Cathodic Protection - page 2

WHERE CATHODIC PROTECTION IS FEASIBLE. .

In order to be technically feasible, cathodic protection requires that the structure to be protected be electrically continuous and be immersed in an electrolyte of sufficient volume to allow the distribution of current onto the structure.

Electrical Continuity of Structure.

Electrical continuity of the structure to be protected may either be through metallic continuity provided by bolting or welding of the structure. Continuity is often achieved or insured by means of electrical connections installed specifically to insure the effectiveness of cathodic protection. These connections are called “bonds.”

Electrolyte.

The electrolyte is water or the water contained in moist earth. The conductivity of the electrolyte is an important factor in the design of cathodic protection systems.

Source of Current.

Cathodic protection also requires the presence of a source of electrical current at the proper voltage or potential to prevent attack on the structure. These sources of current are called “anodes.” As described below, the anodes can either be fabricated from an active metal, such as magnesium or zinc, which provides a high potential source of electrons through corrosion on its surface or from a relatively inert material. This inert material has the ability to pass current from its surface without being consumed at a high rate, but requires using an external energy source to increase the potential of the electrons supplied to the structure being protected. The systems that use active anodes are called “sacrificial” systems as the anode material is sacrificed to protect the structure under protection. The inert anode systems are called “impressed current” systems as the external energy source is used to impress a current onto the structure under protection.

PRINCIPLES OF OPERATION.

Understanding the principles of operation of cathodic protection systems is based on understanding the nature of the corrosion process. Corrosion is basically an electrochemical process. That is, it is a process where chemical reactions take place through the exchange of electrons. By preventing the exchange of electrons, cathodic protection systems prevent the corrosion reactions that would otherwise naturally occur. The principle of this mechanisms is also the basis of the Galvanic Corrosion.

The Cathodic Protection Cell.

Cathodic protection prevents corrosion by making a metal, which would ordinarily behave like an anode and corrode, behave like a cathode and be free from corrosive attack, the basis of the mechanism of Galvanic Corrosion. In cathodic protection this is achieved by providing electrons of a higher energy level (electric potential) than those which would be produced in the corrosion reaction at the anode. This is illustrated in Fig 1. In order to better understand which metal will be the cathode and which the anode is important to understand what is a Galvanic Corrosion Chart or a Galvanic Series of Metals.

On the right side of Fig 1, corrosion is proceeding. Metal atoms are reacting to form metal ions and electrons. The electrons formed have a certain amount of energy as a result of the reaction. The presence of these electrons in the metal give the metal a characteristic electrochemical potential. On the left side of Fig 1, the application of a potential that is higher (more negative) than the potential that was produced by the corrosion reaction prevents the flow of electrons in the ordinary - to + direction and the corrosion reaction is prevented. The metal that was formerly a location from which electrons flowed (an anode) has been changed to a location to which electrons flow (a cathode) by the application of a negative charge.

Corrosion has been stopped. The potential required for cathodic protection depends on the metal being protected and the environment.