Method and composition for electroplating aluminum alloys

Abstract

A method of and a bath composition for conditioning the surface of an aluminum alloy article to substantially increase the adhesion to the conditioned surface of subsequently applied electrodeposited surface layer. More specifically the surface of the article is anodized in a solution having three components, i.e. phosphoric acid, sulfuric acid and an organic acid selected from the group consisting of acetic acid, hydroxy acetic acid and amino acetic acid (glycine). The anodizing operation typically is followed by a nickel strike deposit, and this deposit may be subsequently electroplated under conventional operating conditions.

Description

BRIEF DESCRIPTION OF THE INVENTION

It has recently been proposed that bumpers and other exterior automotive components be made from aluminum alloys, particularly alloys from the "7000" series. Such alloys generally have the following typical analyses:

Alloy 7046 Alloy 7016 ______________________________________ Si 0.4 max 0.3 max Fe 0.35 max 0.1 max Cu 0.1 (0.25 max) 1.0 Mn 0.3 0.3 max Mg 1.3 1.1 Cd 0.12 -- Zn 6.6 - 7.6 4.0 - 5.0 Zr 0.12 -- Ti 0.03 0.03 max Al Balance Balance ______________________________________

Such aluminum alloy components must be finished by electroplating with nickel-chromium or similar bright plating. Yet conventional electroplating techniques do not form completely adherent electrodeposited layers on such alloys.

It has now been discovered that adherent electrodeposited layers can be formed on such alloys by initially anodizing the aluminum surface in an anodizing bath containing a mixture of phosphoric acid, sulfuric acid, and an organic acid.

As used hereinafter, all percentages given are expressed as volume percent.

More specifically, the surface of the aluminum alloy which is to be electroplated is first cleaned, acid etched and then rinsed. Next, the article surface is anodized in a three component bath containing from about 5% to about 12% H.sub.3 PO.sub.4, plus from about 0.4% to about 2.0% H.sub.2 SO.sub.4, plus from about 1% to about 7% of an aliphatic mono carboxylic acid or a hydroxy or amino derivative of such an acid having a solubility of at least 10 grams per liter in the inorganic acid mixture. To be more precise, the three component bath contains, as a preferred ingredient, and in addition to the phosphoric acid and the sulfuric acid, from about 1% to about 7% of an organic acid selected from the group consisting of acetic acid, hydroxy acetic acid, and amino acetic acid.

It should be emphasized that the anodizing treatment proposed by the present invention is a preliminary or conditioning treatment intended primarily for utilization as an undercoating for a subsequently applied electrodeposited final layer or coating.

Thus, the present treatment differs substantially from the earlier proposed anodizing treatments which are intended to form integral colored anodic coatings on the surface of aluminum or its alloys. For example, in British Pat. No. 1,022,423, a final "hard coat" is provided by anodizing aluminum or its alloys in a bath containing a mineral acid, an organic acid, and organic acid salts. Similarly, mixtures of either sulfuric acid or phosphoric acid together with certain organic acids have been proposed to achieve a hard, dense, anodic coating as a final surface treatment in U.S. Pat. No. 3,524,799. In each of these instances, the baths are utilized as totally different concentrations than in the present invention and for entirely different purposes. Also it has been proposed in U.S. Pat. No. 3,349,014 that a mixture of phosphoric acid, acetic acid and sulfuric acid be utilized to impregnate a porous pad which is then incorporated into an anodizing circuit and rubbed over a previously damaged surface of an aluminum alloy part. Here again, the intent is to form a final anodized coating, and the ingredients are utilized in proportions and amounts entirely outside the range of the present invention.

The anodizing conditions of the present invention typically carry out the treatment at a temperature of from about 100.degree. to about 110.degree.F., at a voltage of from 25 to 30 volts and at a current density of from about 15 to about 20 amperes per square foot for a period of time of from about 5 to about 10 minutes.

Following the surface treatment of the present invention, the surface is subsequently plated with nickel and chromium or with any other surface treatment which may be desired. Typically, the surface may be plated with one mil of semi-bright nickel, 0.5 mils of bright nickel, or 0.01 mils of chromium. The resulting deposit is uniformly bright and smooth, and the adhesion of the final electrodeposition to the surface is excellent.

OBJECTS OF THE PRESENT INVENTION

It is, therefore, an important object of the present invention to provide an improved method for electroplating articles made from aluminum alloys, and wherein the surface of the article is anodized prior to electroplating in a bath containing a mixture of phosphoric acid, sulfuric acid, and an aliphatic mono carboxylic acid or the hydroxy or amino derivatives of such an acid.

