UFC 3-570-06
JANUARY 31 2003
present, they may be reduced (gain their electron(s) back) or deposited. The metal
does not become an ion, does not lose an electron, and cannot combine with another
atom or element (oxidize or rust). Although the cathodic reaction must occur for the
corrosion reaction to proceed there is no corrosion occurring at the cathode. This
reduction reaction is normally called protection; since the metal is protected from
becoming an ion, it is protected from corrosion. This process also results in many
factors which would otherwise slow the corrosion rate: the reduction of hydrogen ions
(which causes pH to change in the alkaline direction); the formation of hydroxyl ions
(which also causes pH to change in the alkaline direction); the breakdown of water
(which causes an increase in resistivity in the electrolyte); and the formation of a
hydrogen coating on the cathode (which causes an increase in the cathode to
electrolyte resistance).
The electrons formed at the anode flow through the metallic electron path
and are re-bonded at the cathode. The electrolyte provides the ions necessary for the
cathodic reaction and serves to dissolve the metal ions formed at the anode. The most
common electrolyte is water or a water based solution. The water may be tap water,
seawater, water held in the pores of a soil or water which has precipitated from the air
as rain or dew. It is important to note that corrosion and cathodic protection current
discussed in this publication and NACE International publication is conventional or
positive current flow.
2-2
TYPES OF CORROSION. Basically, there are four ways corrosion can
occur. Corrosion can occur through a chemical reaction or three general types of
electrochemical reactions. The three general types of electrochemical reactions that
occur depend on the cause of the potential difference between the anode and the
cathode. This potential difference can be caused by differences in the environment,
differences in the metal, or by external electrical sources of DC current. Understanding
this principle leads to an understanding of the principles of operation of cathodic
protection systems. Each of these three types of corrosion will be explained in detail,
with examples of each. These three types are general corrosion, concentration cell
corrosion (electrochemical cell caused by differences in the electrolyte), galvanic
corrosion (electrochemical cell caused by differences in the metal), and stray current
corrosion (electrochemical cell caused by external electrical sources).
2-2.1
General Corrosion. This type of corrosion is chemical or electrochemical in
nature. However, there are no discrete anode or cathode areas. This form of corrosion
is uniform over the surface of the metal exposed to the environment. The metal
gradually becomes thinner and eventually fails.
The energy state of the metal is basically what causes this reaction.
Referred to as the "dust-to-dust" process, high levels of energy are added to the raw
material to produce the metal. This high energy level causes an unnaturally high
electrical potential. One law of chemistry is that all materials will tend to revert to its
2-5
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