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What Are the Electrical Properties of Chromate Conversion Coating over Zinc?

October 6, 2008

When we build aerospace cables, one of the tests we perform is a resistance check between the connector backshell and the metal braid used as cable shield. We are finding higher than usual resistance measurements when using a backshell that is zinc plated with chromate conversion coat.
Does anyone know if the conversion coat is affecting our resistance measurement? Is it possible to do the coating in a way that does not affect electrical continuity?
Thanks for any insight into this issue!

Brenda Keiner
Product Engineer - Chandler, Arizona, USA

First of two simultaneous responses -- October 8, 2008

The chromate is a very thin insulator. The thicker the chromate, the more the resistance. You can ask for a lighter coat or go to a clear chromate for better results. This will reduce the anti corrosion properties, so it is a trade off.

James Watts
- Navarre, Florida

Second of two simultaneous responses -- October 8, 2008

Chromate conversion coatings are dielectric to some degree. The darker the color (thicker deposits) the higher the resistance. However, even higher resistance occurs when the zinc is corroded. Therefore, it is a good idea to protect the zinc by using a chromate conversion coating. the coating is mostly a gel and will not interfere with pressure contacts such as clamps rivets screws, etc. because the coating is esy to break through, thus the contact is mate to the zinc coating. The chromate prevents corrosion of the contact area.
The numbers are similar to that of chromated aluminum. MILC-5541 deals with the number resistance. The tests are not reliable because it uses a ball contact of specific wt. a slight vibration will show contact or low resistance.

don baudrand
Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [affil link to the book on: Amazon or AbeBooks ])

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