Sung Yul Lee

Corrosion Behaviors for Galvanizing, Galvalume and Chromate Treated Steels in 1% NaOH Solution

The galvanized steel structures may be inevitably corroded rapidly in the case of exposed to corrosive environments for long time, and these corrosive environments has been accelerated with increasing the environmental contamination due to the rapid development of industrial society. However, since the galvanizing method have the various merits compared to surface coating treatment in economical point of view, the galvanized steel have been extensively used to the numerous constructional steels such as a guard rail of high way, various types of structural steel for manufacturing ship and various industrial fields etc.. Therefore, it has been made an effort to improve the corrosion resistance of the galvanizing film through various methods such as variation of chemical composition of galvanizing bath, chromate treatment and coating treatment etc.. In this study, comparison evaluation on the corrosion resistance of three types of the test specimens, that is, three samples of pure galvanizing, galvalume and chromate treatment were investigated using electrochemical methods in 1% NaOH solution. The samples of chromate treatment and of galvalume exhibited the lowest and highest corrosion current density respectively in 1% NaOHsolution. In addition, the sample of chromate treatment revealed the highest impedance at 0.01Hz, which is considered that the oxide film by chromate treatment is deposited on the surface of pure galvanizing sample. After drawing polarization curves, the corroded surface of the pure galvanizing specimen indicated pattern like as localized corrosion, moreover, the large amount of corrosive products was observed on the surface of galvalume sample. However, the smooth pattern nearly similar to general corrosion was observed at the corroded surface of the sample of chromate treatment. Consequently, it is considered that the chromate treatment is an optimum method compared to pure galvanizing and, galvalume treatment to improve corrosion resistance in 1% NaOH solution.

Evaluation on the Addition of Inhibitor and Surface Treatment to Corrosion Behavior of the Reinforced Steel Bar Embedded in Mortar Specimen

In this study, seven types of mortar test specimens were manufactured with parameters, that is, the surface of the reinforced steel bar was treated with hot dip galvanizing (Zn) and the surface of the test specimen was coated with underwater paint, and four types of inhibitors (DAW, MCI, DCI, and Silcon) were added in mortars respectively. And, the seven types of mortar test specimens were immersed in seawater for 4 years. The corrosion properties of the reinforced steel bars embedded in mortar test specimens were investigated using electrochemical methods. The corrosion potentials of the test specimens with painting on the surface of the specimen and Zn coating on the surface of the steel bar exhibited the noblest and lowest values respectively after one year, however, after 4 years, the specimens of underwater painting and of addition of Silcon inhibitor indicated the noblest and lowest values of corrosion potentials respectively. Furthermore, the painting specimen exhibited the smallest values of corrosion probability as welll as of the corrosion current density, while, addition of MCI inhibitor showed the highest values of both corrosion probability and corrosion current density. Moreover, the painting specimen showed the smallest value of neutralization degree among all the specimens, and the largest value of neutralization degree was observed at the specimen of natural condition (no adding of inhibitor, no painting and no Zn coating). As a result, it is considered that the addition of inhibitors, coating with hot dip galvanizing (Zn), and painting on the surface have the effects not only to inhibit the neutralization degree but also to increase the corrosion resistance of the embedded steel bar.

Evaluation on the Mechanical and Corrosion Resistance of ALDC 12 Al Alloy by Three Types of Heat Treatments

Al is a active metal. However, it has a generally fine oxide film with thin and protective barrier, which is stable in air and neutral aqueous solution. Thus, Al alloys have been widely used in architectural trim, cold & hot-water storage vessels and piping. However, Al and most of their alloys may be easy to be corroded with various patterns of corrosion such as pitting, intergranular and galvanic corrosion etc. in the case of exposure to various industrial and marine corrosive environments. Therefore, an optimum evaluation of corrosion resistance for Al and Al alloys may be more important in a economical point of view. In this study, characteristics evaluation for ALDC 12 Al alloy by solution, normalizing, and aging (natural and artificial) heat treatments was carried out using electrochemical methods etc.. We can see that the hardness of ALDC 12 Al alloy were decreased by solution and aging heat treatments, in particular, the solution and natural aging (SNA) heat treatments indicated the lowest value of hardness. However, the lower hardness by heat treatment, the better corrosion resistance, for instance, the solution and natural aging (SNA) heat treatment exhibited the best corrosion resistance with showing the lowest hardness. Furthermore, the highest hardness was observed after normalizing and artificial aging (NAA) heat treatments. Therefore, it is considered that if improvement of the hardness as well as the corrosion resistance were to be required together, the normalizing and artificial aging (NAA) heat treatment should be performed. On the other hand, the solution and natural aging (SNA) heat treatment is thought to be an optimum heat treatment method for only corrosion resistance improvement.

