Papers by Author: Dong Bok Lee

Abstract: ASTM T22 (Fe-2.25Cr-1Mo), 347HFG (Fe-18Cr-11Ni), and 310H (Fe-25Cr-19Ni) steels were reacted with CO₂-0.3SO₂-6O₂ mixed gas. During corrosion at 700-800 °C for 20-100 h, 347HFG (Fe-18Cr-11Ni), and 310H (Fe-25Cr-19Ni) steels had good protective Cr₂O₃ oxide scale. Corrosion rates increased progressively as the corrosion temperature and time increased. Corrosion resistance increased in the order of T22, 347HFG, and 310H, suggesting that the alloying elements of Cr and Ni beneficially improved the corrosion resistance of steels. Basically, Fe oxidized to Fe₂O₃, and Cr oxidized to Cr₂O₃, some of which further reacted with FeO to form FeCr₂O₄.

Abstract: The high-temperature corrosion behavior of Ni₃Al+2.9 wt% Cr alloy was studied in SO₂-containing environment. Corrosion tests were carried out at 900, 1000, and 1100 °C for 100 h in atmospheric Ar-0.2% SO₂ gas. The alloy corroded relatively slowly due mainly to formation of Al₂O₃ in the scale. Its corrosion kinetics deviated from the parabolic corrosion rate law to a certain extent owing to ensuing scale spallation. This was attributed to (1) stress generated during scaling and the subsequent cooling period, (2) voids that formed due to the Kirkendall effect, and (3) incorporation of sulfur in the scale. The scale that formed after corrosion at 900 °C consisted of the outer NiO scale, middle NiAl₂O₄ scale, and inner Al₂O₃ scale. The increased corrosion rate at 1000 and 1100 °C led to formation of the outer NiO scale, and inner Al₂O₃ scale.

Abstract: Pure Fe₃Al and Fe₃Al+4%Cr alloys were corroded at 1000 °C for up to 200 h in N₂-0.1%H₂S-mixed gas in order to study their corrosion behavior in H₂S-containing atmosphere. The formed scales consisted primarily of α-Al₂O₃, FeAl₂O₄, and Fe₂O₃. In these oxide scales, hydrogen and sulfur dissolved according to the reaction; H₂S→2H+S. Corrosion products of Cr were not identified in the scales from the XRD analysis, indicating that Cr dissolved in the oxide scales. Fe₃Al+4%Cr alloy displayed poorer corrosion resistance than Fe₃Al alloy, indicating that chromium accelerated the corrosion rates of Fe₃Al alloys.

Abstract: The Ti-6Al-4V alloy was nitrided at 950 °C for 8 h by heating under atmospheric nitrogen in order to improve its surface hardness and oxidation resistance. Nitrogen diffused into the Ti6Al4V alloy, and formed ~40 μm-thick coating consisting of TiN as the major phase and Ti₂N as the minor one. Nitriding increased the surface microhardness through the strengthening effect of interstitial nitrogen and the formation of nitrides. Oxidation at 700 °C for 10 h formed a superficial TiO₂ layer on the coating.

Abstract: The pack-cementation is one of economical, efficient coating processes for Fe-base alloys. It can provide good protection against high-temperature oxidation and corrosion. In this study, the high-temperature corrosion behavior of the aluminized diffusion-coating on low-carbon T20 steel (Fe-2.0Cr-0.5Mo-0.8Mn-0.3S in at.%) was studied at 800 °C in N₂/H₂O/H₂S-mixed gas. The aluminized coating consisted of Fe₃Al. The aluminized T20 steel after corrosion at 800 °C for 10~100h in N₂/H₂O/H₂S-mixed gases, the scale formed on the Fe₃Al coating consisted primarily of α-Al₂O₃, Al₂S₃, FeAl₂O₄ and FeO, with relatively slow scaling rates. The Fe₃Al intermetallics has reasonable corrosion-and oxidation-resistance, because it can form a protective alumina scale. Without the aluminized diffusion-coating, T20 steel corroded fast with serious scale failure. At the surface, coarse FeS grains with cracks formed. Since FeS has a very high concentration of cation vacancies, it grew fast through the outward diffusion of Fe²⁺ ions.

