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Zinc plating on steel is rusting -- will annealing help?

May 30, 2011

Hello,
I have a steel nut which is plated Zinc Trivalent chromium 4µ. I did a salt spray test for the plated part and found that there is corrosion or rust formation on the nut surface.
The nut is made from a free cutting steel rod.

Will any sort of annealing the steel before plating prevents the corrosion?
If yes what is the annealing temp and time to be maintained?

Or is there any other process to prevent corrosion or rust formation on zinc plated steel?

Thanks
Best regards,
Abhilash

Abhilash M Raveendran
Product Engineer - Kochi, Kerala, India

Hi, Abhilash.

The purpose of the zinc plating and chromate conversion coating is corrosion protection; so that is what must be fixed. I assume the thickness of the plating is 4 micrometers? (The symbol you typed for units did not translate across the web, so we converted it to the µ symbol)

Although I have heard of people plating to only 4 microns, I have never heard of less and feel that it is is insufficient. Also, without getting into a long discussion of "free cutting" steel and annealing, the question can be reduced to whether you have full coverage and good adhesion or you don't.

I think if you increase the thickness to a minimum of 5 microns (still a minimal "commercial" plating thickness) with a good name-brand trivalent conversion coating, and fully follow the supplier's recommendation, that you will pass a minimal salt spray test (you didn't say how many hours you need). Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 3, 2011

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June 14, 2011

Abhilash,

Please do not jump in to any conclusions.
First try to get your system of Zinc plating set up audited. Pl. check if the plating and chromating are done as per manufacturer's specs.
I am sure you may not have to look further than plating.
I belong to your part of India. from Kerala - the "GOD'S OWN COUNTRY"

Best of luck

T.K. Mohan
plating process supplier - Mumbai, India