| US6209472B1 | 2001-04-03 | |||
| JPH10147894A | 1998-06-02 | |||
| DE3822168A1 | 1990-02-15 | |||
| DE4110504A1 | 1992-10-01 |
2. Description of the Related Art In general, metallic surfaces on the marine structure such as a ship make themselves electrodes. The oxidation reaction occurs at anodic areas on metallic surfaces, and then metals loose their electrons and ionized metals (M) are discomposed, thereby going into water or solution. Formula (1-1) shows this oxidation reaction.
M<M+ e~ (1-1) The reduction reaction occurs at cathodic areas on metallic surfaces, thereby cathode attracts electrons that are generated in the process of oxidation of metals and electrons combine with the ionized hydrogen, thereby hydrogen gas is generated. Formula (1-2) and (1-3) show the generation of molecules of water and hydrogen gas, respectively. Also, formula (1-4) and (1-5) show the generation of hydroxyl.
°2 + 4H+ + 4e~ < 2H20 (1-2) 2HL'+2e'-H (1-3)
02+2HO+4e'4 (OH)' (1-4) 2H20+2e---HZT+2(OH)-(1- It can be thought of surfaces on the marine structure in seawater as an electrolytic model. The reason that surfaces on the marine structure are corroded is mostly because of the oxidation reaction at their anodic areas and the reduction reaction at their cathodic areas. Also, although the marine structures are made of the same metal, they could be corroded because the metal is separated into two electrodes, which are anode and cathode, according to the degree of electrolysis with respect to seawater or the electrolytic concentration in seawater.
Since surfaces on the marine structure are corroded easily by direct exposure to seawater, painting operation is performed so that it could prevent their corrosion. At this time, metals contacted with electrolyte react electrically and then the marine structure is corroded at the spots of defects in painting operation or local harm on painted surfaces. Finally, the corrosion damages the marine structure rapidly. Accordingly, electron-chemical protection methods such as the sacrificial anode method and the impressed current method are performed in order to protect metallic surfaces.
In the sacrificial anode method, two different metals are contacted with each other in electrolyte, and these two metals form a galvanic couple. In this method, one metal having relatively higher corrosion potential is polarized as an anode, whereas other metal having relatively lower corrosion potential is polarized as a cathode, then the active former provides a flux of electrons to the latter, thereby being solved into anodic polarity ; this phenomenon is corrosion. A certain metal has more corrosion potential in active direction, thereby forming a couple together with the other metal for serving as a sacrificial anode. Then, the metal working as a cathode is electro-chemically protected from the corrosion. Generally speaking, magnesium, zinc, and aluminium are used as sacrificial anodes for steel materials in seawater.
However, the sacrificial anode method has problems that its control of the magnitude and density of current is not possible, it is not suitable for a big structure, and a system using this method has a certain life.
The impressed current method is accomplished by polarizing the surfaces on corroded metals as a cathode so that it may reduce corrosion speed. Anodic reaction and cathodic reaction of iron corroded in neutral electrolyte solution are similar to above-described reaction formulas. If metallic surfaces are polarized as a cathode, a flux of excess electrons reverses equilibrium, thereby reducing corrosion speed. The impressed current method is applicable to any metal or alloy.
In a conventional technology using the impressed current method, a reference cell has been used in order to determine a quantity of protection current while the metal serves as a cathode, and it will be described with reference to FIG. 3, FIG. 4a, and FIG. 4b. FIG. 3 is a schematic sectional side view of the marine structure having an impressed current cathodic protection system for a marine structure with a reference cell according to the conventional system. FIG. 4a and FIG. 4b are schematic views for showing the layout of electrodes in FIG. 3. While FIG. 4a and FIG. 4b show the distribution of electrodes at rear and front of hull 112, respectively, FIG. 4a and FIG. 4b are plan views of FIG. 3.
In general, the marine structure 110 using the conventional impressed current cathodic protection system includes respective power supply units 120 at front and rear of hull 112. Furthermore, the marine structure 110 includes anodes 122 and reference cells 124 that are disposed at both sides of hull 112 and connected to the respective power supply units 120. The anodes 122 and the reference cells 124 installed by means of a structure such as a watertight cofferdam are disposed apart a predetermined distance. In the conventional system, potential difference between cathode (not shown) connected to the power supply unit 120 and the reference cell 124 are measured continuously. Also, the power supply unit 120 increases current
progressively until potential impressed to the anode 122 approaches a suitable protection potential.
