• Corrosion Science and Technology
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• 제7권3호
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• pp.182-186
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• 2008

The microstructures of the oxide scale developed at high temperature on steels are very complex and their development depends on many factors including time, temperature, oxidation conditions and alloying elements. The classical model of an oxide scale on steel consisting of wüstite, magnetite and haematite layers, is more complicated in reality and its properties change with the factors that affect their development. An understanding of the oxide scale formation and its properties can only be achieved by careful examination of the scale microstructure. The oxide scale microstructure may be difficult to characterise by conventional techniques such as optical or standard scanning electron microscopy. An unambiguous characterisation of the scale and the correct identification of the phases within the scale are difficult unless the crystallographic structure for each phase in the scale is considered and a simultaneous microstructure-microtexture analysis is carried out. In the current study Electron Backscatter Diffraction (EBSD) has been used to investigate the microstructure of iron oxide layers grown on low carbon steels at different times and temperatures. EBSD has proved to be a powerful technique for identifying the individual phases in the oxide scale accurately. The results show that different grain shapes and sizes develop for each phase in the scale depending on time and temperature.

• Corrosion Science and Technology
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• 제23권1호
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• pp.33-40
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• 2024

The concentration and diffusion coefficient of oxide ion vacancies in the oxide scale formed on Fe-22Cr-0.5Mn ferritic stainless steel with and without molybdenum (Mo) was measured at 800℃ by the electrochemical polarization method. After pre-oxidation for 100 h in ambient air at 800 ℃, the oxide scale on one side was completely removed with sandpaper. A YSZ plate was placed on the side where the oxide scale remained. Platinum (Pt) meshes were attached on the top of the YSZ plate and the side where the oxide scale was removed. Changes in electrical current were measured after applying an electrical potential through Pt wires welded to the Pt meshes. The results were interpreted by solving the diffusion equation. The diffusion coefficient and concentration of oxide ion vacancy decreased by 30% and 70% in the specimen with Mo, respectively, compared to the specimen without Mo. The oxide ion vacancy concentration of chromia decreased due to the addition of Mo.

• 대한금속재료학회지
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• 제50권4호
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• pp.300-309
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• 2012

Low-carbon steels and a stainless steel were oxidized isothermally and cyclically between $1050^{\circ}C$ and $1200^{\circ}C$ for up to 100 min in air to find the effect of alloying elements of Si, Mn, Ni, and Cr on their oxidation. The most active alloying element of Si was scattered inside the oxide scale, at the scale-alloy interface and as internal oxide precipitates beneath the oxide scale. Manganese, which could not effectively improve the oxidation resistance, was rather uniformly distributed in the oxide scale. Nickel and chromium tended to present at the lower part of the oxide scale. Excessively thick porous scales formed on the low-carbon steels, whereas thin but non-adherent scales containing $Cr_2O_3$ formed on the stainless steel.

• 한국초전도ㆍ저온공학회논문지
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• 제21권2호
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• pp.22-25
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• 2019

The superconducting magnetic separation system has been developing to separate the iron oxide scale from the feed water of the thermal power plant. The accumulation in the boiler lowers the heat exchange rate or in the worst case damages it. For this reason, in order to prevent scale generation, controlling pH and redox potential is employed. However, these methods are not sufficient and then the chemical cleaning is performed regularly. A superconducting magnetic separation system is investigated for removing iron oxide scale in a feed water system. Water supply conditions of the thermal power plant are as follows, flow rate 400 t / h, flow speed 0.2 m / s, pressure 2 MPa, temperature $160-200^{\circ}C$, amount of scale generation 50 - 120 t / 2 years. The main iron oxide scale is magnetite (ferromagnetic substance) and its particle size is several tens ${\mu}m$. As the first step we are considering to introduce the system to the chemical cleaning process of the thermal power plant instead of the thermal power plant itself. The current status of development will be reported.

• 한국전기전자재료학회논문지
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• 제34권2호
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• pp.142-147
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• 2021

We present a subthreshold swing model for a symmetric junctionless double gate MOSFET. The scale length λ1 required to obtain the potential distribution using the Poisson's equation is a criterion for analyzing the short channel effect by an analytical model. In general, if the channel length Lg satisfies Lg > 1.5λ1, it is known that the analytical model can be sufficiently used to analyze short channel effects. The scale length varies depending on the channel and oxide thickness as well as the dielectric constant of the channel and the oxide film. In this paper, we obtain the scale length for a constant permittivity (silicon and silicon dioxide), and derive the relationship between the scale length and the channel length satisfying the error range within 5%, compared with a numerical method. As a result, when the thickness of the oxide film is reduced to 1 nm, even in the case of Lg < λ1, the analytical subthreshold swing model proposed in this paper is observed to satisfy the error range of 5%. However, if the oxide thickness is increased to 3 nm and the channel thickness decreased to 6 nm, the analytical model can be used only for the channel length of Lg > 1.8λ1.

