What is Corrosion?
Corrosion is an electrochemical process. It is essentially the tendency of a refined metal to return to its native state. There are certain conditions which must exist before a corrosion cell can function. Figure I illustrates the four essential elements of a corrosion cell. Necessary elements include the anode, the cathode, an electrical path between each and an electrical conductive electrolyte.
The driving force behind the corrosion cell is a potential or voltage difference between the anode and cathode. Once each of the four conditions have been met, an active corrosion cell is set in place. When functioning properly, there will be a measurable DC voltage which can be read in the metallic path between the anode and the cathode. When the two are electrically bonded, the anode is positively charged and the cathode is negatively charged. Conventional current flows from positive to negative and thus current discharges from the anode and is picked up at the cathode through the electrolyte. The current then returns from the cathode to the anode through the electrical path. This flow has a detrimental effect on the anode known as corrosion.
The above stated example is a corrosion cell in its simplest form. It is important to know that each of the four elements of the corrosion cell will effect how severe or mild the effects of corrosion are. We will begin by discussing the four elements of a corrosion cell and how they interact with one another.
Dissimilar Metals and the Galvanic Series
In a corrosion cell, the anode and the cathode will typically be composed of dissimilar metals. Each different metal finds its place in the GALVANIC SERIES. In figure 2, we see where several more commonly used metals are located in this series. The metals placement in the series is a function of the electrical relationship or POTENTIAL the metals have to one another.
If we were to choose two metals from the scale, electrically bond them and immerse them in an electrolyte, we would find that the voltage produced would equal the differences in the two metals' potentials. For example, if we use the mild steel and copper as our anode and cathode, the voltage measured between the two will be approximately -.6 volts.
In corrosion terms, the metal higher on the scale is the anode and the metal lower on the scale is the cathode. In this example, the mild steel is anodic to the copper and therefore will corrode - provided that the other two conditions of the corrosion cell are met.
There is also a direct relationship between the sizes of the anode and the cathode as to the severity of the corrosion cell. If the area of the cathode is very large in relationship to that of the anode, the corrosion cell will be more severe, and thus the faster the anode will deteriorate. On the other hand, if the anode is very large in relationship to the cathode, the effects of corrosion are much less and the anode deterioration is more gradual. Figure 3 illustrates this relationship as it might be seen on a typical tower anchor support.
Effects of Electrolytes on the Corrosion Cell
Many different environments could be considered electrolytic. Tower anchor supports are most commonly located in soil and concrete. This paper will confine its study of electrolytes to only these two.
Each type of soil has a measurable resistivity. Soil resistivity can be measured by using a soil resistivity meter. This measurement is defined in ohm-centimeters (ohm-cm). The method of measuring soil resistivity used most often is the four-pin method. Figure 4 illustrates the soil resistivity meter as it would be set up using the four-pin method.
A lower resistivity measurement typifies a more conducive environment to the flow of current. The absence of oxygen in the soil also contributes to the enhancement of the corrosion cell. For example, clay type soil in a wet climate may measure 1,000 ohm-cm and therefore be a very good electrolyte. Sandy or rocky soil in a dry climate may measure 20,000 ohm-cm and therefore be a very poor electrolyte. The resistivity of the soil is a very important factor in evaluating the variables of the corrosion cell. When all variables of a corrosion cell remain constant, the electrolyte resistivity becomes the determining factor in the design and application of corrosion control measures.
It is also important to know that soils vary drastically from place to place across the globe. Soil type can even vary within inches! The variability of soil can cause multiple corrosion cells on the same structure. Figure 5 illustrates how various soil types can create a corrosion cell on a tower anchor shaft. In this illustration, we find the upper soil layer a looser, somewhat gravelly soil and below that a more dense clay type soil. The portion of the shaft in contact with the clay type soil acts as an anode to the portion of the shaft in contact with the looser gravelly soil, which is consequently the cathode. Again, we have a corrosion cell where the shaft deteriorates in the anodic areas.
When considering the placement of a new tower site, the geotechnical soil investigation should include a determination of soil type including soil resistivity and chloride and sulfate ion presence. These items are critical to the design of corrosion control measures. If a geotechnical study is not required, soil samples can be taken and analyzed separately by a competent corrosion control firm.
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