Historical Articles

April, 1952 issue of Plating


Question Box--readers’ questions of general interest

Q. 139. What is the most effective stop-off for selective nitriding?

A. Tin plate used to be employed for this purpose, but has been largely superceded by bronze plate containing 10-15 per cent tin, remainder copper. The bath is prepared from 5 oz/gal sodium stannate, 7 oz/gal sodium cyanide, 4.75 oz/gal copper cyanide and 1.5 oz/gal sodium hydroxide, and is operated at pH 13 and 140° F. One-third of the anode area is copper, two-thirds tin. The-tank voltage to the tin anodes is about 5 volts, to the copper anodes about 3 volts. The copper plate from the pyrophosphate bath has also been used successfully.—D. G. FOULKE.

Q. 140. Is there any method for removing ammonia from a nickel solution having an excess of nickel ammonium sulfate?

A. Small amounts of ammonium have been removed from bright-nickel solutions by means of an activated clay Super-Filtrol, but where large quantities are involved even a treatment with 6-8 oz/gal Super-Filtrol may not solve your problem. The method requires pumping of the solution into a spare tank, addition of the activated clay, intermittent stirring for 3-4 hr, and settling over night. If a small-scale trial shows that it will not work, you will have to withdraw a calculated portion of the solution, make up the volume with water, and dissolve sufficient nickel sulfate to bring back the metal content.—D. G. FOULKE.

Q. 141. A source of copper is used trolley wire containing 0.9 per cent cadmium, remainder copper. Will the cadmium interfere in cyanide copper plating?

A. The-cadmium will deposit with the copper. It has been reported that as little as 0.1 g/1 of cadmium can be detrimental in that the deposit may become columnar and treed. Blistering has been experienced over brass. However, it should be possible to remove the cadmium, either continuously or by batch treatment, using sodium sulfide to precipitate the cadmium as cadmium sulfide.—D. G. FOULKE.

Q. 142. We use a white-brass plate instead of nickel under chromium, but have encountered networks of cracks that ruin the appearance. What can be done?

A. The white brass is a brittle and probably considerably stressed alloy. However, freedom from cracking is reported when the alloy is no more than 0.0003 inch thick. To apply anything approaching that thickness on recessed parts it is necessary to rack with thieves and shields and use anodes of suitable length to get as uniform deposit as possible. The white brass is usually applied over a copper deposit, but there also seems to be an interest in zinc under coatings, which some believe will give better protection.—K. G. SODERBERG



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