Introduction

Anodic protection was first introduced by Adelino in 1954. In short, the transfer of the metal attacked by corrosion to the rust zone with the help of an external current is called anodic protection.

In anodic protection, the electric current applied is usually equal to the amount of corrosion of the structure under protection. Therefore, it is not only a protection method but also a method of measuring the instantaneous corrosion rate.

In this type of protection, unlike cathodic protection, the attempt is made to corrode the metal to an acceptable extent and then form a rust oxide protective layer on the metal surface. In this way, after the rust layer is formed, the corrosion protection will be performed by this layer. It should be noted that, by applying anodic protection, the corrosion rate is greatly reduced (even up to 100,000 times) by the formation of the protective layer.

Possibility of using the method:

The basic point about the possibility of using anodic protection for a structure is that the material to be protected must show electrochemically active-rust behavior and at the same time the rust potential range must be sufficiently wide and the rust current must be at least ten times lower than the corrosion current of the metal in normal conditions. In practice, a small number of metals or metal alloys meet the aforementioned conditions, but fortunately, iron and many of its alloys, which are widely used in industry, are included in this category.

This method is mostly used for metals that rust easily or have a lower rust current intensity. Therefore, it is only used for metals such as zinc, magnesium, cadmium, silver, copper and their alloys, stainless steel that follow active-rust conditions.

Anodic protection is less important from a practical point of view than cathodic protection due to its high sensitivity and maintaining the potential in the rust range.

For example, a steel tank containing acid cannot be protected by cathodic protection because the applied current will be consumed for the evolution of hydrogen. Therefore, one of the important applications of the anodic protection method is for such tanks.

For anodic protection of steel tanks containing sulfuric acid, a carbon electrode is used as the cathode and the metal body as the anode. Therefore, the protected current must reach all parts of the metal. For anodic protection, a device called a potentiostat is required. A potentiostat is an electronic device that keeps the metal at a constant potential with respect to a reference electrode. The potentiostat has three terminals. One is connected to the steel tank, the other to an auxiliary electrode as the cathode, and the third to a reference electrode. The potential required for electrochemical measurements is determined in advance. This method greatly reduces the corrosion rate. The main advantage of the method is its ability to be used in highly corrosive environments and the need for low electric current.

Comparing cathodic and anodic protection, we find the following points:

1. In cathodic protection, all anodic areas on a metal become cathodes, so that corrosion stops. But in anodic protection, the entire metal surface becomes anodes to the extent that the metal is completely corroded. Therefore, it is clear that this method is applicable to metals that are capable of forming protective rust layers. Since rusted metals still corrode at a low rate, anodic protection often does not completely stop corrosion.
2. Anodic protection is used in very weak to highly corrosive environments. While cathodic protection is related to moderate environments, because with the increase in the intensity of the corrosive environment, the electric currents required for cathodic protection become much higher and will not be economical. Therefore, cathodic protection of metals is not practical in highly corrosive environments.
3. In anodic protection, very low currents are used, and for this reason it can be used in highly corrosive environments.
4. Cathodic protection is economically viable because the components are simple and easy to install. However, anodic protection requires complex devices such as potentiostats, reference electrodes, etc. Therefore, its installation and maintenance costs are higher.
5. In a cathodic protection system, the protected area or the ejection power is small. Therefore, to establish a uniform current, multiple electrodes are required at close distances from each other. Anodic protection systems have high ejection power. Therefore, a long pipeline can be protected with a single auxiliary electrode.
6. In cathodic protection, the metal is thermodynamically stable, but in anodic reaction (anodic protection), the metal itself is not thermodynamically stable.
7. The conditions required for anodic protection can be accurately determined using polarization curves. However, these conditions in the case of cathodic protection are obtained by trial, error, and estimation.