Corrosion
Corrosion is the reaction of a metallic material with its environment. In all electrolytes, as for example, in the ground, in river or sea water Metal atoms go into solution as electrically charged ions during the corrosion reaction. This process produces a more or less loss of metal surfaces. The simultaneous movement of charges in electrolyte and electrons in metal, causes a flow of current in the metalelectrolyte circuit.
Anodec reaction Fe→Fe++ +2e-
Why cathodic corrosion protection
Cathodic protection is a method which protects the metal reliably even at undetected coating holidays. The protective current supplies electrons to the structure which is protected. These electrons cover the electron requirements forthe reduction of oxygen which comes into contact with the metal surface
Without cathodicprotection, these electrons would provide the decomposition of the metal. Thus, the potential of the metal surface is sufficiently reduced to prevent the disassociation of positive ions from the metal, where formerly an anodic reaction took place.
The oxygen is reduced by cathodic reaction. The entire surface of the structure protected is thus transformed into a safe cathode, i. e. the metal is „ cathodically „ protected.
cathodic reaction: ½O + H2O + 2e→2OH
Effectiveness of cathodic corrosion protection
A. Electrolyte
water, soil and atmospheric moisture
B. Element >>> element voltage
A voltage can result in different influences, such as
✔ inhomogeneous steel interface
✔ Temperature differences
✔ different electrolyte compositions
✔ Contact between different metals for example: Fe-CU
To suppress this element voltage is the anode and cathode with an insulating material covered, i. e. coated.
Coating materials: Paints, Bitumen, PE insulation
This element voltage is theoretically prevented by the passive corrosion protection. As always damages (Faults) of the passive protection, active protection (CP) must be used in addition. The protection current flows only at the defects in the steel and prevents corrosion.
Priciple of impreced current protection system
Impreced Current Protection
Cathodic protection after the foreign flow process, the required protection current from the Transformer rectifier is supplied and fed on the foreign power anodes in the object.
The CP System consists of four basic components
1. Rectifier for the power supply
2. Impressed current anodes
3. Anode cable of the rectifier to the anodes, cathode cable of the rectifier to the protection object
4. Protection object Advantage
✔ Protection power output can be easily adjusted or regulated
✔ Low depending on the specific resistance of the soil or water
✔ Long range of the protective effect
✔ Low material removal of anodes
✔ Low effert for measuring the OFF potential
Disadvantage
✔ higher investment costs
✔ Possible interfernce of foreign objects
Procedure of cathodic corrosion protection - galvanic anodes
Corrosion protection with galvanic anodes
The galvanic anodes are used for the protection of smaller objects such as short pipelines, storage tanks (oil, gasoline, liquefied petroleum gas):
Magnesium (Mg) • zinc (Zn) • aluminium (AL)
Basic requirements for a CP with galvanic anodes are:
✔ A good conductive electrolyte, soil
✔ Good insulation of the protection object
Advantage
✔ Low installation costs
✔ Lower maintenance costs
✔ No interference on foreign equipment
Disadvantage
✔ Limited power supplied by the given voltage of the used anodes
✔ For low resistive-soil and water
✔ Protection objects must be well insulated