CATHODIC PROTECTION (CORROSION)

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 CATHODIC PROTECTION 

It is a method of preventing a metallic object from corrosion. It is an electrochemical process in which the metal which is to be protected is forced to act as a cathode.

Corrosion occurs only in the anodic area. Therefore the entire surface of the metal can be turned into a cathode, then corrosion can be prevented. In this case, corrosion is not actually eliminated but moved to a known location by adding an anode, which undergoes slow and controlled corrosion.
There are two main methods of applying  cathodic protection:
  1. Sacrificial anode protection and
  2. Impressed current cathodic protection

SACRIFICIAL ANODE PROTECTION

In this method, the metallic structure which is to be protected is connected to a more anodic metal through a wire, so the entire corrosion is concentrated on the anodic metal, and the metallic structure is protected sacrificially. Anode metals commonly used for this are Mg, Zn, Al, and its alloys.
 
They are based on the position of the metal in the galvanic series. Metals at the top of the series are more prone to corrosion. If we connect one of the metals at the top of the galvanic series with one at the bottom, the top metal will corrode or be sacrificed in preference to the bottom metal. 

Sacrificial anode systems are more reliable than the impressed current systems because a lesser quantity of the component is destroyed and no power supply is required. Sacrificial anode system consists of mainly

  1. The anode is usually a solid block of Mg, Zn, or Al. 
  2. Cable connection to the structure
  3. Test facility to monitor anode current output.
  4. Isolation of material between above or below ground structure.

Applications

Protection of buried pipelines, underground cables, marine structures, ship bodies, open water tank coolers, condensers, etc.

IMPRESSED CURRENT CATHODIC PROTECTION

It is developed to protect a structure that is immersed in a high resistivity electrolyte such as freshwater or sandy soils and for structures where a large amount of current is required.

The impressed current system utilizes a power source, usually a DC source to drive an electric current into the structure. ‘As a result of corrosion an electronic flow occurs from the anodic region to the cathodic part, which is known as corrosion current. An impressed current is applied in opposite direction to neutralize corrosion current and convert the corroding metal from anode to cathode. 

Impressed current is derived from a battery or DC source with an insoluble anode like cathode or silica. It consists of the following parts.
  • A power source, usually a rectifier unit, solar power, or any other DC power source.
  • Anodes like graphite or silica 
  • Cabling and isolated material between above/below ground structures.

Application

  • Open water box, coolers, water tanks, buried pipelines, condensers
  • Concrete buildings, bridges, and similar structures
  • Vessels, pipelines, and tanks are used to store or transport liquids.
  • Marine cathodic protection covers many areas jetties, harbors, offshore structures
Location: United States

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