Historical Articles

January, 1954 issue of Plating



readers’ questions of general interest

Q. 170. We electroplate coatings of copper, nickel and finally gold on heat-treated steel. Little pin holes form on the parts with colored brownish and purplish circles. These can be removed by running through- our cleaning cycle. However, after a few days they reappear. What is causing this?

A. The appearance of pin holes with brown circles around them would indicate corrosion through pores in the thin copper and nickel plates. Many factors are involved.

The heat scale must be removed completely and the surface given as high and uniform a finish as is permissible for the price range. The better the finish and the more uniform the metallurgical properties of the surface, the easier it will be to obtain a good jewelry finish that will not deteriorate rapidly.

Next, dirty plating solutions will give plates with an increased number of pores or thin spots. Solutions out of balance will also tend to give poorer plates.

No mention is made of the thickness of plates, but most plates in this field are too thin. The finish of the base metal and the operating conditions for copper and nickel will dictate the minimum plate thickness. However, it is felt that there should be a minimum of 0.0003 inch for each plate after any polishing or burnishing operation that may be used to develop a higher finish. The gold plate is assumed to be a flash—in the range of 0.000002-3 inch.

There is a small possibility that impurities may be occluded in the plate and leach out, with time, through the pores. In such cases, the items, after final gold ‘plate and thorough rinses, may be immersed in a weak, hot, chromic acid (0.01 per cent) or a slightly stronger hot dichromate solution for one to five minutes. The work is then rinsed thoroughly in cold running water, rinsed in deionized water, dipped in isopropyl alcohol and then dried in a hot-blast drier or maizo tumbled depending on the nature of the’ items.—EDWARD A. PARKER.

Q. 171. Can you furnish information on the process of electropurification of water by the use of silver electrodes?

A. The subject is reviewed at length by Goetz, Tracy and Harris in their article, “The Oligodynamic Effect of Silver” (Silver in Industry—Reinhold Publishing Corp., N. Y.—1940). The Katadyn-Electro process for the sanitation of swimming pools or the treatment of water supplies utilizes an apparatus consisting of a system of silver electrodes operated by direct current at 1.5 volts. The sterilization of water by such a method found successful military applications in the tropics.—A. KORBELAK.



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