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MARCH 2012 / Issue #01

Cathodic Protection

Definition of Terms (as per AWWA Manual M42):

  • Principles of Cathodic Protection: CP systems are used to prevent the corrosion that would naturally occur in a steel water tank. These systems prevent or slow corrosion by altering the electrochemical environment so that the submerged tank shell becomes the cathode of a corrosion cell. When the cathode of a cell does not corrode, the submerged metallic tank shell is protected.
  • Nature of Corrosion: The corrosion of steel in aqueous solutions is an electrochemical process in which a current flows and chemical reaction occurs. A corrosion cell has four basic elements: anode, cathode, electrolyte, and closure path. The anode is the metal that will corrode, that is, metal ions leave its surface and enter the electrolyte solution. The cathode is a metal from which no metal ions enter the solutions. The electrolyte may be any solution, such as drinking water that is capable of conducting electricity. The closure path, also called the return current path, is the electrical conductor, usually metal, that connects the anode and cathode together. If any one of these elements is missing, corrosion does not occur. For example, coating stops corrosion from occurring by providing a barrier to the current flowing between the metal and the electrolyte.

Common Cathodic Protection types:

  • Galvanic or sacrificial anodes
  • Impressed current system
  1. What is corrosion?
  2. Corrosion is a normal occurrence common to all metals not in their natural state. The process itself is electrochemical in nature, very similar to a battery, where electrons flow between anodic and cathodic sites on metals. For corrosion to occur four elements are required:

    • Anode – site where corrosion occurs and current flows from.
    • Cathode–site where no corrosion occurs and current flows to.
    • Electrolyte – a medium capable of conducting electric current (i.e. soil, water or concrete).
    • Metallic Path – connection between the anode and cathode.

  3. What is cathodic protection?
  4. It is defined as the reduction or elimination of the corrosion process, by making the corroding metal a cathode via an impressed direct current, or by connecting it to a sacrificial or galvanic anode.

  5. What is Galvanic cathodic protection?
  6. A sacrificial or galvanic anode system uses a more reactive metal (anode) such as aluminum, zinc or magnesium, to create a current flow. Direct current is generated by the potential between the anode and the reinforcing steel that it is connected to. The sacrificial anode will corrode during the process and is consumed.

  7. Is there any maintenance to the cathodic protection system?
  8. Once the CP system is installed it is necessary to provide routine operation and maintenance. This involves visual inspection of the system and periodic checks. Electrically continuity is checked periodically with a multi-meter and the corresponding voltage is recorded. If no voltage is recorded, the CP is no longer operating and the sacrificial anodes must be replaced.

  9. What are the cost savings?
  10. Cathodic Protection Systems can save a tank from total corrosion and replacement that can occur in as little as five to seven years. By providing considerable technical and economic benefits over traditional, remedial approaches CP will have paid for itself by preventing future repairs or total replacement of storage tanks.

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