Page 594 - Corrosion Engineering Principles and Practice

556 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 557

production is restricted at the anode. The use of carbonaceous backfill
assists in reducing the electrical resistance to ground associated with
the buildup of corrosion products. Periodic flooding with water can
also alleviate resistance problems in dry soils.
Theoretical anode consumption rates are at 9 kg A y . For grey
–1
–1
cast iron, consumption rates may be lower than theoretical due to
the formation of graphite-rich surface films. A full utilization of the
anode is rarely achieved in practice due to preferential dissolution in
certain areas. For long anode lengths, multiple current feed points
are recommended to ensure a reasonably even current distribution
over the surface and prevent premature failure near the electric
connecting points.
Limitations include the buildup of corrosion products that will
gradually lower the current output. Furthermore, in high-density
urban areas, the use of abandoned structures as anodes can have
serious consequences if these are shorted to foreign services. An
abandoned gas main could, for example, appear to be a suitable anode
for a new gas pipeline. However, if water mains are short circuited to
the abandoned gas main in certain places, leaking water pipes will be
encountered shortly afterward due to excessive anodic dissolution.

Semiconsumable Anodes. Semiconsumable anodes such as graphite
and high-silicon iron (HSI) have been in service since the first industrial
electrochemical systems were developed. Graphite is a porous material
and for this reason graphite is often impregnated with resins to reduce
solution ingress and improve mechanical strength. Graphite anodes
are inert when chlorine evolution is occurring, chlorine being produced
efficiently at low polarization. But if oxygen formation predominates,
as in low chlorine media, graphite is oxidized to carbon dioxide.
Graphite deterioration also increases with decreasing pH and
increasing sulfate ions concentration [13].
Graphite is not recommended for use in closed systems because
of the possibility of galvanic corrosion caused by detached pieces. In
addition, graphite suffers high consumption rates in water at
temperatures above 50°C. Consumption rates measured for graphite
depend on the environment and thus range from 0.045 in seawater to
0.45 kg A y in freshwater. Similarly the corresponding operating
−1
−1
−2
current densities vary from 2.5 to 10 A m . The main disadvantages
of graphite compared to other impressed current anodes are low
operating current densities and inferior mechanical strength. Graphite
is generally used in conjunction with carbonaceous backfills in soil-
based ICCP systems [11].
Polymeric anodes are a different type of carbon-based anodes
consisting of flexible wire anodes with a copper core surrounded by a
polymeric material that is impregnated with carbon. The impregnated
carbon is gradually consumed in the conversion to carbon dioxide,
with ultimate failure by perforation of the copper strand. The anodes