Marine Cathodic Protection
A Survey Article by ELGARD Corporation
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.
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.
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.
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.
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.
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.
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.
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.