I've been researching ways to protect my guy anchors from corrosion, and
I'd like to share a few things I've learned. (After almost 30 years in
Wisconsin clay, the 5/8 inch rods were rusty, but showed no decrease in
diameter.) Much of this information comes from corrosiondoctors.org.
Cathodic corrosion protection is accomplished by causing current to flow
from the protected metal to one or more anodes buried in the soil. In
sacrificial anode protection, the anode is magnesium or zinc,
essentially making a battery, which is the source of current. In
impressed current protection, a DC power supply provides the current. In
either case, the current density at the protected steel cathode should
be at least 11 mA per square meter, or 1 mA per square foot.
Magnesium is the best sacrificial anode for protecting steel. The
measured rate of magnesium consumption is about 7 kg per Ampere-year. At
10 mA, one kg should last about 14 years. The surface area of anodes
required depends on soil conductivity. In average soil, for guy anchors,
it seems a couple 1 kg anode would be sufficient. At this time, the
material required to protect 3 anchors, including backfill, is available
for about $200.
The sacrificial anode is buried in a bag of special backfill material,
not just buried in the ground. Probably at least two anodes should be
used, on on either side of the anode for better current distribution.
For typical amateur radio guy anchors, impressed current protection is
probably more cost effective, as the long term cost of the electricity
is far less than that of magnesium anodes. I don't know if the anodes
for impressed current protection need to be surrounded by the special
backfill material. Perhaps one can just drive them into the ground.
Apparently, scrap iron or steel can be used, but it will be consumed at
a rate of 9 kg per ampere-year. High silicon steel anodes are available,
which have a much lower rate of consumption, but I haven't looked into
their cost. I suppose the power supply could be made from a 5 V wall
wart and some resistors. For impressed current protection, there would
be the added cost/effort of burying wires to the anchors. The protected
metal connects to the negative output of the power supply. Don't get
that wrong!
I think this information is all correct, but if anyone knows otherwise,
or has additional information, please let me know. Apparently, the true
source is a set of handbooks which are not online, and my local
engineering library is disfunctional at the moment.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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