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Need expert advise on reducing/ eliminating zinc plating defects

We have small parts made from Cold Rolled Carbon Steel (Mild Steel) and then Zn Plated for 10-12 µ and then trivalent passivated.

Some of the quantity starts having black spot and white rust on surface within 3-4 month. These parts are barrel Zn plated.

Similarly we have screws of M3x12 or M4x15 like small size and they are heat treated for 8.8 grade quality and then Zn plated like above. They also start white rust formation on threads.

Please give me your expert advise on reducing / eliminating the problem.

Chandubhai Patel
group general manager looking after plating shop - India
October 5, 2009

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October 7, 2009

Hi

First, do accelerated salt spray test on your parts to obtain whether your corrosion requirements are fulfilled with your existing process setup and bath parameters or not.

Secondly, being the Cr3+ passivations are not having self-healing properties like Cr6+ passivations, ask your chemical supplier to provide inorganic sealant after Cr3+ passivation for your parts.Which will give longer life against corrosion.

Note: Please try to avoid zinc plating for fasteners where Hydrogen embrittlement issues are very likely to happen for those parts.
Recommendation: Provide Geomet or Magni coating for fasteners.

Regards
Vijendran

Vijendran Sudeendran
- Chennai, Tamil Nadu, India

adv.