Copper sulphate.. gms 7 Triethylene tetramine gms 16 Water ems Percent copper by weight 1.74 Percent triethylene tetramine by Weight 15.5

The pH was over about 10.5.

Copper acetate gms 5 Triethylene tetramine gms 12 Water gms Percent copper by weight 1.64 Percent triethylene tetramine by weight 11.2

The pH was over about 10.

Copper sulphate gms- 1 Triethylene tetramine gms 2 Water gIn 90 Percent copper by weight 0.27 Percent triethylene tetramine by weight 2.15

The pH was over about 9.5.

Other organic amine compositions with which comparable plating resultshave been achieved are given in the following examples:

Example-6 Copper sulphate grams 1 Tetraethylene pentaiuine do 5 Water do 90 Percent copper by weight 0.26 Percent tetraethylene pentamiue 5.2

The pH was over about 9.5.

Copper sulphate grams 7 Tetraethylene pentamine -do 22 Water d 80 Percent copper by weight 1.64 Percent tetraethylene pentamine 20.1

The pH was over about 10.5.

.Cupric acetate grams 3 Tetraethylene pentamine do 14 Water do 85 Percent copper by Weight 1.0 Percent tetraethylene pentamine 13.7

The pH was over about 10.5.

Example 7.

Copper sulphate grarns 1 Tetraethylene pentamine do 2 Diethylene triamine do 0.2 Water do" 90 Percent copper by weight 0.27 Percent tetraethylene pentamine 2.1 Percent diethylene triamine by weight 0.21

The pH was over about 9.5.

It was found that in going from ethylene diamine to its higher'homologs, the ratio of amine to the weight of copper in the bath increases for the maintenance of good deposition. When the amount of amine present in a solution of a higher homolog is not sufficient to effect adequate plating, the addition of a strong alkali, e. g. sodium hydroxide, potassium hydroxide or choline, or the addition of one of the lower homologs has been found to overcome the deficiency. Brightness of the deposit and its adhesion are not affected materially by changing the amine complexing agent used 'in the foregoing series. Maintaining the plating solution substantially alkaline is important if consistent results are to be obtained.

Tests have indicated that aliphatic amines may also be employed as complexing agents provided they are soluble in producing the novel bath of the invention, although these amines are not as efficient as the series based on ethylene diamine. A fair copper deposit was obtained on aluminum with butylamine. When producing a plating solution from copper sulphate by the addition of.

butylamine, a precipitate forms. However, by using excess butylamine as in the following example, a copper deposit is assured.

Example 8 Copper sulphate "grams" 1 n-Butylamine do 12 Water do 90 Percent copper by weight 0.25 Percent n-butylamine by Weight 11.6

The pH was over about 9.5.

Hydroxylated and oxygenated amines were found to give good copper deposits provided a small amount of sodium hydroxide is added. Typical compositions are given as follows:

Percent sodium hydroxide by weight 0.26

The pH was over about 10.

The pH was over about 10.5.

1 10% in water.

The aluminum specimens used in testing the adherenc of the copper plate were as follows:

0.002 inch thick strip, full hard, type 28 0.002 inch thick strip, dead soft, type 2S 0.025 inch thick strip, aluminum alloy, type 245T 0.0625 inch diameter aluminum wire, type 280 The copper plated aluminum was tested for soldering by means of an electric soldering iron to solder lamp cord and #16 solid copper wire to it by using Kesters acid core and resin core solders.

Adhesion of the plate to the base metal was determined qualitatively by pulling the soldered wire and determining qualitatively by comparison tests the force needed to break the soldered joint.

Experiments in precleaning the surface of the aluminum using'carbon tetrachloride for washing off the oils, and/ or sodium hydroxide solutions and/or sulphuric acid solutions with a final water wash indicated that precleaning of the aluminum surface reduced time of deposition by causing aluminum to be wetted easily by the bath. However, the bath itself (in almost all cases) was able to remove the oils normally present on cold rolled unannealed aluminum foil. If excessive scale or dirt is present on the surface of the work, some precleaning of the surface may be required which involves degreasing and dipping in an alkali or acid etching bath to insure a clean surface exposed to the bath.

When the foregoing precautions are taken, the copper deposit obtained is wettable by a commercial grade of solder using either an acid or rosin type flux and the mechanical adhesion of the plate is good.

