How Does Cathodic Protection Work?
Cathodic protection (CP) is an electrochemical means of corrosion mitigation. The process minimizes the anodic dissolution of a metallic structure by reducing the electrical potential energy difference between the anodic and cathodic sites on the surface of a metal when placed into a conductive electrolyte. Theoretically, cathodic protection is achieved when the open circuit potentials of the cathodic sites are polarized to the open circuit potentials of the anodic sites. The goal is to make the entire structure a cathode (current receiver) relative to an expendable or replaceable anode (current provider). Specific criteria have been established by the National Association of Corrosion Engineers (NACE) for various metals in different electrolytes. The corrosion engineer must carefully consider these criteria when deciding on the level of cathodic protection to apply to a metallic structure. There are two basic methods of applying cathodic protection, although there are many variations on installing cathodic protection. These two methods are known as sacrificial (galvanic) anodes and impressed current (rectifier) cathodic protection.
Sacrificial Anode Cathodic Protection
Sacrificial anodes are usually made of an alloy of magnesium, zinc, or aluminum. Different alloys are available for various electrolytes such as seawater, soil, or concrete. Each type of sacrificial anode will provide a source of cathodic protection current due to the higher electrical potential energy of the anode versus the structure intended for protection. The Practical Galvanic Series provides guidance on the selection of anodes that can be used to provide cathodic protection for various metals. The selection of an alloy for a sacrificial anode must be made with consideration for the electrolyte in which the anode and the structure are placed. Additional factors are the size and shape of the anode, any protective coating or wrapping applied to the structure, and the intended design life of the system.
Impressed Current Cathodic Protection
An impressed current cathodic protection system uses inert anodes that are powered by an external source of DC current. Anodes can be materials such as graphite, cast iron, mixed-metal oxide-coated titanium, and platinum-niobium-clad metals. Many sizes and shapes of impressed current anodes are available including wire, rods, tubes, sticks, plates, and disks. The composition of the electrolyte is very important when selecting an impressed current anode. The DC current source is usually a rectifier, although other sources of DC current are available, such as solar voltaic energy systems. In this type of system, the anodes are installed within the electrolyte around the structure and are connected to the positive output terminal of the rectifier. The structure is connected to the negative output terminal of the rectifier. Current flow is from the anodes through the electrolyte and onto the structure. The metallic connection from the structure to the rectifier completes the DC circuit. Impressed current systems can have many anode configurations that are dependent upon the electrolyte and the structure to be protected.
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Operating & Maintenance of Cathodic Protection Systems
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Can You Afford to Not Maintain Your CP System?
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NACE International Standard Practice SP0169, Section 10.3 states:
"The effectiveness of the cathodic protection system should be monitored annually."
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