[TowerTalk] Corrosion Protection

[K9MA]

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 at sdellington.us