• Corrosion Science and Technology
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• 제11권3호
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• pp.82-88
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• 2012

The thickness and chemical composition of oxides on heat recovery steam generator tubes of combined cycle power plant were examined in order to evaluate the corrosion of the tubes. Tubes were removed from the plant after actual operations for 21,482, 42,552 and 56,123 hours respectively. Thickness and growth rate of the oxide scale on reheater inner tube (SA213-T22) were very high compared to those other tubes. The oxide scale was about $250{\mu}m$ thick and uniform. The components of the scale were iron oxides. The oxide scale was mixed oxides consisting of magnetite$(Fe_3O_4)$ and hematite$(Fe_2O_3)$. The oxide on inner tube was removed using many kinds of chemicals and it was found that chelating agents were dissolved faster than other chemicals.

• 동력기계공학회지
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• 제11권2호
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• pp.44-50
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• 2007

The oxidation behavior of commercial pure titanium is investigated in the temperature range of $727^{\circ}C{\sim}950^{\circ}C$ in mixed gases. The weight change is measured by TGA during oxidation in mixed gases. The oxidation behavior indicated by weight gain or the growth of oxide layer is based on the linear rate law at high temperatures. The structure of the oxide scale formed during oxidation is analysed by optical microscopy, electron probe microanalyzer, scanning electron microscope and x-ray diffraction. Oxide scales have a $TiO_2$ structure, and are constituted with multi-layered or two layered porous external one and a dense internal one. Ti-O solid solution region is formed at the interface of metal and scale layer. The formation of oxide scale is influenced by the oxidation temperature, time, crystal structure and the condition of atmosphere.

• 한국전기전자재료학회논문지
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• 제20권7호
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• pp.569-574
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• 2007

In this paper, the reliability (NBTI degradation: ${\Delta}V_{th}$) and device characteristic of nano-scale PMOSFET with plasma nitrided oxide (PNO) is characterized in depth by comparing those with thermally nitrided oxide (TNO). PNO case shows the reduction of gate leakage current and interface state density compared to TNO with no change of the $I_{D.sat}\;vs.\;I_{OFF}$ characteristics. Gate oxide capacitance (Cox) of PNO is larger than TNO and it increases as the N concentration increases in PNO. PNO also shows the improvement of NBTI characteristics because the nitrogen peak layer is located near the $Poly/SiO_2$ interface. However, if the nitrogen concentration in PNO oxide increases, threshold voltage degradation $({\Delta}V_{th})$ becomes more degraded by NBT stress due to the enhanced generation of the fixed oxide charges.

• 설비공학논문집
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• 제24권3호
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• pp.281-287
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• 2012

The internal oxide scale of twelve superheater and reheater tubes were tested which were serviced for 30,000~120,000 hours in thermal power plants. The oxide scale was formed in three layers. The Cr-rich area was observed beneath the original metal surface. The hematite ($Fe_2O_3$) phase was formed on the outer surface. The intermediate layer was magnetite ($Fe_3O_4$). The thickness of Cr-rich layer was about half of the total scale. All layers grew during the operation hour of the plant. The thickness of thickest scale was 0.2mm in superheater tubes. This can increase the tube metal temperature about $7^{\circ}C$ more than initial state. $7^{\circ}C$ tube metal temperature can reduce tube life about 30%, but the boiler tube's design margin is big enough therefore it has been analyzed that it would not effect on the life span.

• Corrosion Science and Technology
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• 제2권5호
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• pp.225-232
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• 2003

High temperature oxidation of SUS430 and SUS304 stainless steels in 16.7 kPa $O_2$ - 20.3 kPa $H_2O$ - balanced N2 atmosphere at 1273 K was studied focused on the scale spalling during cooling after an isothermal oxidation. Spalling of the oxide scale during cooling occurred only for SUS304 stainless steel. The oxide scale was composed of two layers and they detached at the interface between them. The reason for the spalling could not be explained only by thermal stresses applied to the specimen during heating and cooling. A new mechanism for scale spalling was proposed based on combination of thermal stresses and thermal shock caused by a fast Martensite transformation of substrate metal.