Page 580 - Corrosion Engineering Principles and Practice

542 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 543

For ICCP anode systems in soil, it is common practice to use a
coke or graphite backfill to surround anodes (Fig. 13.18). There are
two advantages to this. One is that graphite or suitable coke has a
very low electrical resistivity (lower than most soils) and helps to
produce low resistance between anode and earth. The second
advantage is that if the coke or graphite is solidly packed around
the impressed current anode, the greater part of the current
discharged by the anode passes from the anode to the backfill
through direct contact between the anode material and the backfill
particles. This reduces the consumption rate of the anode. The
current discharging from the outer surfaces of the backfill column
will, however, consume the backfill material at a slow rate. The net
result is a lower resistance, longer life anode installation than
without the presence of backfill.
Suitable coke or graphite backfill materials are available from
suppliers. Coke should be either coal coke or calcined (heat-treated)
petroleum coke. Non-calcined petroleum coke can have high electrical
resistivity and be unsuitable for groundbed backfill use.

FIGURE 13.18 Conventional groundbed using petroleum coke backfill and
5 cm cast iron anodes. (Courtesy of Dean Rookes, West Coast Corrosion
Prevention Ltd.)