If the stains are not easily removed, then it is possible that the stains were formed during or immediately after the plating process. Remember that the chromate film is very soft for the first 24 to 48 hours. Make sure you are handling the parts after plating with clean gloves and change them frequently. It might also be possible (but very highly unlikely) that the fingerprints are under the zinc. To check for good precleaning, check the parts as they leave the acid. The parts should show no signs of waterbreak. If the water sheet does break, the part is not clean. Again, I think this is highly unlikely, as it would generally also show up as poor adhesion.
As for the white stains, it is possible that the chromate is corroding, which will leave a white residue. It is also possible that there is some contaminant in/on the chromate that is causing it to prematurely fail. Again, I would guess that somewhere in your handling procedure after plating you are causing this issue.
Some questions that might help us get a better understanding of your process:
Are you using a predip between the zinc and chromate?
How often are you changing your rinses between the zinc and chromate?
What are the performance specs of the chromate (how many hours to white rust)?
How often are you changing the rinses after the chromate?
How many rinses do you have? Flowing or stagnant?
How often are you changing the chromate?
Hex-chrome or tri-chrome based chromate?
Have you talked to your chemical vendor?
How do you handle your parts after plating?
-Dustin Gebhardt,
CEF
Plating Engineer
Danaher Tool Group
Gastonia, NC
