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The principle of Cathodic Protection (CP) is to lower the corrosion potential of a metal object. This can be done with a protective current or by using sacrificial anodes. In both cases, the metal to be protected is made cathodic and thus impervious to corrosion.
Corrosion is a chemical reaction where the metal reacts and goes back to it’s original oxidized state. The dissolved metal ions are positive ions, usually Fe2+ , the metal object is negative because electrons stay behind when the positive ions leave. All metals in contact with an electrolyte have the tendency to dissolve. The most common electrolyte is (environmental) water. But other conductive materials can also serve as an electrolyte.
The dissolution rate depends on the conductivity of electrolyte and the natural state of the metal, which differs from metal to metal. The more a metal has the tendency to dissolve, the less noble the metal is. Zinc (not passivated) is less noble than iron, so zinc has a greater tendency to dissolve in water and in the same way iron is less noble than copper.
Example
magnesium has a greater tendency to dissolve than iron as Magnesium is less noble than iron. Due to the fact that more Mg 2+ ions dissolve than Fe2+ ions, the Mg bar will become more negative than the Fe bar.
At a certain moment, no further positive ions will dissolve into the solution because the metal object has such a negative potential that it hinders the ions to leave the object. At that moment, there is a balance between the negative charged metal object and the positive ions in the solution. When an equilibrium is reached, the metal object reaches a certain potential which is characteristic for the particular type of metal. This point is called the electrochemical potential of that metal.
This specific potential can be measured against a reference electrode. The reference electrode consists of a metal –
electrolyte combination with a stable and well known potential. The standard reference electrode is usually the standard hydrogen electrode (SHE) such that the following electrochemical reaction is at a equilibrium state: H2 <-> 2 H+. The potential of this equilibrium reaction is stated to be 0 Volt.
In practice it is difficult to use a hydrogen electrode. Therefore, other reference electrodes are used. But their own fixed
potential is of course different from the hydrogen electrode. We have summarized the most common reference electrodes with their specific characteristics.
| Type of electrode | Solvent | Potential versus H2 (mV) |
Temperature dependence (mV/0C) |
Application | |
| Cu/CuSO4 | Cu/Cu2+ | Saturated CuSO4 | +320 | 0,97 | Commonly used in soil |
| Ag/AgCl | Ag/Ag+ | Saturated KCl | +200 | 1,0 | Fresh and sea water |
| Ag/AgCl | Ag/Ag+ | Sea water | +240 | n.a. | sea water |
| Saturated Calomel | Hg/Hg22+ | Saturated KCl | +240 | 0,65 | laboratory |
| Thalamite | Ti/Ti+ | 3,5 M KCl | -570 | <0,1 | High temperature |
So it is possible to fix the balance potentials of metals in their own electrolyte. If this is done in a salt solution with
a fixed concentration of own metal ions, the theoretical electrochemical potential table can be determined. In the
table, the potentials are expressed in Volt against the hydrogen electrode. However, it is more practical to express potentials in Volts against an easier reference electrode such as the Cu/CuSO4 in a certain environment such as soil or sweet / salt water.
In a sacrificial system, cathodic protection is achieved by metallically bonding sacrificial anodes to the metal to be protected. Due to the two different interface potentials that are created, a current will flow, which will affect the metal to be protected. The necessary reduction in potential is thus achieved at the interface.
Metals that can act as a sacraficial anode are always less noble than the object to be protected. We have provided a list of metals and their electrochemical potentials against the theoretical hydrogen electrode and the practical Cu/CuSO4 reference (click here).
Materials often used as sacrificial anodes include:
As described earlier, by making the iron (Fe) object to be protected more negative through a supply of electrons, the dissolution of Fe2+ ions is being prevented. This means, no corrosion is going to take place.
From the viewpoint of the metal object that needs to be protected, there is no difference between cathodic protection with sacrificial anodes or cathodic protection with impressed current. With either of these techniques, the protected object will be be negatively polarised.
From a practical viewpoint from the perspective of the Anode, there is a big difference. When sacrificial anodes are being applied, the driving force is the potential difference between the iron object and the more negative potential of the material of the Anode. During the lifetime of the anode, metals ions will leave the anode. Thereby decreasing the size of the sacrificial anode.
Therefore it is important the predict the environmental behaviour and the required ions to properly protect the object. For moving objects such as vessels, it is not always easy to predict the required mass. In that case, the lifetime of the anode is not meeting the design specifications.
WIth impressed current systems, the power source is provided externally. Usually through a connection with the grid. A rectifier is connected to the grid an thereby provides the required electrons. Wires are used to bring the electrons to the electrolyte (water/soil), in order to protect the object. It is however quite important to choose a wire material with a high resistance to corrosion.
Some of the most common materials used as impressed current wire are:
| Anode | components | Density (g/cm3) | max current (V) | current density (A/m2) | Anode consumption (g/A/Year) |
| FeSi | 14 Si, 1 C, Rest Fe (5 Cr Or 1Mn of 1 – 3 Mo |
7,1 | 300 | 10-50 | 90-250 |
| Fe3O4 | Fe3O4 | 5,2 | n.a. | 90-100 | 1,5-2,5 |
| Graphite | C (100%) | 1,6-2,1 | 50-150 | 10-50 | 30-450 |
| MMO | IrO2 Ta2O5 | 4,5- | 45-75 | 50-600 | 0,25-0,5 |
The advantage of using impressed current is that larger objects can be cathodically protected and larger distances can be bridged.
For underground pipelines, most commonly used inert anode materials are:
In offshore objects, like pipelines, offshore structures (rigs, risers, wind turbines) the type of cathodic protection systems varies with several parameters. How difficult is it to access the system? Due to the depth or other characteristics, it may be difficult to carry out subsequent maintenance on the cathodic protection system. Which coating system is applied? And is there an external power source available? And of course, what is the expected lifetime?
Sacraficial anodes are still widely used, but for offshore objects like wind turbines, ICCP systems are used more and more. Usually MMO wires are used for these type of systems. In comparison with other systems, their lifetime is significantly longer.
In either configuration, it is required to set a protection potential and design the how the asset will be protected. Usually, the surface area of the anodes are significantly smaller, compared to the protected object. In this case the placement and configuration of the anodes is critical. You can be read more in this article about required potentials.