Historical Articles

July, 1953 issue of Plating

 

Mail Box

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Subject: Stripping of Chromium From Zinc

DEAR SIR:
On page 170 of the February issue of PLATING, the answer given to Q. 152 concerning stripping of chromium from zinc is not completely satisfactory. I would like to submit the results of some of the work I did at this Bureau in the course of an A. E. S. project about 20 years ago.

Chromium can be anodically stripped from zinc or cadmium by the use of reversed current in sulfuric acid or in a solution containing sodium sulfide and sodium hydroxide. The former method may be quite satisfactory for commercial work. The latter method permits the chromium to be stripped almost quantitatively so that it could be used for a thickness determination. The details follow:

Stripping of Chromium from Zinc and Cadmium

Chromium from Zinc
For analytically stripping chromium from zinc, a solution of 0.5 normal in NaOH (20 g/l) and 0.25 normal in Na₂S (30 g/l of Na₂S · 9H₂O) is suitable. The object is made anodic at 2 amp/dm², which requires about 3.5 volts. The Na₂S protects the zinc from attack; however, if an Na₂S solution alone were used, the chromate would be reduced to insoluble CrIII compounds on the surface of the zinc and the object would weigh too heavy. In the presence of sodium hydroxide, the chromate formed is only slowly reduced. When all the chromium has been stripped, the current quickly drops to less than 0.01 amp/dm². The object is then removed and the fine silt (of sulfur?) wiped off. The loss in weight of zinc is about 2 to 3 mg/dm², which amounts to an error of 5 to 10 percent for a 0.00002 inch chromium coating (Cr — 0.034 g/dm²). If the chromium has been stripped from a polished zinc base, the surface will be left almost as bright as it was before plating.

Chromium from Cadmium
For analytical stripping, a 1.0N solution of sodium hydroxide with reversed current is employed. The initial current density used is 2 to 3 amp/dm². (Voltage required is about 3.5.) When stripping is completed, the current density drops to about 1 amp/dm². The cadmium surface is left with a dull grey-brown color.

Another reagent which can be used is a solution 1.0N in NaOH (40 g/l) and 0.10N in Na₂S (12.5 g/l of Na₂S · 9H₂O).
It is operated similarly as the bath described above for zinc. After removal from the stripping solution, the cadmium is given a dip in a 0.5N solution of sodium cyanide. It then possesses a smooth gun metal black appearance.

With either reagent cadmium gains in weight about 2 mg/dm², which amounts to an error of 5 to 10 percent in stripping a 0.00002 inch chromium coating. It has not been determined which method is the more reliable. In any case, the article must be removed as soon as stripping is completed, or it will continue to gain in weight.

For the purpose of stripping chromium for replating, the caustic soda solution is suitable for cadmium base, but not for zinc. Chromium may be stripped from both cadmium and zinc in concentrated sulfuric acid with reversed current. In the latter stripping bath, zinc loses 0.02 g/dm² and cadmium 0.04 g/dm². The base-metal should require only a slight amount of buffing prior to replating.

ABNER BRENNER

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Subject: Further Impressions of Australia

We left Melbourne on March 30, spent two weeks in Adelaide, and have now completed one week of a four weeks stay in Sydney. We will return to Melbourne for their convention from May 18 to 20, and after a few days in Brisbane will fly home on May 30, and attend the Philadelphia A. E. S. Convention in June.

In each city, the A. E. S. committee has made detailed plans, not only for the lectures and plant visits, but also for auto trips and visits to their homes. We have made so many good friends in each city that when the time comes to leave, we feel homesick. In Adelaide the Branch gave a dinner at which they presented to Mrs. Blum a large birthday cake, on which were decorated an Australian flag and an American flag, with a dove joining them, a beautiful symbol of their friendship. When we left Adelaide Airport at 7:00 A. M. there were 20 there from the A. E. S. to see us off, including 9 children. At Sydney we were met by a committee of men and one lady. We are literally treated royally everywhere. Friends bring flowers to our room several times a week, and the flowers here are really beautiful.

The attendance and interest at the lectures is very encouraging. Many of these lectures are given at the Universities, with members of Chemical Societies and metallurgical groups as well as A. E. S. members. This cordial association of the platers and other technical groups is an evidence of the progressive spirit in Australia.

I have not visited many plating plants, frankly because of so many other demands on my time. Those visited are not very large by American standards but they are modern and progressive. The largest plating plant in Australia is the General Motors-Holden plant in Adelaide, where the procedures are much like those in Detroit.

I feel that I have come to know the platers here and their interests and problems, as revealed in discussions following lectures, and in personal conversation

Both we and they are grateful that the trip to Australia was made possible by the A. E. S., to the benefit of both countries.
We are looking forward to the A. E. S, Convention in Philadelphia, where we will have more to tell about this interest land ”down under”.

WILLIAM BLUM

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Subject: Length of Membership

DEAR SIR:

Some time ago I read in the ”News from California”, by Fred A. Herr that a total of 134 years of membership in the American Electroplaters’ Society was represented by 4 members seated at the same table at the May meeting (1952) of Los Angeles Branch of the A. E. S. They included:

I wish to submit the names of 4 Newark Branch members who have 221 years of electroplating experience, and have been members of the A. E. S. a total of 166 years.

That is sure a lot of plating.

ROYAL F. CLARK, SR.

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Subject: Electropolishing

DEAR SIR:

In Ezra Blount’s report on Japanese electroplating practice in the May issue of PLATING, he describes electropolishing in phosphoric acid—sulfuric acid—chromic acid—solution on page 482 as part of the Japanese practice. Readers may be interested in knowing that this solution composition is covered in U. S. Patents 2,338,321 for electropolishing - steel - and 2,550,544 for electropolishing aluminum.

CHARLES L. FAUST