Blue Zinc Redux!

Question:

Our customer requires zinc plating with a clear chromate finish. We use an alkaline, non-cyanide zinc plating bath and then apply the chromate. When we apply the chromate finish we get a yellowish color instead of the expected blue bright finish. What’s going on? T.J.

Answer:

A good question that seems to come my way frequently. In spite of what some metal finishers think, chromate conversion coatings do require good control to get consistent results. The important control factors are time, temperature and pH. The vendor of your chromate conversion coating may also specify other control parameters. Always keep in mind that a chromate conversion coating is not a simple dip and rinse operation!

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Having said this, let’s look at the specific issue. What is causing the yellow color instead of the blue bright color? The obvious answer is that the chromate conversion coating is too thick. Are you following the recommendations of your chemical supplier? Too long in the tank, too hot a tank or wrong pH will cause your problem.

Another factor that perhaps is a little more subtle is this. The film left on the surface of the plated part after removal from the alkaline zinc plating bath is difficult to completely remove. If your rinse step after the plating step is not adequate, you will drag alkalinity into the chromate conversion tank and change the pH of the bath. This will affect how the chromate works. Setting up a neutralizing dip between your rinse steps and the chromate step can solve the problem. The neutralizing dip should contain 0.5 - 0.75% nitric acid.