Marine Cathodic Protection
A Survey Article by ELGARD Corporation
Marine Corrosion
Today, much of our infrastructure built upon steel components
is seriously threatened by a destructive and persistent enemy:
corrosion. If not properly protected, your marine facility is
corroding right now as you read this web page. The rate of
corrosion for clean, bare steel is at least 5 mils per year.
This loss in material thickness can be greatly accelerated by
variations in water composition, oxygen concentration cells
associated with marine growth, and water velocity.
Many marine facility owners mistakenly believe that the
coating of all submerged steel eliminates the need for cathodic
protection. However, as much as 50% of the coating on a steel
marine structure can be damaged, or completely removed during
construction. Even undamaged coatings suffer from the presence
of microscopic pinholes, which lead to the development of
concentrated areas of corrosion. This severe form of pitting
corrosion can have corrosion rates as high as 50 mils to 80 mils
per year - resulting in complete penetration of 1/4" steel plate
in under four years.
Cathodic Protection
The preferred technique for mitigating marine corrosion,
based on historical performance and measurable results, is
cathodic protection (CP) - the practice of using electrochemical
reactions to prevent the corrosion of steel structures. The
reason for increased acceptance: cathodic protection prevents
corrosion on underwater structures. In theory and practice, the
implementation of a CP system is quite simple. Assuming you
already have corroding steel in seawater, all you need is an
anode, a power supply, and engineering talent. A protective
circuit is accomplished between the anode, steel (cathode),
power supply and electrolyte (seawater).
Impressed Current versus Sacrificial
Standard industry practice for cathodic protection in new
construction includes the installation of sacrificial anodes,
such as aluminum and zinc. These types of anodes will typically
provide a 10 to 20 year service life before requiring
replacement. When retrofitting or replacing these existing CP
systems, the most cost effective system is impressed current
anodes, which require an external power source for their
operation. The ease of installation, enhanced performance and
lower installed cost of impressed current anodes make them the
ideal CP system for retrofitting most structures.
TYPICAL ANODE DELIVERY SYSTEMS
On the design basis of maximum current distribution for the
lowest possible cost, with the most long-term reliability,
ELGARD has developed numerous anode current delivery systems
suited for a variety of facility types and anticipated operating
conditions. The following are but a sample of the many delivery
systems commercially available. Our design engineers are
likewise capable of creating custom systems to suit the
requirements of your marine facility.
Pile Mounted Anodes
Pile
Mounted Anodes are designed for efficient current distribution
in and around pilings where the complex geometry of the facility
precludes remote placement of the anodes. These delivery systems
are suitable for direct attachment to pilings. The Flat Back
Pile Mounted Anode was designed specifically for H-Piles, and
can also be configured for installation on sheet piling.
Disk Anode
The Disk Anode was designed in conjunction with the U.S. Army
Construction Engineering Research Lab for use on navigational
locks and dam gates. This anode system is also suitable for use
on seawater intake structures, vessel internals, and sheet
piling when shoreside access is possible.
Retractable Mount
For installations where it is deemed necessary to access the
anode for periodic maintenance, or when current is only required
on a periodic basis, the retractable anode allows the user to
easily retrieve the anode. The above illustration is rotated by
90 degrees.
Sled Anodes
Anodes mounted on the sea bed typically afford the
best spread of protection on a marine structure. Sled
anodes can be designed for operation in either seawater
or buried in the mud. The Post Tension Sled was
developed to insure anode operation out of the mud when
resting in silty and soft sea beds. By adjusting the
height of the concrete sled, the mesh anode sled can
also be designed for operation out of the mud. The
advantage of this type of sled is its low profile,
thereby limiting the potential for damage by anchors,
fishing nets, etc.
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Suspension Anodes
Suspension Anode Delivery Systems allow for strategic
placement of anodes in and around a marine facility,
providing optimum distribution of current. Many
suspended anode systems are also suitable for mounting
on pilings, or other structural steel.
Rod Anode
Although incorporated into a variety of anode
delivery systems, the rod anode is most commonly used
for the cathodic protection of seawater intake
structures and vessel internals.
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BACKGROUND ON ELGARD
Marine CP Technology
Proprietary technology, in tandem with advanced products and
engineering, position ELGARD as a leading cathodic protection
materials supplier.
ELGARD, the manufacturer of ELGARD LIDA mixed metal oxide anodes,
benefits from the substantial resources of its parent, ELTECH System
Corporation, which is the world's leading developer and producer of
DSA technology for the chlor-alkali and other electrochemical
industries. ELGARD Corporation's unique products and innovative
systems have been halting corrosion in steel-reinforced concrete and
conventional offshore and in-shore structures since 1974.
Mixed Metal Oxide Anode Technology
ELGARD LIDA anodes are comprised of a titanium substrate with a
mixed metal oxide coating. The mixed metal oxide is a crystalline,
electrically conductive coating that activates the titanium and
enables it to function as an anode. This coating has an extremely
low consumption rate, measured in terms of milligrams per year. As a
result of this low consumption rate, the anode dimensions remain
nearly constant during the life of the anode, providing constant
levels of performance for the duration of the anode design life. Due
to the ductility of the titanium substrate, a wide range of anode
shapes suitable to the structure to be protected are possible, such
as wire, rod, tubular, disk and mesh configurations.