It is possible by using the technique of the invention to plate from a bath containing two metal ions such as copper and cadmium as follows:

Example 10 Copper sulphate grams 2 Cadmium sulphate do 1 Ethylene diamine do 1 2 Sodium hydroxide do 2 2 Water do 90 Percent copper by weight 0.52 Percent cadmium by weight 0.45 Percent ethylene diamine by weight 1.71- Percent sodium hydroxide by weight"; 0.21

The pH was over about 10.5.

Copper sulphate grams 5 Cadmium sulphate do 2 Ethylene diamine do 1 2 Tetraethylene pentamine do 4 Sodium hydroxide do "3. Water "don See footnotes at end of table.

Percent copper by weight 1.32 Percent cadmium by weight 0.92 Percent ethylenediamine by weight 1.73 Percent tetraethylene pentamine 4.2 Percent sodium hydroxide by weight 0.31

The pH was over about 11.

1 83% in water.

'- 10% in water;

Improved adhesion of copper'plate to the aluminum was obtained.

The results of the foregoing tests have indicated that aluminum can be easily immersion plated with copper provided the alkalinity of the plating bath is maintained at least high enough to prevent the polarization of the aluminum in the solution during plating. In order to insure adequate plating conditions a metal complexing amine of sufiicient water solubility must be used which will form a relatively stable amine complex at high pHs, even in the presence of a strong alkali, such as sodium hydroxide.

One of the advantages of the invention is that dual coats can be produced on aluminum and similar metals by utilizing an efiect arising from an incompletely immersion coated article. For example, in immersion plating aluminum with copper it has been found that an adherent but discontinuous coating is obtained during the early part of the process which is not discernible to the eye. The coating appears like a regular solid plating and also appears to cover fully the surface of aluminum. Yet, when the coated aluminum material is immersed in an amine solution containing either tin, zinc or cadmium, or other metals, an adherent over coat is obtained distributed over the copper. Tests have shown that the amine baths have very high throwing power, this being proven' by the fact that the copper coating need only be slightly discontinuous to obtain an adequate over-plate.

One advantage of the dual coat is in preparing aluminum for cladding to a base metal such as steel. In this case the dual coat may comprise zinc on top of copper on top of aluminum. The bond between the steel base and aluminum is effected via the dual coat by means of pressure followed by interfacial diffusion at an elevated temperature.

Examples of solutions employed in producing an over coat on copper plated aluminum are given below. An incompletely immersion coated aluminum is first produced as mentioned above which is thereafter immersed in the following solution:

A. Tin over coat (a) Grams Tin sulphate 1.5 n-Butylamine 8.0- Water 80.0 Percent tin 0.93 Percent n-butylamine 9.0

While a precipitate forms, suflicient tin remains in solution to yield a clean bright deposit of metal over the copper.

(b) Grams Tin sulphate 1.0 Ethylene diamine 3.0 Water 80.0 Percent tin 0.65' Percent ethylene diamine 3.5

B. Cadmium over coat (a) Grams Cadmium sulphate 3.0 Diethylene triamine 4.0 Water 90.0. Percent cadmium 1.1 Percent diethylene triamine 4.12

(b) Grams Cadmium sulphate 3 Tr'iethylene tetramine 4 NaOH (10% water) 2 Water 90 Percent cadmium 0.98 Percent triethylene tetrarnine 4 Percent NaOH 0.2

Both of these baths yielded excellent cadmium deposits on both pure aluminum and type 245 on which a discontinuous coating of copper was first produced by immersion coating. Deposition is substantially speeded up at temperatures in the range of to C. After a heat treatment at 250 C, test pieces coated first with copper and then with cadmium gave excellent soldering adhesion.

C. Zinc over coat These yielded bright adherent deposits of zinc over thecopper deposit.

A short heat treatment at 200 C. caused the zinc to diffuse into the copper underlayer forming a bright uniform deposit of brass, the brass existing as a single layer rather than two separate layers of copper and zinc. Adhesion of the brass coating thus formed is excellent. A one hour heat treatment at 250 C. gives excellent soldering adhesion despite the thinness of the coating.

By the use of these baths, it is possible to deposit layer upon layer of different metals depending on the discontinuity of the copper coating. Thus, starting with the copper deposit, it is possible to deposit tin on the copper, zinc on the tin, and cadmium on the zinc. Finally copper may be deposited on the zinc.

The order of deposition may be changed. Thus, zinc might be deposited first, then tin and then cadmium. Or successive layers of copper-zinc-copper-zinc may be deposited, thus building up the coating thickness. The lead-oil layer must be discontinuous in order to make use of this novel eflect. The high throwing power of the bath takes care of the rest.

The baths may be formulated with any of the amine compounds shown in the copper deposition baths with suitable adjustment of pH.

