Jae-Hong Yoon

Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl−, F−, S2−, and O2−. The presence of Cl− when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.

Optimization of the Adhesion Strength of Arc Ion Plating TiAlN Films by the Taguchi Method

A three-level six-factor (arc power, substrate temperature, pre-treatment bias voltage, working pressure, deposition bias voltage and pretreatment time) orthogonal experimental array (L18) to optimize the adhesion strength of arc ion plating (AIP) TiAlN films was designed using the Taguchi method. An optimized film process, namely substrate temperature 220 °C, arc power 60 A, negative bias voltage -800 V, nitrogen pressure 10-2 Torr, pretreated voltage -450 V and pretreated time 15 minutes was obtained by the Taguchi program for the purpose of obtaining a larger critical load. The critical load of the optimized TiAlN film (53 N) was increased by 43% compared to the film with the highest critical load before optimization. The improvement in the adhesion strength of the films was attributed to the enhancement of hardness and the competitive growth of the (111), (200) and (220) orientations in the film.

Aluminizing and boroaluminizing treatments of Mar-M247 and their effect on hot corrosion resistance in Na2SO4−NaCl molten salt

The effect of surface modifications of Mar-M247 superalloy on hot corrosion resistance was examined in Na2SO4−NaCl molten salt. The Mar-M247 was aluminized and boroaluminized by pack cementation in Ar and underwent a cyclic hot corrosion test in Na2SO4−NaCl molten salt. The XRD results showed that a Ni2Al3 phase was formed between the aluminized layer and the substrate when the surface modification temperature was below 1273 K. However, a NiAl phase formed when the temperature was above 1273 K. The intensity of the XRD peak in the NiAl phase increased after post heat treatment. Hot corrosion resistance increased for the specimens containing NiAl rather than Ni2Al3 phase. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling. Post heat treatment increased the corrosion resistance of the aluminized layer for Mar-M247, which underwent surface modification at 1273 K and above. In the boroaluminized Mar-M247 specimens, corrosion resistance decreased as a result of the blocking of outward diffusion of Cr by boron and decreased cohesion between the oxide scale and the aluminized layer during thermal cycling.

Corrosion resistance of Fe2O3-Cr2O3 artificial passive films formed with LP-MOCVD

Fe2O3-Cr2O3 artificial passive films were formed with a low pressure MOCVD technique using iron (III) acetylacetonate and chromium (III) acetylacetonate. The relationships between the crystal structure, the chemical state of the constituent elements, and the corrosion resistance of the films were examined in acid solutions. The films deposited above 300°C hardly dissolved in 1.0 M HCl and those deposited below 250°C, however, easily dissolved in the same solution. The dissolution rate of the films in solution increased with decreasing substrate temperature. When polarized cathodically in 1.0 M H2SO4, the films deposited below 250°C dissolved due to the reduction of the Fe2O3 component in the films. The reduction of the Fe2O3 component was, however, suppressed on the films deposited above 300°C. Therefore, with increasing crystallinity and the amount of M-O type chemical bonds, the corrosion resistance of the films increases in HCl and H2SO4 solutions.

A Study on Microstructure, Mechanical Properties, Friction and Adhesion of TiN Thin Films Coated on SKD61 and Radical Nitrided SKD61 Substrates by Arc Ion Plating

TiN coating on tool steel has been widely used for the improvement of durability of tools. In this work, radical nitriding(RN) is carried out on SKD61 at for 5 hours in the ammonia gas pressure . The TiN coating is carried out by arc ion plating(AIP) with the process parameters: arc power 150 A, bias voltage -50V, coating time 40 minutes and nitrogen gas pressure . Hardness, elastic modulus, friction coefficient and adhesion of TiN coating on substrates of both TiN/SKD61 and TiN/RN SKD61 coatings are investigated comparatively. The primary crystalline faces of TiN surface are(200) and(111) for TiN/SKD61 and TiN/RN SKD61 respectively. In addition to the primary phase, Fe phase exists in TiN/SKD61 coating, but not in TIN/RN SKD61. The hardness of TiN/RN SKD61 is about 700 Hv, 250 Hv(56%) higher than that of TiN/SKD61 at the near interface of TiN and substrates. At the TiN surface, hardness of TiN/RN SKD61 is 2,149 Hv, 71 Hv(3%) higher than that of TiN/SKD61. The elastic modulus of TiN coating is improved to 26.7 GPa(6%) by radical nitriding. The adhesion is improved by the RN coating showing no spalling. buckling and chipping on the scratch test track which are shown on the non-RN TiN/SKD61.

