• Korean Journal of Metals and Materials
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• v.50 no.5
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• pp.395-400
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• 2012

In this study, we have investigated the surface morphology of hydroxyapatite (HA) coated Ti alloy surface using pulsed laser plating. The HA (tooth ash) films were grown by pulsed KrF excimer laser, film surfaces were analyzed for topology, chemical composition, crystal structure and electrochemical behavior. The Ti-6Al-4V alloy showed ${\alpha}$ and ${\beta}$ phase, Cp-Ti showed ${\alpha}$ phase and the HA coated surface showed HA and Ti alloy peaks. The HA coating layer was formed with $1-2{\mu}m$ droplets and grain-like particles, particles which were smaller than the HA target particle, and the composition of the HA coatings were composed of Ca and P. From the electrochemical test, the pitting potential (1580 mV) of HA coated Ti-6Al-4V alloy was higher than those of Cp-Ti (1060 mV) and HA coated Cp-Ti (1350 mV). The HA coated samples showed a lower current density than non-HA coated samples, whereas, the polarization resistance of HA coated samples showed a high value compared to non-HA coated samples.

• Korean Journal of Metals and Materials
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• v.46 no.10
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• pp.652-661
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• 2008

Reduction of NOx in emission gas, improvement of engine efficiency, and extension of warranty period has made demands for developing materials more corrosively resistant to the inner-muffler environments or predicting the lifetime of materials used in muffler more precisely. The corrosion inside muffler has been explained with condensate corrosion mainly though thermal oxidation experiences prior to condensate corrosion. Hence, the aim of this study is to describe how the thermal oxidation affects the corrosion of stainless steel exposed to the inner-muffler environments. Auger electron spectroscopy and electrochemical tests were employed to analyze oxide scale and to evaluate corrosion resistance, respectively. Thermal oxidation has different role of condensate corrosion depending on the temperature: inhibiting condensate corrosion below $380^{\circ}C$ and enhancing condensate corrosion above $380^{\circ}C$. The low temperature oxidation causes to form compact oxide layer functioning a barrier for penetrating condensate into a matrix. Although though thermal oxidation caused chromium-depleted layer between oxide layer and matrix, the enhancement of the condensate corrosion in high temperature oxidation resulted from corrosion-induced crevice formed by oxide scale rather than corrosion in chromium-depleted layer. It was proved by aids of anodic polarization tests and measurements of pitting corrosion potentials. By the study, the role of high temperature oxidation layer affecting the condensate corrosion of stainless steels used as muffler materials was well understood.

• Corrosion Science and Technology
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• v.21 no.2
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• pp.147-157
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• 2022

Electrochemical characteristics and damage behavior of 6061-T6 aluminum alloy used as a battery housing material for electric vehicles were investigated in solution simulating the acid rain environment with chloride concentrations. Potentiodynamic polarization test was performed to analyze electrochemical characteristics. Damage behavior was analyzed through Tafel analysis, measurement of damage area, weight loss, and surface observation. Results described that corrosion current density was increased rapidly when chloride concentration excceded 600 PPM, and it was increased about 7.7 times in the case of 1000 PPM compared with 0 PPM. Potentiodynamic polarization experiment revealed that corrosion damage area and mass loss of specimen increased with chloride concentrations. When chloride concentration was further increased, the corrosion damage area extended to the entire surface. To determine damage tendency of pitting corrosion according to chloride concentration, the ratio of damage depth to width was calculated. It was found that the damage tendency decreased with chloride concentrations. Thus, 6061-T6 aluminum alloy damage becomes larger in the width direction than in the depth direction when a small amount of chloride is contained in an acid rain environment.

