Environmental Engineering Reference
In-Depth Information
thereby that a pound of material will last for so many hours if 1 ampere of
current is continuously discharged by it.
3.
The self-corrosion of the anode material should be minial so that it is used
up effectively for the current output only.
Among the three common sacrificial anode materials, magnesium has the
largest potential difference with steel, which is increased further by alloying with
1% Mn. Magnesium is often alloyed with aluminum and zinc. Although the
efficiency of magnesium is low (about half of the theoretical efficiency of 1000
amp-h/1b), it is preferred as an anode material because of the advantage of higher
potential. Zinc is usually used in its commercially pure form and has an efficiency
of 95% of the theoretical. The potential difference between steel and pure alumi-
num is not large and under several conditions polarization of aluminum tends to
render it cathodic with respect to steel. To avoid such a situation and also to
increase efficiency, aluminum is alloyed with tin, zinc, mercury, and iron. Newer
alloys of aluminum have also been developed for use as sacrificial anodes, which
are often proprietary.
In soils, the anodes are usually surrounded by a backfill comprising coke
breeze, gypsum, and bentonite. The purpose of the backfill is to provide conduct-
ing surroundings that help in the uniform discharge of current. When aluminum
alloy is used as anode, some amount of NaCl is added to the backfill, which
counters passivation of the alloy.
Cathodic protection should invariably be employed with an insulating coating
on the surface to be protected. A bare surface needs a large amount of current
for protection, which means a shorter life for the sacrificial anode. Coated sur-
faces need to be protected only at the pores or leaks (holidays) and the current
requirement decreases drastically. A single magnesium anode can protect a length
of 8 km of a coated pipeline, whereas only a few meters can be protected by it
when the pipeline is bare. The coatings applied to the buried pipelines are usually
tar- or asphalt-based backed up by layers of adhesive tapes. Thick and rugged
coatings are necessary to resist damage during transport and installation.
Impressed Current System
In an impressed current cathodic protection system the protective current is sup-
plied by an external dc supply, which is usually a rectifier but may be a generator
or a battery as well. Its negative terminal is connected to the structure to be
protected and the positive terminal to an auxiliary anode that discharges the cur-
rent. In soils, the anode or, more often, a ''ground bed'' comprising a number
of anodes is surrounded by backfill for better conductivity. Figure 4.11 shows
the arrangement of an impressed current system protecting an underground tank.
The common materials used for the auxiliary anode are steel scrap, graphite,
aluminum, lead, and high-silicon iron (Duriron). Theoretically, the anodes should
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