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  Cathodic Protection Systems >> Corrosion and Cathodic Protection  
Corrosion and Cathodic Protection


Introduction:   Cathodic  Protection is an electrical  method  of preventing corrosion to metallic structures which are in electrolytes  such as soil or water.  It has widespread application  on underground  pipelines and it has been found effective  on  other storage tanks, lock gates and dams, steel piping, well  casings, ship hulls, water treatment equipment etc.  It operates by  passing  direct  current continuously from electrodes which are  installed in the electrolyte usually known as anodes, to the structure  to be protected. Corrosion is therefore held up when  the current  is of sufficient magnitude and when it is properly  distributed.

Although  the  classic  methods  of  retarding  corrosion,   like coatings, inhibitors, etc., are well known there is some  mystery about  the use of direct current for Cathodic protection.

It  is very  likely that this mystery is a result of unfamiliarity  with the terminology, particularly amongst persons who are responsible for fighting corrosion with familiar methods.  In actual practice Cathodic protection technique is very simple and for a number of applications Cathodic protection has become a routine standard.

The  use  of  electricity for corrosion prevention  will  not  be strange  to those who have an acquaintance with the behavior  of  galvanic  cells, and batteries.  in galvanic cells and batteries direct current is generated by the corrosion of one of the electrodes normally  the anode. Therefore when  direct  current  is properly directed it can be used to prevent corrosion.

Chemistry  of  Corrosion: As stated above when  two  metals  are immersed  in an electrolyte and connected externally by  a  wire, current  will  flow  between  them.  This is  a  simple  case  of electrolysis.   If  one of the electrodes is say copper  and the other zinc, then since zinc is a baser metal it will be dissolved into the solution and it is known to corrode.  This action takes place  because  of potential difference between  the  two  metals according to the called Electro Chemical Force Series.  The baser metal  with  higher negative potential is dissolved or  in  other works  corroded,  and is usually called the  ANODE whereas  the electrode which does not corrode is called the CATHODE

Similarly  if a metallic piece is immersed in an  electrolyte  so that  different  parts  of  the metal are  in  contact  with  the electrolyte of different strength, a difference of potential will be  set up between two parts of the metal and current  will  flow between the two parts of the metal.

Since corrosion reaction is electrochemical by nature  and  if  corrosion currents are reduced,  naturally  the amount of corrosion will also reduce.  This is done by making the structure   to be  protected  most  negative  with  respect to surrounding  soil  or water. The current is passed  through  the electrolyte  from an artificial electrode made up of   cheap  and replaceable material.

Since the structure is made negative or cathodic, this method  of protection is called cathodic protection. Corrosion will then be transferred to the renewable material of the anode called  ground bed  and  is completely prevented at the  structure because  the corrosion currents are nullified by cathodic protection  current.

This is explained as shown in figure `1'.

To understand the mechanism of cathodic protection the  corrosion is  represented by a simple electric circuit figure `B'.  Ec  and Ea are the potentials  of the cathodic and anodic areas  measured relative  to  a standard  reference cell.  Rc  and  Ra are  the resistances associated with the electrolyte close to the cathode and anode respectively and Rm is metal resistance.  Applying Ohms Law the current flowing will be:

 (Ea  -  Ec)   /  (Ra + Rc + Rm)

If the resistance Rm is neglected Electrical Circuit Diagram of   Cathodic  protection System can be represented by fig `C'. if Ex, Rx,  and Ix are the parameters of the external source, it can be proved that 

Ea = Ec + Rc  I x  Or 

I x = (Ea - Ec)  /  R  for I a = 0

when corrosion stops.

Thus it is possible by an externally impressed current to prevent corrosion  by  reversing the voltage of  the  anode.   In  worst conditions  encountered in practice, anode potential is found  to be  less negative than -0.8 volts.  It is normally accepted  that to  prevent  any current flowing from  the  structure  into  the electrolyte, the steel should have a potential more negative than  -0.80  V.  usually this is kept at -0.85 V. In anaerobic  conditions  and in water logged conditions sulphate reducing  bacteria have  a depolarizing action cased structure potential of -0.95  is accepted as satisfactory.

The  power  for cathodic protection is made  available  from  two sources, one  is from metals so placed  in  the  electrochemical series that they are baser to the metal to be protected, such  as Mg,  Al and Zn.  They produce currents protection by forming  two electrodes  of a primary cell.  This is called Sacrificial  Anode System.   Since these anodes have limited life,  replacement  and maintenance cost is heavy and hence have limited applications.

The  other  source for power is by Impressed  Current  System  in which low voltage D.C. Currents are made available usually  from rectifiers.  The  anode or ground bed is  provided  by  graphite blocks.  They  are  impregnated with resin or wax  to  give  them proper bond and are surrounded by carbonaceous backfill.

We have seen that the protective current for cathodic  protection as:-

                     I x   =  (Ea - Ec) /  Rc

Thus  if  Rc  is  very high then the  current  required  will  be proportionately  less.  This is achieved by applying  coal  tar enamel  coatings wrapped with fibreglass to the pipes  when  they pass through corrosive soils.  In fact cathodic protection is the compliment  to and not a substitute for a good  quality coating. It  is found in practice that a  bare, uncoated pipe requires  50 times  the  protective current  that would  be  required  for  a pipeline having a good coating.

Soil  Resistivity  And Corrosivity:

Soil Resistivity in Ohms-cm Corrosivity
0 to 1000 ohms-cm Highly corrosive
1000 to 5000 ohms-cm  Corrosive
5000 to 10000 ohms-cm Mildly Corrosive
Above 10,000 ohms-cm Generally not corrosive

Applications of Cathodic Protection

This  can  be  applied on all  structures  when  continuously immersed  in water or permanently buried in soil.  Oil,  gas and  water  pipes, wharves, jetties,  tanks, tankers,  steel pilings, ships hull, heat exchangers, lead sheathed  cables, etc  are  but a few cased in steel  industry  where  cathodic protection  has  been  used with large measure  of  success.  Water and gas boards, oil industries, ports and sewage plants are large users of this system.


YOUR PIPELINE can be saved from corrosion attacks only by providing Cathodic Protection System with a design life of 20 years or more.

GET STARTED with Cathodic Protection System to prevent corrosion to your pipeline

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