Cathodic Protection is a technique that control the corrosion of a metal surface by making it the cathode of an electrochemical cell. It can be applied to any metallic structure in contact with a bulk electrolyte in theory. It is mainly used in protect steel structures buried in soil or immersed in water. But, Cathodic Protection cannot prevent structural elements from atmosphere corrosion.
Principles of Cathodic Protection:
There are serval cases lead to corrosion due to vapour. For Cathodic Protection, we control the electrons to offset the effect. Here is an example of Anodic and cathodic reactions at a metal surface.
Fig 1- Demonstration of Anodic and Cathodic reactions
We can change the rate of these two reactions by withdrawing electrons or supplying additional electrons to the piece of metal. It is an established principle that if a change occurs in one of the factors under which a system is in equilibrium, the system will tend to adjust itself to annul, as far as possible, the effect of that change.
For Anodic reactions, Fe -> Fe2+ + 2e
For Cathodic reactions, 2H+ + 2e -> H2
Thus, if we remove electrons from the metal, the rate of anodic reaction will increase to attempt to offset our action and the dissolution of iron will increase, whereas cathodic reaction will decrease. Conversely, if we supply additional electrons from an external source to the same metal, anodic reaction will decrease and corrosion started and cathodic reaction will increase. Thus, to prevent corrosion we need to continue to supply electrons to the steel from an external source to satisfy the requirements of the cathodic reaction. Note that the anodic and cathodic processes are inseparable. Reducing the rate of the anodic process will allow the rate of the cathodic process to increase.
By plotting the potential of the metal against the logarithm of the anodic and cathodic reaction rates, we can find a point that anodic and cathodic reactions rates are equal.
Fig 2- Potential of the metal against the logarithm of the anodic and cathodic reaction rates
Methods of Applying Cathodic Protection:
Using Impressed Current
Fig-3 Schematic Drawing of ICCP
The arrangement for protecting a pipeline under ground or water is illustrated in figure 3. The pipe receives current from a direct current power source through an auxiliary inert electrode. Then, the pipe becomes the cathode and the auxiliary electrode the anode. The auxiliary electrode sometimes consists of scrap iron. The iron will dissolve from the anode and the electrode is described as a consumable anode. The surrounding environment will be oxidized and in water reaction will occur.
2H2O -> O2 + 4H+ + 4e
Advantage of Using impressed-current system
1. Non-Destructive and simple Installation
2. Long anode life
3. Maintains structure aesthetics and integrity
4. Cost Saving
5. Long term corrosion control
6. The voltage may be adjusted to allow for environmental and coating changes.
Application of Cathodic Protection in different zone
1. Tidal Zone
The cathodic protection system was installed in the tidal zone of the bottom of Hong Kong Container Berth. Titanium Anode Mesh Encapsulation is adopted. These systems are normally designed and installed such that the average anode current density does not exceed 110 mA/m2. Power is delivered to the mesh through lead wires and titanium current distributor bars. Such systems perform well because the precious metal oxide coating is the active anode, which slowly oxidizes with time.
Fig 4 – Titanium Anode Mesh Encapsulation in HK Container Berth
2. Splash Zone and Tidal Zone
The cathodic protection system was installed in the tidal and splash zones of the front beam of the wharf of the Aveiro Port, Portugal. This area is considered the greatest corrosion risk zone due to alternatively drying and wetting. The wharf has a length of 250 m. The total area of reinforced concrete to be protected was 1046 m2. The Ti/MMO Mesh tapes is adopted as anodes which have 0.5 mm wide and 20 mm thick. The tapes (shown in A) were installed with a spacing of 250 mm and connected by welding to another titanium tape used as a current distributor and fixed to the reinforcement before the concrete casting. Appropriate spacers were used, aiming at avoiding a short circuit. The electric power supply composed of three power supplies, one per zone, voltmeters, and ammeters to measure the intensity of the supplied voltage and current and high impedance voltmeters for the steel/concrete is shown in B. The General view is shown in C.
Fig 5 – Cathodic Protection System in Aveiro Port, Portugal
The relationship between the depolarization values and the current density obtained in each subarea is presented in Figure 6. The required current density for obtaining 100-mV depolarization after, 72 hours of interruption of the current, is about 3.5 mA/m2. The designed current is much higher than the existing current densities. The need to increase the current does not present problems of anode or other system components’ durability. To conclude, the system is stable and effectively protected the front beam in the tidal and splash zones of the wharf from corrosion.
Fig 6 – Mean values of depolarization
3. Zone of continuous erosion
The cathodic protection system was installed in submerged zone of offshore wind turbines at Irish sea. The VSE anode system adopted in this project. It is designed by Deepwater Corrosion Services Inc. Within the envelope, an anode is kept free from mud contamination, allowing it to output the maximum cathodic protection current.
Fig 6 – Cathodic Protection System in offshore wind turbines at Irish sea
 
Comparison between Impressed Current System and Sacrificial System
Impressed Current System Sacrificial System
Anode Life Longer Shorter
Current Can be controlled Cannot be adjusted or controlled
Electrical Isolation Required between anode and steel No requirement
Conclusion
Cathodic protection is a highly adaptable and effective measurement for preventing corrosion on a variety of underground or underwater structures. There are basically two types of systems: Sacrificial Anode and impressed Current System. Each has characteristics which make it more adaptable under given circumstances. Cathodic protection designs can differ considerably depending upon the coating, configuration of the structure, environment and the presence of nearby structural elements. When a system is designed, installed and maintained properly, cathodic protection is one of the most effective and economical methods of preventing corrosion.
Reference
Francis, P. E. (2007).†Cathodic Protectionâ€. National Physical Laboratory, pp.2-10
Daily, S. F. (1999, November). Using cathodic protection to control corrosion of reinforced concrete structures in marine environments. In AUSTRALASIAN CORROSION ASSOCIATION CONFERENCE, 1999, SYDNEY, NEW SOUTH WALES, AUSTRALIA.
Araujo, A., Panossian, Z., & Lourenço, Z.. (2013). Cathodic protection for concrete structures. Revista IBRACON de Estructures e Materials, 6(2), pp 178-193.
ARKLOW FIELD (2016, November 25), Deepwater Corrosion Services Inc, Retrieved from http://stoprust.com/about/experience-overview/project-review-bulletin-11/