Zinc Plating and Crystal Growth Revisited

In the February issue, I answered a question regarding formation of excess crystals in a potassium chloride zinc plating bath. I recently received additional information from one of the plating shops regarding its problem.

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Q. We’re running a potassium chloride zinc plating bath. The bath has heavy crystal formation throughout and a very thick layer of sludge at the bottom of the tank. As you might guess, we are not obtaining satisfactory plating from the bath. I have attached a few photos showing the tank and crystal growth. What is the cause of this, and how can we solve the problem? L.N.

A. I’ve received two emails in the past 30 days asking the same question. In both of these cases, the shops have allowed their plating baths to get completely out of control. Why? Neither shop has performed regular testing. The bottom line is this: You cannot operate a modern plating shop without a testing program. Plating baths must be monitored on a regular-scheduled basis. If you don’t have an in-house testing capability, you must have your chemical vendor or an independent laboratory test your plating bath on a regular basis.

Additional information and response:
An Energy Dispersive Spectrometer (EDS) was used to analyze a sample of the crystals from the plating bath. (While not the most common type of analysis, EDS is great for determining components of small particles.) In addition, a log of pH, temperature and boric acid concentration showed some fluctuation in all three parameters. The EDS showed the presence of oxygen, boron, potassium, chloride and zinc. The presence of boron might be surprising to you, but keep in mind boric acid, H3BO3, is used in this bath as a buffer to help control pH. Boric acid is not very soluble in water and will crystallize out of solution if the temperature of the plating bath drops. This is more likely if the concentration of boric acid is near the upper limit of its solubility. pH also affects the solubility of boric acid in the plating bath. If the pH increases to 6, you will see crystals forming in the plating bath.

In short, a combination of solution saturation, pH change and temperature fluctuation can bring about the formation of excess crystals. Once these crystals form, redissolving them into the plating solution is difficult.