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Black chromate is Rubbing off

I do tri black chromate lately the black has been rubbing of leaving a dull finish sometimes down to the tri clear under coat. should I stop using clear first? or is it another problem

George Smith
employee - Dakota, USA
March 18, 2009

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The silver containing chromate is extremely sensitive to chloride contamination, which if you are chromating on top of chloride zinc, you are probably dragging in too much chloride. Make up a new bath and crack down on the rinsing.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

March 20, 2009

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First of two simultaneous responses -- March 22, 2009

DEAR SIR,NITRIC ACID 1-1.5%(10TO15ML/LR)PRIOR TO PASSIVATION DIP IS RECOMMENDED.
IN PASSIVATION KEEP CONC. PH.,TEMP,DIPPING TIME STRICTLY AS PER CHEMICAL SUPPLIERS RECOMMENDATIONS.
IN TOP COAT CONC.,TEMP,DIPPING TIME AS PER SUPPLIER.
BAKING AS PER SUPPLIER RECOMMENDATIONS.
THANKS,

Ajay Raina
Ludhiana, Punjab, India

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Second of two simultaneous responses --

It appears from your letter that you are first applying a clear passivate and then a trivalent black passivate. All of the trivalent black passivates with which I am familiar are applied directly over an active zinc surface as a 'conversion coating.' I'd check with your supplier (or review the operating data sheet) to see if this is the recommended practice.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.


March 23, 2009

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Hello,

The Trivalent black passivations itself is difficult process to get a dense jet black colour, so the post dip has to be a good one to make the colour appearance better as well the corrosion resistance, but please check the PH as well the concentration of the chromate, it should not be higher.I think its easier and better on acid zinc as compared to cyanide and Non cyanide zinc.

Praveen Kumar
plating process supplier
Mumbai, India
April 7, 2009