UFC 3-570-06
JANUARY 31 2003
2-7.2.3
Aluminum Anodes. Occasionally, aluminum is used as an impressed
current anode for protecting the interior of water tanks. Because it is relatively
inexpensive compared to other impressed current materials, anodes are provided in -
inch-diameter by ten-foot-long screwed-rod sections. These are for use in tanks where
seasonal icing usually destroys the anode lead wire connection, requiring annual
replacement. This type of anode system does not contaminate potable water, but the
consumption rate of 4.1 kilograms (9 pounds) per amp-year limits the cost effectiveness
on the basis of cost per year of service compared to other anode systems.
2-7.2.4
Lead-Silver Anodes. Lead alloy anodes are only used in free flowing
seawater applications and may employ various metals such as antimony lead, tin and
1% or 2% silver. Commonly supplied in rod or strip form, of 1.5-inch diameter by 10
inches long, they have been used extensively in Europe with a 2 percent silver alloy,
which doubles the life. Upon initial startup, the consumption rate is about 1.3 kilograms
(3 pounds) per amp-year, and eventually a black, passive film of lead peroxide forms to
extend the life of the anode surface, resulting in consumption of about 0.09 kilograms
(0.2 pounds) per amp-year. Normal current density ranges from 3 to 25 amps per
square foot. In silting or low chloride conditions, this oxide film does not form, and the
anode is consumed rapidly. Cable connections are made by drilling a hole, and silver
soldering the lead wire at the base of the hole. The connection cavity is then filled with
epoxy to prevent moisture penetration. Installation is accomplished by hanging the
anodes from a structure, dock, or pier, in a perforated FRP pipe or by a support device
to maintain its position. This support is important to prevent ice damage and keep the
anodes from coming in contact with mud or silt.
2-7.2.5
Platinum Anodes. Platinum can be used as an anode coating for almost
every type of cathodic protection installation. Structures in a vast array of environments
such as underground, offshore, concrete, power plants, and the internals of piping,
tanks, and machinery have used platinum for cathodic protection systems. Since
platinum has such a low consumption rate, 0.00008 kilograms (0.00018 pounds) per
amp-year, only a small amount is needed for a twenty-year anode life. Pure platinum,
by itself, would be too expensive. The platinum is normally coated over noble base
metals such as titanium and niobium. When anodes are in the form of wire and rods,
there may be a copper core to increase the conductivity for lengths in excess of 7.6
meters (25 feet), since titanium and niobium are relatively poor electrical conductors
compared to copper. The passive film on titanium starts to break down at 10 volts
anode-to-cathode potential, and is limited to low resistance environments such as
seawater. Niobium has a breakdown voltage of 120 volts anode-to-cathode potential,
and is used in higher resistance electrolytes. Current densities range from 50 amps in
soils to 500 amps in seawater, depending on the anode surface area and thickness of
the coating.
Platinum has been coated on base metals using many techniques, including
sputtering, electrode positioning, cladding, and metallurgically bonding. A consensus of
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