Plating Q&A: Nickel Tank Crystals

Why is there crystalline material at the bottom of the tank?

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Q. We recently pumped out our nickel tank for its annual cleaning and found a couple of inches of crystalline material in the bottom of the tank. Is this material nickel sulfate or something else? The bath is a Watts nickel plating bath which is continuously filtered, monitored and tested on a weekly basis.

A. The crystalline material at the bottom of your Watts nickel plating bath is not nickel sulfate, but is most likely boric acid, which is not very soluble in water. In a typical plating bath, you can have a concentration of approximately 4 ounces per gallon and it tends to drop out of solution fairly readily as the temperature is dropped. If you are allowing the bath to cool down over weekends, the boric acid will crystallize out of the bath. When you bring the plating bath back up to operating temperature, the boric acid is slow to redissolve. The plating operator will often add additional boric acid, thinking that the concentration is low, though it is low because the boric acid at the bottom of the tank has not completely redissolved.

To determine what this material is at the bottom of your tank, send a sample out for analysis. Assuming the crystalline material is boric acid, you must allow additional time when the bath is brought to operating temperature for the boric acid to redissolve.

Surprisingly, the quality of the boric acid that you use can make a difference. In a study performed a few years ago, two different boric acids were used in two identical Watts plating baths. Samples were taken from each of these baths and allowed to cool overnight. One sample gave approximately 30 cubic centimeters of precipitate while the other sample gave approximately 160 cubic centimeters of precipitate. The bottom line is that the quality of chemicals used in your plating baths can and does make a difference. 

Originally published in the October 2015 issue.

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