Cathodic protection is the most expensive method of protection. Engineers usually use it in protecting pipelines in the petroleum industry. Also, it is suitable for use in reinforced concrete structures and also for special structures due to its higher cost and need for special studies, design, execution, and monitoring.
If applied properly, cathodic protection can prevent corrosion of steel in concrete and stop corrosion that is already in progress. This is accomplished by making the steel bar a cathode by using an external anode.
Electrons are supplied to the reinforcing bar from the anode through the ionically conductive concrete. The current supplied should be sufficiently high so that all the local cells are inhibited and the entire steel surface becomes anodic.
The external current can be supplied by connecting the steel to a metal that is higher in the electrochemical series (e.g., zinc). This serves as the anode relative to the cathodic steel.
Principle of Cathodic Protection
Cathodic protection is used in a certain type of construction as a result of the high cost as well as the specific nature of the construction and the need for a system of monitoring. It is used most often in structures exposed to chlorides that exist inside the concrete mix, or penetration of chlorides within the concrete resulting from surrounding environmental conditions.
Chloride influence in the presence of concrete has a special nature in that it starts when corrosion results from chlorides and the part of the concrete contaminated by chlorides must be completely removed.
Sometimes, the work of repair is very difficult. The case of chlorides in the concrete mix is considered impossible. Experience shows that the limited availability of electrical protection is more effective in stopping the process of corrosion
The FHWA in the United States has said that the method of repair proven to be the only way to stop corrosion in concrete bridges with salts is the use of electrical current, irrespective of the chloride content in concrete.
The use of cathodic protection specifically prevents the corrosion of steel reinforcement in concrete or stops the corrosion process when it has already begun; this method relies on making steel with a continuous cathode through the use of an external anode (see Figure 1).
As seen from this figure, when cathodic protection is not used, a cathode with a negative electron will form on the surface of a steel part and the other part will work as an anode; from this, the corrosion process forms.
As shown in Figure 1, the electron will be generated on the steel reinforcement surface when an anode is placed on the concrete surface.
In this case, there exists electrical conductivity between the concrete and the steel reinforcement, so the cathodic protection will be formed on the steel reinforcement.
As shown in the figure, the positive electrons will move to the anode. This method of cathodic protection is called sacrificial protection and the anode in this case is called a sacrificial anode.
Methods of Cathodic Protection
There are two main methods of cathodic protection:
- Sacrificial Protection: This method depends on using a sacrificial anode, which is made of zinc metal that gets corroded instead of the steel reinforcement. The oxidation process will cause the zinc to move to the steel reinforcement, and the negative electron will be formed on it, providing the required cathodic protection.
- Impressed Current Protection: This method generates electrons on the steel reinforcement in concrete by outsourced electricity. The source acts as the anode and is placed inside the concrete, called a fixed anode.
Sacrificial protection is used in submerged structures, where the concrete is immersed in water, minimizing electron movement and reducing the potential voltage between materials, thereby providing long-term cathodic protection.
Figure 2 illustrates the second method of protection which is placing an anode in concrete and using an external source of electricity.
An example of a fixed anode is a wire mesh, placed in concrete at the concrete surface, working as an anode. The connectivity among anodes, steel reinforcement, and batteries using cables is shown in Figure 2. The source of energy is normally batteries.
Some key considerations for effective cathodic protection:
- The electrical conductivity to the steel reinforcement must be continuous.
- The concrete between steel reinforcement and anode must be able to conduct electricity.
- Alkaline aggregates must be avoided.
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