Page 256 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

234 CORROSION CONTROL AND PREVENTION

2. Degradation of conductive coating after extended current passage.
3. Increase in electrical resistance between the anode and the steel because
of insufficient moisture or accumulation of insulating by-products at the
anode/concrete surface.

Of all the systems cited above, only the titanium mesh anode and metallized zinc
are in extensive use at present. The titanium mesh on bridge decks is durable over
long time. The thermal-sprayed zinc is free from debonding problems but suffers
from an increase in resistance over time. However, the Oregon DOT has had sig-
nificant success with thermal-sprayed zinc anode on substructure components. The
thermal-sprayed titanium has shown promise as a new anode.
Some DOTs have investigated alternate energy sources for CP systems, such as
solar power and long-life batteries for use on the substructure elements exposed to
brackish waters.
In some cases, CP offers the only acceptable service life extension as an alterna-
tive to replacement of a critical ridge component. For example, a thermal-sprayed
zinc CP system on historic bridges built in 1930s has been successfully imple-
mented. Missouri DOT leads North America in the use of CP to extend the life of
salt-contaminated and corroding concrete bridges. CP is primarily used for corrosion
control of voided slab structures although it is used on steel frame and stringer type
structures. Conventional methods of corrosion prevention on bridges built in the
1950s and 1960s were unsuccessful. Since 1975, Missouri has installed CP systems
on more than 140 bridges. Many CP systems have been evaluated and used in
Missouri. First introduced in 1986, the activated titanium mesh anode system with
concrete overlay has become the exclusive CP system installed on Missouri DOT
bridges. To date, this system has provided a high level of corrosion control to more
than 30 bridges in the Kansas City and St. Louis areas.

4.9.1.9.2 Sacrificial Anode Cathodic Protection The basic characteristics of a sac-
rificial anode system are the following:
1. No external power source is required.
2. The driving voltage is fixed.
3. The applied current is dependent on the driving voltage and the resistance
between the rebar and the anode.
4. The CP system is limited to relatively low current requirements.
5. The CP system is limited to low-resistivity concrete environments.

Sacrificial anode CP systems have been used for the corrosion control of bridge
decks as long as impressed-current anode systems for corrosion control of bridge
decks. Two of the earliest field trials (1977) for sacrificial anode systems were the
following:

1. Perforated zinc sheets fastened on the deck with a bed of mortar, then covered
with concrete overlay.