Abstract: The nickel-iron-chromium-based alloy, Incoloy alloy 800, was corroded at 600, 700 and 800 °C for 10-70 h under 1 atm of total pressure in three different atmospheres, viz., 1 atm of N₂, N₂/H₂O, and N₂/H₂O/H₂S-mixed gases. The corrosion rates always increased with addition of H₂O and, much more seriously, with the addition of H₂S gas. In N₂ and N₂/H₂O gases, oxidation prevailed. In N₂/H₂O/H₂S gases, sulfidation dominated. The corrosion resistance increased in the T22 steel displayed better resistance to oxidation and sulfidation than Fe-2Mn-0.5Si steel, owing to the presence of Cr. Strong enrichment of Cr and the presence of Ni and Fe were noticeable in the inner scale. Chromium sulfidized to FeCr₂S₄ in N₂/H₂O/H₂S gases, which was responsible for the enhanced sulfidation resistance of T22 compared with Fe-2.0Mn-0.5Si steel.

Abstract: The Ti-6Al-4V alloy was oxidized isothermally and cyclically in air, and its oxidation behavior was compared with that of Ti metal. The isothermal oxidation at 800°C indicated that Ti-6Al-4V and Ti oxidized fast almost linearly, and the oxide scales that formed on Ti-6Al-4V and Ti were non-adherent. The cyclic oxidation indicated that Ti-6Al-4V oxidized faster than Ti at 600°C, and serious scale spallation occurred in Ti-6Al-4V compared to Ti at 800°C. The oxide scales that formed on Ti-6Al-4V and Ti after cyclic oxidation at 800°C delaminated into several pieces owing to excessive stress aroused by the repetitive thermal shock.

Abstract: AZ31magnesium alloys with initially added (0.5, 1, 1.5) wt.% CaO particles were cast, hot extruded, and oxidized between 450 and 650°C in atmospheric air. Initially added CaO particles reacted with the AZ31 melt, and precipitated as Al₂Ca along the grain boundaries of the α-Mg matrix. They increased the oxidation resistance of the AZ31 alloys. The more CaO particles was, the better the oxidation resistance of the alloys was. During oxidation, MgO oxide scales that incorporated CaO formed at the surface of the AZ31 alloys.

Abstract: Three kinds of high-strength steel plates containing (less than 0.07%, or 0.024%, or 0.057%)-Si were oxidized at 700-900 °C isothermally and cyclically in atmospheric air, and their oxidation behavior was compared. The composition of the steels significantly affected the scaling rates, thickness, and adherence of the formed scales. The most important element in terms of oxidation was Si because Si affected the oxidation rates and scale adherence much. Silicon formed quite slowly a growing SiO₂–containing scale around the scale/matrix interface. In the Si-deficient steel, quite thick oxide scales formed, and their adherence was poor. An optimum amount of Si was necessary in order to decrease the oxidation rate, and improve the scale adherence.

Abstract: A low carbon steel was hot-dip aluminized, and corroded in the N₂/0.4%H₂S-mixed gas at 650-850°C for 20-50 h in order to find the effect of aluminizing on the high-temperature corrosion of the low carbon steel in the H₂S environment. A thin Al topcoat and a thick Al-Fe alloy layer that consisted primarily of Al₅Fe₂ and some FeAl and Al₃Fe formed on the surface after aluminizing. The corrosion rate increased with an increase in temperature. Hot-dip aluminizing increased the corrosion resistance of the carbon steel through forming a thin protective α-Al₂O₃ scale on the surface. The α-Al₂O₃ scale was susceptible to spallation. During corrosion, internal voids formed in the Al-Fe alloy layer, where the Al₅Fe₂, AlFe, and Al₃Fe compounds gradually transformed through interdiffusion.