In the conventional system, platinum (Pt) works as the anode 122, whereas zinc (Zn) works as the reference cell 124. Furthermore, impressed current in the conventional system is higher than current obtained from potential difference between metallic surfaces on the marine structure having Fe and the reference cell 124. For example, potential difference between metallic surfaces on the marine structure and the reference cell 124 in seawater having about 2% salinity is assumed as about 320mV. Then, the power supply unit 120 supplies a flux of excess electrons to the surface of the marine structure until the potential difference value is 220mV, which is considered as a fixed target.
However, the impressed current cathodic protection system for a marine structure having a reference cell according to the conventional system requires particular watertight structures in order to install the reference cell.
Furthermore, the conventional system has some problems that an initial cost is expensive, it is difficult to maintain the system, and the system cannot reflect all of the variable environments in seawater.
SUMMARY OF THE INVENTION To solve the above-described problems, it is an object of the present invention to provide an impressed current cathodic protection system for reducing the installation cost by removing the reference cell in the marine structure such as a ship, performing installing operation and maintenance easily, and corrosion protecting the marine structure in optimum condition.
In accordance with an aspect of the present invention, there is provided an impressed current cathodic protection system for a marine structure without a reference cell, the system comprising power supply units disposed at the certain positions of hull, and anodes connected to the power supply units and disposed at both sides of hull.
In accordance with another aspect of the present invention, the most suitable protection potential impressed to the anodes from the power supply units is within the range of 1500 to 2500mV in seawater and 4000 to 6000mV in freshwater in case of a marine structure made of steel material, while 2500 to 3500mV in seawater and 5000 to 7000mV in freshwater in case of a marine structure made of aluminium material.
BRIEF DESCRIPTION OF THE DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1 is a schematic side view of the marine structure having an impressed current cathodic protection system for a marine structure without a reference cell according to the present invention; FIG. 2a and FIG. 2b are schematic views for showing the layout of electrodes in FIG. 1; FIG. 3 is a schematic side view of the marine structure having an impressed current cathodic protection system for a marine structure with a reference cell according to the conventional system; and, FIG. 4a and FIG. 4b are schematic views for showing the layout of electrodes in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic side view of the marine structure having an impressed current cathodic protection system without a reference cell according to the present invention, FIG. 2a and FIG. 2b are schematic views for showing the layout of electrodes in FIG. 1. FIG. 2a and FIG. 2b show the layout of electrodes at rear and front of hull 12, respectively. In the drawing, the marine structure 10 is represented in the form of a ship.
Referring to the drawing, an impressed current cathodic protection system for a marine structure according to the present invention comprise power supply units 20 disposed at a certain positions of hull 12, and anodes 22 connected to the power supply units 20 and disposed at both sides of hull 12. Reference cells in the conventional system are omitted in the present invention. Thus, potential differences between the marine structure 10 to be protected and electrolyte, for example sea water, are measured only by means of anodes 22.
Protection potential to be impressed actually is set to be lower than corrosion potential, which can be obtained from potential difference between any surface of hull 12 and an anode 22, preferably 100-200mV lower than the obtained potential difference. Accordingly, by adding 100-200mV to corrosion potential in order to supply a flux of excess electrons to the surface of hull 12, protection potential can be obtained. In the course of obtaining the protection potential, current impressed from the power supply unit 20 is increased progressively until protection potential approaches the most suitable value.
In the present invention, the most suitable protection potential impressed to the anodes from the power supply units could be obtained by using the trial and error method. In case of a marine structure made of steel material, the value is within the range of 1500 to 2500mV in seawater and 4000 to 6000mV in freshwater. Also, in case of a marine structure made of aluminium material, the value is within the range of 2500 to 3500mV in seawater and 5000 to 7000mV in freshwater.
An additional value 100 ~ 200mV, which is added for the purpose of supplying a flux of excess electrons to the surface of the marine structure, is a reference value recommended by NACE (National Association of Corrosion Engineering) in U. S. A. , which is adopted in marine associations of many other nations. Also, potential between surfaces of various marine structures including marine transportation or underwater structure and anodes is measured continuously in order for the marine structure to be in optimum
corrosion protection, and enabling the system to automatic control by means of the computer.
According to the impressed current cathodic protection system for a marine structure without a reference cell, reference cell disposed by means of a structure such as a watertight cofferdam, which has been installed for four per the marine structure such as a ship, can be removed, thereby reducing the installation cost. Furthermore, installing operation and maintenance of the protection system can be performed easily, and the breakage of the marine structure can be minimized.
Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiment described above will occur to those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
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