Mill scale.- A heavy oxide layer formed on
steel and iron during hot fabrication or heat
treatment of the metal.
Noble metals.- Those metals having the
greatest tendency to remain in the uncom-
bined or free state. The more noble of two
metals in a corrosion cell will be the cathode
and will not corrode.
Polarization.- Production of a back EMF
(electromotive force) or countervoltage in a
corrosion cell as a result of chemical changes
at the electrode produced by the flow of
current. This acts as resistance in the internal
circuit of a corrosion cell.
Stray current corrosion.- Corrosion caused
where current from an extraneous source is
discharged into the electrolyte (i.e., ground
return from a street, railway system, etc.).
Tuberculation.- The formation of knob-like
mounds of corrosion products due to local
2. Galvanic-type corrosion
2.1. Description.- Galvanic-type corrosion oc-
curs as the result of the tendency of metals to
revert to their natural state. If this is to occur,
the metals must be so arranged as to form a
complete cell, which may be termed a battery
or corrosion cell or galvanic cell. Since corro-
sion may stem from other causes, it is import-
ant to note that the type described as
galvanic may be recognized from the fact that
the cell provides the forces causing corrosion,
rather than external currents, etc. The cell is
comprised of an anode and cathode
immersed in an electrolyte. When the anode
and cathode are metallically connected (as
when a wire is connected across the
terminals of a battery), current flows and
corrosion of the anode occurs. When the
anode happens to be a metallic part of a
structure, piping, or cable system, severe
damage may result.
2.2. Natural corrosion cells.- The environment for
many electrical power structures provides
conditions favoring formation of natural corrosion
cells. The metal or metals of a structure serve as
anode, cathode, and the necessary metallic
conductor between the two. Water, either as such
or as moisture in soil, provides the electrolyte
required to complete the cell circuit. Such cells
develop their driving force or electrical potential
from differing conditions at the interfaces between
metal and electrolyte of the anode and cathode.
These differences fall into three categories: (a)
Dissimilar metals comprising the anode and
cathode, (b) in-homogeneity of a single metal,
which causes one area to be anodic to another
area, and (c) inhomogeneity of the electrolyte.
The following are a few of many possible
examples in which the essential requirements of
a complete cell are satisfied in a structure.
(a) Iron will be anodic to copper ground mats
or to brass bolts or other brass parts.
(b) An iron plate having some mill scale
present may rust because the iron is anodic
to the mill scale.
(c) An apparently homogeneous iron plate
may rust because tiny areas of the surface
contain impurities or grain stresses which
cause them to be anodic to other areas of the
(d) Weld areas of a welded pipe may rust
because the weld metal is of different com-
position, may contain impurities, or may
cause stress which make it anodic to nearby
(e) Corrosion may be observed on the bottom
of a pipeline while the top remains nearly
undamaged. This may be attributable to
higher oxygen concentration in the soil
moisture (electrolyte) at the top of the pipe,
leaving the bottom anodic. The soil being
undisturbed at the bottom of the pipe
provides a lower oxygen content and a lower
resistance to current flow than is present in
the backfill covering the top of the pipe.
(f) Exposed iron areas in contact with con-
crete. Encased or embedded iron may rust
because the concrete creates a different and
special electrolytical environment which
causes the exposed iron to become anodic to
the embedded iron.
(FIST 4 - 5)