Corrosion protection of underwater piles is essential in case of steel piles. When steel piles are used in seawater, it reacts chemically to form anodes and cathodes resulting in the flow of electricity, which results in the corrosion of anodic areas of piles.
Chemically active surface areas of underwater steel piles act as anodes and less chemically active surfaces acts as cathodes.
What is Corrosion of Piles?
Corrosion can be defined as the deterioration of the metal due to reaction with water and atmospheric air or the oxidation of metals catalyzed by chemical reaction with environment
What are Underwater Piles?
Piles are the substructure member of the building used to transfer the loads from the super structure to the ground.
When a structure is built in the water, the piles are to driven in the water to underwater strata. As the piles are always inside the water, they are subjected to corrosion. To eliminate this corrosion problem is very hard, but the corrosion can be controlled by using corrosion protection measures.
Zones of Corrosion of Underwater Steel Piles
The piles which are driven into the water strata undergo corrosion in several zones due to the different contact with water. It is convenient to divide these areas into four zones, each having a characteristic corrosion rate as shown in figure below.
Corrosion Protection Methods for Underwater Piles
Protective Coatings for Underwater Piles
The corrosion in piles form when the surface of pile comes in contact with water. To protect the piles from coming in contact with water, the pile is to be coated with non-porous material which is anti-corrosive.
The different types of coatings used for under water piles are:
Inorganic Zinc Silicates Primers
The steel structure which is below the plash zone is always immersed in water are commonly not coated with any cathodic protective layers for obvious reasons.
There are numerous types of anti-corrosive pigmented primers in which inorganic zinc silicate is the best. The best part of this is that it arrests rust creep or undercutting of the coating surrounding the damages area and confines the corrosion to the point of damage.
High Build Epoxy Coatings
These epoxy coating are more abrasion and chemical resistant that that of primers and coats. It is because they not only protect the metal but the zinc primers also from detrimental factors.
It also has a drawback that is poor resistant to sunlight and chalk. When it comes in contact with these factors, they fade quickly which leads to erosion of coating which in turn reduces the barrier protection of the system.
Aliphatic Polyurethane Topcoats
The polyurethane is actually not an anti-corrosion or corrosion barrier, but is provides optimum resistance to UV and high degree of flexibility and chemical resistance.
They also help in retaining the cosmetic glow and color of the material. They provide high level of protection to the coating system.
Zinc Rich Epoxy Primers
In is the mixture of both Inorganic Zinc Silicates Primer and High Build Epoxy Coating. It provides a high level of service and more tolerant ambient weather conditions. It is also most effective in maintaining the damaged area and breakdown of coating system.
Non-Skid Deck Coatings
They are applied in very high film builds and normally without a zinc rich primer. Coatings specifically designed with anti-slip properties normally incorporate very course aggregates for an exaggerated profile.
They are applied in very high film builds and normally without a zinc rich primer. When primers are required they are usually epoxy types.
Cathodic Protection of Underwater Piles
Cathodic protection is the commonly used technique for the overcoming the corrosion on piles. Cathodic protection is the process of using electrochemical reactions to prevent steel from corrosion. It is commonly used and accepted because it prevents the corrosion on steel which is in water.
In theory and practice, the implementation of a cathodic protection 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).
This is a very simple process for installing a cathodic protection for underwater piles. When the steel is corroded, an anode with power supply is inserted into the water near the steel. The insertion or delivery of anode is made in many ways depending upon the various factors. The types of anode delivery are,
Pile Mounted Anode
This type of anode delivery method is used when anode can be attached to the cathode or piles directly. Pile mounted anode are designed for efficiently for the current distribution in and around the piling where the complexity is faced in placing the anode to the pile in remote places.
The Flat Back Pile Mounted Anode was designed specifically for H-Piles, and can also be configured for installation on sheet piling.
Pile Mounted Anode
This type of anode delivery system is used when the anode is to be replaced with new one periodically. When the cathodic protection is needed only time to time but not continuously, this type of system is used so that the anode can be replaced easily.
Sled anodes can be designed for operation in either seawater or buried in the mud. Anodes mounted on the sea bed typically afford the best spread of protection on a marine structure.
By adjusting the height of the concrete sled, the mesh anode sled can also be designed for operation out of the mud. The Post Tension Sled was developed to insure anode operation out of the mud when resting in silty and soft sea beds.
The advantage of this type of sled is its low profile, thereby limiting the potential for, damage by anchors fishing nets, etc.
Application of FRP Composites for Corrosion Protection of Underwater Piles
The use of FRP is simply, that it is mixed with the wet concrete, which makes it economical to conduct repairs on sub structure parts.
Normally the repair of this parts require the enlargement to accommodate new ties. But when FRP are used the completely corroded part of the element is carefully removed and this FRP induced concrete is applied, which provides the lost tensile capacity and also provides the lateral support to the steel.
As the FRP is applied with concrete, spreading of corrosion to other piles can be protected and at the same time it protection from UV coating on the wrap of the right color. The aesthetics of FRP repair is one of its unheralded benefits.
FRP Wrapping for Corroded Bridge Piles