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Letter 1026
Poor adhesion of yellow chromate on
zinc plating
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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
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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.
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Ted Mooney, P.E.
finishing.com
Brick, New Jersey
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Ted,
Thankyou 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 sucess 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
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Richard,
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
mack products - Jefferson,
Wisconsin
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Dear Richard,
You may try a post-dip that improves adhesion of chromates.
Go in touch with Atotech
Australia in Melbourne/Springvale.
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.
regards,
Rudolf Kempf
- Hong Kong
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