The Pulse Rectifier (Pulser) produces
very short duration, very high voltage, direct current electrical pulses
for cathodically protecting buried metallic structures, such as
pipelines and well casings, from external corrosion.
Pipelines, well casings, and other steel
structures buried in soil, corrode. Cathodic Protection has been used to
mitigate corrosion for many years. Sufficient direct electrical current
(DC) is caused to flow from the soil to the surface of the structure.
Typically, the negative lead from a DC rectifier is connected to the
structure to be protected, and the positive lead to an anode bed. The
ground bed usually consists of one or more rods, called anodes,
fabricated from expendable materials such as graphite or silicon steel.
The anodes are buried in deep wells, or shallow holes or trenches, in
the vicinity of the structure to be protected. The process is called
Cathodic Protection, because protective cathodic reactions are caused to
take place on the surface of the structure being protected.
The DC current mitigates corrosion by
electrochemically reducing any corrosive oxygen molecules and hydrogen
ions that may be adsorbed on the surface of the structure in contact
with the soil. As additional molecules and ions diffuse from the soil
and become adsorbed onto the steel surface, they are also reduced by the
continuously flowing direct current. There are certain drawbacks to
conventional cathodic protection, however. Current flows in the ground
continuously in a relatively large three dimensional envelope. If there
are other foreign metallic structures buried in the soil within the
current envelope, damage in terms of metal loss will very likely occur
to the foreign structures. Also, the "throw", or distance down
the pipeline or well casing, that can be protected, is limited by the
magnitude of the current reaching the more distant corroding surfaces.
The corrosive oxygen molecules and
hydrogen ions, adsorbed on the surface of the buried structure, are
electrochemically reduced within microseconds after the application of
cathodic current. Before the corrosion reactions can begin again,
additional molecules and ions must diffuse to the surface of the
structure from the surrounding soil and become adsorbed. The diffusion
and adsorption processes are orders of magnitude slower than the
reduction process, particularly when the ions are not being driven by a
continuously applied cathodic protection voltage.
With Pulse Cathodic Protection, the
corrosive molecules and ions are electro- chemically reduced with very
short duration but very high current pulses. Since throw is a function
of current magnitude, the very high pulsed currents result in greatly
improved throw down the length of the structure. Because of the natural
electrical characteristics of the structure/anode system, the greatly
improved high current throw is further enhanced by the inductive and
capacitive redistribution of that current.
Also, the very short duration pulses are
separated by relatively long "off" intervals. The pulse
voltage is applied for less than 20% of the total time. Thus, the
continuous current envelope and the resulting interference on foreign
structures is greatly reduced.
