How can Corrosion be Mitigated on Anchors?

There are several options available to curtail, if not wholly eliminate, the problems associated with corrosion on anchor supports. In the following section, we will discuss these options. When designing a tower corrosion control system, all of the following examples should be considered. More than one of the options will be required in most instances.

Galvanization and Epoxy Coatings

The first example of corrosion control and the one currently used most often is that of coating the buried steel shaft. Usually anchor shafts are hot dipped galvanized with a zinc coating. This is advantageous as the zinc acts as an anode to the steel shaft. However, if galvanizing is used by itself as corrosion control, the zinc can rapidly deteriorate leaving the exposed steel shaft open to damage. Hot dipped galvanizing in many cases gives a false sense of security because of the satisfactory appearance of the galvanizing above grade.

Epoxy coatings are also often used to protect shafts. This is accomplished by coating the entire shaft with an epoxy that acts as an insulator to protect the steel shaft from direct exposure to the electrolyte. This method is beneficial in protecting against corrosion. However, it has been proven that even the best epoxy coatings cannot guarantee 100% isolation from current. In addition, the coating can be damaged during shipping or installation, leaving small anomalies or "holidays." If the shaft is then buried with these "holidays," the "big cathode, small anode" scenario spoken of earlier comes into play. The shaft is open to accelerated corrosion in small areas that rapidly become larger. This type of concentrated deterioration is worse than if the shaft were left to corrode on its entire surface more evenly.

Concrete Encasement

Another option to prevent anchor shaft corrosion can be seen in Figure 9. In this option, the entire anchor shaft is encased in concrete. This method of anchor design is sometimes used for its structural capacity, but it also has its corrosion control advantages. The greatest advantage is that it all but eliminates one essential element of the corrosion cell. The concrete has such a high resistivity that even with copper grounding electrically connected, current flow is substantially impeded. In addition, if the entire shaft is encased there is no anode/cathode relationship on the shaft itself.

The disadvantage to this alternative is the possibility that the concrete could become cracked or broken. If this should happen and water or soil fills the cracks, a corrosion cell would be created. The anode would be the area inside the cracks exposed to the water or soil, and the cathode would be the portion of the shaft inside the uncracked concrete.

Cathodic Protection

Cathodic protection is a process of using the known variables of a corrosion cell to effectively mitigate the detrimental effects of corrosion. There are two types of cathodic protection commonly used. The first is known as galvanic anode and the second is impressed current. We will discuss each.

Galvanic Anode. Figure 10 illustrates the application of galvanic anode cathodic protection to a typical anchor support. The anchor support without cathodic protection installed is anodic to the copper grounding system and the portion of the shaft inside the concrete. By electrically bonding sacrificial anodes to the anchor support, the current flows away from the sacrificial anode and toward the anchor shaft and copper ground rod. It then returns through the electrical path. In this way, the elements of the corrosion cell have been used to make the anchor shaft the cathode, thereby eliminating corrosion where corrosion is not wanted.

Sacrificial anodes vary widely in their sizes, shapes and make-up. Anodes are typically made of magnesium or zinc. The anode is usually placed in a cotton bag surrounded by a gypsum, bentonite and sodium sulfate mixture. This mixture is used for the purpose of assisting in the activation of the current flow and to ensure that moisture remains around the area of the anode. A wire is attached to the inner core of the anode and is designed to be bonded electrically to the member to be protected.

Following installation of the galvanic anode cathodic protection system, it is essential that it be monitored regularly to ensure its proper operation. A DC volt meter and copper/copper sulfate reference electrode (half-cell) is the most common method of checking the system after its installation. The tip of the half cell is placed in the soil with one lead of the volt meter connected to it and the other to the structure being tested. The measurement should show a voltage shift from the same test conducted on the structure before the system installation.

Impressed Current. Impressed current cathodic protection also uses buried anodes, but in a somewhat different fashion than the galvanic anode system. Rather than relying on the electromotive force of magnesium, DC voltage is impressed on the anodes by means of a voltage rectifier. The rectifier supplies ample current to the anodes to allow current to flow through the electrolyte and toward the protected structure. An electrical connection is made from the rectifier to the structure in which the return current flows. Figure 11 illustrates a typical application of this system at a tower site.

An advantage of impressed current cathodic protection is that the entire system can be centrally located near the base of the tower. The tower and its guy cables are the electrical connections to the anchor supports - provided the guy cables do not have cable insulators. The anodes are placed strategically in the same central location. The rectifier is then mounted on the transmitter building or anywhere AC power is readily accessed.

As with the galvanic anode system, the impressed current system also requires maintenance. It is recommended that the rectifier be inspected regularly and potential measurements be taken in the same fashion as explained in the Galvanic Anode example. This system allows for adjustment in current through changing the rectifier output.

Electrical Isolation

The final example of corrosion control is electrical isolation. Figure 12 shows an anchor support that has been electrically isolated from stray current or galvanic corrosion associated with dissimilar metals. This type of system is commonly used on AM radio towers to isolate the transmitting structure from the ground.

Electrical isolation employs guy wire strain insulators to eliminate the electrical path between the tower and the anchor support. It is an effective way of eliminating the galvanic cell problem associated with copper grounding systems, while at the same time taking advantage of the properties of copper as a means of superior lightning protection. In addition it protects against stray current corrosion. If applied correctly, the lead from the ground rod is attached to the guy cable on the "tower side" of the insulator so as to isolate the ground rod from the anchor shaft. This will ensure lightning protection and at the same time eliminate the copper-steel galvanic relationship. It also breaks the electrical connection required for stray current corrosion to take effect.

Introduction | History | What is Corrosion? | How does the corrosion cell affect anchor supports?
How can corrosion be mitigated on anchors? | Corrosion on existing structures | Summary