JANUARY 31 2003
Resistivity of the Electrolyte. The resistivity of the electrolyte is normally a
significant factor in determining the rate of corrosion. This is an uncontrollable
characteristic of the soil or water (the electrolyte). The definition of an electrolyte is a
material that will allow ions to migrate, and the resistivity is the rate at which it allows
ions to migrate. Resistivity is the inverse of the conductivity and is measured in ohm-
centimeters. Resistivity is inversely proportional to current, and therefore to corrosion,
in an electrochemical cell. If the resistivity is doubled, and all other factors remain the
same, the amount of corrosion is cut in half.
. The contact resistance
of the anode
to electrolyte and
of the cathode to electrolyte has the same effect as resistivity, since it is a measure of
resistance. The lower the resistance, the greater the current (corrosion). If the contact
resistance of the anode or the cathode is doubled, and all other factors remain the
same, the amount of corrosion is cut in half. Note that if the contact resistance of both
the anode and the cathode is doubled, the amount of corrosion is only one-fourth of its
Coating of the Structure. Coating of the structure normally raises the
contact resistance of the anode and the cathode since most coatings are dielectric in
nature (non-conductive). See paragraph 2-3.1.3.
Polarization of the Structure. Polarization is the change of the electrode
potential as a result of the electrochemical current flow and usually results in the
formation of a film on the electrode surface called a "polarization film." Polarization film
and other changes have beneficial effects at the cathode. The layer of hydrogen acts
as an additional coating, water is driven away from the surface of the cathode, ion
concentration in the electrolyte is reduced, the contact resistance of the electrode to
electrolyte is raised, and essentially, corrosion cell current no longer flows or is reduced
to a small fraction of its previous value.
Amount of Current Flow. The amount of current flow directly influences the
rate of corrosion. Corrosion can be determined from the amount of current flow. Each
metal has definite characteristics in the number of electrons given up in the oxidation
process and the number of atoms in a kilogram (pound) of the metal. This can,
therefore, be translated to kilograms (pounds) per amp. The normal unit of measure
encompasses a one year period--kilograms (pounds) per amp-year.
Chemical Effects on the Rate of Corrosion. Any factor that affects the
speed of a chemical reaction will affect the rate of the chemical portion of the
electrochemical reaction (corrosion). Following is a description and example of the
factors affecting the rate of the chemical portion of corrosion.