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Chromate Leaching from Lap Joints

We have a major customer who is constantly sending us parts that have a folded lap joint that will either join two parts or just run along the side of a part. As the point may be, the lap joint will hold the chromate solution and leach out along the surface of the part. Currently we are using a light LPS oil to combat the leaching fluid, but I am certain there must be a better way when this occurs. Is there any post masking product on the market that could be used to absorb the chromate solution as it begins to leach from the lap joint?

Paul King
- Lynchburg,, Virginia
2005

Your problem is a giant pain for everyone. We were able to reduce the problem some by a 1 min dip in a hot water rinse tank and then into the etch/cleaner tank. It partially fills the gap with water which keeps some of the other chemicals out. Some people have used a dip in cold water followed by a dip in warm water, sometimes several cycles.

James Watts
- Navarre, Florida
2005

The previous approach is one I would take but I would also add a wetting agent to the chromate to improve rinseability from the joint of material which enters it.

Gene Packman
process supplier - Great Neck, New York
2005

A wetting agent will help, but a hot rinse and maybe a couple cycles in the dryer can help as well. we get parts that have similar issues, all the above remedy sometimes work, (90%), but sometimes going back to a cleaner rinse or cleaner after rinsing and the rinsing again will work, it is just very time consuming.

Joe Avino
- Allegan, Michigan
2005

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First of two simultaneous responses --

What works for us is to keep the parts on a hot plate, with the oozing out area on the bottom and a filter paper to absorb the leak out.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2005

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Second of two simultaneous responses --

I've found using an agitated rinse helps remove leaching.

Doug Pickerd
- Blairsville, Pennsylvania
2005

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If this is the same process line you had when I did the training seminar there about 10 years ago, then you are cleaning in alkali, etching in alkali, and deox in a mixed acid, which is normally good, but in the case of a folded lap joint, it precipitates salt in the crevice which later leaches out.

Try (1) stay out of the alkali altogether, clean and etch in a proprietary product that is phosphoric acid and detergent, this rinses easier than alkali and does not leave a salt behind. (2) Lower the concentration of the chromate and compensate by raising the temperature and time and lowering the pH.

Do not try to bake it out with heat, this destroys the chromate coating.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2005