US3398028A - Process of forming a red, cuprous oxide coating on copper - Google Patents

Description

States ABSTRACT OF THE DISCLOSURE The method of forming a red, cuprous oxide coating on copper which comprises heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900 F. and cooling said heated alloy to a minimum of 200 F. in an oxygen deficient zone.

The present invention relates to processes for the formation of a red coating on copper.

It is highly desirable, for architectural, decorative and other purposes to provide copper or copper base alloy surfaces which have a durable red appearance.

Generally, copper surfaces when exposed to indoor atmospheres will tarnish and present an undesirable surface appearance. When exposed to outdoor weathering copper surfaces become discolored and unsightly. Heating copper to elevated temperature in air results in a formation of black oxide coating which has no adherence and readily flakes off.

When an esthetic appearance is required in outdoor applications of copper, the formation of a green patina is relied upon. This takes many years to form and untilit does the copper surface is generally discolored and unsightly. For indoor applications where esthetic appeal is required, the copper must be either polished frequently or coated with a clear lacquer. Frequent polishing is obviously inconvenient, time consuming and expensive, Whereas the durability of the lacquer coating is relatively limited and thus cleaning and recoating is required at frequent intervals.

Accordingly, it is highly desirable to devise a method for coloring copper or copper alloy surfaces to provide a durable, esthetically pleasing, red appearance which overcomes the above disadvantages, is immediately available after the practice of the invention and is stable under conditions of outdoor and indoor exposures.

Accordingly, it is an object of the present invention to provide a process for forming a red oxide coating on copper.

It is a further object of the present invention to provide a red coating as aforesaid which is highly decorative and pleasing and is characterized by the retention of its decorative and pleasing appearance over a long period of time under a wide variety of conditions.

It is a further object of the present invention to provide a process as above which is inexpensive and readily usable on a commercial scale.

'It is an additional object of the present invention to devise a process which provides a durable, esthetically pleasing, colored article as above with the durable color being available immediately after practice of the invention and being retentive under a wide variety of outdoor and indoor conditions.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention it has now been found that the foregoing objects may be readily obtained. The process of the present invention forms a durable red cuprous oxide coating on copper by: heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900 F. and cooling in an oxygen- 3,398,Z8 Patented Aug. 20, 1968 deficient zone, preferably oxygen-free or substantially oxygen-free. The cooling should be done in an oxygendeficient zone containing less than 0 .0001% oxygen. Naturally, the greater the amount of oxygen in the oxygendeficient zone, the more rapid ,the cooling must be. In the event very small amounts of oxygen are present in the oxygen-deficient zone, the cooling may be more slowly. In general, it is preferred for practical considerations that the cooling should be as quickly as possible, i.e., cool to ambient temperature in from 10 seconds to three minutes.

In the preferred embodiment, after the heating step the following steps are performed: exposing said heated alloy to oxygen, preferably air, for from 3 to 20 seconds; and cooling said alloy in an inert atmosphere.

It has been found that performance of the foregoing process results in a copper surface having a compact, strongly adherent cuprous oxide coating thereon which has a pleasing red appearance. Furthermore, it has been found that the surface color or appearance is not noticeably aifected by several years indoor exposure, and outdoor exposure results in a softening and mellowing of the color.

By appropriate masking techniques, it is possible to produce designs on the copper alloy surface, such as lettering, numbering or esthetic characterizations.

It has been surprisingly found in accordance with the present invention that the process conditions of the present invention are critical in obtaining the desired results.

In general, any copper base alloy may be used, i.e., any alloy containing a major proportion of copper. Since the process of the present invention depends upon the formation of cuprous oxide, copper must be the main oxide-forming constituent in the alloy which is used. Typical alloys which may be employed include, but are not limited to, the following: high purity copper, tough pitch copper, and OFHC copper (oxygen free high conductivity). Typical alloying su-bstituents include but are not limited to the following: silver; gold; platinum; palladium up to 5%; tin up to 2%; nickel up to 12%; zirconium up to 20%; and zinc up to 10%.

Similarly, the particular form of the copper alloy is not especially critical but is governed by the desired application. For example, copper foil, copper plate, or copper sheet may be readily employed.

The chosen copper base alloy is heated in an inert atmosphere at a temperature of from 1100 to 1900 F. When heated at temperatures under 1100 F, the alloy does not attain the desirable red color of the present invention, but rather achieves a light orange to almost brasslike appearance. It is critical that the alloy be heated in an inert atmosphere. The inert atmosphere heating zone should be substantially oxygen-free since oxygen present during the heating step develops a black cupric oxide coating on the copper.

Typical inert atmospheres wihc-h may be provided are argon, nitrogen, helium, carbon dioxide, carbon monoxide and steam.

The length of time of maintaining the alloy in the temperature range 1100 to 1900 F. is not particularly critical. The alloy may be maintained in the temperature range for longer or shorter times as desired, with the longer times naturally providing the thicker coatings. In general, the alloy is held in the temperature range for at least 3 minutes.

The particular temperature chosen is governed by the particular shade of red desired and the particular alloy used. In general, the higher the temperature, the darker are the colors.

The alloys should be cooled to a minimum of 200 F. in the oxygen-deficient zone, and preferably the alloy is cooled to ambient temperature in the oxygen-deficient zone.

If the copper is then cooled in an oxygen-deficient zone, it may be cooled in the heating zone with the desired amount of oxygen or air metered in and cooled at the chosen cooling rate. Alternatively, the copper may be transferred to a separate zone.

