PRINCIPLE OF CATHODIC PROTECTION
Corrosion is a natural process of a metal back to its original state of ore.
The best known example is the rusting of steel. Corrosion is an electrochemical process that occurs in an electrolyte (water, soil, etc.) and resulting in the deterioration of anodic areas to the benefit of cathodic areas on the surface of the metal.
Anodic and cathodic areas are at different potentials due to the heterogeneity of the metal or the electrolyte. Other phenomena such as stray currents can create anodic and cathodic areas on the same structure.
If the metal to be protected is made more negative than a certain value (criterion of -850 mV/CuSO4Cu), corrosion becomes negligible.
The external current to be applied for polarization can be generated by two methods galvanic anodes or impressed current.
Galvanic anodes: Galvanic or sacrificial anodes produce a current that will move from the anode to the cathode (structure) within the electrolyte.The metals used should be more electronegative than steel such as magnesium and Zinc for soils and aluminum in seawater.
Impressed current: The current is supplied by a DC generator such as a transformer-rectifier. The negative pole is electrically connected to the structure to be protected and the positive pole of the anodes such as graphite, ferrosilicon or Timmo anodes.
The potential criterion can be more negative than -850 mV when the metal is exposed to sulphate-reducing bacteria or less negative depending on the soil resistivity.
Any metallic structure buried or immersed in a conductive electrolyte (water, sols, etc) can be protected.
The applications of cathodic protection are : pipelines, ships, marine structures, steel in concrete, internal surfaces of tanks, etc...
- Steel pipelines (Oil, Gas, LPG, etc).
- Steel water lines
- Underground storage tanks
- Tanks bottoms
- Marine structures in steel