1. General
1.1. Types of corrosion.- Corrosion is an elec-
trochemical phenomenon; that is, it involves
chemical reactions and the flow of current. The
two most common types are corrosion caused by
stray currents from external sources and galvanic-
type corrosion caused by different metals in an
electrolyte or the same metal in different
electrolytes. These will be discussed separately
and possible remedial measures given.
trochemical phenomenon; that is, it involves
chemical reactions and the flow of current. The
two most common types are corrosion caused by
stray currents from external sources and galvanic-
type corrosion caused by different metals in an
electrolyte or the same metal in different
electrolytes. These will be discussed separately
and possible remedial measures given.
1.2. Description of corrosion terms.- Several
corrosion terms frequently used in this bulletin
and in corrosion work are described more in
terms of practical corrosion concepts rather than
as formal definitions.
corrosion terms frequently used in this bulletin
and in corrosion work are described more in
terms of practical corrosion concepts rather than
as formal definitions.
Anion.- Negatively charged ions in the elec-
trolyte. Anions are attracted to and move
toward the anode under influence of a po-
tential gradient. Some may react at the
anode.
trolyte. Anions are attracted to and move
toward the anode under influence of a po-
tential gradient. Some may react at the
anode.
Anode.- The metal which corrodes, consid-
ered as being at the higher potential or pos-
itive terminal of the current source.
ered as being at the higher potential or pos-
itive terminal of the current source.
Cathode.- The metal which is protected by
the anode and does not corrode, considered
as the lower potential or negative terminal of
the current source.
the anode and does not corrode, considered
as the lower potential or negative terminal of
the current source.
Cation.- Positivity charged ion in electrolyte.
cations are attracted to and move toward the
cathode under influence of a potential
gradient. Some may react at the cathode.
cations are attracted to and move toward the
cathode under influence of a potential
gradient. Some may react at the cathode.
Concentration cell.- A corrosion cell whose
voltage is the result of inhomogeneities or
differential chemical conditions within the
electrolyte.
voltage is the result of inhomogeneities or
differential chemical conditions within the
electrolyte.
Corrosion.- The process of oxidation of a
metal due to the interaction of the metal and
its environment.
metal due to the interaction of the metal and
its environment.
Corrosion cell.- Consists of an anode and a
cathode which are both metallically con-
nected and immersed in an electrolyte. Dry
and wet cell batteries are common exam-
pies (when shorted across the terminals).
cathode which are both metallically con-
nected and immersed in an electrolyte. Dry
and wet cell batteries are common exam-
pies (when shorted across the terminals).
Electrical resistivity.- The resistance offered
to the passage of current by a unit volume
of the material. Units are ohm-centimeters
or ohm-feet.
to the passage of current by a unit volume
of the material. Units are ohm-centimeters
or ohm-feet.
Electrolyte.- The medium (such as water or
moist soil or solution of special chemicals)
through which the internal circuit current or
a corrosion cell flows from the anode to the
cathode by migration of anions and cations.
moist soil or solution of special chemicals)
through which the internal circuit current or
a corrosion cell flows from the anode to the
cathode by migration of anions and cations.
Electron flow.- Is in the opposite direction to
"conventional" current flow.
"conventional" current flow.
External circuit.- The part of a corrosion cell
circuit in which the current flows through the
metal of the anode, cathode, and metallic
conductor between them (the metallic part of
the circuit).
circuit in which the current flows through the
metal of the anode, cathode, and metallic
conductor between them (the metallic part of
the circuit).
Internal circuit.- The part of a corrosion cell
circuit in which the current flows through the
electrolyte (the solution part of the circuit).
circuit in which the current flows through the
electrolyte (the solution part of the circuit).
Galvanic cell.- A corrosion cell in which the
anode is of a different metal than the cath-
ode.
anode is of a different metal than the cath-
ode.
Galvanic-type corrosion.- Corrosion similar
to that produced by a galvanic cell.
to that produced by a galvanic cell.
Galvanic series.- A listing of metals and alloys
arranged in increasing order of their
resistance to corrosion when any two of them
are the electrodes of a complete cell (table 1).
arranged in increasing order of their
resistance to corrosion when any two of them
are the electrodes of a complete cell (table 1).
Ion.- An electrically charged atom or group
of atoms.
of atoms.
Local cell corrosion.- Corrosion caused by
local inhomogeneities in a metal surface
which creates small anode and cathode
areas.
local inhomogeneities in a metal surface
which creates small anode and cathode
areas.
Long-line corrosion.- Corrosion occurring
where the anode and cathode are widely
separated, sometimes by several hundred
feet. It is usually caused by inhomogeneity in
the electrolyte or metal at these locations.
where the anode and cathode are widely
separated, sometimes by several hundred
feet. It is usually caused by inhomogeneity in
the electrolyte or metal at these locations.
1 (FIST4 -5)