US2133255A - Process of electroplating copper - Google Patents

Description

Patented 11, 1938 UNITED STATES PATENT OFFICE to Percy A, E. Armstrong, 'Westport, Conn.

No Drawing. Application May 19, 1937, Serial No. 143,515

"6 claims. (on. 204-10) when electroplating copper onto a ferrous metal base it is generally accepted that if a coat ing of good adherence and good quality is to be obtained, it is essential to use a cyanide bath at least for the initial application of copper. There are many recognized disadvantages to this practice, but thus far it has been considered necessary.

I have discovered a satisfactory method where by copper can be plated onto ferrous alloys such as those of chromium and nickel, without the use of cyanide, by using a chloride bath. In carrying out this invention, it is essential that the under metal be thoroughly cleaned to remove 16 adhering oxide, and ordinarily precautions should be taken to prevent the reformation of such an oxide coating. The clean metal is then immersed in the plating bath which differs from usual plating baths primarily in that it should 20 have an unusually high acidity. Thus I have found that the bath should have an acidity equal to at 'least a 5 normal hydrochloric acid solution,

and preferably the acidity should be much higher, ranging up to a strength from 6 normal to above 9 normal; Though for commercial operations the 9 normal solution is about as strong as is readily available, I have obtained excellent results with a 12 normal solution. The copper'is calculated in the solution as cuprio chloride (Cum-2.21120) and it is highly advisable that the proportion of cupric chloride to free acid be kept relatively low in the bath, for this chloride must be converted into the cuprous form before the best operating conditions are reached and the '35 cuprous chloride is not highly soluble in a solution of low acidity. Further, it willbe found that the rate at which the anodes go into solution is greatly increased in the case of baths of high copper chloride content. The ordinary amount of cupric chloride which maybe employed with acid ranging in strength from 6 normal up to 9 normal will be between and grams per liter, but this amount of copper may be greatly in-' creased, and in some circumstances may range up to as much as 150 grams per liter. This is particularly true of baths having very high acidity.

In preparing the metal for plating, it may be I given a preliminary cleaning in any generally recognized manner but I have found it advisable to give the metal a final pickling with hydrochloric acid, for this is a non-oxidizing acid and leaves the surface substantially oxide-free. Due

to the fact that the plating bath used is a chloride bath, washing subsequent to the final pickling is not essential, and where the metal is one which forms an oxide film very rapidly, as in the case of the so-called stainless steels, such washing is very undesirable, and it is preferred to carry the metal over to the plating bath while 6 wet with the acid. The same procedure may also be followed in the case of the less readily oxidizable compositions, though in such case it is not so essential. I

The cathode current density used will vary 10 a more or less with the amount of cupric chloride employed in the bath. Thus with a bath containing 37 grams of cupric chloride per liter and a 9 normal acid, I may use a current density of about 8.5 amperes per square'foot or less, where 15 as, with a cupric chloride content of 150 grams per liter, I have found that a current density of 35 amperes per square foot gives the best results, and under some circumstances this may be even further increased. High temperatures are 20 not necessary for carrying out my process and an eflicient operating temperature is slightly above room temperature, namely, about 25 C. though of course this value is in no way critical and is subject to wide variation. 25

Whenusing copper anodes it will be found that the anode goes into solution even when the current is not turned on. Of course, when the current is on, the rate of increase of the copper concentration in the bath will be even greater, 30 and accordingly precautions should be used to prevent this and care should be taken to withdraw the anodes from the bath when plating is discontinued. Apparently there is a tendency for the anodes to dissolve at a greater rate just 3.; below the surface of the liquid than elsewhere. This undesirable tendency is much more noticeable when there is a high concentration of copper in the bath and this is an added reason why it seems advisable to keep the copper concentra- 40 tion low. Obviously various means may be employed for keeping the copper concentration down to the desired point such as the following:

1. The plating solution may be run gradually into a tank in which the material to be plated is 45 being pickled anodically. In such case the copper would be plated out on the cathodes and could be melted and re-cast into anodes. The acid would help to keep the pickling bath at the desired acidity.

