How does the Corrosion Cell Affect Anchor Supports?

For many years, the tower industry has used a basic anchor design similar to the one shown in Figure 6. This design presents a large number of benefits, especially in its use of the soil as a means to contain the anchorage. However, it is now becoming increasingly evident that the problem of corrosion on the anchor has not been sufficiently addressed in this design. In this section, we will discuss anchor support designs as they relate to the corrosion cell.

Galvanic Corrosion

Example 1. Figure 6 shows the basic design of a typical anchor support. The anchor support has all the necessary elements of a corrosion cell. The shaft itself acts as both anode and cathode as well as the electrical path between the two. The concrete and soil act as dissimilar electrolytes. The soil has less oxygen just above the concrete anchorage and consequently, less resistivity. The electrical path is the shortest between the anode and the cathode at the same point as the oxygen deficient soil, thus the deteriorating action of corrosion is most strongly in effect in this area. Experience has shown that this is the area most likely to deteriorate and cause the tower to fail.

Example 2. The second example shown in Figure 7 has the same basic anchor support design, however, now a copper grounding rod is incorporated as a means of lightning protection. The electromotive force series notes that copper is lower on the scale than steel and when coupled with steel will produce the measurable current previously spoken of. The two dissimilar metals activate a corrosion cell in which the steel anchor shaft is the anode and the copper ground rod is the cathode. Depending on the resistivity of the soil, this combination can contribute to rapid deterioration of the anchor shaft.

Experience has shown that by using galvanized grounding rods for lightning protection, this particular corrosion cell can be avoided. However, changing ground rod type alone does not preclude the possibility of the existence of other corrosion cells, such as those mentioned in example 1.

Stray Current Corrosion

The third common example of anchor support corrosion is known as "stray current" corrosion. Figure 8 illustrates how stray current can adversely effect the buried components of a tower facility.

Many buried structures such as petroleum pipelines are protected against corrosion by means of impressed current cathodic protection systems. (See the following section for a more detailed explanation of Cathodic Protection.) Electrified railways and welding or plating operations will also put direct current into the ground. If a tower is near any one of these, it could be exposed to stray current corrosion. In Figure 8, the tower is used as a path of least resistance for current flow through the electrolyte. Stray current is picked up on one anchor support and travels by means of the guy cables to the alternate anchor where it subsequently discharges. The structure rapidly deteriorates at the place where the current discharges. This problem can be alleviated by electrically bonding the tower to the structure generating the stray current, essentially making it part of the protected structure.

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