INTRODUCTION TO CATHODIC PROTECTION

PROBLEMATIC

Naturally, most of metallic structures are buried or stay immersed for many years. In such environments, the installation has a normal tendency to return to its original ore or rust state due to the corrosion processes.

To prevent this “underhand” degradation and penalizing damage (rupture of pressure equipment, downtime, repairs …), some methods of corrosion protection are used:

• The metal surface coating forms a protective insulating barrier whose efficiency is limited by any degradation due to imperfections, injuries and environmental ageing.
• The cathodic protection ensures a permanent protection of the metal exposed to the conductive surrounding. It is used with or without the coating application.

PRINCIPLE

Cathodic protection consists in obtaining an electrochemical immunity through a sufficiently electronegative potential of the structure.

Such immunity is achieved when the potential measured between the steel and an electrode placed in the immediate vicinity of the metal to be protected:

• To –850 mV with a saturated Cu/CuSO4 electrode.
• To –800 mV with an Ag-AgCl/seawater electrode, also used in chloride-loaded water.

The steel potential can be reduced in two ways:

• By coupling it to a less noble metal (magnesium, zinc, aluminum). It is then named sacrificial anodes protection.
• By the action of a dc current generator connected to an anodic ground bed. It is then named impressed current protection.

Sacrificial anode protection

Impressed current protection