Storage tank systems, metallic pipelines, and other submerged structures of metallic nature are protected from corrosion by employing the electrochemical principle governing galvanic anodes such as the very common zinc anode. The idea lies on the physical phenomenon of electrochemical reaction wherein a dissimilar metal pair submerged in a conductive electrolyte such as seawater tends to form an electrical circuit. On this circuit, the metal with a more active voltage becomes the anode where corrosion would occur; the other metallic component with a less active voltage becomes the cathode. Due to the current originating from the anode to the electrolyte and onto the surface of the cathode, the latter is protected from corrosion.
In practical applications, the galvanic anode is also referred to as the sacrificial anode since it is purposely installed to be sacrificed in order to save the metallic cathode; thus, the latter is said to have received cathodic protection. For the anode material to be effective in the electrochemical system, it must be marginally more active than the cathode material. In most practical cases, the use of zinc anode suffices most applications. Aside from the intrinsic metallic properties checked basically for suitability, the physical dimensions and installation methods as well are important considerations.
Submerged metallic systems prefer the protection offered by galvanic anodes because virtually no power is required. Additionally, installation costs are very minimal since the only crucial requirement is that the zinc anode must be installed near the structure to be protected with enough proximity to form the required electrochemical reaction. As long as the anode is installed properly on the structure, less maintenance is needed on account of the fact that this type of system is not prone to electrical or mechanical malfunction.
Although the cathodic protection system comes with a number of benefits, certain limitations somehow beset the galvanic system due to the naturally small driving potential. In systems where there is a high resistivity area separating the cathode and the anode, galvanic protection is ineffective. Large and poorly coated structures are among the limitations of the system.
Aside from the zinc anode material, magnesium and aluminum are also popular choices for the anode. On applications with high resistivity, magnesium or certain alloy of the metal is preferred due to its highly negative electrical potential. Nonetheless, overly negative potential results to disbanding of the coated surface. On the other hand, aluminum is recommended for applications requiring lightweight structures. However, this metal is easily passivated in the presence of other chemicals particularly chlorine. In conclusion, zinc anode is generally the optimal choice except in high-temperature applications.