Papers by Author: Dong Bok Lee

Abstract: SUS316L, TiN and CrN films were corroded at 500 °C for up to 30 h in 1 atm of (N₂/3.1%H₂O/2.42%H₂S)-mixed gas. SUS316L displayed poor corrosion resistance due to the formation of FeS. It corroded to (Fe, Ni)-mixed sulfides and FeS, forming highly fragile, porous, and nonadherent scales. TiN and CrN films suppressed the sulfidation of SUS316L.

Abstract: The thermal barrier coating that consisted of ZrO₂-8Y₂O₃ top-coat and Ni-22Cr-10Al-1Y bond-coat corroded in Ar-1%SO₂ gas at 1000-1200°C for up to 300 h. The top-coat and bond-coat consisted primarily of β-ZrO₂ and (γ-Ni, α-Al₂O₃), respectively. During corrosion, Al oxidized preferentially to α-Al₂O₃ to protect TBC, and sulfidation occurred to a small extent.

Abstract: The composite coating that consisted of 50%(Ni-22%Cr-10%Al-1%Y) and 50%(ZrO₂-8%Y₂O₃) was fabricated by air plasma spraying, and corroded at 800 and 900°C for 50 h in Na₂SO₄ or 75%NaCl-25%Na₂SO₄ salt. The scales consisted primarily of Cr₂O₃, NiO and α-Al₂O₃. However, most of them continuously dissolved off as complex ions into the salt during hot corrosion.

Abstract: The oxidation of Ti₃Al_0.7Si_0.3C₂ carbide at 1100 and 1200°C in air for up to 50 h resulted in the formation of rutile-TiO₂, α-Al₂O₃ and SiO₂. Depending on the location of the carbide, either thin, dense, single layered oxide scales or thick, porous, bi-layered oxide scales formed. The single layered scales consisted primarily of protective Al₂O₃. The bi-layered scale consisted primarily of an outer (TiO₂+Al₂O₃)-mixed oxides, and an inner TiO₂-rich, (Al₂O₃+SiO₂)-containing oxides. Since the testing temperatures were high, the oxidation rates varied much depending on the location of the carbide.

Abstract: SUS 310 stainless steels were corroded at 600, 700 and 800°C for up to 30 h in 1 atm of N₂/H₂O/H₂S-mixed gas. The formed scales consisted primarily of (Fe_0.5Ni_0.5)_0.96S, FeNiS₂, and FeCr₂S₄. They were fragile, porous, and susceptible to spallation. During corrosion, Fe, Ni, and Mn diffused outward, while sulfur and a lesser amount of oxygen diffused inward. The diffusion of Cr and Si was not conspicuous. The formation of sulfide scales and the presence of hydrogen and water vapor in the mixed gases made SUS 310 corrode fast.

Abstract: 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.

Abstract: The ASTM T23 steel with a composition of Fe-2.25Cr-1.6W-0.1Mo in wt.% was corroded in the Na₂SO₄ salt at 800 and at 900°C, and its corrosion behavior was studied. It formed thick, porous oxide scales that consisted primarily of outer Fe-rich oxides and inner (Fe, Cr)-rich oxides. The corrosion of the T23 steel progressed mainly via oxidation.

Abstract: Hot rolled mild steel was corroded in air, Ar/0.2%SO₂ and N₂/0.4%H₂S gases for 20 and 50 h. The corrosion in air at 800 and 1000°C resulted in the formation of Fe₂O₃/Fe₃O₄/FeO layered scales from the surface. The corrosion in Ar/0.2%SO₂ gases resulted in the formation of (FeS, Fe₂O₃, Fe₃O₄, FeO)-mixed scale at 800°C, and (Fe₂O₃, Fe₃O₄, FeO)/FeS layered scale at 1000°C. The corrosion in N₂/0.4%H₂S gases at 650 and 750°C led to the formation of (Fe₂O₃, Fe₃O₄, FeO)/FeS layered scales.

Abstract: 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.

Abstract: 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 Na₂SO₄ and NaCl were incoporated.