Cathodic Protection

   

      Principle              Criteria for Applied Potential         Overprotection and Underprotection

 Computer Modeling of cathodic protection             Back to Material Science    

 

 

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  Eappl  as shown in polarization diagram fig.1.1.

 

Overprotection and Underprotection

 

 

 

 It is seen that from applying a potential Eappl  ,the corrosion rate can be brought down from icorr  to icorr1 by supplying a current Iappl . By applying a higher and higher potential than Eappl , corrosion rate can be reduced much lower than icorr1 . But at much negative potential beyond Ep., another cathodic reaction occurs

                    H2O + 2 e- > H2 + 2OH-   

One of the major problems that people face while designing the cathodic protection is the applied potential or current changes from the point its of application over the surface of the structure. While the distributed anode arrays on space frame such as production platform tend to provide relatively uniform polarization, the one-dimensional pipe gives rise to problem of potential attenuation with increasing distance. Thus the portion of the surface furthest from the anode gets under protected and the portion of it closest to the anode gets overprotected, giving rise to undesirable other reactions. 

Thus 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-  

 

Criteria for Applied Potentia

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

Criteria
Structure
Reference
Cathodic polarization of the steel structure to a potential of  0850 mV vs. Cu/CuSO4  or more negative potential with IR present
 Buried steel & C.I.
NACE standard RP0169-83
Off shore Pipe lines
NACE standard RP0675-75
Cathodic polarization of the steel structure to more than 300 mV to corrosion potential with IR present
 Buried steel & C.I.
NACE standard RP0169-83
Off shore Pipe lines
NACE standard RP0675-75
Cathodic polarization of the steel structure to more than 100 mV to corrosion potential with IR present
 Buried steel & C.I.
NACE standard RP0169-83
Off shore Pipe lines
NACE standard RP0675-75
0850 mV vs. Cu/CuSO4   in aerobic environment
Buried steel & C.I
British Standard CP1021-1973
0950 mV vs. Cu/CuSO4   in anaerobic environment
Buried steel & C.I
British Standard CP1021-1973

 

 

Prepared by S Paul