It is possible to electroplate a wide variety of other metals on the surface of any of the chemically deposited metal plates, e. g. nickel or chromium.

While the present invention has been described with respect to producing an adherent copper plate on aluminum to improve its solderability, it will be appreciated that the invention is applicable to the plating of other metals capable of forming a complex with an amine complexing agent. Such metals include zinc, cadmium, nickel, etc. Likewise while aluminum has been given as an. example as the base metal upon which an adherent plating can be produced, other base metals are contemplated within the scope of the invention. For example, such base metals may include zinc, tin, lead, etc. When reference is made to" aluminum herein, it is meant also to include aluminum-base alloys;

' As has. been pointed out hereinbefore, the plating solutions may range over a broad range of composition. Generally, these solutions will contain at least 0.1% by weight of platable metal such as copper, at least 011% by weight of a metal complexing amine and have an alkalinity above 9- pH, usually at least about 9.5. Generally the pH- of the solution will range from-"about 9.5 to 12.5 and preferably about 10 to 12.. I

It will also be appreciated from the foregoing, that the invention has a broad field of application beyond just improving the solderability of a metal surface. For example, the novel plating bath may be usedto produce a metal strike on aluminum as a basis for further electroplating in another solution which normally polarizes aluminum. In this respect, the novel plating bath has great utility. I

Although the present invention has been described in conjunction with preferred-embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

l. A method for the immersion plating of copper on aluminum which comprises establishing an alkaline aqueous bath comprising a copper complex of a water-soluble organic amine selected from the group consisting of alkylene, alkyl and alkylol amines, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, adjusting the pH to above 9.5 at a value at least suflicient to depolarize said aluminum, and contacting the aluminum with said solution whereby copper is chemically plated thereon.

2. The method of claim 1 wherein the coppercomplex is based on an alkylene amine.

3. The method of claim 2 wherein the copper complex is based on ethylene diamine.

4. The method of claim 2 wherein the copper complex is based on diethylene triamine.

5. A method for the immersion plating of copper on aluminum which comprises establishing an alkaline aqueous bath comprising a copper complex of at least one water-soluble organic amine selected from the group consisting of alkylene, alkyl and alkylol amines, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, adjusting I the pH to above 9.5 at a value at least depolarizing to said aluminum, and contacting the aluminum with said solution to produce by chemical plating a copper strike thereon and further plating a metal on said copper-plated aluminum electrolytically from a metal electroplating bath normally polarizing to aluminum.

6. A method for the immersion plating of copper on aluminum which comprises establishing an alkaline aqueous bath comprising a copper complex of ethylene diamine, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, adjusting the pH to a'value above 9.5 at least depolarizing to said aluminum, and contacting the aluminum with said solution to produce thereon by chemical.

plating a copper strike and further plating. a metal on said copper-plated aluminum electrolytically from an electroplating bath normally polarizing to aluminum.

7. A method of plating aluminum which comprises contacting aluminum with an alkaline aqueous bath comprisinga copper complex of at least one water-soluble amine selected from the group consisting of alkylene, alkyl and alkylol amines of pH above 9.5 at least sufficient to depolarize aluminum for a time at least suflicientto produce by chemical plating a discontinuous plating of copper thereon, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, and then contacting said plated aluminum with a metal plating bath containingat: least one metal from, the. group: consisting of'zinc, tinandcadmium asa metal complex of at least one water-soluble amine selected from the group consisting of alkylene, alkyl and alkylol amines, thereby producing ametal plate over the copper. Without the passage. of current.

8. A method for soldering aluminumwhich comprises contactingthe surface of said aluminum with an alkaline aqueous bath comprising a copper complex of at least one water-soluble organic amine selected from the. group consisting of alkylene, alkyl andalkylol' amine, of pH above 9.5 at least suificient to effect adeposit' of copper on said aluminum by chemical plating, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, followed by contacting the copper-plated surface of said aluminum with molten solder under normal soldering conditions.

9. A method for soldering aluminum which comprises contacting the surface of said aluminum with an alkaline solution comprising a copper complex of ethylene diamine of pH above 9.5 at least sufiicient to effect a de posit of copper on said aluminum by chemical plating, said bath containing at least about 0.1% by weight of copper and at least about 0.1% by weight of said amine, followed by contacting the copper-plated surface of said aluminum with molten solder under normal soldering conditions.

