Criteria for Protected Potential
The fundamental criteria for cathodic protection is to cathodically polarize the structure to a potential of –0.80V vs. silver-silver chloride electrode or more negative potential. Criteria for Cathodic protection recommended by National Association of Corrosion Engineers (NACE) are given in table .
The ideal potential for cathodic protection of steel structure would be –850 to –950 mV vs. Cu/CuSO4 . Above this level it is over protection and at a potential over –1100 mV , increased overprotection with coating disbondment and risk of hydrogen embrittlement will occur
The anodic reaction 1.1 which is the corrosion reaction of steel structure release electrons which are consumed by the cathodic reaction 1.2 so that net charge is zero. Now if any of the reactions cathodic or anodic are discouraged, the rate of corrosion is reduced. The principle of cathodic protection is to pump electrons to the steel structure so that anodic reaction 1.1 is forced to move to the backward direction and hence rate of corrosion will decrease. This can be achieved by cathodically polarization of the structure by supplying a current under a applied potential. Fig1 shows the polarization to find the required applied potential and current to bring down the corrosion rate from Icorr.
if protected potential at the point A (fig.1 ) on a pipe, the potentials at B decreases and at C it further decreases below the required potential. If the potential at A is increased, the region around may well be protected, but is underprotected at C and overprotected at A.
The problems with overprotection are
(i) Higher than necessary current and anode consumption
(ii) Damage of protective coatings, if any
(iii) Hydrogen embitterment due to initiation of a second cathodic reaction .
2H2O + 2 e- à H2 + 2 OH-
Current demand I is the current required to cathodically polarize the surface area of the structure under corrosive environment
to above protected potential Ep, as described above.
I= I1xA1 + I2xA2 + I3xA3 + ........
where A1, A2 , A3 etc parts of the surface area of the structure in 3 corrosive environments , adjacent to each other
for example a submerzed pipe line may be passing through 3 zones of soil of different resisitivity or the pillar of an off shore structure may be submerzed in sea bed, deep water and tidal water , having 3 different corrosion tates.
I1, I2, I3 are the corresponding current density in the 3 zones that required to polarize the parts of the structure above protection potential. This is to be found out from experimental polarization curve as shown in Fig1 above.
Current generated Ic from the cathodic protection system is given by ohm's law
Ic=(Esteel-Ea)/Ra where Ea and Ra are the potential and resistance of anode, Ra is determined from Dwight's formula
The criteria for satisfactory CP is that Ic shold be greater than current demand I.
Material like Zn , Mg, Al and their alloys can generate galavanic current to polarize the structure to above
protection potential and hence can protect the structure without any external power supply. But they thselves corrodes and get degraded with time thus sacricfice to protec the structure. In this case life of structure is dependent on the amount of and shape of sacrificial anode.
L = years of life
W= anode weight in Kg.
C = energy capability in amp-hrs per Kg.
I = current output in Amps.
U = Efficiency factor as a decimal
8760 = hours in 365 days
Property Mg Zn
C amphr/kg 2200, 810 2000
Ecorr v,SCE -1.68 -1.1 -1.05
Efficiency 0.5-0.6 0. 9 0.9
Density g/cc 1.7 7.1
For larger structures, Impressed Current Cathodic Protection (ICCP) systems are more common because sacrificial anodes generally will not economically deliver enough CP current to protect pipelines longer than several several dozen kilometers.
Anodes for ICCP systems may be may be Silicon-Cast Iron, Mixed Metal Oxide, Graphite, Platinum or Titanium coated alloys. Silicon-Cast Iron anodes are the most economical, but also crack easily.
typical ICCP system for a long distance pipeline
would include an AC (or in some cases solar)
powered rectifier with a maximum DC output between
10 - 50 A at 50 – 100 V. Typical protection spans for ICCP
anode groundbeds (also called anode boreholes) are
25-50km. The higher potentials from the ICCP
rectifier require the ICCP anode groundbed to be seprated
further away from the pipeline to reduce the ground potential
rise near the pipeline. Typically, ICCP anode to pipeline
separation is 80 - 150m. ICCP for tanks use lower anode
potentials and typical separation from the tank base may
be anywhere from 8-100m. The approximate minimum
separations are be calculated by the CP engineer based
on soil resistivity profiles at the CP station area.