Available Sizes. HSCBCI anodes are available in a wide variety of
standard sizes and shapes as shown in Tables 8 and 9. Special configurations
can be produced at extra cost and are usually practical when standard anodes
have been shown to be unsatisfactory for a particular application and where a
large number of special configuration anodes are required. Typical HSCBCI
anode configurations are shown in Figures 45 through 49. The cable-to-anode
connection is, as in the case of all impressed current anodes, critical.
Three common methods of achieving the cable-to-anode connection and seal are
shown in Figures 50, 51, and 52. The use of the center connected tubular
anode as shown in Figure 53 is preferable as necking of the anode at the
connection point is avoided and life of the anode is extended 90 percent (50
percent anode material expended before failure versus 95 percent anode
material expended before failure for center connected anode).
Operation. HSCBCI anodes are consumed at a rate of 1 lb/A yr when
used at a current not exceeding their nominal discharge rates. The potential
difference between steel and HSCBCI can be neglected in the selection of
impressed current rectifiers. HSCBCI anodes will operate without backfill in
most applications, but backfill will reduce the anode-to-electrolyte
resistance and extend the life of the anodes. Because metal-to-metal contact
is made between the anode and the round particle calcined petroleum coke
breeze, the outside of the coke breeze becomes the anode. Also, the lower
output voltage required will save power and reduce the initial cost of the
rectifier unit. Because of these reasons, petroleum coke backfill is
recommended where it can be feasibly installed.
Aluminum. Aluminum anodes are sometimes used for the protection of
the interior of water storage tanks. They are consumed at a fairly high rate
of approximately 9 lbs/A yr in most applications. The main advantages of
using aluminum anodes in the protection of water storage tanks is their low
cost, light weight, and lack of water contamination from the products of
deterioration of the anodes. They are commonly used when seasonal icing of
the tank would damage the anodes. The aluminum anodes are sized to last 1 year
and are replaced each spring. HSCBCI and graphite anodes are more commonly
used in water tanks and, when installed on a floating raft, can be made
resistant to icing conditions.
Platinum. Pure platinum wire is sometimes used for impressed
current cathodic protection anodes where space is limited. Platinum is
essentially immune to deterioration in most applications. In seawater its
consumption rate at current densities as high as 500 A/ft is 0.00001 lb/A yr.
Due to the high cost of platinum, this material is more commonly used as a
thin coating on other metals as described in para. 6.6.6.
Platinized Anodes. Platinum can be bonded or deposited on other
materials for use as an impressed current cathodic protection anode. The
substrate materials, namely titanium, tantalum, and niobium have the special
characteristic of being covered with a naturally formed stable oxide film
which prevents current flow from their surfaces, even when exposed to high
anodic potentials. All of the current flows from the platinum coated portion
of the anode surface. These "platinized" anodes, although high in initial
unit cost, can be used at very high current densities and have had wide