Cathodic Protection

Stray Current Effects

Before preparing a cathodic protection (CP) design, the possible presence of stray currents must be considered. Stray currents are defined as those which follow a path other than the one intended. Where stray currents discharge from a structure into the electrolyte environment in order to return to the source, corrosion will occur.

Effects of Coating on Corrosion and Cathodic Protection

The four basic elements of a corrosion cell are an anode, a cathode, and the metallic and electrolytic pathways between them. Corrosion control can be achieved by eliminating (or reducing) any of these elements. One such method is to modify the electrolytic pathway by introducing a barrier between the threatened metal surface and the corrosive medium (i.e., by applying some kind of coating).

Cathodic and Anodic Protection

A metallic structure in contact with an electrolyte (typically soil or water) usually includes anodic sites, where oxidation (corrosion) occurs, and cathodic sites, where reduction (protection) occurs. Cathodic protection (CP) is a technique to reduce the corrosion of a metal surface by making that entire structure the cathode of an electrochemical cell—that is the derivation of the term. This is typically accomplished by discharging current from an external anode so that current will flow through the electrolyte to, instead of away from, the original anodic sites on the structure surface.

Polarization

As is the case with other chemical reactions, the driving force of a corrosion reaction is related to the difference in energy between an initial equilibrium that is higher in energy than the final equilibrium. As corrosion action proceeds, this difference in energy tends to decrease as a result of the effects of the products of anodic and cathodic reactions in the vicinity of the corrosion sites. The cathodic reaction, and with it the overall corrosion reaction, would slow down if, for example, the hydrogen product of the cathodic reaction were not removed by evolution as gas or some reaction involving oxygen. This slowing down is said to be the result of cathodic polarization.

Principles of Electrochemistry Applied to Corrosion

Although corrosion can take several forms, the mechanism of attack in aqueous environments involves some aspect of electrochemistry. There is a flow of electricity from certain areas of a metal surface to other areas through a solution capable of conducting electricity, such as seawater or fresh water. The term anode is used to describe that portion of the metal surface that is corroded and the term cathode is used to describe the metal surface from which current leaves the solution and returns to the metal.