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Brown staining of Cr III conversion coating with a lithium based sealant

I have a plating process which deposits acid zinc with a thick film Cr III passivate. This is finally sealed with a lithium based sealant. I have a problem whereby after a few weeks the parts which initially look a good shiny silver start to get brown stains all over the surface. EDX analysis can find no difference between the stained areas and the ok areas.

Anyone had any similar experiences or importantly had this problem and solved it ?

Mark Dando
Automotive - Wales, UK
2006

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2006

Dear Dando,
Yes.Yellow stains appear if the control over the process is not maitained as per the suppliers instructions.
Advanced plating, especially trivalent passivations require constant monitoring of process control.
Kindly give full detailes of the process you follow such as:
Conc,pH,immersion time,type of water used for passivation and rinse, Maitanance/make up schdule of passivation bath,"thickness"-what do you mean by "thick".
I may be able to help you if you give full proceess detailes.
The type of sealent does not give yellow tint.
Regards

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