Cathodic Protection of Above Ground Storage Tank Bottoms

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  • 8/12/2019 Cathodic Protection of Above Ground Storage Tank Bottoms

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    Cathodic Protection of Above Ground Storage Tank Bottoms

    The Principles of Cathodic Protection

    Coating the storage tank is the first line of defense against corrosion. Cathodic -protection is the second line of defense.Cathodic protection is a proven method of controlling corrosion of buried, partially buriedand submerged metallic structures. This is done by making the tank a cathode.

    A properly designed, installed and operated cathodic protection system will eliminatethe corroding areas by passing direct current to the metal surface. The direct current isdischarged from electrodes installed in the electrolyte adjacent to or near thestructure being protected. In this manner, corrosion is arrested when the direct currentis of sufficient magnitude and is properly distributed over the entire surface to beprotected.

    Types of Cathodic ProtectionThere are two basic types of cathodic protection.

    alvanic anode system.

    Impressed current system.

    Galvanic anode system:alvanic anode protection is often called !sacrificial" because the anode is thought of as!sacrificing" itself to protect the structure. This type of protection utili#es a galvanic cellconsisting of an anode made from a more active metal than the structure. The anode isattached to the structure, either directly or, to permit measurement of the anode output

    current, through a test station.Anode materials which are commonly used for galvanic anode systems aremagnesium, #inc and aluminum.

    Advantages:

    o $o e%ternal power re&uired.

    o 'ase of installation (and relatively low installation costs).

    o *ittle maintenance.

    o *ittle chance of cathodic interference.

    o +ystem is essentially self-regulating.

    o elatively low risk of overprotection.

    o elatively uniform potential distributions.

    Limitations:o *imited current and power output.

    o ossible high replacement costs

    o $eed for electrical isolation of protected structure

    o Anodes can increase structural weight if directly attached to a structure.

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    mpressed current system:Impressed current protection provides dc from a power source. The anodes areinstalled in the soil or water and are connectedto the positive terminal of a .C. powersource such as a rectifier. The structure to be protected is connected to the negative

    terminal of the power source and electrical current is forced to flow from the positiveterminal to the anodes through the electrolyte to the structure.This type of cathodic protection system uses long life anode materials such as

    high silicon chromium cast iron, graphite, and mi%ed metal o%ide coated titanium .

    Advantages:o Adjustable output.

    o *arge current available.

    o Applicable to poorly coated or bare structures.

    o Applicable to non isolated structures.

    o *ow-cost method of replacing spent galvanic anodes.

    Limitations:o Constant power re&uired.

    o /ore maintenance, inspections, and recordkeeping than with galvanic systems.

    o '%perienced electrical personnel may be needed for installation.

    o ossibility of cathodic interference.

    ConclusionCathodic protection for corrosion control of tank bottoms has been applied using avariety of approaches with varying degrees of success. The use of galvanic anode

    system is typically limited to those applications where the tanks are of small diameterand where they are electrically isolated from other underground metallic structures.Impressed current systems have been used where more protective current is re&uiredand where a longer system life is desired. /oreover, the e%ternal cathodic protection ofthe tank is impressed current cathodic protection and the internal cathodic protection ofthe tank is galvanic anode cathodic protection.