The authoritative public forum
  for Metal Finishing since 1989
-----

Post-plating treatment

 

My query is that in some cases of cyanide zinc-plated and yellow passivated auto parts which are threaded outside, the passivation color fades away after some hours of a storage only on the threaded area or sometimes knurlings. In the worst instance, the white color appears immediately after plating. We have investigated the problem threadbare from the process angle and there are no non-conformities in the processing. This is also confirmed by the fact that other components behave ok. Does this point to base material or machining defect? In such a case is there any method of overcoming this problem while plating? The cyanide bath is 80-90 gm/l NaCN.,30-40 GMS/L Zn and 72-80 gms/l NaOH.

Thanks for any suggestions.

Subramanian Ramajayam
consultant - Bangalore, India

---

You might cross-section a sample part and see what the zinc thickness is in the areas where the chromate isn't yellow or doesn't stay yellow. It might be that by the time the conversion coating is done there is no zinc left in the roots of the threads. Try longer plating on a sample and see if the problem goes away.

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

---

Thank you Mr Mooney. I doubled the plating time and at the same current I measured on the significant surfaces a thickness of 15 to 20 microns as against the specified 8 to 10microns. After 24 aging, I observed the yellow passivation has turned whitish at the base of the threads whereas other areas are ok. Hence the problem appears to be open for further discussion.

Thanks,

Subramanian Ramajayam
consultant - Bangalore, India

Based on that, I think that you need better rinsing before and after the chromate. I think that a thin film of chemical left in the bottom of the thread is "reacting" with the chromate film.

James Watts
- Navarre, Florida