Anode materials in marine applications.
Cathodic protection of marine pipelines
Cathodic protection of offshore structures
Cathodic protection of ship hulls.
Cathodic protection can be defined as a technique of
reducing or eliminating the corrosion of a metal by
making it the cathode of an electrochemical cell and
passing sufficient current through it to reduce its
The choice of anode material depends on whether
active (ICCP) or passive (SACP) systems are under
Sacrificial anode :
Should be anodic to steel.
inexpensive and durable
Commercial sacrificial anodes are magnesium,
aluminum, or zinc or their alloys like Al-Zn-In alloy
insoluble and corrosion resistant,
sustain high current density,
exchange current density,
lower power consumption
examples: High-silicon caste iron, precious
metals, Mixed-metal oxide, polymer anodes.
Effective coating resistance in
for one average square foot
Current required in amperes
Bare Pipea 500
Perfect coating 0.000058
The CP system supplements coatings and is intended to
control corrosion at holidays in coatings.
Large pipelines : ICCP at one/ both ends.
Most marine pipelines are protected by SACP.
Zn, or Al sacrificial anodes
Electrical contact by insulated copper cables.
The average cathodic protection current density
required to protect a marine pipeline will depend
the type of coating applied,
the amount of damage,
whether or not burial is specified, and
the location of the pipeline.
Large-diameter pipe lines can be protected by
installing an ICCP system atone or both ends of
Bracelet-type zinc or aluminum /alloys sacrificial anodes
Spacing between anodes
o Small dia pipeline(≤14in.) = 150m
o Large dia pipeline = 300m.
Calculation of current:
Surface area of pipe segment
Fraction of steel assumed to be bare.
Anodes are then sized to fit the condition:
W/C > IL ;
where W is the anode weight (kg), C is the alloy consumption rate in
kilogram/amp year (kg/Ayr), I is the anode current output (A), and L is the
desired design life in years.
I=E/R and R=0.315r/√A (McCoy’sEquation) ;
where r is the electrolyte resistivity (Ω cm), and A is the anode area
(cm2), R= anode –electrolyte resistance, E= net driving voltage.
Cathodic Protection of Offshore Structures
• Most plateforms are not painted below the
waterline, CP system causes change in pH near
cathode which causes precipitation of minerals and
formation of protective scale.
• Problems with new technology:
Offshore structures are now being built in deeper,
colder water where mineral deposits are less likely to
form. mineral deposit require current densities 750 to
1000 mA/m2 (70 to 93 mA/ft2).
Under-protection as current requirement change with
depth (Gulf of Mexico and Santa Barbara Channel).
carbonate scales are harder to deposit in deep, cold
Methods used for protection
SACP systems: simple and rugged, do not depend on
external electric power supplies.
the weight of sacrificial anodes can be a serious
consideration for deep-water platform.
Hybrid designs: ICCP (primary CP) + SACP. Example
Cathodic protection of ship hulls
Ships normally have protective coatings as their
primary means of corrosion control.
Cathodic protection systems are then sized so that an
adequate electric current will be delivered to polarize
the structure to the desired level.
Holidays in coatings.
Aluminum anodes are available for ship hulls,
but they can passivate and become inactive on
ships that enter rivers or brackish estuaries. For
this reason, zinc anodes are almost universally
used in commercial service.
ICCP systems are used on very large ships.
by di-electric shield
Impressed-current cathodic protection systems can produce
overprotection in some cases. Organic coatings can disbond because
of the formation of hydrogen gas bubbles underneath coatings.
Coating disbondment can produce increased surface areas that
require more cathodic protection and is controlled by placing
dielectric shields between the impressed-current anode and the