Another important object of the present invention is to provide a bath composition for subjecting an aluminum surface to a pre-electrodeposition treatment, the bath containing from about 5% to about 12% phosphoric acid, from about 0.4 to about 2% sulfuric acid, and from about 1 to about 7% of an organic acid selected from the group consisting of aliphatic mono carboxylic acids, hydroxy derivatives and amino derivatives of such acids having a solubility of at least 10 grams per liter in the phosphoric and sulfuric acid mixture.

It is a further important object of the present invention to provide a method of electroplating articles made from aluminum alloys by anodizing the article in a three component aqueous bath containing phosphoric acid, sulfuric acid, and an organic acid selected from the group consisting essentially of acetic acid, hydroxy acetic acid and amino acetic acid, and then electroplating the anodized article.

It is yet another object of the instant invention to provide a method of electroplating an article made from an alumimum alloy by anodizing the article prior to electrodeposition by placing the article as an anode in an aqueous bath consisting of from about 5 to about 12% phosphoric acid, from about 0.4 to about 2% sulfuric acid, and from about 1 to about 7% of an organic acid selected from the group consisting of acetic acid, hydroxy acetic acid and amino acetic acid, and anodizing the article at a temperature of from about 100.degree. to about 110.degree.F. at a voltage of about 25 to about 30 volts, at a current density of about 15 to about 20 amperes per square foot, for a period of time of from about 5 to about 10 minutes.

DETAILED DESCRIPTION OF THE INVENTION

As above explained, the present invention proposes a specific anodizing treatment for aluminum alloys as a pre-treatment for a subsequent plating operation. It has been found that such initial anodizing greatly increases the adherence of the subsequently applied electrodeposit to the underlying aluminum alloy surface.

Specifically, the anodizing solution consists of three components, namely phosphoric acid (H.sub.3 PO.sub.4) at a concentration of from 5 to about 12%, and at a preferred concentration of about 7%. The second ingredient is sulfuric acid (H.sub.2 SO.sub.4) at a concentration ranging from about 0.4% to about 2% and at a preferred concentration of about 1%. The third ingredient is a saturated aliphatic mono carboxylic acid or a hydroxy derivative of such an acid or an amino derivative of such an acid, any one of these third ingredients necessarily having a solubility of at least 10 grams per liter in the acid solution. Preferred specific saturated aliphatic mono carboxylic acids and derivatives are acetic acid (CH.sub.3 COOH), hydroxy acetic acid (HOCH.sub.2 COOH) and glycine or amino acetic acid (NH.sub.2 CH.sub.2 COOH). The saturated aliphatic mono carboxylic acid or its derivative is present in an amount ranging from about 1% to about 7% of the solution with a preferred composition containing about 3.5%.

The anodizing operation is carried out under operating conditions which generally include a temperature of from about 100.degree.F. to about 110.degree.F., and preferably 105.degree.F.; at a voltage of from about 25 to 30 volts; at a current density of from about 15 to about 20 amperes per square foot, and for a period of time of from about 5 to about 10 minutes, preferably for a period of about 7 to 8 minutes.

Of course, the surface must be initially cleaned, which can be carried out by either soaking or by power spraying with an alkaline solution or by any other preferred cleaning method. After cleaning, the surface is rinsed and preferably is acid etched. This etching is carried out at room temperature for a sufficient period of time, on the order of 1.5 to 2 minutes and the etching solution preferably contains 50% by volume nitric acid (HNO.sub.3) and 50 grams per liter of ammonium bifluoride (NH.sub.4 F-HF).

The article to be anodized, after cleaning and etching, is immersed in the anodizing solution, and a direct current is passed between the aluminum article as the anode and a cathode immersed in the solution. The anodizing is carried out under the conditions set forth above.

Next, the panel is rinsed and then plated. The plating can be carried out in a number of different ways. For example, a nickel strike may be made from a low chloride Watts nickel bath, and subsequent plating with semi-bright nickel, bright nickel and chromium may be applied. Alternatively, the anodized article may be plated directly with nickel using a Watts nickel solution, preferably containing coumarin, to produce a semi-bright deposit, followed by a bright nickel deposit. Subsequent chrome plating onto the bright nickel deposit results in an adhesive, integral, composite electrodeposited layer.

The following examples point out specific operating conditions and illustrate the practice of the invention, but these examples are not to be considered as limiting the scope of the invention.

EXAMPLE I

A 4 inch by 6 inch panel of 7046 aluminum alloy was soaked and cleaned in an alkaline cleaner for one-half hour. Next, the panel was etched for 2 minutes in an acid solution containing 50% by volume nitric acid and 50 grams per liter of ammonium bifluoride.

The panel was then prepared for electroplating by placing it, as the anode, in an aqueous anodizing solution having the following volumetric composition:

H.sub.3 PO.sub.4 7% H.sub.2 SO.sub.4 1% Acetic acid 3%

The panel was then anodized in the solution for 6 minutes at a temperature of 105.degree.F. The anodizing was carried out at a constant potential of 25 volts.