2. Pickling and plating could be carried out in the same tank. Thus a sheet of metal such as an alloy containing 18% of chromium and 8% nickel could be anodically pickled while the sheet which went before it is being copper plated. This 1s would increase the concentration of ferrous iron in the bath, but my investigations have shown that there is no great disadvantage in this, and

in fact under some circumstances it may even be an advantage.

3. From time to time a portion of the plating solutions can be withdrawn and diluted with water. This will result in the precipitation of a certain amount of cuprous chloride. The clear solution remaining can then be mixed with additional concentrated acid and run back into the plating tank. The cuprous chloride precipitated can be utilized in any desired manner.

4. Anodes can be used coated in such manner that only the ends of the electrodes will go into solution.

Even if the copper content gets too high, this can be corrected at least temporarily by the addition of very small quantites of an oxiding agent such as peroxide of hydrogen (H202) which will keep the copper on the cupric side. This expedient is particularly useful when plating on a ferrous metal of a relatively active nature.

The efficiency of the bath may be increased somewhat by certain additions. Ammonium chloride appears to increase the cathode current efliciency and to decrease the amount of treeing. The addition of ferrous chloride seemed to produce a finer grained more compact deposit and does not appear to have any noticeable effect upon the bath. Aluminum chloride added to the bath seems to give a finer grained deposit and to decrease the tendency toward treeing." Gelatine gives a much finer grained deposit which contains very few trees but unfortunately there is danger that the gelatine will react with other constituents of the bath, producing undesirable compounds and gelatine should only be used toward the low end of the permissible range of acid strength. Benzoic acid has also been found to have some added benefits.

Specific examples of my process are as follows: I

Example No. 1 A sheet of chromium nickel steel containing about 18% of chromium and 8% of nickel was treated anodically in 'a hydrochloric acid bath (approximately 6 normal) until on scrubbing a good clean crystalline surface was presented. The sample was then rinsed thoroughly, scrubbed and given a further anodic treatment in the hydrochloric acid for 30 seconds. It was then taken, without rinsing, from the pickling bath to the copper pating bath and copper deposition was begun immediately. The bath employed was made up of 6 normal hydrochloric acid in which was dissolved about 37 grams of cupric chloride per liter. per square foot was employed and while the initial cathode current efficiency was not high, this increased as the plating went on and reached a A current density of about 12 amperes value of 92.5%. An excellent deposit was obtained which was strongly adherent and of good appearance.

Example No. 2

This process was carried on as in Example No. 1, but here the acid was 9 normal. As in Example No. l, 37 grams of cupric chloride per liter were used and the current density was about 8.5 amperes per square foot. The deposit was rougher and not as uniform as in Example No. 1 but had excellent adherence.

Example No. 3

In this case the acid employed was 9 normal and the amount of copper chloride was 150 grams per liter. The current density was. 35 amperes per square foot. A current efficiency of 91% was obtained and the deposit consisted of a good adherent layer of copper next to the base metal. This also was covered with very small copper grains more or lessclosely packed.

It is understood that these examples are given only by way of illustration and are not intended to limit the scope of my invention.

What I claim is:

l. The method of electroplating copper onto ferrous alloys of chromium and nickel which comprises pickling the surface to be treated with hydrochloric acid and then electroplating copper onto such treated surface from a copper chloride bath containing hydrochloric acid in amount equal to at least a 5 normal hydrochloric acid solution, with the amount of copper chloride present ranging between about 35 grams and 150 grams per liter and the ratio between the metal ion and acid being relatively low.

2. A process as specified in claim 1, in which the acid strength is between about 6 normal and about 12 normal.

3. A process as specified in claim 1, in which the acid strength is between about 6 normal and about 9 normal and the amount of copper chloride present is between about 35 and 40 grams per liter.

4. A process as specified in claim 1, in which the surface to be plated is transferred direct from the pickle to the plating bath without washing.

5. A process as specified in claim 1, in which the copper chloride is present initially as cupric chloride and is reduced in situ to cuprous chloride to provide effective electroplating.

6. In a method of electroplating copper. onto ferrous alloys, the improvement of pickling the surface of said alloys with hydrochloric acid and then electroplating copper onto such treated surfaces from a copper chloride bath containing hydrochloric acid in amount equal to a5 to 12 normal hydrochloric acid solution and in which the proportion of cupric chloride to free acid in the