JANUARY 31 2003
PRINCIPLES OF OPERATION
THE CORROSION PROCESS. Understanding the principles of cathodic
protection systems is based upon understanding the nature of the corrosion process.
The corrosion of metals is an electrochemical process. That is, it is an electrical circuit
where the exchange of electrons (electricity) is conducted by chemical reactions in part
of the circuit. These chemical reactions occur at the surface of the metal exposed to the
electrolyte. Oxidation reactions (corrosion) occur at the surface of the anode and
reduction reactions occur at the surface of the cathode. Corrosion control systems
which relocate these oxidation reactions, by making the protected structure a cathode in
a larger corrosion cell, is called a "cathodic" protection system." The cathodic protection
anodes are installed to become the anode in this larger corrosion cell and provide the
location for all oxidation reactions in the cell. To describe the principles of operation of
cathodic protection in detail, the exact nature of the corrosion process must be
described in detail.
is the deterioration
of a material through reaction
with its environment. In the case of a metal, this deterioration occurs mainly through an
electrochemical process. The electrochemical process consists of four distinct parts:
anode, cathode, electrolyte, and metallic path. These four parts constitute what is
called the "corrosion cell". Electrochemical corrosion occurs only when all four parts of
the corrosion cell are present. To understand the operation of a cathodic protection
system, it is extremely important to understand these four parts of the electrochemical
Anode. The most obvious part of the corrosion cell is the anode. This is the
location where corrosion occurs. The anode is the point in a corrosion cell where
electricity is passed by chemical means from the surface of the metal to the electrolyte.
This chemical reaction is an oxidation reaction, which is characterized by the metal
loosing an electron and combining with another element, usually oxygen. In the case of
steel, the resulting material is iron oxide (rust).
Cathode. The second part of the corrosion cell is the cathode. This is the
location where protection occurs. The cathode is the point in a corrosion cell where
electricity is passed by chemical means from the electrolyte to the surface of the metal.
This chemical reaction is a reduction reaction, which is characterized by the metal
passing electrons to the electrolyte.