Evaluation on the Corrosion and Wear Resistance of Welding Zone Welded with Plasma Transferred Arc Welding Method

There are many welding methods which have been currently performed to prolong the life time of exhaust valve of marine engine from an economic point of view. In this study, one of these welding methods, plasma transferred arc (PTA) welding was performed at the base metal of Nimonic 80A which would be used as the material of exhaust valve with three kinds of filler metals such as Stellite 6, Inconel 625 and Inconel 718. The mechanical and corrosion characteristics were investigated with electrochemical and wear loss test methods. The Nimonic 80A as the base metal had a better corrosion resistance than those of the weld metals welded with three types of the filler metals. However, after post weld heat treatment, all these weld metals mentioned above exhibited better corrosion resistance compared to the base metal, shifting the corrosion potential to the noble direction, and pitting corrosion was more or less observed at the surface of Nimonic 80A after post weld heat treatment. In particular, Inconel 625 showed the best corrosion resistance among the filler metals after heat treatment. The wear ratio of Stellite 6 showed the lowest value due to its highest hardness among the filler metals, however, its ratio by cavitation test exhibited the highest value. It is considered that increasing of embrittlement with increasing the hardness was resulted in increasing the wear loss by cavitation test.

Coating Thickness of Reinforced Concrete Affecting to Cathodic Polarization and Cyclic Voltammogram of Reinforced Steel

The reinforced concretes are often exposed to severely corrosive environments such as sea water, contaminated water, acid rain and seashore etc.. Therefore, the reinforced steel bar embedded in the concrete is increasingly corroded in various environments mentioned above, and this corrosion problem is thought to be very important in terms of the safety and economic points of view. In this study, a multiple mortar test specimen(W/C:0.5) with variation of coating thickness was prepared and immerged in flowing seawater for five years. And, the effects of coating thickness affecting to cathodic polarization and cyclic voltammogram were investigated using electrochemical methods. The thinner coating thickness, both invasion and diffusion of dissolved oxygen, water and chloride ion on the surface of reinforced steel bar is more easily compared to the thicker coating thickness. Thus, at the beginning of immersion, the rate of corrosion in the case of the thinner coating thickness is higher than that of the thicker coating thickness. However, it is considered that corrosion products deposited on the surface due to higher corrosion rate played the role as a resistance polarization, and increased the diffusion layer, as a result, decreased the corrosion current density compared to the thicker coating thickness. Consequently, the relationship between corrosion current density and the coating thickness were not well in good agreement with each other due to the corrosion products after being immersed for 5 years. Therefore, in order to more optimum evaluate for corrosion possibility of the reinforced steel embedded in the concrete, not only corrosion potential but also other parameters such as coating thickness, W/C ratio, and other corrosion environment should be investigated in the case of immersed in seawater for long years.

Characteristics of Inconel 718 Filler Metal Affecting to Welding Metal Zones of Cast Steel for Piston Crown Material

Since the oil price has been significantly jumped for recent some years, the diesel engine of the merchant ship has been mainly used the heavy oil of low quality. Thus, it has been often exposed to severely corrosive environment more and more because temperature of the exhaust gas of a combustion chamber is getting higher and higher with increasing of using the heavy oil of low quality. As a result, wear and corrosion of the most parts surrounded with combustion chamber is more serious compared to the other parts of the engine. Therefore, an optimum repair welding for these parts is very important to prolong their lifetime in a economical point of view. In this study, Inconel 718 filler metal was welded with GTAW method in the cast steel which would be generally used with piston crown material. And the corrosion properties of weld metal zone, heat affected zone and base metal zone were investigated using electrochemical methods such as measurement of corrosion potential, anodic polarization curves, cyclic voltammogram and impedance etc. in 35% H2SO4 solution. The weld metal and base metal zones exhibited the highest and lowest values of hardness respectively. Furthermore, the corrosion current density of the weld metal zone indicated the lowest value, having the highest value of hardness. The corrosive products with red color was more or less observed in the base metal zone, while the weld metal zone exhibited the general corrosion without corrosive products of red color. The microstructure of the pearlite with black color was predominantly observed in the base metal zone, however, the microstructure of ferrite with white color increased increasingly in the heat affected zone, and the microstructure of ferrite with white color was significantly observed in the weld metal zone. In particular, the polarization characteristics such as impedance, polarization curve and cyclic voltammogran associated with corrosion resistance property were well in good agreement with each other. Keywords : Repair welding, Inconel 718 filler metal, GTAW, Weld metal zone, Electrochemical method, Corrosion current density, Hardness. Microstructure

Evaluation of Welding Metal Zones in the Case Welded with 0.5 Mo Filler Metal to Forged Steel for Piston Crown Material

