Tower Corrosion Protection
Lightning Protection Information
Galvanic action. Corrosive effects of soil conductivity. Active/passive cathodic protection.
Corrosion Protection for Towers
Most people have a tendency to use copper for grounding because it is readily available, relatively economical, a good conductor, and one of the more noble metals. However, it does have a significant drawback. Since it is near the upper end of the table of noble metals, copper when put in direct contact with most common metals,
which are lower in nobility, will cause accelerated corrosion of the lesser metal. The significance of being more noble means any other metal buried and connected to your copper ground system will be more anodetic and thus become sacrificial. (Also see Topic: Dissimilar Metals.)
A galvanized (hot dipped zinc) tower in a rebar reinforced concrete base is not at risk with a copper ground system.. Since concrete is conductive and forms a Ufer ground, and rebar is a very hard (heat treated) steel which will rust, the rust on the surface of the rebar, which is a non-conductive oxide when dry, will prevent
further oxidation. Since concrete readily retains moisture for long periods of time, the rust is actually a good conductor. Further corrosion reduction of the rebar, due to galvanic action, will be limited by this oxide layer.
A buried galvanized tower section (not the "J" bolts) or guy anchor embedded in concrete will have some galvanic currents that could cause the depletion of the zinc coating into the concrete. This will leave exposed steel that will continue to pit and may de-alloy. This type of corrosion may take 20 to 30 years or more before the
structural failure of the tower may occur.
In 1990, several towers were lost in Minnesota alone due to guy anchor corrosion and failure. Some contractors and manufacturers have now gone to the extent of tar/pitch dipping the anchors so this galvanic corrosion does not occur. This means that the anchor is now insulated from the surrounding Ufer ground! Proper guy wire and
anchor grounding is essential under these (or any) circumstances. An improperly grounded insulated guy anchor can arc through the anchor’ s pitch coating, cracking the concrete with resulting structural failure.
The better the soil conductivity, the more galvanic corrosion could occur. Doping soil with salts can increase the speed of the corrosion. Consequently, it is better to have an extensive radial and ground rod system rather than a smaller ground system with doped soil.
For existing towers, look at the ratio of surface areas and ground resistance. The current density for a given current will be greater for material with a smaller surface area. The more extensive the copper grounding system close in around the anchor or tower base, the more the current density and galvanic corrosion on the
anchors or tower base for a given earth resistance.
If you follow the recommended PolyPhaser method of using radials and ground rods that lead away from the tower base, guy anchors, and equipment building, the resultant distributed surface area and current must return through ever increasing ground distance/resistance. This makes the currents smaller than a ground
system using concentric rings or a ground grid.
It may appear obvious that the use of similar materials would eliminate the galvanic problem While this is true, galvanized ground wire and ground rods are not normally recommended since the electric utility company will probably use a copper clad steel ground rod. Replacing the utility rod may be dangerous or impractical.
Another alternative is to install an active cathodic protection system. The system consists of a power supply (lightning protected of course) and a buried sacrificial anode element, such as zinc. The power supply will electrically elevate the tower section or anchor (negative) forcing galvanic currents through the sacrificial
zinc anode (positive) element. The anode will deplete over time in the soil instead of your tower and guy anchors. If magnesium were substituted for zinc, the power supply can be eliminated since magnesium is more anodetic that galvanized steel.