Non-destructive tests for distinguishing between objects formed of dissimilar metals

United States Patent Office _P;_lented J·*¹*

2,981,607

NON-DESTRUCTIVE TESTS FOR DISTINGUISHING BETWEEN OBJECTS FORMED OF DISSIMI' LAR METALS

John Danaczko, Jr., Oak Park, Rl., assignor to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York

No Drawing. Filed July 11,1958, Ser. No. 747,838

Claims. (Cl. 23—230)

This invention relates to non-destractive methods of distinguishing between articles formed of different metals having similar physical appearances, and more particularly to a non-destructive test for distinguishing between relay contacts formed of silver or palladium.

In designing contacts for electromagnetic devices, such as relays, problems, such as contact resistance and contact wear, are encountered and must be solved. When the problem is contact resistance, silver contacts are normally used in order to take advantage of the extremely high conductivity of silver. When the problem is contact wear, palladium is frequently used as the contact forming material because palladium’s ability to resist wear substantially exceeds that of silver. Frequently, because of the specific circuitry involved, it is desirable to include both silver and palladium contacts in a single relay. When the relay is completely assembled, it is desirable to be able to ascertain whether the silver and palladium contacts have been properly locked in the relay structure. Since silver and palladium both present similar physical appearances, any test involving visual techniques will not suffice. It goes without saying, of course, that a suitable test technique must be one which is non-destructive.

It is therefore a primary object of this invention to provide a non-destructive process for distinguishing between articles formed of dissimilar metals.

It is another object of this invention to provide a nondestructive test process for distinguishing between articles formed of dissimilar metals which present similar physical appearances.

It is a further object of this invention to provide a nondestructive method of distinguishing between relay contacts formed of silver or palladium.

With these and other objects in view, the present invention contemplates a method including the steps of rubbing the contacts to be identified on an abrasive surface until marks caused by the deposition of minute quantities of the contact metals appear thereon and then treating these marks with an aqueous solution of ammonium hydroxide and ammonium persulfate. The aforementioned solution will dissolve the marks created by silver contacts, while leaving untouched those marks caused by the palladium contacts, thus distinguishing between the two.

A specific example of a method by which the invention has been successfully practiced is as follows. A sheet of filter paper was immersed in an aqueous suspension of a polishing grade of alumina and then permitted to dry to form an abrasive surface thereon. The filter paper was then brushed to remove the excess alumina from the surface. Although filter paper and polishing grade alumina have been described here, various other types of paper or cloth and various other abrasives of polishing grade may also be used. It is necessary that the materials used be such that they will not react in any manner with the reagent. Next, 2.5 grams of chemically pure ammonium persulfate were added to 5 cc. of distilled water and stirred until the salt had been completely dissolved. Tap water or well water may also be utilized ' ² '· although distilled water is preferred. Finally, 2.5 cc. of chemically pure ammonium hydroxide were added to the salt solution and thoroughly mixed therewith to complete the preparation of the reagent.

δ A set of relay contacts of known compositions was then rubbed on the surface of the abrasive impregnated filter paper until each of the contacts had produced a mark thereon. Each of the marks was then treated with several drops of the aforedescribed reagent. After a few 10 minutes, the marks caused by the silver contacts had completely disappeared, whereas those caused by the palladium contacts persisted indefinitely.

The chemical reactions which take place to effect the above results are as follows:

¹⁵ (1) 2Ag+(NH₄)₂S₂O₈+H₂O ?^Ag₂O+(NH₄)₂SO₄+H₂SO₄ (2) Ag₂O+4NH₄OH^2Ag(NH₃)₂OH4-3H₂O (3) H₂SO₄+2NH₄OH<^(NH₄)₂SO₄+2H₂O

In reaction No. 1 above, the silver reacts with the ammonium persulfate in the presence of water to yield silver oxide, ammonium sulfate and sulfuric acid. This reaction is relatively slow; taking perhaps two or three minutes. As soon as the silver oxide has been produced by reaction

No. 1, reaction No. 2 immediately takes place. In reaction No. 2 the silver oxide reacts with the ammonium hydroxide to yield water and a colorless soluble silver ammonia complex. The entire test takes several minutes because all of the silver is not immediately converted to 30 silver oxide in accordance with the relationships in reaction No. 1.

Reaction No. 3 is a side reaction which has no effect upon the outcome of the test, but is set forth because it does in fact occur due to the particular constituents of 35 the reagent. As soon as reaction No. 1 produces any sulfuric acid, it is immediately neutralized by some of the ammonium hydroxide which is present in the reagent; the results of this reaction being the production of ammonium sulfate and water.

A reagent which has been found to be eminently suitable for distinguishing between silver and palladium relay contacts comprises the following constituents present in quantities within the ranges stipulated:

Ingredients: Parts by weight, percent

Distilled water, H₂O__25-50

Ammonium persulfate, (NH₄)₂S₂O₈____.20-40

Ammonium hydroxide, NH₄0H__.20-50

Deviation from the above range is possible, but not de60 sirable because of the adverse effects (e.g., too long, incomplete, etc.) upon the practicability of the test.

It is to be understood that although the method described herein is utilized to distinguish between silver and palladium relay contacts, its utility is not limited thereto. 55 It is obvious that the method disclosed herein is readily adaptable to other processes involving other articles formed of various metals. Additionally, it is manifest that modifications other than those suggested above may be made to the disclosed method to achieve a similar 60 process which still embodies the principles of the present invention and which is within the spirit and scope thereof.