Concept of cathodic protection

• CATHODIC PROTECTION

In wet corrosion conditions, it is rarely possible to use coatings with noble metals that do not corrode, for economic reasons.

One of the most widely used methods is cathodic protection, which is generally inexpensive and effective over a long period of time when this technique is controlled and accompanied by a good knowledge of the environment conditions.

Industrial applications of cathodic protection mainly concern :

• buried or submerged pipelines
• underground tanks, capacity interiors, in combination with passive protection
• protection of vessels, buoys,
• protection of port structures and offshore platforms

This technique is developing in new sectors such as reinforced or prestressed concrete overhead structures (bridge decks, piers) and offshore wind turbine structures.

• PRINCIPLE OF CATHODIC PROTECTION

Corrosion of metals in contact with aqueous media (water, soil, concrete) is electrochemical in nature. In these media, the application of the cathodic protection technique is intended to reduce the polarization potential of the metal to a level where the corrosion rate of the metal is significantly reduced. The cathodic protection threshold can be determined thermodynamically. It corresponds to the immunity threshold as shown in the Pourbaix diagrams (Figures 2) corresponding to a state from which corrosion becomes theoretically impossible.

To modify the potential of the metal to be cathodic protected, an anode is used installed in the same electrolyte. Anodes can be of two types :

• either anodes with a more electronegative potential than the metal to be protected (galvanic anode or sacrificial anode)
• or anodes coupled to a current generator imposing a difference in potential between the two metals (imposed current method)

The choice of one system over another must be made taking into account technical comparisons (cf. Table 6) and economic criteria (design, equipment, installation, maintenance, material and energy consumption).

cathodic protection systems table

The purpose of the cathodic protection by sacrificial anodes is to take advantage of the effect of the galvanic couple (battery) created by the association, in the same electrolyte, of two different metals: the metal to be protected and a less noble metal that constitutes the anode. In this type of protection, the anode is consumed by dissolution by carrying the surface of the metal to be protected beyond its immunity potential.

For protection to exist, it is necessary that :

• the battery flows, so that the anode is electrically connected to the metal,
• the surface to be protected and the anode are all immersed in the same electrolytic environment,
• the installation is adapted to the surface to be protected (dimensions, number, distribution of anodes).

Protection by sacrificial anodes does not last indefinitely. It stops with the disappearance of sacrificial material or by electrochemical blocking. The evaluation of its lifetime is estimated by calculation of anode sizing and by return of experience (from a few months to several decades).