Chromate Color Change

Question:

We use a blue-bright type chromate on parts that are plated in an alkaline, non-cyanide zinc bath. The parts look great after the chromating step but change to a yellowish color with time. This does not happen with all the parts, only certain batches. Our chromate solution vendor has suggested that there may be a problem with how the parts are stored. Do you have any suggestions? G.R.

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Answer:

The only thing that can happen to your parts during storage is that the chromate coating may lose a small amount of water (dehydrate). This might cause some of the trivalent chromium in the coating to oxidize and give you a yellow color. If the chromate film was a little on the heavy side to start with, the yellowing would be more pronounced.

The trick here is to put the proper thickness of chromate on the zinc plate. This means better control of the chromating step. In typical chromating solutions, the thickness will increase as the pH of the solution goes up. With an alkaline zinc plating bath, this happens because of drag in from the plating bath to the chromate solution. A commonly recommended solution is to use a dilute nitric acid dip after the zinc plating step and before the chromating step. Of course you should be checking the pH of the chromating solution frequently.