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Conductivity of bare Aluminum vs. Chemical Conversion Coating

Our Engineering group thinks it's necessary to chemical convert (chromate) Al 6061-T6 prior to anodizing machined parts in order to have uniform electrical grounding over the life of the product. So typically, they chemical convert the part and then mask any blind threaded holes that may provide grounding paths. I say they are wasting money and bare Al will provide a hundreds of years good electrical grounding since the instruments are used in clean room environment or pharma applications. Your thoughts?

Brad [last name deleted for privacy by Editor]
Operations - Boulder, Colorado, USA
February 11, 2009

Either of you may be correct. The prints may call for electrical connection points to be chem-filmed where the bolt head contacts the part. Past practice can have the effect of a contract unless the contract specifies otherwise. An external star washer will provide adequate grounding in virtually all cases. James Watts - Navarre, Florida February 12, 2009 I would agree that the only reason to chromate the contact area is corrosion resistance. Leo Herringon consultant - Grand Rapids, Michigan, US February 12, 2009 February 12, 2009 Brad Your engineering department is wise in this case. Consider that bare aluminum oxidizes quite readily, naturally. If you initially make electrical contact, and maintain it, it is probably not going to cause resistance issue. But if you have not prepared the surface prior to installing the screw, how will you know you have the best ground? Then what if you have to remove and then re-install the grounding screw? Will you get the screw back in exactly the same location/contact? Will particulate generation from the oxidation cause problems for your cleanroom? Or is it your customer's cleanroom? Refer to 6.10.1.1 of MIL-A-8625 if you are having your parts anodized to this spec. Willie Alexander - Colorado Springs, Colorado

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February 18, 2009

Thanks for the responses. Let me clarify a little.

I understand the need for chromating between bare Al surfaces especially if the parts are conducting electricity. This is for bare Al in pure contact with another Al surface. What if hardware (male threads to female threads) is used between the parts for the electrical connection?

We are using the contact for grounding purposes only. Also, the bolt head is not providing the ground since the mating part is anodized. The threads of the bolt are proving the grounding path. Since, we are using Type III chromate (Type III is a thin layer) I argue that the SS bolt is removing most of the chromate in the thread so why bother chromating. I doubt that an oxide layer could build up between the SS bolt threads and the female blind threads.

Thoughts?

Brad [returning]
Operations - Boulder, Colorado, USA