CATODIC PROTECTION

To prevent metal loss due to corrosion, all underground fuel storage and conduction facilities must have this anti-corrosion system.

Soil characteristics substantially affect the type and rate of corrosion of a structure in its contact. For example, dissolved salts influence the current-carrying capacity of soil electrolytes and help determine reaction rates over anodic and cathodic areas. Moisture content, pH, oxygen concentration and other factors interact in complex ways, influencing corrosion.

CATHODIC PROTECTION METHODS FOR CORROSION CONTROL

Cathodic protection is a widely accepted method for corrosion control. Corrosion of steel storage tanks with above-ground bottom can be reduced or eliminated by the appropriate application of cathodic protection. Cathodic protection is a technique that makes the entire surface of the metal to be protected act as the cathode of an electrochemical cell. There are two cathodic protection systems:

Sacrificial anodes

Printed current

SACRIFICIAL ANODES

Sacrificial anode systems, using galvanic anodes, which are made of magnesium or zinc in the form of bars, the anodes buried directly in the ground surrounded by a chemical mixture and packed in special bags are installed.

The anodes are connected to the system individually or in groups. Galvanic anodes are limited in their output current by system voltage and circuit resistance. Systems of the printed current type should be used for the cathodic protection of large structures, uncovered or with deteriorated coating.

The number of anodes that are required for the cathodic protection of above-ground storage tanks depends on the total current requirement and the individual average discharge current of the anodes in the soil. When calculating the placement of anodes, the factors that influence the distribution of current over the geometry of the structure must also be considered.

Generally, better current distribution and more uniform polarization are obtained, distributing anodes evenly around the tank or under the newly built tanks.

PRINTED CURRENT SYSTEMS

Printed current anodes should be of materials such as graphite; cast iron with high silicon content; platinized metals; magnetite and mixture of metal oxides, among others. These anodes are installed naked with special filler material (such as pulverized graphite, with high carbon content). They are connected, individually or in groups, to a positive terminal or to a direct current source, by means of isolated conductors. The structure is connected to the negative terminal, from a direct current source.

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