In the preferred embodiment, after the alloy is heated at the desired temperature, the alloy is then exposed to oxygen and preferably air or air containing oxygen for from 3 to 20 seconds. If the heated alloy is exposed in excess of 20 seconds, a dark or blackened coating is attained. After the exposure for the desired period of time, the alloy is then cooled in an inert atmosphere. The alloy should be cooled in the inert atmosphere to a minimum of 200 F, preferably in less than three minutes. Naturally, if desired, the alloy may be cooled in the inert atmosphere to room temperature.

The principle of the present invention is the heating of high copper content metal in an oxygen-free or inert atmosphere and then limited exposure to air or oxygen so that cuprous oxide coating is formed rather than a cupric oxide coating. The highly protective nature of the cuprous oxide is due to the fact that when it forms on the surface, it is under compressive stress resulting in a strongly adherent continuous coating.

Naturally, the present process may be performed either in batch or continuous manner. For example, strip may be preheated in an inert atmosphere, passed through an air or oxygen atmosphere for limited oxygen exposure and then passed through a cooling zone deficient in oxygen.

The present invention and the improvements resulting therefrom will be more readily apparent from a consideration of the following illustrative examples.

EXAMPLE I A coupon of tough pitch copper measuring 2%" x /2" x .022" was cleaned with steel wool until it was bright in appearance. It was then placed in a stainless steel tubular chamber about seven inches long, made of one inch pipe through which argon gas flowed at the rate of cubic feet per hour. The tubular chamber was, in turn, located in a mufile furnace which was set at 1800 F. When the specimen was being transferred to or from the tubular chamber, argon flow was momentarily increased to cubic feet per hour.

After heating at 1800 F. for ten minutes, the specimen was removed from the furnace and immediately transferred to another tubular chamber which was the duplicate of the first, but which was located outside the mufile furnace, with the air exposure being 6 to 8 seconds. Argon also flowed through this chamber at a rate of 10 cubic feet per hour. After cooling for two minutes the specimen was removed.

After this treatment the specimen had an adherent red oxide coating of pleasing appearance. The same procedure was repeated with nine other specimens and the same results were obtained.

A specimen was submitted for electron diffraction analysis of the red coating. It was identified as cuprous oxide.

To determine environmental effects on the coating, three specimens were wrapped in aluminum foil and stored for one year, three were left in the laboratory atmosphere for one year and three were placed on a rack on a building roof and allowed to weather for one year. The specimens in the laboratory retained their pleasing red color and the weathered specimen developed a deep rich red color.

EXAMPLE II In order to determine the effect of temperature on the color of the coating obtained, a series of tests were made in the same way as in Example I, except that furnace temperatures from 900 F. to 1900 F. in 100 F. increments were used. The results obtained in terms of color related to temperature were as follows:

A mask was made from copper foil in which the letters OLIN were cut. This was placed over a heavier piece of copper and the assembly was treated according to the procedure outlined in Example I. This resulted in the word OLIN being formed in cuprous oxide in a pleasing red color on the copper surface.

EXAMPLE IV Two coupons of tough pitch copper measuring 2 /8" x /2 x 0.022 were cleaned with steel wool until they were bright in appearance. They were then placed in a muflle furnace containing an air atmosphere at a temperature of 1800 F.

When the specimens had reached the furnace temperature, one of the specimens was transferred to a tubular chamber through which argon flowed at room temperature at a rate of 20 cubic feet per hour. The other specimen was placed outside the furnace in the air and allowed to cool.

When both specimens had cooled to room temperature, they were both covered with a black oxide scale that readily flaked ofl.

EXAMPLE V Tests were carried out as in Example I except specimens were allowed to be exposed to air during transfer from the heating to the cooling zone for times of 30, 45 and seconds. When cooled to room temperature, all specimens were covered with a black non-adherent scale.

EXAMPLE VI A test was carried out as in Example I except that the specimen was allowed to cool in air instead of in an inert atmosphere. When the specimen had cooled to room temperature, it was coated with a loosely adherent black scale of cupric oxide.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. The method of forming a red, cuprous oxide coating on copper which comprises: heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900 F.; and cooling said heated alloy to a minimum of 200 F. in an oxygen deficient zone containing up to about 0.0001% oxygen.

2. 'I he method of claim 1 wherein said alloy is cooled to ambient temperature in an oxygen-deficient zone.

3. The method of claim 1 wherein said oxygen-deficient zone contains substantially no oxygen.

4. The method of claim 1 wherein said copper base alloy is cooled to ambient temperature in less than three minutes.

5. The method of forming a red, cuprous oxide coating on copper which comprises: heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900" F.; holding said alloy at said temperature for at least thre minutes; and cooling said alloy to ambient temperature in an oxygen-deficient zone containing less than 0.0001% oxygen.

6. The method of claim 5 wherein said alloy is cooled to ambient temperature in less than three minutes.

7. The method of forming a red, cuprous oxide coating on copper which comprises: heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900 F.; exposing said heated alloy to oxygen for from 3 to 20 seconds; and cooling said alloy in inert 5 atmosphere.

8. The method of forming a red, cuprous oxide coating on copper which comprises: heating a copper base alloy in an inert atmosphere at a temperature of from 1100 to 1900 F.; holding said alloy at said temperature 10 for at least three minutes; exposing said heated alloy to oxygen for from 3 to 20 seconds; and cooling in an inert atmosphere.

9. The method of claim 7 wherein said alloy is cooled to a minimum of 200 F. in less than 3 minutes.

RALPH S. KENDALL, Primary Examiner.