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Types of Cathodic Protection Systems

1/11/2013 12:34:13 PM | by Anonymous

Cathodic Protection Systems

Cathodic protection system is a method of preventing corrosion of metal structures. This usually applies to metal, commonly steel, which easily corrodes when constantly exposed to air and water, a process called oxidation reaction. The reaction occurs when the metal got in contact with water that has any type of salt dissolved in it and caused it to release electrons. In order to protect metal from rust, cathodic protection is applied. The metal is protected by attaching another metal that corrodes more quickly. The second metal is commonly referred to as sacrificial anode patterned to its role on the corrosion control process. It sacrifices itself by corroding first preventing the more expensive metal from rusting. The anodes to use should be more positively charged than the object being protected for it to effectively serve its function. The most commonly used metals as sacrificial anodes are magnesium, zinc and aluminium.

 

There are various applications where cathodic protection systems are utilized such as in protecting fuel pipelines, ship, home water heaters and storage tanks from corrosion. The type of technique employed for these structures may differ as the method applied on ships may not be suitable for underground pipes. The main types of cathodic protection systems are galvanic protection and impressed current systems.

 

Galvanic Protection

Galvanic protection is one of the two main methods of cathodic protection system. It is the process of preventing corrosion by attaching a more positively charged metal. The metal or the anode will serve as an alternative source of electrons. The anode corrodes when it loses electrons and transfers them to the cathode or the main structure. The best example where galvanic protection applies is on ships. Ships are partially submerged making it one of the most prone materials to rust. Installing pipes is a complex task so they are protected to make sure they last for a long time. Pipes buried underground are protected by placing a bundle of metal next to it so electric charge will first attack the sacrificial anode. Since sacrificial anodes corrode, there will be a time when it will completely dissolve and no longer provides protection. To ensure that there is still an anode functioning properly, the galvanic protection has to be tested every two to four years.

 

Impressed Current Systems

Impressed current systems are applied at times when galvanic protection is inadequate. This method is similar to galvanic protection except for the fact that an external electrical current is provided through a power supply. It gives more lasting protection as the anode does not rely to its environment for it to react. This way, the life of the anode lasts longer for it becomes less vulnerable to corrosion and damage. Impressed current systems are done by connecting a power supply to the material, for an instance long pipelines being protected and a positive electric current to the sacrificial anode. The distance of the anode from the pipe is determined by a civil or design engineers. They will need to check the soil condition and run tests as the special metal bars or anodes are buried under the ground. This type of cathodic protection systems can provide longer protection to the cathode compared to galvanic protection.

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