Summary for trivalent chrome passivation black spots Discussion
Black spot on parts after 1–2 months is discussed in the context of zinc plating (8–12 micron) with trivalent chrome passivation. Shoban Kesarapu asks whether the spotting is rust or a normal effect of trivalent passivation. Devendra Kumar Sharma advises using only DI water in the passivation and post-rinses, and stresses proper neutralization of plating solution before trivalent passivation. J.K. Sharma says part life depends on coating thickness and chromating type, but process control is critical; he recommends keeping trivalent passivation within the proper parameters, such as pH 1.8–2.0, and routinely testing iron content because excess iron can harm results. M. Deepan agrees with J.K. Sharma and adds that high iron in the passivation solution is a main concern; for large lines, decanting the solution and disposing of the bottom portion may help, while manual systems may need a full dump and remake.
Q. We are facing problem of black spots on parts After 1~2 month period, the parts are plated with Zn 8~12 micron & passivated trivalent chrome, please let us know this the problem of rust of it is the nature of Trivalant Passivation.
Tarun Kumar
Product designer – Gurgaon
August 29, 2008
A. MR.TARUN,
Generally trivalent chrome based passivations gives more life than hexavalent chrome based passivations.In the trivalent based passivation pH control is very important. Generally trivalent passivations works at pH range from 1.8-2.2.And also depends on the process cycle.
Shoban Kesarapu
plating chemicals supplier – Secunderabad, A.P., India
September 2, 2008
A. Dear Tarun, you should use only DI water in passivation and in post rinses and a proper neutralisation of plating solution prior to trivalent passivation should be there.
Devendra Kumar Sharma
– Delhi, India
September 5, 2008
A. Hi,Tarun
The life of the plated part is depend on the coating thickness and type of chromating.The most important thing is process control, your trivalent passivation can give a batter salt spray life with recommended parameters.e.g. pH 1.8-2.0, and iron contents must be controlled by routine testing.If the iron contents is higher,they can affect your results.
J.K.Sharma
process and support supplier – Ludhiana, Punjab, India
September 15, 2009
A. DEAR MR.Tarun Kumar,
I go with Mr.J.K Sharma’s suggestion.
Mainly because of high Iron in the Passivation solution, decant the solution and the bottom solution you dispose thus would be fine; if it is very big line.
If its is manual, decant the whole solution and prepare and new one.
M.Deepan
– Chennai, India
February 3, 2013