Synthesis and characterization of cubic boron nitride thin films using the ME-ARE method

Cubic boron nitride (cBN) films were deposited by a magnetically enhanced activated reactive evaporation (ME-ARE) technique. Pulsed DC instead of r.f. power was used to bias the substrate. The effect of deposition parameters such as substrate bias voltage, plasma discharge current and gas flow ratio on the formation of cBN was investigated. BN films were characterized by FTIR and TEM. CBN films with a high content of cubic phase were successfully synthesized. It was found that formation of cBN film requires the bombardment of ions with both high flux and high energy. TEM observation showed that the cBN film had grain sizes of 15-50 nm and that a non-cubic phase BN, 10-15 nm was initially grown.

Wear Property of HOVF WC-CrC-Ni Coating Prepared by Optimal Coating Process

WC-CrC-Ni coatings were prepared by nine processes of the Taguchi program with three levels for the four spray parameters: spray distance, flow rates of hydrogen and oxygen, and powder feed rate. The optimal coating process (OCP) was oxygen flow rate of 38 FMR, hydrogen flow rate of 53 FMR, powder feed rate of 25 g/min, and spray distance of 7 inches. Hardness of 1150 Hv and porosity of 1.2 %, were obtained by OCP; these are better results compared with the highest 1033 Hv and the lowest 1.5% porosity obtained by nine processes of the Taguchi program. Friction coefficient of the WC-CrC-Ni coating decreased from 0.36 ± 0.07 at 25 oC to 0.23 ± 0.07 at 450 oC. These values were smaller than those of the EHC (electrolytic hard chrome) plating at both temperatures due to lubrication from the oxide debris. The wear trace and wear depth of the coating are smaller than those of the EHC at both temperatures. Pitting was not found in the WC-CrC-Ni coating sample, while it did appear in the EHC sample.

Effect of post-N+ implantation on the microstructure of the interfacial non-cubic BN layers

Plasma source ion implantation has been applied as a post treatment in order to modify the interfacial sp2 bonded layer of the cubic BN (cBN) films. The effect of ion irradiation on the microstructure of the noncubic BN layer was investigated. From HRTEM observation, the thickness of the BN layer of the representative sample was about 100 nm including 15 nm of initially grown non-cubic layer at the interface between a substrate and the cubic layer. At optimal plasma source implantation conditions, an acceleration voltage of 50 keV and an ion dose of 2×1016 ions/cm2, the microstructure of the interfacial non-cubic BN layer was changed. It was noticed that the ion irradiation caused the recrystallization of the amorphous phase and transformed the small crystallites into another phase. The micro-hardness of the complex consisting of a hard and soft layer increased as a result of phase transformations. This was caused by atomic displacements in the initially grown amorphous and hexagonal layers. The atomic displacements calculated by the TAMIX code were in the range of 0.31–0.52 nm per atom within 70–100 nm in depth.

Study of Surface Alloying of TiC, TiB 2 and VC with Carbon Steel Using High Energy Electron Beam Irradiation

Surface alloying using TiC, and VC ceramic particles on carbon steel has been performed using high voltage electron beam. Each type of ceramic particles was mixed with flux of Al and in 1 to 4 ratio. The microstructures of the surface alloyed layers consisted of melted region, interface region. heat affected region and the unaffected matrix. The surface layer of the TiC surface alloyed had a cubed primary and a eutectic type of TiC. in surface layer of surface alloyed were incompletely melted with particles as observed before the alloying. On the surface layer of the VC surface alloyed, very well defined cell structure was observed with VC on the cell boundary. In addition, ~50 nm in diameter VC particles in high density were ubiquitous in the matrix. Those fine VC particles prominently improved the hardness and wear resistance of the surface layer of the VC surface alloyed.

Corrosion Resistance of Artificial Passive Film and Real Passive Film on Fe-Ti Alloy

The corrosion resistances of Fe2O3-TiO2 artificial passive film and real passive film on the sputter-deposited Fe-Ti alloy films were examined in acid solutions. The Fe2O3-TiO2 films containing less than XTi=0.70 have a spinal structure, the films containing more than XTi=0.70 have an amorphous structure. The dissolution rate of the Fe2O2-TiO2 films in 5 M HCI decreased with an increase in the titanium cationic fraction in the films. The Fe2O3TiO2 films dissolved at cathodic potentials in 1 M H2SO4 and 1 M HCl owing to the selective reduction of Fe2O3 components in the films. Sputter-deposited Fe-Ti alloy films containing more than 39 at.% Ti passivated anodically in 1 M H2SO4 and 1 M HCl. and showed high corrosion resistance. The Fe2O3TiO2 artificial passive films have a higher corrosion resistance than real passive films on the sputter-deposited Fe-Ti alloy films in 5 M HCl.