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Shannon Knoepflein wrote:
....
How do you think this spray on copper paint I used would stack up in
this comparison? It sounds like it might perform favorably, as it is on
the outside of the carbon, and it's certainly easy to apply. It's
called Electrodag, and the spec sheet is attached. Comments?
The spec sheet says a 1 mill thick coat achieves 0.25 ohms/sq. This probably is not much higher at frequency (it’s already pretty thin.) Silver is good, but I think the environmental questions are the key here. Note, it is “silver plated copper.” So the silver is very thin. I would want the aging information. Exposure to the atmosphere, in particular sulfur compounds, goes after the silver. So how long before the coating goes high-resistance? You need a lot more info from Electrodag regarding this material in outdoor use. It’s one thing to use it on a PC in a desktop environment and another to put it on a plane. I should also mention that coatings like this are subject to electrolytic corrosion, which being used on carbon is another interesting question. Silver on plastic – no problem. A couple of URLs of interest:
http://www.corrosion-doctors.org/Aircraft/galvdefi.htm <http://edis.ifas.ufl.edu/HE605>
http://www.metal-mart.com/Guides/Galvanic.htm
http://corrosion.ksc.nasa.gov/html/galcorr.htm
which discusses the corrosion series.
http://www.coastalfasteners.co.nz/galvanic_corrosion.htm
interestingly shows graphite below gold. Connection between items close together on the list is good, far apart is bad. The one on top dissolves in preference to the one below. (I looked up in the Chem/Phy Handbook some of the voltages against a hydrogen PH electrode which don’t totally fit the typical corrosion series – usually defined in salt water)
Magnesium -2.375
Zinc -0.7628
Aluminum -1.706
Untreated iron or steel -0.036
Solders (lead-tin) +0.15
Copper, brass, bronze +0.522
Silver +0.7996
Graphite (A zinc/carbon cell would argue that the number is about +0.8)
Gold +1.68
Platinum
So this says that the electrolytic problem may not be much of a problem (but keep in mind the coating is very thin and it doesn’t take much to adversely effect it.) Although the one table that show graphite below Gold would be worrisome as this indicates a potential difference > 0.8 volts, more than enough to drive electrolysis.
A good test would be to take a swatch of carbon cloth and an old (real) silver dime or quarter, immerse the bottom of each in carbonated water, but not the clips that connect to the swatch or dime, then measure the voltage potential with a high impedance meter like a DVM. Carbonated water because one of the basic corrosion mechanisms is the CO2 from the air dissolved in water forming a mildly acidic electrolyte. Greater than few tenths of a volt would be a bad sign, particularly if the silver is the negative terminal, as this says you may electrolytically dissolve the silver coating.
Maybe I can get a swatch or one of you can do the test and let us know how it comes out.
Chas
LNC2 360JM
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