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Nitric acid neutralization of Zinc plating before chromating

2007

Hi Friends,

This is Maria raj from Singapore. I am working as Process Engineer in an Electroplating company.

We are doing alkaline zinc plating with blue, yellow and black chromate conversions. Presently Nitric acid(0.5-1%) is used to neutralize the parts before chromate conversion.Is there any particular reason behind this? Other acids like HCl or sulfuric can be used for same purpose?

I have some basic ideas about trivalent chromates. The chromate itself contains nitric acid, if I am not wrong.So to neutralize the alkaline zinc, Nitric acid is uesd. But our black chromate which is hexavalent chrome based chemical,can I use nitric acid to neutralize before chromating? It contains Silver Nitrate also.Our supplier recommended to use sulfuric acid for pH adjustments.

My questions are:
1.What is the necessity of acid pre dip(Nitric acid)? Apart from neutralization, is there any chemistry behind this?
2.Can I use other acids for pre dip before chromating?
3. In our black chromating, can I use sulfuric acid pre dip instead of Nitric acid?

Please help me with suitable answers or reference. Thank you very much for any guidance, reference, or assistance

with thanks,

Maria raj
Plating shop employee - Singapore

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2007

Hai!
This is Karthick from Bangalore, In your question itself answer is there. That is nitric acid for shiny finish, to nutralise the alkai. Then Cr coversion coating chemical is also Nitric acid base.So we are using nitric acid dip before to do Cr coating,and to avoid other acid contamination.
Go ahead as per the procedure, don't do RD on this process I did several R&D on it. But I failed.
Have A Nice Day

P.KARTHIKEYAN
- Bangalore, Karnataka, India

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Dear Maria Raj,

Sometimes the nitric hardens the brightener film instead of removing it completely on parts originating from chloride based plating baths.

If your supplier has asked you to use sulfuric acid instead, there must be a good reason for it.

Use a rinse after the sulfuric and follow it up with a dip in a very dilute nitric acid if you are using hexavalent black chromate as nitric is friendlier to Hex black passivations.

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2007

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I think you will find your black chromate is based on a formulation that does not have nitric acid in it, apart from the small ammt of nitrate in the silver nitrate. These formulations are effectively an olive drab chromate to which silver is added to give the black colour and nitric acid is a contaminant

Geoffrey Whitelaw
- Port Melbourne, Australia
2007

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Keep chloride away from silver containing black chromate !

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007