To get corrosion in a metal structure, it has to be in contact with the corrosive medium, it could be the atmosphere as in the case of atmospheric corrosion, or an electrolyte (earth, water or another hostile medium), in the case of galvanic corrosion.
The first method that can be seen to avoid corrosion is to insulate the metallic structure from the corrosive medium using an insulating, or more stable, coating. These types of protection are called PASSIVE PROTECTION.

In modern industry we use many types of insulated coverings: resins, asphalt, vinyl paint, epoxy, and chlororubbers, etc. In all of these, the value of resistance, flexibility, adherence, point of softening, power of absorption of water, etc., plays an important role in the selection of this class of protection.

Passive protection is the system in which a metal is covered by another of greater resistance to the corrosion, or is capable of neutralizing the medium which surrounds it.

There are many methods of achieving it: with electrolytes, by immersion, by sprinkling etc. To choose the metal and the method of covering, a series of factors have to be taken into consideration, for example the porosity of the carrier material and its electrochemical behaviour with respect to the base metal.

Another method of anticorrosive protection, the most important, is CATHODIC PROTECTION.

In the formation of galvanic batteries, by introducing a metal structure in an electrolyte, the destruction of the structure comes from the dissolving of the anodes opposite the cathodes, as electrons are carried from one to the other and this dissolving the metallic ion.

So therefore, this process give us the idea of the method that we have to use to protect the cathode, converting the metal structure in need of protection into the cathode of a galvanic battery or electric circuit. We can do this by turning to the electrochemical series of the metals and choosing a more electronegative metal to act as the anode or by connecting the structure to the negative pole of a continuous current generator, whose positive pole is introduced into the electrolyte in question, using an anode which generally doesn’t dissolve or dissolves very slowly. With this method we can communicate to the structure to be protected a controllable tension at any moment during the life of the installation.

The first method described is known by the name Protection by SACRIFICIAL ANODES and the second method is known as IMPRESSED CURRENT.

In the protection of the Cathodes by SACRIFICIAL ANODES, the polarized current is supplied by Anodes, which are used up instead of the structure (the Cathode) which remains unalterable.

The materials used as Sacrificial Anodes are diverse, however alloys of Zinc, Aluminum and Magnesium, are the most common. Magnesium not in an alloy can not be used in systems of cathodic protection in seawater due to its rapid deterioration, although some of its alloys are used.

Certain alloys of Aluminum are used, but the most used Sacrificial Anodes are of Zinc, which you do not have to control and which also delivers a continuous and efficient current. An imperative for this type of Anode is the purity of the base metal; the composition must be in keeping with the current specifications. Iron is one of the most damaging impurities to Zinc’s activity as an Anode; A maximum of 50 ppm of Fe can be tolerated if at the same time certain quantities of Cd and Al are present.