CORROSION AND PROTECTION

CATHODIC PROTECTION

Principle          Design Criteria              Current Demand      Over and Under Protection            ICCP           SACP

Modeling and Software for Cathodic Protection

Design Criteria

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

 

 

Cathodic Protection Fundamental

When a steel structure is in contact with an open aqueous environment or in underground soil , an electrochemical cell is formed with following anodic and cathodic reactions. 

Fe = Fe++  + 2e        …….. ……..1.1        Ea  - Half cell potential 

[O] + H2O  + 2e = 2OH-   ……… 1.2       Ec - Half Cell potential

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.

 

Overprotection and Underprotection

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- H2 + 2 OH-

 

Current Demand

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.

 

Sacrificial Anode Cathodic Protection (SACP)

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.

 Life of the structure 

Mg Anode             Mg anode in Backfill bag

     

L=(W*C*U)/(8760*I )

 Where
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      Al                                                         
                                                            
 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        8                               
                                                      

 

Impressed Current Cathodic Protection (ICCP)

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. 

Graphite Anode                             Backfill

  

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.

 A 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 transformer        
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.