방청도료의 내식성에 미치는 염분농도와 삼투압의 평가

Abstract

Due to rapid increasing of environmental contamination, the steel structures have been easily exposed to the severe corrosive environment. Therefore, the corrosion problem of these steel structures has generated numerous safety problems and economical damages in the economical and safety point of view. Furthermore, it has been revealed that the protection cost by coating method for corrosion control of the these steel structures estimates approximately 63% of total protection cost. Although in Korea, the percentage of the protection cost by the coating method was not clearly verified, it is expected that the cost may be nearly same as that of Japan. Therefore the corrosion control is being generally significantly accepted as an important issue in not only in economical point of view but also in safety reason. Therefore, those steel structures should be protected by an optimum protection method. Many protection methods such as surface coating, electric protection or some other methods etc. have been applied to numerous steel structures which have been widely used in continental and marine areas. Most of those steel structures are mainly protected by coating method. However, some steel piles being under seawater are generally protected by electric protection method, that is, either as an impressed current or as a sacrificial anode method. Furthermore, environmental contamination may be resulted in severe corrosive environment, which, in turn, cause to be accelerated corrosion of the steel structures. Subsequently, the coated steel structures would deteriorate more rapidly than the designed life time due to acid rain caused by air pollution etc. Therefore, when the marine plant is often exposed to the seawater, that is, severe corrosive environment, the corrosion resistance of anti-corrosive paint may be more and more important and its corrosion resistance property have been increasingly required. So it is thought that the development of anti-corrosive paint increasing the corrosion resistance is important, and the choice of an optimum method which is to evaluate rapidly and correctly the corrosion resistance of anti-corrosive paint is also important and is necessary in economical point of view. In this study, four types of epoxy series paints are prepared with different resin series and additives. And, after these specimens coated with four types of paints(DFT(dry film thickness) : 25㎛) made by different epoxy are submerged in various salt concentration(0.1%, 0.3%, 3%, 6%, 9% and 15%), their corrosion and osmotic resistances were investigated with electrochemical methods such as corrosion potential, polarization curves, impedance and cyclic voltammogram measurements etc.. Chapter. 3.1 was investigated with electrochemical methods such as measurement of corrosion potential, cathodic and anodic polarization curves, cyclic voltammogram and AC impedance when the test specimens are coated with 4 types of anti-corrosive paints with different resin series and additives and submerged in various salt concentration(0.1%, 3% and 9%). The corrosion current density of these samples submerged in 0.1% NaCl solution exhibited a higher value than those immersed in 3% NaCl solution because it makes easier for water, dissolved oxygen and chloride ion etc. to invade toward inner side of coating film due to increasing of the osmotic pressure compared to 3% NaCl solution. However, when chemical additive of amine series was involved in paint, the corrosion resistance decreased with increasing of the salt concentration, for example, the corrosion current density in the case of submerged in 9% NaCl solution indicated a higher value than that submerged in 0.1% NaCl solution. Consequently, it is considered that the mechanism on the corrosion resistance of the coated steel plate is surely different with bare steel plate due to the osmotic pressure between the salt solution and the coating film. Moreover, it can be known that if a special additive were included in paint, the higher salt concentration regardless of the osmotic pressure, the corrosion resistance may be decreased. Chapter 3.2 was also investigated with electrochemical methods such as measurement of corrosion potential, cathodic and anodic polarization curves, cyclic voltammogram and AC impedance when the test specimens are coated with 4 types of anti-corrosive paints with different resin series and additives and submerged in various salt concentration(0.3%, 6% and 15%). The corrosion resistance of the sample submerged in 6% NaCl solution exhibited the best condition compared to the samples which were submerged in 0.3 and 15% NaCl solutions. It is considered that the osmotic pressure in the case of submerged in 6% NaCl solution is to decrease than that of the 0.3% NaCl solution, and the concentration of chloride ion is to become a smaller value compared to 15% NaCl solution, which is resulted that the corrosion current density of the 6% NaCl solution indicated the smallest value compared to 0.3% and 15% NaCl solutions. Consequently, in the case of 0.3% NaCl solution, it is easier for the water, dissolved oxygen and chloride ion to invade into the painting film compared to 6% NaCl solution due to higher osmotic pressure, and in the case of 15% NaCl solution, also it is easier for the water, dissolved oxygen and chloride ion to invade into the painting film compared to 6% NaCl solution due to higher value of chloride ion. The sample coated with ceramic epoxy series(AC specimen) exhibited the highest resistance against the osmotic pressure and chloride ion compared to other samples.

Chapter 3.3 was also investigated with electrochemical methods such as measurement of corrosion potential, cathodic and anodic polarization curves, cyclic voltammogram and AC impedance when the test specimens are coated with 4 types of anti-corrosive paints with different resin series and additives and submerged in various salt concentration(0.1%, 0.3%, 6% 9% and 15%). The corrosion current densities of these samples submerged in 3% NaCl solution exhibited the smallest values compared to other salt solutions. However, in the case of lower values of salt solutions than 3% NaCl solution, the corrosion current density increased again because it makes more easy for water, dissolved oxygen and chloride ion etc. to invade toward inner side of coating film due to increasing of the osmotic pressure than 3% NaCl solution, but in the case of higher values of salt solutions than 3% NaCl solution, the coating film is easily deteriorated due to high concentration of chloride ion rather than the osmotic pressure, which is resulted in increasing the corrosion current density. On the contrary, AC sample with ceramic epoxy indicated the best corrosion resistance in both lower and higher salt solutions than 3% NaCl solution compared to other samples. Consequently, it is considered that the mechanism of the corrosion resistance for the coated steel plate is completely different with bare steel plate due to the osmotic pressure between the salt solution and the coating film, and the corrosion resistance of coating film against the osmotic pressure as well as the chloride ion depend on various types of epoxy for paint.