Nace CP 1, Course Manual

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RP0575-2001
NACE International 3
4.1.6 Current density requirements can range from 50
to 400 mA/m2 (5 to 40 mA/ft2) of bare water-immersed
steel. In the absence of specific current density data,
100 mA/m2 (10 mA/ft2) is commonly used for design.
However,
vessels handling water containing
depolarizers, such as H2S and oxygen, or operating at
high temperatures and/or high flow rates, could require
higher
current densities to maintain protective
potentials. Internal coating of vessels decreases the
area of bare steel in contact with water and reduces the
amount of current required for corrosion protection.
4.2 System Selection
4.2.1 Cathodic protection can be provided by
impressed current systems or galvanic anode systems.
Typical
performance data for commonly used
impressed current and galvanic anodes are shown in
Table 1. Prior to the application of either impressed
current or galvanic anodes, it must be ensured that
treated electrolytes are chemically compatible with the
anode.
Table 1—Typical Performance Data for Commonly Used Impressed Current and Galvanic Anodes
Type of Anode A-h/kg
(A)
A-h/lb
(A)
Impressed Current Anodes
Linseed oil-impregnated graphite 13,000 to 15,000 (14,000) 6,000 to 7,000 (6,500)
High-silicon cast iron with chromium 18,000 to 24,000 (19,000) 8,000 to 11,000 (8,500)
Galvanic Anodes
Magnesium 1,000 to 1,100 (1,000) 450 to 500 (450)
Aluminum
(B)
400 to 2,000 (1,300) 200 to 950 (600)
Zinc 700 to 800 (800) 300 to 350 (350)
(A)
Values in parentheses are commonly used in design calculations.
(B)
Caution: The performance and efficiency of aluminum anodes vary with the alloy, and with certain alloys they vary with the heat
treatment.
Note: Anode efficiencies vary widely, particularly for galvanic anodes. Factors influencing this include anode locations,
position and surface area, water composition and temperature, and selective electrochemical attack.
4.2.2 Impressed current systems have greater
flexibility if high current demand is anticipated.
4.2.2.1 Impressed current systems can be used in
any water, but are usually the most practical in
high-resistivity
waters where an appreciable
amount of current is required to achieve
protection.
4.2.2.2 Impressed current systems typically
require more monitoring and maintenance than
galvanic anodes.
4.2.2.3 Automatic potential rectifier systems
decrease the likelihood of underprotection or the
excessive use of power and coating disbondment
resulting from overprotection (see Paragraph
4.3.2).
4.2.2.4 Impressed current anodes should be
provided with individual lead wires to the rectifier
for control and measurement of current output
from each individual anode.
4.2.3 Galvanic anodes are commonly used when
electrical power is not feasible to use or is not available
and may be preferred for low-current-requirement
installations, even if electrical power is available. The
effects of the produced fluid chemistry on the
performance
of a galvanic anode should be
considered. The pH of produced fluids containing
dissolved CO
2
and/or H
2
S can be lowered to acidic
levels that can attack zinc, aluminum, or magnesium.
Large concentrations of H
2
S may also react with the
anode to alter its performance. The possibility of the
presence
of other impurities, such as amines,
emulsions, or even small quantities of mercury, should
be considered.
Consideration should be given to
effects of the various oilfield treating chemicals and
workover fluids that may flow through the vessel being
protected. Residual acids from stimulation treatments
may cause severe attack on all galvanic anodes. The
potential effects of demulsifiers, scale and corrosion
inhibitors, drilling mud, and other material that might be
added to production should be considered.
4.2.4 Galvanic anode materials most commonly used
are aluminum, magnesium, and zinc alloys.
The