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Enquiry on conductivity issues for black zinc plating

July 15, 2009

Hi I have a few questions as below.

1.)Difference between black zinc plating and black chromate plating? Additional layer?

2.)I have some aluminium cast parts that requires black chromate plating (rohs) for conductivity purposes, however after plating, resistance via measurement of voltmeter fluctuates greatly between different areas, what are the possible causes of this? Poor plating process? Surfaces has been bead blasted too for uniformity. Is there any standards for resistance values with tolerance for black chromate plating in the industry?

3.)Will conductivity vary between bead blasted surface and non bead blasted surface? Does bead size matters?

Thanks

Jimmy Go
engineer - Singapore

Hi, Jimmy. When inexact terms like 'black chromate' are used very casually, and provided without sufficient context, it simply isn't possible to know what was meant.

When speaking of aluminum parts, 'black chromate' probably means a black colored chromate conversion coating directly on the aluminum. But it could> mean black chromate conversion coating on top of zinc plating. 'Black zinc' would probably mean zinc plating followed by black chromate conversion coating.

If the parts are to be RoHS compatible, the chromate conversion coating must be trivalent chromium rather than the traditional (but rapidly going out of style) hexavalent chromium.

RoHS compatible chromates are virtually all proprietary, and vary from one vendor to the other. I think you need to call in your chromate supplier to work with you on this. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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July 16, 2009

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