MIL-HDBK-1004/10
Characteristics. All products from the operation or deterioration
6.6.1.3
of graphite anodes are gasses. In fresh water or non-saline soil, the
principal gasses produced are carbon dioxide and oxygen. In saline soils or
in seawater, chlorine is also produced and is the major gas produced in
seawater applications.
The gasses generated, if allowed to collect around the anode, can
displace moisture around the anode which results in a local increase is soil
resistivity and an increase in circuit resistance.
Operation. Graphite anodes must be installed and operated properly
6.6.1.4
in order to insure optimum performance and life.
a) Current Densities. The current densities in the following
table should not be exceeded in order to obtain optimum anode life:
RECOMMENDED MAXIMUM CURRENT DENSITY FOR GRAPHITE ANODES
FRESH WATER
SOIL
SEAWATER
Maximum Current
Density
3.75
0.25
1.0
(A/ft)
Equivalent Current
on 3" x 60" Anode
15 A
1A
4A
Equivalent Current
on 4" x 80" Anode
36.6 A
1.7 A
7.1 A
b) Operating Potentials. Since the potential difference between
steel and graphite is approximately 1.0 V with the graphite being the cathode,
this potential difference must be overcome before protective current will
begin to flow in the impressed current cathodic protection system circuit.
This 1.0 V must be added to the other voltage and IR drop requirements during
the selection of proper power supply driving voltage.
c) Consumption Rates. Assuming uniform consumption, the rate of
deterioration of graphite anodes in soil and fresh water at current densities
not exceeding the values in the table above will be approximately 2.5 lbs/A
yr. The deterioration rate for graphite anodes in seawater ranges from 1.6
lbs/A yr at current densities below 1 A/ft to 2.5 lbs/A yr at current
densities of 3.75 A/ft.
d) Need for Backfill. The deterioration of any point on a
graphite anode is proportional to the current density at that point. If the
resistivity of the environment at any one point is lower than the resistivity
at other points, the current density and attendant deterioration will be
higher there. This can result in uneven consumption and premature failure of
graphite anodes, particularly if the low resistivity area is near the top of
the anode. In this case, "necking" of the anode at the top occurs and the
connection to the lower portion of the anode is severed. The use of backfill
of uniform resistivity is used when graphite anodes are used in soil in order
to prevent uneven anode deterioration.
High Silicon Cast Iron. Cast iron containing 14 to 15 percent
6.6.2
silicon and 3/4 to 1 percent other alloying elements such as manganese and
carbon, form a protective film of silicon dioxide when current is passed from
their surface into the environment. This film is stable in many environments,
with the exception of chloride rich environments. The formation of this film
reduces the deterioration rate of this alloy from approximately 20 lbs/A yr,
as for ordinary steel, to 1 lb/A yr. Due to the lack of resistance of this
alloy to deterioration in environments containing chloride, a chromium bearing
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