Next, the panel was rinsed and plated in a low chloride Watts nickel strike containing 2 ounces per gallon of NiCL.sub.2 .6H.sub.2 O. This operation was carried out at a pH of 4.5, at 105.degree.F. for 5 minutes. After the nickel strike, the panel was subsequently plated with 1 mil of semi-bright nickel, 0.5 mils of bright nickel, and 0.01 mils of chromium. The resulting deposit was uniformly bright and smooth, and the adhesion of the total electrodeposit to the panel was excellent.

EXAMPLE II

A 6 inch by 4 inch aluminum panel of 7046 alloy was cleaned, rinsed and etched as explained in Example I.

The panel was then anodized in a solution having the composition:

Phosphoric acid 97.4 g/l Hydroxy acetic acid 33.6 g/l

During anodizing, the panel was immersed in the bath for 10 minutes at 105.degree.F. The bath was operated at 25 volts and at a current density of 15 to 20 amperes per square foot.

After anodizing, the work piece was rinsed with water and plated directly with nickel. An initial plate was made from a Watts nickel solution containing coumarin to produce a semi-bright deposit which constituted 70% of the total desired nickel deposit. Following the semi-bright deposit, a bright nickel deposit was made incorporating the remaining 30% of the total nickel deposit. After nickel plating, the work piece was rinsed with water and chromium plated.

Adhesion tests were made by grinding the panel, and these tests showed that the deposit adhered very well to the basic metal.

EXAMPLE III

The procedure of Example I was followed except that the aqueous anodizing solution had a volumetric composition comprising 7% H.sub.3 PO.sub.4, 1% H.sub.2 SO.sub.4 and 50.0 g/l of amino acetic acid.

The panel was then anodized for 6 minutes at a temperature of 105.degree.F. at a constant potential of 25 volts. The panel was next rinsed and placed in a low chloride (2 oz/gal NiCl.sub.2.6H.sub.2 O) Watts nickel strike for 5 minutes at a pH of 4.5 and 105.degree.F. After the nickel strike, the panel was plated with one mil of acid copper, 0.5 mils of bright nickel, and 0.01 mils of chromium. The resulting deposit was uniformly bright and smooth, and the adhesion of the total electrodeposit was excellent.

EXAMPLE IV

The above tests were repeated using the identical conditions mentioned except that 5% formic acid was used instead of the glycine (amino acetic acid). The resulting deposit was uniformly bright and smooth and the overall adhesion was again excellent.

It will be seen from the foregoing that the present invention provides a new, novel, and effective processing cycle for the plating of aluminum alloys, particularly aluminum alloys of the 7000 series. The processing cycle includes the anodizing of the aluminum surface which is subsequently electroplated, this anodizing treatment being carried out as a preliminary to the final electroplating operation.

Claims

What is claimed is:

1. The method of electroplating articles made from aluminum alloys having a zinc content of at least 3% by weight and a copper content of not more than about 1.2% by weight, comprising the steps of acid etching the surface of the article, anodizing the article in a three component aqueous bath containing from about 5% to about 12% phosphoric acid, from about 0.4% to about 2% sulfuric acid, and from about 1 to 7% of an organic acid selected from the group consisting of a saturated aliphatic mono carboxylic acid, saturated aliphatic mono carboxylic amino acids and saturated aliphatic mono carboxylic hydroxy acids, the organic acid being sufficiently soluble in the bath to provide at least 10 grams per liter, and electroplating the anodized article.

2. The method as defined in claim 1, wherein the organic acid ingredient is selected from the group consisting of acetic acid, hydroxy acetic acid and amino acetic acid.

3. In a method of electroplating an article made from an aluminum alloy having a zince content of at least 3% by weight and a copper content of not more than about 1.2% by weight, the improvement of anodizing the article as a pre-plate prior to electrodeposition by the steps of (1) placing the article as an anode in an aqueous bath consisting essentially of from about 5% to about 12% phosphoric acid, from about 0.40% to about 2% sulfuric acid, and from about 1% to about 7% of an organic acid selected from the group consisting of acetic acid, hydroxy acetic acid, and amino acetic acid, and (2) anodizing the article at a temperature of from about 100.degree.F. to 110.degree.F., at a voltage of from about 25 to 30 volts, and at a current density of from about 15 to about 20 amperes per square foot, for a period of time of from about 5 to about 10 minutes.

4. In a method of electroplating an article made from an aluminum alloy having a zinc content of at least 3% by weight and a copper content of not more than about 1.2% by weight, the improvement of anodizing the article as a pre-plate prior to electrodeposition by anodizing the article in an aqueous bath consisting essentially of from about 5% to about 12% phosphoric acid, from about 0.40% to about 2% sulfuric acid, and from about 1 to 7% of an organic acid selected from the group consisting of acetic acid, hydroxy acetic acid, and amino acetic acid.