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 whichare 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 andbatteries 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 arereduced, 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
|0 to 1000 ohms-cm
|1000 to 5000 ohms-cm
|5000 to 10000 ohms-cm
|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