Dong Bok Lee

High-Temperature Oxidation of WC-20%TiC-10%Co Carbides

The oxidation characteristics of WC-20%TiC-10%Co sintered carbides were studied by oxidizing at 700, 800 and 900 °C for 3 h in air. The samples oxidized fast with large weight gains, displaying quite poor oxidation resistance. The formed oxide scales that consisted primarily of CoWO4, WO3, and TiO2 were porous, and prone to cracking.

High Temperature Oxidation of CrAlSiN Thin Films

Nano-multilayered, crystalline CrAlSiN thin films were deposited on either steel or WC-10%Co substrates by the cathodic arc plasma deposition. Their oxidation characteristics were studied at 800-1000°C for 50 h in air. The film deposited on steel displayed good oxidation resistance, due mainly to formation of Cr2O3 and α-Al2O3. The film deposited on WC-10%Co displayed poor oxidation resistance, due mainly to the oxidation of the substrate.

High-Temperature Corrosion of Incoloy Alloy 800 in N2/H2S Gas

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.

High Temperature Oxidation Behavior of High Strength Steel Plates

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.

Isothermal and Cyclic Oxidation of Ti-6Al-4V Alloy

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.

Oxidation of Hot Extruded AZ31 Magnesium Alloys Containing Initially Added CaO Particles

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.

High Temperature Corrosion of Al Hot-Dipped Low Carbon Steels in N2/H2S-Mixed Gases

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.

Oxidation of TiAlCrSiN Thin Films at 1000°C in Air

TiAlCrSiN thin films consisting of alternating TiCrN and AlSiN nanolayers were deposited by cathodic arc plasma deposition, and oxidized at 1000°C in air. When oxidized for 10 h, about 1 μm-thick oxide sale formed, and its surface was covered with numerous tiny oxide crystallites. When oxidized for 30 h, about 2.5 μm-thick oxide scale formed, and began to spall from the surface. When oxidized for 80 h, the oxide sale was about 12.2 μm-thick. The film had a reasonable oxidation resistance due mainly to Al, Cr, and Si, which formed protective oxides.

Corrosion of T92 Steels in Na2SO4 Salts at 800 and 900°C

The corrosion behavior of the ASTM T92 steel in the Na2SO4 salt was studied at 800 and 900°C. At 800°C, the T92 steel formed thin (Fe, Cr)-rich oxide scales, indicating a good corrosion resistance due to 9%Cr presented in the alloy. However, at 900°C, the T92 steel displayed poor corrosion resistance, because the scales dissolved off rapidly owing to the increased corrosion rate.

Hot Corrosion of Inconel 740 Alloys in Na2SO4-NaCl Salts

The Ni-Cr-Co based superalloy, Inconel 740, was corroded between 700 and 900°C for 20 h in 75% Na2SO4-25% NaCl salt. The scale consisted primarily of Cr2O3, (Ni,Co)Cr2O4, NiO, and Ni3S2. During corrosion, oxidation dominated rather than sulfidation. The scale was neither dense nor compact owing to the formation of the (oxides, sulfides)-mixed scale where Na2SO4and NaCl were incoporated.