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Some of the details about corrosion in sea water and cathodic protection methods are as follows to emphasize the importance of cathodic protection.
CORROSION IN SEA WATER
Sea water is a strong electrolyte as containing average % 3-4 dissolved form of salt in it’s composition. Percentage of salt is varies according to the seas. Therefore ,ph value ,resistance of sea water ,sodium chloride value and intensity of dissolved salt vary according to the seas.
Except all these ,various dissolved gases (oxygen ,carbon dioxide ,ozone ,iodine ,bromine) are availablein sea water. In terms of corrosion ,oxygen is the most important of all these. The dissolved oxygen concentration in area close to the surface area is around 8 mg/litre at 20OC.
Sea water ph value varies between 7.5 and 8.0 . In these circumstances ,corrosion proceeds to the steel surface under control of the oxygen diffusion. While Fe = Fe+2 + 2e- reaction is formed in the anode , equivalent amount of H20 + 1/2O2 +2e- = 2OH reaction should occur in the cathode. Thus ,the electrons liberated at the anode are wasted. Sufficient amount of oxygen must be diffused to the metal surface for the progress of the cathodic reaction. Sea water takes oxygen from the atmosphere. Dissolved oxygen is more abundant in area close to the sea surface. Oxygen diffusion from the atmosphere ,gets difficult when the depth is deepened. This situation is so important for corrosion of pier piels in sea water.
Pier piels ,platforms and other steel piles in sea water are under severe corrosive effects of sea water. Corrosion rate is particularly exacerbated in water impact area for these metal structures. Corrosion events and precautions to be taken are different for the metal structure’s parts situated in sea water and parts situated on sea water.
In summary of corrosion events’ general characteristics for the fixed structures in sea water are as follow ;
a) The high amount of chloride ions in the sea water ,revent inactivation of steel. Therefore ,the protective oxide layer does not occur on the steel surface situated in sea.
b) The electrical conductivity of sea water is too high. This situation causes corrosion cells’ that occur with galvanic action on steel surface or due to the concentration difference to be more effective. The corrosion rate of steel situated in sea water is about 10 times more than corrosion rate of steel situated in any ground.
c) Numerous live plants and animals live in sea water. These biological assets create a natural shell by adhering to metal surface. This shell shows a heterogeneous feature with effect of corrosion product and carbonate ,silicates in sea water.
The shell that formed within a certain time on staked piles into sea, has two negative effects in terms of corrosion.
1) The metal surface covered with biological assets takes less oxygen than uncovered surface and covered part will act as anode. However ,this cover causes reduction of required current during cathodic protection.
2) Decay occurs when biological assets die. As a result of plant organisms’ decay acid occurs. Also as a result of animal organisms’ decay hydrogen sulfide and nitrogen compounds occur. In both cases corrosion rate incereses in that area.
As it is explained above ,the corrosion rate in sea water directly depends on oxygen diffusion. This situation causes the formation of three seperate area on staked steel piles into the sea.
Corrosion areas for a staked steel pier’s pile form three seperate areas as shown in the figure below.
Corrosion rate in sea water remains constant regardless of the depth. Corrosion rate is even smaller than the sea area at the bottom of seabed where the pile has gone into ground. Corrosion rate rises nearly doubled near the water surface area where oxygen diffusion is easier than the other areas. Corrosion rate is the highest level above the sea level where the area getting wet and dry from time to time due to waves or tide. Tidal area is the most influenced area of a pile by corrosion.
When the researches are examined ,it is understood that the corrosion rate varies between the seas. Most severe corrosion area is just below the sea water level as the common result of the researches.
CORROSION PREVENTION OF THE PIER’S STEEL PILES
Corrosion occurring on steel piles is not in a constant character and is distributed in places, in the form of cavities on the surface .Some areas where the cavities are dense on the pile surface ,can be drilled in a short time. Therefore it is very difficult to predict the life time of staked piles into the sea. Occurrence of unanticipated perils is possible ,if necessary measures are not taken to prevent the corrosion.
Painting or coating is not an effective solution for the staked piles into seas to protect from corrosion. The main reason of that is even the most perfect paint can not sustain in the sea more than a few years. Renewal of surface paint or touch up in water is not discussed. During staking of the pile into the seabed ,previously made coating crumbles and loses it’s credibility when coating method is used.
Cathodic protection is the most appropriate and certain method to protect piles against corrosion. Lifetime of steel piles shall be increased for the long term by using cathodic protection. However ,the part of piles that permanently staying under the water can be protected by cathodic protection. It is not possible to protect the area above the sea level or the area wetted and dried from time to time due to wave effect cathodic.
In practice, after the piles’ cathodic protection is provided ; special protective coating is applied to -/+ 1,00 m. wide section of water level.This coating may be a concrete ,plastic or corrosion resistant metal.
Cathodic protection can be applied in two different ways for the staked piles into the sea water.
a) Galvanic Anode Cathodic Protection
b) Impressed Current Cathodic Protection
There is no difference in term of protection reliability for these two systems which have same theoretical principles.However,the application technique is quite different.
Sufficient current is applied to pile to form the cathode for both system. Therefore ,first of all required current must be determined. Needed current of staked piles into the sea ,depends on temperature and the resistance of sea water against to electrical current. Different sea current needs are measured at different seas.
This required current values are for the initial state of the piles. After a while of cathodic protection implementation ,alkaline environment formed as a result of cathodic reaction ,creates crusting on metal surface and that causes the reduction of needful current. The rate and duration of the needful current depends on the environment ,nature and thickness of the shell formed on the metal surface.
Needful current for piles into the sea can be determined experimentally same as pipeline systems. A fully insulated sample pile with special connections is required for this aim. It is very difficult to carry out this experiment on the completed piers. It is necessary to conduct long term studies on the samples ,in laboratory conditions ,to determine the required current.
a) Galvanic Anode Cathodic Protection
Required electrons are obtained from the galvanic cells (such as magnesium ,zinc etc.) to protect the metal surface. There is no need for external electric current for this system ,so protection made with this system is called as bouncer systematic protection. Although the cost of first facility installation is more expensive ,these systems requires minimal operating and maintenance costs. In continuous operations ,the subjects are not discussed such as power consumption and rectifier failure. When suitable places can not be found for the anode bed or the management factors are considered ,this system must be applied. It is not possible to get more current from the anode than it gives in bouncer systematic protection. Current can be increased as required with potentiometers on Tr/Red in algebra system protection.
b) Impressed Current Cathodic Protection
This protection system consist of the main source (Tr/Red) and connected anode to this source. Feed current is externally provided. On anode bed ,the iron / silicon – metal oxide coated titanium – graphite and scrap iron are used as the anode in this system. Anode beds can be done horizontally and vertically. Metal parts remain unprotected during the power cuts and T / R failure.