Since the oil price has been significantly jumped for recent some years, the diesel engine of the merchant ship has been mainly used the heavy oil of low quality. Thus, it has been often exposed to severely corrosive environment more and more because temperature of the exhaust gas of a combustion chamber is getting higher and higher with increasing of using the heavy oil of low quality. As a result, wear and corrosion of the most parts surrounded with combustion chamber is more serious compared to the other parts of the engine. Therefore, an optimum repair welding for these parts is very important to prolong their lifetime in a economical point of view. In this study, 0.5Mo filler metal was welded with SMAW method in the forged steel which would be generally used with piston crown material. And the corrosion properties of weld metal zone, heat affected zone and base metal zone were investigated using electrochemical methods such as measurement of corrosion potential, anodic polarization curves, cyclic voltammogram and impedance etc. in 35% H2SO4 solution. The heat affected and base metal zones exhibited the highest and lowest values of hardness respectively. And, the corrosion current density of the heat affected zone indicated the lowest value, having the highest value of hardness. It appeared that the corrosive products with red color was wholly covered on the surface of the base metal zone, while its products was not observed in the heat affected zone. The microstructure of the pearlite with black color was more or less observed in the base metal zone with patterns such as crystal and needle, in particular, the crystal pattern of ferrite microstructure with white color was considerably included in the base metal and heat affected zone, and the ferrite microstructure was significantly observed in the weld metal zone. In particular, the polarization characteristics such as impedance, polarization curve associated with corrosion resistance property were well in good agreement with each other. Keywords: Forged steel, Microstructure, 0.5Mo filler metal, SMAW, Weld metal zone, Electrochemical method, Corrosion current density, Hardness.

Passing Aged Years to Polarization Characteristics of the Steel Bar Embedded in Mortar Specimen (W/C:0.4)

The structures of reinforced concrete has been extensively increased with rapid development of industrial society. Furthermore, these reinforced concretes are often exposed to severely corrosive environments such as sea water, contaminated water, acid rain and seashore etc.. Thus, the corrosion problem of a steel bar embedded in the concrete is very important in terms of the safety and economic points of view. In this study, a multiple mortar test specimen (W/C:0.4) that had six types of cover thickness was prepared and immerged in flowing seawater for five years. And the effects of cover thickness and immersion years on the corrosion properties of the steel bars were investigated using electrochemical methods such as measuring corrosion potential, anodic polarization curve, and impedance. At the beginning of immersion (0 year), corrosion potentials exhibited increasingly nobler value with increasing the cover thickness. However, after being immersed for 5 years, the corrosion potentials conversely shifted in the positive direction with decreasing the cover thickness. As a result, the relationships between corrosion potential and cover thickness were not in good agreement with each other after 5 years. In addition, after 5 years, the thinner cover thickness, the higher value of impedance at 0.01Hz. It is considered that corrosive products deposited on the surface of the steel bar embedded in mortar specimen in the case of thinner cover thickness played the role as a resistance polarization which resulted in decreasing the corrosion current density. Consequently, it seemed to be somewhat problem that evaluation on the corrosion property in reinforced steel would be estimated by only the corrosion potential measurement. Therefore, it is suggested that we should take into account various parameters such as cover thickness, corrosion potential, and immersed years etc. for its accurate assessment. Keywords : Cover thickness, Corrosion potential, Anodic polarization curve, Impedance, Resistance polarization

Evaluation of Thermal Conductivity and Heat Flux by Insulation Analysis of Double-Wall for Cryogenic Storage Tank

The double-walled steel vessel filled with insulation powder in the space between two walls was used to minimize heat transfer by radiation and conduction in cryogenic storage tank. And, evaluation of the insulation effect by the thermal conductivity of perlite powder and the effect of heat flux by welding conditions were carried out with insulation analysis of double-wall. The solid supports were also used to bear the weight of the inner container. Furthermore, thermal and structural analysis of the tank was performed to study the effect of vacuum and weldment geometry of the internal supports. If the degree of vacuum were low, the thermal conductivity of perlite increases rapidly and the heat flux should be increased in proportion to the thermal conductivity. Two types of solid supports did not show significant differences in the aspect of working stress or heat flux. Therefore, it is considered that the heat flux is less sensitive against the weldment geometry.

The Effect of Cover Thickness to Corrosion Characteristics of Reinforced Steel Bar Emedded in Mortar Specimen (W/C:0.6) Aged 5 Years in Seawater

Reinforced concrete structures have been increasingly widely used through numerous industrial fields. These structures are often exposed to severely corrosive environments such as sea water, contaminated water, acid rain, and the seashore. Thus, corrosion problems of the steel bars embedded in concrete are very important from a safety and economic point of view. In this study, the effects of cover thickness on the corrosion properties of reinforced steel bar embedded in mortar specimen (W/C:0.6) were investigated using electrochemical methods such as corrosion potentials, polarization curves, cyclic voltammograms, galvanostat and potentiostat. Corrosion potentials shifted to the noble direction, and the value of AC impedance also exhibited a higher value with increasing cover thickness, furthermore, polarization resistance also increased with increasing cover thickness. This is probably that the thinner cover thickness, seawater solution is easy to arrive at embedded steel compared to other thicker cover thickness, so, its steel bar may be easily corroded due to chloride ion, which is resulted in shifting corrosion potential to negative direction, decreasing polarization resistance. Consequently, it is considered that the relation between corrosion resistance of reinforced steel and cover thickness is nearly matched with each other. However, its corrosion resistance estimated by measurement of corrosion potential was not well in agreement with value obtained by polarization curves.