• Corrosion Science and Technology
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• v.22 no.1
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• pp.64-72
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• 2023

Ni-Ti shape memory alloy for dental nerve treatment devices was prepared by adding Mo to Ni-Ti alloy to improve flexibility and fatigue fracture characteristics and simultaneously increase corrosion resistance. Surface properties of the alloy were evaluated. Microstructure analysis of the Ni-Ti-xMo alloy revealed that the amount of needle-like structure increased with increasing Mo content. The shape of the precipitate showed a pattern in which a long needle-like structure gradually disappeared and changed into a small spherical shape. As a result of XRD analysis of the Ni-Ti-xMo alloy, R-phase structure appeared as Mo was added. R-phase and B2 structure were mainly observed. As a result of DSC analysis, phase transformation of the Ti-Ni-Mo alloy showed a two-step phase change of B2-R-B19' transformation with two exothermic peaks and one endothermic peak. As Mo content increased, R-phase formation temperature gradually decreased. As a result of measuring surface hardness of the Ti-Ni-Mo alloy, change in hardness value due to the phase change tended to decrease with increasing Mo content. As a result of the corrosion test, the corrosion potential and pitting potential increased while the current density tended to decrease with increasing Mo content.

• Korean Journal of Materials Research
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• v.33 no.7
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• pp.309-314
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• 2023

This study aimed to address the limitations of traditional plasma nitriding methods by implementing a short-term plasma oxy-nitriding treatment on the surface of AISI 420 martensitic stainless steel. This treatment involved the sequential formation of nitride and oxide layers, to enhance surface hardness and corrosion resistance, respectively. The process resulted in the formation of a 20 ㎛-thick nitride layer and a 3 ㎛-thick oxide layer on the steel surface. Initially, the hardness increased by 2.2 times after nitriding, followed by a subsequent decrease of approximately 31 % after oxidation. While the nitriding process reduced corrosion resistance, the subsequent oxidation process led to the formation of a passive oxide film, effectively resolving this issue. The pitting corrosion of the oxide passive film started at 82.6 mVssc, providing better corrosion resistance characteristics than the nitride layer. Consequently, the trade-off between surface hardness and corrosion resistance in plasma oxy-nitrided AISI 420 martensitic stainless steel is anticipated to be recognized as an innovative and comprehensive surface treatment process for biomedical components.

• Corrosion Science and Technology
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• v.23 no.2
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• pp.166-178
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• 2024

The localized corrosion resistance of UNS N07718 alloy was investigated after solution heat treatment. When the alloy was heat-treated at 1050 ℃ for 2.5 hours, it experienced an increase in average grain diameter, a reduction in grain boundary area, and the dissolution of delta phases along grain boundaries. Additionally, primary metallic nitrides (MN) and metallic carbides (MC), enriched with either Ti or Nb, were identified and exhibited a random distribution within the microstructures. Despite the solution heat treatment, the composition, diameter, and abundance of MNs and MCs remained relatively consistent. The critical pitting temperature (CPT), as determined by the ASTM G48-C immersion test, revealed similar values of 45 ℃ for both treated and untreated alloys. However, a decrease in maximum pit depth and corrosion rate was observed after the solution heat treatment. The microstructural changes that occurred during the heat treatment and their potential implications were discussed to understand the influence of the solution heat treatment.

• Nuclear Engineering and Technology
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• v.56 no.3
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• pp.1091-1099
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• 2024

Fluoride-salt-cooled High-temperature Reactor (FHR) is a promising nuclear reactor technology. Among many challenges presented by the molten fluoride salts is the corrosion of salt-facing structural components. Higher moisture contents, in the FLiNaK (LiF-NaF-KF, 46.5-11.5-42 mol%) salt, aggravate intergranular corrosion and pitting for the given alloys. Therefore, several thermal dehydration tests of FLiNaK salt were performed with a batch size suitable for thermal-hydraulic experiments. Thermogravimetric Analysis (TGA) was performed for the three constituent fluoride salts individually. Preliminary thermal dehydration plans were then proposed for NaF and KF salts based on the TGA curves. However, the dehydration process may not be required for LiF since its low mass loss (<1.3 wt%). To evaluate the performance of these thermal dehydration plans, a batch-scale salt dehydration test facility was designed and constructed. The preliminary thermal dehydration plans were tested by varying the heating rates, target temperature, and holding time. The sample mass loss data showed that the high temperatures (>500 ℃) were necessary to remove a significant amount of moisture (>1 wt%) from NaF salt, while relatively low temperatures (around 300 ℃) with a long holding time (>10 h) were sufficient to remove most of the moisture from KF salt.

