What is Cathodic Protection?
Cathodic protection(CP) is a method of corrosion control that can be applied to buried and submerged metallic structures.
It is normally used in conjunction with coatings and can be considered as a secondary corrosion control technique. The primary corrosion control method on any given structure is normally a coating system which can be between 50 and 99% efficient depending upon age, type, method of installation, etc. A properly designed and maintained cathodic protection system will take up the remainder resulting in a 100% efficient corrosion protection system.
Simply CP works by preventing the anodic reaction of metal dissolution occurring on the structure under protection.
Corrosion occurs at the anode as this is where electrons are released. In order to complete the electrical circuit the electrons must flow to the cathode, as per the illustrations below:
In the illustration below the principle of corrosion is demonstrated on a buried metallic structure:
Two different types of Cathodic ProtectionThere are two methods of applying of cathodic protection and these are:
- Impressed current cathodic protection (ICCP); and
- Sacrificial (or galvanic) anode cathodic protection (SACP).
The main difference between the two is that ICCP uses an external power source with inert anodes and SACP uses the naturally occurring electrochemical, potential difference between different metallic elements to provide protection.
What structures can it protect?
Cathodic protection can protect all types of buried and submerged metallic structures including:
- Cross country pipelines
- In plant piping
- Above ground storage tank bases
- Buried tanks and vessels
- Internal surfaces of tanks, vessels, condensers and pipes
- Well casings
- Piling – tubular, sheet steel and foundation
- Marine structures including jetties, wharfs, harbours, piers, platforms
- Reinforcing steel in concrete
The cathodic protection system can be designed to prevent both oxygen controlled and micro- biologically controlled corrosion.