UFC 3-570-06
JANUARY 31 2003
knowledgeable engineers finds that metallurgically bonded anodes seem to experience
fewer failures because the metals are compressed together in an oxygen-free vacuum.
This provides an oxide-free, low resistance, and complete bond between the metals,
thereby maximizing design life. Cladding involves wrapping a thin sheet of platinum
around a rod and spot-welding the platinum to the base metal at the overlap area. The
limited weld area allows the underlying base metal to oxidize, thus increasing resistance
and achieving minimal design life. The smooth surface has little bearing on the life,
because the surface becomes irregular once current is discharged. Electro-deposition
techniques plate a film of platinum on the base metal, but the process results in a
porous surface that is less likely to achieve full life due to high resistance oxide film
formation. Thermal decomposition and welded techniques exhibit the same problems
as cladding and, as of the late 1980s, are rarely used.
The anode-to-cable connection is critical, and improper connections can
result in premature failure. Users should assure that the anodes are manufactured in
compliance with their specifications by skilled personnel under the guidance of
established quality control methods. The major disadvantage of platinum is its poor
resistance to anode acid evolution in static electrolytes, rippled direct current, and half
wave rectifiers. Use of a three-phase transformer rectifier in seawater systems has
been known to double the life of platinum anodes by reducing the ripple on the DC
output.
2-7.2.6
Ceramic and Mixed-Metal Oxide Anodes. Mixed-metal oxide anodes were
developed in Europe during the early 1960s for use in the industrial production of
chlorine and caustic soda. The first known use of the technology for cathodic protection
occurred in Italy to protect a seawater jetty in 1971. These anodes exhibit favorable
design life characteristics while providing current at very high-density levels. The oxide
film is not susceptible to rapid deterioration due to anode acid generation, rippled direct
current, or half wave rectification, as is common with other precious metal anodes. The
composition of the anode consists of a titanium rod, wire, tube or expanded mesh with
the oxide film baked on the base metal. Sometimes they may be referred to as
dimensionally stable, ceramic, or linear distributed anodes. In oxygen evolution
environments such as soils, the oxide consists of ruthenium crystals and titanium halide
salts in an aqueous solution that is applied like paint, on the base metal, and baked at
400 C to 800 C, forming a rutile oxide. In chlorine evolving environments such as
seawater, the oxide consists of an aqueous solution of iridium and platinum powder that
is also baked at high temperatures to achieve a desirable film. After baking, the rutile
oxide develops a matte black appearance and is highly resistant to abrasion. Some
manufacturers produce variations of the oxide films specifically for chloride or
non-chloride electrolytes and they are not interchangeable. Normally, titanium will
experience physical breakdown around 10 volts, but the oxide film is so highly
conductive (0.00001 ohm-cm resistivity), that the current, which takes the path of least
resistance, is discharged from the oxide rather than the base metal even with a rectifier
voltage of 90 volts in soils. This is in contrast to the insulating titanium dioxide film that
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