10. A method for soldering aluminum which comprises contacting the surface of said aluminum with an alkaline aqueous bath comprising a copper complex of a water-soluble, organic amine selected from the group consisting of alkylene, alkyl and alkylol amines of pH above 9.5 at least sutficient to eifect a deposit of copper on said aluminum by chemical plating for a time sufficient to produce a discontinuous coating of said copper, said bath containing at least about 0.1% by Weight of copper and at least about 0.1% by weight of said amine, contacting said plated aluminum with a metal plating bath containing at least one metal from the group consisting of zinc, tin and cadmium as a metal complex of at least one water soluble, organic amine selected from the group consistingvof alkylene, alkyl and alkylol amines, thereby producing a metal plate over the copper without the passage of current, subjecting said plated aluminum to a heat treatment to effect the dilfusion of the coatings one into the other, and then contacting the thus-treated aluminum surface with a molten solder under normal soldering conditions.

11. The method of claim 10 wherein the metal plate deposited over the copper is cadmium which is obtained from a solution comprising a cadmium complex of said water soluble, organic amine.

12. A method for producing an alloy metal plating on aluminum which comprises contacting aluminum with an alkaline aqueous bath comprising a copper complex of at least one water-soluble amine selected from the group consisting of alkylene, alkyl and alkylol amines of pH above 9.5 at least suflicient to depolarize said aluminum for a time suflicient to produce a discontinuous plating of copper thereon by chemical plating, said bath containing at least about 0.1% by Weight of copper and at least about 0.1% by weight of said amine, contacting said plated aluminum with a metal plating bath containing at least one metal from the group consisting of zinc, tin and cadmium as a metal complex of said water-soluble amine, thereby producing a metal plating over the copper plate without the passage of current and then subjecting the thus plated aluminum to a heat treatment to effect the diffusion of the coatings one into the other to form an alloy layer on the aluminum.

13. The method of claim 12 wherein the second metal coating step is followed by at least another coating step in a bath comprising another metal complex of a Watersoluble organic amine from said metal group, the plural coated aluminum then being subjected to heat treat- 11 ment to eflect the diffusion of the coatings one into the other to form a substantially adherent alloy layer on the aluminum.

14. The method of claim 12 wherein a brass surface layer is produced on aluminum by dipping the coppercoated aluminum in an alkaline bath comprising a zinc complex of said Water-soluble amine thereby producing a zinc coating on the copper which is then followed by subjecting the thus-coated aluminum to heat treatment to effect the diffusion of one coating into the other resulting in an alloy layer of brass.

12 References Cited in the file of this patent UNITED STATES PATENTS 1,989,764 Meltsmer Feb. 5, 1935 5 2,217,912 Saukaitis Oct. 15, 1940 2,602,757 Kantrowitz et a1. July 8, 1952 2,726,969 Spaulding Dec. 13, 1955 OTHER REFERENCES 10 Char et al.: Journal of the Electrochemical Society,

vol. 100, #5, May 1953, pages 227 to 231.

Scientific American, April 1945, pages 245 to 246.

Claims

12. A METHOD FOR PRODUCING AN ALLOY METAL PLATING ON ALUMINUM WHICH COMPRISES CONTACTING ALUMINUM WITH AN ALKALINE AQUEOUS BATH COMPRISING A COPPER COMPLEX OF AT LEAST ONE WATER-SOLUBLE AMINE SELECTED FROM THE GROUP CONSISTING OF ALKYLENE, ALKYL AND ALKYLOL AMINES OF PH ABOVE 9.5 AT LEAST SUFFICIENT TO DEPOLARIZE SAID ALUMINUM FOR A TIME SUFFICIENT TO PRODUCE A DISCONTINUOUS PLATING OF COPPER THEREON BY CHEMICAL PLATING, SAID BATH CONTAINING AT LEAST ABOUT 0.1% BY WEIGHT OF COPPER AND AT LEAST ABOUT 0.1% BY WEIGHT OF SAID AMINE, CONTACTING SAID PLATED ALUMINUM WITH A METAL PLATING BATH CONTAINING AT LEAST ONE FROM THE GROUP CONSISTING OF ZINC, TIN AND CADMIUM AS A METAL COMPLEX OF SAID WATER-SOLUBLE AMINE, THEREBY PRODUCING A METAL PLATING OVER THE COPPER PLATE WITHOUT THE PASSAGE OF CURRENT AND THE SUBJECTING THE THUS PLATED ALUMINUM TO A HEAT TREATMENT TO EFFECT THE DIFFUSION OF THE COATINGS ONE INTO THE OTHER TO FORM AN ALLOY LAYER ON THE ALUMINUM.