Page 598 - Corrosion Engineering Principles and Practice

560 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 561

For horizontal surfaces, anodes without overlay can be recessed in
the concrete surface. Nonuniform current distribution is a fundamental
concern in these systems. Anodes in the form of a titanium mesh, with
proprietary surface coatings of precious metals are commonly used in
concrete structures, in conjunction with cementitious overlays. These
systems are applicable to both horizontal and vertical surfaces and
generally provide uniform current distribution.
Magnetite is a cheap and naturally occurring material. It is a non-
−1
−1
stoichiometric oxide with an electrical conductivity of 1.25 W m .
Due to its brittleness, the anode is cast as a hollow cylinder and closed
at one end. The inner surface is then copper plated and the cylinder is
filled with polystyrene. Epoxy resin is used to fill any remaining
space. The anode cable is soldered to the copper plate. Magnetite
anodes have been successfully used in the cathodic protection of
buried structures and those immersed in seawater. The maximum
operating current density is 0.12 kA m and the anode consumption
−2
−1
−1
rate is approximately from 1 to 4 g A y .
13.5.3 Rectified Current Sources
The most common ICCP power source is a rectifier that converts AC
power from electric utility lines to lower voltage DC power by means
of a step-down transformer and a rectifying circuit based on selenium
or silicon elements (Fig. 13.28). These elements have a low resistance
to current flow in one direction and a high resistance in the opposite
direction. Figure 13.29 shows the simplified circuit of a rectifier utiliz-
ing a bridge connecting rectifying elements. The arrowhead symbols
on the rectifying elements show the direction that current can flow
readily. For the usual 60-cycle AC power source, the direction of cur-
rent flow is reversed 120 times per second. Referring to Fig. 13.29, at
some instant in time, current may be originating at connection (1) on
the transformer secondary. The only path that this current can take
will be through leg (C) of the bridge-connected rectifier, through the
external circuit (groundbed to protected structure), and through leg
(B) of the rectifier to return to the secondary winding at (2).
A 120th of a second later, the direction of the AC flow will have
reversed and it will originate at connection (2). Under this condition,
the only path that it can take to reach connection (1) will be through
rectifier leg (D), the external circuit, and rectifier (A). For either
direction of AC flow, the DC flow through the external circuit is in
one direction only.
Rectifiers operate at less than 100 percent efficiency, meaning that
the DC output power is less than the AC input power from the supply
line. Rectifiers operate at maximum efficiency when operated at full-
rated load. Where large rectifiers are used (as for large, bare structures),
the rectifier rating should be selected so that it will operate at close to
the full-load rating.