Metal finishing Q&As since 1989

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Conductivity of Black Oxide

We have some PH17-4 parts that need to be coated by a Non-Porous and Non-conductive Coating. Parts were sent to be Black Oxide Coated, Once the process was done and these parts were returned from the Shop, all of them were completely Conductive (about 4 Ohms of resistance). I had read on this website that Black oxide is nonconductive, and some black oxide coated 4340 parts we worked with before were completely nonconducitve also. the plating company was contacted and they claim blak oxide is a conductive coating and showed some references to back their claim.
ANy suggestions on who is right here.

Dan Fallah
Testing Lab - Ca, USA
September 28, 2009

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September 28, 2009

Hi, Dan. Black oxide is a metallic conversion coating millionths of an inch thick, and is certainly not insulative, although people would not choose it as a conductive coating either. If you can point us to the thread on the website that misled you we would like to correct it.

Regards,

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

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Thanks for the response Ted,
In letter 28747 this was your quote:
"A black oxide finish is basically a controlled rust finish. It is very thin (millionths of an inch) but is still non conductive because oxides are not metallic. A zinc-iron finish with black chromate, a black nickel, or a black chrome plating are examples of conductive finishes; all are aesthetic finishes. Good luck."

I'm not posting this to be rude I'm doing so because I agree with your comments above. In our previous tests for Black Oxide coating on Steel we found it to be Non-porous (water cannot seep through the coating) and Non-Conductive, since there was no OCP ( Open Circuit Potential) reading for the base metal in a Salt Water solution.
Any Ideas?

Dan Fallah
- Newport Beach, Ca US
October 2, 2009

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October , 2009

Thanks, Dan. I've edited that page with a note so that it can't be read the way you apparently read it. If organic coatings like PVC and Teflon will not suffice, there are probably flame-sprayed ceramic coatings available that would meet your needs, but you haven't said what breakdown voltage you require, or the general nature of your requirement for non-conductivity.

Regards,

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

Thanks for your response. We are trying to run some environmental tests on some metallic samples in salt solution. unfortunately the slots which the samples sit in are metallic. We have been trying to isolate the samples from the test frame in order to reduce the discharge of the electrons, which build up on the sample due to corrosion, to the test frame. We have tried Teflon but it allows water to pass through it and create a conductive path through the coating. And to make matters worst the coating cannot be extremely brittle or soft.( so it cracks or rubs off easily with sample movements). So far we have also tried different sulphate coatings but they rub off very easily. Black Oxide seem to be the knight in shinning armor but it failed to produce the non-conductivity or at the least the very high resistivity which we expected. The voltages on the samples are -1<V<0

Dan Fallah
testing lab - Newport Beach, California, USA
October 7, 2009