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Zinc/Iron black chromate problem

We are zinc/iron plating some forged and machined components. The parts get a hex chrome silver free black passivation. The problem is that sometimes after leaving the parts rest for 24 hours in order to get the chromated layer "cured" the chromate can be wiped of with a cloth leaving the zinc/iron plating with out the black color our customer needs. Any ideas why this might be happening ?.

Guillermo Luna
- Mexico City, Mexico
2003

Dear Guillermo,

You may need to reduce brightener additions to your zinc -iron bath, and treat or plate out the existing brightener levels. Your problem is that the zinc iron is already passive before entering the passivation bath.

Regards,

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

2003