Basic Principles of Corrosion and its Prevention by cathodic Protection

Basic Principles of Corrosion in a bi-metallic structure and in single metallic structure  :

Corrosion is basically a deterioration of materials usually metals from any part of its surface when in contact partly or fully with soil or water by way of an Electro- chemical reaction. Electro-chemical corrosion of metallic parts of process equipment is a process  in which the metal surface is in contact with an electrolyte which may be a film of moisture containing dissolved salts, ( example: atmospheric corrosion), or may be the whole or part of the surrounding medium, e.g. when metal is immersed in fresh water or seawater or buried in the soil (under ground pipe line). In the last case, the electrolyte is the water in the soil,containing dissolved salts.
Corrosion in presence of bi-metallic structure:
When two metals having different energy levels or potentials are coupled together, current will flow. The direction of positive current flow will be from the metal with the more negative potential through the soil to that which is more positive. Corrosion will occur at the point where positive current leaves the metal surface.
 Different metals have different potentials in a specific electrolyte.
Corrosion in single metallic structure:
The same metallic structure, when placed in an electrolyte (e.g. soil) can develop differences in potential as a result of metal grain composition, milling imperfections, scratches, threads, etc., being exposed. Those portions will usually be, anodic to the remainder of the surface and will corrode.
Corrosion in same metallic structure can also occur due to differences in the electrolyte. These differences may be in the soil resistivity, oxygen concentrations, moisture content and various ion concentrations. The variations produce current flow from one location, through the electrolyte, to another portion of the same metallic structure.
 At the surface of single metal corroding in an electrolyte, there are anodic and cathodic areas, which have small differences in potential. They form active electrochemical cells in which current flows from the anodic areas into the electrolyte, and from the electrolyte into the cathodic areas .Corrosion takes place at anodic area where dissolution of metal takes place.

 The corrosion cell
At anodic locations of metal part, positively charged metal ions migrate the metal surface, where as at cathodic locations,electrons leave the metal surface. Thus, corrosion takes place at the anodic locations

where metal ions react with the electrolyte to form the typical corrosion products. The basic
electrochemical reaction is:

Fe à Fe⁺⁺ + 2e^-

At the cathodic areas, dissolution of metal does not take place, but reduction reactions
occur in the electrolyte. Depending on the pH and presence of oxygen, the basic
electrochemical reaction can be:

2H ⁺ + 2e^-     à  H₂     or    ½O₂ + H₂O +2e^-    à 2OH^-

At the anode the electrochemical current leaves the metal surface and at the cathodic
areas the current enters the metal surface.As the reaction involves the flow of electrons, the reaction rate can be expressed as an electric current. The path of the current will be from the corroding metal, through the electrolyte (soil) to the non-corroding metal and then back through the connection (conductor) between the two metals. The corroding metal is the one where the current leaves to enter the electrolyte and is called an anode. The metal that receives the current is called the cathode.

Cathodic Protection

It has been established that electric current can generate corrosion, corrosion, in turn can generate electric current. As indicated by these phenomena, it has become possible to prevent corrosion by use of electrical current. This, then, is the basis for cathodic protection. When direct current is applied with a polarity which opposes the natural corrosion mechanisms, and with sufficient magnitude to polarize all the cathodic areas up to the open circuit potential of the anodic areas, corrosion is arrested.
The theoretical considerations indicate that the basis for cathodic protection is relatively simple not difficult to understand. However, practical designs for various applications can vary considerably based on the type of structure to be protected and the conditions encountere
There are basically two methods of applying cathodic protection. One of these methods makes use of anodes which are energized by an external DC power source. In this type of cathodic protection system, anodes are installed in the electrolyte and are connected to the positive terminal of a DC power source and the structure which is to be protected is connected to the negative terminal of that source. Because the power source is almost always a rectifier unit, this type of system is often referred to as a rectifier or impressed current cathodic protection.
The second method of protection makes use of galvanic anodes which have a higher energy level or potential with respect to the structure to be protected. These anodes are made of materials, such as magnesium or zinc, which are naturally anodic with respect to steel structures and are connected directly to these structures.
In most cases, the impressed current  type system is designed to deliver relatively large currents from a limited number of anodes, and the galvanic anode type system is designed to deliver relatively small currents from a large number of anodes. Each method of applying cathodic protection has characteristics that make it more applicable to a particular problem than the other.
Regardless of the type of system used, current flows from the cathodic protection anode through the soil to the structure to be protected. Where this current flows onto a structure from the surrounding electrolyte (soil), the potential of the structure is made more negative. Cathodic protection is achieved when this change in potential is sufficient to arrest corrosion.

It would appear that cathodic protection can be achieved merely by the application of current of sufficient magnitude. Although this statement is true, it is deceptively simple because there are very large differences in the design of cathodic protection systems. These differences result from the infinite variety of structures that are to be protected and from the large assortment of environments in which those structures are located. Because of the large differences in the designs of systems necessary to achieve protection, it is often necessary for existing structures that each system be custom designed for a given location.
In order to prevent corrosion using cathodic protection, current must flow from the electrolyte onto the structure at all locations. If a portion of the structure does not receive current, the normal corrosion activity will continue at that point. If any of the cathodic protection current picked up by the structure leaves that structure to flow back into the electrolyte, corrosion will be accelerated at the location where the current is discharged. As an example, when mechanically coupled piping is used, this can be discontinuous from one pipe section to the next. If a galvanic anode type system is used for protection, it may be necessary to install an anode on each pipe length or to electrically bond across each joint. If one length of pipe is neglected, that length will receive no cathodic protection and the normal corrosion activity will continue. When a rectifier type system is installed on an underground storage tank system, it is even more important that the tank and lines be electrically continuous. If there are non-continuous joints, it is possible for the cathodic protection current to leave the pipe or tank to flow around the electrically discontinuous joint causing corrosion at each point where the current leaves the pipe surface. Similarly, if cathodic protection current is applied to one structure in an area, it is possible for other structures in the neighborhood to be exposed to damage unless proper steps are taken. Potential measurements are used to determine whether such damaging exposure exists. Just as protection is indicated when the potential of a structure is made more negative, stray current corrosion is indicated when the potential of a structure is made less negative as a result of the application of cathodic protection current.


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