Poor adhesion of yellow chromate on zinc plating
We are experiencing a problem with the adhesion of yellow chromate film to our zinc plating deposit.
A particular product we barrel zinc plate in a potassium chloride solution is compressed in a post plating assembly process. Although prior to compression of the part the chromate film passes all 'normal' tests for adhesion, after compression the film sometimes loses adhesion and can be removed with the lightest touch in the area that the base metal has been deformed. In these cases the chromate tends to look powdery in the compressed area.
The problem occurs erratically. Often a barrel load apparently free of this problem is followed by a barrel in which all parts fail on this criteria. Variance within a single barrel load is also sometimes encountered. The plating is carried out on an automatic line - so process variance is relatively minimal.
Our efforts to link the problem to any specific process variable have so far failed.
The parameters with which we have experimented include:
1. Five different proprietary chromate dips have been trialled at various concentrations and pH.
2. Three different proprietary chloride zinc brightener systems have been trialled.
3. With and without pre-dip in a sour rinse (dilute nitric acid).
4. With and without pre-dip in a clear chromate dip.
5. Vary the heat used in the drying process.
6. Vary the pH in the post dip rinses.
7. Vary the temperature of the chromate dip.
8. Use of wetting agents in post plating rinses to negate the effect of any oily films.
We are unsure if the problem is due to variation in the composition of the chromate film or a lack of adhesion of the film to the zinc. Any ideas or suggestions would be most appreciated.Richard Gianatti
Zinc is not ductile, which is why cadmium was traditionally used for such applications as hose fittings destined for crimping.
Still, and this is just intuition and experience, the drying temperature is probably the critical factor here. I say that simply because irrespective of the other factors you vary in the list that you gave us: let the temperature get too high and any of those chromates will dehydrate and slough off. It's a common problem.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Thank you for your response.
We are now drying these parts at ambient temperature, but even so this has not eliminated the problem.
We have had some success by introducing a fairly strong water based lacquer dip. Parts from within one batch which have been lacquered seem OK whilst unlacquered parts still exhibit the sloughing off problem after compression. We are not sure if the lacquer holds the chromate on or prevents the chromate film from dehydrating and becoming brittle.Richard Gianatti
This is just a thought, but you might look into the possibility that your compression process is builing up large amounts of heat which could be destroying the chromate film. Look closely at every step these parts take in the production process after chromate.Mike McDonald
- Jefferson, Wisconsin
You may try a post-dip that improves adhesion of chromates.
Go in touch with Atotech Australia.
I am not sure if it will help, as the problem you describe very much depends upon the mechanical and physical conditions during further processing of the plated article.
- Hong Kong
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