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Black Spots observed on hexagon face of Non Cyanide Zinc Plated Parts

We are observing the delayed appearance of black Spots on The hexagon face of Non Cyanide zinc plated parts. This part is being made out of low carbon Hexagon steel bars. The black spots seem to be appearing only on the hexagon area and not on the turned areas. The parts are Trivalent passivated and subsequently dipped in a top coat/sealant for 35 sec at 50 °C.


Please suggest solution for this problem.

Anantha R.K
Plating Shop Manager - Bangalore , India
September 17, 2010

Dear Mr. Anantha,
Please check for metallic impurities like Iron in Both Zinc plating and Passivation baths.

All the best

Shankaranarayanan N

- Bangalore KARNATAKA
September 19, 2010

It is quite probably slag or heat treat material or spots of carbon that are in the surface metal. Note that you do not have any problem where the surface has been removed.

You might try a pilot run of pickling the rod before doing any of the machine work. Grit blast is another option.

James Watts
- Navarre, Florida
First of two simultaneous responses -- September 20, 2010


Just try these components in acid zinc bath with same passivations and sealant,just to check whether you get rid of such problems.

praveen kumar
Praveen Kumar
plating process supplier
Mumbai, India

Second of two simultaneous responses -- September 21, 2010

Dear Anantha,
I agree with Mr James Watt that it is directly related to the raw material surface rather than plating bath or passivation. It is easy to conclude this way since machined surfaces are free from the black spots. It is also well-known that if the pits' depth is more than 2 microns, the material has greater tendency to rust due to absorbed solution.Hence Rz (Average Depth of the pits on raw material surface) value will have to be checked besides Ra(Average Root Mean Square roughness which is less important in surface treatment process like zinc plating. All platers will agree that any delayed appearance of surface defects during storage is mostly attributed to raw material defects like inclusions and internal defects. Thanks,


Subramanaian Ramajayam
consultant - Bangalore, India
October 13, 2010

I was also plating the similar parts, The problem you are facing is due to base metal not due to plating or passivation and this the final conclusion.

Thanks & Regards,

Parashar S R

Sanjeev Raman Parashar
- PUNE, Maharashtra, India
January 2, 2011

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