• Journal of Electrochemical Science and Technology
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• v.15 no.2
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• pp.220-241
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• 2024

Due to its unique advantages, electrochemical machining (ECM) is playing an increasingly significant role in the manufacture of difficult-to-machine materials. Most of the current ECM research is conducted at room temperature, with studies on ECM in a cryogenic environment not having been reported to date. This study is focused on the electrochemical dissolution characteristics of typical iron and nickel base alloys in NaNO₃ solution at low temperature (-10℃). The polarization behaviors and passive film properties were studied by various electrochemical test methods. The results indicated that a higher voltage is required for decomposition and more pronounced pitting of their structures occurs in the passive zone in a cryogenic environment. A more in-depth study of the composition and structure of the passive films by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy showed that the passive films of the alloys are modified at low temperature, and their capacitance characteristics are more prominent, which makes corrosion of the alloys more likely to occur uniformly. These modified passive films have a huge impact on the surface morphologies of the alloys, with non-uniform corrosion suppressed and an improvement in their surface finish, indicating that lowering the temperature improves the localization of ECM. Together with the cryogenic impact of electron energy state compression, the accuracy of ECM can be further improved.

• The korean journal of orthodontics
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• v.30 no.4 s.81
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• pp.453-465
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• 2000

The purpose of this study was to investigate the change of mechanical properties, surface topography and frictional force of various nickel titanium wires after recycling. Three types of nickel-titanium wires and one type of stainless steel wire were divided to three groups: as-received condition(T0:control group), treated in artificial saliva for four weeks(T1) and autoclaved after being treated in artificial saliva(T2). Some changes were observed for the selected mechanical properties in tensile test, surface topography by means of SEM and 3D profilogram, and frictional coefficient. The findings suggest that: 1. Nickel-titanium wires demonstrated no statistically significant differences in maximum tensile strength, elongation rate and modulus of elasticity, but stainless steel wire demonstrated statistically significant differences in maximum tensile strength, elongation rate and modulus of elasticity between the groups(p<0.05). 2. NiTi, Optimalloy, Stainless Steel wires demonstrated increased pitting and corrosion in SEM finding. 3. Recycled NiTi, Optimalloy and stainless steel wires demonstrated significantly greater surface roughness(Ra and Rq) through 3D profilogram when compared with the control wires(p<0.05), but Sentalloy didn't demonstrate significant difference. 4. Recycled NiTi, Optimalloy and stainless steel wires demonstrated significantly greater maximum frictional coefficient when compared with the control wires(p<0.05), but Sentalloy didn't demonstrate significant difference The changes of surface roughness and frictional coefficient in NiTi and Optimalloy had no clinical implication. Consequently recycled nickel titanium wires demonstrated no clinical problem in tensile properties, surface topography and frictional coefficient.

• Korean Journal of Metals and Materials
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• v.47 no.2
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• pp.129-137
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• 2009

Corrosion behaviors of TiN coated dental casting alloys have been researched by using various electrochemical methods. Three casting alloys (Alloy 1: 63Co-27Cr-5.5Mo, Alloy 2: 63Ni-16Cr-5Mo, Alloy 3: 63Co-30Cr-5Mo) were prepared for fabricating partial denture frameworks with various casting methods; centrifugal casting(CF), high frequency induction casting(HFI) and vacuum pressure casting(VP). The specimens were coated with TiN film by RF-magnetron sputtering method. The corrosion behaviors were investigated using potentiostat (EG&G Co, 263A. USA) in 0.9% NaCl solution at $36.5{\pm}1^{\circ}C$. The corrosion morphologies were analyzed using FE-SEM and EDX. Alloy 1 and Alloy 2 showed the ${\alpha}-Co$ and ${\varepsilon}-Co$ phase on the matrix, and it was disappeared in case of TiN coated Alloy 1 and 2. In the Alloy 3, $Ni_2Cr$ second phases were appeared at matrix. Corrosion potentials of TiN coated alloy were higher than that of non-coated alloy, but current density at passive region of TiN coated alloy was lower than that of non-coated alloy. Pitting corrosion resistances were increased in the order of centrifugal casting, high frequency induction casting and vacuum pressure casting method from cyclic potentiodynamic polarization test.