JANUARY 31 2003
Graphite Anodes. Graphite rods have been used as an impressed current
material for many years. The basic configurations consist of round or square rods,
manufactured from a slurry of powdered petroleum coke and coal tar resin. The coal tar
is used as a bonding agent to hold the graphite particles together and then baked for a
month at high temperatures to fuse the mixture. This process increases the resistance
to oxidation and subsequent breakdown. There are many types of graphite
compositions and the type used for cathodic protection groundbeds is one of the most
porous. The porosity allows moisture penetration to eventually migrate to the
connection, causing failure at the cable connection. A denser anode is expensive;
therefore, the porosity is reduced by impregnating the rods with an inexpensive filler of
linseed oil, microcrystalline wax, or a phenolic-based resin. There is controversy
concerning the best type of filler and even whether filler really reduces moisture
penetration over long periods of time. Some fillers have caused deep well anode
ground beds to become fouled and develop a high circuit resistance when the anodes
have been driven at high current density levels.
It is very important to have a proper anode-to-cable connection with any
anode material. For years, the manufacturers of graphite anodes had no specification
for a proper anode lead wire connection. This led to varying procedures and numerous
failures attributable to poor material, poor workmanship, and no quality control
parameters prior to shipment. Many users have developed their own assembly
standards that make graphite anodes a customized and consequently, more expensive
product than it could be if an industry standard existed. Additional specification details
Type of container - lead, brass, molten, compression.
Connection depth/diameter 76 mm (3 in), 102 mm (4 in), 127 mm (5 in),
Connection sealant - thermoplastic, thermosetting (epoxy).
Cable sealant - TFE tubing, shrink cap, encapsulation.
Impregnation - wax, linseed oil, resin.
Sizes - 3"x30", 3"x60", 4"x40", 4"x80".
Graphite should not be operated at current densities exceeding 10.76
amperes per square meter (1 ampere per square foot) in soil or 2.70 amperes per
square meter (0.25 amperes per square foot) in water. Unfortunately, graphite is brittle
and may be easily damaged during transportation, either bare or packaged. Special
handling and padding is necessary to prevent cracking and breaking.