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Neutralization of Zinc plating before Cr3 - coating

Hi Friends,
This is mariappan from India, I am working as a process engineer in a rubber to metal bonded molding shop. Here we are doing compression (Rubber)molding with Cr3 - Coated metal parts & our process flow follows the below,
1) Acid wash - 2) water rinse - 3) zinc plating - 4) Cr3 coating - 5) adhesive coating - 6) molding.

Frequently, we are facing bonding failure ( between the adhesive & the plated surface). We consulted here with the adhesive supplier.According to them the failure is because of not doing the neutralization process after zinc plating before chromate-coating.
They say, presence of charged particles over the plated metal hinders the bonding activity. Is this statement is right, suggest me how to do the neutralization or guide me how to come out of this issue.

thanks,

Kamatchi Mariappan
Product designer - Madurai, Tamilnadu, India
September 17, 2011

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September 21, 2011

Dear Mr.Mariappan,
As per my understanding from your process flow, Adhesive coating need a very good porous surface before the coating.
With the porous surface only your Adhesive coating will stick properly.
For your process, instead of Zinc plating + Cr3 passivation (as per your term Cr3 coating) you can try with Phosphating then adhesive coating.
Because, after the Cr3 passivation all the porosity will get arrested in the zinc plating, so you can get the smooth finish. With the smooth finish adhesive coating will not stick properly, you may get the inconsistent peel off.
Or
You can try the adhesive coating without the Cr3 passivation, then check the peel off
But you need to do the Adhesive coating, as soon as possible after the Zinc plating.
If you are keeping the long period for the parts with Zinc plating alone, part will get rusty.

Kannan Boopathi
- Salem, TamilNadu, India