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,
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APPENDIX 2E
CORROSION CONSIDERATIONS
Chapter 2
Helical Foundation Systems
CORROSION CONSIDERATIONS
The term “corrosion” is used to describe the
degradation of a material or its properties due
to reaction with its environment. Although
most materials are known to corrode over
time, corrosion is typically considered as the
destructive attack of a metal by chemical or
electrochemical reaction. During this process,
ions from the base metal migrate from the
surface, resulting in material loss. As the
corrosion process and metal loss continues,
there can be a reduction in material thickness
and area, which could result in loss of structural
capacity of a given member.
Romanoff (1957): “For electrochemical corrosion
to occur there must be a potential difference
between two points that are electrically
connected and immersed in an electrolyte.
Whenever these conditions are fulfilled, a small
current flows from the anode area through the
electrolyte to the cathode area and then through
the metal to complete the circuit, and the anode
area is the one that has the most negative
potential, and is the area that becomes corroded
through loss of metal ions to the electrolyte. The
cathode area, to which the current flows through
the electrolyte, is protected from corrosion
because of the deposition of hydrogen or other
ions that carry the current.”
The following conditions must be met in order
for corrosion to occur:
1) There must be two points (anode and
cathode) on a metal structure with different
electrical potential and these two points must
be electrically connected to complete the
circuit. The difference in electrical potential
could be caused by inconsistencies in the
metal, varying stress/strain points, contact
with dissimilar metals or materials, etc.
2) There must be an electrolyte to carry current,
and for below ground pile applications, soil
moisture serves this purpose. The presence of
solublesaltsincreasestheelectricalconductivity
(or lowers resistivity) of the electrolyte, thereby
increasing corrosion potential.
There is still much discussion and debate about
how much corrosion actually occurs for buried
metal, with the central argument typically being
the amount of available oxygen. The amount of
oxygen within soil decreases significantly just a
few feet from the surface, unless the material is
loosely-placed fill or an open-graded, granular
soil. Relatively speaking, we would then expect
these materials to present a higher potential for
corrosion than undisturbed clayey soils. The
presence of a water table further complicates
the discussion as you’d expect less oxygen
below the water table than above. It is also
important to note that although oxygen-starved
environments inhibit rusting, which is a specific
type of corrosion, other types of galvanic or
bacterial corrosion are still possible.
ICCEvaluationService,LLC(ICC-ES)Acceptance
Criteria 358 (AC358) and ICC-ES AC406 define
corrosive soil environments by: (1) soil resistivity
less than 1,000 ohm-cm; (2) soil pH less than 5.5;
(3) soils with high organic content; (4) soil sulfate
concentrations greater than 1,000 ppm; (5) soils
located in landfills, or (6) soil containing mine
waste. In such environments, the steel can be
protected with a hot-dip galvanized zinc coating
or with other means such as sacrificial anodes.
A site-specific evaluation of the soil can be
conducted in order to determine an appropriate
level of protection. Foundation Supportworks
®
,
Inc. (FSI) recommends that a corrosion engineer
be consulted when site or project conditions
warrant further evaluation.
FSI helical products and hardware may be ordered
as plain (black, uncoated) steel or with a protective
coating to further prolong the anticipated service
life. Helical pile capacity ratings are therefore
provided for plain, plain corroded, and galvanized
corroded pile sections. Scheduled corrosion
losses are for a period of 50 years and are in
accordance with ICC-ES AC358. Helical products
