GB2272001A

GB2272001A - Low toxicity preparation bath for electroplating

Info

Publication number: GB2272001A
Application number: GB9223502A
Authority: GB
Inventors: Louw Jacobs
Original assignee: ZINEX CORP
Current assignee: ZINEX CORP
Priority date: 1992-10-27
Filing date: 1992-11-10
Publication date: 1994-05-04
Also published as: GB9223502D0

Abstract

An aqueous preparation bath for use in electroplating a metallic substrate eg. Al, Zn with eg. Cu contains potassium hydroxide, sodium stannate and zinc chloride. This bath is low in toxicity, particularly when compared with a conventional cyanide zincate preparation bath. Electrical current may be applied.

Description

LOW TOXICITY PREPARATION BATH FOR ELECTROPLATING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the field of electroplating metallic substrates, and specifically to a low toxicity, non-cyanide preparation bath.

2. Prior Art Electroplating processes typically involve the use of a number of highly toxic chemicals, particularly cyanide. Such chemicals create serious environmental hazards, both in the plating facility when they are used and in the outside world if proper procedures for their disposal are not followed.

Conventional processes for plating aluminum and die cast zinc utilise a number of toxic chemicals. For example, a typical conventional process for electroplating aluminum begins with immersion in a cleaning solution followed by a rinse and then a hot caustic dip to desmut the substrate. The aluminum substrate is then subjected to an acid bath comprising nitric acid, sulfuric acid and/or ammonium diflouride, the latter being particularly toxic. Following another rinse, the substrate is immersed in a preparation bath that is mostly commonly a zincate solution containing approximately 3t cyanide. Such a preparation bath is sold commercially under the trademark DIPSOL. The preparation bath is repeated and the substrate is then typically plated with copper in a cyanide process, after which a final plating is deposited over the copper.

Conventional processes for die cast zinc are generally similar, particularly with respect to the use of a cyanide zincate preparation bath.

The present invention permits a considerably shorter processing cycle than conventional processes for aluminum and die cast zinc, as well as other metallic substrates. In particular, the present invention provides a low toxicity, non-cyanide alkaline preparation bath that is highly effective for aluminum, die cast zinc and other metallic substrates.

SUMMARY OF THE INVENTION The present invention provides an aqueous preparation bath containing potassium hydroxide, sodium stannate and zinc chloride. This bath is low in toxicity, particularly when compared with a conventional cyanide zincate bath.

DETAILED DESCRIPTION OF THE INVENTION In the following description, for purposes of explanation and not limitation, specific substances, concentrations, etc. are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known processes are omitted so as to not obscure the description of the present invention with unnecessary detail.

The preferred preparation bath is an aqueous solution of potassium hydroxide, sodium stannate and zinc chloride in the concentrations that are set forth in Table 1 below. Minimum, nominal and maximum values are given for the concentration of each constituent. The nominal values are known to provide satisfactory performance, and it is believed that comparable performance may be achieved with concentrations anywhere within the indicated ranges or even beyond. Thus, this invention is not strictly limited to the values given in Table 1. All values are given in ounces per gallon of bath.

TALE 1

CONCENTRATION (oz. per gal.) CONSTITUENT MINIMUM NOMINAL MAXIMUM potassium hydroxide (KOH) 4.0 16.0 96.0 sodium stannate (Na2SnO3 3H2o) 4.0 12.0 20.0 zinc chloride (ZnC12) 0.3 0.7 1.2 The bath of the present invention is prepared by adding the constituent chemicals to deionized water in small increments while stirring continuously until the salts are fully dissolved. The zinc chloride, in particular, must be added very slowly during the mixing process as it is not highly soluble in the alkaline bath and will tend to remain as a precipitate.Care should be taken during the mixing process since the salts may react violently with the deionized water and may cause an eruption of the solution.

Although not essential for use of the preparation bath, superior performance is realized by adding a wetting agent such as TRITON X-100 manufactured by Rohm & Haas Company.

