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.
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.
Graphitization is a corrosion phenomenon
associated with grey
cast iron. This material consists
essentially of flakes of graphite in
an iron matrix. The iron constituent corrodes
out because of the same multitude of
corrosion reactions that affect ferrous metals,
leaving behind the graphite and some
Cathodic protection expert John Fitzgerald uses an
experience early in his career to illustrate the
consequences of not heeding the advice of corrosion
professionals when designing and implementing
corrosion control systems—many times because of
reluctance to bear the expense. Cutting corners to
save money up front can lead to unexpected failures
and expensive repair and replacement in the future.
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.
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.
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).
There are times when it is necessary to use impressed
current cathodic protection (ICCP) in urban areas,
in spite of concerns about extensive interference. This
article describes three ICCP systems that can be used
safely in urban environments. They are distributed
anode (parallel) groundbeds, deep anode
groundbeds, and low-output surface groundbeds.
Being a cathodic protection corrosionist is a lot like
being a detective. It is necessary to examine all the
evidence when solving a mystery. Sometimes it is
necessary to establish a stakeout. That’s what was
done to find the source of intermittent shorts in this
incident. Careful use of instrumentation, data
analysis, and observation solved the problem.