With the rapid development of industry corrosion of steel structures exposed to the severe environment has generated numerous social and economical problems in the economical point of view. Furthermore, it has been revealed that the economical loss by corrosion of steel structures was approximately 2~4% of GNP in the U.S. in 1998. Although in Korea, the percentage of the economical loss by the corrosion was not clearly verified, it was assumed that the loss would be much greater than the U.S’. Therefore a corrosion control is being generally accepted as important issue not only in economical point of view but also in safety reason and those steel structures should be protected by an optimum protection method. And there are many protection methods for steel structure one of them is a cathodic protection including both impressed current method and sacrificial anode method, which are being mainly used for steel structures in marine environment.
Recently, the corrosion property of sea water was being changed the increase of environmental contamination, so the optimum protection design should be changed corresponding to some parameters such as degree of contamination, temperature, velocity (flow rate) of sea water and surface condition of steel structure.
In this study, the optimum protection design for steel structures in marine environment was investigated with some parameters such as variation of supplied cathodic current density, surface condition of cleaned or rusted and solution state of stirring or no stirring through the simulation technology, followed by both laboratory and field experiment.
Chapter. 3.1 is to investigate the optimum protection potential and protection current density with some parameters such as variation of supplied cathodic current density, surface condition of cleaned or rusted and solution state of stirring or no stirring.
The corrosion current density of the rusted surface was considerably amall compared to the cleaned one and the cathodic polarization potential of cleaned specimen was more negative than that of the rusted one at the same applied cathodic current density.
However the protection potential of cleaned specimen was below -770 mV (SCE), on the other hand its potential of the rusted one was lower than -700 mV (SCE) irrespective of solution stirring condition and in the case of no stirring condition, the protection current density was 100 mA/m2 at both cleaned and rusted specimens, however, it was revealed that the protection current density in stirring condition was about 200 mA/m2 due to increasing of oxygen diffusion on the cathode surface. It is suggested that the optimum protection potential and current density should be controlled by the surface condition of structures as well as solution stirring condition.
Chapter 3.2 is to reconfirm the experimental results of chapter 3.1 by field experiment. As the results of chapter 3.2, the consumption rate and producing current of anode in the case of rusted steel pile was much greater than the cleaned steel pile and the applied current density for protection potential (-770 mV (SCE)) was also much larger compared to the cleaned steel pile.
Chapter 3.3 is to investigate the variation of anode producing current and cathodic polarization potential of some steel piles in case of their steel piles was protected by sacrificial anode in field experiment. When some steel piles electrically connected with the other adjacent piles which was protected or not by sacrificial anode was being also cathodically protected by sacrificial anode, anode producing current between sacrificial anode and their steel piles was varied with corresponding to whether the other adjacent piles were cathodically protected because anode producing current was consumed to the other adjacent piles for protection or saved due to their steel piles were protected slightly by other anode producing current between sacrifical anode and the other adjacent piles for protection. Therefore it is suggested that life time of sacrificial anode for some steel piles protection can be also varied with increasing or decreasing of anode producing current according to the other adjacent piles’ protection or not.
Consequently it is concluded that for the optimum protection design, the surface condition as well as the contamination degree of sea water should be considered to determine some factors such as life time, current efficiency and protection current density.