Page 595 - Corrosion Engineering Principles and Practice

558 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 559

are typically used in combination with carbonaceous backfill, which
reportedly increases their lifetime substantially. Because these anodes
are typically installed over long lengths, premature failures are
common when soil resistivity varies widely [11].
HSI anodes are widely used for ICCP (Figs. 13.26 and 13.27).
These anodes contain approximately 14.5 percent silicon and
certain alloys contain additional 4.5 percent chromium replacing
molybdenum as an alloying element in such anodes. The chromium
alloying additions are particularly useful in chloride-containing
environments to reduce the risk of pitting damage while the high
silicon content ensures that the alloy forms a protective film
containing silicon dioxide (SiO ). A prerequisite for the formation of
2
the silicon dioxide film is that the anode must initially corrode during
the first few hours of operation. Silicon dioxide is highly resistant to
acids but it is readily dissolved in alkaline conditions.
HSI anodes are extremely hard and cannot be machined easily.
They are generally cast and then stress relieved by annealing.
Although they are brittle these anodes have superior abrasion and
erosion characteristics compared to graphite. HSI anodes are widely
used usually in conjunction with carbonaceous backfills in soils. They
have also found limited use in marine and freshwater environments.
The maximum operating current density is determined by the type of
alloy and the environment. For instance, in groundbeds with backfills
−2
the current density is limited to between 10 and 20 A m because of
problems caused by gas entrapment. In marine environments, a high
iron chromium anode can be operated up to 50 A m .
−2

FIGURE 13.26 Sleds with six cast iron anodes, integrated cable, and
protective hose for an operation capacity of 75 amperes for fifteen years.
(Courtesy of Dean Rookes, West Coast Corrosion Prevention Ltd.)