Masts, Poles and Electrical Distribution Pylons

Differential aeration – a corrosion

inducing mechanism that is controllable
Differential aeration or necking corrosion, the exposed portion. The result is a corrosion of street light poles can be
as it is often referred to in this instance in corrosion cell where the steel exposed to attributed to man’s best friend who
Masts, Poles and Pylons, as a corrosion an oxygen lean environment constitutes sniffs and raises a leg at every pole that
form which frequently occurs on partially the anode in relation to the exposed steel he passes. While this canine custom
buried steel at the interface between the which is the cathode. does contribute to the corrosion
buried portion and that exposed to the problem, it is not the root cause.
atmosphere. It can also occur but to a This form of corrosion is frequently
lesser degree, where steel structures are observed just above ground level while it What then is the solution? Corrosion
cast into concrete bases and slight should not be confused with other which is brought about by differential
shrinking of the concrete during curing corrosive conditions associated with the aeration is prevented entirely, simply by
results in a small gap or crevice at the presence of corrosion inducing applying a paint coating onto the buried
interface between the steel and the substances in some soils. portion of a hot dip galvanized structure,
concrete. prior to erection. The paint film should
Perhaps the best example of corrosion extend to about 300mm above ground
The cause of this corrosion mechanism is brought about by differential aeration is level. Coal tar epoxy has proved to be
a depletion of oxygen where the steel the street lighting pole, the base of which singularly effective for this purpose while
the surrounding compacted soil ensures
surface is buried, compared with the is normally buried in soil. There is a
that the paint coating is not disturbed
oxygen rich atmosphere in contact with misconception that the reason for the
after erection.

An additional benefit obtained from this

duplex concept is the enhanced
protection from normal corrosion attack
in aggressive soils.

Municipal engineers have adopted this

procedure over the years with
outstanding results. Hot dip galvanizing
has become the generally accepted
A paint film extending some 200 - 300mm above and below the ground level will prevent method of corrosion control for lighting
differential aeration. masts and poles. With the added
protection of a duplex coating on the
buried section, it would seem that the
major cause of damage to these
structures can now be attributed to errant
and perhaps inebriated motorists. As an
alternative to painting, some pole
manufacturers have added a steel sleeve
in this area, clearly providing a double
layer of steel, resulting in an extended
maintenance free life of the pole.
Less effective prevention of differential Differential aeration may also be prevented by
aeration. an additional steel sleeve situated above and
below the ground level. Eskom is an organization that has over
the years, thoroughly researched the
various causes of corrosion and how best
to control them. Eskom power
transmission towers which spread eagle
the entire country are hot dip galvanized
with an expected maintenance free life of
50 years in most environments. All base
legs of these galvanized towers are coated
with a familiar band of coal tar epoxy at
the interface between the galvanized steel
Prevention against differential aeration after erection may not have the desired long term benefit. and the concrete foundations.

6 Hot Dip Galvanizing Today Volume 3 Issue 1 2006