This should be added in a quantity of approximately 1 ml per gallon of bath, although a quantity anywhere within the range of 0.1 - 2 ml/gal would be acceptable.

A preferred process for the preparation and plating of an aluminum substrate comprises the following sequence of operations: 1. Degrease the substrate with a suitable non silicon degreaser.

2. Cold water rinse the substrate in three consecutive dips. One cascade rinse is recommended.

3. Dip the substrate in a solution of 1%-3% V/V hydrochloric or sulfuric acid (nitric acid may also be used).

4. Cold water rinse the substrate in three consecutive dips. One cascade rinse is recommended.

5. Immerse the substrate in the preparation bath described above for 30 seconds to 2 minutes at room temperature (50 -70 F). It is recommended that steps 3-5 be repeated a second or third time if the substrate is a complex part to insure that all surfaces are adequately treated.

6. Cold water rinse the substrate in three consecutive dips. One cascade rinse is recommended.

7. Electroplate with copper. The preferred process uses a cyanide-free plating bath such as ZinexTH Alkaline Copper for 2-60 minutes.

8. Rinse substrate and apply final coating.

A preferred process for the preparation and plating of a die cast zinc substrate comprises the following sequence of operations: 1. Degrease the substrate with a suitable degreaser followed by a mild electro cleaner or ultrasonic cleaner.

3. Dip the substrate in a solution of 1%-3% V/V hydrochloric acid for 15-30 seconds.

5. Immerse the substrate into the preparation bath described above while the current is on for 1-2 minutes at 1250-1300 F. A current of 10-80 ASF is recommended at 2-8 volts using a stainless steel anode and an anode to cathode ratio of 1:1. The substrate should be grey/white.

7. Electroplate with copper. The preferred process uses a cyanide-free alkaline plating bath such as ZinexN Alkaline Copper for 15-60 minutes. The temperature of the plating bath should be raised to 1400-1600 F if the deposition rate needs to be increased.

8. Cold water rinse the substrate in three consecutive dips before the final plating.

The above-described aluminum and die cast zinc processing may both be performed in conventional rack or barrel equipment. Indeed, the same line utilizing the same baths may be employed to process both materials as well as other metallic substrates. Lined or PVC tanks are recommended with air agitation of the preparation bath and 15 micron filtration.

The above-described processes provide a much shorter cycle than conventional preparation and plating processes.

Essentially the same process has been used for other typical metallic substrates, although titanium and magnesium have not been attempted.

It will be recognized that the above described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.

Claims

1. An improved process for electroplating a metallic substrate, wherein the improvement comprises the step, prior to electrodeposition, of immersing the substrate in an aqueous preparation bath comprising: (a) approximately 4-96 ounces per gallon potassium hydroxide; (b) approximately 4-20 ounces per gallons sodium stannate; and (c) approximately 0.3-1.2 ounces per gallon zinc chloride.

2. The improvement of claim 1 wherein the step of immersing the substrate in the aqueous preparation bath includes passing an electrical current through the substrate and preparation bath.

3. A process for electroplating a metallic substrate comprising the steps of: (a) acid dipping the substrate; (b) rinsing the substrate; (c) immersing the substrate in an aqueous preparation bath comprising: (i) approximately 4-96 ounces per gallon potassium hydroxide; (ii) approximately 4-20 ounces per gallons sodium stannate; and (iii) approximately 0.3-1.2 ounces per gallon zinc chloride.

(d) rinsing the substrate; and (c) electroplating the substrate with copper.

4. The process of claim 3 wherein an electrical current is passed through the substrate and the preparation bath in step (c).

5. The process of claim 3 wherein step (e) is performed with a cyanide-free alkaline plating bath.

6. The process of claim 3 wherein the acid used in step (a) is selected from the group of hydrochloric, sulfuric and nitric acid.

7. An improved process for electroplating a metallic substrate, substantially as hereinbefore described.