Looking for expert opinion, or better yet an ancient tome describing this phenomenon in detail.
We have a contractor building a vehicle out of aluminum alloy, with no coating of any kind. Standard practice in our industry is MIL-C-5541 class 3 chromate conversion coating when good rf bonding is required. Concern is what happens with no coating. My understanding until very recently was that absent the coating, the aluminum oxide that forms would act as an insulator at low voltages. That is, the issue of a MIL-B-5087 class H bond (shock hazard – green wire connection) is not at issue, because vehicle primary power voltage will punch right through the few nanometers of aluminum oxide. But for class R (rf concerns) the oxide layer might look like an open circuit, because the rf potentials are millivolts, and cannot punch through. I procured a couple identical plates of 6061 aluminum, with the intent of dipping one of them, leaving the other uncoated, and measuring the difference in resistance across bonds with these sheets, using a milliohm meter with controlled applied voltage. The controlled voltage is worst case 20 mv, but with careful attention, can be held to 200 uV. Careful attention was applied. To my surprise, I got single digit milliohm resistance across a bond between some copper shim stock and the bare, uncoated aluminum plate with no more than 200 uV applied potential. So the question is, is the chromate conversion only for environmental factors, such as salt fog? Or maybe to mitigate dissimilar metal connections? Is the fact that it is softer an aid in achieving a gas-tight spot? Why are chromate conversion coatings almost universally applied to vehicle sheet aluminum? Thank you, -- Ken Javor Ph: (256) 650-5261 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
