UFC 3-570-06
JANUARY 31 2003
ANODE
CATHODE
An electrode where oxidation reactions
An electrode where reduction reactions
(corrosion) occurs
(protection) occurs
Antonym - Cathode
Antonym - Anode
2-1.1.3
Anode/Cathode Relationship. An electrode becomes either an anode or a
cathode in an electrochemical corrosion cell depending upon its electrical potential
compared to the other electrode. This electrical potential difference is the electromotive
force of the cell and is the voltage difference between the anode and the cathode. The
electrode which is more electrically active, or more negative in voltage, undergoes the
corrosion, so by definition is the anode. The electrode that is more noble (less negative
in potential) passes electrons to the electrolyte (reduction reactions) and by definition is
the cathode and does not undergo corrosion (oxidation reactions). As previously
discussed, there are four distinct parts to the electrochemical corrosion cell, all four
parts must be present for a complete circuit to exist and for current to flow (corrosion to
occur).
2-1.1.4
Electrolyte. The third part of the corrosion cell is the electrolyte. The
electrolyte is the location where ions flow. The electrolyte is any material in contact with
both the anode and the cathode that will allow ions to migrate. The electrolyte is the
part of a corrosion cell which allows oxidation and reduction reactions to occur. The
electrolyte includes the source of elements or atoms that are required for ion transfer to
and from the metal electrodes (anode and cathode).
2-1.1.5
Metallic Path. The fourth part of the corrosion cell is the metallic path. The
metallic path completes the circuit and allows the electrons to flow. The metallic path is
any metal that contacts both the anode and the cathode and allows electrons to flow.
This electron flow must be present for electrochemical corrosion to occur. In the case of
a tank or pipeline, this can be the tank or pipe itself, or it can be a metallic bond to
different metallic structure.
ELECTROLYTE
METALLIC PATH
Any soil or liquid adjacent to and in contact
Any conductor that allows electrons to flow
with the anode and the cathode that allows
from the anode to the cathode
ions to migrate (flow)
Again, all four parts of the corrosion cell must be present for electrochemical
corrosion to occur. When all four parts are present, a closed circuit condition exists,
and current will flow through this circuit. Corrosion only occurs at the anode of the cell,
which is where the oxidation reactions occur. A familiar example of the corrosion cell is
the common dry cell battery. In this case, two different metals, one being an anode and
2-2
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