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Old 03-08-2010, 03:09 PM   #1
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White crud on bronze/copper ground clamp


Hey all;
Last summer (around July), I installed an amateur radio antenna on my house.
I used 1" EMT for the mast, and used a standard bronze/copper ground clamp.

When I removed the clamp today so that I could install a lightning arrestor onto the ground wire I found a white substance on the ground clamp where it was making contact with the EMT pipe.

Since the pipe is not aluminum, I did not expect any problem with corrosion.
But this was exposed to weather, so perhaps there is another issue I need to know about.

Is there a gel I am supposed to use for this type of connection?

Thanks

FW
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Old 03-08-2010, 04:13 PM   #2
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The conduit (EMT) has corroded where the bronze clamp was attached. When two different metals are bonded and subject to moisture (or immersion in water) the more active metal has a tendency to corrode, or literally dissolve. I believe that the makeup of the EMT is galvanized (zinc coated) steel and that the white substance is zinc oxide.

Zinc is the most active of the three metals zinc, copper, and iron.

Although I am not sure what might be used to retard such corrosion, I believe that the protective jelly used on aluminum wire fittings will work.
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Old 03-08-2010, 04:17 PM   #3
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I know this as "galvanic" corrosion. Where two dissimilar metals meet ( the EMT is galvanize coated, and the copper) you will get "galvanic" corrosion caused by a chemical reaction between the metals, and being exposed to atmosphere will accelerate the problem. Certain pieces of equipment will use "sacrificial anodes" to prevent this. Most electric water heaters have one. Maybe someone will help me out on this: I'd try using "No-Lox" (sp) which is used on electrical connections to help prevent this. David
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Old 03-08-2010, 04:27 PM   #4
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Wrapping it with plastic tape may help keep the metals from touching each other for a time. A rubber bushing of some sort might do the trick as well.
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Old 03-08-2010, 05:30 PM   #5
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Thanks for the help guys;
Funny I never thought of galvanic action. When I was a kid, I had one of those chemistry sets where I zinc plated a copper penny using a solution of some sort, and an electric current.

Wrapping electricians tape seems like the most simple solution, since I won't have to remove the clamp to apply it.
It's going to take quite a bit of tape to make a good seal, but it's pretty cheap after all.

I had thought of applying a layer of silicon seal (after making the connections), as I did to the coax connections (just the outsides of course), but thought that might actually impede electrical current, since silicon is an insulator.
Even though the connection between pipe and clamp, and clamp to wire is supposed to be gas-tight, I wasn't sure about the silicone. Then, the tape will be a lot less messy, and much easier to remove when I need to service the installation.

Since the clamp is easy to reach from the attic window, I'll try the tape tomorrow.

Thanks

FW
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Old 03-08-2010, 05:43 PM   #6
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Quote:
Originally Posted by sdsester View Post
Wrapping it with plastic tape may help keep the metals from touching each other for a time. A rubber bushing of some sort might do the trick as well.
The metals must touch each other to have an electrical connection.
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Old 03-08-2010, 08:58 PM   #7
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Tar


A long time ago an old timer told me that after you make your connection to your ground rod and tighten the clamp, then you should spread tar or roofing cement over the ground clamp/ground rod to stop any corrosion. I never knew if this was the proper way to do it or not but I have done it several times and it does stop the corrosion and it is easier to remove than you might think if you have to make changes.
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Old 03-08-2010, 10:20 PM   #8
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Quote:
Originally Posted by regularguy View Post
A long time ago an old timer told me that after you make your connection to your ground rod and tighten the clamp, then you should spread tar or roofing cement over the ground clamp/ground rod to stop any corrosion. I never knew if this was the proper way to do it or not but I have done it several times and it does stop the corrosion and it is easier to remove than you might think if you have to make changes.
Don't happen to have either of those on hand.
Another thing that comes to mind is that pourable plastic; the stuff used to coat the handles of tools, to make a better grip.
I think you can brush it on as well, and it dries a nice rubbery finish. A lot less messy to work with than roofing cement or tar, and probably a lot cheaper, since I wouldn't have to buy so much of it.
I could also use that stuff around the house for other things, not yet specifically identified

FW
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Old 03-09-2010, 06:48 AM   #9
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Use a copper clad ground rod with a bronze clamp. They are compatible metals.

Mark
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Old 03-09-2010, 09:04 AM   #10
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Quote:
Originally Posted by KE2KB View Post
Thanks for the help guys;
Funny I never thought of galvanic action. When I was a kid, I had one of those chemistry sets where I zinc plated a copper penny using a solution of some sort, and an electric current.
Ironically, today we copper-plate our zinc pennies.
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Old 03-09-2010, 01:25 PM   #11
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Quote:
Originally Posted by busman View Post
Use a copper clad ground rod with a bronze clamp. They are compatible metals.

Mark
Not an option. It's at the mast pipe, not the ground rod where I have this problem. I could have used copper pipe for the mast, but I don't think it would have been as strong as the EMT.

I wrapped it with 3M electrical tape for now. I guess next fall, I'll pull the tape off and inspect the clamp. Or maybe I'll just check ground continuity between my radio feed cable's shield and the house ground system.
Although, now that I have installed the surge arrestor in the feed line, i have a second path to ground through it, so I won't notice if I get a bad connection from the mast pipe to the clamp just by ohmming it out.
I would need to look at the VSWR on the feed line.

Perhaps what I should have done when installing the antenna is to use copper pipe for the mast, and then run a smaller dia EMT pipe up inside the copper to give it strength. I could have bolted the copper through the EMT pipe using brass screws. That way, my ground system would have been all copper and bronze.

FW
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