Manny,
Thank you. This is getting at what I need to know. My understanding is that aluminum oxide a few nanometers thick forms almost instantaneously on exposure of an uncoated aluminum sample to air, and then it’s done. Does your message below imply that exposure to salt fog increases the amount of corrosion of bare aluminum alloy, above and beyond that which forms instantaneously upon exposure to air? Thank you, -- Ken Javor Ph: (256) 650-5261 From: Manny Barron <[email protected]> Date: Wednesday, December 4, 2024 at 10:01 PM To: Ken Javor <[email protected]> Cc: <[email protected]> Subject: Re: [PSES] Chromate conversion coatings on aluminum alloy, need for When I worked in the military defense industry many years ago, I specified that 6061 aluminum alloy be coated in accordance with MIL-C-5541 Class 3, this requirement had been around for many many years (1950s ?). The purpose of the thin Class 3 coating was two-fold: 1) to provide corrosion protection, and 2) to provide some level of electrical conductivity (although the exact value was never specified), while the purpose of the much thicker Class 1A coating was primarily to provide robust corrosion protection. But I understand that MIL-C-5541 has been retired and has been replaced with MIL-DTL-5541 (2006?). And the purpose of the Class 3 section is similar to MIL-C-5541, except that: a) other non-hazardous coatings are now allowed in order to comply with RoHS, and b) a numerical value is now specified for the electrical conductivity requirement along with how to test its compliance. Plus there is now an additional requirement for electrical conductivity after 168 hours of salt-fog testing. This is my understanding of these Mil-Specs. Manny Barron On Wed, Dec 4, 2024 at 3:36 PM Ken Javor <[email protected]> wrote: 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 at: [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 - ---------------------------------------------------------------- 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
